MICROSTRUCTURE AND CREEP-BEHAVIOR OF AN ORTHORHOMBIC TI-25AL-17NB-1MOALLOY

Microstructural evolution during three heat-treatment schedules and th e terminal microstructures in an orthorhombic alloy of Ti-25Al-17Nb-1M o were observed and analyzed with optical microscopy, transmission ele ctron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The creep behavior of the alloy with three differen t microstructures (a coarse-lath, fine-lath, and fine equiaxed microst ructure) was studied over a temperature range of 600 degrees C to 750 degrees C and over a stress range of 150 to 400 MPa in air. The steady -state creep rates, apparent stress exponents, and apparent creep acti vation energies of the various samples have been determined. The resul ts show that creep behaviors in the alloy are strongly influenced by m icrostructure. The effect on creep by some of the microstructural feat ures, such as the multivariants within the coarse laths and the interf aces of the laths and the equiaxed grains, is also discussed.