The Effect of Mg Content on the Microstructure and Open Porosity of a Porous FeAl Intermetallic Compound

Abstract

In this study, a porous FeAl intermetallic compound with high porosity was synthesized via vacuum sintering using Mg powder as a pore-forming agent, leveraging its high saturated vapor pressure and almost non-reactivity with Fe. The influence of the addition of Mg powder on pore characteristics and microstructure evolution was systematically investigated. The results indicate that variations in Mg content within sintered compacts exhibit a negligible impact on primary phase composition, with the FeAl phase remaining predominant. However, excessive initial Mg content induces the encapsulation of the FeAl phase by minor Fe₂Al₅ and Al₃Mg₂ phases, compromising the phase’s purity. The porosity positively correlates with Mg content, and porous material with a porosity of 72.8% is obtained (40 at.% of Mg as an additive). Moreover, the pore structure manifests as an interconnected hole morphology. These findings provide valuable insights for further exploration of the design of porous FeAl material and its performance enhancement in emerging applications.

1. Introduction

Porous Fe-Al intermetallic compounds combine the mechanical robustness of conventional metallic porous materials with ceramic-like characteristics, including exceptional oxidation resistance, sulfidation resistance, and corrosion stability [1,2]. These unique properties make them particularly suitable for manufacturing filtration components that withstand harsh operating conditions involving high temperatures and corrosive environments [3]. Notably, the porosity represents a crucial determinant for porous Fe-Al intermetallic compounds when employed as filter media, as it directly influences filtration efficiency [4].

Currently, several preparation methods for porous Fe-Al intermetallic compounds have been developed, including the reaction synthesis method, the pore-forming agent addition method, and the self-propagating high-temperature synthesis method, among which the reaction synthesis method has been most widely employed [5,6,7,8]. Nevertheless, these established methods exhibit inherent limitations. The reaction synthesis method, while commonly used, typically produces materials with a relatively low porosity (generally below 50%), which falls short of filtration efficiency [9,10,11]. The pore-forming agent method yields materials with a higher porosity, while inevitably leaving residual agents that contaminate the porous matrix and degrade mechanical properties, such as NaCl [12], Eosin Y [13], and crystalline oxalic acid [14]. The self-propagating high-temperature synthesis method achieves a notable enhancement in porosity, but its excessively violent reaction process frequently induces shape distortion and structural collapse in sintered compacts, consequently resulting in an unsatisfactory product yield [15]. These inherent drawbacks underscore the urgent need for developing other preparation technologies that can balance the relationship between the open porosity and purity of porous Fe-Al intermetallics. Mg powder, as a pore-forming agent, is used in the preparation of porous titanium alloys and porous TiNi alloys due to its low melting point and high oxygen affinity [16,17,18]. However, there are few reports on the application of Mg powder in the pore-forming of porous Fe-Al intermetallic compounds. Moreover, diffusion reactions occur between Al and Mg, and the impact of this on the microstructure and open porosity of porous Fe-Al intermetallic compounds is unknown.

In this work, based on the extremely limited mutual solubility of Fe and Mg and their inability to form intermetallic compounds [19,20], Mg powder was selected as the pore-forming agent. Taking advantage of its high saturated vapor pressure characteristic, FeAl porous intermetallic was successfully fabricated. The influence of Mg content in sintered compacts on the microstructure, weight loss rate, and porosity of the porous materials was investigated. This work clarifies the mechanisms governing the formation of pores in the Fe-Al-Mg system. The findings provide novel strategies and a theoretical framework for fabricating FeAl intermetallic compounds as high-porosity porous materials.

2. Materials and Methods

2.1. Powder Preparation

Carbonyl Fe, atomized Al, and Mg elemental powders were employed as the raw materials, with their respective morphologies illustrated in Figure 1. Specifically, the irregularly shaped carbonyl Fe particles exhibit an average particle size of 40 μm, while the Al and Mg elemental powder particles demonstrate near-spherical morphologies, averaging 20 μm and 35 μm in particle size, respectively.

To systematically investigate the effect of Mg content on the microstructure and properties of FeAl porous materials, five different Mg-containing powder blends with varying compositions were prepared. The metallic powder system maintained a fixed Fe/Al atomic ratio of 1:1, while Mg was introduced at controlled atomic percentages of 0%, 10%, 20%, 30%, and 40%. The weighed powder mixtures were homogenized using a drum mixer (QM-5, TENCAN, Changsha, China) operating at 300 rpm for 10 h.

