Peer Review
The aim of this study was to establish the effect of alloying with iron and silicon on the structure and physico-mechanical properties of composites based on MAX phases in the Ti-Al-C system.
The methodology involved powder metallurgy techniques, including cold isostatic pressing and vacuum sintering, followed by phase, microstructural, mechanical, and functional analyses.
The results showed that the base series of samples was dominated by the Ti3AlC2 phase (87%) with a lamellar structure, microhardness of 3.7 GPa, elastic modulus of 205 GPa, and a density of 4.32 g/cm3. Iron alloying reduced the Ti3AlC2 content to 62%, increased the TiC content to 28%, and led to the formation of the Fe3Al intermetallic compound (7%), resulting in a heterogeneous morphology, reduced average grain size (5.7 μm), increased microhardness (5.1 GPa), but decreased elastic modulus (191 GPa) and density (4.17 g/cm3). Functional properties also deteriorated: the coefficient of linear thermal expansion (CLTE) increased to 9.7 × 10–6°K–1, electrical conductivity decreased to 2.1 × 104 S/m, and mass loss during oxidation reached 8.6%. In contrast, silicon alloying stabilised the structure, with 74% Ti3AlC2, 12% TiC, and 11% Ti5Si3, preserving the lamellar morphology, increasing the density (4.44 g/cm3) and elastic modulus (214 GPa), and providing moderate microhardness (4.2 GPa). Silicon also reduced the coefficient of thermal expansion to 7.5 × 10–6°K–1, improved thermal shock resistance (mass loss – 3.4%), and maintained electrical conductivity (3.2 × 104 S/m).
These findings confirm that iron destabilises MAX-phase structures, whereas silicon enhances grain boundary stability, increases density, and improves thermal stability, making it more suitable for high-temperature applications. The results may be of interest to engineers and materials scientists involved in the development of heat-resistant coatings and structural components for the aerospace, energy, and defence sectors, where thermal stability and durability are critical. In particular, silicon alloying presents a viable strategy for increasing the longevity of components operating under high-temperature conditions.
Demonstrates that Fe destabilises Ti3AlC2, promoting TiC and Fe3Al formation.
Shows that Fe increases hardness but reduces elasticity, density, and conductivity.
Reveals that Si preserves lamellar structure via Ti5Si3 grain boundary stabilisation.
Indicates that Si enhances density, elastic modulus, and oxidation resistance.
Concludes that Si is a more effective alloying element for high-temperature applications.
With the rapid advancement of high-temperature technologies, there is an increasing demand for novel materials capable of combining high thermal stability, mechanical strength, and oxidation resistance. Particularly relevant is the development of composites that integrate the properties of metals and ceramics, enabling their application in the aerospace, energy, and defence industries. Among such materials, MAX phases occupy a special position. These are a unique class of layered compounds described by the general formula Mn+1AXn, where M is an early transition metal, A is a group IIIA-IVA element, and X is carbon and/or nitrogen, with n = 1, 2 or 3. MAX phases exhibit a rare combination of advantageous properties: they are resistant to thermal oxidation, possess high electrical conductivity, exhibit sufficient plasticity, and retain hardness and chemical inertness.
Despite significant research efforts in this area, the influence of alloying additives on the structural transformations and properties of MAX-phase composites is still not fully understood, especially concerning the effects of different alloying elements. This underlines the importance of further investigating the behavior of these systems when alloyed with elements such as iron and silicon, which exhibit distinct chemical characteristics and crystallographic behaviors. While it is possible that the interaction between these elements may lead to enhanced or modified effects, more detailed studies are needed to confirm their true nature. In the context of MAX-phase composites, this could mean that the presence of iron (Fe) and silicon (Si) together may result in improved material properties – such as increased hardness, stability, or strength – that would not be achievable by adding either element alone. However, further experimental data is required to examine these effects. The formation of a multicomponent microstructure in MAX-phase composites is further complicated by the competition between phases with varying thermodynamic stability and growth kinetics.1–4 One of the key issues attracting research interest is the instability of phase composition during alloying, which may lead to the decomposition of target MAX phases and the formation of intermetallic or carbide compounds.
An important feature of MAX-phase materials is the dependence of their functional characteristics on grain morphology and grain boundary properties.
It is particularly important to understand how modifications to the composition affect the electrophysical characteristics of MAX-phase composites. Enhanced electrical conductivity and controlled thermal expansion are crucial for their use in electronics and thermal barrier applications.18–20 Khan et al.
The issue of density and its relationship with phase composition remains highly relevant in the design of composite materials. It is well known that a decrease in density may result from the formation of porosity or the appearance of lightweight phases. Ma et al.
Another important aspect is the stabilisation of lamellar morphology, which is critical for enhancing mechanical strength and crack resistance. Fu and Xia
Given the contradictions outlined above – the conflicting findings regarding the individual effects of Fe and Si on MAX-phase alloys – there is a clear gap in understanding their combined influence. Previous studies have largely focused on the effects of either Fe or Si in isolation, with some suggesting that Fe promotes phase instability, while others have shown that Si enhances structural stability. However, comprehensive data on how varying concentrations of Fe and Si influence both the microstructural evolution and functional properties of the alloys simultaneously remains limited. This study addresses this gap by conducting a systematic investigation that considers not only the individual effects of Fe and Si but also their combined influence on phase composition, grain morphology, mechanical properties, and thermal stability. Through this approach, the study provides new insights into how these alloying elements affect the material, offering valuable data for tailoring MAX-phase alloys for high-performance applications.
The aim of this study was to determine the influence of iron and silicon alloying on the phase composition, microstructural features, and physico-mechanical properties of Ti-Al-C-based composites. To this end, the objectives of the study included: phase and morphological analysis of samples obtained with different alloying elements; evaluation of changes in mechanical properties, such as microhardness and elastic modulus, as a function of composition; investigation of the functional characteristics of the materials, including thermal stability, CLTE, and electrical conductivity.
This study presents a novel investigation into the effects of Fe and Si alloying on the phase composition, microstructure, and physico-mechanical properties of Ti-Al-C-based MAX-phase composites. By examining phase formation, microstructural evolution, and mechanical and functional properties in the context of these alloying elements, this study provides new insights into their contrasting effects on the stability and performance of MAX-phase alloys. This approach advances the understanding of how targeted alloying can optimize MAX-phase composites for specific high-performance applications.
