Enhanced heat resistance of a sand-cast Mg-Sm-Zn-Zr alloy due to the co-precipitation of basal and prismatic phases by minor Nd addition

Minor Nd addition into a sand-cast Mg-4Sm-0.6Zn-0.4Zr alloy enhances the creep resistance, with a reduction of the minimum creep rate by nearly one order of magnitude, in comparison to Nd-free alloy. And the high-temperature tensile strength is also improved by more than 30 MPa. At the duration of creep, both basal and prismatic precipitates were dynamically precipitated in the grain interior of Nd-modified alloy, whilst only basal precipitate was developed in Nd-free alloy. The co-precipitation of basal and prismatic precipitates constructs a closed volume due to the vertical distribution of each other, which blocks dislocation motions more effectively than a single basal precipitate, as the most crucial reason for enhanced creep resistance through Nd addition. In addition, RE atoms at high temperatures can intrinsically offer stable atomic size-dependent solute hardening, whereas a large atomic misfit of Nd with Mg is expected to provide more strengthening effect in favor of the creep resistance of Nd-modified alloy. And the Nd is of lower diffusivity in Mg than that of Sm, beneficial to the creep resistance as well. These results will shed a light on the development of low-cost and creep-resistant Mg-RE-based cast alloys.