Notch sensitivity in pure nickel determined by two mechanisms of hydrogen-assisted crack propagation: sub-/main-crack coalescence versus main-crack growth

<p>The effects of short/long notches on crack initiation and extension were studied under static tensile loading and electrochemical hydrogen charging in pure nickel. In the hydrogen-charged smooth specimen, multiple cracks were initiated on grain boundaries after significant plastic deformation and were subsequently arrested by crack blunting in the interior of neighboring grains. With the assistance of sub-cracks on specimen surface, quasi-cleavage (QC) crack propagation occurred soon after, causing the final failure. The introduction of a notch did not change the crack initiation site (grain boundary). However, crack extension exhibited significant notch-length dependence. A 2.5 mm notch caused QC crack propagation without sub-cracks initiation, because the notch root acted as a strongly preferential site for crack extension. However, with a shorter notch, sub-cracks were necessary to motivate the main crack propagation behavior and to provide the sufficient plasticity which could not be met in the initial stage from the notch root even with hydrogen charging. Thus, the crack extension from notch root is determined by two mechanisms: 1. Intergranular (IG) sub-crack initiation and subsequent coalescence with main crack; 2. Transgranular (TG) main crack extension. The former shows notch insensitive due to a constant threshold of unstable crack extension, while the latter shows notch sensitive since the unstable crack extension is promoted by the increase in notch length.</p>