Localized vs homogeneous deformation in Fe 82 B 15 Si 3 amorphous alloy
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2. E X P E R I M E N T A L M E T H O D S
1. I N T R O D U C T I O N A unique feature of amorphous alloys, also known as metallic glasses, is the lack of regular crystalline structure, and thereby grain boundaries as well. For this reason, the deformation mechanisms known for crystalline metals over a wide range of temperatures are no longer applicable to amorphous alloys. It is known, for example, that at well below the glass transition temperatures amorphous alloys deform by intense, localized shear bands without revealing any microstructural discontinuities across the band. ]-5 With increasing temperature or decreasing strain rate, the deformation is believed to occur in a viscous and homogeneous manner; ~l~ however, the mechanism responsible for the transition from localized to homogeneous deformation is not well established at the present time. Phenomenological models describing deformation mechanisms of amorphous alloys have been proposed where the change in deformation behavior is suggested to be due to either transition to Newtonian viscous behavior u or a qualitative difference in the mode of shear transformation? 3 However, some of the experimental data describing the stress-strain rate behavior are inconsistent with Newtonian viscous behavior. ]4,15 F o r these reasons, an experimental approach has been undertaken to establish the strain rate dependent plastic flow characteristics and to identify conditions which encourage homogeneous deformation in an amorphous alloy, Fes2B~sSi3. Based on these experimental observations, some of the key features of the proposed model have been examined. It is shown that the bulk of plastic flow behavior could be explained on the basis of the changes in material parameters that are known to influence the nature of nonuniform deformation in crystalline metals. D. LEE is with Metallurgy Laboratory, General Electric Corporate Research and Development, Schenectady, NY 12301. Manuscript submitted March 19, 1980. METALLURGICAL TRANSACTIONS A
2.1 Preparation of A m o r p h o u s Ribbons The samples were prepared from an ingot consisting of 82Fe, 15B, and 3Si by at. pct using the single wheel chill block melt-spinning method? 6 The cross section of the rapidly quenched ribbon was about 0.53 cm in width and 0.0025 cm in thickness. Results of X-ray diffraction and T E M diffraction analysis have indicated that the ribbons were noncrystalline in character. The glass transition temperature, Tg, of the alloy is estimated to be 417 ~ and the temperature of crystallization, Tx, is reported to be 344 ~ after 2 h anneal and 475 ~ when heated at the rate of 20 ~ 2.2 Load Relaxation Tests Since amorphous alloys have limited tensile ductility, the load relaxation test is ideally suited for determining the stress-strain rate relationship at different levels of plastic strain. The load relaxation test was made using a machined uniform gage section specimen where the HEATING
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