A Study of the Effects of Processing and Alloying on the Microstructure and Deformation Behavior of Wrought Magnesium Alloys

Lightweight magnesium (Mg) alloys are being progressively incorporated into structural applications where weight reduction is an important design priority. Higher strength and better formability can be achieved by the development of new processing methods and new alloy composition, which entail a comprehensive understanding of the processing-microstructure-property relationship and deformation mechanisms. A systematic study on the processing-microstructure-property relationships was performed on a Mg-6Al-0.3Mn (wt%) (AM60) alloy. The effect of thermomechanical processing and subsequent heat treatment on the tensile, fatigue, and creep behavior was investigated. The specific processing conditions investigated were: (1) as-Thixomolded® (as-molded), (2) Thixomolded® then thermomechanically processed (TTMP), and (3) TTMP then annealed (annealed). Compared to the as-molded material, both the TTMP and annealed materials exhibited a significant increase in the tensile yield strength, the ultimate tensile strength, and the fatigue strength, but a decrease in the creep resistance. The altered microstructure was responsible for the change in the mechanical properties. In particular, the resulting mechanical properties were correlated with reduced porosity, texture modification, work hardening, and grain refinement effects introduced by the processing. A detailed analysis was performed on the small fatigue crack growth behavior. The effect of processing and mechanical loading parameters (including maximum applied stress and stress ratio) was evaluated. The applicability of a dislocation-based fracture mechanics relationship to the material was demonstrated by successfully modeling the effect of stress ratio on the crack growth rate using this relationship. Near surface pores were found to be the most preferential sites for fatigue crack initiation. Dilute rare earth (RE) additions to Mg alloys can lead to randomization of texture and potentially improved formability to wrought Mg alloys. A novel testing method was employed to study the tensile deformation mechanisms in a conventional wrought Mg alloy Mg-3Al-1Zn (wt%) (AZ31) and a newly-developed RE-containing Mg alloy Mg1Mn-1Nd (wt%) (MN11) for the temperature range of 50°C to 250°C. Twinning and dislocation slip activity were identified by a combination of in-situ tensile testing and electron backscatter diffraction (EBSD) analysis. GBS activity was evaluated using atomic force microscopy (AFM). For the highly-textured rolled AZ31, prismatic slip was prevalent at all testing temperatures and exhibited an increased activity with increasing temperature. The plastic strain ratio was found to decrease with increasing temperature, and was attributed to the increase in both second-order pyramidal slip activity and GBS activity. For the weakly-textured extruded MN11, basal slip was prevalent at all testing temperatures and exhibited an increased activity with increasing temperature. It is believed that the addition of RE provides effective strengthening to the basal slip system. The strengthening effect was reduced with increasing temperature due the faster diffusion of RE solutes, which led to a decrease in the critical resolved shear stress (CRSS) of basal slip. Overall the work performed in this dissertation provided new insight into the processing-microstructure-property relationships and deformation mechanisms in Mg alloys, which can serve as a guidance for alloy development and microstructural optimization, and the information and data provided in this dissertation can also be incorporated into future modeling efforts for predicting the deformation pathway and mechanical properties of simulated microstructures. Persons with disabilities have the right to request and receive reasonable accommodation. Please call the Department of Chemical Engineering and Materials Science at 355-5135 at least one day prior to the seminar; requests received after this date will be met when possible.