Electron Microscopy Study of Structural Changes During Isothermal Annealing of a Ni4Mo-Based Alloy Containing Chromium

Transmission electron microscopy has been used to study phase transformations in a Ni4Mo alloy containing 2.08 wt pct chromium following isothermal annealing at 850, 800 and 700°C. At these temperatures, the DO22 phase forms initially, which then transforms to Ni3Mo. At 700°C, the DO22 phase forms from bands composed of Ni4Mo and Ni2Mo aligned parallel to {111} fcc planes, and is relatively stable. A second mechanism for the formation of Ni3Mo occurs at 700°C. Large Ni4Mo domains form at grain boundaries. These subsequently transform to Ni3Mo, with the simultaneous presence of stacking faults on (010) Ni3Mo planes. These observations are correlated to the optical microstructure. Grain size influences the type, volume, and distribution of second phases. Sequences and mechanisms of transformations are related to the experimental observations.