Mechanisms of nucleation and growth of metastable phases in Mg-Gd and Mg-Gd-Nd alloys

The α-Mg (S.S.S.S.) in Mg-Gd-Nd and Mg-Gd alloys decomposes during aging into the following four successive phases: β'' (Mg₃RE, DO₁₉)→ β' (Mg₇RE, BCO) → β₁ (Mg₃RE, FCC) → β (Mg₅RE, FCC). β'' phase nucleates homogeneously from the supersaturated Mg matrix, while β' nucleates heterogeneously on the β'' phase and involves additional Mg atoms from the adjacent Mg matrix. β'' and β' are coherent with the α-Mg matrix, β₁ is semi-coherent with the matrix. The nucleaton of β₁ was associated with β' precipitates, resulting in formation of K-like and V-like particle associations. Transformation β' → β₁ occurs with decrease of precipitated volume providing decrease of microhardness. The platelets β' and β₁ have (0-110)_α-Mg habit planes; β₁ grows along three close packed directions of the matrix <1-2-10>_α-Mg, while β' grows along <0001>_α-Mg. Hardening occured with three-peaks in the Mg-Gd-Nd alloy and two-peaks in Mg-Gd alloy. Additional first peak was connected with faster diffusion of Nd and β'' (Mg₃Nd) formation. (author)