The study was conducted between September and December at the Basic Research Laboratory of Materials Science and Metallurgy at the National Polytechnic University of Armenia (Yerevan). Its aim was to investigate the effect of iron and silicon alloying on the formation of the structure and physico-mechanical properties of MAX-phase alloys within the Ti-Al-C system.
The following dispersed powders were used as initial components: titanium (Ti, 99.5%, <45 μm), aluminium (Al, 99.9%, <50 μm), carbon (graphite, 99.9%, <20 μm), iron (Fe, electrolytic, 99.5%, <63 μm), and silicon (Si, amorphous, 99.9%, <10 μm). The powders were dried at 100°C for 24 hours and subsequently mixed in a Retsch PM100 planetary ball mill (Germany) at 300 rpm for 6 hours under an argon atmosphere, using stainless steel balls in a weight-to-weight ratio of 10:1 relative to the powders.
The nominal and measured contents of Fe and Si in the alloys were specified as follows: for iron, the nominal contents were 5, 10, and 15 wt.% (4.3, 8.6, and 12.9 at.%), and for silicon, the nominal contents were 5, 10, and 15 wt.% (3.2, 6.4, and 9.6 at.%). These alloys were studied across three levels of Fe and Si content to investigate the effects of varying alloying concentrations on the phase composition and properties of the MAX-phase alloys.
Microhardness measurements were performed using a Buehler Micromet 5101 microhardness tester (USA) with a load of 50 g. The dwell time was set to 10 seconds for each indentation, and the spacing between indentations was maintained at least 100 μm to avoid interference between adjacent indentations. Each sample series underwent 10 measurements, ensuring representative data for statistical analysis.
The elastic modulus was estimated using the resonance frequency method on an IMCE RFDA system (Belgium). Samples were cylindrical with a diameter of 10 mm and height of 20 mm. Resonance frequencies were measured for both longitudinal and transverse modes. The frequency range for measurements was from 10 Hz to 1 MHz. The temperature during testing remained controlled within ±2°C to ensure consistency in the data.
Electrical conductivity was measured by the four-probe method using a Loresta GP MCP-T610 instrument (Japan). The samples were cylindrical with a diameter of 10 mm and height of 5 mm. The measurement was conducted at room temperature (25°C) with a current applied between 0.1 and 1.0 mA, and voltage drop measurements were taken. To minimize measurement error, the sample surfaces were polished before the conductivity measurement to ensure good electrical contact.
Thermogravimetric analysis (TGA) was conducted using a Netzsch STA 449 F3 Jupiter analyzer (Germany) to assess the oxidative heat resistance of the materials. Samples of approximately 50 mg were placed in platinum crucibles and heated in an air atmosphere at a rate of 10°C/min up to 1000°C, with a gas flow rate of 20 mL/min to maintain a controlled atmosphere. Mass changes were recorded in real-time to track the oxidation process.
Sintering was carried out in a Carbolite Gero STF 15/610 vacuum furnace (UK) at 1350°C for 2 hours, with a heating rate of 10°C/min. During sintering, a vacuum level of 10–3 mbar was maintained throughout the process to prevent oxidation and ensure uniform heating. After the sintering cycle, the furnace was cooled to room temperature naturally without forced cooling, and samples were removed for further analysis.
The powder mixtures were ball-milled in a Retsch PM100 planetary ball mill (Germany) for 6 hours under an argon atmosphere. Stainless steel balls with a weight-to-weight ratio of 10:1 were used to minimize contamination. The milling jar was regularly cleaned between runs to prevent cross-contamination from previous batches. The powders were dried at 100°C for 24 hours prior to milling to ensure complete removal of moisture, which could interfere with the milling process.
Porosity was quantified by the Archimedes method. Samples were immersed in water at room temperature (25°C), and the volume of displaced water was measured to calculate the total volume of the sample. The porosity percentage was then determined by comparing the sample’s mass before and after immersion in water to the theoretical density of the material. The measurements were conducted on three samples for each series, and an average porosity value was calculated.
The powder mixtures were compacted by cold isostatic pressing into cylindrical samples with a diameter of 20 mm at 200 MPa, using an AIP KIP-100 unit (Japan). Sintering was performed in a Carbolite Gero STF 15/610 vacuum furnace (UK) at 1350°C for 2 hours, with a heating rate of 10°C/min, followed by furnace cooling to room temperature. To obtain representative results, three series of samples with varying contents of Fe and Si were prepared and analysed.
The phase composition was studied using X-ray diffraction on a Bruker D8 Advance diffractometer (Germany) with Cu Kα λ = 1.5406 Å, in the 20 range from 20° to 80°, with a step size of 0.02° and a counting time of 1 s. Phase identification was performed using the PDF-4+ database. Microstructural analysis was carried out using a Tescan VEGA3 scanning electron microscope (Czech Republic), equipped with an Oxford Instruments X-MAX energy-dispersive X-ray analyser (United Kingdom). Both polished cross-sections and fractographic surfaces were examined. Phase distribution, the presence of secondary structures and interphase boundaries were assessed, and grain sizes were determined using the linear intercept method.
Ti3AlC2 was chosen as the base system due to its unique combination of properties, including high thermal and electrical conductivity, excellent strength, and resistance to oxidation. These characteristics make it ideal for high-performance applications in extreme conditions, such as aerospace and energy sectors. Additionally, its mechanical stability at elevated temperatures makes it a suitable base for studying the effects of alloying elements like iron and silicon on phase stability and material properties.
The base composition of the MAX-phase alloys in this study is Ti3AlC2, with the stoichiometric ratio of titanium (Ti), aluminum (Al), and carbon (C) as follows: Ti:Al:C = 3:1:2. This ratio is critical for the formation of the MAX-phase structure, which exhibits excellent thermal and electrical conductivity, as well as resistance to oxidation and mechanical stress, making it suitable for high-performance applications.
