Effects of strain rate on martensitic phase transformation in TRIP assisted multiphase steels studied in-situ with X-ray diffraction

In this work, the stability of austenite during tensile deformation of two industrially produced TRansformation Induced Plasticity (TRIP) assisted steels was studied using uniaxial tensile tests and analyzing the phase fractions in-situ using high-energy X-ray diffraction at three strain rates of 10⁻³, 10⁻¹, and 1 s⁻¹. The tests were carried out continuously without interrupting the experiment between the diffraction measurements and thus producing relevant data describing the effects of the adiabatic conditions at the higher strain rates. The two studied steels, the DH800 steel and the TRIP700 steel, had different retained austenite volume fractions and morphologies, with DH800 steel having a smaller austenite volume fraction and a finer morphology. Based on the results, the austenite in the DH800 steel is less stable than that in the TRIP700 steel. For both steels, the rate of the phase transformation was the highest at the strain rate of 10⁻¹ s⁻¹ and the lowest at the highest studied strain rate of 1 s⁻¹. The transformation rate was the highest after yielding and the phase transformation rate reduced considerably with plastic strain in both materials and at all strain rates.