Neutron diffraction under in-situ multi-axial loading

Fig. 1: Neutron diffraction patterns with increasing the applied strain for (a) uniaxial loading and (b) equibiaxial loading showing the martensite formation is supressed under equibiaxial tension (Σ=0.67), compared to uniaxial tension (Σ=0.33).

Most of the existing studies, implement kinetic laws where the triaxiality factor, Σ, defined as the ratio of the hydrostatic stress to the von Misses equivalent stress, is used as a fit parameter affecting the kinetics of the martensitic transformation. However, our investigations using the novel biaxial rig at POLDI, a discrepancy has been observed whether the transformation is a monotonic function of stress triaxiality, as shown in Fig. 1.

Using the novel biaxial machine at POLDI, it was possible to study the transformation induced plasticity (TRIP) effect under equiaxial or non-proportional biaxial loading states. It was possible to unveil the effect of loading state on the TRIP behavior in different grades of technologically important steels. In specific, the effect of uniaxial/multiaxial loading on the TRIP effect was studied in situ with neutron diffraction, in two metastable austenitic stainless steels with different stacking fault energy (SFE) [1,2]. Moreover, the effect of different stress triaxiality factor on the TRIP effect was investigated experimentally and modelled on a low-alloyed Quenched and Partitioning (Q&P) TRIP-Bainitic Ferrite steel with dispersed metastable austenite particles [3].

These studies indicate that the stress triaxiality, which is associated with the growth mechanism of martensite, cannot solely account for the differences in the transformation kinetics between different loading states, but the loading state plays a key role on the nucleation mechanisms in these materials [4]. The nucleation mechanisms of martensite in these steels, is moreover, strongly dependent on the stacking fault energy of the material.