Effect of tensile stress on the annealed structure of a metallic glass
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. INTRODUCTION One of the interesting features of metallic glasses is the phenomena of structural relaxation that results from exposure of these materials to elevated temperatures. Changes in the mechanical and physical properties following the thermal annealing of amorphous alloys have been extensively reviewed.1"3 Additional insight into the relaxation process can be obtained by observing the effect that different external conditions have on this phenomena. Of particular interest in this investigation are the structural changes caused by the simultaneous application of a tensile stress during heat treatment of the metallic glass. Most interest in the effects of stress annealing has centered around optimizing the magnetic properties of selected amorphous alloys. For example, Nielsen4 discussed the role of stress annealing on the magnetic anisotropy of a cobalt-iron magnetic alloy. Improvements in the low-field magnetization, as well as an increased sensitivity of the magnetic properties to elastic stresses applied at room temperature, have been noted by Egami et al.5 following the stress annealing of nickel-iron metallic glasses. The goal of the present study is to examine the structural changes that occur during heat treatment with an applied tensile stress by means of carefully conducted microhardness measurements made on the heat-treated specimens. As demonstrated in an extensive study by Stubicar,6 the iron-nicka)
Now at Sandia National Laboratories, Albuquerque, New Mexico 87185.
J. Mater. Res. 2 (4), Jul/Aug 1987
el-based metallic glasses similar to those used in this investigation, show an appreciable increase of microhardness following thermal annealing, thus making them suitable materials for observing the differences, if any, between the stressed and nonstressed heat treatments. To augment these measurements, differential scanning calorimetry (DSC) techniques were also conducted to further analyze structural changes in the annealed ribbons. II. EXPERIMENTAL A. Sample preparation The material used in this study was Metglas® 2826B (Metglas is a registered trademark of the Allied Corporation) D wire with the chemical composition Ni49Fe29P14B6Si2 (subscripts denote atomic percents), which was fabricated by the single-roller technique. A preliminary investigation indicated that for the time and temperature parameters used in this study, a greater change in microhardness accompanied this alloy as compared to the more popular 2826 material (Ni 40 Fe 40 Pi 4 B 6 ). Microhardness test samples to be annealed under zero load were cut from the stock and placed directly into the vacuum furnace. Following the thermal treatment, the specimens were mounted in cold-setting epoxy, exposing that side of the ribbon that contacted the spinning wheel during fabrication. The samples were next polished on a 600 grit abrasive paper and then finally polished to a 0.05 ^ m alumina finish. The preparation of specimens annealed under stress was
0003-6951 /87/040461-10$01.75
© 1987 Materials Research Society
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P. T. Vianco an
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