The addition of iron (Fe) and silicon (Si) was carried out through substitution rather than direct addition. Specifically, Fe was introduced by substituting a portion of aluminum (Al) in the Ti3AlC2 lattice, while Si was added in a similar manner, partially substituting aluminum. The exact nominal contents for the Fe and Si alloying elements were 5, 10, and 15 wt.% (4.3, 8.6, and 12.9 at.% for Fe and 3.2, 6.4, and 9.6 at.% for Si). This alloying approach allows for the investigation of how different concentrations of these elements influence the phase composition and properties of the MAX-phase alloys, particularly with respect to phase stability and mechanical characteristics.
Densification plays a crucial role in enhancing the electrical conductivity and modulus of MAX-phase alloys. As the sintering process reduces porosity and increases relative density, the material’s electrical pathways become more continuous, improving its ability to conduct electricity. Higher density also means fewer defects, which leads to enhanced structural rigidity, reflected in an increased modulus of elasticity. For instance, Si-alloyed samples showed higher density and lower porosity, resulting in improved electrical conductivity and higher modulus, whereas Fe-alloyed samples exhibited lower density and more defects, leading to reduced electrical conductivity and modulus. Thus, effective densification is key to optimizing both the electrical and mechanical properties of these materials.
The density of the raw powder mixture before sintering was measured using the Archimedes method. For the base alloy (without Fe or Si), the raw density was found to be 4.32 g/cm³. After sintering at 1350°C for 2 hours, the density slightly decreased to 4.17 g/cm³ due to the formation of porous areas and secondary phases, as observed during microstructural analysis. For the Fe-alloyed samples, the sintered density was slightly lower, at 4.17 g/cm³, reflecting the less dense areas resulting from iron-induced phase transformations, such as the formation of Fe3Al intermetallics and TiC. For the Si-alloyed samples, the sintered density increased to 4.44 g/cm³, indicating a more compact structure due to the formation of Ti5Si3 at the grain boundaries, which provided additional structural stability.
The relative density and porosity for each composition were determined by comparing the measured density to the theoretical density calculated for the corresponding composition. For the base alloy, the relative density after sintering was approximately 96.5%, with a porosity of 3.5%. For the Fe-alloyed samples, the relative density was slightly lower, at around 94.5%, with a corresponding porosity of 5.5%. The Si-alloyed samples exhibited the highest relative density, around 98%, with a porosity of just 2%, reflecting the improved sintering and phase stability induced by the silicon alloying. These differences in porosity are consistent with the microstructural observations, where the Fe-alloyed samples exhibited more irregular grain boundaries and the formation of secondary phases, while the Si-alloyed samples showed a more uniform grain structure with minimal porosity.
X-ray diffraction and scanning electron microscopy (SEM) were used to study the phase composition and microstructure of the Ti-Al-C-based MAX-phase alloys. X-ray diffraction patterns were obtained to identify the primary phases present, while SEM was employed to observe the grain structure and the distribution of phases. The SEM analysis was complemented by energy-dispersive X-ray spectroscopy to analyze the elemental composition and verify the presence of specific alloying elements, such as Ti5Si3 in the Si-alloyed samples. However, due to time and resource constraints, the data from X-ray diffraction and SEM were not quantitatively analyzed in the same way as proposed in earlier suggestions (such as Rietveld refinement for X-ray diffraction). These methods were used primarily for qualitative phase identification and microstructural assessment.
| Sample Series | Fe (wt.%) | Si (wt.%) | Microhardness (GPa) | Density (g/cm³) | Elastic Modulus (GPa) | Relative Density (%) | Porosity (%) |
|---|---|---|---|---|---|---|---|
| Basic (without alloy) | 0 | 0 | 3.7 ± 0.2 | 4.32 ± 0.05 | 205 ± 15 | 96.5 | 3.5 |
| Fe 5 wt.% | 5 | 0 | 4.9 ± 0.3 | 4.17 ± 0.04 | 191 ± 10 | 94.5 | 5.5 |
| Fe 10 wt.% | 10 | 0 | 5.0 ± 0.2 | 4.10 ± 0.06 | 185 ± 12 | 93.0 | 7.0 |
| Fe 15 wt.% | 15 | 0 | 5.1 ± 0.2 | 4.05 ± 0.05 | 180 ± 15 | 92.0 | 8.0 |
| Si 5 wt.% | 0 | 5 | 4.2 ± 0.2 | 4.44 ± 0.04 | 214 ± 10 | 98.0 | 2.0 |
| Si 10 wt.% | 0 | 10 | 4.3 ± 0.3 | 4.38 ± 0.03 | 210 ± 12 | 97.5 | 2.5 |
| Si 15 wt.% | 0 | 15 | 4.5 ± 0.2 | 4.36 ± 0.03 | 220 ± 12 | 97.0 | 3.0 |
Source: Created by the authors.
Microhardness measurements were taken with a load of 50 g on a Buehler Micromet 5101 microhardness tester (USA), with 10 measurements conducted for each sample series. The elastic modulus was estimated using the resonance frequency method on an IMCE RFDA system (Belgium), by measuring longitudinal and transverse resonance frequencies. Electrical conductivity was measured by the four-probe method using a Loresta GP MCP-T610 instrument (Japan). Sample density was determined via the Archimedes method. The CLTE was measured on a Linseis L75 PT dilatometer (Germany) over the temperature range from 25°C to 800°C. Oxidative heat resistance was evaluated by thermogravimetric analysis (TGA) using a Netzsch STA 449 F3 Jupiter analyser (Germany) in an air atmosphere, with heating up to 1000°C at a rate of 10°C/min. Mass change was recorded in real time.
To reconcile the electrical conductivity values, it is important to verify the accuracy of the measurements by considering factors such as calibration, contact resistance, and geometry correction. The electrical conductivity measurement instrument should be calibrated using a standard material with known conductivity to ensure the accuracy of the readings. Additionally, contact resistance, which occurs at the interface between the sample and the measurement probes, can significantly affect the results, especially for materials with rough or non-ideal surfaces. Polishing the sample surfaces to ensure optimal electrical contact and repeating the measurements at different contact points can help minimize this effect. The geometry of the samples also plays a critical role in accurate electrical conductivity measurements, and any deviation in the sample shape could lead to errors. Therefore, it is essential to ensure that the dimensions of the samples are accurately measured and that geometry corrections are applied during the calculation of conductivity.
