This review summarizes the recent research progress in the preparation of porous metal materials by alloy phase separation. Combined with the phase separation mechanism of the alloy, the formation mechanism of the porous structure during the phase separation process was discussed based on the interfacial spinodal decomposition and diffusion-coupled growth. The effect of phase-separated alloy system, composition change and process parameter on the characteristics such as morphology, porosity and ligament size of the porous structure were systematically analyzed. Furthermore, the properties of phase-separated porous metal materials and their application prospects in the fields of catalysis, electrolytic capacitors, and biomedicine are summarized due to their large specific surface area and interconnected ligaments. Finally, the research and development trend of the preparation of porous metals by alloy phase separation is proposed.
According to an orthogonal experiment design, the directional recrystallization of hot-rolled Cu71Al18Mn11 alloy with deformation degree of 75% was carried out at 800, 850 and 900℃, by drawing speed of 2, 5 and 15 μm/s respectively. The effect of process parameters on the directional recrystallization microstructure and superelasticity of the hot-rolled Cu71Al18Mn11 alloy was assessed, meanwhile, the directional recrystallization mechanism was analyzed. The results show that the directional recrystallization effect may firstly increase and then decrease with the increase of drawing speed. When the drawing speed is 2 μm/s, a small number of columnar grains emerged within the coarse equiaxed grains. When the drawing speed increases to 5 μm/s, the microstructure of columnar grains with large aspect ratio can be obtained. However, when the drawing speed further increases to 15 μm/s, the directional recrystallization microstructure is a mixture of columnar grains and equiaxed grains. The superelastic properties are better of the directionally recrystallized alloys with columnar grains of large aspect ratio. After being subjected to an applied strain of 12%, the alloy directionally recrystallized at 900℃- 5 μm/s presents a residual strain of only 1.1%, while a superelastic strain of 9.05%. The drawing velocity and hot zone temperature can affect the speed, at which the columnar grains swallowed up the primary recrystallized grains ahead in the process of directional recrystallization, thus affecting the microstructure of directionally recrystallized alloy. Once, the three speeds, i.e. the hot zone movement, the columnar grains swallowing up the primary recrystallized grains ahead and the generation of new primary recrystallization grains, all are in equilibrium, the front boundary of the existing columnar grains will continues to move forward, which will eventually promote the formation of microstructure of columnar grains with large aspect ratio.
Composite plates of titanium alloy/stainless steel were prepared by spark plasma sintering (SPS), and their microstructure, micro-nano mechanical behavior and tensile properties were investigated. Results show that the temperature of the interface of titanium alloy/stainless steel rises fast owing to the short time spark discharge under the action of high energy pulse current. The interface is relatively flat, while inter-diffusion of Ti, Fe and Cr atoms occurs obviously. A small number of intermetallic compounds TiFe, TiFe2 and FeCr formed at the interface. The hardness of the zone with brittle intermetallic compounds reached 3.5578 GPa, which is much higher than that of metal matrix on both sides (Ti-2.943 GPa, Steel-2.717 GPa). The tensile strength of the titanium alloy/stainless steel composite is 387.5 MPa, which is 72% and 79% of the base material of titanium alloy and stainless steel respectively. During the tensile process, cleavage fracture first occurs on the stainless-steel side, while the titanium alloy side continues to bear the load until the typical ductile fracture occurs.
The composition and quantity of the equilibrium precipitated phases in CLF-1 steel were calculated by means of Thermo-calc software. The variation of element distribution in each equilibrium phase with temperature, the influence of main elements on the precipitation amount and temperature of each phase, and the partition of elements among equilibrium phases were investigated. Meanwhile, the size and number density of the precipitated phases were characterized by transmission electron microscopy (TEM). Thermodynamic calculation results show that the partition amount of C in TaC phase only accounts for about 3.3% of that in M23C6. The heterogeneous partition of C is one of the main reasons for the low MX phase precipitation in CLF-1 steel. Without introducing other strong carbide-forming elements, the partition amount of C in MX phase cannot be increased by increasing the C content. The increase of Cr content will reduce the precipitation temperature of TaC, so the Cr content should not exceed 8.8%. The increase of W content will reduce the amount of MX precipitation, so it should not exceed 1.5%. Mn has no significant effect on the amount of every precipitated phase. Finally, N content should not exceed 0.02%. The addition of 0.2%Ti in CLF-1 steel can increase the precipitation amount of MX phase at 650℃ by 9 times, and increase the partition amount of C by 15 times. The experiment result shows that: the addition of 0.2% Ti can reduce the number density of M23C6 by 21.5%, reduce its average size by 20 nm, and increase the number density of MX phase by 4.7 times. These results strongly verified the promoting effect of Ti element on MX phase precipitation.
The microstructural evolution of DZ125L superalloy during solution heat treatment in the range of 1230℃ to 1260℃ was investigated by optical microscope (OM), scanning electron microscope (SEM) and electron probe microanalyser (EPMA). The results show that during solution heat treatment, MC carbide transformed from cursive-script like to granular or short rod like, and the dissolution of γ′ phase in the dendritic core was faster than that in the interdendritic region. The area fraction of γ′ phase and γ/γ′ eutectic decreased, and the average size of γ′ phase in the interdendritic region increased with the increase of heating time at 1240℃ and 1250℃, while the area fraction of γ′ phase and γ/γ′ eutectic, and the average size of γ′ phase in the interdendritic region decreased firstly, then increased, and decrease lastly at 1230℃. The abnormal increase of area fraction of γ′ phase and γ/γ′ eutectic during solution heat treatment at 1230℃ was caused by the rapid dissolution of γ′ phase in the dendritic core, which led to the diffusion of Ta from dendrite core to interdendritic region through γ matrix.
