Study On Microstructure Stability And Mechanical Behavior Of Bulk Nanocrystalline Pure Magnesium

The bulk nanometal materials have excellent and special properties,such as ultra-high strength and hardness as well as good plasticity and toughness,etc.However,their preparation techniques are so complex that they are usually synthetized in the extreme conditions.Therefore,the development of a low-cost,mass-producible bulk nanometal material preparation technique has always been the focus of attention.In addition,the nanocrystallization of magnesium is very difficult due to its high chemical activity.Compared with other nanomaterials,there are few reports about the preparations,structures,properties and their relationship of magnesium based materials.This paper provides an effective and feasible method to solve this problem.In the study of magnetic materials,it was found that when the coarse powder of metal material was hydrogenated and disproportionation reaction occured,the microstructure of the material was greatly refined to the nanometer level.And when the material was dehydrogenated and recombined,its grains remained at the nanometer level.The process was called HDDR(hydrogenation-disproportionation-desorption-recombination process)treatment.In this paper,pure magnesium metal powders were treated by the optimized hydrogenation dehydrogenation technology to prepare ultra-thermal stable nanocrystalline pure magnesium powders with grain size less than 20 nm.Combining with spark plasma sintering and hot extrusion technology,the nanocrystalline pure magnesium bars with dimensions of ?10×320 mm were obtained.And on this basis,the macroscopic mechanical behavior of nanocrystalline pure magnesium was studied.The OM,XRD,SEM,TEM,tensile and compression tests were performed to reveal the effects of different extrusion temperatures,deformation temperatures and strain rates on the mechanical properties of nanocrystalline pure magnesium materials.The nanocrystalline pure magnesium with excellent mechanical properties that the tensile yield strength(TYS)of 259 MPa and compressive yield strength(CYS)of 157 MPa was obtained,which are approximately twice as large as the corresponding micron-sized coarse-grained samples(125 MPa for TYS and 84 MPa for CYS).At the same time,in order to study the grain size stability of the nanocrystalline materials,the nanocrystalline powders and bulk materials obtained in this paper were subjected to high temperature long-time annealing heat treatment and severe plastic deformation,such as ball milling,equal-path angular extrusion and so on.The results showed that the nanocrystalline pure magnesium material have excellent stability,although it was treated at a high temperature of 550°C for 20 h,or ball-milled at a 60:1 ratio of a ball mill for 9 h,even treated at an equal channel angle of 400°C with eight passes.A preliminary analysis of the grain boundary stability mechanism of nanocrystals was conducted.It is believed that the stability of the grain boundary is mainly attribute to the fact that hydrogenation-dehydrogenation(HD)is an equilibrium reaction process at a high temperature,and the order of the atoms in the nanocrystalline grain boundary that formed during this process is higher than that in the grain boundary formed during non-equilibrium processes such as large plastic deformation.Thus the grain boundary stability is also higher.