In-situ investigation of crack initiation and propagation in roll-bonded five-ply ASS/Al/Mg/Al/ASS laminated composites during tensile test

Abstract

In this study, 304 austenitic stainless steel (ASS)– 1060 aluminum alloy (Al)– AZ31 magnesium alloy (Mg) five-ply laminated metal composites (LMCs) ASS/Al/Mg/Al/ASS were prepared by hot rolling and subjected to annealing at different temperatures. Both the lamellar interface between Mg-layer and Al-layer (Mg/Al) and the interface between Al-layer and ASS-layer (Al/ASS) bonded well, and no new phases were observed at the lamellar interfaces after hot rolling. However, intermetallic layers of Mg17Al12 and Al3Mg2 would form at the lamellar interface Mg/Al when laminated composites were annealed at 300 °C or above temperature. The mechanical properties of LMCs could be significantly improved by annealing at 200 °C for 1 h. With the annealing temperature further increasing up to 300 °C or more, however, both the tensile strength and the uniform plastic elongation decreased due to the presence of intermetallic layers and their premature rupture. For the LMCs in the rolled state or annealed at 200 °C, the tensile curves exhibited a two-step fracture; while for the LMCs annealed at 300 °C or 400 °C, the tensile curves presented a three-step fracture. In-situ tensile test showed that the fracture processes of the LMCs without intermetallic layers included necking, crack initiation at the interfaces Al/Mg and their partial delamination, crack propagation into Mg-layer along 45° angle, fracture of the Mg-layer and finally of the Al-layer and ASS-layer. The obvious necking of the LMCs before cracking and interlaminar delamination indicated a strong combination between the layers. However, no necking was observed before delamination but a premature failure of intermetallic layer occurred in the LMCs annealed at temperature of 300 °C or above. For the LMCs annealed at 300 °C, cracks first initiated and propagated along the intermetallic layer; while for the LMCs annealed at 400 °C, cracks first generated in crosswise in the thick intermetallic layer which then fractured into segments, resulting in an early delamination and fracture of Mg-layer.