Viscoplastic Forming of Mg Bulk Metallic Glasses in the Supercooled Liquid Region
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BULK metallic glasses (BMGs) are of particular interest due to their favorable properties at room temperature. They usually exhibit high stresses to failure with large elastic strains and high hardness as well as improved corrosion behavior. Important efforts have been devoted to the study of such amorphous alloys. Among all compositions, Mgbased BMGs are of special interest for providing new light alloys for structural applications. The Mg-based BMGs have been produced for quite some time, in particular based on Mg-Cu-Y compositions.[1] Other amorphous compositions, with an improved glass forming ability (GFA), were later successfully elaborated. For example, the partial substitutions of copper by silver or zinc as well as palladium addition were reported to have a beneficial effect on the critical amorphous diameter.[2–4] More recently, it has been shown that the substitution of yttrium by gadolinium could also improve the GFA of ternary compositions.[5] For such alloys, fracture strengths higher than 800 MPa have been obtained in compression.[6] The same interesting effect of the substitution of the rare earth (RE) was also observed on the quaternary compositions containing Ag which shows improved fractures stress and some limited plasticity.[7] S. PUECH, Ph.D. Student, and J.J. BLANDIN, Research Director, are with SIMAP-GPM2, Institut National Polytechnique de Grenoble (INPG), CNRS/UJF, BP 46, 38402 Saint-Martin d’He`res, France. Contact e-mail: [email protected] J.L. SOUBEYROUX, Research Director, is with Laboratoire de Cristallographie/CRETA, CNRS Grenoble, BP166, 38402 Grenoble Cedex 09, France. This article is based on a presentation given in the symposium entitled ‘‘Bulk Metallic Glasses IV,’’ which occurred February 25– March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee. Article published online January 30, 2008 1874—VOLUME 39A, AUGUST 2008
In their supercooled liquid state, BMGs display promising forming possibilities due to a particularly large plastic stability. The viscoplastic deformation of BMGs with wide supercooled liquid regions (SLRs) has received large attention in the recent past. It has been shown that the homogeneous viscous flow behavior of BMGs deformed in the SLR can be Newtonian (i.e., flow stress linearly dependent on the applied strain rate or viscosity independent on strain rate) or non-Newtonian, depending on testing temperature and strain rate. The Newtonian flow is typically obtained at high temperature and low strain rates. The transition from Newtonian to non-Newtonian flow behavior when the strain rate is increased or when the temperature is reduced, has been attributed to stress-induced formation of defects in the bulk glassy alloy,[8] which limits the increase of the flow stress when strain rate is increased. Nevertheless, contrary to the case of zirconium-based BMGs, high-temperature deformation of magnesiumbased BMGs remains poorly documented.[9–14] These works were carried out mai
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