DIC-based experimental study of fracture toughness through R-curve tests in a multi-layered Al-Mg (LZ91) composite fabricated by ARB

Abstract

Adding Li to Mg, dual-phase Mg-Li with very low density and high formability at room temperature was produced and used as a reinforcement phase for fabricating a three-layered metal matrix Al-Mg composite through accumulative roll bonding (ARB) process. In this study, due to the inherent nature of the ARB process, which is associated with high strain, and low strength of the magnesium-lithium alloy, the fracture toughness test and energy absorption were used in addition to the tensile test to observe the interaction effect of the increasing strength and the reducing elongation, which were caused by the ARB. By preparing the compact tension (CT) specimens and conducting R-curve tests, the fracture behavior of the processed composite in various ARB cycles (primary sandwich and four ARB cycles) was investigated. Meanwhile, a Digital Image Correlation (DIC) system was also employed to measure the variation of crack growth, crack opening, and corresponding forces and increase the accuracy and quality of the results. The surrounded area under the force-displacement of crack opening curves was measured and calculated as the toughness and absorbed energy. Tensile strength and total elongation of the processed composite improved gradually, 275.7% and 38%, respectively, compared to the primary phase as the number of cycles increased and the thickness of the layers decreased. Plastic instability was more prominent in the last cycles because of the increasing trend of imposed strains and the reduction in thickness which reached their highest amounts at the final step. Although obtained results confirmed the general improvement in fracture toughness of the ARBed composite, its value varied slightly from one cycle to another. The highest values were achieved for total energy absorption and crack opening at maximum force after the second ARB cycle. In contrast, the maximum amount of fracture toughness, 29.08 MPa m1/2, was belonged to the last cycle of the process. Dimples and micro-cavities, seen in both SEM images of Al and Mg layers, resulted in a ductile failure.