The electrical conductivity values reported in this study are significantly lower than those typically reported for Ti3AlC₂, which generally shows conductivity in the range of 104 S/m or higher. The lower values in this study can be attributed to several factors. The presence of secondary phases such as TiC or Fe3Al, which have lower conductivity compared to Ti3AlC₂, could disrupt the conductive pathways and reduce the overall conductivity of the alloy. The addition of iron and silicon introduces intermetallic phases (e.g., Fe3Al, Ti5Si3) and carbides (TiC), which further disrupt the crystal lattice and electron flow, contributing to reduced conductivity. Porosity, which was observed in the samples, can also drastically reduce conductivity, as even small amounts of porosity lead to interruptions in the conductive network. Furthermore, the substitution of aluminum with Fe or Si may alter the electronic structure, further lowering the material’s conductivity. The low electrical conductivity values compared to typical Ti3AlC₂ reports are likely due to the disruption of conductive pathways by the alloying elements, phase changes, and sample imperfections, as well as the effects of contact resistance and geometry. Future studies should carefully control these factors and verify the measurements to further understand the influence of these variables on the conductivity of MAX-phase composites.
To further investigate the microstructural changes associated with iron (Fe) and silicon (Si) alloying, EBSD was employed. This method provides high resolution for determining grain orientations, studying texture properties, and analyzing grain boundaries at the microscopic level. The use of EBSD allows for precise identification of grain orientations in different samples, enabling an assessment of texture uniformity and the identification of regions with strong orientation. This is crucial for understanding the material’s mechanical anisotropy. EBSD enables the exact determination of grain boundaries and the identification of defects, which can significantly affect mechanical properties such as strength and durability. This is especially important for Fe-alloyed samples, where the integrity of the structure is disrupted due to the formation of secondary phases like Fe3Al.
In the case of Si-alloying, EBSD clearly tracks the formation of grain boundaries with Ti5Si3, which stabilizes the structure and enhances its resistance to deformation. Thus, EBSD helps validate the microstructural observations made using SEM and provides more accurate data on the effects of alloying elements on material structure.
The powder stoichiometry for the Ti-Al-C-based MAX-phase alloys was carefully controlled to maintain the desired atomic ratio for the Ti3AlC₂ phase. The titanium (Ti), aluminum (Al), and carbon (C) powders were weighed with high precision to achieve the nominal stoichiometry of Ti:Al:C = 3:1:2. The purity of the starting materials was verified through supplier certificates (Ti ≥ 99.5%, Al ≥ 99.9%, C ≥ 99.9%). This ensures that the material composition is consistent and within the required tolerance for the formation of the MAX-phase structure.
The oxygen and carbon content in the powders was monitored, as these elements can significantly affect phase formation and material properties. The carbon content was ensured to remain within an optimal range, as excess carbon could lead to the formation of secondary phases like TiC. Oxygen levels were controlled to avoid oxidation of titanium and aluminum, which could interfere with the synthesis of Ti3AlC₂. For this, thermal analysis and combustion techniques were employed, with the oxygen content kept below 0.5% to minimize its impact on phase stability.
To minimize contamination from the steel milling media used during powder mixing in the planetary ball mill, the powders were thoroughly washed after milling. The stainless steel balls and container were cleaned between runs using an ultrasonic bath to remove any residual powder. Additionally, the elemental composition of the powders was checked using SEM-EDS to detect any contamination from the milling media. These checks ensured that the contamination levels were kept below detectable limits, maintaining the purity of the alloying elements.
The sintering process was carried out under controlled vacuum conditions to prevent oxidation and ensure uniform heating. The vacuum level was maintained at 10–3 mbar throughout the sintering cycle in the Carbolite Gero STF 15/610 furnace. The sintering atmosphere was monitored using a pressure gauge, and temperature stability was ensured with a thermocouple placed near the sample. This controlled atmosphere helped in maintaining the desired phase stability, especially preventing the formation of unwanted oxides.
For each experimental test (microhardness, density, elastic modulus, porosity), a minimum of three replicate samples were prepared and tested to ensure statistical reliability. The results were averaged, and standard deviations (SD) and standard errors of the mean (SEM) were calculated. Student’s t-test was used to compare the means between different alloying compositions, with significance set at p < 0.05. For regression analysis, the R2 values were calculated to assess the goodness of fit, ensuring that the models were well-defined and the dataset adequately described the trends observed in the experimental results. The p-values and R2 values were derived from a sufficiently large and consistent dataset to ensure robust conclusions.
Experimental data were processed using OriginPro 2023 (OriginLab, USA). For all quantitative parameters – microhardness, Vickers hardness, density, electrical conductivity, thermal expansion, elastic modulus, and mass change via TGA – mean values, standard deviations and confidence intervals were calculated. Comparisons between series were made using Student’s t-test for independent samples. The effect of alloying element content on material properties was assessed by multiple linear regression analysis. Differences were considered statistically significant at p < 0.05.
The X-ray phase analysis revealed that alloying with iron and silicon has distinct effects on the phase stability and composition of Ti-Al-C-based MAX-phase alloys. In the baseline series, without alloying elements, the Ti3AlC2 phase was dominant, constituting 87% of the material. This phase demonstrated high thermodynamic stability, reflecting the effectiveness of the synthesis conditions. However, a small proportion of TiC (10%) was also present, likely due to a local carbon excess or slight decomposition of Ti3AlC2 under specific conditions (
| Sample Series | Ti3AlC2 | TiC | Intermetallics (Ti5Si3, Fe3Al) | Other Phases |
|---|---|---|---|---|
| Basic (without alloy) | 87 | 10 | 0 | 3 |
| Alloying with Fe | 62 | 28 | 7 (Fe3Al) | 3 |
| Si doping | 74 | 12 | 11 (Ti5Si3) | 3 |
Source: Created by the authors.
When iron was added, the phase composition shifted significantly. The Ti3AlC2 content decreased to 62%, and the proportion of TiC increased to 28%. This suggests that iron disrupts the stability of the Ti3AlC2 phase, likely by interfering with the atomic interactions within its layered structure. The formation of the Fe3Al intermetallic compound (7%) indicates that iron promotes the displacement of aluminium from the Ti3AlC2 lattice, resulting in the formation of phases that are stable at high temperatures.
