The analysis of bainitic plates in Cu-Zn and Cr-Ni
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I.
INTRODUCTION
THE determination of the
mechanism of the formation of "bainitic" plates in substitutional alloys, whether by diffusional growth or by martensitic shear, is a subject of debate. In binary Cu-Zn alloys containing approximately 40 wt pct Zn, samples quenched from the high-temperature ordered bcc/3' phase to the aging temperature precipitate al. The aL phase has a platelike morphology, t~j and several characteristics which are consistent with its formation by a shear (martensitic) mechanism. The al plates yield an invariant plane strain (IPS) surface relief t~,2,31 and they have an irrational habit plane, {2, 11, 12}, which is identical to that of martensite formed at subzero temperatures in the same alloys. 1~,4,5JThe lattice orientation relationships are irrational Ill and are the same as those of the martensite. Initially, the plates are internally faulted .[6,7} The critical information which is required to determine whether the a~ plates form by a diffusioncontrolled or shear mechanism is the composition of the plates. While there is general agreement regarding the morphological and crystallographic nature of the a~ plates, there is considerable disagreement concerning their composition. Flewitt and Towner, I71 using conventional microprobe analysis of bulk specimens, found no difference between the composition of the a~ plates and the matrix during the early stages of growth. Cornelis and Wayman m repeated the experiments of Flewitt and Towner tT~using thin specimens of a Cu-40.5 wt pct Zn alloy analyzed with a JEOL JX50A electron-probe microanalyzer, and they, too, found that the initially formed ct~ plates had the same composition as the /3' G. CLIFF, Senior Experimental Officer, and G.W. LORIMER, Reader, are with the Manchester Materials Science Centre, University of Manchester/UMIST, Gmsvenor Street, Manchester M1 7HS, United Kingdom. F. HASAN, formerly Research Assistant, Manchester Materials Science Centre, University of Manchester/UMIST, is Professor with the Department of Metallurgical Engineering, Engineering University, Lahore, Pakistan. M. KIKUCHI, Professor, is with the Department of Metallurgical Engineering, Tokyo Institute of Technology, Tokyo 152, Japan. This paper is based on a presentation made in the symposium "International Conference on Bainite" presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee. METALLURGICAL TRANSACTIONS A
matrix. Lorimer e t al. t8j used the analytical electron microscope EMMA-4 to examine thin specimens of a Cu-41.5 wt pct Zn alloy and samples of the same alloy which had been heat-treated and examined by Cornelis and Wayman. I~1For both specimens, Lorimer e t al. fsl were careful to analyze only those plates which protruded over the edge of the thin foil. Both of the specimens yielded similar results: even from the earliest stages of growth which could be analyzed, 30 seconds at a reaction tempe
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