Formation Mechanism And Regulation Of Nano-domains In Rich-Ni NiTi Memory Alloy

The rich-Ni NiTi alloy(Ni?51.5 at.%)after solution treatment produces many anomalies during the cooling process:the structure does not change with temperature,the resistance increases abnormally,the modulus softens,and the electron diffraction shows diffuse scattering stripes.Since the 1970s,people have observed these anomalies during the study of the pre-martensitic transformation,and found that nano-domains are formed in the corresponding microstructure.The mechanism of nano-domain formation is currently inconclusive.The current mainstream view is that defects impede the B2 phase long-range ordered martensite transforms to form short-range ordered nano-domains.In addition,people have also proposed pseudo-amplitude decomposition(coupling of fast lattice structure changes and slow composition changes)to form nano-domains through simulation calculations,but there is no experimental confirmation.In this paper,the formation mechanism of nano-domains analyzed by three-dimensional atomic probe technology for Ni₅₂Ti₄₈ alloy after solution treatment:there is microscopic composition unevenness in the alloy,forming a poor,rich Ni about8-10nm region(nano-domain).With the temperature decreases,the Ni-depleted region has a tendency to preferentially martensite transformation.Due to the constraint of the surrounding Ni-rich B2 on the Ni-depleted martensitic transformation,the long-range ordered martensitic transformation of the alloy is affected and suppression the formation of nano-domains.And it was found that with the increase of temperature and time,the volume fraction is almost contant and size of nano-domains decreased,the corresponding nano-domain formation temperature T_nd increased,and the ideal freezing temperature T₀ of nano-domains decreased.At the same time,in this paper,through large deformation cold drawing of rich-Ni NiTi alloys that can occur nano-domain transition,combined with appropriate annealing process and low temperature aging,a comprehensive study of the surface defects and body defects for rich-Ni NiTi shape memory alloy nano-domain regulation.It is found that this may be due to the existence of a large number of surface defects,which enriches the Ni element in the grain boundaries of the nanocrystals,causing the intracrystalline Ni content to be much lower than the average composition and causing the R phase transformation of the alloy.Due to the spatial regionalism of the crystal grains,the nanocrystals still cannot control the nanodomains even after aging.Based on this,increasing the grain size through short time annealing at high temperature and performing low-temperature aging,it was found that The alloy with martensitic transformation occur strain glass transformation after low-temperature aging.Furthermore,by changing the type of doping elements in the Ni-rich alloy,it was found that the doping Fe element has a more pronounced control effect on nano-domain transition than the doping Ni element.