High Cycle Fatigue Behavior Of Cast Mg-Gd-Zn-Zr Alloy Fabricated By Semi-continuous Casting

At present, many researches are conducted on magnesium alloys dueto the increasing demand of lightweight and fuel saving. Mg-Gd-Zn-Zralloy is a recently developed magnesium alloy which h as high tensilestrength and creep resistance, and thus is very attractive as structuralmaterials in automotive and aerospace industries. As structural materials, itis very necessary to evaluate its fatigue properties. However, to date thereare no available literatures of the fatigue behavior of this alloy.This paper is mainly to study the influence of solution treatment (T4)and solution plus aging treatment (T6) on the microstructure, tensileproperties and fatigue properties of cast Mg96.32Gd2.5Zn1Zr0.18alloyproduced by semi-continuous casting. The following conclusions can bemade.The microstructure of as-cast Mg96.32Gd2.5Zn1Zr0.18alloy prepared bysemi-continuous casting is composed of α-Mg matrix, eutectic compounds(Mg, Zn)3Gd and some fine lamellar LPSO phases at grain interiors. Theaverage grain size of the as-cast alloy is about12.2μm. Solution treatmentleads to the modification of eutectic (Mg, Zn)3Gd compounds and LPSOstructured X phases, the increase of the average grain size and theprecipitation of Zn–Zr phases. The shape of eutectic (Mg, Zn)3Gdcompounds changes from the dendritic to elliptic, and their contentdecreases a little. After solution treatment, the X phases become larger insize. Compared with as-cast alloy, the solution content of α-Mg matrixonly varies a little. Most of the newly precipitated Zn–Zr phases locatenear grain boundaries, which may have resistance to grain growth together with the residual (Mg, Zn)3Gd compounds. The modification effect ofsolution treatment on tensile and fatigue properties are not obvious. Thetensile properties of as-cast and solution treated alloys are comparable,while the T4-treated alloy has a little bit higher fatigue strength (+7MPa).Subsequent aging does not leads to the modification of the eutectic(Mg, Zn)3Gd compounds, LPSO structured X phases, the average grainsize and the Zn–Zr phases. The meta-stable β’ phases with orthorhombicstructure and β1phases with fcc structure are precipitated when aged at200°C for128h. The precipitations greatly improve both the yield strengthby90MPa and the fatigue strength by18MPa.Basal slip lines are observed on the surface of as-cast and T4-treatedfatigued samples, and are believed to be the main deformation mechanismduring the high cycle fatigue, although they are not investigated on thesurface of T6-treated alloy.There exists life gaps in as-cast and T4-treated alloys, where there isno failure of fatigue samples between105and107cycles, while it does notexists in T6-treated alloy. The plastic zone sizes of as-cast and T4-treatedalloys are larger than their average grain size, and lead to a fasterpropagation rate of fatigue cracks, which is probably the reason to generatethese life gap phenomenon.