Role of applied uniaxial stress during creep testing on precipitation in Mg-Nd alloys

The coupled effects of an applied stress and temperature, during creep testing at 90 MPa/177 °C, on intragranular second phase (orthorhombic β′ and fcc β1 phases) precipitation in a binary Mg-0.5 at% Nd alloy were investigated by comparing and contrasting with second phase precipitation under isothermal (177 °C) annealing conditions. In the early stages of creep deformation, after 5 h, homogeneous precipitation of fine scale β′ was observed within the α-Mg matrix, while on the other hand, only small pockets of Nd enrichment were observed in the isothermally annealed sample after 5 h, indicating that β′ precipitation is accelerated during creep testing. With respect to β1 precipitation, these precipitates appeared to decorate dislocation lines primarily, which is indicative of a dislocation stress-field assisted precipitation mechanism, in both creep tested and isothermally annealed samples. Interestingly, the β1 precipitates appear to grow and coarsen more rapidly in the case of the isothermally annealed sample than in the creep tested sample. The differences between the precipitation behavior of β′ and β1 phases under the influence of stress and temperature (creep testing) vs. only under isothermal annealing conditions have been rationalized based on the crystal structures of these phases and their elastic interaction energies with the stress-fields associated with dislocations as well as bulk applied stress.