Abstract: Department of Mechanical and Industrial Engineering, University of Manitoba, Winnipeg, Manitoba,Canada, R3T 5V6(Received March 16, 2000)(Accepted in revised form April 21, 2000)Keywords: Superplasticity; Strain rate sensitivity index; Activation energy; SuperalloyIntroductionIN718 superalloy is reported [1–5] to exhibit superplasticity in the temperature range of 1173–1273 K.Superplastic behavior is characterized by high strain rate sensitivity index (m $ 0.3) and low activationenergy (Q), which is generally comparable with that for grain boundary diffusion. However, the stress(s) 2 strain («) curves obtained under superplastic condition are known  to exhibit strain hardeningand/or strain softening, especially, in the early part of deformation. Under such non-steady state flowcondition, the values of m and Q can vary with strain [6–7], making them as the non-unique parametersof the constitutive relationship  for high temperature deformation. Fortunately, the s-« curves for alarge number of superplastic materials  exhibit steady state flow behavior subsequent to the largeinitial transient stage of strain hardening/softening and concomitant microstructural evolution. Hence,the stress (s)–strain rate («) data obtained  beyond a certain strain level represent a reasonable steadystate condition. Therefore, the aim of the present work was to evaluate the m and Q parameters, of theconstitutive relationship for superplastic deformation of IN718 superalloy after such prestraining.ExperimentalIN718 superalloy of superplastic forming (SPF) grade was obtained in the form of 1.3 mm thick sheet.The analyzed composition of the alloy is given in Table 1.Tensile specimens with gage length and width of 20 mm and 5.3 mm, respectively, were machinedand tensile tested by Instron Universal Testing Machine. Test temperatures were controlled to anaccuracy of 61 K, and the specimens were soaked at the test temperature for 30 min. prior todeformation. Metallographic samples were prepared by mechanical polishing followed by and electro-lytic etching with 10% oxalic acid in water. Grain size was measured by mean-linear intercept methodand the error in the grain size data reported here were within 65%.