The Prepare And The Investigation Of Deformation Behavior Of Nanocrystalline Ni₅₂Ti₄₈ Shape Memory Alloy

With the raising demand of miniaturization and lightweight of device,new challenges have been raised to extreme superelastic stress and environmental service of NiTi shape memory alloy.Nanocrystalline NiTi shape memory alloys exhibit higher yield strength and superelastic strength than coarse-grain NiTi.Increasing Ni content of NiTi alloy can increase the superelastic stress,lower the inferior limit and widen the superelastic temperature range.In this paper,by using"solid solution atom precipitation caused plastification+mechanical re-dissolution+crystallization annealing",nanocrystalline Ni₅₂Ti₄₈（at.%）memory alloy was prepared for the first time and its microstructure,phase transformation and deformation behavior of it were investigated.The features mentioned above of materials after aging treatment were also investigated.The main research contents and results of this paper are as follows:NiTi shape memory alloys with high Ni content（Ni content>51.5 at.%）cannot be nanocrystallized by normal methods due to its poor plasticity.We were inspired by the principle in pearlitic steel which dissolves cementite back to matrix by large deformation,and come up with a new design concept,precipitate Ni₄Ti₃by aging treatment,lower the Ni content of Ni₅₂Ti₄₈shape memory alloy which has a high Ni content,and raise its plastic deformation ability,then dissolve the Ni₄Ti₃precipitation by large deformation back to matrix,following by rapid crystallization annealing（<10 min）,eventually obtained nanocrystalline NiTi shape memory alloy with high Ni content.The average grain size of gained nanocrystalline Ni₅₂Ti₄₈ SMAs was 10 nm,the Ni atoms converged at grain boundary,and the Ni content of grain boundary reaches 62 at.%.The material shows high strength and excellent superelasticity over a wide temperature range from-180℃ to 120℃.At room temperature,the superelastic stress of nanocrystalline Ni₅₂Ti₄₈ SMAs reaches 1.1 GPa and the fracture strength reaches 2.1 GPa,filling the vacancy in the field of high-strength shape memory alloy in the wide temperature range.Due to the grain boundary segregation,Ni₅₂Ti₄₈ shape memory alloy shows lower annealing temperature sensitivity than Ni_50.2Ti_49.8 SMAs.We found the Ni₄Ti₃ precipitation in the aged nanocrystalline Ni₅₂Ti₄₈ shape memory alloy（with an average grain size of 29 nm）,and directly observed the existence of Ni₄Ti₃ phase in the nanocrystalline NiTi memory alloy material for the first time.Due to the increase of grain size and the decrease of Ni content,the superelastic and superelastic temperature range of aged nanocrystalline Ni₅₂Ti₄₈memory alloy decreased compared to which before aging.In the process of matrix deformation,Ni₄Ti₃ precipitation exhibited an extremely large lattice strain（～4.3%）,which was much higher than the maximum lattice strain（～2.7%）of the precipitated phase of Ni₄Ti₃ in coarse-grained NiTi SMAs.