The structure, thickness and chemistry of antiphase domain boundaries in heat-treated, rapidly solidified Ni3Al

Author： Perez J. F. Shang P. Morris D. G. Jones I. P.

ISSN： 0141-8610

Source： Philosophical Magazine. A. Physics of Condensed Matter. Defects and Mechanical Properties, Vol.79, Iss.1, 1999-01, pp. : 179-192

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Abstract

Antiphase boundaries (APBs) in a rapidly solidified Ni3Al alloy have been examined by transmission electron microscopy in the as-solidified state and after annealing. Dark field imaging suggests that the APB thickness increases with temperature during annealing and that a dark, disordered phase appears at temperatures above about 800oC. The boundary region at high temperatures has a chemical composition close to that expected from the phase diagram for the disordered phase, namely containing about 15 at.% Al. Examination of APBs by high-resolution electron microscopy does not confirm the disorder, however, and the so-called disordered regions showing dark contrast may in fact be caused by the significantly modified chemistry of these ordered regions. It is not clear whether disorder may form at high temperature when the average alloy composition enters the two-phase ordered disordered region, or whether simply extensive segregation takes place to the APB without the true appearance of a second, disordered phase. Similar effects occurring at grain boundaries would lead to a structure dependent on both composition and temperature, with the mechanical behaviour of the material depending on the structural state.