High temperature deformation behaviour and microstructure of cast in-situ TiAl matrix composite reinforced with carbide particles

Abstract

High temperature deformation behaviour and microstructure were investigated in cast in-situ TiAl matrix composite reinforced with carbide particles. Samples of the in-situ composite were prepared by vacuum induction melting followed by centrifugal casting of Ti-44.6Al-7.9Nb-3.6C-0.7Mo-0.1B (at.%) alloy. Compression, tensile and creep tests were carried out up to a temperature of 950 °C in air. The initial microstructure of the test specimens consists of coarse primary Ti2AlC particles and the matrix composed mainly of γ(TiAl) with small amount of retained α2(Ti3Al) lamellae and fine secondary P-Ti3AlC and H-Ti2AlC precipitates. During compressive deformation, the work hardening of the composite results from an increment of dislocation density and formation of deformation twins in the matrix. The work softening is caused mainly by dynamic recovery (DRV) and dynamic recrystallization (DRX) of the matrix and to a less extent by fracture of some coarse Ti2AlC particles. The composite shows brittle tensile behaviour below 850 °C. The brittle-ductile transition temperature (BDTT) is determined to be between 850 and 900 °C at a strain rate of 1 × 10−4 s−1. The creep deformation curves exhibit a primary creep stage, which is directly followed by the tertiary creep at temperatures ranging from 760 to 900 °C and applied stresses from 120 to 300 MPa. The coarse primary and fine secondary carbide particles act as effective obstacles to dislocation motion. The kinetics of creep deformation is controlled by diffusion assisted climb of dislocations at strains corresponding to minimum creep rates.