Experimental evaluation of the plane stress fracture toughness for ultra-fine grained aluminum specimens prepared by accumulative roll bonding process

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 MPam1/2 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