Effect on the microstructure of aging Hastelloy B2 from 550 to 850°C for 1,200 hours

A microstructural examination (using optical and scanning electron microscopy) has been conducted of samples of a Hastelloy B2 alloy, which have been aged from 550 to 850°C for 200, 400, 800, and 1,200 hr. The nickel-based alloy contained 27.7% Mo, 1.0% Cr, 0.8% Fe, 0.3% Mn, and 0.002% C (wt. %). In the solution-annealed condition, the structure is facecentered cubic α. The study used samples of 50 and 800 μm initial α grain sizes. At all aging temperatures, the hardness increased and approximately doubled after aging for about 400 h at 550, 650, and 750°C. At 850°C, it was still increasing after 1,200 h. In the 50 μm initial grain-sized material, aging produced increasing amounts of a Widmanstatten plate structure, with the most prolific amounts at 750°C and the least at 550°C. The formation of this structure was considerably slower in the larger-grained size material since it initiated at the α grain boundaries. The initial formation was by a plate of high Mo content (relative to the matrix) surrounded by a region of low Mo content. This structure developed into a lamellar product, growing lengthwise by the movement of an interface. It appears that the plates are the γ phase (Ni₃Mo). Since considerable hardening occurs even when the Widmanstatten structure is not prominent, it is believed that the α has, at least initially, transformed to the β phase (Ni₄Mo), as this is known to cause hardening. The structures are discussed in terms of the known Ni-Mo-Fe and Ni-Mo-Cr ternary phase diagrams.