The strain-rate sensitivity of the flow stress represents a crucial parameter for characterizing the deformation kinetics of a material. In this work a new method was developed and validated for determining the local strain-rate sensitivity of the flow stress at different plastic strains. The approach is based on spherical nanoindentation strain-rate jump tests during one deformation experiment. In the case of ultrafine-grained Al and ultrafine-grained Cu good agreement between this technique and macroscopic compression tests has been achieved. In contrast to this, individual spherical nanoindentation experiments at constant strain-rates resulted in unrealistically high strain-rate sensitivities for both materials because of drift influences. Microstructural investigations of the residual spherical imprints on ultrafine-grained Al and ultrafine-grained Cu revealed significant differences regarding the deformation structure. For ultrafine-grained Cu considerably less activity of grain boundary sliding has been observed compared to ultrafine-grained Al.

This paper aims to develop an integrated method to extract elastic–plastic parameters from a single instrumented spherical indentation curve. The expression of unloading work is chosen to be combined with the previous work [P. Jiang, T.H. Zhang et al, J. Mater. Res.24(3), 1045 (2009)]. An extensive numerical study was performed to examine the effectiveness of the method. Refitting Jiang's similarity solution based on the numerical study was also performed to simplify the form of the expression and improve the accuracy of the elastic–plastic parameters extracted. The results show that the error of our solution was less than ±5%. We also examined its sensitivity by assessing levels of artificial error introduced into the testing parameters used in the method. These results show that this method can provide reasonable estimates of the elastic–plastic parameters for most common metals.

Methods for identifying the hardening behavior of solids described by power law through indentation testing have been well developed. However, many important engineering materials deviate from the power law description significantly. After fitting the uniaxial curves of several typical materials, the three-parameter relationship proposed by Voce [E. Voce, J. Inst. Met.74, 537–562 (1948)] is chosen to describe the hardening behavior for its fitting performance and inherent simplicity. Based on the Voce law, an efficient method is formulated to extract the hardening data from a single spherical indentation curve. Improved identifying performance is manifested by applying the method to four metallic materials. It is validated that the present method has the ability to effectively identify the plastic properties of materials through spherical indentation testing.