Nanomechanical Behavior of Nanocrystalline Hign-Entropy Alloys

Abstract

High entropy-alloys (HEAs) that contain at least five principal elements have received considerable attention in the structural materials community since the concept of HEA was first introduced in 2004. The interesting nature of HEAs, including their simple structure, severe lattice distortion and sluggish diffusion, leads to many promising mechanical properties such as high strength-ductility combination, good tribological properties, and excellent resistance to environmental degradation. Over the last decade, much effort has been made to find new compositions of HEAs exhibiting the most desirable mechanical properties as their properties are usually determined by the constituent elements. On the other hands, it is well accepted that the mechanical performance of a material can be optimized by controlling their microstructure. In this context, achieving a nanocrystalline (nc) structure in the material has attracted much attention, since nc metals and alloys exhibit superior mechanical properties in comparison with their coarse grained (cg) counterparts due to the large portion of grain boundaries. Thus, one can expect the maximized mechanical performance by incorporating both the advantages of HEA and nc structure. However, despite the significant advances in the processing, characterizing, and structural applications of HEAs, only a few studies have been carried out for exploring the mechanical properties of nc HEAs. With these in mind, in this thesis, I attempt to extend our current knowledge over the mechanical behavior of nc HEA. Main topics examined are divided into three, as follows. First, a CoCrFeNiMn HEA, in the form of a face-centered cubic (fcc) solid solution, was processed by high-pressure torsion (HPT) to produce a nc HEA. Significant grain refinement was achieved from the very early stage of HPT through 1/4 turn and an nc structure with an average grain size of ~40 nm was successfully attained after 2 turns. The feasibility of significant microstructural changes was attributed to the occurrence of accelerated atomic diffusivity under the torsional stress during HPT. Nanoindentation experiments showed that the hardness increased significantly in the nc HEA during HPT processing and this was associated with additional grain refinement. The estimated values of the strain-rate sensitivity were maintained reasonably constant from the as-cast condition to the nc alloy after HPT through 2 turns, thereby demonstrating a preservation of plasticity in the HEA. In addition, a calculation of the activation volume suggested that the grain boundaries play an important role in the plastic deformation of the nc HEA where the flow mechanism is consistent with other nc metals. Transmission electron microscopy (TEM) showed that, unlike conventional fcc nc metals, the nc HEA exhibits excellent microstructural stability under severe stress conditions. Second, time-dependent plastic deformation behavior of nc and cg CoCrFeMnNi HEAs was systematically explored through a series of spherical nanoindentation creep experiments. Indentation creep tests revealed that creep deformation indeed occurs in both cg and nc HEAs even at room temperature and it is more pronounced with an increase in strain. The creep stress exponent, n, was estimated as ~3 for cg HEA and ~1 for nc HEA and the predominant creep mechanisms were investigated in terms of the values of n and the activation volumes. Through theoretical calculations and comparison of the creep strain rates for nc HEA and a conventional face-centered-cubic nc metal (Ni), the influence of sluggish diffusion on the creep resistance of nc HEA was analyzed. In addition, sharp indentation creep tests were performed for comparison purposes and the results confirmed that the use of a spherical indenter is clearly more appropriate for investigating the creep behavior of this HEA. Finally, the influence of annealing on the constitutive stress-strain response of nc CoCrFeMnNi HEA was investigated through a series of nanoindentation experiments using five different three-sided pyramidal indenters. The nc HEA, produced by HPT, was subjected to annealing at 450 oC for 1 and 10 h. Microstructural analysis using TEM showed that three different nano-scale precipitates (NiMn-, FeCo-, and Co-rich phases) form in the primary single-phase matrix of nc HEA after annealing. The strain-dependent plastic flow response of nc HEA pre- and post-annealing was estimated using the indentation strain and constraint factor, revealing a significant strain softening in nc HEA, which becomes pronounced after annealing. TEM analysis of the deformed material underneath the indenter suggests that the plastic deformation aids in the dissolution of the annealing-induced intermetallic precipitates, which could be the mechanism for the pronounced softening. The dissolution mechanismwas rationalized by the destabilization of precipitates during plastic deformation due to the increase in interface energy.