Materials Science and Engineering A 387–389 (2004) 1005–1011 Homogeneous deformation of bulk metallic glasses in the super-cooled liquid state M. Bletry a,b,c,∗ , P. Guyot a , Y. Brechet a , J.J. Blandin b , J.L. Soubeyroux c a LTPCM ENSEEG, BP 75, 38402 Saint-Martin d’Hères Cedex, France b GPM2 ENSPG, BP 46, 38402 Saint-Martin d’Hères Cedex, France c Laboratoire de Cristallographie/CRETA CNRS, 25 avenue des martyrs BP 166, 38042 Grenoble Cedex 9, France Received 25 August 2003; received in revised form 30 January 2004 Abstract Different Zr–Ti–Al–Ni–Cu bulk metallic glasses have been processed. They have been characterized by neutron and X-ray diffraction in the glassy state and after DSC heating up to crystallization. The homogeneous visco-plasticity of the glasses has been studied in the under-cooled liquid state by compression tests at constant strain-rate and by differential mechanical tests with strain-rate jumps at different temperatures. The flow stress appears to be strongly thermally activated. Another characteristic feature of the deformation of the fully amorphous state is that the flow stress is independent of the strain-rate path followed to reach a given level of strain. The stress–strain curves and their dependence on temperature and strain-rate are then discussed in terms of Newtonian and non-Newtonian viscosity. It is shown that a reduced viscosity can be scaled on a master curve, independently of the strain-rate and temperature. A second analysis of the results is made, in terms of free volume. The temperature and stress variation of the plastic flow are compared with Spaepen model. A constitutive equation of the deformation is proposed. Finally, the effect of crystallization on mechanical behaviour of metallic glasses is treated trough a model involving backstress due to a composite behaviour. © 2004 Elsevier B.V. All rights reserved. Keywords: Bulk metallic glasses; Compression; Viscosity; Master curve; Free volume model; Constitutive equations; Crystallization 1. Introduction For some years now, it has been possible to produce metal- lic glasses with critical cooling rates less than 10 K/s, which allows bulk amorphous products to be obtained (typically with the smallest dimension higher than a few millimeters). This was made possible thanks to the discovery of composi- tions optimized to retard the nucleation of crystalline phases during cooling. In particular, zirconium based bulk metallic glasses (BMGs) have been produced, with particularly in- teresting mechanical properties, for instance yield strength up to 2 GPa combined with good fracture toughness (about 50 MPa m 1/2 ) [1]. It was shown that their formability between the glass tran- sition temperature T g and the crystallization temperature T X ∗ Corresponding author. Tel.: +33 4 76 82 66 69; fax: +33 4 76 82 66 44. E-mail address: mbletry@ltpcm.inpg.fr (M. Bletry). was outstanding. As a result, high temperature deformation of BMG is the topic of a growing number of studies [2–4]. The aim of the present study is to present and to discuss the results concerning the high temperature deformation of two such BMG between T g and T X . Results are discussed in terms of viscosity and stress–strain relation within the frame of the free volume model. The effect of crystalliza- tion on the mechanical response and the effect of strain on crystallization kinetics are also discussed. 2. Sample preparation and characterization BMG of composition Zr 52.5 Al 10 Cu 27 Ti 2.5 Ni 8 (hereafter called D ′′ ) and Zr 52.5 Al 10 Cu 22 Ti 2.5 Ni 13 (hereafter called D ′ ) were prepared. The pure metals were melted in a cold cru- cible in an argon atmosphere and injected under pressure in a water-cooled copper mould. The degree of amorphicity was characterized by X-ray (copper wavelength, using theta 0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.02.085