Sulphide/phosphide precipitation associated with carbon saturation in Ti–15V–3Cr–3Sn–3Al–0.2C

Abstract

Titanium sulphide precipitates, Ti 3S, and sulphur-enriched titanium phosphide precipitates, Ti 3(P, S), have been observed in a Ti–15V–3Cr–3Sn–3Al–0.2C alloy with sulphur less than 10 wt ppm and phosphorus of about 50 wt ppm. No such precipitates have been found in standard low C (90 ppm) Ti-153. The titanium sulphide has a formula of Ti 3S and an orthorhombic structure with lattice parameters of a = 1.01 nm, b = 0.79 nm and c = 0.32 nm determined using TEM diffraction analysis. The sulphur-enriched titanium phosphide has a formula of Ti 3(P, S) with a P:S ratio of about 2:1. It has the same tetragonal structure as does the Ti 3P and similar lattice parameters. Titanium sulphide precipitates have a well defined orientation relationship with the beta matrix which is ( 0 1 0 ) T i 3 S //{1 3 0} β and [ 0 0 1 ] T i 3 S //〈0 0 1〉 β. Three variants of Ti 3S precipitates, perpendicular to each other, were observed and their habit planes in the beta matrix are {1 3 0}. Ti 3S has a morphology of a thin layer with a thickness along [0 1 0] axis of less than 50 nm and up to 50 and 20 μm in other two axes, respectively. The precipitation behaviour of the titanium sulphide during both continuous cooling and isothermal treatment is strongly affected by the carbon solution in the beta matrix.