Effect of Stress on Creep of Lamellar Near γ-TiAl

Abstract

ABSTRACTThe creep behaviour of a binary Ti-48%A1 intermetallic is presented as a function of stress for two fully lamellar microstructures. The two lamellar conditions differ in terms of the lamellar interface spacing and grain boundary morphology. Air cooling (AC) from the single phase a region causes planar grain boundaries and lamellar spacing of 90 to 130 nm, while furnace cooling (FC) causes interlocked lamellae along grain boundaries and 350 to 550 nm lamellar spacing. Monotonie and stress increment creep tests at 760°C indicate that the AC condition exhibits a lower mimimum creep strain rate at stresses between 105 MPa and 290 MPa. The stress exponent increases from ∼ 1 at low stress to ∼ 10 at high stress. Consecutive stress reduction creep tests indicate that the internal stress required for dislocation glide is higher for the AC condition. The results suggest that at low stress the creep rate is controlled by recovery mechanisms, while at high stress the creep rate is predominantly controlled by dislocation glide. It is postulated that at high stresses the lower creep rate of the AC condition, compared to the FC condition, results from the reduced lamellar interface spacing, which increases the internal stress required for dislocation glide.