Heating and Structural Disordering Effects of the Nonlinear Viscous Flow in a Zr55Al 10 Ni5Cu30 Bulk Metallic Glass
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MM8.19.1
Heating and Structural Disordering Effects of the Nonlinear Viscous Flow in a Zr55Al10Ni5Cu30 Bulk Metallic Glass Hidemi Kato1, Akihisa Inoue1 and H. S. Chen2 1 Institute for Materials Research, Tohoku University, Katahira 2-1-1, Sendai 980-8577, Japan 2 (Ret.) Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974, U.S.A. ABSTRACT Heat evolution of stress-induced structural disorder, ∆H s (ε ) , of a Zr55Al10Ni5Cu30 bulk metallic glass (BMG) during constant ram-velocity deformation at the glass transition region (Tg= 680 K) was deduced from in-situ measurements of temperature change of the deforming sample. At the transition from the linear to nonlinear viscoelasticity, the behavior of viscosity change with strain, η (ε ) , is qualitatively consistent with the enthalpy evolution of the structural disordering, ∆H s (ε ) , but not with the temperature change, ∆T (ε ) . It is concluded that the initial softening deformation is due to the stress-induced structural disordering. The change in the nonlinearity, - log η~ ≡ − log η η N , is found to be proportional to the ∆H s and the slope of ∆H s (− log η~ ) can be estimated to ~ 400 J/mol, where η N is the Newtonian viscosity. On the other hand, the temperature raise, ∆T (ε ) , is pronouncedly delayed as compared with the η (ε ) and ∆H s (ε ) at the transition, but is determined by the product of stress and plastic strain-rate, σ ⋅ ε& p , and is nearly proportional to it at the steady-state. The slope of ∆T (σ ⋅ ε& p ) can be estimated to 5.2×10-2 K mol/W. INTRODUCTION Over a broad temperature range in the region of the glass transition, the Newtonian viscosity, η N , of metallic glasses as well as many non-metallic glasses decreases with increasing temperature rapidly in the range 1010 – 105 Pa s and is described by the Vogel-Fulcher-Tammann (VFT) expression [1-4]. The high stability and fluidity of these metallic glasses opens the possibility of forming bulk materials of various shapes, at elevated temperatures with straightforward glass processes, such as casting, forging, extrusion, blowing and consolidation [5-7]. In these processes, the glassy materials are subjected to extremely high strain rates and high stresses, and the viscosity of the glass becomes non-Newtonian. Understanding of the transition between the Newtonian – non-Newtonian viscous flows of a supercooled liquid of BMGs is of both a scientific and technological importance. For this problem, Spaepan introduced a transition model based on two competing processes of stress-driven free volume creation and diffusion controlled free volume annihilation [8]. One of the typical nonlinear viscoelasticity phenomena, the stress-overshoot effect, was observed in some systems of metallic glass [9-11].
MM8.19.2
de Hey et al. observed specific heat change associated with the additional free volume creation during stress-overshoot phenomenon in a Pd40Ni40P20 metallic glass [12]. A master flow curve for a Pd-based BMG in terms of the normalized viscosity, logη η N (= nonlinearity) and the product, η
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