The effect of cold rolling on the creep behavior of INCONEL® alloy 718

The creep behavior of INCONEL (R) alloy 718 (IN 718) was investigated to identify processing-creep property relationships. The alloy was cold rolled (CR) to 0, 10, 20, 30, 40, 60, and 80% followed by annealing and aging. In addition, this alloy can be superplastically formed (IN 718SPF) to a significantly finer grain size and the corresponding microstructure and creep behavior were evaluated. The creep behavior was evaluated in the applied stress range of 300-758MPa and the temperature range of 638-670 degrees 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 (sigma(o)). Creep-rupture time (T-r) and elongation-to-failure (epsilon(f)) were also evaluated at 649 degrees C and 758MPa. The lowest sigma(o) values (300MPa <sigma(0)< 310MPa) were exhibited for the most severely CR microstructures (60%, 80%, and IN 718SPF), while the baseline 30%CR microstructure exhibited a significantly greater sigma(o) value (540MPa). The greatest sigma(o) values, 645 and 630MPa, were exhibited by the 20% and 30%CR conditions, respectively. The sigma(o) values were related to the overall creep resistance as the 20%CR condition exhibited the lowest secondary creep rates for a given applied stress (sigma(a)), while the samples CR to more than 40% exhibited the greatest creep rates. The values for the effective stress exponent suggested that the transition between the rate-control ling creep mechanisms was dependent on effective stresses (sigma(e)=sigma(a)-sigma(0)) and the transition occurred at sigma(e)=135MPa for a temperature of 638 degrees C. Increased CR tended to increase T-r and epsilon(f), and the 30%CR condition exhibited the greatest creep rupture properties. Overall, the 20%CR and 30%CR microstructures exhibited the greatest creep strength, while the most severely CR materials exhibited the poorest creep strength.