2.2. Press and Sinter

The powder mixtures with varying Mg content were uniaxially pressed at 200 MPa with a 2 min holding time to form green compacts for subsequent sintering. The compaction was followed by vacuum sintering (<5.0 × 10⁻³ Pa) in alumina crucibles using a tubular furnace (TL1600, BYT, Nanjing, China). A multi-stage sintering protocol involving sequential heating and isothermal holding steps was implemented to mitigate the potential high-temperature self-propagating reaction between Fe and Al. The detailed sintering parameters, including temperature ramps and holding times, are systematically presented in Figure 2.

2.3. Characterization

When the saturated vapor pressure of Mg surpasses the furnace pressure, significant Mg volatilization occurs during vacuum sintering [21,22]. This leads to a weight reduction in the sintered compacts compared to their pre-sintered counterparts. The extent of weight loss can be quantitatively evaluated using the weight loss rate [23], as defined by Equation (1).

$$R={\displaystyle \frac{m_0-m_1}{m_0}}\times 100\%$$

(1)

where $R$ is the weight loss rate of the sintered compact, $m_0$ is the weight of the green compact before sintering, and $m_1$ is the weight of the sintered compact after sintering.

The open porosity of the sintered porous FeAl materials was measured using Archimedes’ principle according to the standard ASTM B0962-23 [24]. The microstructure and elemental distribution of the sintered compacts with a varying initial Mg content after high-temperature sintering were examined using a scanning electron microscope (SEM, FEI Quanta 450FEG, FEI, Hillsboro, OR, USA) equipped with an energy-dispersive spectrometer (EDS). Before SEM observation, the samples were sequentially ground with P280 to P3000 abrasive papers, polished with 1 μm and 0.25 μm diamond suspensions, and subsequently ultrasonically cleaned for 5 min. The phase compositions of the sintered compacts with a different initial Mg content were analyzed using a X-ray diffractometer (XRD, Ultima III, Rigaku Corporation, Tokyo, Japan) with Cu Kα radiation (λ = 1.5406 Å). The diffraction patterns were acquired in the 2θ range of 25–85° with a scanning rate of 3°/min and a step size of 0.01° under continuous scanning mode.

3. Results and Discussion

3.1. Influence of Mg Content on Phase Composition

Figure 3 presents the XRD patterns of Fe-Al-Mg sintered compacts with a varying Mg content. The results show that after the vacuum sintering of the initial compacts with a different Mg content, only diffraction peaks corresponding to the FeAl intermetallic phase are observed, with no detectable signals from elemental Fe or Al, indicating complete interdiffusion and reaction between the constituent elements during sintering. Additionally, the elemental Mg phase is absent from the XRD patterns of all the sintered compacts. This demonstrates that during the vacuum sintering process, Mg in the Fe-Al-Mg mixed powder has been removed via high-temperature sublimation and evaporation, with residual Mg content below the XRD detection limit. Consequently, no elemental Mg and Mg-related intermetallic compounds are detected.

3.2. Influence of Mg Content on Weight Loss Rate

Figure 4 illustrates the correlation between the weight loss rate of Fe-Al-Mg sintered compacts and their corresponding initial Mg content. The data for each group of compacts was obtained by averaging the results of three measurements. The results shown in Figure 4a indicate that in the absence of Mg as a pore-forming agent, the sintered compacts exhibit an extremely low weight loss (0.072 wt.%), confirming the high thermal stability of the Fe-Al system during sintering. For the Fe-Al-Mg ternary system, the weight loss rate of the sintered compacts increases with Mg content. Specifically, the Fe-Al-10 at.% Mg compact exhibits a weight loss rate of 5.90 wt.%, which closely matches its initial Mg mass fraction (6.04 wt.%) in the green compact. Similarly, when the Mg content in the compact reaches 20 at.%, the weight loss rate increases to 12.12 wt.%, also aligning approximately with the initial Mg mass fraction (12.63 wt.%). However, at higher Mg concentrations (30 at.% and 40 at.%), a gradual deviation emerges between the weight loss rate and the initial Mg mass fraction, suggesting incomplete sublimation and increased Mg retention in the sintered product. The fitting curve in Figure 4b regarding the relationship between the difference (i.e., the difference between the mass fraction of initial Mg and the weight loss rate) and the initial Mg content (at.%) indicates that the residual amount of Mg (wt.%) has a quadratic function relationship with the amount of Mg added (at.%). It can be found that the difference between the mass fraction of initial Mg and the weight loss rate increases with the increase in the initial Mg content (at.%). This behavior can be attributed to the thermodynamic properties of Mg under sintering conditions. Studies have shown that the saturated vapor pressure of Mg reaches ~40 Pa at 550 °C, exceeding the vacuum level of the sintering environment (5.0 × 10⁻³ Pa), and rises rapidly with increasing sintering temperatures [25,26]. This results in the rapid sublimation and evaporation of Mg in both solid and liquid states. Consequently, the weight loss rate escalates with Mg content, confirming that Mg volatilization is the primary mass loss mechanism under the given sintering parameters.

