Twinning in CoCrFeNiMn high entropy alloy induced by nanoindentation

Abstract

The nanoindentation tests of CoCrFeNiMn high entropy alloy prepared by mechanical alloying route have been performed. Pop-ins on loading curves have been characterized by their indentation load, depth and length. Locations of indents within the grains, grain sizes and their orientation were obtained by the Scanning Electron Microscope/Electron Back Scattered Diffraction analysis. The stress state under the indenter was analyzed by Finite Element Method simulations using von Mises plasticity. The analysis has shown that pop-in events can be attributed to twin initiation as the pop-in length can be correlated with the twin thickness. The estimated critical twin shear stress corresponds to the one measured by the tensile test when the indentation size effect is taken into account. Fine grained HEA has very close Critical Resolved Shear Stress values for slip and twinning.

Introduction

High entropy alloys (HEAs) became one of the main research topics in materials science in the last decade and a half since the introduction of novel alloying strategy suggested by Cantor et al. [1] and Yeh et al. [2]. This new class of alloys represents a new concept in alloying in which several elements, often five and more, are mixed in equal or close atomic proportions (5–35 at.%). The concept assumes that for such alloys the entropic contribution to the total free energy overcomes the enthalpic contribution and, thus, stabilizes the solid solution. HEAs are able to combine different mechanical/physical properties, such as very high fracture toughness at cryogenic temperatures [3], high strength [4], significant resistance to fatigue, corrosion and oxidation [[5], [6], [7]], outstanding high-temperature strength [8], superconductivity [9], etc. Those remarkable properties of HEAs are results of their multi principal-element character leading to so-called ‘core effects’, that is, high configuration entropy, sluggish diffusion, large lattice distortion, and cocktail effect [10]. There are already several reviews that summarized the research in this field [11,12]. The most analyzed HEA is the equiatomic CoCrFeNiMn alloy (Cantor's alloy). This FCC (face-centered cubic) alloy is particularly interesting due to its mechanical properties as it combines high strength with extensive stable deformation. This behavior is explained by the TWIP (twin induced plasticity) effect that causes the “dynamic Hall-Petch” effect, which progressively introduces new interfaces in the microstructure, and thus strengthen the alloy and stabilize deformation [[13], [14], [15], [16], [17]]. The extended twinning is caused by low stacking fault energy [11,18,19]. General conditions for mechanical twins can be summarized as: large deformation strains, high strain rates, low temperatures and large grain sizes [20]. These findings were confirmed by the work of Laplanche et al. on Cantor's alloy. They observed nanotwins during the tensile test at temperature 77 K and 7% strain or at temperature 293 K and 25% strain [14].

The nanoindentation technique offers interesting possibilities to characterize local deformation processes of material at the micro/nanoscale. Reviews of nanomechanical tests on HEA's can be found in Refs. [21,22]. Several studies have been performed on Cantor's alloy [[23], [24], [25]]. The pop-ins were observed on indentation curve load–displacement and they were attributed to the bursts of dislocations that either nucleated or had been activated on defects. Studies on other types of HEAs (BCC-FCC) also showed the pop-ins that have been attributed to dislocations bursts [26,27]. Presented analyses have been performed on polycrystalline materials produced by melting with large grain sizes (60–120 μm) and low initial dislocations densities (10¹⁴ m⁻²) [24]. However, a numerical study has shown that twinning can be also activated during the nanoindentation of HEAs [28]. Such twin initiation was caused by increased stress level which can be achieved either by small indenter radius or by higher critical resolved shear stress values for slip due to decreased grain size and/or increased dislocation density [29]. These conditions can be fulfilled in HEAs prepared by mechanical alloying, i.e. high-energy milling of elemental metallic powders in ball mills.

The aim of this paper is to examine the hypothesis that twin nucleation and growth can occur during nanoindentation of mechanically alloyed Cantor's HEA via analysis of pop-ins observed on indentation curves. The nanoindentation testing is complemented with a scanning electron microscope (SEM) and an electron back scatter diffraction (EBSD) analysis of microstructure and finite element method (FEM) analysis of stress state inside the material. Combination of tested material and listed experimental and numerical techniques prove the twin nucleation and growth in fine grained high entropy alloys. Numerical analysis of experimental data also allows estimating mechanical properties of tested material such as critical resolved shear stress (CRSS) for slip and twinning.