In-Situ X-ray Diffraction Analysis of Metastable Austenite Containing Steels Under Mechanical Loading at a Wide Strain Rate Range

This paper presents and discusses the methodology and technical aspects of mechanical tests carried out at a wide strain rate range with simultaneous synchrotron X-ray diffraction measurements. The motivation for the study was to develop capabilities for in-situ characterization of the loading rate dependency of mechanically induced phase transformations in steels containing metastable austenite. The experiments were carried out at the DanMAX beamline of the MAX IV Laboratory, into which a custom-made tensile loading device was incorporated. The test setup was supplemented with in-situ optical imaging of the specimen, which allowed digital image correlation-based deformation analysis. All the measurement channels were synchronized to a common time basis with trigger signals between the devices as well as post-test fine tuning based on diffraction ring shape analysis. This facilitated precise correlation between the mechanical and diffraction data at strain rates up to 1 s⁻¹ corresponding to test duration of less than one second. Diffraction data were collected at an acquisition rate of 250 Hz, which provided excellent temporal resolution. The feasibility of the methodology is demonstrated by providing novel data on the kinetics of the martensitic phase transformation in EN 1.4318-alloy following a rapid increase in strain rate (a so-called jump test).