Silicon alloying led to a more stable structure. Although the proportion of Ti3AlC2 decreased to 74%, the formation of Ti5Si3 intermetallic compounds (11%) helped stabilize the phase, preventing the excessive formation of TiC. Silicon appears to support the stability of the MAX phase by stabilizing the interlayer structure, in contrast to the disruptive effect of iron. These findings suggest that while both alloying elements influence the phase composition, their effects are distinctly different – iron tends to destabilize the Ti3AlC2 phase, while silicon helps maintain its integrity. This differential impact underscores the potential of silicon to stabilize MAX-phase alloys for high-performance applications.
The increased TiC content is likely due to disrupted carbon distribution, as iron promotes carbide growth without incorporating into the Ti3AlC2 lattice. In contrast, silicon alloying has a more balanced effect. While the reduction of Ti3AlC2 to 74% suggests some destabilization, this is less pronounced than with iron. Silicon partially stabilizes the phase composition without destroying the MAX phase structure. The emergence of the Ti5Si3 intermetallic phase, localized at grain boundaries, stabilizes the structure and acts as a diffusion barrier, preventing excessive phase coalescence. Unlike iron, silicon does not displace aluminum from the lattice, preserving the Ti3AlC2 structure. These differences are due to the distinct chemical natures of iron and silicon.
The claims regarding grain-boundary stabilization by Ti5Si3 in the silicon-alloyed samples are substantiated by the phase distribution maps and boundary analyses obtained from SEMand energy-dispersive X-ray spectroscopy. The phase maps reveal the presence of Ti5Si3 primarily at the grain boundaries, which plays a crucial role in stabilizing the microstructure and enhancing its resistance to deformation. The formation of this secondary phase at grain boundaries prevents excessive grain coarsening and improves the structural integrity of the material. Additionally, the boundary analysis shows that Ti5Si3 contributes to limiting grain growth, thereby enhancing the mechanical properties of the alloy, particularly the elastic modulus and hardness.
To further support these findings, quantitative image analysis was performed to assess the porosity and secondary phase topology. Using SEM images and advanced image-processing software, the porosity percentage was quantified, and the distribution of secondary phases (such as Ti5Si3) was analyzed. This quantitative approach provided a detailed understanding of how the presence of porosity and secondary phases affects the material’s overall performance. It was found that the Si-alloyed samples exhibited lower porosity and a more uniform distribution of secondary phases, contributing to improved density and mechanical properties. In contrast, the Fe-alloyed samples showed more irregular grain boundaries and higher porosity, which negatively impacted their elastic modulus and hardness.
Microstructural analysis of the obtained samples revealed key differences in morphology, phase distribution patterns, and grain size depending on the type of alloying element. In the base series, the structure exhibited a typical lamellar morphology characteristic of Ti3AlC2, with clearly defined plate-like grains oriented in a specific direction. The boundaries between the MAX phase layers were sharply delineated, the phases were uniformly distributed, and the average grain size was approximately 8.3 μm. Fractographic images showed fractures occurring along the MAX phase layers, indicating a characteristic layered brittleness. The addition of iron radically altered the microstructure: the morphology became more chaotic, with pronounced phase dispersion. Rounded inclusions of intermetallic phases and finely dispersed carbides were observed within the matrix, and the phase boundaries appeared blurred. The grains were smaller – averaging 5.7 μm – and in some areas showed clear signs of secondary phase growth, disrupting the continuity of the main structure. In the silicon-alloyed series, the structure retained its lamellar character, but the boundaries of the Ti5Si3 intermetallic phase were clearly visible between the grains, predominantly localised at the layer junctions. The average grain size in this case was 6.9 μm. These features indicate the influence of silicon in slowing grain growth through grain boundary stabilisation (
| Sample Series | Morphology | Phase Distribution | Average Grain Size (µm) |
|---|---|---|---|
| Basic (without alloy) | Lamellar, plate-like | Uniform | 8,3 |
| Alloying with Fe | Chaotic, with inclusions | Heterogeneous | 5,7 |
| Si doping | Lamellar, with granular phase | Localised | 6,9 |
Source: Created by the authors.
A detailed microstructural study using scanning electron microscopy revealed that the base series of samples exhibited a stable lamellar structure typical of Ti3AlC2, with an average grain size of 8.3 μm. The addition of iron disrupted this structure, reducing the grain size to 5.7 μm and causing phase segregation, with the formation of Fe3Al and TiC phases. These intermetallics and carbides caused local stresses and defects, negatively impacting the mechanical properties. In contrast, silicon alloying preserved the lamellar structure and led to the formation of Ti5Si3 at grain boundaries, stabilizing the microstructure and limiting the average grain size to 6.9 μm. Silicon enhanced structural stability without damaging the aluminium layer, in contrast to iron. These differences highlight the contrasting effects of iron and silicon on the microstructure and properties of the alloys.
The mechanical characteristics of the studied alloys varied depending on the type of alloying element, reflecting the effect of microstructural transformations on the physicomechanical parameters. In the base series, where the Ti3AlC2 phase with an ordered lamellar structure prevailed, the microhardness averaged 3.7 GPa, the density was 4.32 g/cm3, and the elastic modulus was approximately 205 GPa. These values correspond to typical parameters of MAX phases and indicate the presence of moderate brittleness combined with high rigidity. Alloying with iron was accompanied by a noticeable increase in microhardness to 5.1 GPa, attributable to the formation of hard carbide phases and intermetallics (TiC, Fe3Al), which possess higher hardness than Ti3AlC2. However, the density decreased to 4.17 g/cm3, associated with the formation of less dense porous areas observed during microstructural analysis. The elastic modulus decreased to 191 GPa, suggesting disruption of the integrity of the layered structure and the presence of interphase defects. In the samples alloyed with silicon, the microhardness was 4.2 GPa, representing an intermediate value between the two series. The density increased to 4.44 g/cm3, explained by the compacted structure with grain boundary intermetallic compounds Ti5Si3, and the elastic modulus reached 214 GPa due to the additional rigidity imparted by the secondary phase and the preservation of a continuous layered structure (
| Sample Series | Microhardness (GPa) | Density (g/сm3) | Modulus of Elasticity (GPa) |
|---|---|---|---|
| Basic (without alloy) | 3,7 | 4,32 | 205 |
| Alloying with Fe | 5,1 | 4,17 | 191 |
| Si doping | 4,2 | 4,44 | 214 |
Source: Created by the authors.
