Accession Number : ADA330577

Title :   Creep Resistance of Gamma TiAl Microstructures.

Descriptive Note : Final rept. 1 Jul 96-30 Jun 97,

Corporate Author : CARNEGIE-MELLON UNIV PITTSBURGH PA DEPT OF METALLURGICAL ENGINEERING AND MATE RIALS SCIENCE

Personal Author(s) : Pollock, Tresa M.

PDF Url : ADA330577

Report Date : SEP 1997

Pagination or Media Count : 3

Abstract : The developtment of microstructure and its influence on creep properties has been studied in two Ti-48Al-2Cr-2Nb alloys. The addition of 0.9 atomic % Mo to the Ti-48Al-2Cr-2Nb composition results in the formation of the ordered B2 phase. The presence of this phase along with a small amount of alpha2 at grain boundaries was found to effectively limit grain growth at 1125 deg C during heat treatments that produce equiaxed gamma microstructures. The gamma -> alpha transformation produces a2 plates with several orientation variants within gamma grains during subsequent anneallng of the equiaxed gamma microstructures below the a-transus. Formation of this a2 morphology results from rapid up-quenching and this structure persists through annealing, cooling, and creep testing. Differences in minimum creep rates for several microstructures containing varying amounts multi or single variant gamma/a2 grains are shown to be minimal. The presence of Mo has also resulted in improved creep resistance in equiaxed gamma, and gamma + a2 + B2 structures as compared to similar microstructures in the Ti-48Al-2Cr-2Nb alloy. Deformation during creep at 760 deg C at stresses between 200 and 400 MPa occurs by a combination of twinning and dislocation glide without recrystallization, resulting in power-law stress exponents in the range of 6 to 9. On!y minimal strain path dependence of the minimum creep rate was detected in a comparison of creep rates in stress jump, stress drop and single stress tests.

Descriptors :   *MICROSTRUCTURE, *RESISTANCE, *DEFORMATION, *TITANIUM ALUMINIDE, *CREEP TESTS, ANNEALING, STRESS TESTING, MOLYBDENUM, COOLING, HEAT TREATMENT, ALUMINUM, NIOBIUM, CHROMIUM, GRAIN GROWTH, RECRYSTALLIZATION, TITANIUM ALLOYS, TRANSFORMATIONS, TWINNING(CRYSTALLOGRAPHY), CREEP STRENGTH.

Subject Categories : Inorganic Chemistry
      Properties of Metals and Alloys
      Crystallography

Distribution Statement : APPROVED FOR PUBLIC RELEASE