Effect Of Nano-Precipitation On Mechanical Properties Of CoCrFeNiTi_0.2 High Entropy Alloy At Room And Cryogenic Temperatures

A new alloy design concept,high-entropy alloys（HEAs）,has been widely concerned since its discovery.Traditional alloy design is based on one or two elements,supplemented by adding other alloy elements,while high-entropy alloys are composed of a variety of elements by means of equal atomic or near equal atomic ratios.With its novel design concept and special microstructure,HEAs obtain many excellent performances,such as high strength,excellent corrosion resistance,and high temperature resistance.However,the mechanical properties of single-phase or dual-phase high-entropy alloys are not enough for practical application,in order to improve both the tensile strength and ductility of the alloys,various strengthening mechanisms were introduced by means of deformation and heat treatment to improve the properties of the alloys.In the present study,by adopting appropriate heat treatment process,nano-precipitates are produced in the CoCrFeNiTi_0.2 HEAs.The effect of precipitation on the mechanical properties of the present HEAs at room and cryogenic temperatures,and the micro-deformation mechanisms of the alloys are studied.The main results are as follows:（1）（Co,Ni）₃Ti phases in different size were precipitated in CoCrFeNiTi_0.2 high entropy alloy by aging at 800℃and 900℃for different time,respectively,and their coarsening kinetics was studied.The results show that the coarsening behavior of precipitated phase is in good agreement with the PV model.（2）Tensile tests at room and cryogenic temperature were carried out on the specimens before and after the formation of precipitates.Compared with the tensile test data at room and cryogenic temperature,it can be concluded that the yield strength and tensile strength were significantly improved,while good plasticity was still maintained,which was mainly due to the strengthening effect provided by a large number of dispersed nano-precipitates.The strength and plasticity of the alloy have been improved significantly at cryogenic temperature,and the deformation mechanism has changed from dislocation dominant mode to stacking fault control mode.（3）Since the aging specimens do not fully recrystallize at 800℃,there are a lot of slip bands in the matrix.The slip bands can be regarded as sub-grain boundaries,which can improve the strength of the alloy through grain boundary strengthening.（4）The plasticity of existing alloys was predicted by Whitehouse and Clyne model in ceramic particles-reinforced metal-matrix composites.It was concluded that the plasticity of unreinforced alloys was in good agreement with the experimental results.