Study On The Behavior Of Abnormal Grain Growth Of CuAlMn Superelastic Alloys

In the process of stress-induced martensitic transformation in polycrystalline CuAlMn superelastic alloy,CuAlMn alloy is easy to produce stress concentration and grain boundary cracking in the process of deformation due to strong anisotropy,which seriously limits the application of CuAlMn alloy.In this paper,CuAlMn/CuAlMn Co Ni superelastic alloy with large grain size or even single crystal structure was prepared by adjusting the microstructure of and using directional recrystallization and cyclic heat treatment,to reduce the grain boundary and improve the deformability of the alloy.Based on CuAlMn alloy,the effects of Ni and Co alloy elements on the abnormal grain growth and superelastic properties of CuAlMn alloy were studied,and the ways to improve CuAlMn alloy were discussed.In this paper,the effects of directional recrystallization process parameters and microstructure of initial materials on grain structure evolution and grain boundary selective migration mechanism of CuAlMn superelastic alloy during directional recrystallization were studied.It is found that the directional recrystallization technology is suitable for the preparation of columnar grain CuAlMn superelastic alloy with large aspect ratio.In the process of directional recrystallization,the temperature of each hot zone above 800℃corresponds to the optimal drawing velocity of columnar grains with the maximum aspect ratio.For each of the hot zone temperatures of 800℃,850℃,900℃,there is a corresponding optimum drawing velocity of 2μm/s,5μm/s and 15μm/s,respectively.At the same time,in the process of the sample moving along the hot zone,there is a drawing upper limit to obtain the columnar grain structure,beyond this drawing velocity,abnormal grain growth cannot occur.In addition,in the directional recrystallization of CuAlMn,abnormal grain growth can occur only when the temperature is higher than the remelting temperature of the second phaseαphase.The mechanism of directional migration of grain boundaries in the process of directional recrystallization of CuAlMn superelastic alloy was studied.It is found character of grain boundary between columnar grains of CuAlMn superelastic alloy after directional recrystallization is mainly concentrated in the high-angle grain boundary,and the grain boundary between the columnar grain and the front equiaxed grain tends to the high-angle grain boundary,and the high-angle grain boundary has a higher grain boundary mobility than the low-angle grain boundary.During the growth of columnar grains,the fraction of low-angle boundaries between the orientation of equiaxed grains along<111>direction is significantly higher than the fraction of high-angle boundaries.Thus,the growth along<111>direction is constrained due to the low-angle boundary pinning while the surrounding random orientation grains can grow into columnar grains.The apparent activation energy of the columnar grain growth is estimated as 177 k J/mol and the driving force for grain boundary migration mainly comes from the grain boundary energy reduction induced by grain growth.The CuAlMn-based superelastic alloy with centimeter single crystal can be well prepared by cyclic heat treatment,and the addition of Ni has little effect on the abnormal grain growth behavior of CuAlMn superelastic alloy.The addition of 0.5at.%Co can refine the sub-grain size and enhance the driving force of sub-grain on the abnormal grain growth of CuAlMn-Ni₂ alloy,so the addition of 0.5at.%Co plays an obvious role in promoting the abnormal grain growth of CuAlMn-Ni₂ alloy.After the addition of 1at.%Co and 2at.%Co,not onlyαphase but also massive Co Al-rich phase precipitated in the process of cyclic heat treatment,which is difficult to completely remelt into the matrix during cyclic heat treatment,resulting in the decrease of plasticity and superelastic recovery of the alloy after the addition of 1at.%Co and 2at.%Co.The addition of Ni element has little effect on the superelastic properties of CuAlMn alloy,while the addition of 0.5at.%Co element can improve the superelastic properties of CuAlMn-Ni₂ alloy.