ON THE CREEP DEFORMATION OF A CAST NEAR GAMMA TIAL ALLOY TI-48AL-1NB

The steady-state creep deformation behavior of a cast two phase gamma TiAl alloy having the composition Ti-48Al-1Nb (at.%) has been studied. Tension creep tests using the stress increment technique (sigma-1 < sigma-2 < sigma-3) were conducted over the temperature range of 704-850-degrees-C at constant initial applied stress level of 103.4-241.3 MPa. The activation energy for creep over the temperature and stress regime of this study varied from 317.5 kJ/mol (137.8 MPa) up to 341.0 kJ/mol (206.8 MPa) with an average value of 326.4 kJ/mol. This is well within the range of values previously measured for gamma TiAl alloys where creep controlled by volume diffusion has been suggested as rate controlling. The stress exponents measured were 5.0 at 704-degrees-C, 4.9 at 750-degrees-C, 4.7 at 800-degrees-C and 4.46 at 850-degrees-C. Using the activation energy of 326.4 kJ/mol, the temperature compensated steady-state creep rate was plotted against log stress with all temperatures collapsing onto a single line having a slope equal to 4.95. Using conventional creep analysis, this value of the stress exponent can be taken as suggestive of dislocation climb controlled power law creep as the operative deformation mechanism within the stress and temperature regime of the present study. The boundary separating the lamellar grains in two phase gamma TiAl alloys having the duplex microstructure may be a very important aspect of this microstructure with respect to creep deformation resistance. The interlocking gamma/alpha-2, laths making up these boundaries are expected to be very resistant to grain boundary sliding which may contribute to creep deformation in the dislocation creep regime. Finally, some previous observations along with a comparison of the creep behavior of the Ti-48Al-1Nb alloy to that of a Ti-50.3Al binary have been discussed in terms of the pre-exponential constant A in the power law creep equation. TiAl alloys having similar stress and temperature dependencies but differing steady-state strain rates over comparable stress-temperature regimes may be rationalized on the basis of differing power law creep constants which may reflect differences in stacking fault energies.