Symmetry aspects of pretransformation behavior in metallic alloys
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I.
INTRODUCTION
THIS paper
is concerned with a general survey of pretransformation behavior as observed in metallic alloys and minerals. This behavior is frequently observed in alloy systems on cooling, and prior to displacive (martensitic) transformations. Although it has not been described in exactly this way, similar behavior has long since been recognized in minerals. 1'2 In this paper we will be concerned with attempting to derive a general theory of the origin of these phenomena, and it would seem to be highly desirable to bring together all the important data on both alloy and mineral examples, since they differ a great deal both in symmetry and in the nature of the structural changes involved. A. The Evidence for Pretransformation Behavior Careful comparison of the details of pretransformation behavior in metallic alloys and in minerals shows that they appear to have a great deal in common. Evidence for pretransformation behavior is usually obtained from diffraction studies by X-rays or electrons. It normally comprises local sharp intensity streaks that occur in directions at right angles through the Bragg maxima. In the case of the mineral potassium feldspar these streaks are particularly well developed. ~-4 Similar intensity streaks have been described for certain cubic alloys. 5'6 In electron-optical experiments these streaks give rise to a distinctive cross-hatched texture which is familiarly known as "tweed". In the electron microscope it is possible to demonstrate that the corresponding fluctuations correspond to transverse waves by dark-field imaging techniques, or by the study of absent intensity in certain positions in reciprocal space. ~-3.5In certain cases, although the intensity streaks are not readily observed, it is possible J. DESMOND C. McCONNELL, formerly with Schlumberger Cambridge Research, P.O. Box 153, Cambridge CB2 0HG, is Professor, Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, United Kingdom. This paper is based on a presentation made in the symposium "Pretransformation Behavior Related to Displacive Transformations in Alloys" presented at the 1986 annual AIME meeting in New Orleans, March 2-6, 1986, under the auspices of the ASM-MSD Structures Committee. METALLURGICAL TRANSACTIONS A
to establish the existence of the phenomena from electronoptical study of the tweed texture. This is the case for the mineral cordierite, P6/mcc, where it has been possible to establish the existence of two transverse distortion waves in the electron-optical contrast pattern in dark-field images using g" b criteria. 7 In the dot product g is the chosen reciprocal lattice vector used in dark-field imaging, and b is the amplitude vector of the fluctuation wave. Contrast is absent in the image where the amplitude vector is normal to the reciprocal lattice vector. In both potassium feldspar, C2/m, and cordierite it is possible to demonstrate from such criteria that the waves are transverse, and polarized in the plane defined by the two suites of orthogonal wave vectors.
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