A Comprehensive Study on the Microstructure, Texture, and Mechanical Properties of Mg and its Alloys during Grain Growth

Abstract

In the present study, the microstructure, and texture evolution during grain growth of pure Mg and its alloys, such as AM30 (Mg 3 wt.% Al 0.3 wt.% Mn) and AME300 (Mg 3 wt.% Al 0.3 wt.% Mn 0.2 wt.% Ce) have been investigated. The objective of the present study has also been to establish the correlation between microstructure, texture, and mechanical properties in these materials. Hot rolled pure Mg samples were subjected to isothermal annealing at temperatures of 150, 200, 300 and 400 °C for different soaking times ranging from 1 min to 1440 min (i.e., from 1 min to 1 day). A grain growth exponent of n = 13 was observed and the activation energy for grain growth kinetics was found to be 95.6 kJmol. Further, broadening of the normalized grain size distributions, indicating abnormal grain growth, was also observed at all temperatures of annealing. The samples had dominant basal texture before and after annealing. However, alleviation of basal texture was observed in the samples after annealing up to a temperature of 300 °C. Annealing further at a temperature of 400 C formed a strong basal texture in the samples. The mobility of high angle grain boundaries, which is proportional to correlated misorientation distribution, was observed to be responsible for texture strengthening of the material. The boundary mobility changes during grain growth led to the growth of either small or large grains. It was further observed that the growth of small grains caused the formation of basal fiber and large grains led to the weakening of basal texture. AM30 alloy samples were subjected to annealing at temperatures ranging from 200 to 450 °C for different time periods of 1 1440 min respectively. The effects of both annealing temperature, T, and time, t, on the overall grain growth process have been evaluated. A grain growth exponent of n = 9 and activation energy of Q = 105.6 kJmol were observed. It was also observed that the presence of precipitates, such as Mg17 Al12, Al11 Mn4, and Al8 Mn5, in the alloy significantly affect the grain growth kinetics during annealing. Both the volume fraction and the average size of the precipitates decreased as a function of annealing temperature/time. The precipitates were further appeared to be dissolved at a temperature of 450 °C of annealing and this led to the abnormal grain growth of the samples during annealing. The effects of solute drag and dislocation density on the grain growth behavior were found to be insignificant during annealing, as estimated through a mesoscale model based on cellular automata. The strong basal texture in the starting material was observed to be weakened up to 480 min of annealing. However, annealing for 1440 min regained the basal texture which may be attributed due to the abnormal grain growth in the samples. AME300 alloy samples were subjected to annealing at temperatures ranging from 200 to 450 °C for different periods to evaluate the microstructure and texture developments in the alloy. The grain growth exponent of n = 13 and the activation energy for grain growth, Q = 75 kJmol were observed. A large number of precipitates were observed in the samples. They were found to be Mg17 Al12, Al8 Mn5, Al11 Mn4, Al11 Ce3, Al3Ce, Al4 Ce, Mg17 Ce2, Al10 Mn7 Ce2, Mg2 Ce, Mg3 Ce, and Mg12 Ce, as confirmed by electron probe microanalysis (EPMA) and X-ray Diffraction (XRD). The precipitates were further found to be dissolved or disappeared with an increase in the temperature of annealing. As a result, the grain boundary mobility was enhanced during annealing at high temperatures and increased the grain growth of the samples. The initial basal texture in the alloy was observed to be tilted (by ~ 30°) towards the transverse direction of the samples during annealing. Such a texture weakening in the alloy during annealing was attributed to the nucleation and growth of new grains formed in the deformed zone surrounding the precipitates. The correlation between microstructures, textures, and mechanical properties of pure Mg, AM30 and AME300 alloys was further investigated in the present study. The results revealed that the ductility of pure Mg is dependent on the reduction in basal texture intensity. However, its tensile strength is dependent on the average grain sizes of the samples. The same has also been observed in AM30 and AME300 alloys after annealing at higher temperatures of 400 and 450 °C. However, annealing at lower temperatures (i.e., 200 and 300 °C) did not show any correlation between grain size, texture, and mechanical properties of the alloys. This has been attributed to the presence of precipitates in the alloys. It was further found that AME300 alloy had the best combination of tensile strength and ductility compared to AM30 and pure Mg after annealing.