Size effects and plastic deformation mechanisms in single-crystalline CoCrFeNi micro/nanopillars

Q Zhang and RR Huang and JX Jiang and TQ Cao and YP Zeng and JG Li and YF Xue and XY Li, JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS, 162, 104853 (2022).

DOI: 10.1016/j.jmps.2022.104853

As emerging and revolutionary alloy materials, high entropy alloys (HEAs) have been extensively studied because of their unique composition, microstructures and excellent mechanical properties and performances. However, there have been limited studies on the deformation behaviors and mechanisms of HEA single crystals at the micro/nanoscale. Here, we fabricated single-crystalline CoCrFeNi HEA pillars with typical orientations of <100>, <110> and <111> and diameters ranging from 272 nm to 1,253 nm and then conducted in situ uniaxial compression tests on these fabricated micro/nanopillars inside a scanning electron microscope. The in situ compression tests showed pronounced size effects on yield and flow stresses in the <100>-, <110>- and <111>oriented micro/nanopillars, i.e., both yield and flow stresses follow a scaling law with pillar diameter. The scaling exponents of <110>- and <111>-oriented micro/nanopillars are approximately -0.60, which is close to those of face-centered cubic (FCC) pure metallic micro/nanopillars. However, the <100>-oriented pillars, having a scaling exponent of -0.32 similar to-0.17, exhibit weaker size effects on yield and flow stresses compared with the <110>- and <111>-oriented micro/nanopillars. Combining observations and analyses using transmission electron microscopy, and large-scale atomistic simulations, we revealed that the nucleation and slip of full dislocations dominate the plastic deformation in the <110>- and <111>-oriented micro/nanopillars, while deformation twinning is a governing mechanism in the <100>-oriented micro/nanopillars. These underlying deformation mechanisms are responsible for the size effects of single-crystalline HEA micro/nanopillars. Large-scale atomistic simulations further reveal that twin nucleation in <100>-oriented pillars is initiated by the slip of pre-existing partial dislocations from an internal source. By considering the free energy change during partial slip, we used a theoretical model to predict the scaling law for the size effects of <100>-oriented micro/nanopillars. The predicted scaling exponent for the size effect of <100>-oriented micro/nanopillars is consistent with our experimental results. Our current study sheds light on the underlying deformation mechanisms in FCC HEA single crystals at the micro/nanoscale, which provides a guide for the design and fabrication of HEAs with high strength and remarkable plasticity.

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