Age hardening response and microstructure of Mg–8Sn–4Al alloy with Ag, Sc addition

Abstract

The age hardening response and microstructure characterization of Mg–8Sn–4Al alloys with 0.8 wt% Sc and its combination with 1.0 wt% Ag addition was investigated by Vickers hardness, optical microscope (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The Mg–8Sn–4Al–1Ag-0.8Sc alloy obtains a noticeable age hardening response that the hardness value reaches to 80.7 HV (increased by 24.3%) after ageing treatment at 200 °C for 120 h. Microstructural studies reveal that as-cast Mg–8Sn–4Al alloy mainly composed of ɑ-Mg matrix and Mg2Sn phase distributed along grains boundaries. With Ag and Sc addition, the average grain size decreased to 70.1 μm from 142.7 μm. The addition of Ag and Sc contributes to the grains refinement. Meanwhile, the block-shaped Al3Sc phase with an excellent heat resistance is newly observed in the as-cast, solution treated and peak-aged alloys containing Sc element. After solution treatment, the Mg2Sn phase is dissolved into ɑ-Mg matrix. The Mg2Sn precipitates are observed in the peak-aged alloys. The orientation relationship of Mg2Sn precipitates with ɑ-Mg matrix is mainly detected as (01-1)β∥(0002)ɑ, (−111)β∥(11–20)ɑ in the TEM results of peak-aged alloys. It is obviously found that Ag and Sc addition increases the number density of Mg2Sn precipitates and decreases the size of Mg2Sn precipitates. The strength enhancement of peak-aged Mg–8Sn–4Al–1Ag-0.8Sc alloy is attributed to the grains refinement, second phase strengthening (Al3Sc phase), solution strengthening (Al, Ag and Sc) and a noticeable aging precipitation (Mg2Sn). The corresponding mechanism is discussed in detail.