Detection of several ω-phase morphologies in β-III Ti by small-angle neutron scattering and transmission electron microscopy

Two previously unreported morphologies of the ω phase in β-III Ti (Ti-11·09Mo-6·0Zr-4·5Sn-0·15O 2 ) have been detected by small-angle neutron scattering (SANS) and transmission electron microscopy (TEM). SANS iso-intonsity contour plots from more than 30 different ageing conditions were shown to fit into two distinct groups, one exhibiting pronounced biaxial anisotropy and the other approximately isotropic. The radially averaged intensity profiles of the anisotropic group showed relatively stationary interference peaks at low q (wave-vector) values on varying the time and temperature of ageing. TEM observations indicated the correspondence of this group to rod-like and spindle-shaped (fusiform) precipitates which have not previously been reported. In the isotropic group, a shift of the peak position to lower q with increasing ageing times and temperatures was noted. TEM observations on this group revealed the familiar ellipsoidal morphology. In contrast to the low aspect ratio of these particles (c :α≈2 : 1), rod-like precipitates attained ratios as high as 10 : 1. The origin of the morphological differences was traced to the heating rate in bringing the specimen to the ageing temperature. Abrupt heating was shown to produce rod-like and fusiform particles, whereas slow heating resulted in ellipsoids. It was shown that both rods and ellipsoids may be formed independently in the same specimen, and that both may simultaneously be observed by TEM. The crystal structure of the new morphologies, as detected by electron diffraction was found to be identical to that for the ellipsoidal precipitates, namely c.p.h.