Influence of the High-Temperature Mechanical Loading Mode on the Flow Behavior and Microstructural Evolution of a Nb-Stabilized Austenitic Stainless Steel

Abstract

The present work addresses the effect of three high-temperature deformation modes, namely, torsion, uniaxial compression, and plane strain compression, on flow stress vs. strain curves, as well as on post-dynamic and static recrystallization of 316Nb austenitic stainless steel. Using a Hosford criterion instead of the classical von Mises criterion enables a unified description of the stress–strain curves obtained under different loading modes. This work also revealed that the loading mode had no significant effect on post-dynamic and static recrystallization phenomena. The amount of niobium atoms in solid solution might be preponderant in the control of recrystallization of 316Nb.

Appendices

Appendix A: Estimation of the Hosford Exponent Using a Self-consistent Viscoplastic Model

Polycrystalline models can be used to simulate yield surfaces, and therefore, to compare flow curves using different yield criteria. In this work, we used the Berveiller–Zaoui self-consistent polycrystalline model [A1] with the Méric–Cailletaud crystal plasticity model [A2, A3]. These models are implemented in the Z-set software [A4]. No crystal plasticity parameters are available from literature for 316Nb or even for 316-type steels at elevated temperatures considered in this work. Therefore, we used the parameters given in [A5] for a 316L(N) steel at ambient temperature.

100 grains were chosen according to an isotropic orientation distribution function to represent the initial isotropic polycrystal. Pure compression and pure shear deformation at strains up to $\varepsilon =0.05$ were considered to represent initial stages of uniaxial compression and torsion, respectively. Then, post-processing was done using Tresca, von Mises, and several values of the Hosford criterion exponent. Flow curves are given in Figure A1 for Hosford exponents n = 8 and n = 20 (Hosford 20 thereafter). One can see that for very small amounts of strain (lower than 0.005), the prediction from Hosford 8 is very close to the uniaxial compression curve but it departs for larger values of strains. On the contrary, Hosford 20 gives slightly different predictions at very small strains but enables very good agreement between uniaxial compression and torsion curves at somewhat larger strains ($\varepsilon =0.05)$. Therefore, based on these numerical simulations, we choose the Hosford exponent $n=20$ to describe experimental flow curves.

Fig. A1

Effect of the equivalent criterion chosen on the flow curves from a 100-grain simulation (a) Hosford exponent n = 8; (b) Hosford exponent n = 20. In addition to colors, one specific symbol was added at the final point of the curves for easier reference to the testing conditions, except for compression

References for Appendix A

• 1. M. Berveiller and A. Zaoui: J. Mech. Phys. Solids, 1978, vol. 26, pp. 325–44. https://doi.org/10.1016/0022-5096(78)90003-0

• 2. L. Méric, P. Poubanne, and G. Cailletaud: J. Eng. Mater. Technol., 1991, vol. 113, pp. 162–70. https://doi.org/10.1115/1.2903374

• 3. L. Méric, P. Poubanne, and G. Cailletaud: J. Eng. Mater. Technol., 1991, vol. 113, pp. 171–82. https://doi.org/10.1115/1.2903375

• 4. Z-set, material and structure analysis suite. http://www.zset-software.com/, accessed on 21. May, 2021.

• 5. Y. Guilhem: Numerical investigation of the local mechanical fields in 316L steel polycrystalline aggregates under fatigue loading. PhD thesis, École Nationale Supérieure des Mines de Paris, France, 2011 (in French). https://pastel.archives-ouvertes.fr/pastel-00732147