PNNL

Abstract

Austenite mechanical stability, i.e., retained austenite volume fraction (RAVF) variation with strain, and transformation behavior were investigated for two third generation advanced high strength steels (3GAHSS) in quasi-static uniaxial tension: a 1200 grade, two-phase medium Mn (10 wt%) TRIP steel, and a 980 grade, three-phase TRIP steel produced with a quenching and partitioning heat treatment. The Mn (10 wt%) TRIP steel deforms inhomogeneously via propagative instabilities (Lüdering and Portevin Le Châtelier-type bands) while the 980 grade TRIP steel deforms homogenously up to necking. The dramatically different deformation behaviors of these steels required the development a new in-situ technique that couples synchrotron X-ray diffraction measurement of RAVF within a material volume associated with the beam impingement area with surface strain measurement with stereo digital image correlation over the beam area. With the new technique, data is logged when bands of localized deformation propagate in and out of the beam area. Results from the new technique are compared from those with a more conventional approach wherein strains are measured over the entire gauge region while RAVF measurement is the same as that in the new technique. A determination is made as to the appropriateness of the different measurement techniques in measuring the transformation behaviors for steels with homogeneous and inhomogeneous deformation behaviors. Extension of the new in-situ technique to the measurement of austenite transformation under different deformation modes and to higher strain rates is discussed.