Twinning in CoCrFeNiMn high entropy alloy induced by nanoindentation
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.
Section snippets
Material
The one phase equi-molar CoCrFeNiMn high entropy alloy was prepared by mechanical alloying from pure elemental powders (Sigma Aldrich, USA) with purity ≥99.9% (Co), ≥99.7% (Ni) and ≥99% (Cr, Fe, Mn). The 100 g blend of powders (each element was added with nominal 20 at%) was mechanically alloyed using a planetary ball mill (Pulverisette P-6, Fritsch). The alloying was carried out in a hardened steel vial with steel balls for 24 h under vacuum at 350 rpm rotational speed of the main disc. The
Experiment
The nanoindentation tests were performed using the nanoindentation device Anton Paar NHT2 (Anton Paar TriTec, Peseux) with diamond Berkovich indenter. The shape of the indenter tip was characterized by the methodology described in Refs. [31]. Loading was applied with controlled constant force increase to maximal load for 30 s. Measurements were done at room temperature and air atmosphere. Two sets of measurements were performed. The first one contains a 10 × 10 matrix (100 indents) with maximal
FEM model
The FE method was used to estimate the stress level under the indenter. The 2D axisymmetric model was created to represent the system. The set up is shown in Fig. 4. The indenter shape was approximated using data measured on the real indenter. The FEM indenter radius at a given depth was calculated from the measured projected area [31]. The indenter is modeled as a purely elastic diamond. The HEA material behavior contains isotropic elasticity and von Mises plasticity. The elastic constants
Results
The longest pop-ins that were identified on each indentation curves were characterized by the indentation depth and load at which they initiate and by their length. The size of each grain that was indented was estimated as a diameter of the equivalent circle with the same area. The size distribution of indented grain together with their crystallographic orientation are shown in Fig. 5a and b). Because of the small grain size, many indents hit an area close to the grain boundary. Thus, an
Discussion
The pop-ins during nanoindentation of HEA are commonly observed both in FCC and BCC lattices [23,24,26,33]. These pop-ins occur at the elastic-plastic transition and they are related to the bursts of dislocations that nucleate inside material or are activated at defects. However, these tests were performed on casted materials with large grain size and low dislocation density. In our case, the plastic transition occurs much earlier as can be analyzed combining elastic stress distribution
Conclusions
The nanoindentation tests of CoCrFeNiMn high entropy alloy prepared by mechanical alloying were performed. The analysis of pop-ins was done using SEM, EBSD techniques complemented with FE and phenomenological models. The main results can be summarized in the following points:
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The pop-ins during nanoindentation of fine grained HEA can be related to the twin initiation.
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The pop-in length on the indentation curve can be related to the thickness and the length of the induced twin.
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The critical
CRediT authorship contribution statement
Filip Siska: Conceptualization, Investigation, Formal analysis, Methodology. Jaroslav Cech: Investigation, Methodology. Petr Hausild: Investigation, Formal analysis, Methodology. Hynek Hadraba: Investigation, Formal analysis, Methodology. Zdenek Chlup: Investigation, Formal analysis, Methodology. Roman Husak: Investigation. Ludek Stratil: Investigation.
Acknowledgment
The authors would like to acknowledge Ing. Pavla Roupcová Ph.D. for XRD analysis. This work was supported by the Czech Science Foundation via the project 17-23964S and European Structural and Investment Funds via the project CZ.02.1.01/0.0/0.0/16_019/0000778 (Centre of Advanced Applied Sciences).
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2021, Journal of Alloys and CompoundsCitation Excerpt :Compared with casting, PM has the advantages of lower cost, higher dimension accuracy and mass production capability [24]. In this approach, HEA powders were firstly produced by mechanical alloying (MA), and bulk HEA materials were subsequently fabricated using different consolidation methods, such as hot pressing sintering (HPS) [17,19,25–27], spark plasma sintering (SPS) [28–33], hot isostatic pressing (HIP) [1,34–36], or microwave vacuum sintering [37]. When SPS was used to consolidate the HEA powder, materials with a single FCC phase could be obtained, which exhibited good mechanical properties [28,29]; while consolidation of the HEA powders by the hot-pressed sintering process might produce precipitates, normally M23C6 (M=Cr, Fe, Co) carbides or σ phase, in the HEA matrix [20,33,34,36], which proved to have adverse effects on the mechanical properties of the HEA samples [35].