V. Seetharaman et Sl. Semiatin, INFLUENCE OF TEMPERATURE TRANSIENTS ON THE HOT WORKABILITY OF A 2-PHASE GAMMA-TITANIUM ALUMINIDE ALLOY, Metallurgical and materials transactions. A, Physical metallurgy andmaterials science, 27(7), 1996, pp. 1987-2004
The hot deformation behavior, microstructure development, and fracture
characteristics of a wrought two-phase gamma-titanium aluminide alloy
Ti-45.5Al-2Nb-2Cr containing a fine, equiaxed microstructure were inv
estigated with special reference to the influence of temperature trans
ients immediately preceding plastic deformation. Specimens were soaked
at 1321 degrees C or 1260 degrees C, cooled directly to test temperat
ures of 1177 degrees C and 1093 degrees C, and upset under conditions
of constant strain rate and temperature. Plastic flow behavior and mic
rostructure evolution occurring in tests involving prior temperature t
ransients were compared with those occurring in specimens which were d
irectly heated to the test temperature and upset under identical defor
mation conditions. Flow curves associated with prior exposure at 1321
degrees C exhibited very sharp peaks and strong flow softening trends
compared to those obtained under isothermal conditions, i.e., involvin
g no temperature transients. During cooling from 1321 degrees C, the m
etastable Lu phase undergoes limited or complete decomposition into al
pha/alpha(2) + gamma lamellae, depending on the final temperature (117
7 degrees C/1093 degrees C). Subsequent hot deformation leads to parti
al globularization of the lamellae together with extensive kinking and
reorientation of lamellae. In contrast, isothermal deformation at 117
7 degrees C/1093 degrees C preserves the fine, equiaxed microstructure
, through dynamic recrystallization of the gamma grains. Cracking obse
rved in specimens deformed at 1093 degrees C and 1.0 s(-1) after expos
ure at 1321 degrees C has been attributed to the low rate of globulari
zation as well as the occurrence of shear localization. Plastic flow b
ehavior observed in this work is compared with that observed in severa
l single-phase and two-phase gamma titanium aluminide alloys in order
to identify mechanism(s) responsible for flow softening.