The comparison of Fe and Si alloying in Ti-Al-C-based MAX-phase composites reveals their contrasting effects on mechanical properties. In the base series without alloying, the Ti3AlC2 phase dominated, offering a balanced combination of properties, with a microhardness of 3.7 GPa, elastic modulus of 205 GPa, and density of 4.32 g/cm³. This structure, characterized by alternating titanium, carbon, and aluminium layers, efficiently dissipates mechanical energy, ensuring high rigidity while maintaining moderate ductility.
The addition of iron significantly increased microhardness to 5.1 GPa, due to the formation of TiC carbides and Fe3Al intermetallic compounds within the matrix. These phases, while hard, increase resistance to indentation and plastic deformation. However, the presence of these phases disrupts the continuity of the layered Ti3AlC2 structure, reducing the elastic modulus to 191 GPa and causing a loss of directional rigidity. This results in the formation of voids and defects, particularly along grain boundaries, which is reflected in the decreased density (4.17 g/cm³) and suggests incomplete sintering. Thus, while iron alloying enhances local hardness, it compromises overall structural integrity and elasticity.
On the other hand, silicon doping led to a more favorable distribution of properties. The microhardness increased to 4.2 GPa, which is higher than in the base series but lower than in the Fe-alloyed series. Silicon stabilizes the Ti3AlC2 structure by forming the Ti5Si3 intermetallic compound at grain boundaries. This reinforcement enhances resistance to deformation without significantly degrading the layered morphology of Ti3AlC2. The elastic modulus increased to 214 GPa, the highest value observed among all series, reflecting the high rigidity of the composite. The density also increased to 4.44 g/cm³, indicating a more compact structure with no volumetric defects and successful phase synergy. This demonstrates that silicon not only enhances mechanical stability but also preserves the material’s structural integrity, in contrast to iron.
Iron increases microhardness but decreases structural integrity and elasticity, making it suitable for applications requiring high surface hardness but limited overall stability. In contrast, silicon enhances mechanical stability, elastic modulus, and density, making it ideal for applications where overall rigidity and durability are crucial without compromising hardness. These contrasting effects highlight the importance of choosing the right alloying element based on the specific performance requirements of the application.
Evaluation of the functional properties of the studied alloys revealed a pronounced effect of the type of alloying element on thermal stability, electrical conductivity, and behaviour under oxidative heating conditions. In the base series without alloying additives, the CLTE was 8.1 × 10–6 1/°C, electrical conductivity was 3.5 × 104 S/m, and mass losses during TGA reached 5.8% at 1000°C. These parameters correspond to the characteristics of pure MAX phases, exhibiting moderate thermal stability and metallic conductivity due to the presence of metal layers in the crystal lattice. Alloying with iron led to an increase in the CLTE to 9.7 × 10–6 1/°C, a reduction in electrical conductivity to 2.1 × 104 S/m, and an increase in mass loss to 8.6%. These changes can be attributed to the high thermal and chemical instability of the intermetallic and carbide phases (Fe3Al, TiC), as well as to the disruption of the electronic structure caused by aluminium substitution and the breakdown of layered bonds. By contrast, in the samples alloyed with silicon, the CLTE decreased to 7.5 × 10–6 1/°C, electrical conductivity remained at 3.2 × 104 S/m, and mass loss was only 3.4%. This indicates the stabilising effect of the Ti5Si3 phase, which contributes to the suppression of oxidation and a reduction in thermal expansion (
| Sample Series | CLTE ( × 10–6 1/°C) | Electrical Conductivity (S/m) | Mass Loss During TGA (%) |
|---|---|---|---|
| Basic (without alloy) | 8.1 | 3.5 × 104 | 5.8 |
| Alloying with Fe | 9.7 | 2.1 × 104 | 8.6 |
| Si doping | 7.5 | 3.2 × 104 | 3.4 |
Source: Created by the authors.
The functional characteristics of MAX-phase alloys, including CLTE, electrical conductivity, and oxidative heat resistance, are closely related to their microstructure, electronic structure, and phase composition. These parameters are critical for assessing alloy performance under high temperatures and cyclic thermal loads. In the base series, containing mainly Ti3AlC2, the CLTE of 8.1 × 10–6 1/°C reflects moderate thermal expansion anisotropy. Electrical conductivity (3.5 × 104 S/m) indicates metallic conductivity, while mass loss during heating to 1000°C reached 5.8%, attributed to oxidation of titanium and aluminium.
The study on the influence of 5, 10, and 15 wt.% Fe and Si on Ti-Al-C-based MAX-phase composites showed that in the base series (without alloying elements), the phase composition included 87% Ti3AlC2, which decreased with the addition of Fe (to 62% at 15 wt.% Fe) and slightly decreased with Si (to 74% at 15 wt.% Si). The phase composition changes were statistically significant (p < 0.05 for all comparisons between Fe and Si alloys). Grain size measurements revealed that the average grain size in the base series was 8.3 μm. The addition of Fe reduced the grain size to 5.7 μm, while Si doping resulted in a grain size of 6.9 μm. Regression analysis showed a significant impact of alloying on grain size (p < 0.01 for Fe, p < 0.05 for Si).
Microhardness in the base series was 3.7 GPa, whereas Fe alloying increased the hardness to 5.1 GPa due to the formation of harder phases such as TiC and Fe3Al. Silicon doping increased the hardness to 4.2 GPa, representing an intermediate value between the two series. Statistical analysis showed significant increases in hardness (p < 0.001 for Fe, p < 0.05 for Si). Regression analysis revealed a strong relationship between Fe content and microhardness (R2 = 0.92, p < 0.001), while Si alloying showed a weaker relationship (R2 = 0.68, p = 0.03).
Elastic modulus in the base series was 205 GPa, which decreased to 191 GPa with Fe alloying, likely due to disruption of the lamellar structure. Silicon doping increased the modulus to 214 GPa, indicating improved rigidity. The differences in modulus were statistically significant (p < 0.01 for Fe, p < 0.05 for Si). Regression analysis showed a negative correlation between Fe content and elastic modulus (R2 = 0.89, p < 0.001) and a positive correlation with Si (R2 = 0.84, p = 0.004).
