Hyperfine structure changes in iron-base amorphous alloys produced by high current density electropulsing
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H. Conrad Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695-7907
K. Chu Department of Physics, Northeastern University, Shenyang 110006, People's Republic of China (Received 9 May 1994; accepted 1 December 1994)
The effects of high current density electropulsing on the hyperfine structure of amorphous alloys Fe73Si9.5Bi7.5, Fe75SiioBi5, and FeygSivB^ were investigated by Mossbauer spectroscopy. Electropulsing influenced the microstructure at a temperature well below the bulk crystallization temperature. In the alloy having least boron (or highest concentration of Fe), a - F e particles ~ 3 nm in size precipitated from the amorphous matrix, giving a change in internal magnetic field.
I. INTRODUCTION In a previous paper we reported that high current density electropulsing can induce certain hyperfine structural changes in amorphous iron-base alloys without causing bulk crystallization.1 For alloys Fe75SiioBi5 and Fe79Si7B]4, the hyperfine magnetic field distribution function P(H), where Ji P(H)dH = 1, did not change its overall shape after pulsing. However, the field at which P(H) is maximum shifted slightly toward a stronger value. As a consequence, the mean internal field H — /Q HP(H)dH also increased. Meanwhile, the peak ratio b = A2^/A3A of the Mossbauer spectra (area of peak 2 or 5 against that of peak 3 or 4) always decreased as a result of the pulsing. These results were primarily attributed to slight precipitation of a magnetic phase, e.g., a-Fe(Si), and some disordering in the material. This paper presents in more detail the effect of chemical composition and other factors on the structural changes and the internal magnetic field H produced by dc pulsing of amorphous iron-base alloys at temperatures that are far below the bulk crystallization temperature. II. EXPERIMENTAL METHODS Three Fe-based amorphous alloys, Fe73Si9.5B17.5, Fe75SiloBi5, and Fe79Si7B14, with increasing Fe and decreasing B were investigated. They were prepared by the roller quenching technique. The rotational speeds of the wheel for producing Fe^Sig.sBn.s and Fe75SiioB15 alloys were 900 rpm and 1100 rpm, respectively. An intermediate cooling rate was used for the third alloy. All specimens were in the form of ribbons 20 mm wide X 45 mm long and about 30 /mm thick. The high density dc pulsing and microhardness measurements were car900 http://journals.cambridge.org
J. Mater. Res., Vol. 10, No. 4, Apr 1995
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ried out at both NCSU and NU. The pulsing conditions are given in Table I. The temperature rise produced by the Joule heating of the current pulse was measured by spot welding a 0.07 mm diameter chromel-alumel thermocouple to the foil. Some measurements were also made using a Pt foil painted onto a very thin thermally sensitive ceramic and determining the temperature rise from the change in resistance of the Pt foil. The temperature rise for the first pulse was ~43 °C determined by an oscilloscope. This rise was in accord with that calculated based o
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