Transmission electron microscopy identification of a new Ti–Al–Fe intermetallic compound

Abstract

A new intermetallic phase has been discovered in the Ti–Al–Fe system. It was first found in a commercial ferrotitanium alloy and then confirmed in a specially prepared experimental alloy. Its crystal structure and chemical composition were investigated using various transmission electron microscopy (TEM) techniques, namely selected area and convergent beam electron diffraction, high-resolution lattice imaging, energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy (EELS). TEM investigations were complemented by other characterization techniques—scanning electron microscopy, electron-probe microanalysis with wavelength spectrometers, X-ray diffraction and scanning Auger microscopy, as well as quantitative metallography and microhardness measurements. The compound contains 68–74 at.% Ti, 20–24 at.% Fe and 3.5–7 at.% Al. Its crystal lattice is body-centred orthorhombic with periods a ≈ b and c/a ≈ 1.04. The lattice parameters are about four times larger than those of β-Ti (bcc with a = 0.325 nm) and of the TiFe intermetallic (CsCl-type structure with a = 0.298 nm). Apparently, the crystal unit cell of the compound is composed of 4 × 4 × 4 body-centred subcells and contains 128 atoms; the Pearson symbol of the crystal structure is oI128. The new phase was designated β2, thus hinting at its possible relation to β-Ti.