Structural material fatigue is a leading cause of failure and has motivated fatigue-resistant design to eliminate risks to human lives. Intrinsic microstructural features alter fatigue deformation mechanisms so profoundly that, essentially, fatigue properties of structural materials become deviant. With this in mind, we initiated this project to investigate the microstructural effect on fatigue behavior of potential structural high entropy alloys. With a better understanding of the effect of microstructure features on fatigue properties, the ultimate goal was to engineer the microstructure to enhance the fatigue life of structural materials. The effects of two major deformation mechanisms presented here are twinning-induced fatigue crack retardation, and transformation-induced fatigue crack retardation. The fundamental principle of both mechanisms is to delay the fatigue crack propagation rate by altering the work hardening ability locally within the crack plastic zone. In ultrafine grained triplex Al0.3CoCrFeNi, nano-sized deformation twins were observed during cyclic loading in FCC matrix due to low stacking fault energy (SFE). The work-hardening ability of the material near the crack was sustained with the formation of twins according to Considere's criteria. Further, due to the ultrafine-grained (UFG) nature of the material, fatigue runout stress was enhanced. In a coarse-grained, dual-phase high entropy alloy, persistent slip bands formed in FCC matrix during cyclic loading due mainly to the slight composition change that affects the SFE in the FCC matrix and eventually alters the deformation mechanism. Another way known to alter an alloy's work hardening (WH) ability is transformation-induced plasticity (TRIP). In some alloys, phase transformation happens due to strain localization, which alters the work-hardening ability. iii In a fine-grained, dual-phase metastable high entropy alloy, gamma (f.c.c.) to epsilon (h.c.p.) transformation occurred in the plastic zone that was induced from cracks. Thus, we designed a Cu-containing FeMnCoCrSi high entropy alloy that exhibited a normalized fatigue ratio of ~ 0.62 UTS (ultimate tensile strength). Our design approach was based on (a) engineering the gamma phase stability to attain sustained work hardening through delayed gamma (f.c.c.) to epsilon (h.c.p.) transformation to hinder fatigue crack propagation, (b) incorporating an ultrafine-grained microstructure to delay crack initiation, and (c) forming deformation twins to reduce the crack propagation rate. We verified that a UFG gamma dominant microstructure could provide opportunities for exceptional fatigue resistance, as sustained WH activity strengthened the material locally in the crack plastic zone, thereby validating our expectation that the combination of UFG and TRIP is a path to design the next generation of fatigue-resistant alloys.

Ab initio theory provides a powerful tool to understand and predict the behavior of materials. This thesis contains both of these aspects. First we use ab initio alloy theory to investigate a new kind of complex alloy (high-entropy alloy). Second we introduce a novel potential (interlayer potential), which can be extracted from ab inito total energy calculations using the Chen-Möbius inversion method. High-entropy alloys (HEAs) are composed of four or more metallic elements with nearly equimolar composition. In spite of the large number of components, most of the HEAs have a simple solid-solution phase rather than forming complex intermetallic structures. Extensive experiments have reported the unique microstructures and special properties of HEAs. Single-phase HEAs may be divided into three types, i.e. the 3d-HEAs adopting the face centered cubic (fcc) phase, the refractory-HEAs with a body centered cubic (bcc) phase, and the HEAs with the duplex fcc-bcc structure. We employ the exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA) to investigate the electronic structure, the equilibrium volume and the elastic properties of these three-type HEAs. First we compare the CPA with the super cell technique (SC) to assess the performance of the EMTO-CPA method. As typical fcc 3d-HEAs, we consider the CuNiCoFeCrTix systems in the paramagnetic state. Starting from the calculated electronic structure, we give an explanation for the observed magnetic states. Furthermore, we provide a theoretical prediction for the elastic parameters and polycrystalline elastic moduli for CuNiCoFeCrTix (x= 0.0−0.5, 1.0) and NiCoTeCrTi. A detailed comparison between the theoretical results and the available experimental data demonstrates that ab initio theory can properly describe the fundamental properties of this important class of engineering alloys. Refractory-HEAs are composed of Ti, Zr, Hf, V, Nb, Ta, Mo, and W. These HEAs have a simple bcc structure. Taking the TiZrNbMoVx and TiZrVNb HEAs as examples, we provide a detailed investigation of the effect of alloying elements on the elastic parameters and the elastic isotropy. Our results indicate that vanadium enhances the anisotropy and ductility of TiZrNbMoVx. As an application of the present theoretical database, we verify the often quoted correlation between the valence charge concentration (VEC) and the micro-mechanical properties in the case of multi-component alloys. Furthermore, we predict that the present HEAs become elastically isotropic for VEC ≃ 4.72. With increase of the aluminum content, phase transformations (fcc→(fcc+bcc)→bcc) occur in NiCoFeCrAlx HEAs. Our ab initio results predict that at room temperature the paramagnetic NiCoFeCrAlx HEAs adopt the fcc structure for x ≤ 0.60 and the bcc structure for x ≥ 1.23, with an fcc-bcc duplex region in between the two pure phases. The calculated single- and polycrystal elastic parameters exhibit strong composition and crystal structure dependence. Based on the present theoretical findings, it is concluded that alloys around the equimolar NiCoFeCrAl composition have superior mechanical performance as compared to the single-phase regions. Many modern materials and material systems are layered. The properties related to layers are connected to interactions between atomic layers. We introduce the interlayer potential (ILP), a novel model potential which fully describes the interaction between layers. The ILPs are different from the usual interatomic potentials which present inter- action between atoms. We use the Chen-Möbius inversion method to extract the ILPs from ab initio total energy calculations. The so obtained ILPs can be employed to investigate several physical parameters connected with the particular set of atomic layers, e.g. surface energy, stacking fault energy, elastic parameters, etc. As an application, we adopt the supercell method and the axial interaction model in connection with the ILPs to calculate the stacking fault energy along the fcc ⟨111⟩ direction, including the intrinsic stacking fault energy, extrinsic stacking fault energy and twin stacking fault energy as well as the interactions between the intrinsic stacking faults. We find that the data derived from ILPs are consistent with those obtained in direct ab initio calculations. Along the fcc ⟨111⟩ direction, we study the surface energy and surface relaxation using the ILPs. The phonon dispersions are also described. Our conclusions are as follows the EMTO-CPAab initioalloy theory can be used to understand and predict the fundamental properties of multicomponent alloys. the interlayer potentials based on the Chen-Möbius inversion method may provide a new way to investigate the properties related to layers in layered materials, the EMTO-CPA alloy theory combined with the Chen-Möbius inversion method offers a powerful technique to study the properties of complex alloys. / <p>QC 20131108</p>

