Investigation On Mechanical Properties And Bio-corrosion Resistance Of Zr-2.5Nb Alloy Processed By Laser Shock Peening

The requirements for the implant metal materials has been increasing due to the increasing of aging population and injury by accidents.Zr-2.5Nb alloys present excellent mechanical properties,good corrosion resistance and biocompatibility.Besides,Zr-2.5Nb alloys have better magnetic resonance compatibility than other metal materials.Hence,it has been regarded as one of the potential materials used for implants.However,its modulus is still much higher than that of the bone,which results in the serious stress shielding after implanting.Moreover,the poor wear resistance of Zr-2.5Nb alloys will lead to the aseptic infection due to the abrasive dust.In recent years,the development and application of MRI precise detection technology have put forward higher requirement for the magnetic susceptibility of the implants.All of the three factors challenge the comprehensive properties of Zr-2.5Nb alloy implants.The clinical application of Zr-2.5Nb alloy as the implants was restricted.Hence,it is urgent to develop a new method to reduce the modulus and magnetic susceptibility,as well as enhance its wear and corrosion resistance.In this thesis,a cold working process,named laser shock peening(LSP)was introduced to improve the surface properties of Zr-2.5Nb alloy.It was expected to reduce the magnetic susceptibility and modulus,improve the wear and corrosion resistance,and make progress in improving the comprehensive properties of Zr-Nb alloys.The microstructure and phase constituents of the Zr-2.5Nb processed by LSP were investigated to reveal the mechanism of microstructure and phase transformation.The mechanical properties,wear and corrosion resistance of the laser shock peened Zr-2.5Nb were examined to explore the relation between microstructure,phase transformation and the properties.The main contents and conclusions are as following.Laser shock peening induced the plastic deformation on the surface of the Zr-2.5Nb alloy.The deformation depth increased with increasing of laser pulse energy and impact times which result in a higher shock-wave strength.As a result of the plastic deformation,α to β,and α toα’ phase transformation occurred.The microstructure in the deformation zone is different from that of the Zr-2.5Nb substrate.The plastic deformation result in the needle-like or lath-like martensite rather than the network structure in the substrate.The martensite transformation and the microstructure transformation produced by laser shock peening is benefit for reducing the modulus of the Zr-2.5Nb alloy.The tensile results indicated that the elongation,the yield strength and the tensile strength of the Zr-2.5Nb processed by LSP present no obvious changes,the modulus,however,decreased significantly.Among the samples processed with different parameters,the maximum reduction of the modulus reached 31.86%.Meanwhile,the fracture morphology illustrated that the fracture mechanism for the substrate can be characterized as ductile fracture due to the micro pore aggregation.The layer crack between the deformation layer and the substrate occurred in the laser shocked Zr-2.5Nb alloy.The plastic deformation leads to the grain refinement on the surface of the Zr-2.5Nb alloy.The microhardness was significantly improved due to the grain refinement.The Gaussian distribution of the laser beam energy resulted in the soft-hard alternative structure on the surface.As a result of such structure,the coefficient of the friction and the wear rate were reduced.The wear resistance of Zr-2.5Nb was improved by laser shock peening treatment.The wear mechanism of the alloy before and after LSP can be characterized as adhesive wear and abrasive wear.The results of corrosion resistance against SBF solution showed that the resistance of the passive film on the Zr-2.5Nb surface with LSP is higher than that of the control without LSP.The corrosion current Icorr corresponding to the Zr-2.5Nb after LSP is lower than that of the control Zr-2.5Nb before LSP,the corrosion voltage Ecorr corresponding to the Zr-2.5Nb after LSP is higher than that of the control Zr-2.5Nb before LSP.The results indicated that the corrosion resistance of the Zr-2.5Nb alloy against SBF solution was improved by LSP.