On the occurrence of a nano-scale fcc phase in thin-foil specimens of titanium alloys

This paper reports an unknown phase occurring in the thin-foil specimens of a range of metastable titanium alloys prepared by conventional ion-beam milling. The phase demonstrates an apparent face-centered-cubic (f. c. c. ) lattice structure and exists in foil specimens prepared from alloys having an intermediate to high [Mo_eq](i. e. >11). This phase can be distinguished from the conventional hydride phase(s) that are known to arise in Ti alloys thinned with acid-based electrolytes, or the ion-beam-induced fee Ti phases due reportedly to interfacial stresses associated with deposited multilayered structures. Electron microscopy revealed that the f. c. c. phase assumes a nano-scale domain network rather than the isolated structural entities that would be expected from a nucleation and growth reaction. The f. c. c. phase maintains an orientation relationship with the (matrix such that [110]_β || [111]_fcc;the (002)_β and (220)_fcc, plane sets are related by a rotational angle δ, where δ varies from 0 to 2°. It is shown that the phase gives rise to diffuse scattering in the electron diffraction pattern, strongly resembling that exhibited by the athermal ω phase, yet with a dissimilar distribution of intensity maxima along the loci of the diffuse scattering. The effect of specimen preparation temperature on the occurrence of the f. c. c. phase is reported. The possibility of the phase originating from a lattice displacement controlled reaction is discussed.