Cyclically induced grain growth within shear bands investigated in UFG Ni by cyclic high pressure torsion
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Thomas Leitner Department of Materials Physics, Montanuniversität Leoben, Leoben 8700, Austria
Pradipta Ghosh, Bo Yang, and Reinhard Pippan Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Leoben 8700, Austria (Received 1 May 2017; accepted 13 June 2017)
Structural instabilities of nanocrystalline and ultrafine-grained (UFG) materials have been recognized as a major challenge during cyclic loading, especially in the low cycle fatigue regime. Although a severe deterioration of the mechanical properties has been reported during cyclic deformation, quantification of the softening portion solely due to grain coarsening was not possible. It will be demonstrated that cyclic high pressure torsion (CHPT) is a versatile method to enable direct measurement of the impact of grain coarsening on cyclic softening, as failure of the sample is prevented. Here, CHPT experiments have been performed on 99.99% UFG nickel. Grain coarsening similar to conventional uniaxial fatigue experiments was observed and could be studied up to large cyclic accumulated macro strains of 50. The correlation of electron back scatter diffraction images with microhardness measurements facilitated quantification of the cyclic softening as a consequence of grain growth for the very first time. Further, structural investigations revealed distinctly enhanced grain coarsening within shear bands. Thus, the cyclic strain seems to be the most important parameter controlling mechanically driven boundary migration during cyclic loading at low homologous temperatures.
I. INTRODUCTION
Extensive grain refinement of metallic structures by severe plastic deformation (SPD) yields an enormous enhancement of the strength of materials. This improvement of the mechanical properties is also reflected in the high cycle fatigue (HCF) performance, making this material class ideal for frequent load reversal applications. Unfortunately, this holds true only for cases where the plastic strain amplitude is low. Under low cycle fatigue (LCF) conditions i.e., large plastic strain amplitudes, nanocrystalline (NC) or ultrafine-grained (UFG) structures become unstable due to their large amount of grain boundary area.1 Various types of structural instabilities have been reported for static, dynamic and cyclic loading situations, when higher strains were applied. For instance, during the SPD process itself two recovery processes take place, namely grain boundary migration2 and triple junction motion.3 These processes continuously modify the microstructure in a way that a further refinement is impeded, even when the strain during SPD is increased. Contributing Editor: Yuntian Zhu a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.273
It seems that these processes not only determine the saturation grain size of a particular SPD process, but also the grain shape.4 The design of structural components is dictated by the mechanical properties of a material, such as the yield, fracture or fatigue strength. Changes
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