Superplastic Formation And Diffusion Behavior Of A Zr-based Bulk Metallic Glass In The Super Cooled Liquid Region

Bulk metallic glasses (BMGs) have many potential applications due to their unique properties, such as superior strength and hardness, excellent corrosion resistance and improved wear resistance. In their supercooled liquid regions (SLR), BMGs usually exhibit superplastic behavior. This feature allows BMGs to be shaped in the SLR. Furthermore, the superplastic forming is often combined with other processing techniques in manufactures. For example, by combining superplastic forming (SPF) with diffusion bonding (DB), a novel manufacturing technology (SPF/DB) can be developed, which can realize the diffusion bonding of BMGs during forming.In this work, Zr₆₅Al_7.5Ni₁₀Cu_17.5 amorphous alloy was prepared by the water- cooled copper mould casting method. The compressive deformation behavior of the alloy under different temperatures and strain rates was investigated, and the deformation mechanisms in the SLR was discussed based on the free volume model. In the mean time, the diffusion bonding behavior between Zr₆₅Al_7.5Ni₁₀Cu_17.5 and Cu was investigated. A diffusion bonding model to describe the diffusion bonding process of BMGs was suggested.The results indicate that both temperature and strain rate have a signi?cant influence on the deformation behavior of the BMG. The alloy exhibits high yield stress but low ductility (<2%) at the low temperature regime (i.e. below 623K) which below its glass transition temperature (Tg). However, when the alloy deforms in the SLR, it shows a good ductility. The deformation is in Newtonian flow mode (m≈1) at low strain rates (<1×10^-2 s^-1) but in non-Newtonian mode at high strain rates. The alloy can exhibit superplastic deformation behavior at the conditions of 673K and 2×10^-4 s^-1. However, The low strain rate may result in the crystallization of the alloy during the deformation in the SLR. The transition state theory based on the free volume model proposed by Spaepen can be used to describe the deformation behavior of the BMG in the SLR.The formation of the bond between Zr₆₅Al_7.5Ni₁₀Cu_17.5 and Cu is strongly dependent on the bonding temperature and time. Prolonging the bonding time and/or raising the bonding temperature can reduce the voids at the bonding interface of Zr₆₅Al_7.5Ni₁₀Cu_17.5/Cu, and increase the width of the bond. The complete metallurgical bonding of Zr₆₅Al_7.5Ni₁₀Cu_17.5 and Cu can be realized by diffusion bonding at the temperature of 400℃with bonding time of 3.6 ks in 150 MPa. The diffusion coefficients and active energies of Zr and Cu atoms in the bond are determined.Based on the plastic deformation and atom diffusion features, a diffusion bonding models to described the diffusion bonding processes of BMGs are suggested in the present work. In this model, the diffusion bonding consists of two stages: one is the plastic deformation stage and the other is the void shrinkage stage, in which, the atom diffusion and superplastic deformation are responsible for the void shrinkage. Applying this model to the diffusion bonding of a Zr₆₅Al_7.5Ni₁₀Cu_17.5 metallic glass, the predicted bonding time for the high quality diffusion bonding is 1.53Ks under the conditions of 400℃/150MPa.