V. Seetharaman et Sl. Semiatin, PLASTIC-FLOW AND MICROSTRUCTURE EVOLUTION DURING HOT DEFORMATION OF AGAMMA-TITANIUM ALUMINIDE ALLOY, Metallurgical and materials transactions. A, Physical metallurgy andmaterials science, 28(11), 1997, pp. 2309-2321
The hot workability of a near gamma titanium aluminide alloy, Ti-49.5A
l-2.5Nb-1.1Mn, was assessed in both the cast and the wrought condition
s through a series of tension tests conducted ever a wide range of str
ain rates (10(-4) to 10(0) s(-1)) and temperatures (850 degrees C to 1
377 degrees C). Tensile flow curves for both materials exhibited sharp
peaks at low strain levels followed by pronounced necking and flow lo
calization at high strain levels. A phenomenological analysis of the s
train rate and temperature dependence of the peak stress data yielded
an average value of the strain rate sensitivity equal to 0.21 and an a
pparent activation energy of similar to 411 kJ/mol. At low strain rate
s, the tensile ductility displayed a maximum at similar to 1050 degree
s C to 1150 degrees C, whereas at high strain rates, a sharp transitio
n from a brittle behavior at low temperatures to a ductile behavior at
high temperatures was noticed. Dynamic recrystallization of the gamma
phase was the major softening mechanism controlling the growth and co
alescence of cavities and wedge cracks in specimens deformed at strain
rates of 10(-4) to 10(-2) s(-1) and temperatures varying from 950 deg
rees C to 1250 degrees C. The dynamically recrystallized grain size fo
llowed a power-law relationship with the Zener-Hollomon parameter. Def
ormation at temperatures higher than 1270 degrees C led to the formati
on of randomly oriented alpha laths within the gamma grains at low str
ain levels followed by their reorientation and evolution into fibrous
structures containing gamma + alpha phases, resulting in excellent duc
tility even at high strain rates.