Study Of Mechanical Properties Of AZ91D Magnesium Alloy After Surface Nano-Crystallization

Currently, magnesium alloys have been known as the lightest metallic structured materials, which have high thermal conductivity, damping characteristics, dimensional stability as well as good electromagnetic shielding and mechanical properties. They are also easily recycled.In actual application, the instabliity of materials always occur on the surface, for example, the fatigue, corrosion and wearing are very sensitive to the materials’structure and properties. Consequently, the surface characteristics of material are the key factors that determine its whole service behavior.The research results show that materials’surface nano-crystallization can produce compact and non-polluted nano-meter structure layer, which is supposed to change surface micro-structure and attributes of the material. However, most of current studies are focus on the evolution mechanism of material microstructure, on the contrary, the further observation, on material mechanical properties such as elastic modulus, hardness and strain harding exponent, etc., are seldom.The main work of the thesis is summarized as following aspects: Firstly, a series of nanoindentation tests were carried on the nano layers, transition layers and matrix of AZ91D magnesium alloy and pure magnesium, respectively. The values of elastic modulus and hardness were measured, while the load-displacement curves, depth-elastic modulus curves and depth-hardness curves were recorded. The variation of mechanical properties from nano layers to matrix can be observed from those test results. The finite element software-ANSYS was used to simulate the process of indentation, and the simulating results are identical with the date obtained from the indentation tests. Besides, the simulation can also dynamic observe the change of the stress field between indenter and contact area, which is very difficult to be observed in the indentation tests.According to the method that was proposed by J.M.Antunes, the results of experimental hardness tests acquired with single indenter geometry to be used to determine the plastic properties of materials. Forward and reverse analyses of high deformation two-dimensional numerical simulations of Berkovich tip indentation tests are used to determine different mechanical properties of materials:Young’s modulus, yield stress and strain-hardening exponent.First of all, establish the elastic-plastic constitutive model of material by a power law description. Then, comparing the loading part of load-displacement curves that obtained from experiments and simulation, the values of the representative plastic stress and strain were determined, respectively; comparing the values of stiffness that was calculated at the deepest point, the value of the strain-harding exponent was determined. When the value of strain-hardening exponent is given, the value of yield stress can be determined from elastic-plastic constitutive model.