Phase Transformation Behavior And Mechanical Properties Of NiTiHf(Fe) Shape Memory Alloys

In NiTi-based shape memory alloys,there is a strain glass state formed by short-range ordered strain nanodomains.This structure formed when the transformation of the parent phase to the long-range ordered martensitic phase was inhibited.Recently,a new strain glass transition mechanism has been discovered in NiTi Pt:a large number of grain boundaries plane defects and large atomic point defects are co-doped to form strain glasses.In this study,the doping of Hf atoms with larger atomic radius is proposed to verify the applicability of this principle.In addition,the H phase is a normal precipitation phase in the NiTiHf alloy system after aging,and this precipitation phase can introduce a coherent strain field similar to that of Ni₄Ti₃.It is generally believed that the coherent strain field of the precipitated phases in Ni-rich NiTi alloys is the inducement of R-phase transformation,however,R-phase transformation does not occur in already reported NiTiHf alloys with precipitation of H-phase.In this study,a series of NiTiFeHf alloys were designed based on the NiTi Fe system with R-phase transformation.By changing the content of Hf and Fe and the aging process,the influence of precipitation relative to R-phase transformation was studied.However,the R-phase transformation cannot occur in materials with H-phase provide coherent strain field similar to Ni₄Ti₃,probably because the inhibition of the R-phase transformation by the Hf element is greater than the induction of the R-phase transformation by the coherent strain.The mechanical properties test of NiTiFeHf alloy shows that when the Hf element content is greater than 5 at.%,the brittleness of the material increases.Properly increasing the Hf element content can improve the material properties.The nanocrystalline Ni_51-xTi Fe_xHf_a alloy reinforced by grain boundaries,Ni elements,Fe elements and Hf elements exhibits good mechanical properties:the recoverable strain reaches 6.8%at room temperature,and the maximum of d-spacing strain of the parent phase（110）crystal plane along the loading direction can approach 6.0%;When the temperature is-100℃,the recoverable strain reached 7.8%,and the minimum stress hysteresis is about 70 MPa,and the material has stable recovery strain about 7.7%after 30 cycles.