Issue34

S. Takaya et alii, Frattura ed Integrità Strutturale, 34 (2015) 355-361; DOI: 10.3221/IGF-ESIS.34.39 356 air without corrosion [7]. Fig. 1(a), (b) and (c) show typical fracture surfaces in SCC [5], FCP in corrosive environment and in dry air [6. 7], respectively. It should be noted that parallel lines are recognized under SCC and FCP in corrosive environment, while not under FCP in dry air. Mg alloy has hexagonal close-packed (HCP) structure, and it is known that only basal slip or twinning can operate during plastic deformation. That is because critical resolved shear stresses (CRSS) of the other slip systems such as pyramidal or prismatic slips are much higher than the basal slip. The characteristic plastic deformation behavior would be related to the formation of parallel lines on SCC or FCP fracture surfaces. However, the formation mechanism of parallel lines is not clear from crystallographic point of view. In the present study, electron back scattered diffraction (EBSD) technique was applied directly on the fracture surface, and the formation mechanism of parallel lines was discussed based on the EBSD- assisted fractography. Figure 1 : Typical fracture surfaces; (a) SCC under -1.4V cathodic potential, (b) FCP under -5V cathodic potential, (c) FCP in dry air. E XPERIMENTAL PROCEDURE he material used is T5-treated AZ61 cast Al alloy. The chemical composition (wt. %) is as follows, Al: 6, Zn: 0.67, Mn: 0.34, Si: 0.01, Cu: 0.002, Ni: 0.001, Fe: 0.004, Mg: balance. All SCC and FCP tests were performed using compact tensiotn (CT) specimen in accordance with ASTM E647. The detailed information of SCC and FCP test procedures is in the references [4, 5] and [6, 7], respectively. EBSD assisted fractography was performed by the following procedure. As schematically shown in Fig.2, when characteristic parallel lines are found on the fracture surface by SEM, small Vickers indentations are put near the lines. Subsequently, the fracture surface was polished by buffing followed by ion milling. The depth of polishing was identified from the change of the size in Vickers indentation. In all cases, the depth was identified as 10-20 μ m. EBSD analysis gives Euler angle of each grain. Schmid factors and the angles of slip lines on the specimen surfaces were calculated from Euler angles. Figure 2 : Schematic illustration of EBSD assisted fractography procedure. T 10  m 10  m 10  m (a) (b) (c)