Microstructure, Phase Evolution and Precipitation Strengthening of Mg-3.1Nd-0.45Zr-0.25Zn Alloy

The microstructure of the Mg-3.1Nd-0.45Zr-0.25Zn (wt.%) alloy has been investigated after solution treatment at 540°C for 24hr, followed by isothermal aging at 175°C up to 32 days. Various electron microscopy techniques, like EDX mapping and TEM with SAED, have been used to characterize the phase composition and orientation relationships between different phases. After solution treatment, the BCT (Mg_1-xZn_x)₁₂Nd phase present in the as-cast alloy dissolved, and small tetragonal Zn₂Zr₃ rod-like particles precipitated in the oc-Mg grain interiors. Zn₂Zr₃ particles are elongated along [001] direction, have an orientation relationship with the Mg matrix [−2110](0001)_Mg||[001](110)_Zn2Zr3 and appear to be stable in the Mg matrix. Precipitation during isothermal aging involves the formation of metastable phases β″(Mg₃Nd)_HCP (DO₁₉ structure) and β′(Mg₃Nd)_FCC (DO₃ structure). The β″ precipitates formed during the first 8 days of aging have a plate shape and are fully coherent with the Mg matrix, with the orientation relationship [−2110](0001)_Mg || [−2110](0001)_β″ and [−1100]_Mg|| [−1100]_β″. The Zn₂Zr₃ rods serve as additional nucleation sites for precipitation during aging. The heterogeneous nucleation occurs in two ways: precipitates nucleate on the basal planes and on the side planes of the Zn₂Zr₃ rods. After 8 days of aging, these precipitates were identified as β′. During the 16÷32 days of aging, β” precipitates in the grain interior transform into β′ precipitates with an FCC structure. The β′ precipitates are semi-coherent ith the Mg matrix and have the following orientation relationship: [0001](2-1-10)_Mg || [101](11–1)_β, and [−1100]_Mg||[−112]_β′. In the late stage of aging, the β′ precipitates transform into a stable incoherent β (Mg,Zn)₁₂Nd phase. The growth, coarsening and phase transformations were followed by microhardness tests.