High-resolution transmission electron microscopy investigation of the face-centered cubic/hexagonal close-packed martens
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I.
INTRODUCTION
THE purpose of this study was to investigate a facecentered cubic/hexagonal close-packed (hcp) martensite phase transformation by high-resolution transmission electron microscopy (HRTEM). The fcc/hcp martensite transformation is one of the simplest transformations crystallographically.[1,2] Both the fcc and hcp phases are close-packed; the crystallographic relationship between them is (111)\(0001) (habit plane) and ^110&\^1120&, where the Miller–Bravais notation is used for the hcp phase. The crystallography of the fcc/hcp martensite transformation can be described by the passage of 1/6^112& Shockley partial dislocations along every other (111) plane of the fcc matrix, which transforms the fcc matrix into the hcp structure,[1–8] as illustrated schematically in Figure 1. There are four hcp variants defined by the four (111) planes (or habit planes) in the fcc matrix. One (111)\(0001) variant can be obtained by rotating the basal plane of another by 70.53 deg with respect to the corresponding ^011& direction.[4] The hcp martensite can form directly by the generation and movement of extended dislocations or stacking faults in the fcc matrix. Although there is no difference between stacking faults and the hcp phase crystallographically, in this article, a stacking fault is described as being two (0002) planes thick, while an hcp martensite plate is thicker than two (0002) planes. The stacking fault energy can be thought of as being proportional to the free energy difference between the fcc and hcp phases.[9] The stacking fault energy
decreases as the temperature is lowered to the martensite start (Ms) temperature,[10,11] which suggests that Ms marks a transition from positive to negative stacking fault energy, thus making the hcp phase energetically favorable.
II.
EXPERIMENTAL PROCEDURES
A single-crystal fcc sample of Co-31.8 wt pct Ni alloy was used in this investigation. This alloy has been described in detail previously.[12] The Ms temperature of the alloy is approximately 210 7C, and the austenite start (As) temperature is approximately 100 7C. In order to partially transform the fcc phase to the hcp phase, a piece of the sample was quenched in liquid nitrogen at a temperature of approximately 2195 7C. Thin slices of the alloy were then cut so that both ^112& (for conventional TEM) and ^110& (for HRTEM) orientations could be obtained. The slices were mechanically thinned to approximately 20 mm, and 3mm disks of the thinned alloy were punched and electropolished in a twin-jet Fischione apparatus using an electrolyte of 20 pct perchloric acid in ethanol at about 220 7C. The thin foils were rinsed in ethanol prior to TEM examination. Conventional TEM examination was performed on a PHILIPS* EM400T at 120 kV. The HRTEM examination was performed on a JEOL** 4000EX at 400 kV. *PHILIPS is a trademark of Philips Electronic Instruments Corp., Mahwah, NJ. **JEOL is a trademark of Japan Electron Optics Ltd., Tokyo.
D.W. BRAY, Graduate Student, is with Materials Science and Engineering, North Carolina Sta
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