Study On Crystal Evolution Mechanism Of Al/ZN And Al/Cu Bimetallic Gradient Alloys

Al/Zn and Al/Cu gradient alloys,which combine a lot of important properties of themselves,that concentration gradient is an important factor affecting their bonding interface.A large number of crystals will be formed during the solidification diffusion process,which was constrained by concentration gradient.The growth and evolution of crystals have great significance for the mechanical properties of materials.In this paper,liquid-liquid composite method was used to prepare Al/Zn and Al/Cu gradient alloys with different concentrations at different cooling rates.The crystal structure on the longitudinal section of its sample was observed with optical microscopy and scanning electron microscopy.The results showed that the morphology of a-Al dendrite in Al/Cu gradient alloys revealed the characteristics of directional solidification,but the morphology of a-Al dendrite in Al/Zn gradient alloys changed continuously with Zn concentration.The a-Al dendrite crystal orientation transition angle Al/Zn alloys with different concentration gradients was calculated,results show that the change of concentration gradient affects the starting and ending position of the crystal orientation transition angle change.As the concentration gradient is larger,the starting and ending position of the change of orientation transition angle of a-Al dendrite crystal is higher,and the distance range of the change is smaller.The angle of the primary and secondary dendrite arms of a-Al dendrites was measured,and it was found that there were 60°,900 and irregular angle.The cooling rate is different,and the angle of the primary and secondary dendrite arms has irregular starting and ending position,and the distance range is different.Different concentration gradients and cooling rates change the crystal growth morphology of a-Al crystals.At the whole longitudinal section,the angle of the primary and secondary dendrite arms decreases with the decreasing of Zn concentration.Within the range of experimental parameters,the concentration gradient formed by diffusion can control the directional growth of a-Al dendrites.The low cooling rate and high Cu concentration are beneficial to the stability of the S/L interface.The relationship between solidification parameters(concentration C,concentration gradient Gc,cooling rate RT)and microstructure parameters(primary dendrite arm spacing ?1,secondary dendrite arm spacing ?2was established based on theoretical calculations and experimental results.Primary dendrite arm spacing decreases with the decrease of Cu concentration,decreases with the increase of absolute value of concentration gradient,and decreases with the increase of cooling rate.The mathematical relationships between primary dendrite arm spacing(?1)and concentration(C),concentration gradient(Gc),cooling rate(RT)are:(?).Secondary dendrite arm spacing decreases with the increasing of Cu concentration,decreases with the increasing of absolute value of concentration gradient,increases with the increasing of local solidification time,and decreases with the increasing of cooling rate.The mathematical relations between secondary dendrite arm spacing(?2)and cooling rate(RT)and local solidification time(tf)are as follows:?2=3.196 RT-0.022tf0.5.