The Constant-Stress, Constant-Heating-Rate Test: A Novel Method for Characterizing Transient Mechanical Behavior of Metallic Materials

A novel mechanical test for characterizing the transient, on-heating mechanical behavior of metallic structural materials was developed. It comprises the continuous heating of a specimen at a constant rate while loaded under a constant true stress. Key features of the approach consist of quantifying the thermal expansion ("thermal compliance") and mechanical compliance of the load train, modeling the plastic flow and onset of localized necking of the test specimen, and the simulation of concurrent phase changes for multi-phase alloys. From such experiments and analyses, the instantaneous, true plastic strain and strain rate as a function of temperature can readily be determined and used to establish constitutive parameters (such as the apparent activation energy, Q(app), for plastic flow) and temperatures at which rapid deformation may occur. The test technique was validated for three alloys, Ti-7Al, Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti6242S), and superalloy 718. For the single-phase-alpha titanium alloy, Ti-7Al, semi-log plots of strain rate vs inverse temperature (ln (epsilon)over dot vs 1/T) were linear, and yielded a value of Q(app) comparable to that for self diffusion of hcp titanium. The corresponding plots for Ti6242S with either a fully equiaxed or duplex (equiaxed/colony alpha) microstructure were also linear, but exhibited higher values of Q(app), a finding rationalized using simulations of the kinetics of dissolution of equiaxed a as a function of temperature and the so-called "mechanical contribution" to the activation energy of two-phase alloy systems. Last, ln (epsilon)over dot vs 1/T plots of 718 were either linear (in the solution-treated condition) or approximately bilinear (in the solution-treated-and-aged condition). These observations were interpreted with the aid of simulations of precipitation and dissolution of gamma '' and gamma' during continuous heating and the retarding effect on straining of a threshold (back) stress associated with the precipitates. (C) The Minerals, Metals & Materials Society and ASM International 2021