Growth Behaviour Of Faceted Phase During Directional Solidification Of Al-Y Alloys

In this study,Al-15wt%Y eutectic alloy and Al-53wt%Y peritectic alloy have been chosen as main research objects. The growth behavior of faceted phase during directional solidification has been invesigated.First,Al-15wt%Y alloy has been selected as the primary phase from the melt during directional solidification at different growth rates,revealing the effect of growth rate on the behavior of Al3 Y phase. During directional solidification,at a growth rate of 1μm/s,Al3 Y phase,exhibiting irregular and having a branching structure on the top,precipitates in a faceted growth with sharp edges and corners. As the growth rate increases,the morphologies of Al3 Y phase transit to elongated prism. Al3 Y phase distributes dispersively in the eutectic structure at a higher growth rate,presenting a crossing shape with two prisms crossed vertically. In addition,during laser rapid solidification of Al-15wt%Y alloy,the microstuctures are more refined,which are quite different with those in directional solidification. The mophologies of Al3 Y phase are hexagon particles,petal shaped and equiaxed crystal shaped.Due to the temperature gradient,a mushy zone is created between the complete liquid zone and solid zone. Thermal stabilization experiments on Al-Y peritectic alloy ranging from 0 to 1h have been carried out. With the thermal stabilization time increase,the volume fraction of the liquid phase in the mushy zone decreases,the mushy zone become shorter and more stable,Al2 Y phases distributes more tightly,and Y concentration in the melt before solid-liquid interface decrease.Directional solidification experiments at different growth rates on Al-53wt%Y peritectic alloy have been performed,in order to investigate the growth behavior of Al3 Y phase as a peritectic phase. At a low growth rate(V=1μm/s),the primary phase and the peritectic phase both grow continuously,the microstructure which is parallel to the solid-liquid interface has been found and explained. With the freezing distance increase,the precipitating solid phase from the liquid at the quenching solid-liquid interface transforms from primary Al2 Y phase to peritectic Al3 Y phase. The interface is consist of coarse Al3 Y phase without Al2 Y.During the directional solidification at higher growth rate,the morphology of Al2 Y phase transit from continuous growth to cellular phase,and further to dendrites with the growth rate increase. First,the primary phase is enclosed with the serrate peritectic phase,and at the same time,Al3 Y phase precipitates from the liquid in needle shape. Faster the Al3 Y phase grows,more and smaller it is. With the freezing distance increase,Al3 Y phase become thicker and more. In addition,the Al3 Y phase precipitated from liquid transit from needle shape to short rod and lump shape,which distributes around the peritectic structure. In the end,the microstructures of the directional solidification of two alloys have been compared,and the difference of the growth behavior between two alloys has been analysed.