Effect Of Heat Treatment On Deformation-induced Phase Transformation Of Zr-17Nb Alloys

There is a contradiction between mechanical compatibility and shape formability of traditional materials used for spine fixation rods.Metastable ?-type Zr-17Nb alloys has excellent biocompatibility,low magnetic susceptibility,and initial elastic modulus close to human bone,which can ensure the biological and mechanical compatibility of materials.Further,deformation induced ??? phase transformation during deformation can increase the local elastic modulus of the deformed part,reduce the springback radios of the alloy,thereby improving the shape formability during surgery.The XRD,TEM and in situ synchrotron radiation observation are used to characterize the phase transformation and microstructure of the alloy.The effects of grain size,heat treatment temperature and quenching rate on deformation-induced phase transformation were systematically studied.The mechanism of deformation directions(tensile,compressive)on deformation induced phase transformation was compared.The experimental results indicate that Zr-17Nb alloy undergoes deformation-induced phase transformation during deformation.The transformation product includes small amount of ?' phase and ? phase,and there is a significant difference in the deformationinduced phase transformation of Zr-17Nb alloy with different heat treatment conditions.Deformation induced phase transformation are significantly enhanced with increasing grain refinement.For the large-size grain sample with long holding time,?-stabilizing Nb element has sufficient diffusion,thereby increasing the stability of the ?phase.In addition,the possibility for oxygen diffused into matrix is higher,leading to the suppression of ?phase formation.Further,large grain samples have fewer grain boundaries,and the dislocations at the grain boundaries during the tensile process result in fewer potential nucleation sites of the new phase.The heat treatment temperature(700?,800?,900?)has an effect on the deformation-induced phase transformation of Zr-17Nb alloy,the deformation-induced phase transformation is enhanced as the heat treatment temperature increases.At higher heat treated temperature,the ? phases formed during air cooling after hot-rolling are dissolved into matrix,this ? phase is Nb-leak phase comparing with matrix.Therefore,at higher temperature,the matrix contains fewer Nb content than that at lower temperature,in other words,the ?stability of matrix at higher temperature is lower than that at lower temperature.The athermal ? phase will be promoted by the low ? stability,further the athermal ? phase provides potential nucleation sites for the deformation-indcued phase transformation.The high quenching rate can promote the deformation-induced phase transformation of Zr-17Nb alloy during tensile deformation.The difference between quenching temperature and ? phase transformation starting temperature is large,promote the precipitation of athermal ? phase,which can be the potential nucleation sites of deformation-induced ? phase.In situ high energy X-ray observation found that the deformation induced ??? phase transformation occurred in the elastic deformation stage during the tensile deformation process,indicating that the phase transformation is stressinduced transformation.With the increase of stress during the loading process,the amount of ? phase increase,however,during the unloading process,and the reverse phase transformation occurs the reverse transformation is incomplete,which indicate the stressinduced ??? phase transformation is a partially reversible phase transformation.Zr17Nb alloy also undergoes deformation-induced ? phase transformation during compression,but deformation-induced phase transformation is suppressed under compressive stress.