[en] In this research, the plane stress fracture toughness of ultra-fine grained aluminum specimens produced through accumulative roll bonding (ARB) process was investigated for the first time. The specimens were produced successfully by the ARB process up to 7 cycles with the amount of 50% thickness reduction in each cycle at room temperature without using lubricant. The fracture toughness was evaluated for the annealed and different ARB cycles using ASTM E561 standard and compact tension specimens. Additionally, mechanical properties, tensile fracture surfaces and crystallite size of ultra-fine grained aluminum ARBed specimens were evaluated by uniaxial tensile tests, microhardness measurements, scanning electron microscopy and X-ray diffraction. By increasing the number of the ARB cycles, fracture toughness was increased and the maximum value of this parameter was achieved in the last cycle, which was approximately 25.4 MPam^1/2 that it increased by 155% higher than the annealed specimen. Results of X-ray diffraction demonstrated that by increasing the number of the ARB cycles, crystallite size decreased so that it reached 175 nm for the 7th cycle ARBed specimen from 1341 nm for annealed samples. Furthermore, by increasing the number of the ARB cycles up to the 7th cycle, tensile strength and microhardness of ultra-fine grained aluminum increased to 232 MPa and 51VHN, respectively. At first, the value of elongation decreased and then increased. The SEM results showed that ductile fracture mode with large dimples occurring in the annealed specimen, changed to shear ductile fracture with elongated sophomoric shear and fine dimples after the ARB process.

[en] In this study, for the first time, the effect of applied strains and volume percentage of components of layered composite on the mechanical properties and fracture toughness of Al/Mg were investigated experimentally. The multilayered Al/Mg were produced by the accumulative roll bonding (ARB) process. For the investigation, three Al/Mg composites with different volume percentages (25%, 50%, and 66.6%Al) at different applied strains (0.8–3.2) were produced. The experimental evaluation included microscopic examination by optical microscope imaging, uniaxial tensile test, and plane strain fracture toughness. As the applied strain for all three composites increased, plastic instability in the magnesium reinforcement intensified, but due to the low thickness of the Al layers compared to the Mg layer, uniform structure of Mg distribution in Al for all three composite was not achieved. Also, by adding Al layers to the primary composite, a lower shear strain was applied to the magnesium reinforcement, and instability intensity in the reinforcement layer decreased. For this reason, as Al layers increased, plastic instability diminished. By raising the exerted strain, the values of tensile strength increased, and by adding Al layers, the elongation increased. The maximum amount of tensile strength and elongation for each composite was achieved in the same ARB pass (last pass) and the highest values of UTS and elongation were reached to 384.1 MPa and 1.95% for Al25%Mg, respectively. However, the highest amount of fracture toughness for each composite was obtained in the different exerted strains and the maximum value of 41.4 MPa·m^1/2 was achieved for Al33.3% in the third pass. The present phenomena indicated that many factors such as higher Mg volume with higher energy absorption, plastic instability, thickness ratio, plastic instability, and value of applied strain affected the fracture toughness. In summary, the relationship between fracture toughness with applied strain and also with volume percent of Al was not always straightforward. It depends on other factors, such as how the reinforcement was distributed, the thickness of the layers, the workability, and the addition of aluminum. Also, the applied strain has a more significant effect on increasing fracture toughness in multilayered composite if they cause a uniform distribution of reinforcement particles in the field or continuity in the reinforcement layer. (paper)