Tweed microstructures I. Characterization in β-NiAl

We describe an electron microscopic investigation of an as-quenched 63 at. % Ni-Al alloy. This material exhibits strong tweed contrast and associated diffraction effects, commonly called ‘premartensitic effects’. Anomalous diffraction effects in martensite-forming β3 Hume-Rothery phases have been studied extensively, but the mottled microstructures have received little attention. This paper describes an attempt to characterize the microstructure systematically. In addition to some well-known characteristics, we have discovered that in many ways tweed contrast resembles the fringe contrast observed on stacking faults. This includes the variation of fringe spacing with foil orientation, with the extinction distance of the diffraction vector and with the deviation from the Bragg condition. Such behaviour provides a basis for understanding the origin of premartensitic effects in Hume-Rothery phases, and tweed microstructures in general.