Transmission Electron Microscopy Investigations Of The Precipitated Phases In As-cast And Rapidly Solidified Mg-Sn And Mg-Gd-Zn Alloys

Among age-hardening magnesium-based alloys, the magnesium-tin (Mg-Sn) based alloys and magnesium-gadolinium-zinc (Mg-Gd-Zn) based alloys have been attracting increasing attentions of researchers due to their excellent castability, mechanical strength and creep resistance at elevated temperatures. Now, reseachers especially discuss and study the precipitates’crystallography and the affection mechanism of mechanical properties during aging treatment.In this thesis, the precipitation process, the precipitates’crystallography and the age-hardening mechanism for heat-treated as-cast Mg-9.76wt.%Sn alloy and as-cast Mg-9.05wt.%Gd-2.85wt.%Zn alloy have been investigated using X-ray diffraction (XRD) technology, scanning electron microscopy (SEM) and advanced transmission electron microscopy (TEM) including high-resolution TEM (HRTEM) and Cs-corrected atomic resolution high-angle annular dark-field scanning TEM (HAADF-STEM). The referential Mg-9.76wt.%Sn alloys prepared by rapidly solidification technology have been studied and compared with those as-cast alloys at the same time.The as-cast Mg-9.76wt.%Sn alloys, which were dissolved at 823K for 12 hours and quenched in water, have been heat-treated at 573K and 453K respectively using traditional aging method. The aging hardness curves for the two aging processes had been measured. The experiments revealed that, the peak harness is 63.5Hv for the as-cast Mg-9.76wt.%Sn alloy which was heat-treated at 573K for 5h; and the peak harness is 67.0Hv for the as-cast Mg-9.76wt.%Sn alloy which was heat-treated at 453K for 425h.The orientation relationships between the precipitated β-Mg2Sn particles and the matrix a-Mg had been analyzed and counted statistically in peak hardness samples for as-cast Mg-9.76wt.%Sn alloy aging at 573K and 453K using the selected-area electron diffraction method in TEM. The results aging at 573K showed that, the proportion of the precipitated β-Mg2Sn particles which had the orientation relationship of OR-3 ((110)β//(0001)α, [111]β//(1210)α) was 75.1%; the proportion of the precipitated β-Mg2Sn particles which had the orientation relationship of OR-4 ((110)β/(0001)α, [001]β//(2110)α) was 24.3%; the proportion of the precipitated β-Mg2Sn particles which had the orientation relationship of OR-1 ((111),//(0001)α, [110]β//(2110)α) was 0.6%. The results aging at 453K showed that, the proportion of the precipitated β-Mg2Sn particles which had the orientation relationship of OR-4 was 77.4%; the proportion of the precipitated P-Mg2Sn particles which had the orientation relationship of OR-3 was 22.6%.According to the crystallographic theory in solid-state phase transformation, the crystallographic factor which determines the predominance of OR-3 in precipitated P-Mg2Sn particles aging in 573K was explained by a three-dimensional invariant line model constructed using a transformation matrix from a-Mg (HCP) to β-Mg2Sn (FCC) in reciprocal space.The interface structure models between the precipitated β-Mg2Sn particles with OR-3 and the matrix a-Mg, the interface structure between the precipitated P-Mg2Sn particles with OR-4 and the matrix a-Mg, had been constructed using HRTEM and HAADF-STEM images, and had a good greement with the experimentals.The microstructure investigation and mechanical strength (micro-hardness) measurement were carried out on the rapidly solidified Mg-9.76wt.%Sn ribbons. The result showed that, the average micro-hardness was 69.4 Hv, and the grain size of a-Mg was 3.824 μm and decreased sharply compared with as-cast samples. According to the Hall-Petch relationship σs=σ0+kd-1/2 and owing to the large value of k of the magnesium alloys, the strength of the magnesium alloys would be greatly improved by the deep drop in the grain size d. A non-equilibrium phase β"-Mg3Sn with the structure of DO19 had been found in this rapidly solidified Mg-9.76wt.%Sn ribbons. It has a close-packed hexagonal structure (HCP) with αβ"=2αα, cβ= cα and the orientation relationship between the DO19 phase and the matrix a-Mg is (0110)β"//(0110)α, (0001)β"//(0001)α, (2110)β"//(2110)α.HRTEM and HAADF-STEM method were carried out to observe and characterize these 14H long period stacking ordered (LPSO) phase in as-cast Mg-9.05wt.%Gd-2.85wt.%Zn that was heat-treated at 773K for 16h, and the atomic structure model was improved and had a good greement with the experimental images by summarizing the results of previous studies.