Tensile superplasticity in a nanocrystalline nickel aluminide

Tensile superplasticity has been demonstrated in a nanocrystalline nickel aluminide processed by severe plastic deformation at 650-725 degrees C and a strain rate of 1 x 10(-3) s(-1). The flow curves exhibit an unusually large strain hardening stage and very high flow stresses as compared to the flow stress of microcrystalline Ni,AI alloy of the same composition at typical superplastic strain rate and temperature. It is suggested that the origin of high flow stresses and extensive strain hardening stage is likely to be linked to the role of grain size on slip accommodation during grain boundary sliding. It is proposed that part of the high ductility in nanocrystalline materials processed by severe plastic deformation arises from the 'Exhaustion Plasticity' mechanism. (C) 1998 Elsevier Science S.A. All rights reserved.