Effect Of Cyclic Heat Treatment On Abnormal Grain Growth And Properties Of CuAlMnNi Shape Memory Alloy

As an important functional material,Cu-based shape memory alloy has been widely used in biomedical,aerospace,mechanical manufacturing,civil construction and other fields.However,common polycrystalline Cu-based shape memory alloy has many problems,such as cracking at the grain boundary,short fatigue life,low superelasticity and large residual strain.In order to promote the development of Cubased shape memory alloys,it is necessary to further improve the properties of the alloys.In this paper,the abnormal grain growth in CuAlMnNi shape memory alloy was studied by CHT process,and the general rule of abnormal grain growth in CuAlMnNi memory alloy induced by CHT process was preliminarily obtained,and the influence of abnormal grain growth on the properties of the alloy was further studied.The results showed that:(1)The time of low-temperature annealing in CHT process will affect the precipitation process of α phase,and then affect the subsequent abnormal grain growth process.The larger the volume fraction of α phase,the easier the abnormal grain growth behavior of CuAlMnNi memory alloy,but when the volume fraction of α phase is too high,due to the number of abnormal grain increased,it is not conducive to obtain large abnormal grain.Further analysis shows that the mismatch dislocations are introduced due to the formation of semi-coherent interface between the precipitated phase α and the matrix β phase.Different volume fraction of α phase leads to different dislocation densities in the matrix,which results in different abnormal grain growth.(2)The time of high-temperature annealing in CHT process directly affects the abnormal grain growth process.With the increase of annealing time,the abnormal grain growth becomes more and more obvious.The initial abnormal grain growth process is basically completed at 120 min.In this paper,the driving force of the abnormal grain growth process is mainly composed of three parts: the interface energy of large Angle grain boundary,the interface energy of subgrain boundary and the indication energy.Among them,the interface energy of subgrain boundary formed by dislocation rearrangement is the dominant driving force.(3)The dominant driving force of forming abnormal grain growth only depends on the heat treatment,so the abnormal grain growth can be repeated induced by CHT process,the microstructure of CuAlMnNi shape memory alloy was modified by optimized CHT process,after multiple cyclic heat treatment,it have been able to obtain super large grain.(4)Further study on the properties of CuAlMnNi shape memory alloy showed that the superelastic strain and residual strain of CuAlMnNi shape memory alloy after CHT process were significantly better than those of ordinary polycrystalline memory alloy.The superelastic strain of CuAlMnNi shape memory alloy after three times CHT could reach 11.7%,while the residual strain was only 0.3%.It is much higher than that of ordinary polycrystalline memory alloy with superelasticity of about 4.8% and residual strain of 2.5%.At the same time,since there is no trigeminal grain boundary,its elongation(including superelastic strain)can reach about 42%,but its tensile strength decreases to a certain extent.