Creep Behavior of Fine Grain, Low Carbon Allvac 718

Abstract

The aerospace materials community has demonstrated considerable interest in achieving improved properties in Alloy 718 by finer grain microstructures and better cleanliness. Its composition has been investigated in the past, in combination with standard and novel fine grain processing techniques, to improve low cycle fatigue capabilities. Lower carbon content is one compositional change that has been evaluated and debated for a number of years. Investigators are in agreement that tensile strength is not effected by low carbon, and that improved carbide distribution provides an opportunity for better fatigue properties. However, the effect on creep properties remains unresolved.Production ingots of standard (0.025 wt% C) and low carbon (0.010 wt% C) Allvac 718 were processed to 5-inch diameter forged bar. Processing and heat treatment was aimed at producing fine grain microstructure and maximizing tensile properties. Two process techniques were used; 1) a tine grain process employing super and sub 6-solvus temperature forging, 2) a mini-grain process which involves an intentional &phase precipitation and sub 8-soivus forging. All material was direct age heat treated. Preliminary creep properties showed no difference between low carbon Allvac 718 and standard carbon Allvac 718. Supporting previous studies, the material displayed high tensile strength at room and elevated temperatures.Creep and low cycle fatigue testing were not completed by the deadline for the written presentation.