Study On R Phase Deformation Behavior Of Nanocrystalline NiTi Shape Memory Alloy

Nanocrystalline NiTi shape memory alloys are regarded as very promising functional material due to their excellent mechanical properties and functional characteristics.However,the inhibition effect of nanocrystalline grains on B19'martensite phase transformation makes R phase transformation inevitable and the R phase existed in a very wide temperature range.Therefore,studying the reorientation deformation behavior of the R phase and its micro-mechanism in nanocrystalline shows of great significance for understanding the nanocrystalline NiTi shape memory alloys and their practical application.In this thesis,NiTi alloy wires with fully nano-scale grains were prepared by severe cold-drawing deformation of NiTi alloy wires to amorphization and annealing,and combined with in-situ HE-XRD to study the R phase reorientation deformation and its micro-mechanical behavior in nanocrystalline.In addition,the effect of deformation temperature and annealing temperature on the R-phase reorientation deformation behavior were also explored.The main research contents and conclusions of this thesis are as follows:The thermally induced B19 'martensitic transformation is fully suppressed,and the B2?R transformation continues with a wide temperature range and always exists as R phase at low temperatures.The R phase variant reorientation deformation in tension was revealed to proceed in a Lüders-manner firstly,as evidenced by in-situ HE-XRD data.The variant reorientation obeys the principles to yield maximum crystallographic strain in the direction of the applied load,thus,the variants with directions of larger d-spacing values aligned to the axis of tension grew at the expense of those of lower d-spacing values,and the internal load transfer occurred among the different crystallographic variants during the R phase reorientation.The thermally formed R phase in the nanocrystalline NiTi wire shows two clear fiber textures,[100] and [001],which are a result of the pre-existing [111] texture of the B2 parent phase.After the reorientation deformation,the [100] texture is enhanced,and the [001] texture is weakened,its micromechanism is the orientation with larger d-spacing values [100] instead of the orientation with lower d-spacing values [001].R-phase reorientation strain and reorientation stress increase with decreasing deformation temperature.The structure of the R phase was found to change continuously with decreasing temperature after its formation,which includes a continuous extension of the c-axis and contraction of the a-axis,the d-spacing of favorable orientation almost perpendicular to the c-axis increases with the elongation of the c-axis,and unfavorable orientation almost perpendicular to the a-axis is reverse,caused the the crystallographic strains increase with decreasing temperature.The nanocrystalline NiTi alloys obtained by annealing at different temperatures all can undergo thermal induced B2(?)R transformation,and as the grain size increases,the R transformation temperature range gradually narrows due to the reduction of the grain boundary restraint effect.In addition,the smaller the grain size of the nanocrystalline NiTi alloy,the better its driving performance and shape memory effect.