3.3. Influence of Mg Content on Macroscopic Characteristics and Open Porosity

Figure 5 presents the macroscopic morphology and porosity of Fe-Al-Mg sintered compacts with a varying Mg content. The open porosity of each group of samples was obtained by the mean value of three measurements. As shown in Figure 5a, the Mg-free compact exhibits volumetric expansion after sintering, whereas compacts containing 10–40 at.% Mg maintain comparable dimensions, indicating that the Mg content has no pronounced influence on the overall dimensional expansion of the material. Figure 5b quantifies the porosity’s dependence on Mg content. The sintered Mg-free compact displays a baseline porosity of 38.1 vol.%, which increases sharply to 61.9 vol.% at 10 at.% Mg and further rises to 72.8 vol.% at 40 at.% Mg. This substantial enhancement in porosity arises from two concurrent mechanisms: (1) Kirkendall porosity due to Fe-Al interdiffusion and (2) the additional formation of pores via Mg sublimation/evaporation, driven by its high saturated vapor pressure under sintering conditions. The synergistic effect of these processes significantly exceeds the porosity generated by Fe-Al interdiffusion alone, demonstrating the critical role of Mg as a multifunctional pore-forming agent.

3.4. Influence of Mg Content on Microstructure

Figure 6 presents the cross-sectional microstructure and EDS elemental mapping of a sintered compact derived from the Mg-free Fe-Al powder compact. As shown in Figure 6c, Fe and Al are homogeneously distributed, indicating that complete diffusion/reaction and compositional homogenization have been achieved after sintering at 1100 °C.

Table 1. Composition analysis by EDS at selected locations in Figure 6.

Point	Fe (at.%)	Al (at.%)	Possible Phase
1	53.53	46.47	FeAl
2	55.91	44.09	FeAl
3	56.39	43.61	FeAl

reveals consistent Fe/Al atomic ratios (≈55:45) at multiple measurement points (Points 1–3 in Figure 6b). Combined with the Fe-Al binary phase diagram [27], this confirms the formation of a single-phase FeAl intermetallic compound. This finding agrees well with the XRD pattern presented in Figure 3. Furthermore, the microstructural examination in Figure 6a reveals an interconnected pore network, formed primarily through the Kirkendall effect due to the unequal diffusion rates of Fe and Al [28]. These pores could theoretically facilitate subsequent Mg sublimation/evaporation in Mg-containing systems. The porosity measurement in Figure 5b demonstrates that this intrinsic pore-forming mechanism alone yields a limited porosity (38.1 vol.%).

Figure 7 presents the cross-sectional microstructure and the corresponding EDS elemental mapping result of the Fe-Al-Mg ternary powder compact with the addition of 10 at.% Mg after sintering. Figure 7a reveals more pronounced interconnected pores compared to the Mg-free sintered compact in Figure 6a. These pores are both the residual traces of Mg volatilization and the pathways facilitating further Mg removal. In general, the pore formation mechanism involves the following three key stages. First, the pre-existing Kirkendall pores from Fe-Al interdiffusion establish a continuous network, providing preferential pathways for Mg vapor migration. Subsequently, Mg’s sublimation creates vacancies at atomic lattice sites, which then coalesce into microscopic voids. Finally, these voids grow and interconnect, forming macroscopic channels that facilitate rapid Mg vapor egress, thereby significantly enhancing the porosity of the sintered compacts. This hierarchical pore development mechanism explains the substantial 23.8 vol.% porosity increase observed in the 10 at.% Mg-containing compact relative to the binary Fe-Al system. The self-amplifying nature of this process—where initial pores enable faster Mg removal, which in turn generates additional porosity—accounts for the dramatic enhancement in the overall void fraction.

As shown in Figure 7c and the EDS data of each point in

Table 2. Composition analysis by EDS at selected locations in Figure 7.

Point	Fe (at.%)	Al (at.%)	Mg (at.%)	Possible Phase
1	64.65	35.31	0.04	FeAl
2	60.67	39.32	0.01	FeAl
3	63.11	36.87	0.02	FeAl

, the uniform distribution of Fe and Al elements indicates that interdiffusion and reactions between Fe and Al are completed during sintering at 1100 °C, achieving compositional homogenization. Notably, EDS point analyses (Points 1–3 in Figure 7) detect residual Mg concentrations below 0.1 at.%, verifying near-complete Mg removal through sublimation and evaporation under vacuum conditions. This observation is also corroborated by the absence of diffraction peaks corresponding to Mg or its compounds in the XRD patterns (Figure 3) and the weight loss measurements in Figure 4.

