Research On The Precipitates Crystallography In Several Mg Alloys

Large-scale development and application of magnesium alloys have been greatly stimulated by the demand for lightweight and environment-friendly materials in the automotive industry in recent years. However, conventional magnesium alloys cannot meet the demand of high strength both at room and elevated temperatures. Therefore, a lot of attention has been paid on how to improve the elevated temperature properties. There is a large number of magnesium resources in China, and it is necessary for us to design new magnesium alloys with high performance both at room and elevated temperatures. After literature survey and systematic analysis on microstructures of Mg alloys, we have chosen Mg-Sn based alloys with additions of Mn and Si to investigate the heat-treatment process, microstructures and crystallography. Optical microscopes, SEM and TEM were used to examine the newly developed alloy. After solid solution and age-hardening for a period of time, a large number of Mg₂Sn precipitated in the matrix and they have more than eight types of orientation relationships (OR) with the matrix, some of them are irrational. We have analyzed these ORs thoroughly. Although a lot of plate and rod shaped Mg₂Sn precipitates lie in the basal plane of (0 0 0 1)α, the additions of Mn and Si have stimulated the precipitate of polygonal Mg₂Sn precipitates. This kind of polygonal precipitates is covered by about 4 pairs of facets. Most of the facets are not parallel with any low-indexed plane, however, they are found to be parallel with certainΔg's. The orientation and matching status of these facets are analyzed by usingΔg's and morie fringe theory, and the calculated result is consistent with the measured results. The relative planes match exactly at the facets. This will provide theoretical support in designing new alloys.There are also several Mg₂Si and Mn₅Si₃ particles in the present Mg-Sn-Mn-Si alloy. To our knowledge, no publilcation has reported the presence of Mn₅Si₃ precipitates in any Mg alloy before. The Mn₅Si₃ precipitates grow into plates with about 50 nm in thickness and 200 nm in width. In Mn₅Si₃/Mg system, the OR is not fully constrained in 3-dimensional space, but the following OR is often observed: g(1 –1 0 0)Mn5Si3//Δg1//Δg2, whereΔg1 = g(2–1–1 0)Mn5Si3– g(0 0 0 2)Mg,Δg2 = g(3–2–1 0)Mn5Si3– g(0 0 0 2)Mg. When satisfyingΔg parallelism rule, the rotation angle between g(2–1–1 0)Mn5Si3 and g(0 0 0 2)Mg is calculated to be 7.8°, which is consistent with experimental results. The broad facet of the precipitate plate is normal to the parallelΔg's, agreeing with theΔg parallelism rule.By combiningΔg parallelism rules and O lattice theory, small deviations of about 0.5°(in Mg-Al alloy) and 0.29°(in Mg-Y-Nd alloy) from exact Burgers OR were calculated and interpreted. The lattice matching status of the habit planes of the precipitates, at the Burgers OR and at the calculated OR, is compared. The calculated results are consistent with experimental work.The microstructures of Mg-Zn-Y alloys and effects of heat-treatment in hardness are analyzed. It is found that the as-cast samples have better hardness since there are a large number of eutectic quasi-crystals in grain boundaries. A kind of Zn₂Zr phase is detected, and an intermetallic compound with Mg and rich Y is also detected in Mg-Zn-Y alloy.