Study On The Microstructures And Thermophysical Properties Of Mg-Zn-Y Alloys

The phase compositions, microstructures evolution and thermal expansion coefficients of conventionally solidified and rapidly solidified Mg-Zn-Y alloys with high magnesium, low zinc and yttrium were investigated using optical microscope (OM), X-ray diffraction (XRD) and scanning electronic microscope (SEM) equipped with an energy-dispersive X-ray spectroscopy (EDS), differential thermal analyzer (DTA), differential scanning calorimeter (DSC) and thermal mechanical analysis(TMA). Based on these analyzing results, the effects of reciprocating extrusion (RE) on the microstructures and thermal expansion coefficients of Mg-Zn-Y alloys are studied. The main results are as follows:(1) In the conventionally solidified Mg-Zn-Y alloys, yttrium and zinc tend to segregate at grain boundaries and satisfy the compositional condition that forming ternary compound phase. So the ternary compound phase is formed at grain boundaries even with relatively low content of Y and Zn. When Zn: Y is about 6:1 (at%), a majority of Zn atoms segregate at grain boundaries to form Z phase and small W phase with Y and Mg; when Zn: Y is about 2:1, only W phase precipitated; when Zn: Y is about 1:2, only X phase formed.(2) The principal ternary phases in conventionally solidified Mg-Zn-Y alloys consisted of W-phase (Mg₃Y₂Zn₃), Z-phase (Mg₃YZn₆) and X-phase (Mg₁₂YZn). W-phase form by eutectic reaction at 520℃; Z-phase is the peritectic phase and appeared at 445℃; X-phase formed at 540℃.(3) As the temperatures increase, the thermal expansion coefficient of conventionally solidified alloys increase fast between 30℃and 100℃, whereas from 100℃to 250℃, the increasing rate decreased. When the content of Zn is changes little, the expansion coefficients of alloys decrease with the increase of high melting-point Y element. Within 30²00℃, the expansion coefficients decrease from 28.5×10^-6 /℃to 25.3×10^-6/℃, as the content of Y increase from 1at.% to 2at.%.(4) The microstructures of rapidly solidified Mg-Zn-Y alloys are apparently refined, and are composed of supersaturatedα-Mg grains and Z phase distributed around them. But theformation mechanism of Z phase of rapidly solidified alloys is different from that of conventionally solidified alloys, and Z phase is nucleate from liquid phase.(5) Reciprocating extrusion can break coarse dendrites of conventionally solidified alloys, improve their distribution and refine the microstructures.(6) The changing tendency of thermal expansion coefficient of reciprocally extruding conventionally solidified alloys at different temperatures is invariable. From 30℃to 130℃, the thermal expansion coefficients increased very fast with the increasing of temperatures, and the expansion coefficient is very low at 30℃, being about 8.1×10^-6/℃, whereas from 130℃to 300℃, the thermal expansion coefficients decrease. The expansion coefficients of reciprocally extruding rapidly solidified alloys is 10²3% lower than that of reciprocally extruding conventionally solidified alloys.