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Pure Cu was deformed at different strain rates and temperatures, i.e. with different Zener Hollomon parameters (Z) ranging within InZ - 22 66, to investigate the effect or Z on its microstructures and mechanical properties. It was found that deformation twinning occurs when InZ exceeds 30, and the number of twins increases at higher Z. The average twin/matrix lamellar thickness is independent of Z, being around 50 nm. Deformation-induced grain refinement is enhanced at higher Z, and the mean transverse grain size drops from 320 to 66 nm when In Z increases from 22 to 66. The grain refinement is dominated by dislocation activities in low-Z processes, while deformation twinning plays a dominant role in high-Z deformation. An obvious increment in yield strength from 390 to 610 MPa was found in deformed Cu with increasing Z, owing to the significant grain refinement as well as the strengthening from nanoscale deformation twins. (c) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.