Mechanistic Understanding of Stress Corrosion Cracking of Mg Alloys

Transgranular Stress Corrosion Cracking (TGSCC) is the inherent mode of SCC in Mg alloys; however, the mechanism for TGSCC is not properly understood. A mechanistic understanding of the influences of microstructure, environment and mechanical loading is in high demand due to the rapidly growing interest in Mg alloys for stressed automobile components. This thesis is aimed at identifying key issues pertaining to the mechanisms for TGSCC in Mg-Al alloys, such as to establish a basis for the development of a methodology for minimising SCC susceptibility by microstructural modification. The most commonly proposed mechanism for TGSCC of Mg alloys is Delayed Hydride Cracking (DHC) [1, 2, 3, 4]. A numerical model for predicting the maximum stress corrosion crack velocity for DHC in Mg alloys was developed, with input parameters taken from the literature. Crack velocities ~10-7 m/s were calculated. The result is consistent with measured values for SCC of Mg alloys in distilled water but slower than those for other aqueous environments, indicating that DHC is a possible mechanism. The SCC characteristics of the Mg-Al alloys AZ91, AZ31 and AM30 (in the form of cylindrical tensile specimens) in distilled water were evaluated using the Linearly Increasing Stress Test (LIST) and the Constant Extension Rate Test (CERT). Crack initiation was detected using the DC Potential Drop (DCPD) technique. The LIST and CERT techniques were compared with respect to fractography and the measurement of SCC parameters. During the LIST, fast fracture ensues a relatively short time after the threshold stress, σSCC, is reached. During the CERT, crack growth over a much longer time period is facilitated by a reduction in stress. Consequently, the LIST is typically shorter in duration, whereas the CERT produces a larger SCC fracture surface. Comparisons of the SCC characteristics and fractography of AZ91 (consisting of an α-matrix with extensive β-particles), AZ31 (consisting of an α-matrix with similar Al concentration to that in AZ91) and AM30 (consisting of an α-matrix with similar composition to AZ31, but with lower Zn concentration) showed that the mechanism for TGSCC of Mg-Al alloys is dependent on alloying and microstructure. SCC initiation in AZ31 and AM30 occurs by transgranular localised dissolution (tunnelling). The mechanism for SCC initiation in AZ91 is uncertain, but likely to involve fracture of β-particles close to the surface. The mechanisms for SCC propagation in AZ31 and AM30 involve microvoid coalescence and cleavage respectively. A model for SCC propagation in AZ91 has been proposed, involving: (i) H trapping by β-particles ahead of the crack tip; (ii) fracture of β-particles upon reaching some critical H concentration; (iii) release of trapped H due to the reduction in internal hydrostatic stress; and (iv) HE of the surrounding matrix as per the AZ31 α-matrix. AZ91 had a lower σSCC and was less sensitive to changes in strain rate than AZ31 and AM30. This was associated with the proposed role of β-particles as crack nucleation sites and internal H sources. The stress corrosion crack velocities for AM30 (~6x10-10 m/s) were slower than those for AZ91 (~7x10-9 m/s) and AZ31 (~4x10-9 m/s). This was attributed to the influence of Zn and second phase particles (which are more concentrated in AZ31) on H diffusivity in the α-matrix. The fractography for AZ91 specimens tested in distilled water at very slow strain rates and pre-charged in gaseous H2 was indicative of a mechanism involving hydride formation. This mechanism was postulated as: (i) nucleation and growth of MgH2 particles; (ii) sudden fracture through the MgH2 particles at some critical stress; and (iii) [1] E.I. Meletis, R.F. Hochman, Corrosion, 1984, vol. 40, pp. 39-45 [2] D.G. Chakrapani, E.N. Pugh, Metall. Trans. A, 1976, vol. 7, pp. 173-178 [3] A.J. Bursle, E.N. Pugh, in: P.R. Swann, F.P. Ford, A.R.C. Westwood (Eds.), Mechanisms of Environment Sensitive Cracking of Materials, Materials Society, London, 1977, pp. 471-481 [4] G.L. Makar, J. Kruger, K. Sieradzki, Corros. Sci., 1993, vol. 34, pp. 1311-1342 [5] B. Cox, in: Proc. Metallography and Corrosion Symp., IMS Annual Conf., Calgary, NACE, 1986, pp. 153-174 [6] B. Cox, J. Nucl. Mater., 1990, vol. 170, pp. 1-23 [7] K. Nuttall, A.J. Rogowski, J. Nucl. Mater., 1979, vol. 80, pp. 279-290 [8] G.K. Shek, M.T. Jovanovic, H. Seahra, Y. Ma, D. Li, R.L. Eadie, J. Nucl. Mater., 1996, vol. 231, pp. 221-230 [9] L.A. Simpson, Mechanical Behaviour of Materials, 1979, vol. 2, pp. 445-455

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