Homogeneous and heterogeneous precipitation mechanisms in a binary Mg–Nd alloy

Abstract

Parallel modes of precipitation mechanisms in an Mg–Nd alloy were revealed by examining an isothermally annealed high pressure die cast alloy at 177 °C for up to 100 h. Broadly, precipitate evolution was observed to occur concurrently on dislocations and within the surrounding α-Mg matrix. However, it was observed that the presence of dislocation accelerated the precipitate formation kinetics significantly. In contrast to the accepted precipitation pathway in the Mg–Nd system, i.e., SSSS → GP zones → β″ → β′ → β₁ → β → β_e, dislocations were found to preferentially facilitate the formation of β′ and β₁ precipitates even at the very early stages (5 h) of annealing. Within the same time frame, a homogeneous distribution of Nd-rich pockets was observed throughout the α-Mg matrix, along with the β′ and β₁ precipitates decorating dislocation lines. Results further indicate that these Nd-rich regions initiated precipitation within the parent α-Mg matrix. The formation of these Nd-rich pockets was explained on the basis of a miscibility gap in the α-Mg phase at 177 °C. Our results demonstrate that the presence of dislocations influence strongly the phase-transformation pathways in Mg-rare earth alloys by facilitating the formation of selective precipitate phases.