Determination of the strain-rate sensitivity of ultrafine-grained materials by spherical nanoindentation
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he 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 ultrafinegrained Al.
I. INTRODUCTION
Ultrafine-grained (ufg) metallic materials often exhibit an outstanding combination of high strength and high ductility compared to their coarse-grained counterparts.1–3 This is usually attributed to the pronounced strain-rate sensitivity of the flow stress for these materials.4–6 For a given plastic strain, the strain-rate sensitivity m is usually expressed as m¼
dðln rÞ dðln e_ Þ
;
ð1Þ
where r and e_ represent the flow stress and strain-rate, respectively. m is generally obtained from macroscopic uniaxial tensile or compression tests, either via using distinct constant strain-rates (CSRs) in different tests or by implementing strain-rate jumps (SRJ) during individual deformation experiments. The latter approach typically delivers more reliable results, since the test is performed on the same sample with the identical experimental setup. However, all these methods only provide a strain-rate sensitivity at a macroscopic length scale. In Ref. 7, Maier et al. recently developed a method using nanoindentation strain-rate jump tests to determine the strain-rate sensitivity at a rather local length scale. In the case of nanocrystalline nickel good agreement between Contributing Editor: George M. Pharr a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.69
nanoindentation and macroscopic compression tests could be achieved. Compared to experiments at CSR, this technique has the decisive advantage to minimize the influence of thermal drift by applying the lowest strain-rate only toward the end of the test. Nevertheless, due to the geometrically self-similarity of the used Berkovich indenter geometry, this technique only provides the strainrate sensitivity at a given plastic strain of typically 8%. In contrast to this, spherical tips offer the possibility to determine material properties continuously as a function of strain, due to the lack of self-similarity of the contact, resulting in a number of applications of this nano
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