The Solid Solution and Grain Boundary Hardening due to Mg in an Aluminum Alloy System at Room and Elevated Temperatures

Abstract

The solid solution and grain boundary hardening due to Mg in Al-1.5wt pct Mn-0.5wt pct Cu system with 0.1 to 2.1 wt pct Mg has been studied at 25 °C to 345 °C. A transition temperature around 200 °C was found, while below it, the frictional stress, solid solution hardening from Mg and grain boundary hardening are little affected by temperature. This is due to the enhanced solute pinning on dislocations, by Cottrell clouds or other extensive segregation of solutes to dislocations. Above the transition, the solid solution hardening is controlled by the temperature-dependent shear modulus, which decreases linearly with the increasing temperature up to ~ 300 °C. Therefore, both the frictional stress and the solution hardening from Mg decrease linearly with the increasing temperature above ~ 200 °C. The grain boundary hardening obeys the Hall–Petch equation over the whole temperature range. Below the transition point the Petch slope is mainly controlled by the solute Mg, while above this temperature it is dependent on both the solute Mg level and temperature. The Petch slope increases linearly with solute Mg level, and decreases proportionally with the inverse of temperature. Semiempirical expressions for the yield strength at 25 °C to 345 °C were derived, including both solid solution and grain boundary effects.