Analytical electron microscopy of C-free and C-containing Ti–15–3

Abstract

Optical microscopy, analytical scanning and transmission electron microscopy have been used to interpret the influence of C on the ageing response of Ti–15–3 (Ti–15V–3Al–3Sn–3Cr (wt.%)). It has been found that the addition of carbon reduces the extent of oxygen segregation to grain boundaries and thus reduces the tendency for grain boundary alpha to form during ageing. The ageing response and the scale of precipitation at 600 °C have been found to depend on the heating rate used. The as-quenched microstructure is characterised by striations typical of pre-martensite-type contrast with a spacing of about 20–25 nm. Diffraction patterns in as-quenched samples show diffuse scattering in addition to the maxima associated with this large spacing. The striations and diffuse scattering anneal out at ageing temperatures above 400 °C. Contrary to earlier work no evidence has been obtained for omega in as-quenched or aged samples. The alpha precipitation is on a finer scale than can be accounted for by the carbides or by the dislocations punched out by the carbides. This conclusion, taken together with the absence of any evidence for omega, leads to the view that the presence of carbon in solution, rather than the carbides, limits diffusion of oxygen and provides additional nucleation sites for alpha – perhaps through vacancy–carbon–oxygen complexes.