The microstructure,chemical composition and morphology evolution of icosahedral quasicrystalline phase of Mg67.4Zn28.9Y3.7 ternary alloy were investigated in detail at different pouring temperatures by X-ray diffraction(XRD),scanning electron microscopy(SEM) and energy-dispersive spectrum(EDS).Low interfacial energy favors the formation of I-phase.The experimental results show that the primary I-phase reveals petal-shaped with five and six branches,where each branch has facetted growth morphology with the size ranging from 50 to 100 μm.As the temperature decreases,the polygon-shaped I-phase forms,attributed to the decomposition of branch of petal-shaped I-phase,and then it grows bigger and some of the fine polygons join together to form large polygons.Besides these,(α-Mg+I-phase) eutectic structures disappear and the relative amount of Mg7Zn3 phase increases as the pouring temperature decreases.The chemical composition and morphology evolution of I-phase were also discussed.
Man Zhu Gencang Yang Diqing Wan Suling Cheng Yaohe Zhou
In the present investigation, the microstructures and growth morphology of Mg32(Al,Zn)49 Frank-Kasper phase in rapidly solidified Mg32Al17Zn32 ternary alloys were studied in detail. The samples were characterised by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), field-emission scanning electron microscopy (FE-SEM) and energy dispersive spectrum (EDS). The results show that the microstructures mainly consist of Mg32(Al,Zn)49 Frank-Kasper phase and interdendritic Mg-rich Φ-phase. Under rapid solidification condition, Mg32(Al,Zn)49 Frank-Kasper phase reveals a perfect faceted dendritic characteristic in the shape of a three-fold symmetric microstructure with doublet tips in the axes direction. Observations for fracture surfaces show that the growth morphology of Mg32(Al,Zn)49 grains was truncated cubic, and its growth mechanism was also discussed.
ZHU Man YANG Gencang WAN Diqing WANG Zhijun ZHOU Yaohe
The mechanical properties (σb,σ0.2,and δ) and fracture behavior of tensile specimens of the refined A356 alloys were investigated as a function of the addition level of Al-Ti-B master alloy under both as-cast and T6 heat-treated conditions. The results show that as the addition level of Al-5Ti-1B master alloy increases from 0.1 wt.% to 5.0 wt.%,the mechanical properties of refined A356 alloys improve steadily and then decrease slightly under both as-cast and T6 heat-treated conditions. Also,they display excellent mechanical properties with σb = 231.30-258.30 MPa,σ0.2 =134.00-155.50 MPa,and δ = 8.5%-11.75% at T6 heat-treated state. The excellent mechanical properties of refined A356 alloys are ascribed to the formation of α-Al equiaxed dendrites,the improvement of eutectic structure from needle/plate-like to short-lathy/block-shaped,and the Mg2Si aging precipitation phase after T6 heat treatment. The fracture surface examined by SEM exhibits a mixed fracture mode of refined A356 alloys at as-cast state,while it reveals a ductile fracture mode after T6 heat treatment.
