Surface hardening through oxygen diffusion in niobium: The defining role of stress inhomogeneity in tensile embrittlement

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Vol. 870; p. 144883
• Main Authors: Dhole, Ashish, Patra, Anirban, Gupta, Rohit Kumar, Gokhale, Amol, Samajdar, Indradev
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 12.04.2023
• Subjects: Microstructure; Modeling; Niobium; Oxygen; Plasticity; Surface hardening; Oxygen; Surface hardening; Microstructure; Modeling; Niobium; Plasticity
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2023.144883

Surface hardening, through oxygen diffusion, enforced significant tensile embrittlement of a commercial niobium (Nb) alloy (C-103). This was explored with microscopic digital image correlation and crystal plasticity finite element (CPFE) modeling. In particular, the presence of an oxygen-rich surface layer provided a gradient in hardness and elastic stiffness. These coincided with an increase in yield and tensile strength but a significant drop in ductility. The latter was reflected in the strain localization(s) and crack(s) on the hard gauge region, which ultimately led to a brittle failure. Full-field CPFE simulations were conducted on the actual microstructures. Our model, without crack and damage initiation, predicted extreme stress inhomogeneity during the tensile deformation. In brief, ∼5 times higher stress concentration was noted in the oxygen-rich hard surface grains. This appeared to be responsible for the subsequent crack initiation and embrittlement. •Surface hardening through oxygen diffusion in a commercial Niobium alloy.•Surface cracking, sub-surface strain localizations, and loss of tensile ductility.•Extreme stress inhomogeneities were predicted by full-field crystal plasticity finite element modeling.•∼5 times higher stress on the hard surface grains appeared responsible for the loss in tensile ductility.