Investigation of Thermomechanical Coupling in Inconel 718 at Homologous Temperatures of 0.2 and 0.5

Abstract

A study was conducted to investigate the temperature dependence of thermomechanical coupling in Inconel 718 (IN718). IN718 was selected as a model material due to deformation being predominantly accommodated by planar slip. Split-Hopkinson (or Kolsky) tension bar experiments were conducted at a nominal strain rate of 750 s\(^{-1}\) at room temperature and \(450\,^{\circ }\)C, representing homologous temperatures (\(T_H = T/T_{melt}\)) of \(T_H=0.2\) and \(T_H=0.5\), respectively. During deformation, specimen gauge sections were imaged with a high-speed infrared camera. Using one-dimensional wave analysis, the transient heat conduction equation, and temperature-dependent specific heat capacity values, the temperature rise as a function of plastic strain was used to calculate plastic work, thermal work, and the plastic work to heat conversion efficiency, commonly known as the Taylor–Quinney coefficient (TQC). As expected, a significant reduction in plastic work was observed during testing at elevated temperatures. The temperature rise due to plastic deformation was observed to be lower at room temperature compared to elevated temperature experiments. It is reported here for the first time that the TQC is a temperature-sensitive quantity. At \(T_H=0.5\), a nearly complete conversion of plastic work to heat was observed (TQC \(\approx\,1.0\)). Under ambient conditions of \(T_H = 0.2\), a much lower efficiency TQC \(\approx\,0.4\) was observed.