Strain Rate Sensitivity of Superplastic Inconel 718

Abstract

The strain rate sensitivity, m, of a superplastic-quality (SPF) INCONEL 718 alloy has been measured in the temperature and strain rate ranges of 925–1000°C and 10⁻⁴–10⁻² s⁻¹ using both the classical method of instantaneous strain rate changes and a new method based on the continuous response to a sinusoidal strain rate input. The novel method furnishes continuous information from the whole stress–strain curve on the dependence of m on strain rate transients.
In agreement with other recent studies, our results show that the maximum m values of the 718 alloy, although interesting enough for industrial superplastic forming, are lower than those shown by other superplastic materials for typical aeronautical application. A texture analysis indicates that crystallographic slip does not merely act here as an accommodation mechanism for grain boundary sliding. It significantly contributes to the total plastic strain, being thus responsible for the relatively low values observed for the strain rate sensitivity m.