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I.

INTRODUCTION

THE purpose of this study was to investigate an fcc/hcp martensite phase transformation by high-resolution transmission electron microscopy (HRTEM). The HRTEM was used to study the structure and properties of intersections between martensite plates and other defects observed in the alloy such as stacking fault tetrahedra (SFT) and Z-type defects. The HRTEM was also used to attempt to determine if any of various proposed mechanisms for the face-centered cubic/hexagonal close-packed (fcc/hcp) martensite transformation were operating in this Co-Ni alloy. With this in mind, a brief review of various nucleation mechanisms for the fcc/hcp martensite transformation follows. Mechanisms for the fcc/hcp Martensitic Transformation Several mechanisms have been proposed to explain the fcc/hcp martensite transformation. These mechanisms include the reflection mechanism of Bollmann,[1] the dipole mechanism of Hirth,[2] various pole mechanisms by Hirth,[3] Venables,[4] and Seeger,[5] and a six-plane mechanism by Mahajan et al.[6] Each of the different mechanisms of transformation is summarized subsequently.

D.W. BRAY, Graduate Student, is with Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695. J.M. HOWE, Associate Professor, is with the Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903-2442. Manuscript submitted November 27, 1995.

METALLURGICAL AND MATERIALS TRANSACTIONS A

The basic configuration of the reflection mechanism[1] is the junction of two stacking faults on different (111) planes. If the two stacking faults intersect, steps are produced in each of the stacking faults, and this requires energy for their formation. The approaching fault is therefore repelled as it runs into the fault already present, producing a local stress field. The approaching stacking fault can remain in an equilibrium position away from the other fault, or if there is sufficient driving force, it can join the existing fault. Stress is produced at the intersection, and this can be released by the production of new stacking faults bounded by 1/6[121] or 1/6[211] Shockley partials on (111) planes two layers away from the original intersection. This compensates in part for the shift produced by the 1/6[112] Shockley partial of the first stacking fault. These faults may run into other faults having any of the other three possible (111) plane orientations and operate the same mechanism at the intersection as before, thus creating new faults and continuing the transformation. The configuration of the dipole mechanism[2] consists of the intersection of a stacking fault by a moving dislocation, partial dislocation, or another stacking fault. The intersection of a stacking fault by dislocation DA on plane (c) (using Thompson notation) creates the dipole configuration. Along the line of intersection, a dislocation dipole Ad -d A is formed. If one considers the nucleation of a partial loop Ad, elastic interactions with the adjacent d A partial reduce the el