Effect of multiple strain-anneal cycles on the 1000° C creep behaviour of γ/γ′-α

Abstract

An investigation of the influence of multiple strain-anneal cycles on the 1000° C creep behaviour of the directionally solidified eutectic alloy γ/γ′-α has been undertaken. Cycles consisted of swageing at room temperature or 900° C by about 5 to 10% per pass followed by annealing at 900° C, and were repeated to total strains of approximately 10, 30 and 50%. Transmission electron microscopy (TEM) of strain-annealed materials revealed that three-dimensional dislocation networks were introduced into the γ′ matrix, but very little work remained in the α fibres. Constant-velocity creep testing indicated that all thermomechanical processing schedules improved the creep strength for strain rates ⩾ 2 x 10⁻⁶sec⁻¹; however only strain-annealing with a total of 13% work at room temperature (RT13) improved the behaviour at strain rates ∼ 2 x 10⁻⁷sec⁻¹. The advantage of RT13 processing over as-grown materials at lower strain rates was confirmed by constant-load creep testing. It was also shown that 900° C annealing slightly improves the 1000° C creep properties in comparison to as-grown alloys. TEM of crept materials indicated that the active creep mechanism had been changed from dislocation pile-ups at fibres in as-grown alloys to dislocations being stopped by sub-boundaries in the γ′ matrix for RT13.