Incipient plasticity and dislocation nucleation of FeCoCrNiMn high-entropy alloy
Introduction
Instrumented nanoindentation has been extensively used to study incipient plasticity in crystals in recent years [1], [2], [3], [4], [5], [6]. Experimentally, the incipient plasticity is marked by a distinct displacement burst (or pop-in) on the load–displacement curve which typically occurs at an indentation depth of about 10–20 nm or less. Since the indented volume is small (of the order of nanometers), and thus probably devoid of dislocation [7], the pop-in event is therefore attributable to the nucleation of dislocations in the crystal [5], [8], [9], [10], [11], [12], [13]. The pop-in stress is generally of the order of 1/30μ–1/5μ, where μ is the shear modulus, corresponding to the “ideal” or “theoretical” strength of the crystal [14], [15]. The activation energy for the nucleation of dislocation was also observed to depend upon the crystalline structure and, specifically, is lower in face-centered cubic (fcc) metals than that in body-centered cubic (bcc) metals since nucleation of partial dislocations is energetically favored in fcc structures in contrast to bcc where nucleation of full dislocations is favored [16].
Structural defects leading to dislocation nucleation have been discussed by Mason et al. [1]. Based on a stress-assisted, thermally activated process, they proposed a statistical approach to analyze dislocation nucleation in Pt and successfully extracted the activation energy and volume of incipient plasticity during nanoindentation. From the obtained data, they concluded that heterogeneous rather than homogeneous dislocation nucleation was the prevalent process.
In the past decade, high-entropy alloys (HEAs) have attracted much research interest because of their unusual structural properties [17], [18], [19], [20]. Traditionally, in designing an alloy, the major component is selected based on a specific property requirement and other alloying components are added in small amounts to achieve secondary properties without sacrificing the primary one. By contrast, HEAs are multicomponent alloys containing several components (>5) in equal atomic proportions. However, it is of particular interest to note that, despite containing a large number of components, HEAs actually exhibit a significant degree of mutual solubility to form a single fcc or bcc phase, instead of complex ordered intermetallics [21]. As a result of the different atomic sizes and chemical bonds of the constituent elements, HEAs possess a highly distorted lattice structure [22]. Dislocation lines in HEAs due to structural distortion are not straight, but wiggled. Thus elastic as well as plastic deformation in HEAs is expected non-traditional. In this study, we applied instrumented nanoindentation to study incipient plasticity in a FeCoCrNiMn HEA with a simple fcc structure.
Section snippets
Experimental
The material used in this study was prepared by arc-melting a mixture of constituent metals (purity > 99 wt.%) with a nominal composition Fe20Co20Cr20Ni20Mn20 (at.%) in a Ti-guttered high-purity argon atmosphere. The ingot was remelted at least four times to ensure homogeneity before it was drop-cast into a mold. The as-cast ingot was further rolled by 70% reduction in thickness.
Rectangular samples were sliced from the rolled plate, ground and polished to a mirror finish of 0.01 μm. The samples
Nanoindentation tests at room temperature
Typical load–displacement (P–h) curves at shallow indentation are presented in Fig. 2a. Each curve is displaced along the x-axis and only the loading portions are shown for clarity. The P–h curves are from three different grains and represent the general trend observed for all other grains. These P–h curves are observed to exhibit a displacement burst (or pop-in) at nearly constant load. The deformation before pop-in is elastic, as confirmed by a reversible loading–unloading behavior slightly
Effect of surface oxide
It has been recognized that pop-in can be triggered by the breakthrough of a passivated thin oxide layer on the sample surface [4], [30], [31]. In such a case, the pop-in depth at high temperatures would be larger than that at low temperatures since a higher-temperature exposure would result in more oxidization. In addition, in the current study, samples were held at the test temperature for more than 60 min before testing to ensure thermal equilibrium. However, as indicated in Fig. 6, the
Conclusion
Instrumented nanoindentation experiments were performed on a HEA FeCoCrNiMn at different temperatures (22–150 °C) and loading rates (25–2500 μN s−1) to examine the nature of incipient yielding. Experimental data were analyzed and further compared with existing models for dislocation nucleation. The following conclusions are drawn.
- 1.
Indentation pop-in, which marks the onset of yielding, is observed at every temperature and loading rate conducted in this study.
- 2.
The observed pop-in phenomenon is not
Acknowledgements
This work was supported by the National Science Foundation under Contract DMR-0905979. Instrumentation for the nanoindentation work was jointly funded by the Tennessee Agricultural Experiment Station and UT College of Engineering. Z.P.L. acknowledges the financial support from the National Natural Science Foundation of China under the Grant Nos. 50901006, 51010001 and 51001009, the Program of Introducing Talents of Discipline to Universities under the Grant No. B07003, and the Program for
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