Growth kinetics of grain boundary ferrite allotriomorphs in Fe-C-X alloys
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THE influence of alloying elements upon the growth kinetics of proeutectoid ferrite in Fe-C-X alloys, where X is a substitutional alloying element, is a problem of long-standing scientific and industrial importance. During recent years there have been substantial improvements in the methods used to measure such growth kinetics and in the formulation of theoretical analyses of ferrite growth. In the present investigation an effort was made to extend the experimental measurements to Fe-C-X alloys containing representative alloying elements and reacted over the widest experimentally accessible temperature range. Use is made of recent improvements in experimental technique t-3 designed to reduce the scatter which complicates the interpretation of data secured by other methods.*-6 Attempts have been made to account for the present data with the principal theoretical analyses of ferrite growth kinetics in Fe-C-X alloys. Use of these analyses has been greatly facilitated by the availability of the Hillert-Staffanson7 regular solution treatment of ternary phase equilibria and its application to many Fe-C-X systems by Hillert and his colleagues,7-1~as reviewed recently by Uhreniu~? ~ The growth kinetics measurements in this investigation have been conducted on grain boundary ferrite allotriomorphs. This morphological choice appears to maximize the proportion of the interphase boundary J, R. BRADLEY, formerly graduate student, Department of Metallurgical Engineering, Michigan Technological University, is now with the Physics Department, General Motors Research Laboratories, Warren, MI 48090, and H. I. AARONSON, formerly Professor at Michigan Technological University is now R. F. Mehl Professor, Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15212. Manuscnpt submitted March 26, 1979. METALLURGICAL TRANSACTIONS A
area which has an essentially disordered structure and thereby minimizes (though does not completely eliminate TM)the influence of the structure of the austenite:ferrite boundaries upon growth kinetics. A number of investigations has been reported on the effects of a single alloying element upon the growth of ferrite allotriomorphs or simulations of this morphology in high-purity Fe-C-X alloys?-5,n-~9 Most of these studies have employed one of the following two approaches. Kinsman and Aaronson 4,5 measured the growth kinetics of ferrite allotriomorphs with thermionic emission electron microscopy (THEEM) in Fe-C, Fe-C-A1, Fe-C-Si, Fe-C-Co, Fe-C-Mo, and Fe-C-Mn alloys. The THEEM technique affords the distinct advantage of providing precise data on the growth of individual allotriomorphs but also results in extensive scatter in the data as a result of both stereological and crystallographic factors. Purdy et a113.2~used austenite/ferrite diffusion couples to simulate the growth of ferrite allotriomorphs in high-purity Fe-C and Fe-C-Mn alloys. Although their method largely eliminates the stereological and, perhaps to a lesser extent, the crystallographic problems
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