4D imaging of void nucleation, growth, and coalescence from large and small inclusions in steel under tensile deformation

Abstract

• High spatial and temporal resolution synchrotron X-ray CT enabled 4D imaging of void nucleation, growth, and coalescence with unprecedented details. • Large inclusion particles in SA508 steel led to large and prolate voids which, while striking, did not affect crack formation. • The final fracture was induced by densely populated carbide precipitates that led to small but closely spaced voids which quickly coalesced to form micro-cracks. • The results challenge established theories on ductile fracture and could potentially lead to new strategies for steel making. Samples of SA508 grade 3 nuclear pressure vessel ferritic steel were subjected to tensile straining whilst being simultaneously imaged in 3D in real time using high resolution, high frame rate time-lapse synchrotron computed tomography (CT). This enabled direct observation of void development from nucleation, through growth to coalescence and final failure validating many inferences made post-mortem or by theoretical models, as well as raising new points. The sparse, large inclusions were found to nucleate voids at essentially zero plastic strain (consistent with zero interfacial strength); these became increasingly elongated with straining. In contrast, a high density of small spherical voids were found to nucleate from the sub-micron cementite particles at larger strains (> 200%) only in the centre of the necked (high triaxiality) region. An interfacial strength approaching 2100 MPa was inferred and soon after their nucleation, these small voids coalesce to form internal microcracks that lead to the final failure of the specimen. Perhaps surprisingly, under these conditions of generally low triaxial constraint the large voids are simply cut across and appear to play no significant role in determining the final failure. The implications of these results are discussed in terms of ductile fracture behaviour and the Gurson model for ductile fracture.