Characterisation of stress corrosion cracking (SCC) of Mg–Al alloys

Stress corrosion cracking (SCC) of the Mg–Al alloys AZ91, AZ31 and AM30 in distilled water has been characterised using constant extension rate tests (CERTs) and linearly increasing stress tests (LISTs). AZ91 consists of an α-matrix with extensive β-particles, whereas AZ31 and AM30 consist only of an α-matrix with an Al-concentration similar to that in AZ91. The presence of β-particles in AZ91 was associated with: (i) a lower threshold stress, σSCC, for AZ91 (55–75 MPa) relative to AZ31 (105–170 MPa) and AM30 (130–140 MPa); and (ii) a different SCC initiation mechanism for AZ91 relative to AZ31 and AM30. The SCC velocity, Vc, for AM30 (3.6 × 10−10 to 9.3 × 10−10 m/s) was lower than that for AZ91 (1.6 × 10−9 to 1.2 × 10−8 m/s) and AZ31 (1.2 × 10−9 to 6.7 × 10−9 m/s). This was attributed to the influence of Zn and second phase particles, which are more concentrated in AZ31, on the diffusivity of H in the α-matrix.