The effect of processing conditions and subsequent heat treatment on the transformation behavior of some rapidly solidif
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I.
INTRODUCTION
THE amount of published
literature in both the field of shape-memory alloys and rapid solidification is extensive and is well served by books, reviews, and bibliographies (e.g., References 1 to 10). The initial reason for employing the technique of rapid solidification for shape memory alloys 11 was to obtain a small grain size in wire and sheet which is difficult to achieve by hot working where grain growth is extremely rapid. A grain size of between 2 to 5/~m has been observed in Cu-Zn-AI alloys 11'12'13and also in Cu-AI-Ni alloys 12 prepared by melt extraction and melt spinning respectively. 1~ In addition to this reduction in grain size there is probably an increase in 'quenched in' vacancy concentration compared with samples that have been quenched from the solid state. 14'15'16This excess concentration is accommodated either by the formation of dislocation loops 14'16'17 or some form of solute-vacancy clustering. 18,19In alloys that undergo a martensitic reaction, the larger vacancy population can affect the nature of the reaction2~or the form of any premartensitic phenomena. It would also be expected that such an increase would reduce the driving force required for the martensitic reaction. These would lead to an increase in Ms temperature. 3'11'2~ By contrast, the effect of reducing the grain size in many alloys (e.g., References 3, 22) gives a reduction in Ms temperature, and this has been observed in some rapidly solidified alloys. 12,13Clearly, there is a tension between the grain size and the vacancy contribution effect to any movement in Ms temperature, which is displayed in conflicting results for Cu-Zn-A1.13,23,24 The grain size of rapidly solidified samples is determined by the amount of undercooling prior to crystallization,23 and for chill block methods is generally small (a few microns). J. V. WOOD is a Lecturer in Materials Science, Faculty of Technology, The Open University, Walton Hall, Milton Keynes MK7 6AA, United Kingdom. P. H. SHINGU is Professor in the Department of Metal Science and Technology, Kyoto University, Kyoto, Japan. Manuscript submitted February 7, 1983.
METALLURGICALTRANSACTIONSA
However, the preservation of a large potential excess vacancy concentration is established by retaining a high quench rate during the whole quench down to ambient temperature. Both theoretical and experimental studies 26,27have shown that the cooling rate during rapid solidification processing can be comp~atively slow toward the final part of the quench which could allow diffusion of vacancies to sinks. Solid state quenching studies of a wide range of alloys shows that Ms temperature is depressed as the cooling rate is increased (for a given grain size) until a limiting value of M~ is obtained. 3 The occurrence of premartensitic phenomena has been extensively investigated (e.g., Reference 28) and the various, apparently conflicting results, reviewed by Wayman. s Whether any of the phenomena actually influence the martensitic transformation characteristics is by no means certain al
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