The Research On Microstructure And Phase Transformation Behavior Of Remelting NiTi Shape Memory Alloy

In the NiTi shape memory alloy products processing,it will have a lot of waste(leftover material),and the material utilization rate was about 20%,which results in a great deal of materials waste.Studying and solving the remelting and recycle of NiTi alloy leftover material will greatly reduce production cost of NiTi alloy products.It will be significance practical meaning for larger scale application of NiTi alloy products.In this paper,the NiTi alloy leftover material was remelted by using vacuum arc melting (VAM)and vacuum induction melting(VIM).Heat treatment of solution and aging were carried out for remelted alloy.The chemical compositions were tested;the phase, microstructure and morphology characteristic were discussed;as well as the phase transformation behavior of the NiTi alloy after remelting was analyzedThe content of Ti was slightly lower than that of the experimental design,but the content of Ni was higher in the alloy after vacuum arc remelting(VARM).The contents of Ti and Ni were consistent with the experimental design in the alloy after vacuum induction remelting (VIRM)using graphite crucible,or zirconia crucible.The contents of C,O,N and H met the demands of ASTM F 2063-00 standard in the alloy after three melting ways.The content of C was higher than O,and which gradually increase with the increasing of leftover material in the alloy through VIRM using graphite crucible.But the content of O was higher than C,and which gradually increase with the increasing of leftover material in the alloy through VIRM using zirconia crucible or VARM.The contents of N,H in the alloy were very low after three melting ways.NiTi B2,TiC phase were coexisted at room temperature,also along with leftover material increase,continuous and cross-distribution TiC gradually increased in the alloy through VIRM leftover material using graphite crucible.B2,Ni₂Ti₄O,TiC phase were existed at room temperature,and the content of Ni₂Ti₄O gradually increased with the increasing of leftover material in the alloy after VIRM using zirconia crucible or VARM leftover material, but the content of TiC was low in the alloy.The alloy through three melting ways showed different preferred orientation.The microstructure characteristic was similar in the alloy aider three melting ways.When the leftover material and Ni,Ti were mixed and melted together, microstructure was coarse dendrite,and serious composition segregation occurred.When the all leftover material or Ni,Ti were respectively melted,the alloy showed small and homogeneous microstructure.But the microstructure morphology in the alloy was quite different through three melting ways.The alloy obtained by melting all Ni,Ti or leftover material by three melting ways showed a certain martensitic transformation and inverse transformation except the alloy obtained by remelting all leftover material through VIRM using zirconia crucible.The transformation heat peaks were flat like steamed bread peak,and the transformation temperature range of martensitic reverse transformation was wide.The remelting alloys after solution and aging had obvious phase transformation behavior.Two-step phase transformation,R and B19' phase transformation appeared in the thermal cycle,and each transformation peak was acuter.The transformation temperature range of martensitic reverse transformation was narrower,and the beginning temperature of martensitic transformation and martensitic inverse transformation were higher in the alloy through heat treatment than those in the cast alloy.The alloy through VIRM using graphite crucible or VARM had obvious phase transformation behavior.But the alloy through VIRM using zirconia crucible had not obvious phase transformation behavior.