Local Deformation and Fracture Behavior of High-Strength Aluminum Alloys Under Hydrogen Influence
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Local Deformation and Fracture Behavior of HighStrength Aluminum Alloys Under Hydrogen Influence KAZUYUKI SHIMIZU, HIROYUKI TODA, KENTARO UESUGI, and AKIHISA TAKEUCHI The local deformation and fracture behavior of high-Zn Al-Zn-Mg(-Cu) alloys under hydrogen influence were investigated by in situ tests through synchrotron X-ray tomography. Intergranular and quasi-cleavage fractures were induced by hydrogen, and strain localization by the presence of cracks was not observed by 3D strain mapping. These results suggest that the strain localization at the crack tip is smaller than the measurement limit of 3D strain mapping. The average crack-tip-opening displacements, which are one of the crack driving forces specified by fracture mechanics, directly measured from the tomographic slice were 0.14 and 0.23 lm for intergranular cracks and quasi-cleavage cracks, respectively. The crack driving forces of the intergranular and quasi-cleavage cracks were small. The local deformation behavior at the crack tips was analyzed based on fracture mechanics. The local deformation field of the crack tip, which was characterized using the Rice–Drugan–Sham (RDS) solution rather than the Hutchinson–Rice–Rosengren (HRR) solution, was located within 20 lm of the crack tip, and its size was limited. The results of this work clarify that the intergranular and quasi-cleavage crack growths are caused by small driving forces; however, this behavior is not perfectly brittle, accompanying local deformation at the crack tip. https://doi.org/10.1007/s11661-019-05304-y Ó The Minerals, Metals & Materials Society and ASM International 2019
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INTRODUCTION
AL-ZN-MG(-CU) alloys are high-strength aluminum alloys that are widely used in aircraft and rolling stock, which require high strength-to-weight ratios. When the Zn or Mg content is increased to improve strength, susceptibility to stress corrosion cracking (SCC) increases, leading to delayed fracture.[1–3] Therefore, the SCC susceptibility of commercial alloys is suppressed both by adding trace elements such as Cr and Zr and by heat treatments such as overaging and retrogression-and-reaging treatments; the latter is a three-step heat treatment consisting of pre-aging, retrogression, and re-aging.[4] The SCC of Al-Zn-Mg(-Cu) alloys, particularly for crack growth behavior, has been speculated to be strongly related to hydrogen embrittlement (HE).[1,5] The SCC and HE of aluminum alloys have been investigated by a slow strain rate test (SSRT).[6–8] The degradation of mechanical properties
KAZUYUKI SHIMIZU and HIROYUKI TODA are with the Department of Mechanical Engineering, Kyushu University, Motooka 744, Nishi-ku, Fukuoka-City, Fukuoka, 819-0395, Japan. Contact email: [email protected] KENTARO UESUGI and AKIHISA TAKEUCHI are with the Japan Synchrotron Radiation Research Institute, Sayo-gun, Hyogo, 679-5198, Japan. Manuscript submitted December 28, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
by hydrogen depends on the strain rates in the SSRT. Kuramoto et al. reported that the true fracture strain
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