TENSION CREEP OF WROUGHT SINGLE-PHASE GAMMA-TIAL

The minimum strain rate, tertiary creep and damage behavior of a single phase gamma (gamma) TiAl alloy over the temperature range 760-900 degrees C at initial applied stress levels ranging from 32 to 345 MPa are reported. Two regions of creep deformation are identified. These consist of a region having a stress exponent of 6 and an activation energy of 560 kJ/mol and a region having a stress exponent of 1 and an activation energy of 192 kJ/mol. These are postulated to represent dislocation and boundary diffusion dominated creep respectively. The activation energy for dislocation creep is suggested to represent the energy to generate an appreciable density of dislocations in the minimum strain rate region. In the diffusional regime the minimum strain rates at 760 degrees C lie well below the predicted minimum strain rates when compared to the Coble creep equation. In addition, a natural transition from diffusional creep to glide dominated deformation occurs at 760 degrees C with increasing stress level. Tertiary creep of this material is found to correlate well with a two state variable approach. The initial stage of tertiary creep is dominated by an increase in the mobile dislocation density with increasing creep strain. Tertiary creep is found to obey a power law relationship with a stress exponent of 3 and an activation energy of 304 kJ/mol and is explained by the coupling of an increasing mobile dislocation density in the early stage of tertiary with constrained cavity growth in the late stage which leads to specimen failure.