Nucleation and Growth of Metastable Phases in Mg-Nd, Mg-Gd and Mg-Gd-Nd Based Alloys

Abstract

High supersaturation of Mg matrix by rare earth (RE) elements after quenching of solution treated Mg-Nd, Mg-Gd and Mg-Gd-Nd alloys provides sequential precipitation of different metastable phases. Precipitation of β″, β′ and β₁ phases during aging was observed. Homogeneous nucleation of fully coherent metastable β″ (Mg₃RE, DO₁₉) phase during aging is followed by its diffusional growth, coarsening and transformation to the coherent β′ (Mg₇RE, BCO) phase or directly to the semicoherent β₁ (Mg₃RE, FCC) phase. The β″/ β′ phase transformation involves additional Mg atoms from the adjacent Mg matrix resulting in nearly twice increase of precipitate volume. Heterogeneous nucleation of semicoherent β₁ (Mg₃RE, FCC) phase may occur on the β′-precipitates as in the Mg-Gd based alloys, or internal defects such as Zn₂Zr₃ rods and point defects at grain boundaries as in the Mg-Nd based alloy. The β′/ β₁ phase transformation is accompanied by reduction of precipitate volume for ~ 44.5% due to release of Mg atoms joined to Mg matrix. The β″ and β₁ precipitates grow in $\{ 01\bar 10\}$ habit planes, while β′ — in the $\{ \bar 2110\}$ plains. The combination of HRTEM, TEM with SAED and JEMS simulation was carried out to characterize the nanometer-scale precipitates. Microhardness measurements were performed to reveal the effect of precipitation on the mechanical properties of the alloys.