Solid-solution structure and the weakly first-order displacive transformation in Fe-Pd alloys
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I.
INTRODUCTION
IT is well established that diffusional first-order solidstate phase transformations can occur via two principal pathways: (a) nucleation and growth and (b) continuous modulation or spinodal decomposition. The competition between these pathways in the case of diffusionless displacive transformations is the subject of current research. Acknowledging these two possibilities, a classification scheme for displacive transformations[1,2] separates lattice-distortive transformations (dominated by strain rather than intracell shuffle displacements) into (a) martensitic transformations occurring by nucleation and growth and (b) quasi-martensitic transformations occurring by continuous strain modulation. A theory of the dynamics of quasi-martensitic strain modulation has been developed[3,4] for the case of an unstable system where modulation wavelength selection is governed by heat transfer. The fast dynamics of displacive transformations makes the unstable regime (corresponding to a negative shear modulus) difficult to access experimentally, however, and thus a subject of current interest is the extent to which the continuous strain-modulation mechanism can compete with nucleation and growth in the metastable regime, particularly if aided by interactions with fine-scale defects. Fine-scale defects are known to reduce the mobility of J.J. FELTEN, formerly Research Assistant with the Department of Materials Science and Engineering, Northwestern University, is now Materials Scientist with Containerless Research Inc, Evanston, IL 60201. T.J. KINKUS, instructor, is with Du Sable High School, Chicago, IL 60615. A.C.E. REID, Research Associate , and G.B. OLSON, Professor and Associate Chairman, Department of Materials Science and Engineering, and J.B. COHEN, Professor and Dean, McCormick School of Engineering and Applied Science, are with Northwestern University, Evanston, IL 60208-3101. Manuscript submitted August 5, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
martensitic interfaces,[5,6] and the roles of both dislocationforest hardening and solid-solution hardening have been treated quantitatively in ferrous alloys.[7,8] In contrast, it is also well known in the case of second-order/higher-order phase transitions that fine-scale defect interactions can spread the transition over a range of temperatures, thus promoting local transformation at higher than normal temperatures.[9,10,11] Interest in the possibility that such effects could also be manifested in weakly first-order displacive transformations has been fostered by observations of apparent transformation precursor effects in the form of strain modulations termed ‘‘tweed’’ structures.[12] Rigorous calculations have shown that such structures cannot exist as equilibrium states of a uniform perfect crystal,[13] and it is now generally accepted that they are stabilized by defect interactions[14] in line with their earliest interpretation.[12] The possibility remains, therefore, that fine-scale defects may inhibit the martensitic nucleation and growth me
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