Figure 8 presents the SEM images and EDS elemental mapping of the cross-sectional Fe-Al-Mg sintered compact with 20 at.% Mg content. As evidenced by the elemental composition at Points 1–3 in

Table 3. Composition analysis by EDS at selected locations in Figure 8.

Point	Fe (at.%)	Al (at.%)	Mg (at.%)	Possible Phase
1	55.78	44.22	0	FeAl
2	55.51	44.43	0.06	FeAl
3	60.84	38.99	0.17	FeAl

for Figure 8b and the EDS mapping result in Figure 8c, high-temperature sintering remains effective in substantially eliminating Mg, with residual content maintained below 1 at.%, even when the initial Mg content increases to 20 at.%. This confirms the successful synthesis of purified porous FeAl intermetallic, even at elevated Mg concentrations. In addition, a microstructural comparison shows a substantial pore coarsening relative to the 10 at.% Mg sample (Figure 7a), in agreement with the porosity measurements in Figure 5b. The enhanced porosity primarily originates from increased sublimation and evaporation losses associated with an elevated Mg content.

A comparative analysis of SEM images (Figure 6a, Figure 7a and Figure 8a) of the sintered compacts containing 0–20 at.% Mg demonstrates that increasing Mg content promotes a progressive refinement of the matrix skeleton. This microstructural evolution is manifested in the simultaneous formation of abundant through-pores and a porous architecture composed of loosely packed fine particles. According to the Al-Mg binary phase diagram [29], this phenomenon can be primarily attributed to the following factors. First, the low-melting-point characteristics of Al-Mg intermetallic compounds facilitate the formation of a liquid phase at relatively low temperatures. Concurrently, Fe atoms from the metallic matrix continuously react with Al in the Al-Mg liquid phase to produce high-melting-point Fe-Al intermetallic compounds. The subsequent reduction in Al content induces solidification of the Al-Mg liquid, followed by re-liquefaction upon an elevation in temperature. This cyclic solid–liquid phase transition drives a continuous contraction of grain volume, ultimately resulting in the formation of a refined particulate structure and significantly enhanced porosity in the sintered compact.

Figure 9 presents the cross-sectional microstructure and elemental distribution of the sintered Fe-Al-Mg compact with 30 at.% Mg content. As shown in Figure 9a, the sintered specimen exhibits further refined matrix framework dimensions and a more dispersed porosity distribution, accompanied by a further increase in total porosity. The elevated Mg content intensifies the diffusion/reaction processes between Al and Mg, promoting the formation of transient liquid phases during sintering. This facilitates a greater participation of Al and Fe atoms in the initial diffusion stages. The intensification of the diffusion/reaction process and increased liquid–solid phase transitions ultimately lead to increased porosity in the sintered compact.

It is noteworthy that Mg aggregation is observed near the FeAl matrix, as evidenced by the Mg concentrations of 4.86 at.% and 11.83 at.% at Points 3 and 4 in

Table 4. Composition analysis by EDS at selected locations in Figure 9.

Point	Fe (at.%)	Al (at.%)	Mg (at.%)	Possible Phase
1	54.71	45.29	0	FeAl
2	34.47	65.40	0.13	Fe2Al5
3	20.32	74.82	4.86	Fe2Al5 + Al3Mg2
4	18.72	69.44	11.83	Fe2Al5 + Al3Mg2

for Figure 9b, respectively. This phenomenon may be attributed to the relatively short sintering duration, which prevents Mg from being fully dissipated through sublimation and evaporation. Furthermore, according to the Al-Mg phase diagram, Mg exhibits a solid solubility limit of 19.2 at.% in Al. The surplus Mg exceeding this solubility limit reacts with Al to form the Al₃Mg₂ intermetallic compound. Consequently, Mg predominantly exists in the sintered compact matrix as both elemental Mg and the Al₃Mg₂ intermetallic compound.

