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 t he near-gamma TiAl alloy Ti-49Al-1V. Specifically, microstructures wit h varying volume fractions of lamellar constituent were produced throu gh thermomechanical processing. Creep studies were conducted on these various microstructures under constant load in air at temperatures bet ween 760 degrees C and 870 degrees C and at stresses ranging from 50 t o 200 MPa. Microstructure significantly influences the creep behavior of this alloy, with a fully lamellar microstructure yielding the highe st 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 fract ions. Examination of the creep deformation structure revealed planar s lip of dislocations in the equiaxed gamma microstructure, while subbou ndary formation was observed in the duplex microstructure. The decreas e in creep resistance of the duplex microstructure, compared with the equiaxed gamma microstructure, is attributed to an increase in disloca tion mobility within the equiaxed gamma constituent, that results from partitioning of oxygen from the gamma phase to the alpha(2) phase. Di slocation motion in the fully lamellar microstructure was confined to the individual lamellae, with no evidence of shearing of gamma/gamma o r gamma/alpha(2) interfaces. This suggests that the high creep resista nce of the fully lamellar microstructure is a result of the fine spaci ng of the lamellar structure, which results in a decreased effective s lip length for dislocation motion over that found in the duplex and eq uiaxed gamma microstructures.