A study of deformation behavior and stability of retained austenite in carbide-free bainitic steel during nanoindentation process

Different scale microstructures at different scales were obtained by one and two-step isothermal transformation. The deformation behavior and stability of retained austenite under the action of nanoindentation stress was investigated in these different micro-structures. The results demonstrate that the boundary fraction and density of the K-S relationship increase significantly after two-step isothermal transformation, promoting the nucleation of bainite and resulting in a refined microstructure. Changes in the nano -hardness as a function of indentation depth and morphology confirmed the quantity of RA transformation and the composite deformation/transformation behavior of blocky martensite/retained austenite (M/RA). A higher dislocation density is enriched in RA with higher KAM with the two-step isothermal process. This resulted in higher stability to delay the occurrence of the pop-in phenomenon. The stability of RA is highly dependent on the crystal orientation, with (001) oriented RA having an increased propensity to undergo mechanical transformation to martensite, with additional pop-ins generated during loading.(c) 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).