Changes in Microstructure and Deformation Resistance during Creep of the Die-Cast Mg–Al-Base Alloy AZ91hp at Intermediate Temperatures up to 150 °C

Abstract

Compression tests at constant stress and temperatures of 70 and 150 °C were used to study the change of microstructure during creep of die-cast AZ91hp by light optical, scanning and transmission electron microscopy up to large strains without interference by fracture. At all stresses from 130 to 240 MPa there is a transition from work hardening (primary creep with decreasing creep rate ε) to softening (tertiary creep with increasing ε). The work hardening can be associated with increase in dislocation density as the dislocation structure approaches a steady state subgrain structure and with precipitation of β-phase, which occurs in quite different forms in the primary α-phase and the surrounding eutectic regions. Twinning contributes to deformation at high stresses above 200 MPa. The pronounced softening in tertiary creep is associated with coarsening of β-phase. The instability of the phase structure has the consequence that the creep resistance of AZ91hp depends on thermomechanical history.