On the effect of fine grain size on the M s temperature in Fe-27Ni-0.025C alloys
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quenching work by Rayment et al.13 further supports the Ansell theory. 11 The present article relates some pertinent recent work, which was itself part of a larger study. ~4'15 By use of a mechanical alloying technique, 16 a series of Fe-27Ni0.025C alloys was fabricated with incremental additions of an inert oxide dispersion (Y203). By holding matrix chemistry constant, it was possible to evaluate the effect of grain size and yttria dispersion strengthening on the Ms temperature of these alloys. Parameters pertinent to this study included the Ms temperature, prior austenite grain size, and austenite yield strength at Ms. Determination of the Ms temperature was by resistivity techniques, similar to those described by Strife, Carr, and Ansel117and by Fisher and Corduroy. 18Taking into account sample-to-sample variations, Ms is considered accurate to better than ---3 ~ Due to low carbon content and generally fine grain size of alloys in this series, prior austenite grain boundaries could not be identified using standard metallographic etching techniques. To overcome this difficulty, a modification~4of the selective oxidation technique of Tobin and Kenyon 19'2~was employed. Grain size was then determined by the line~ intercept method2~ applied to micrographs taken at appropriate magnifications. The average linear intercept measured in this manner is considered accurate to within +- 10 pct. Austenite yield stress can not be measured directly at Ms due to the formation of stress- and/or strain-induced martensite. The austenite 0.2 pct offset yield stress was determined at a series of temperatures above the Md temperature for selected alloys. Linear extrapolation of these data gave an estimate of the yield strength of the austenite at Ms .3:1:6 Strain rates used w e r e 10 -3 per second. Variations of the austenite grain size with austenitizing temperature are shown in Figure 1 for both the base alloy (no dispersion) and an alloy containing 2 vol pct yttria.
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600 P.J. BROFMAN is Project Engineer with the IBM Corporation, East Fishkill Facility, Hopewell Junction, NY 12533. G. S. ANSELL is Dean of Engineering with the Rensselaer Polytechnic Institute, Troy, NY 12181. Manuscript submitted January 24, 1983. METALLURGICAL TRANSACTIONS A
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700 800 900 I000 AUSTENITIZlNG TEMR(*C)
Fig. 1--Prior austenite grain size as a function of 15 min at austenitizing temperature for the base alloy and an alloy containing 2 vol pct yttria dispersion. VOLUME 14A, SEPTEMBER 1983-- 1929
Curves of the Ms temperature vs austenitizing temperature (Figure 2) are similar to data reported by other workers. 3'4'9 Ms temperature is plotted as a function o f the reciprocal square root of the average linear intercept, L, in Figure 3. The reasonably good linear fit confirms Nichol's 3 earlier grain size vs Ms temperature studies and strongly suggests the linkage of the Ms temperature to a Hall-Petch strengthening mechanism. In addition, Figure 3 provides a direct comparison between the dispersi
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