The formation of the ω phase in Ti-Nb alloys

Abstract

The as-quenched and the aged form of theω phase in Ti-Nb alloys were examined using high-resolution dark field electron microscopy and diffraction. The as-quenchedω morphology was shown to be based upon <111> rows of equiaxed particles which are 12 to 20Å in diameter and spaced 20 to 22Å apart. After short aging times the <111> row length increased, and eventually the isolated rows were replaced by clusters of rows. With continued aging the clusters increased in size and evolved into an ellipsoidal shape. The as-quenched Ti-18 at. pct Nb alloy exhibited sharpω reflections and {111} planes of diffuse intensity in reciprocal space. As the niobium content of the alloy increased theω reflections broadened, and in alloys containing 34 at. pct Nb or more, the 0001 and 0002ω reflections were shifted toward each other along an 〈0001〉 direction. The forbidden 10–10 and 20–20ω reflections were shown to be real in aged alloys, indicating that the aged form of theω phase is ordered.