Microstructure And Texture Evolution And Its Effect On Mechanical Properties In Dilute Magnesium Based AZ21 Alloy

Abstract

Dilute Mg alloys are exclusively identified for wrought structural applications in automotive industry. Any improvement in mechanical properties of alloys is possible only by grain size refinement and by the development of suitable texture. The grain size, grain size anisotropy and texture in these alloys affect the compatibility stresses in a very complex manner. To launch a full scale study towards understanding the complex deformation mechanisms operating in these alloys, it is necessary to understand the effect of grain size and texture on the mechanical behavior of Mg alloys in a broad or semi-quantitative manner first. Current literature lacks such broad study. In this present study, the effect of grain size, grain size anisotropy and texture evolution on the mechanical properties are examined in order to develop an understanding of the deformation mechanism that control the mechanical properties of a dilute conventionally extruded Mg alloy, AZ21. The approach adopted was to first study the microstructure and texture evolution in this conventionally extruded alloy. Since the grain sizes in these alloys vary over a wide range, it is hence necessary to study the microstructure evolution in a highly quantitative manner. In understanding texture, the present study is only limited to qualitatively evaluating the evolution of fibre component of texture using X-Ray Diffraction spectra. For truly quantitative microstructure evolution results in materials were grains sizes are spread over a wide range, it is critical to study a statistically enough no. of grains. Hence to avoid any sampling error, large montages (about 0.3 sq. mm) were constructed out of a series of high resolution images captured using an optical microscope. The montages so constructed are subjected to extensive image enhancement and various other operations are performed to convert these coloured to binary montages. Information like grain size, diameter etc., can be easily extracted from these binary montages and used for further analysis. Fibre texture in these conventionally extruded dilute Mg alloys generally develops due to alignment of basal planes along the direction of extrusion. The Critical Resolved Shear Stress for basal slip is very low when compared to that of non-basal planes. And also since there are very limited primary slip systems in these dilute Mg alloys, the development of strong fibre texture drastically changes the compatibility stresses and hence the mechanical properties . To broadly study the effect of microstructure-texture on mechanical proerties, after post extrusion annealing, heat treatments representing typical microstructure-texture combinations were identified. Effect of each microstructure-texture combination on the tensile and completely reversed cyclic fatigue properties are studied and qualitatively interpreted. The fibre texture showed pronounced effect on tensile ductility but it hardly affected the yield strength. With just 10% reduction in BPI, the ductility reduced by about 50%. A small change in average grain size did not alter the yield strength. Unlike tensile ductility, fatigue endurance stress was not altered drastically by the change in grain size or texture. But there appeared to be a significant effect of residual stress. In ending, a small change in microstructure-texture combination in these conventionally extruded alloys has a pronounced effect on ductility or in other words plastic properties. But a it has minimal effect on yield strength and fatigue endurance stress.