Preparation And Superlasticity Of Biomedical Graded β Titanium Based Shape Memory Alloys

As a result of their distinguished properties such as low elastic modulus,good biocompatibility,non-toxic and non-allergenic side effect,excellent elasticity,new β Ti based shape memory alloys has been used widely in medical hard tissues implant field.But as a kind of long-term bearing material,the deformation reinforcement effect of shape memory alloy is unavoidable.The stress increment of gradient material was decreased significantly when the times of deformation increased on the contrary.Therefore,the titanium alloy which has large deformation was dealt with the laser heat treatment firstly,and then the recovery and recrystallization behavior of β Ti based shape memory alloys can be evaluated.Functional Graded β Titanium based Shape Memory Alloy(FG-SMA)can be prepared by changing laser scanning rate,and the superelasticity of shape memory alloy under various loading conditions in service was evaluated at last.The main conclusions are as follow:1.By changing laser scanning power,the recovery and recrystallization behavior about Ti-7.5Nb-4Mo-2Sn based shape memory alloys can be investigated symtematically.The research results show that with the increase of laser power,the hardness of titanium alloy specimens reduce on the contrary.Alloy specimens dealt with the power of 300~500 W is at the recovery process,while specimens of 600 ~ 800 W is in the process of recrystallization.Among them,the depth of hardness distribution of cross section have a gradient tendency when the power is 700 W.700 W,therefore,may be the best parameters of preparation of gradient material.Laser power for 300~500W at recovery process showed excellent superelastic performance.2.By changing laser scanning rate,the pareparation process of functional graded β titanium based shape memory alloy was investigated.It is found that the laser scanning rate has a significant effect on the alloy’s hardness gradient.There is a comparatively ideal hardness gradient trends when the laser rate is 60 mm/s,with hardness difference value of up to 50 HV between Scanning and no-scaning surface.So laser scanning rate for 60 mm/s may be the best parameters to prepare gradient materials.Thus,optimum parameters of preparation of gradient materials is p=700 W,V = 60 mm/s.The superelasticity of Gradient materials greatly exceeds of that homogeneous material.3.The superelasticity of functional graded β titanium based shape memory alloy was investigated by changing loading condition.It is discovered that the thermomechanical conditions including the plastic deformation,strain rate and cyclic loading have a strong influence on the superelasticity of functional gradient shape memory alloy.And in 80 times of loading and unloading tests,every maximum stress level of gradient material all tends to be a constant value of 600 MPa,while that of homogeneous material slowly rise to a constant value of 750 MPa.It can be learned that work hardening rate of the gradient material is greatly reduced compared with homogeneous material.After 80 th cyclic loading and unloading,the recovery deformation of gradient material is 5.986%,and the recovery rate is as high as 99.77% with residual strain tends to zero.