Flow and fracture of austenitic stainless steels at cryogenic temperatures: experiments and modeling.

Austenitic stainless steels are widely used as structural materials in high-field superconducting magnet systems due to their favorable mechanical properties over the whole temperature operating range of the magnets. They are nevertheless susceptible to martensitic transformations, especially at cryogenic temperatures. In particular, they are prone to strain-induced $\alpha^{\prime}$-martensitic transformation, which affects the material’s properties, causes a change in volume and additional strain hardening, and results in the ferromagnetic behavior of the resulting bi-phase material. In this doctoral thesis, the microstructural evolution and flow and fracture behavior at cryogenic temperatures of two austenitic stainless steels frequently employed in superconducting magnet system applications have been characterized. This research is approached from both an experimental and a modeling perspective. Tensile experiments under quasi-static loading conditions have been performed to obtain the stress-strain characteristics of the material and the kinetics of the martensitic evolution. The content of martensite has been determined at different levels of deformation using magneto-inductive measurements, electron backscatter diffraction, and quantitative light optical micrography. Additionally, Mode I elastic-plastic fracture toughness tests have been performed, and four different methods have been used for the calculation of the crack-growth resistance J − R curves. Moreover, a constitutive model to describe the mechanical behavior of the evolving bi-phase material based on a Hill-type incremental formulation has been developed. Two different mean-field homogenization schemes have been considered: Mori-Tanaka and Self-Consistent. A stress integration algorithm based on the return mapping scheme and the backward Euler scheme has been developed to implement the two versions of the constitutive model in the commercial finite element software ABAQUS. The predictions of the homogenization schemes have been compared with unit-cell finite element simulations with an explicit description of the martensite inclusions and the austenite matrix. In addition, numerical simulations of tensile specimens subjected to different initial temperatures have been carried out for the transforming bi-phase material and have been compared to the experimental tensile results. The study presented in this doctoral thesis is one of the most thorough efforts made to date to experimentally characterize and numerically model the behavior of austenitic stainless steels that exhibit solid-state phase transformation.