Density in the base series was 4.32 g/cm³. Fe alloying reduced the density to 4.17 g/cm³, while Si doping increased the density to 4.44 g/cm³. All these changes were statistically significant (p < 0.05 for both Fe and Si). Regression analysis revealed a significant negative correlation between Fe content and density (R2 = 0.75, p = 0.02), while Si showed a positive correlation (R2 = 0.80, p = 0.01).
The CLTE increased from 8.1 × 10–6 1/°C in the base series to 9.7 × 10–6 1/°C in the Fe-alloyed samples, reflecting thermal expansion induced by the formation of Fe3Al and TiC. Si doping reduced CLTE to 7.5 × 10–6 1/°C. Statistical analysis showed significant effects of alloying elements on CLTE (p < 0.05 for Fe, p < 0.01 for Si). Regression outputs confirmed a positive correlation between Fe content and CLTE (R2 = 0.81, p = 0.03) and a negative correlation with Si (R2 = 0.88, p = 0.01).
Electrical conductivity decreased from 3.5 × 104 S/m in the base series to 2.1 × 104 S/m in the Fe-alloyed samples, reflecting disruption of electron pathways. Si doping maintained conductivity at 3.2 × 104 S/m. The differences in conductivity were statistically significant (p < 0.001 for Fe, p < 0.05 for Si). Regression analysis showed a strong negative correlation between Fe content and electrical conductivity (R2 = 0.94, p < 0.001), while Si showed a weaker effect (R2 = 0.66, p = 0.04).
Mass loss during thermogravimetric analysis (TGA) for the base series was 5.8% at 1000°C. Fe alloying increased mass loss to 8.6%, reflecting higher oxidation rates of Fe. Si doping reduced mass loss to 3.4%, indicating improved oxidative stability. These differences were statistically significant (p < 0.001 for Fe, p < 0.05 for Si). Regression analysis confirmed a positive correlation between Fe content and mass loss (R2 = 0.92, p < 0.001) and a negative correlation with Si (R2 = 0.79, p = 0.02).
To substantiate the claim of SiO2 passivation during oxidation, post-TGA surface and cross-sectional analyses were carried out using SEM, EDS, XRD, and XPS. SEM imaging of both the surface and cross-sections revealed the formation of a uniform oxide scale, consistent with the development of a passivating SiO₂ layer. EDS analysis confirmed a significant concentration of silicon at the surface, supporting the presence of a silicon-rich passivation layer.
High-resolution cross-sectional SEM images were analyzed to estimate the thickness and continuity of the oxide scale, which was found to be well-defined and continuous, with thicknesses measured in micrometers. XRD patterns showed SiO2 as the primary phase on the surface, with no significant formation of other oxide phases. XPS analysis confirmed the oxidation of silicon to its +4 oxidation state, typical for SiO2, further validating the presence of a stable passivation layer. These findings collectively demonstrate the formation of a continuous SiO2 layer, enhancing the material’s oxidation resistance.
Iron alloying increased the CLTE to 9.7 × 10–6 1/°C due to the formation of Fe3Al and TiC phases, which induced thermal expansion and disrupted electron pathways, reducing conductivity to 2.1 × 104 S/m. Mass loss increased to 8.6%, caused by intensified oxidation of iron. Silicon doping reduced the CLTE to 7.5 × 10–6 1/°C, stabilizing the structure with Ti5Si3 phases at grain boundaries. Electrical conductivity remained at 3.2 × 104 S/m, and mass loss was lower (3.4%), indicating enhanced oxidative heat resistance due to the formation of a passivating SiO2 layer. Fe and Si have opposing effects: Fe increases thermal instability and reduces conductivity, while Si stabilizes the structure, enhances oxidative resistance, and preserves conductivity, making Si a preferred alloying element for high-temperature applications.
The results demonstrate the distinct effects of iron and silicon on Ti-Al-C-based MAX-phase alloys. Alloying with iron destabilized the Ti3AlC2 phase, increasing TiC content and forming Fe3Al, disrupting the structure. In contrast, silicon stabilized the phase, forming Ti5Si3 at grain boundaries, which improved thermal stability, reduced CLTE, and enhanced oxidative resistance. Iron alloying increased microhardness but decreased elastic modulus, density, and electrical conductivity, while silicon doping provided a balanced improvement in mechanical properties, with increased elasticity and density. Functional properties showed that silicon enhanced oxidative resistance and maintained conductivity, making it preferable for high-temperature applications. These findings highlight the contrasting roles of iron and silicon in influencing the structure and properties of MAX-phase alloys.
In the analysis of Ti-Al-C-based MAX-phase composites, the benchmarking of CLTE, Electrical Conductivity, and oxidation loss reveals significant insights. The CLTE values for the composites range from 7.5 × 10–6 1/°C (Si alloyed) to 9.7 × 10–6 1/°C (Fe alloyed), with a base range of 8.1 × 10–6 1/°C. The lower CLTE in Si-alloyed versions makes them suitable for thermal stability in applications such as turbine blades, contrasting with Fe-alloyed samples which show higher expansion. While other elements like Vanadium (V), Chromium (Cr), Niobium (Nb), and Zirconium (Zr) can improve thermal properties, they tend to increase CLTE, making Si alloying a more effective choice.
Regarding EC, the measured values for the base series is 3.5 × 104 S/m, with Si alloying at 3.2 × 104 S/m, and Fe alloyed materials at only 2.1 × 104 S/m. High electrical conductivity is essential for applications reliant on energy transport, and the Si-alloyed composites maintain conductivity comparable to base materials, while Fe alloying detrimentally affects it. Comparatively, Nb and Zr alloying often leads to lower conductivity due to resistive phases they form.
Oxidation resistance is crucial for longevity in high-temperature conditions. The base series shows a mass loss of 5.8% at 1000°C, which rises to 8.6% with Fe alloying, but drops to 3.4% with Si alloying, highlighting the protective SiO₂ layer that enhances durability in oxidizing environments. While Nb- and Zr-alloyed materials exhibit lower mass loss rates (2–4% and 1–2% respectively) due to better oxidation resistance, they may compromise thermal conductivity and mechanical properties. Cr-alloyed MAX phases also offer good oxidation resistance but tend to increase CLTE. Thus, Si alloying emerges as a well-rounded solution providing improved thermal stability, maintained electrical conductivity, and strong oxidation resistance across various high-performance applications.
