Issue34

S. Takaya et alii, Frattura ed Integrità Strutturale, 34 (2015) 355-361; DOI: 10.3221/IGF-ESIS.34.39 361 The parallel lines are generally formed on the fracture surfaces when SCC or FCP occurred under hydrogen charged conditions [4-7]. However, FCP fracture surface in dry air did not show those parallel lines. Thus it is considered that hydrogen charging is related to the formation of parallel lines. As mentioned above, parallel lines were formed due to the operation of basal slip lines, implying that basal slip operation was enhanced by hydrogen charging. It has been proposed that the local plasticity was enhanced by hydrogen charging in steel materials, which is known as hydrogen-enhanced localized plasticity (HELP) model [8]. In the HELP model, it is considered that the stress distribution induced by the solubility of hydrogen would result in the enhancement of dislocation mobility. Consequently, it is considered that HELP had operated even in Mg alloy and resulted in the formation of characteristic parallel lines in the fracture surfaces under hydrogen charged conditions. C ONCLUSION tress corrosion cracking (SCC) and fatigue crack propagation (FCP) fracture surfaces in AZ61 Mg alloy under hydrogen charged conditions were analyzed based on EBSD-assisted fractography. The conclusions are as follows. (1) Characteristic parallel lines were formed on the SCC and FCP fracture surfaces under hydrogen charged conditions, while the feature was not observed on the fracture surfaces of FCP in dry air. (2) EBSD-assisted fractography revealed that the parallel lines were formed due to the operation of basal slip systems. Twining was not recognized on the fracture surfaces. It is considered that the basal slip was activated under hydrogen charged condition. (3) Hydrogen-enhanced localized plasticity (HELP), known in steels, might have operated in Mg alloy. R EFERENCES [1] Songa, R.G., Blawert, C., Dietzel, W., Atrens A., A study on stress corrosion cracking and hydrogen embrittlement of AZ31 magnesium alloy, Mater. Sci. Eng., A399 (2005) 308-317. [2] Winzer, N., Atrens, A., Dietzel, W., Raja, V.S., Song, G., Kainer, K.U., Characterisation of stress corrosion cracking (SCC) of Mg–Al alloys, Mater. Sci. Eng., A488 (2008) 339-351. [3] Kannan, M.B., Singh Raman, R.K., Evaluating the stress corrosion cracking susceptibility of Mg–Al–Zn alloy in modified-simulated body fluid for orthopaedic implant application, Scripta Materialia, 29 (2008) 175-175. [4] Uematsu, Y., Kakiuchi, T., Nakajima, M., Stress corrosion cracking behavior of the wrought magnesium alloy AZ31 under controlled cathodic potentials, Mater. Sci. Eng., A531 (2012) 171-177. [5] Kakiuchi, T., Uematsu, Y., Nakajima, M., Nakamura, Y., Miyagi, K., Stress corrosion cracking behavior of wrought magnesium alloy AZ31 and AZ61 under controlled cathodic potentials, Proc., 13th Int. Conf. on Fracture (ICF13), Beijing, China, 2013. [6] Tokaji, K., Nakajima, M., Uematsu, Y., Fatigue crack propagation and fracture mechanisms of wrought magnesium alloys in different environments, Int. J. Fatigue, 31-7 (2009) 1137-1143. [7] Uematsu, Y., Kakiuchi, T., Nakajima, M., Nakamura, Y., Miyazaki, S., Makino, H., Fatigue crack propagation of AZ61 magnesium alloy under controlled humidity and visualization of hydrogen diffusion along the crack wake, Int. J. Fatigue, 59-2 (2014) 234-243. [8] Birnbaum, H.K., Sofronis, P., Hydrogen-enhanced localized plasticity—a mechanism for hydrogen-related fracture, Mater. Sci. Eng., A176 (1994) 191-202. S