Strain heterogeneities between phases in a duplex stainless steel. Comparison between measures and simulation

Abstract

Many studies aimed at understanding the strain and damage micro-mechanisms in low-cycle fatigue in duplex stainless steels. Different methods allow the evaluation of the strains on the microstructural scale, some are surface techniques (AFM, EBSD, DIC) and others are bulk techniques (TEM, XRD). Recently, some works have analyzed the plastic strain sharing between austenitic and ferritic phases using one or several of these experimental methods (TEM, XRD, AFM, EBSD, etc.). The present work proposes to analyze the strain fields measured by digital image correlation (DIC) on the microstructural scale at the surface of a specimen cyclically strained in low-cycle fatigue, and to compare it with that obtained by a polycrystalline bi-phased microstructure numerical calculation.A cyclic mechanical test was carried out at room temperature on a forged duplex stainless steel. Before the test, a surface crystallographic orientation measurement was performed by EBSD analysis in the central part of the specimen. A specific in-situ optical microscopic device equipped with a CCD camera was used in order to observe and take digital images of the specimen surface on the microstructural scale, at different number of cycles. The displacement and strain fields of a representative surface zone were obtained by DIC using the specific software CORRELIQ4. The strain distribution and the strain sharing between austenitic and ferritic phases were analyzed from mechanical fields obtained between images taken at different number of cycles. In parallel, a quasi-2D microstructure finite element calculation of a small zone experimentally analyzed was performed. Numerical microstructure and grain orientations were obtained from EBSD measures. Two crystal plasticity constitutive laws were used to model the behaviors of austenitic (FCC) and ferritic (BCC) grains, respectively. The mechanical fields and the strain distributions per phase were compared to those obtained by DIC on the first hysteresis loop.