Pseudoelastic behaviour of cast magnesium AZ91 alloy under cyclic loading-unloading

Large stress-strain hysteresis loops are observed under cyclic loading after a small plastic prestrain. Loops have been observed in sand-cast material in a variety of tempers under tension or compression, and in high-pressure die-cast material with different cross-section thickness tested in tension. The loops are first observed after a nucleation strain of between 0.001 and 0.01% and grow to a maximum width after 1-2% plastic strain, becoming slightly narrower afterwards. When fully developed, the loops add a large (0.3-0.45%) pseudoelastic strain to the elastic strain, effectively decreasing the elastic modulus of the alloy by up to 70%. In sand-cast material of a given temper, the effects tend to be more pronounced in compression than in tension. Further, the effect is slightly larger in die-cast or aged sand-cast as compared to as-cast sand-cast material. The phenomenon is discussed in terms of the partial reversal of {10 (1) over bar2} twins upon unloading. (C) 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.