Additionally, localized Al enrichment exceeding the stoichiometric Al content of FeAl is observed in specific regions of the sintered compact. This suggests that Al may exist in an elemental form or as specific compounds adhering to the periphery of the FeAl matrix. As demonstrated by the line scan data in Figure 9e, the intermetallic compound formed between Fe and Al is identified as Fe₂Al₅ rather than FeAl. The Fe₂Al₅ intermetallic phase encapsulates the FeAl matrix. This phenomenon can be attributed to the following mechanism. The increased Mg content in the compact enhances the proportion of the Al-Mg liquid phase during sintering. This expansion of the liquid phase prolongs the diffusion distance required for Fe atoms to achieve a homogeneous distribution, thereby extending the reaction duration needed for complete formation of the intermetallic compound and resulting in an incomplete reaction of Fe and Al within the experimental time. Consequently, the phenomenon of the Fe₂Al₅ intermetallic compound encapsulating the FeAl matrix is observed. Specifically, Al-rich areas appear in the skeleton contour region.

Figure 10 presents the cross-sectional SEM images and EDS elemental mapping of the sintered Fe-Al-Mg compact with 40 at.% Mg content. Figure 10a shows that the pore size of the sintered compact has been further expanded. Combined with Figure 5b, the porosity at this point exceeds 70 vol.%. The elevated initial Mg content induces intensified sublimation and evaporation losses, where the small pores generated at the original Mg particle locations progressively coalesce into larger voids. This coalescence mechanism constitutes the predominant factor responsible for the enhanced porosity observed in the sintered compact containing 40 at.% Mg.

With a further increase in Mg content, Figure 10c demonstrates an increased presence of elemental Mg and residual Al-Mg intermetallic compounds in the final product. Notably, the EDS point scan data in

Table 5. Composition analysis by EDS at selected locations in Figure 10.

Point	Fe (at.%)	Al (at.%)	Mg (at.%)	Possible Phases
1	65.52	34.48	0	Fe + FeAl
2	16.76	66.23	17.01	Fe2Al5 + Al3Mg2
3	18.61	67.33	14.06	FeAl + Al3Mg2
4	19.78	19.73	60.49	FeAl + Mg + Al3Mg2
5	56.78	42.91	0.32	FeAl

for Figure 10b reveals a significant Mg concentration aggregation (60.49 at.%) at Point 4. To elucidate this anomaly, a line scan analysis is conducted across Region B in Figure 10b, with the corresponding result presented in Figure 10e. The line scan analysis reveals that the bright regions in Figure 10d predominantly consist of FeAl intermetallic, while the dark areas are primarily composed of elemental Mg and Al₃Mg₂ intermetallic compounds. The elevated residual Mg content at Point 4 can be explained by the following reasoning. During the sintering process, Fe atoms from elemental Fe continuously react with Al atoms from elemental Al and the Al-Mg liquid phase to form intermetallic compounds. However, excess Mg becomes encapsulated by high-melting-point Fe-Al intermetallic compounds before the completion of sintering, thereby inhibiting its complete sublimation and evaporation.

While the Fe-Al-40 at.% Mg powder compact facilitates the production of a sintered compact with enhanced porosity, the microstructural quality of this compact is notably inferior to that of its low-Mg-content counterparts, accompanied by the formation of unintended secondary phases. The unexpected intermetallic compounds (e.g., Fe₂Al₅, Al₃Mg₂) are small and dispersed around the FeAl matrix particles, which may weaken the particle bonding strength.

4. Conclusions

This work systematically examines how the initial Mg content influences the weight loss, porosity, and microstructure of Fe-Al-Mg powder compacts under vacuum sintering, with Mg powder acting as the pore-forming agent. The main findings are summarized as follows.

(1) All the sintered compacts with varying initial Mg content are mainly composed of the FeAl phase. However, when Mg content reaches 30 at.%, secondary phases, including Fe₂Al₅ and Al₃Mg_2, gradually emerge around FeAl phase boundaries.

(2) After sintering, the weight loss rates of the sintered compacts with the initial Mg content of 10 at.% and 20 at.% are 5.90 wt.% and 12.12 wt.%, respectively, which is consistent with the initial Mg mass fractions of green compacts. This phenomenon indicates that Mg is almost completely removed. However, when the initial Mg content increases to 30 at.% and 40 at.%, the difference between the weight loss rate and the initial Mg mass fraction becomes more significant, indicating an increase in the amount of residual Mg.

(3) The open porosity of the sintered compact with 10 at.% initial Mg content increases significantly to 61.9 vol.%, compared to 38.1 vol.% for the Mg-free sintered compact. Furthermore, the open porosity progressively rises to 72.8 vol.% as the initial Mg content is elevated to 40 at.%. However, considering the poor homogenization of elements in the compact with 30 at.% Mg content and the looser structure of the compact with 40 at.% Mg content, the addition of 20 at.% Mg is the most suitable experimental parameter for manufacturing FeAl filter materials.