Mesoscopic origin of damage nucleation in dual-phase steels

Abstract

The damage modes in ferrite-martensite dual-phase (DP) steels include ferrite grain boundary (F/F) decohesion, ferrite/martensite interface (F/M) decohesion and martensite cracking. To explore the mesoscopic origin for each damage nucleation mode, we investigated ferrite-martensite dual-phase (DP) steels with different volume fractions of martensite, and characterized mesoscopic strain and stress distributions using microscopic-digital image correlation in SEM (μ-DIC) and finite element (FE) calculations. We performed in-situ tensile testing in a SEM and observed that the dominant damage nucleation mode changed from ferrite grain boundary (F/F) decohesion to ferrite/martensite interface (F/M) decohesion and finally to martensite cracking, with increasing martensite volume fraction (Vm) and varying martensite distribution. Mesoscale stress and strain analysis based on μ-DIC and FE calculations clearly reveal mesoscale origins of these damage modes: 1) F/F decohesion is caused by high strain which promotes accumulation of regular dislocations near the grain boundary and residual dislocations in the grain boundary; 2) F/M decohesion is attributed to high strain gradient which is associated with the accumulation of geometry necessary dislocations; 3) Martensite cracking stems from high stress that is partitioned to martensite. Our work demonstrated the pivot of microstructure engineering to improve the damage resistance of composite-like alloys consisting of soft and hard constituent phases, such as dual-phase steels.