A variant selection model for predicting the transformation texture of deformed austenite
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I.
INTRODUCTION
THE inheritance of preferred orientations during the gto-a transformation has been under investigation for a considerable time. Comprehensive reviews of transformation textures have been published recently by Inagaki,[1] Yutori and Ogawa,[2] Ray and Jonas,[3] and Ray et al.[4] covering broad aspects of this topic. These surveys have shown that the transformation of austenite to ferrite, martensite, and bainite frequently involves only a restricted set of the 24 possible Kurdjumov–Sachs (K–S) variants. This phenomenon is known as variant selection and its origins have been investigated by many authors. Several different methods have been used to predict the product textures resulting from the g-to-a transformation. The texture of the g phase has either been expressed as a discrete set of ideal orientations[5,6] or as a continuous orientation distribution function (ODF). The ODF-based methods require analytical operators to transform the parent texture as a whole; these can be rotation operators, as suggested by Roe[7] and used by Davies et al.,[8] or generalized correspondence functions, such as the misorientation distribution function (MODF) proposed by Sargent[9] and used by Bateman and Davies[10] and Davies and Bateman.[11] While mathematically more efficient, the latter methods are unable to deal with the physical nature of the phase transformation on an individual crystal basis. The discrete orientation methods, on the other hand, allow variant selection mechanisms to be introduced on the level of individual parent orientations, making it possible ´ N-GUILLE´N, formerly Research Associate, Department of M.P. BUTRO Metallurgical Engineering, McGill University, is Visiting Professor, Military Institute of Engineering, 270 Rio de Janeiro-RJ, Brazil. C.S. DA COSTA VIANA, Professor, is with the Instituto Militar de Engenharia, IME. J.J. JONAS, Professor, is with the Department of Metallurgical Engineering, McGill University, 3610 University St., Montreal, PQ, Canada H3A 2B2. Manuscript submitted November 21, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
to investigate the peculiarities of texture transformation in different systems. Of particular interest is the prediction of the transformation textures formed from deformed austenite, especially pancaked grains. This is because variant selection observations are generally associated with the deformed microstructures present in microalloyed steels.[12,13] Texture prediction using the K–S relation without variant selection leads to textures that differ qualitatively and quantitatively from those observed experimentally, thus leading to the firm conclusion that some form of variant selection must be taken into account. Although many variant selection rules have been proposed in a series of attempts to account for the observations, none of these has turned out to be wholly satisfactory.[8,14,15] It was therefore the aim of this investigation to consider some of the selection criteria that have been put forward and, in particular, to suggest that a co
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