Effect of Intense Rolling and Folding on the Phase Stability of Amorphous Al-Y-Fe Alloys
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INTRODUCTION
DEFORMATION-INDUCED crystallization reactions differ from thermally-induced crystallization processes in key characteristics. For example, thermally-induced crystallization affects samples in their entirety, while deformation-induced crystallization reactions are involved with shear bands.[1–3] Usually, thermally-induced crystallization processes develop in samples that are thermally equilibrated or nearly equilibrated without significant thermal gradients. The highly localized deformation of metallic glasses at near-ambient temperatures and high stress levels, on the other hand, involves regions in the amorphous samples that remain without significant deformation, while shear bands undergo substantial deformation.[4,5] Moreover, it has been suggested that very rapid localized heating occurs during shear-band formation that could add thermal gradients of up to 109 K/m over timescales of tens of ns.[6] During annealing, metallic glasses initiate devitrification reactions at temperatures near or at their crystallization onset temperatures. While some metallic glasses crystallize during intense deformation at bulk sample temperatures of R.J. HEBERT, Assistant Professor, is with the Chemical, Materials, and Biomolecular Engineering Department, University of Connecticut, Storrs, CT 06269, USA. Contact e-mail: [email protected] J.H. PEREPEZKO, Professor, is with the Department of Materials Science and Engineering, University of Wisconsin–Madison, Madison, WI 53706, USA. 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, FL, under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee. Article published online December 21, 2007 1804—VOLUME 39A, AUGUST 2008
100 to 150 K below their thermally-induced crystallization onset temperatures,[7,8] other metallic glasses remain amorphous after deformation to very high strain levels. Indeed, deformation experiments with Pd-based glasses even indicate an increase in the crystallization temperature with cold rolling while showing extensive shear banding.[9] High-pressure torsion-straining experiments with Pd-based bulk metallic glasses demonstrated that, despite strain levels of the order of 200, crystallization did not occur.[10] A comparison between the effects of temperature and strain on the crystallization behavior of metallic glasses certainly requires caution. For thermally-induced crystallization reactions, the annealing temperature and time largely determine the onset of crystallization, but the role of strain and strain rate for deformation-induced crystallization reactions has not yet been established in detail. Deformation- and thermally-induced crystallization reactions have been examined in some detail for Al-based amorphous alloys. Previous results have demonstrated that primary crystallization occurred for amorphous alloys such as Al88Y7Fe5 during both annealing and deformation.[3,11–13] For other amorphous al
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