Abstract:
The grain size, creep, and elevated temperature tensile behavior of INCONEL alloy 718 (IN 718) were investigated to identify processing-microstructure-property relationships. The alloy was cold rolled (CR) 0-80% followed by annealing at 954°C or 1050°C and then by aging using the traditional aging schedule for this alloy. This alloy can be superplastically formed (IN 718SPF) to a significantly finer grain size and this condition was also evaluated. The creep behavior was evaluated in the applied stress range 300-758 MPa at a temperature of 638°C. Constant-load tensile creep experiments were used to measure the values of the steady-state creep rate and the consecutive load reduction method was used to determine the values of backstress, σo. Tensile tests were also performed at 650°C. For the 954°C annealed samples, The 20%CR and 30%CR samples exhibited the greatest σ0 values, 645 and 630 MPa, respectively, while the 60%CR and 80%CR conditions exhibited the lowest σ0 values, which were less than half those of the highest σ0 values. The creep resistance after 60-80%CR was significantly worse than conventionally processed and 10%CR conditions, which is expected based on the finer grain size of the heavily CR samples. For the 1050°C annealed samples, the SPF sample exhibited the greatest σ0 value, 630 MPa, while the 0%CR condition exhibited the lowest σ0 value, 450 MPa. The backstress values for most of the 1050°C annealed samples fell in a narrow band, 550-590 MPa. For most samples, the backstress was over 50% of the yield stress at 650°C. Overall, the 20%CR and 30%CR conditions exhibited the best elevated temperature properties in the 954°C annealed condition while the IN 718SPF exhibited exceptional strength and creep resistance in the 1050°C annealed condition, on par with the 20-30%CR 954°C annealed samples. A transition in the effective stress exponent occurred independent of annealing temperature at an effective stress of approximately 135 MPa. The data suggested that the creep mechanism was dependent on the effective stress and indicated that diffusional creep or grain boundary sliding may be active at effective stresses lower than 135MPa.
