Observation of low temperature superplasticity in an ultrafine grained AA6063 alloy

In the present work, mechanical properties and low temperature superplasticity behavior of a nano/ultrafine grained AA6063 alloy fabricated by accumulative roll bonding (ARB) was investigated. To that end, superplasticity was evaluated at various deformation temperatures and for different strain rates in an AA6063 alloy ARBed up to seven cycles. Results showed that the nano/ultrafine grained AA6063 alloy exhibited an excellent low temperature superplasticity (low peak stress of 40 MPa and maximum elongation up to 270%) at 300 degrees C & 350 degrees C and under the nominal strain rates of 5 x 10(-3)s(-1), 5 x 10(-2)s(-1), 5 x 10(-1)s(-1). The best condition of superplasticity, together with a stable microstructure, was obtained at 300 degrees C. Results also showed that at the deformation temperature of 250 degrees C, and under all strain rates, the elongation does not exceed 95%, hence for the absence of proper superplasticity at this temperature. It was found that although high amount of total elongation can be obtained at 350 degrees C and under low strain rates of 5 x 10(-2)s(-1), 5 x 10(-3)s(-1), the microstructural instability of these cases made them unsuitable for industrial applications. During superplastic deformation, low angle grain boundaries (LAGBs) gradually transformed into high angle grain boundaries (HAGBs) to sustain grain boundary sliding and to accommodate dynamic recovery. Constitutive equations were built, and strain rate sensitivity, as well as apparent activation energy variation were calculated. Using Zener-Hollomon parameter, the dominant hot deformation mechanism(s) in each deformation temperature was explained in conjunction with work hardening, dynamic recovery, dynamic recrystallization (DRX) and grain boundary sliding. As well, results indicated that the grain boundary sliding was the predominant deformation mechanism at 350 C. Finally, a truly superplastic regime was achieved at the temperature of 300 degrees C and for the strain rate of 5 x 10(-2)s(-)(1).