Study On Inlfuence Of Deformation On Transition, Tensile And Memory Properties For Ti Rich Tini Shape Memory Alloys

TiNi shape memory alloys have attracted considerable attention because they combine excellent mechanical and shape memory properties in many shape memory alloys. With the ongoing development of equipment manufacturing technique, TiNi shape memory alloys are required to have higher mechanical and shape memory performances. Thus, in this paper, cold deformation and subsequent low-temperature annealing are used to increase mechanical and shape memory properties of TiNi alloys. The purpose is to provide reference basis for manufacture of high performance TiNi memory alloy parts.Microstructure of TiNi shape memory alloys with Ni content ranging from49.8to51at.%, and phase transition, mechanical and shape memory performances of Ti-49.8at.%Ni alloy at different deformation amounts and annealing temperatures in350～500℃range are studied systematically by DSC, optical microscope, XRD, TEM and tester with a program temperature controlling condition room in order to provide reference data for application of TiNi alloys in engineering.The following results are obtained. With the decrease of Ni content, TiNi alloys consist of B2and Ti4Ni2O oxide, B2and Ti4Ni2O and martensite, martensite and Ti4Ni2O and few B2phase. Compared with alloy quenched at850℃, phenomenon of multi-orientation of TiNi alloys annealed at450℃disappears and no second phase precipitates.For cold-drawn and annealed after cold-drawing Ti-49.8at.%Ni alloys, Ms and Mf decrease with the increase of deformation. For annealed alloys, R phase transition occurs on decreasing temperature, and characteristic and location of R phase transition peak don’t vary with deformation, but martensite phase transition peak moves to low temperature zone with increasing deformation. With raising heat treatment temperature, Ms and Mf of cold-drawn alloys increase, and characteristic and location of R phase transition peak do not change, and M transition peak gradually moves to high temperature zone. On the stress-strain curves of cold-drawn alloys, only one plastic plateau appears. While for annealed alloys after cold-drawing, two plateaus are shown and with the increase of deformation, martensitic reorientation plateau becomes short and steep, after deformation reaches27.8%, with continuously raising deformation, martensitic reorientation plateau extends and becomes flatter; Yield strength σs and rupture strength σb enhance and elongation percent drops with raising deformation for cold-drawn and annealed at low temperature after cold-drawing alloys, and martensitic reorientation critical stress σm first reduces and then increases, and drops again after reaches the maximum in annealed alloys.σm,σs and σb enhance with increasing deformation at120℃, while they decrease and elongation percent raises with the increase of heat treatment temperature at27.8% deformation. σM,σs and σb are minimum for hot-drawn alloy. σs and σb are maximum for cold-drawn alloy, and σM at120℃is obviously greater than that at room temperature for alloys annealed after cold-drawing, but their as and σb are near.With the increase of defromation, recovery force memory increases, reverse transformation temperature A’s drops, recovery strain ε1first increases and then reduces, and raises again after deformation is more than27.8%at8%pre-strain; Recovery strain εr and reverse transformation temperature A’s increase with the increase of pre-strain at27.8%deformation; Reverse transformation temperature A’s increases, and recovery strain εr first raises and then drops with the increase of annealing temperature at8%pre-strain and27.8%deformation.