CREEP DEFORMATION IN NEAR-GAMMA TIAL .1. THE INFLUENCE OF MICROSTRUCTURE ON CREEP DEFORMATION IN TI-49AL-1V

The influence of microstructure on creep deformation was examined in the near-gamma TiAl alloy Ti-49Al-1V. Specifically, microstructures with varying volume fractions of lamellar constituent were produced through thermomechanical processing. Creep studies were conducted on these various microstructures under constant load in air at temperatures between 760 degrees C and 870 degrees C and at stresses ranging from 50 to 200 MPa. Microstructure significantly influences the creep behavior of this alloy, with a fully lamellar microstructure yielding the highest creep resistance of the microstructures examined. Creep resistance is dependent on the volume fraction of lamellar constituent, with the lowest creep resistance observed at intermediate lamellar volume fractions. Examination of the creep deformation structure revealed planar slip of dislocations in the equiaxed gamma microstructure, while subboundary formation was observed in the duplex microstructure. The decrease in creep resistance of the duplex microstructure, compared with the equiaxed gamma microstructure, is attributed to an increase in dislocation mobility within the equiaxed gamma constituent, that results from partitioning of oxygen from the gamma phase to the alpha(2) phase. Dislocation motion in the fully lamellar microstructure was confined to the individual lamellae, with no evidence of shearing of gamma/gamma or gamma/alpha(2) interfaces. This suggests that the high creep resistance of the fully lamellar microstructure is a result of the fine spacing of the lamellar structure, which results in a decreased effective slip length for dislocation motion over that found in the duplex and equiaxed gamma microstructures.