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Figure 1. A white spot in Alloy 718 revealed by the SNECMA etching technique. The SNECMA etch contains 10 ml HCL, 80 ml FeCI 3'6HP and 10 ml~O, and was applied for ten minutes at 11O°t-. INTRODUCTIONIt is approximately 30 years since H.L. Eiselstein' developed Inconel 718. Initially used by General Electric as a turbine disk material in an aircraft jet engine, this superalloy's most important properties at that time were resistance to creep and stress rupture. As new applications were unveiled in various industries, Alloy 718 gained wider acceptance because of its properties, fabricability and cost effectiveness. Beyond the original (and still largest) application in aircraft engines, it is currently being used as a generic alloy in nuclear and cryogenic structures and where environmental cracking resistance is required. Applications vary from rotating and static components in engines to high-strength bolting and fasteners, components for sour-gas wells and pipelines, nuclear reactors and space vehicles, marine shafting, and even high-temperature tooling for extrusion and shearing. As a result, Alloy 718 is the most widely used superalloy, accounting for 35% of all superalloy production.The nickel-iron base alloy is strengthened principally by ordered, body-centered tetragonal t" from niobium addition, and ordered, face-centered cubic t' from titanium and aluminum additions. It is perhaps the best alloy for applications below 1,200°F (650°C). Future applications will depend on reliability-oriented process developments and alloy modifications that could extend service tempera-

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The hot deformation behavior of Alloy 718 was characterized on the basis of the dynamic materials model and compression data in the temperature range of 900 to 1177°C and strain rate range of 0.005 to 5s'. The flow curves at all temperatures and strain rates showed little dependence on strains larger than 0.3. Constitutive equations were used to characterize the dependence of flow stress on strain, strain rate and temperature. A threedimensional distribution of strain rate sensitivity with strain rate and temperature revealed a decrease in sensitivity with an increase in strain rate and a decrease in temperature. A processing (power dissipation) map constructed from the dynamics materials model and the corresponding isoefficient contours at a strain of 0.5 exposed a domain of peak efficiency of 35 % at temperatures of 1132 to 1177°C and the strain rates of 0.05 to 0.5 set", which would be optimum parameters for hot working. These results are in good agreement with previous recrystallization-temperature-time maps reported in the literature. The activation energy for plastic flow in the 900 to 1177°C range was about 483 KJ/mole, and the constitutive relationship between flow stress and temperature compensated strain rate (Zener-Hollomon parameter) was found to be valid in the temperature range of 900 to 1177°C.