High-Resolution Transmission Electron Microscopy Investigation of the Face-Centered Cubic/Hexagonal Close-Packed Martensite Transformation in Co-31.8 Wt Pct Ni Alloy: Part 1. Plate Interfaces and Growth Ledges

摘要

The face-centered cubic/hexagonal close-packed (fcc/hcp) martensite transformation in a Co-31.8 wt pct Ni alloy was studied by high-resolution transmission electron microscopy (HRTEM). High-resolution transmission electron microscopy was used to study the structure and properties of growth ledges, the tips of martensite plates, and martensite nucleation sites. The HRTEM image simulations were performed in order to determine the effects of both beam and crystal tilt on the experimental images. In the investigation, it was determined that the fcc/hcp martensite transformation in Co-Ni occurs by the passage of Shockley partial dislocation ledges (b = 1/6 < 112 >) along every other (111) plane in the fcc matrix. The hcp martensite thickens by the lateral movement of ledges across the fcc/hcp interface. Althou'gh superledges were observed, the majority of the ledges were two (0002) planes and this is the basic ledge height. Image simulations show that both beam and crystal tilt can have a marked effect on HRTEM images of fine hcp martensite plates. The effects of tilt must be minimized in order to unambiguously resolve the interfacial structure.