Effects of precipitate origin and precipitate-free zone development on the tensile creep behaviors of a hot-rolled Mg-13wt%Gd binary alloy

Abstract

In heat-resistant Mg Gd based alloys, the relationship between the collective effects of precipitate origin and precipitate-free zone (PFZ) development on the creep resistance is still unclear. In this work, a hot-rolled Mg-13wt%Gd binary alloy was treated by solid solution (SS sample) and peak aging (AA sample). After peak aging, static precipitated β ′ phases were widely generated and PFZs were directly formed along both sides of the grain boundaries in AA sample. Tensile creep tests were performed at 523 K on both SS and AA samples. During creep testing, dynamic precipitated β ′ phases were gradually generated in SS sample while the β ′ phases persisted as large scales in AA sample. The dominant creep mechanisms of SS sample were twinning and cross-slip composed by < a > dislocations, since the late occurrence and low density of β ′ phases failed in hindering the movement < a > dislocations at the beginning of steady creep stage. On the contrary, the large-scale β ′ phases reduced the space for dislocation movement and succeeded in retarding cross-slip, leading to pyramidal < c + a > slip being the main creep mechanism of AA sample. Besides, PFZs were formed in SS sample (from zero to ~1.1 μm) while they became narrower in AA sample (from ~5 μm to ~1.7 μm). The large decreased ratio of PFZ width, the large scale of static precipitated β ′ phases and the resultant pyramidal < c + a > slip effectively enhanced the creep resistance of AA sample. Accordingly, AA sample behaved a higher creep resistance in comparison to SS sample at 523 K. It is hoped that the roles of precipitate origin and PFZ development in the creep properties can enrich the theories and methods in improving the service performance of Mg Gd series at medium to high temperatures. • Wide PFZs and large-scale static precipitates are obtained after peak aging. • For peak-aged alloy, PFZs are narrowed during creep and creep resistance rises. • For solid solute alloy, PFZs are widely developed and creep resistance falls. • Static precipitates can retard a -cross slip whereas dynamic precipitates fail. • As a -cross slip is delayed, pyramidal slip is activated to enhance creep resistance.