Plastic Deformation-Induced Nanocrystalline Aluminum in Al-Based Amorphous Alloys
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ABSTRACT We report the first direct observation of crystallization induced in the slipped planes of aluminum based amorphous alloys by bending the amorphous ribbons. Nanometer-sized crystalline precipitates are found exclusively within a thin layer (shear band) in the slipped planes extending across the deformed amorphous alloy ribbons. It is also found that the nanocrystalline aluminum can be produced by ball-milling. It is likely that local atomic rearrangements within the shear bands create the nanocrystals which appear after plastic deformation. INTRODUCTION Amorphous alloys or metallic glasses produced by rapid solidification are thermodynamically unstable. There is always a thermodynamic tendency for the amorphous state to transform to the 2 crystalline state. The crystallization of amorphous alloys has been studied for many years1' . Crystallization was usually found to occur homogeneously in the bulk of amorphous alloys, and the initiation of crystallization is usually due to thermal treatment or a change of cooling rate in melt-spinning. There are several reports of surface crystallization induced by various techniques, both chemical and mechanical, e.g., polishing, scratching, bending, cold rolling, beating, and deposition of other elements on the amorphous alloys surface. Surface crystallization was also induced by isothermal annealing of amorphous alloys at a temperature well below the normal crystallization temperature prior to or followed by these surface treatments 3-7 . Some of the mechanisms proposed to explain surface crystallization include atom segregation, oxidation, surface diffusion, plastic deformation, decrease of total surface energy and the stress relaxation on the surface. Aluminum-based amorphous alloys were first found in 19888-17. These amorphous alloys, containing up to 90% aluminum and in the form of Al-RE-TM where RE=lanthanides and Yttrium, TM=Fe, Co, Ni and Rh, are made by rapid solidification. They show unusual mechanical properties, with many of the original alloys having fracture tensile strength over 800 MPa, which greatly exceeds that of the conventional high strength aluminum alloys. The fracture mode is ductile. The combination of those characteristics -- low density, high strength and ductility -- gives these alloys great potential as engineering materials. Their excellent mechanical properties can even be improved sometimes by partial crystallization' 4 -16 . The microstructure of the partially crystallized aluminum based amorphous alloys is that of tiny13crystalline precipitates (7-10 nm in diameter) homogeneously embedded in an amorphous matrix . EXPERIMENTAL PROCEDURES Several aluminum-based amorphous alloys, A190Fe 5Gd 5 , A190 Fe 5Ce 5 and A187Ni 87.Y 3.4 , were studied. All showed similar characteristics. Alloy ingots were prepared by melting nominal amounts of high purity elements in an arc furnace under an argon atmosphere. Amorphous alloy ribbons were obtained by using a single roller melt-spinner in a partial helium atmosphere. Typical circumferential speed of
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