Activation energy for superplastic deformation of IN718 superalloy
Introduction
IN718 superalloy is reported 1, 2, 3, 4, 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 activation energy (Q), which is generally comparable with that for grain boundary diffusion. However, the stress (σ) − strain (ε) curves obtained under superplastic condition are known [6] to exhibit strain hardening and/or strain softening, especially, in the early part of deformation. Under such non-steady state flow condition, the values of m and Q can vary with strain 6, 7, making them as the non-unique parameters of the constitutive relationship [8] for high temperature deformation. Fortunately, the σ-ε curves for a large number of superplastic materials [7] exhibit steady state flow behavior subsequent to the large initial transient stage of strain hardening/softening and concomitant microstructural evolution. Hence, the stress (σ)–strain rate (ε) data obtained [9] beyond a certain strain level represent a reasonable steady state condition. Therefore, the aim of the present work was to evaluate the m and Q parameters, of the constitutive relationship for superplastic deformation of IN718 superalloy after such prestraining.
Section snippets
Experimental
IN718 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 machined and tensile tested by Instron Universal Testing Machine. Test temperatures were controlled to an accuracy of ±1 K, and the specimens were soaked at the test temperature for 30 min. prior to deformation. Metallographic samples were prepared by
Results
Five tensile specimens, whose initial microstructure has been presented elsewhere [5], were deformed to true strains of 0.10, 0.30, 0.50, 0.70 and 1.00 at a true constant strain rate of 1 × 10−4 s−1 and the temperature of 1248 K. The grain sizes attained at these strain levels were 6.9, 7.9, 7.9, 8.0 and 8.2 μm, respectively. This suggests that the microstructure remained reasonably stable beyond the strain of 30%. Therefore, the prestraining of ε = 0.30 was assumed to provide steady state flow
Discussion
It is to note here that, in spite of the small change in temperature (1173–1248 K with an interval of 10 K), m was found to increase from 0.37 to 0.58 with temperature. The earlier investigations on superplasticity of this material exhibited the maximum value of m to be 0.90, with the most common values being between 0.50–0.63 1, 2, 3, 4, 5. Based on the stability of grain size obtained here beyond ε = 0.30, to which the tensile specimen was prestrained, the variation in m with temperature
Conclusions
The study of flow behavior of IN718 superalloy over the strain rate range of ∼1 × 10−5–3 × 10−4 s−1 and the temperature range of 1173–1248 K leads to the following conclusions.
- 1.
Log (σ)–log (ε) plot delineates predominantly superplastic region over the entire temperature range, with the strain rate sensitivity index increasing from 0.37 at 1173 K to 0.58 at 1248 K.
- 2.
The Arrhenius plot reveals two values of activation energy for superplastic deformation with the transition temperature being at 1218
Acknowledgements
The authors would like to thank the consortium of Manitoba aerospace industries and the Natural Science and Engineering Research Council of Canada for financial support. One of the authors (BPK) would like to express thanks for the award of an Association of Commonwealth Universities Development Fellowship and to the Indian Institute of Technology, Bombay, for granting leave to avail the same.
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Professor Kashyap was on leave from Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay, Mumbai 400 076, India.