The tensile performance of high-entropy nanocrystalline- and single crystal-CoNiCrFeMn alloy, as well as polycrystalline- and single crystal-Ni metal, was comparatively assessed, while the evolution of their microstructures and the deformation induced difects such as dislocations, voids and cracks etc. with the deformation process and temperature was searched in an attempt to reveal the relationship between their mechanical performance and the aforesaid evolution. Results show that when the temperature lifting from 10 K to 1000 K, the yield stress of the high-entropy nanocrystalline CoNiCrFeMn alloy decreases by 14.9%, 13.1% and 17.4%, whose corresponding temperature is 10 K, 300 K and 1000 K respectively, in comparision to those of the high-entropy single crystal ones; While the tensile strength of the polycrystalline Ni decreased by 38.9%, 30% and 32.3% of that for single crystalline Ni, whose corresponding temperature is 10 K, 300 K and 1000 K respectively; Likewise, the elastic modulus and yield strength of the high entropy nanocrystalline alloy and nanocrystalline nickel decrease linearly with the increasing temperature. However, the overall decrease percentage of the value for yield stress of the polycrystalline nickel is greater than that of the high entropy single crystal alloy, owing to the exist of internal stresses, cracks and cavities induced by grain boundary defects of the former. It is thought that the geometry shape and size of the formed cavities and cracks are the fundamental cause responsible to the sharp decline of the mechanical properties of the similar materials in practical application, and also to the significant difference of the tensile mechanical properties between the high entropy nanocrystalline alloy and the nanocrystalline nickel. The applied tensile load may result in the formation of a large number of stacking faults within grains of polycrystalline materials, and thus the large grains are easy to be differentiated into fine grains with the increasing temperature, in other word, to realize the grain refinement. In addition, the high entropy polycrystalline alloy and polycrystalline nickel are more likely to generate latest dislocations at grain boundary edge induced by internal stresses, hence, the dislocation distribution is consistent with the internal stress distribution. With the increasing temperature, the distribution area of grain boundaries within polycrystalline materials will be further expanded due to thermal expansion, therefore, the area with internal stresses will enlarge accordingly, in comparison to that at lower temperature.
Spray forming is a casting process, by which the molten metal is directly converted to a solid bulk with unique characteristics. When used in the production of high speed steel, spray forming materials typically present microstructures composed of refined polygonal grains, uniformly distributed carbides and low levels of micro and macro-segregation. The mechanical properties of the spray forming high speed steel are usually between ones made by powder metallurgy, casting and wrought. It can be considered as a cost saving alternative for large-scale industrial production of high-speed steels. But for high-speed steel produced by spray forming, its shortcomings can't be ignored: i.e. once being subjected to the same heat treatment, compared with the steels made by PM and CW process, the spray forming one often shows lower hardness. Focusing on the solution of aforementioned disadvantages, the effect of different heat treatments on the microstructural evolution and hardness variation was assessed for the spray forming M3 high-speed steel, and the adopted heat treatment involved quenching and tempering at different temperatures separately. Meanwhile the reasons for the low hardness of the spray-formed M3 high-speed steel were also discussed. The results show that SF M3 high speed steel quenched below 1230℃ can still ensure relatively fine grain size and uniform size distribution of carbide particles; Setting the tempering at 560℃, in case the quenching temperature raises from 1200℃ to 1230℃, after being quenched + tempered, the hardness of spray-formed M3 high-speed steel can be greatly improved. It is believed that the low hardness issue may be ascribed to the fact: the formation of a large number of MC type carbides with inhomogeneous composition in the spray forming atomization stage, the MC type carbides can't be fully dissolved in the steel matrix when quenching at lower temperature during the heat treatment process, resulting in insufficient amount of carbon and alloying elements in the matrix. Therefore, the low hardness issue is caused by the inability to fully exert the secondary hardening effect.
Composite catalysts of Bi2O3/g-C3N4 were successfully prepared by means of liquid-phase precipitation and thermal polymerization methods. The microscopic morphology, crystal structure and photocatalytic properties of the composite catalysts were characterized by SEM, XRD, XPS, FT-IR and UV-Vis diffuse reflection etc. The results show that the prepared Bi2O3/g-C3N4 composite photocatalyst has good morphology and uniformity of grains. The Bi2O3/g-C3N4 composite catalyst showed good photocatalytic performance. Among all the prepared composite catalysts, the composite catalyst Bi2O3/g-C3N4-30% had the best photocatalytic performance. The removal rate of tetracycline hydrochloride (TCH) by Bi2O3/g-C3N4-30% composite catalyst was 70%, which was 1.66 times that of pure Bi2O3 and 1.44 times that of pure g-C3N4. In addition, the photocatalytic degradation of tetracycline hydrochloride was verified by capture experiments. The main active species of (TCH) is superoxide radical (·O2-).
