Abnormal creep stress exponents in a high-pressure die casting Mg-Al- RE alloy

Abnormal creep stress exponents (n) are frequently reported for heat-resistant magnesium alloys, but hitherto without in-depth elucidation or rationalization from the underlying creep mechanisms. In this work, tensile creep behavior of a high-pressure die casting Mg- 3.76Al- 3.52RE- 0.23Mn (wt.%) alloy at 200 degrees C were studied, adopting small applied stress intervals of 1-2.5 MPa. The result indicates a roller-coaster n-value evolution of 5 -* 23 -* 38 -* 8 -* 45 -* 76, as the applied stress increases from 80 MPa to 100 MPa. Microstructural analysis using transmission electron microscopy suggests that basal slip of a dislocations is dominant at lower stresses while dislocation climb at higher stresses and, simultaneously, twinning becomes more prominent and other submicrostructures such as dislocation bands gradually occur as the applied stress increases such as to 87 MPa. By simultaneously introducing a threshold stress and an internal stress, the abnormal n-values are rationalized to 3.0 at relatively lower stresses and gradually to 7.7 at higher stresses, corresponding basal slip of dislocations to dislocation climb. Therefore, we suggest that alloy design aimed to improve creep resistance of magnesium alloys should pay more attention to impeding dynamic twinning during creep.

Automated summary

Abnormal creep stress exponents were reported for heat-resistant magnesium alloys without detailed explanation on underlying creep mechanisms. The study on a Mg-3.76Al-3.52RE-0.23Mn alloy showed varying n-values at different stresses, with microstructural analysis suggesting changes in dominant creep mechanisms. Introducing threshold stress and internal stress helped rationalize the abnormal n-values, providing insights for improving creep resistance in magnesium alloys.