Study On The Microstructure And Properties Of Cr-TaCr₂ Alloy Prepared By Mechanical Alloying And Hot Pressing

Laves phase TaCr2 is expected to work as novel high-temperature structural materials due to its high melting temperature, moderate density and good high temperature mechanical propertis. However, its application is hindered by the brittleness, especially, the severe room-temperature brittleness. One of the solutions to reservation the good high-temperature mechanical propertis of Laves phase and overcome the brittleness by soft second phase is to fabricate in situ composites containing reinforcing Laves phases in Cr matrix of Ta-Cr base alloy. In addition, the alloy has excellent oxidation resistance potential causes the presence of large amounts Cr. Therefore, Cr-TaCr2 is generally considered the most worthy of research as Laves phase alloy.This study used-200 mesh high purity Ta and Cr powder as the raw materials. Nominal composition were Cr-5Ta, Cr-9.25 Ta, Cr-13 Ta, Cr-20 Ta, Cr-25 Ta, Cr-30 Ta content of Laves-phase TaCr2 were 12%, 23%, 33%, 53%, 67%, 87%. Take advantage of mechanical alloying and hot pressed sintering technology prepared the Cr-TaCr2 alloy. In-situ reaction synthesized Laves-phase TaCr2 in the course of powder densifying via hot pressed sintering. Adopted the toughening mechanism of grain refining and ductile phase addition, obtained two phase Cr-TaCr2 alloy with different Laves phase contents. Used OM, XRD, SEM and EDS analysis means to study the effect of different ingredients and hot pressed process on microstructure and properties of the Cr-TaCr2 alloy.The result of study show that Cr-TaCr2 sample prepared under the HP process with the temperature of 1250℃ and the time of 2h can enough synthesized the Laves phase TaCr2. With increased of the Ta content from 5% to stoichiometric 30%, the relative density, the fracture toughness and the room temperature compressive properties of Cr-TaCr2 sample decrease and the Vicker’s hardness increase. The Cr-9.25 Ta sample has the optimum microstructure and properties due to it has plastic strain in compressive test at room temperature and has the largest content Laves phase.With the improve of hot pressed temperature from 1250℃ to 1300℃, the relative density and the fracture toughness of the Cr-TaCr2 sample increased and the Vicker’s hardness unchanged. But improve HP temperature to 1350℃, the grains begin growth observed by SEM, the result is the fracture toughness and the Vicker’s hardness of the Cr-TaCr2 sample decreased. With the extension of hot pressed time from 1h to 3h, the relative density and the compressive properties of the Cr-TaCr2 sample slight increase, but not beneficial to obtain fine grained structure. Therefore, the Cr-TaCr2 sample obtained the best microstructure and properties prepared under the HP process with the temperature of 1300℃ and the time of 1h.Under the determinate HP process, the relative density, the Vicker’s hardness, the ultimate compressive strength, the yields compressive strength and strain of the Cr-9.25 Ta alloy are 98. 5%, 4. 7GPa, 1370 MPa, 1310 MPa and 2% respectively, the fracture toughness exceed 6.5MPa · m1/2, The Cr-9.25 Ta alloy with fine and homogeneous microstructure is obtained and uniformly spaced Laves phase and soft second phase like a net. Compared with the Laves phase TaCr2 alloy prepared by arc-melting, which has the fracture toughness of 1MPa?m1/2, the fracture toughness in this research is increased significantly. Compared with the Cr-9.25 Ta alloy prepared by directional solidification, which has thick stick of microstructure. The Cr-9.25 Ta alloy has uniform fine granular microstructure prepared by mechanical alloying and hot pressing process. The effect of fine grain toughening is fully realized in the process.