Investigation On As-cast Microstructure Of Mg-Al-Zn Based Magnesium Alloys

Mg-Al-Zn based alloys are the most widely magnesium alloy series applicated currently, but at present what block it from further utilization problems are as follows: â‘ Casting magnesium alloys (for example AZ91D) have poor creep resistance at temperature above 120℃, which has made them inadequate for the parts which require excellent creep resistance at high temperature; â‘¡Wrought magnesium alloys (for example AZ31) have poor plastic deformation property, so they can not meet different needs in different situations. In order to solve the existing problems of Mg-Al-Zn based magnesium alloys, lots of research about alloying, grain refinement, melt treatment, heat-treatment and so on, had been carried out for improving the properties. Because the properties of alloys are determined by their microstructure, it is of great importance to carry out the investigation on the microstructure of Mg-Al-Zn based magnesium alloys, especially mechanism of forming and transforming of alloy phases in Mg-Al-Zn based magnesium alloys. The morphology, category, amount and distribution of alloy phases in the Mg-(2⁸%)Al-(0.5¹2%)Zn magnesium alloys and the effects of trace strontium and yttrium on the as-cast microstructure of the Mg-3Al-1Zn alloy were investigated by metallurgical phase analysis, scanning electron microscopy (SEM) analysis, XRD analysis, energy diffraction (EDX) analysis and DSC in this paper. The research results that can be used to guide the alloy phases controlling and the optimization of composition and processing parameters, are expected to obtain. The research results indicate that â‘ The microstructure of the casting Mg-(2⁸%)Al-(0.5¹2%)Zn magnesium alloys consists of the matrix(α-phase) and the second phases whose morphology are bone shape-like, lump shape, fishbone shape-like, particle shape and layer shape-like, which mainly distribute at grain boundaries and only a few are inside the grains; â‘¡The change of Al and Zn content has very obvious effect on the amount and distribution of alloy phases. With the increasing of Al or Zn content, the amount of primary α-phase is decreased, the amount of compounds is increased and their distribution become more continuous; â‘¢The ratio of Zn to Al also has important effect on category of the second phase. With the increasing of the ratio of Zn to Al, the category of second phase has the trend by Mg₁₇Al₁₂ →Mg₁₇Al₁₂+Mg₃₂(Al,Zn)₄₉ →Mg₃₂(Al,Zn)₄₉+Mg₅Al₂Zn₂ →Mg₃₂(Al,Zn)₄₉+MgZn. The second phase is Mg₁₇Al₁₂ under the condition of Zn/Al<1/2 , the second phase is Mg₁₇Al₁₂+Mg₃₂(Al,Zn)₄₉ under the condition of 1/2≤Zn/Al<5/6, the second phase is Mg32(Al,Zn)49+Mg5Al2Zn2 under the condition of 5/6≤Zn/Al<1, and the second phase is Mg32(Al,Zn)49+MgZn under the condition of Zn/Al≥1; â‘£The change of Al and Zn content has also effect on the solid us temperature, the liquidus temperature and phase transformations during solidification processing of alloys. The solidus temperature and the liquidus temperature of the Mg-3Al-1Zn alloy are 442.7 ℃and 648.0℃respectively, the phase transformations existing during the solidification processing of the alloy are: ( )L1 →αMg + L2, ( )L2 →αMg + γ+ L3 and L3 →γ+ Ï„; The solidus temperature and the liquidus temperature of the Mg-8Al-4Zn alloy are 401.5℃and 603.4℃respectively, the phase transformations existing during the solidification processing of the alloy are: ( )L1 →αMg + L2, ( )L2 →αMg + γ+ L3, ( )L3 →αMg + Ï„+ L4 and L4 + γ+ τ→φ; The solidus temperature and the liquidus temperature of Mg-6Al-5Zn alloy is 343.3℃and 595.6℃respectively, the phase transformations existing during the solidification processing of the alloy are: ( )L1 →αMg + L2, ( )L2 →αMg + φ+ L3 and ( )L3 + φ→αMg +Ï„; The solidus temperature and the liquidus temperature of Mg-3Al-5Zn alloy is 342.1℃and 611.2℃respectively, the phase transformations existing during the solidification processing of the alloy are ( )L1 →αMg + L2 and ( )L2 →αMg+ ε+ Ï„; The solidus temperature and the liquidus temperature of Mg-2Al-10Zn alloy is 332.2℃and 609.4℃respectively, the phase transformations existing during the solidification processing of the alloy are ( )L1 →αMg + L2 and ( )L2 →αMg+ ε+ Ï„. In addition, the research results also indicate that 0.01% or 0.05% strontium addition to the Mg-3Al-1Zn alloy do not result in obvious microstructure change. But when the amount of strontium is increased to 0.10%, the morphology change of second phase from fishbone shape-like to particle shape was found, and the Al4Sr phase was also found in the Mg-3Al-1Zn-0.10Sr alloy. Moreover, a small amount of yttrium addition to the Mg-3Al-1Zn alloy results in obvious microstructure change, it was found that yttrium addition not only makes the dendrite develop, but also results in change of dimension of eutectic groups which are distributed at dendrite boundary, and the Al3Y phase was also found in the Mg-3Al-1Zn-0.90Y alloy.