Plastic-flow and microstructure evolution during hot deformation of a gamma titanium aluminide alloy

Abstract

The hot workability of a near gamma titanium aluminide alloy, Ti-49.5Al-2.5Nb-1.1Mn, was assessed in both the cast and the wrought conditions through a series of tension tests conducted over a wide range of strain rates (10 Ϫ4 to 10 0 s Ϫ1 ) and temperatures (850 ЊC to 1377 ЊC). Tensile flow curves for both materials exhibited sharp peaks at low strain levels followed by pronounced necking and flow localization at high strain levels. A phenomenological analysis of the strain rate and temperature dependence of the peak stress data yielded an average value of the strain rate sensitivity equal to 0.21 and an apparent activation energy of ϳ411 kJ/mol. At low strain rates, the tensile ductility displayed a maximum at ϳ1050 ЊC to 1150 ЊC, whereas at high strain rates, a sharp transition 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 coalescence of cavities and wedge cracks in specimens deformed at strain rates of 10 Ϫ4 to 10 Ϫ2 s Ϫ1 and temperatures varying from 950 ЊC to 1250 ЊC. The dynamically recrystallized grain size followed a power-law relationship with the Zener-Hollomon parameter. Deformation at temperatures higher than 1270 ЊC led to the formation of randomly oriented alpha laths within the gamma grains at low strain levels followed by their reorientation and evolution into fibrous structures containing ␥ ϩ ␣ phases, resulting in excellent ductility even at high strain rates.

Intergranular fracture of gamma titanium aluminides under hot working conditions

“…The size and volume fraction of the cracks decreased gradually with distance from the fracture zone. [10] Figure 4(a) also provides evidence for the occurrence of dynamic recrystallization along gamma grain boundaries. It is noteworthy that large voids/cracks are associated with boundaries between lamellar and gamma grains.…”

“…These tables list the peak stress/fracture stress data and qualitative information on the type of fracture observed at different test conditions. The significance of the parameter p (where ͌d d denotes the grain size) will be discussed subsequently in Section F. In view of the severe flow localization observed in the current set of specimens, [10] hot ductility represented by the reduction in area at the fracture region provides a reliable measure of hot workability. Both the cast and wrought materials showed a complex dependence of hot ductility on temperature and strain rate ( Figure 1).…”

Microstructure, creep, and tensile behavior of a Ti–21Al–29Nb(at.%) orthorhombic+B2 alloy

“…The B2 grain size could be significantly varied using supertransus heat treatments followed by water quenching. Under isothermal heat-treatment conditions, normal grain growth for single-phase materials is well described by the following empirical equation [10,11]:…”

Deformation and recrystallization behavior during hot working of a coarse-grain, nickel-base superalloy ingot material

“…This value of m is within the typical range for deformation controlled by the glide and climb of dislocations. [27] The apparent activation energy Q app was calculated from the expression Q app ϭ 2.303 (RS/m), [28] in which R is the gas constant, and S denotes the temperature sensitivity of the flow stress, S ϭ [Ѩ log 10 րѨ(1/⌻)P , ]. The value of S for the cast-and-homogenized Waspaloy was estimated to be 4023 K, resulting in Q app ϳ367 kJ/mol.…”