The identification of Ti3AlC2 as the predominant phase in the base series confirms its high thermodynamic stability, consistent with Swadźba et al.,
Silicon, unlike iron, demonstrated a stabilizing effect on Ti3AlC2, despite a slight reduction in its proportion. Silicon doping preserves the lamellar structure of Ti3AlC2, with the Ti5Si3 intermetallic phase localized at grain boundaries, stabilizing the microstructure. This improves thermal stability and reduces CLTE, while enhancing oxidative resistance, as shown by lower mass loss in TGA. In contrast, iron alloying disrupts the microstructure, forming TiC and Fe3Al, which increases microhardness but reduces elastic modulus, density, and electrical conductivity. These findings highlight the need to control phase composition and microstructure to optimize the functional properties of MAX-phase alloys.
The moderate formation of TiC and the appearance of the Ti5Si3 intermetallic compound in the boundary zones explain the observed increase in elasticity and density. Structural analysis showed that the Ti5Si3 phase is located predominantly along interphase boundaries, forming grain boundary barriers that inhibit grain coarsening and preserve the layered morphology of the principal MAX phase. This distribution contributes to the formation of a composite architecture, wherein each phase fulfils a distinct functional role: Ti3AlC2 ensures directional rigidity, while Ti5Si3 provides structural stabilisation. These observations are corroborated by the findings of Dong et al.
The phase localization also varied: silicon-containing samples showed a well-defined intergranular structure, while iron-containing samples exhibited structural disorder. These results support the work of Edrisi et al.
Silicon, by contrast, decreases the coefficient of thermal expansion and enhances thermal stability.42–44 This is consistent with the findings of Alam et al.
The reduction in electrical conductivity in Fe-containing samples aligns with Laska et al.,
The data obtained demonstrate that alloying with silicon results in enhanced structural, mechanical, and functional properties of Ti-Al-C alloys compared with iron. This study highlights the potential of Si as an effective alloying element for developing stable MAX-phase composites characterised by high thermal stability and directional rigidity. Iron, despite providing a localised increase in hardness, induces microstructural degradation and reduces elasticity, thereby limiting its suitability for such applications.
The article has certain limitations regarding the functional properties of materials provided. In particular, the original CLTE curves, TGA curves of mass change as a function of temperature with derivative graphs, and data on electrical conductivity, including temperature dependence, contact resistance estimation, and geometry and porosity corrections, were not included. For a more detailed analysis and provision of relevant data, additional experimental measurements and corrections should be carried out in the future to include these important functional properties of materials.
As a result of the study, the phase, microstructural, and functional features of MAX-phase alloys of the Ti-Al-C system alloyed with iron and silicon were established. The main phase in all series of samples was Ti3AlC2, characterised by a typical hexagonal structure, though its content varied significantly depending on the alloying element. In the base series without additives, the Ti3AlC2 content reached 87%, indicating high phase purity. Alloying with iron reduced this proportion to 62% due to an increase in the TiC carbide phase and the formation of the Fe3Al intermetallic compound. This reflects the destabilising effect of Fe, which disrupts the integrity of the MAX-phase lattice. In contrast, silicon provided greater stability of Ti3AlC2 (74%) and contributed to the formation of the Ti5Si3 phase at grain boundaries, which exerted a stabilising effect on the microstructure.
Morphological analysis showed that the base series had a lamellar structure with a grain size of 8.3 μm. Iron alloying caused structural disorder, reducing the grain size to 5.7 μm and forming secondary phase inclusions. Silicon preserved the lamellar structure with a grain size of 6.9 μm, demonstrating its stabilizing effect. These differences were reflected in the mechanical properties: iron increased microhardness to 5.1 GPa but reduced both elastic modulus and density. Silicon balanced these properties with an elastic modulus of 214 GPa, density of 4.44 g/cm3, and hardness of 4.2 GPa. In functional properties, iron increased the CLTE to 9.7 × 10–6 1/°C and reduced conductivity, while silicon reduced the CLTE to 7.5 × 10–6 1/°C, maintained conductivity at 3.2 × 104 S/m, and lowered mass loss to 3.4%, attributed to the formation of a passivating SiO2 layer.
The results of this study highlight the practical significance of silicon as an alloying element for industries requiring high thermal stability and oxidative resistance, such as aerospace, automotive, and energy sectors. Silicon stabilizes the Ti3AlC2 phase, improving performance at high temperatures, making it ideal for applications like turbine engines and heat exchangers. The formation of a passivating SiO2 layer enhances oxidative resistance, reducing mass loss and extending component lifespan. Silicon maintains electrical conductivity, boosts mechanical strength, and controls grain growth, ensuring long-term structural stability. These findings can help material engineers optimize MAX-phase alloys for advanced applications.
A limitation of this study is that the analysis was conducted under fixed synthesis conditions, with a limited range of alloying concentrations for iron and silicon. Factors such as pressure, heating rate, sintering time, and other process parameters were not varied or systematically explored, which may influence phase formation, microstructure, and overall material properties. The current findings, while valuable, only provide a partial view of the complex relationship between alloying elements and MAX-phase characteristics. Furthermore, the study focused on a narrow range of Fe and Si concentrations, and it remains uncertain how broader compositional variations may affect the performance of these materials.
Future research should explore the effects of varying sintering temperatures, pressures, and heating rates on the phase stability, microstructure, and mechanical properties of MAX-phase alloys. A more extensive range of alloying concentrations should be investigated to better understand the full scope of the synergistic or antagonistic effects of Fe and Si. Another important area for future work would be to examine the long-term performance of these alloys under real-world conditions, including their behavior under cyclic thermal and mechanical loads, to assess their potential for high-temperature applications.
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Marine Sasuntsyan – Conceptualization, Methodology, Writing – Original Draft, Supervision; Suren Aghbalyan – Data Curation, Formal Analysis, Visualization, Writing – Review & Editing; Nina Sahakyan – Investigation, Resources, Validation, Writing – Review & Editing; Serob Hayrapetyan – Software, Project Administration, Funding Acquisition, Writing – Review & Editing
Marine Sasuntsyan
Unsolicited and externally peer-reviewed
The data supporting this study’s findings are available upon reasonable request from the corresponding author, with access granted in accordance with data protection regulations