Fig. 1. Schematic diagram of preparation mechanism of porous intermetallics based on the physical process of pore formation. (a) Pore formation from the interstitial space between particles. (b) Pore formation from removal of space holder.

Fig. 2. Pore structure and properties of porous intermetallic compounds based on physical process mechanism of pore formation. (a) SEM image of porous Fe20Cr7Al with pores derived from interstitial space between particles [49]. (b) Permeability performance of porous Fe20Cr7Al compared with that of reactively synthesized porous Fe-Al-Cr intermetallic [49]. (c) Pore morphology of cross section of porous Ni3Al with macro-pores derived from the removal of space holder [56]. (d) Roughness factor of porous Ni3Al [56].

Fig. 3. Schematic diagram of combustion synthesis mechanism of porous intermetallics based on the violent chemical reaction process to form pores. a) Pore formation from the CS procedure and the interstitial space between particles. (b) Addition of space holder or foaming agent to increase the porosity.

Fig. 4. Morphology of pore structure formed in the CS process. (a) Microscopic image of cross-section pore structure of porous Fe-40Al synthesized by the TE reaction, and inset: fracture surface microstructure [71]. (b) SEM image of pore structure of porous Ti5Si3/TiAl composite prepared by the SHS process using TiH2 as pore-forming agent, and inset: macrograph of porous composite [79].

Fig. 5. Schematic diagram of the RS mechanism of porous intermetallics based on the Kirkendall effect, in which the sintering procedure includes 2 stages: asymmetric diffusion at medium temperature (arrow marks: movement direction of fast diffusing element) and composition homogenization at high temperature.

Fig. 6. Stage characteristics of pore structure of reactively synthesized porous intermetallic compounds. (a) Microstructure of porous Fe-Al compact sintered at medium temperature of 600 °C [102]. (b) Pore structure of porous FeAl intermetallic after composition homogenization stage at high temperature of 1200 °C [102]. (c) Microstructure of porous Fe-Si compact sintered at medium temperature of 800 °C [27]. (d) Pore structure of fracture surface of porous FeSi intermetallic sintered at high temperature of 1150 °C [27].

Fig. 7. Schematic diagram of reactive synthesis mechanism of porous intermetallics based on the phase transition process, in which pores are formed during the phase transition procedure due to the volume shrinkage of resultants.

Fig. 8. Pore structure of porous intermetallics prepared through the RS process based on the pore formation mechanism of the phase transition effect. (a) SEM image of pore structure of porous Ti3SiC2 MAX phase, and inset: laminate microstructure [108]. (b) Microscopic structure of porous Ti3(Si,Al)C2 quaternary intermetallic, and inset: nanolaminate microstructure of MAX phase [111].

Fig. 9. Pore structure morphology of some typical porous intermetallics and alloys. (a) Optical micrograph of the section of porous Ni3Al with the pore formation through the Kirkendall effect [56]. (b) SEM image of the section of reactively synthesized porous FeAl [97]. (c) SEM image of the fracture surface of porous Ti3SiC2 with the pore formation by the phase transition process [114]. (d) 3D morphology of porous Ni-Cu prepared through the Kirkendall effect by 3D X-ray diffraction microscope (Xradia VersaXRM-500) [104].

Fig. 10. Linear relationship between the pore structure parameters and the preparation parameters in some typical reactively synthesized porous intermetallics. (a) Effect of powder size on the pore structure parameters of porous FeSi intermetallics [27]. (b) Relationship between the open porosity and Al content in porous Fe-Al intermetallics [98]. (c) Relationship between the permeability and cold pressing pressure in porous FeAl intermetallics [102]. (d) Relationship between tortuosity factor and powder size in porous Ti3(Si,Al)C2 compound [113].

Fig. 11. Material properties of typical porous intermetallics. (a) Current-potential curves for porous Ni3Al-Mo and Ni3Al intermetallic HER electrodes after different electrolysis time [44]. (b) Oxidation and sulfidation kinetics curves at 600 °C of porous FeAl and FeAl-Cr intermetallics [41]. (c) Potentiodynamic polarization curves for porous Ti3SiC2 and TiAl intermetallics in NaCl solution [114]. (d) Relationship curves between the bending strength and the average pore size of porous Ti3SiC2 [108].