Effect Of SMAT On Microstructure And Mechanical Properties Of AZ31 Magnesium Alloy After Cryogenic Treatment

Science and technology has made great progress nowadays, but metal mineral resources such as copper, aluminum, iron, zinc and so on are becoming poor, environmental pollution is worsening, people's awareness of energy conservation and environmental protection gradually increased, lightweight construction materials attract increasingly concern. Magnesium alloy is recognized as green engineering materials of new century, compared to other metal alloys, such as aluminum alloy, and copper alloy, it has a specific strength and stiffness, damping performance, good cutting performance, excellent magnetic shielding and other unique advantages. However, magnesium alloy has poor plasticity, low absolute strength, bad wear and corrosion resistances, these shortcomings limit its industrial application greatly. Surface mechanical attrition treatment(SMAT) is a newly developed surface modification technology after new century, which form a form a certain thickness of gradient nanostructure on the material surface, thereby the mechanical properties of metallic materials were improved. However, after SMAT process, although the strength of the alloy has been significantly improved, the elongation of alloys sharply reduced. Cryogenic treatment is an extension process of the conventional cold treatment, which can improve the uniformity of the structure and promote the precipitation of second phase particles of magnesium alloys, thereby the strength, ductility and wear resistance or other properties were enhanced.Based on the purpose of improving the overall mechanical properties, AZ31 magnesium alloys were subjected to cryogenic treatment previously, and then treated by SMAT for different duration. By optical microscopy, X-ray diffraction, scanning electron microscopy and transmission electron microscopy to characterize the microstructure of the alloy changes before and after cryogenic and cryogenic SMAT process, mechanical properties were also measured before and after handling the alloy hardness, strength and elongation, etc., Influence of cryogenic and cryogenic SMAT on the microstructure and mechanical properties of AZ31 magnesium alloy was discussed, the mechanism of the evolution of microstructure and the improvement of mechanical performance of alloys were also subscribed, the main conclusions are as follows:1. AZ31 magnesium alloy after cryogenic treatment, grains were refined slightly and much tiny second phase particles precipitated out in the Mg matrix. The strength and hardness increased slightly, and the elongation enhanced significantly. After cryogenic 7d, the average grain size of the sample is refined from the 6.8?m to 6.2?m, the volume fraction of the second phase particles increased from 1.1 to 4.2%, and the average particle diameter is reduced by 40%. The slightly refinement of grain size makes the strength and hardness of the material increased, more and more homogeneous second phase precipitates make the AZ31 magnesium alloy more susceptible to deformation twins during plastic deformation process, ductility of the alloy subjected to CT 7d improved 42.8% significantly.2. After treated by cryogenic treatment SMAT, the grain of the surface layer AZ31 magnesium alloy was refined to nanoscale, and formed a gradient nanostructure from outside to inside which the grain size gradually increasing; Tensile strength and hardness were increased significantly, but the ductility was declined sharply, mechanical properties of sample treated by cryogenic treatment for 7d and SMAT for 3min was best. With the increasing of SMAT time, the grain of the surface layer of specimens treated by cryogenic treatment for 7d refined gradually, hardness and gradient nanostructured layer thickness of materials' surface increased. The grain size of the surface layer after SMAT 6min refined to about 38 nm, the gradient nano-layer thickness reach to 147?m, hardness of top surface increased by about 51%. Severe plastic deformation during SMAT process significantly increases the dislocation density, dislocation motions were hindered, work hardening rate was enhanced, thereby the plasticity of AZ31 magnesium alloy was reduced, but yield strength and ultimate tensile strength were substantial improved simultaneously and reached the maximum after treated by SMAT for 3min with an increase of about 33.7% and 32.3% respectively.3. Alloys treated by cryogenic treatment for 7 days were selected to subjected to SMAT for different duration, after the complex process of cryogenic treatment and SMAT, yield strength and ultimate tensile strength of AZ31 magnesium alloy were significantly improved and the degree of decreasing of elongation was reduced. A good matching of strength and plastic was achieved, mechanical properties of AZ31 magnesium alloy became much better. Cryogenic treatment before SMAT can make more second phase particles precipitated out and distributed more uniform in the AZ31 magnesium alloy matrix. These second phase particles can promote twinning nucleation and inhibit the twins grew up, favor the release of the stress during alloys subjected to severe plastic deformation and slow down decline of work hardening rate, and reduce the damage of SMAT to plasticity of AZ31 magnesium alloy thereby. Besides, the second phase particles can hinder dislocation slipping and staggering and other dislocation activities during the process of tensile. Thus, the strength AZ31 magnesium alloy subjected to SMAT process was enhanced.