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Shaping of Bulk Metallic Glasses by Simultaneous Application of Electrical Current and Low Stress A.R. Yavari1, M. F. de Oliveira2 and W.J. Botta F.2 1 LTPCM, Institut National Polytechnique de Grenoble, LTPCM CNRS umr 5614, BP 75, St-Martin d’Hères Campus, 38402 France, [email protected] 2 Department of Materials Engineering, Federal University of Sao Carlos, SP, Brasil

ABSTRACT Using the intrinsic materials properties of bulk metallic glasses (BMG), namely electrical resistivities two orders of magnitude higher than good conductors and a Newtonian viscous-flow regime of deformability, a new electromechanical process has been developed for shaping, joining and engraving of BMGs. The wider the liquid supercooled region between the glass transition temperature Tg and the crystallisation temperature Tx of the bulk metallic glass, the easier the application of the new process. In this range, the undercooled liquid deforms in a quasi-Newtonian way, allowing thermomechanical shaping in the low viscosity range as for oxide glasses. The new electromechanical processing technology has been used for economical and rapid shaping at low applied stresses by eliminating the thermal mass of the furnace and the need to heat the deformation dies. The process parameters are adaptable for the full maintenance of the glassy state or when desired, for appropriate compositions, for nanocrystallisation during the joining or shaping operation.

INTRODUCTION The first bulk metallic glasses were Pd-based alloys developed by the team of David Turnbull at Harvard in the early 1980s (see for example [1]). With thicknesses of about 3 mm, PdNiP-type glass demonstrated that the previous thickness limits were surmountable but did not attract much attention due to the high cost of palladium. The real breakthrough came from 1989 onwards when A. Inoue discovered multicomponent liquid alloys with very deep eutectics capable of freezing to a glassy state several mm to several cm thick by conventional cooling such as in copper die casting (for a review, see [2]). Beginning with La- and Mg-based quaternary alloys, Inoue extended BMG formation to Zr, Fe, Ni and other alloy families. W.L. Johnson et al. developed ZrTi-based BMGs with up to 25% beryllium [3-4] and other thick glassy alloys. Bulk metallic glasses have been proposed for a range of applications that include dies, ornaments, solders (deep eutectic compositions), electrodes, medical and dental implants and tools, bullet-proof jackets, tank armour perforators, coatings and more. Fe-based BMGs have saturation magnetisations up to 1.3 T (which is in the application range) together with very high permeabilities (see for example [5]) and can be introduced in ring-shaped form into small motors. Due to their 2% elastic strain range (as compared to 0.2% for crystalline materials), BMGs currently have the best known values for the Ashby performance index σ2/E (where σ and E are the yield strength and Young’s modulus). High values of this performance index (reversible storing of elastic energy) open