A study of a cellular phase transformation in the ternary Ni- Ai- Mo alloy system

Abstract

Orthorhombic Ni3Mo has been observed as a product of a cellular precipitation reaction occurring at grain boundaries in both Ni-12Al-15Mo and a ternary eutectic Ni-12.8Al-22.2Mo (compositions in at. pct) when heat treated at intermediate temperatures (600 to 850 °C). Convergent beam electron diffraction and energy dispersive X-ray spectroscopy techniques have been employed to characterize the structure and composition of this phase in these alloys. Since this phase is usually heavily faulted, the diffraction symmetries are reduced, causing ambiguities in analysis. Therefore, a binary (nominally) stoichiometric Ni3Mo alloy was produced and heat treated in a manner designed to minimize faulting. Convergent beam patterns obtained from this material exhibited symmetries that identify it as having the mmm point group; further analysis of the diffraction patterns led to the conclusion that the space group is Pmmn. These patterns were then used to aid in the interpretation of those taken from the ternary alloys. Orthorhombic Ni3Mo was also observed within the grains of the ternary eutectic alloy following prolonged aging treatments; the orientation relationship between the Ni3Mo and the fee matrix was determined to be identical with that in the cellularly transformed regions.