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On the High Temperature Creep and Relaxation Behaviour of Zr-based Bulk Metallic Glasses B. S. S. Daniel, M. Heilmaier, A. Reger-Leonhard, J. Eckert and L. Schultz IFW Dresden, Institut für Metallische Werkstoffe, Postfach 270016, D-01171 Dresden, Germany ABSTRACT Creep tests under constant load as well as constant true strain rate were carried out at near the glass transition temperatures (Tg) to study the time dependent flow behaviour of a Zr-based bulk metallic glass (BMG). The strain rate - stress relation over a wide strain rate-range (10-7 to 10-2 s-1) was established for different temperatures. The high temperature deformation behaviour is explained on the basis of stress induced creation of free volume versus diffusion controlled annihilation processes. It was found that the creep kinetics near Tg is controlled by the mobility of atoms with an activation energy value Q = 410 kJ/mol . INTRODUCTION Zr-based BMGs have excellent room temperature properties including high strength (1800 MPa), low Young’s modulus (70 GPa) and high fracture toughness (67 MPa⋅m1/2) [1] but very little plastic strain before failure. Current research on hydrogen charging of BMGs have shown encouraging results such as increased hardness, enhanced toughness and fatigue crack growth retardation [2,3] with a slight decrease in thermal stability against crystallisation. For example, by varying the hydrogen charging ratio one can preferentially nucleate different phases in the BMG [4] to achieve desired property requirements pertaining to specific applications [5]. At room temperature, inhomogeneous flow behaviour is observed such that the deformation is confined to a narrow slip band. However, at temperatures close to the glass transition (Tg), homogeneous flow occurs such that plastic strain is manifest in the bulk. The extended homogeneous deformation may allow for easy formability and near net shape fabrication of structural components. Hence, our aim in the present work is to describe the high temperature deformation behaviour of metallic glass with free volume changes as the underlying basis. EXPERIMENTAL DETAILS The processing details and characterisation of bulk amorphous Zr55Cu30Al10Ni5 alloy was reported earlier in ref. [6]. Cylindrical samples of 3 mm diameter and 6 mm length were used for high temperature compression tests at 20K below the onset ( Tgon − 20 = 665 K ), the onset ( Tgon = 685 K ) and the inflection point ( Tgip = 705 K ) of the glass transition. These temperatures were graphically estimated from a differential scanning calorimetry (DSC) scan of an amorphous sample at 20 K/min (see ref. [7]). For conformity, the same initial heating rate was employed while carrying out the mechanical tests. A specially designed high temperature furnace with a laser extensometer could provide contact free continuous monitoring of the creep strain in Argon atmosphere. Constant strain rate tests (CCRT), creep tests (CREEP), isothermal viscosity

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measurements (TMA) and stress relaxation (SRT) experiments were carried out to s