Investigation On The Melting Process,Microstructure And Property Of TiNi Shape Memory Alloy Prepared Using BaZrO₃ Crucible By Vim

The contamination from the crucible has been recognized as the main drawback of using vacuum induction melting(VIM)method to prepare TiNi shape memory alloys(SMAs).By far,there still lacks high-stablity oxide refractory material for re-melting titanium alloys due to their high chemical activity.In the present work,the melting parameters of VIM were optimized,and the prepared TiNi ingot were processed into alloy sheets via thermomechanical and annealing treatments.Subsequently,in-situ synchrotron radiation X-ray diffraction(XRD),transmission electron microscopy(TEM),scanning electron microscopy(SEM),differential scanning calorimetry(DSC),tensile test and other analytical measurements were employed to characterize the underlying mechiansims,including(1)the reaction between TiNi alloy and BaZrO₃ crucible,(2)the existence of impurity elements in TiNi alloy and its effect on the microstructure of the alloy,(3)the mechanical behaviors,shape memory effect and the stress-induced martensitic transformation in the prepared TiNi shape memory alloys.The results can be summarized as follow:1.The impurity contents in the as-cast TiNi alloy prepared by using industrial-scale BaZrO₃ crucible meets the ASTM F2063-05 standard for the first time.There are about 300 ppm of oxygen and 400 ppm of zirconium detected in the TiNi alloy.In addition,the as-cast alloy presents a good chemical compositional homogeneity.The fluctuation of transformation temperatures at different postions of TiNi ingot could be controlled within 4?.2.The microstructure of the as-cast TiNi alloy is mainly composed of the TiNi matrix phase with B2 cubic structure and a small amount of Ti₂Ni phase distributing along the grain boundaries,resulting in the morphological morphology of the as-cast TiNi alloy.The residual oxygen element after melting is mainly concentrated in the Ti₂Ni phase,while a small amount of zirconium element exists in both the matrix phase and the second phase,and no barium element was found in the as-cast alloy.3.The as-cast TiNi alloy exhibits poor resistance to slip deformation during tensile deformation and cyclic deformation,along with the accumulation of dislocation defects during stress-induced martensitic transformation.The mechanical properties of the samples at different positions are not uniform,and the fracture morphology of the alloy samples shows intergranular fracture.The alloy in the center of the ingot exhibited the highest fracture elongation.After cyclic tensile deformation of the as-cast TiNi alloy,the dislocation density increases significantly,and work hardening caused by slip deformation exists in the samples.Nonetheless,the TiNi alloy ingot showed good plasticity and machinability during the thermomechanical process.The grain size of the TiNi alloy sheet after rolling and vacuum annealing reached about10?m,and showed good mechanical property during the tensile deformation.The tensile strength and elongation rate reached 880 MPa and 37.63%,respectively.The resistance of TiNi alloy samples to slip deformation was also significantly improved,and the transformational strain reached about 8%.A large number of tough dimples appeared on the fracture surface of the alloy after fracture,indicating that the plasticity of the alloy has been significantly improved after the thermomechanical processing.4.Inhomogeneous Lüders-type mechanism in Ni Ti SMA could give rise to a sudden and intense increase of martensite at the expense of austenite in local region.However,there still remained a mass of austenite with high lattice strain inside the Lüders band area,which could transform into full martensite progressively beyond the stress plateau of the strain-stress curve.However,along with the intense accumulation of dislocation defects in the material,the inhomogeneous Lüders-type manner turned into a uniform transformation manner,which was realized by progressively homogeneous nucleation and growth of martensite in local region.The accumulation of residual martensite and transformation-induced dislocation slip occurred concurrently with the uniform SIM transformation initially.After the residual martensite in the material was stabilized under 15%strain,the deformation behavior under the uniform manner switched to a mixture of(1)a balanced forward and reverse phase transformation between austenite and martensite,as well as(2)the elastic deformation of the aggregate.The dislocation density in the material didn't show an apparent increase during further cycling.