Post-uniform elongation and tensile fracture mechanisms of Fe-18Mn-0.6C-xAl twinning-induced plasticity steels

• Published in: Acta materialia 2017-06, Vol.131, p.435-444
• Main Authors: Yu, Ha-Young, Lee, Sang-Min, Nam, Jae-Hoon, Lee, Seung-Joon, Fabrègue, Damien, Park, Myeong-heom, Tsuji, Nobuhiro, Lee, Young-Kook
• Format: Article
• Language: English
• Subjects: Crack propagation; Deformation twinning; Fracture mechanism; Mechanical behavior; X-ray tomography
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/j.actamat.2017.04.011

The objective of the present study was to elucidate the complicated interrelationship between necking, post-uniform elongation (epu), strain rate sensitivity (SRS), fracture mechanism and Al concentration in Fe-18Mn-0.6C-xAl twinning-induced plasticity steels. Many tensile tests were conducted for in- and ex-situ observations of necking, fracture surfaces, crack propagation and the density and size of micro-voids with the assistance of a high-speed camera and X-ray tomographic equipment. The addition of Al increased epu, SRS and reduction ratios in dimension of the neck part of tensile specimens, and also changed fracture mode from quasi-cleavage to ductile fracture at the edge part. The quasi-cleavage surface of Al-free specimen was induced by edge and side cracks occurring along grain boundary junctions and twin boundaries within the edges and side surfaces where local deformation bands meet. The ductile-fracture surface of 1.5 %Al-added specimen was formed by the coalescence of micro-voids. While the side-to-middle crack propagation occurred in Al-free and 1 %Al-added specimens due to side cracks, the middle-to-side crack propagation was observed in 1.5 %Al-added specimen. The Al-free specimen had the larger size of the 20 largest voids compared to the 1.5 %Al-added specimen despite its lower void density and local strain due to the accelerated growth of voids near the tips of side cracks. Evaluating the negligible epu of Al-free specimen by SRS is not deemed to be reasonable due to its inappreciable necking and side cracks. The improvement of epu in 1.5 %Al-added specimen is primarily due to disappearance of edge and side cracks. [Display omitted]