Issue34

S. Takaya et alii, Frattura ed Integrità Strutturale, 34 (2015) 355-361; DOI: 10.3221/IGF-ESIS.34.39 355 Focussed on Crack Paths EBSD-assisted fractographic analysis of crack paths in magnesium alloy S. Takaya Graduate School of Engineering, Gifu University, Japan Y. Uematsu, T. Kakiuchi Faculty of Engineering, Gifu University, Japan yuematsu@gifu-u.ac.jp A BSTRACT . Magnesium (Mg) alloys are attractive as structural materials due to their light weight and high specific strength. It is well known that Mg alloy has hexagonal close-packed (HCP) structure and only basal slip or twinning can operate during plastic deformation because critical resolved shear stresses of the other slip systems such as pyramidal or prismatic slips are much higher than the basal slip. Thus sometimes characteristic fracture surfaces are formed during stress corrosion cracking (SCC) or fatigue crack propagation (FCP) in Mg alloys, where many parallel lines are formed. These lines are different from so-called fatigue striations, because they are formed even under sustained load condition of SCC. Consequently, electron back scattered diffraction (EBSD) technique was applied on the fracture surface, and the formation mechanism of parallel lines was investigated. EBSD-assisted fractography had revealed that the characteristic parallel lines were formed due to the operation of basal slips, not twining. It is considered that hydrogen-enhanced localized plasticity (HELP) mechanism had been activated under corrosive environment. K EYWORDS . EBSD-assisted fractography; Magnesium alloy; Fatigue crack propagation; Stress corrosion cracking; Corrosive environment. I NTRODUCTION agnesium (Mg) alloys draw attention as light weight structural material because of light weight and high specific strength. To use Mg alloys for mechanical components, it is important to understand mechanical properties, such as fatigue strength and fatigue crack propagation (FCP) resistance. In addition, it is well known that the resistance of Mg alloys against corrosion is low comparing with other light weight alloys. Consequently, stress corrosion cracking (SCC) [1-5] and FCP [6, 7] behaviors under corrosive environment should be understood. The authors had performed SCC tests using AZ31 and AZ61 Mg alloys [4, 5], and indicated that crack propagation rates were faster in AZ31 than in AZ61. The fractographic analyses revealed that characteristic fracture surfaces were formed in which many parallel lines were recognized within grains. Furthermore, the authors had conducted FCP tests [6, 7] using T5-treated AZ61 under corrosive conditions and dry air, showing that FCP rates were accelerated by hydrogen diffusion near the crack tip [7]. The parallel lines were formed under corrosive conditions similar to SCC fracture surfaces, which were different from so-called fatigue striations. Such parallel lines were not so prominent on the fracture surfaces of FCP in dry M