Microstructure And Mechanical Properties Of Nanocrystalline Magnesium Alloys Preparated By HDDR-SPS Processing

Magnesium alloys with a series of outstanding advantages are the lightest metal structure material,and possess an extremely broad application prospects.However,the poor mechanical properties severely restricts their applications.And grain refinement is the most effective way to improve the mechanical strength and plasticity of Mg alloys.Generally,the yield strength ?y?or hardness?can be related to the grain size d by the Hall-Petch relationship: ?y=?0+kd_-1/2.For Mg alloys,the value of k is relatively high due to the HCP crystal structure and less slip system.Thus fine-grain strengthening is significant.At present,the grain size of Mg alloys can be refined to 0.5¹?m by the usual grain-refining methods,such as rapid solidification and severe plastic deformation etc..But it is difficult to refine the grain size to 100 nm and simultaneously prepare larger samples.In this work,the experiment performed following the process route,"commercial Mg and Mg alloys powder?HDDR treatment ?SPS sintering ? hot extrusion?microstructure analysis and mechanical properties".The commercial pure Mg and AZ91 Mg alloy cutting powders were chose as the initial materials.The nanocrystalline Mg and Mg alloy powders,bulks and bars were successively prepared by HDDR treatment,SPS and extrusion process.And the phase transition and microstructure evolution of Mg alloy powders during HDDR treatment were analyzed by XRD,SEM and TEM.And the optimum HDDR process parameters was determined and the mechanism of grain refinement was studied.The microstructure and mechanical properties of SPS nano bulks and bars were studied by TEM,universal testing machine and hardness testing machine etc..And the thermal stability of the nano grains was investigated.The results show that nanocrystalline Mg and Mg alloys with the average grain size of 15 nm are successfully prepared by the method combination of HDDR,SPS and hot extrusion.Compared with the as-cast and coarse-grained materials,the yield strength of nano Mg alloys has been greatly improved.And the nanocrystalline materials prepared by this method have high thermal stability,which opens up a new way for the preparation of high performance nanocrystalline Mg alloys.