微重力条件下Cu-Zr共晶合金的液固相变研究

Liquid-solid phase transition of Cu-Zr eutectic alloy under microgravity condition

采用落管方法实现了液态Cu-10 wt.%Zr亚共晶、Cu-12.27 wt.%Zr共晶和Cu-15 wt.%Zr过共晶合金在微重力无容器条件下的快速共晶与枝晶生长.Cu-12.27 wt.%Zr共晶合金的凝固组织随液滴直径减小由层片规则共晶向不规则共晶转变,且层片间距减小;Cu-10 wt.%Zr亚共晶合金的初生(Cu)相随液滴直径减小由粗大树枝晶向棒状晶转变,且所占体积分数增加,部分区域形成花状凝固组织,(Cu)相枝晶辐射向外生长;Cu-15 wt.%Zr过共晶合金初生相则为金属间化合物Cu_9Zr_2相,呈条状生长,随液滴直径减小冷却速率增大,凝固组织由宏观弯曲生长向球状晶胞转变.理论计算表明,三个合金液固相变枝晶与共晶的生长均由溶质扩散控制.测定Cu-10 wt.%Zr亚共晶合金初生(Cu)相显微硬度随液滴直径减小而增大,三个合金的共晶相随合金初始成分增大而增大.

Eutectic phase transition involves the competitive nucleation and coupled growth of two solid phases within one liquid phase.Phase selection especially under unequilibrium condition,may result in novel microstructures and thus affects the performances of eutectic alloys.Liquid Cu-10 wt.% Zr hypoeutectic,Cu-12.27 wt.% Zr eutectic and Cu-15 wt.%Zr hypereutectic alloys are rapidly solidified in the containerless process in a 3 m drop tube.During the experiments,the Cu-Zr alloys are heated by induction heating in an ultrahigh vacuum chamber and further overheated to 200 K above their liquidus temperatures for a few seconds.Then the liquid alloys are ejected out from the small orifice and dispersed into tiny droplets after adding the argon gas flow.The solidified samples are analyzed by Phenom Pro scanning electron microscope and HXD-2000 TMC/LCD microhardness instrument.The competitive nucleation and growth among（Cu） dendrite,Cu9Zr2 dendrite and（Cu＋Cu9Zr2） eutectic phase become more and more intensive as droplet diameter decreases.The layer spacing in Cu-12.27 wt.% Zr eutectic alloy decreases when the undercooling increases.And the microstructural transition takes place from lamellar eutectic to anomalous eutectic.The microstructure of Cu-10 wt.% Zr hypoeutectic alloy is characterized by（Cu） dendrite and lamellar eutectic.Whereas the microstructure in Cu-15 wt.% Zr hypereutectic alloy consists of Cu9Zr2 dendrite and lamellar eutectic.For the Cu-10 wt.% Zr hypoeutectic alloy,with the decrease of droplet size,the primary（Cu） phase transforms from coarse dendrites into equiaxed grains,and the volume fraction of（Cu） dendrite becomes larger and larger.As for Cu-15 wt.% Zr hypereutectic alloy,the primary Cu9Zr2 intermetallic compound grows in a band manner,and with the decrease of droplet size and increase of cooling rate,the solidified microstructure transforms from band Cu9Zr2 dendrite plus lamellar eutectic into spherical cell structure.The three alloys reach maximal undercooling at 177 K,156 K and 204 K,respectively.The Trivedi-MagninKurz and Lipton-Kurz-Trivedi/Boetinger-Coriell-Trivedi models are used to analyze the dendritic and eutectic growth as a function of undercooling.Theoretical analysis indicates that both dendritic growth and eutectic growth are controlled by solute diffusion during liquid-solid phase transition.To further investigate the effects of cooling rate and undercooling on the mechanical properties of Cu-Zr eutectic alloys,the microhardness of each of different phases is determined.The microhardness of the primary（Cu） phase within Cu-10 wt.% Zr hypoeutectic alloy is strengthened with the increase of cooling rate.The microhardness of eutectic within the three alloys also increases with increasing the cooling rate and the initial alloy composition of the alloy.