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In recent years, series of high-entropy alloy have been well developed with high hardness and high temperature stability. These properties could apply in hard surface welding technology. The previous research showed that Al0.5CoCrCuFeNi based alloy contained excellent abrasive and adhesive wear resistant properties. According to the results of first year project, the post heat treatment is required for Type A (Al0.3CrFe1.5MnNi0.5 ) and B (Al0.5CrFe1.5MnNi0.5) alloys. It is not suitable for the industrial field service in certain repairing application. This research project will modify the Al0.5CrFe1.5MnNi0.5 base high-entropy alloy in the alloy content of Cr and Ni. These new alloy called Type D high entropy alloys include BCC and FCC two phases. We expect BCC part will provide the wear hardness and FCC part could improve the ductility during the wearing stage. The FCC phase may improve the manufacture of welding rods, also. The research contents include, (1) Type D high entropy alloys weld rod evaluation, (2) wear test, (3) microstructure analysis using electron micro-probe

Structure-tribological property relations have been studied for five high entropy alloys (HEAs). Microhardness, room and elevated (100°C and 300°C) temperature sliding friction coefficients and wear rates were determined for five HEAs: Co0.5 Cr Cu0.5 Fe Ni1.5 Al Ti0.4; Co Cr Fe Ni Al0.25 Ti0.75; Ti V Nb Cr Al; Al0.3CoCrFeNi; and Al0.3CuCrFeNi2. Wear surfaces were characterized with scanning electron microscopy and micro-Raman spectroscopy to determine the wear mechanisms and tribochemical phases, respectively. It was determined that the two HEAs Co0.5 Cr Cu0.5 Fe Ni1.5 Al Ti0.4 and Ti V Nb Cr Al exhibit an excellent balance of high hardness, low friction coefficients and wear rates compared to 440C stainless steel, a currently used bearing steel. This was attributed to their more ductile body centered cubic (BCC) solid solution phase along with the formation of tribochemical Cr oxide and Nb oxide phases, respectively, in the wear surfaces. This study provides guidelines for fabricating novel, low-friction, and wear-resistant HEAs for potential use at room and elevated temperatures, which will help reduce energy and material losses in friction and wear applications.

High entropy alloys (HEAs) are a relatively new class of solid solution alloys that contain multiple principal elements to take advantage of their high configurational entropy, sluggish diffusion, lattice distortion, and the cocktail effect. In recent development, work hardening mechanisms known as twinning induced plasticity (TWIP) and transformation induced plasticity (TRIP) have been found active in Al0.3CoCrFeNi (molar fraction) and Fe50Mn30Co10Cr10 (at %) HEA compositions. Friction-stir processing was done to increase the mechanical properties and improve the microstructure of the alloys for the purpose of high strain rate performance. Quasi-static tensile tests as well as top-hat geometry Split-Hopkinson pressure bar tests were conducted to view the mechanical properties as well as view the microstructural evolution at dynamic strain rates. Overall, the Al0.3CoCrFeNi condition after friction-stir processing and heat treatment has proved to have the best mechanical properties, and selecting from the conditions in this study, Al0.3CoCrFeNi has better shear localization resistance.

In the current study, we have investigated the cyclic loading behavior of conventional as well as novel alloy system exhibiting fine and ultrafine-grained structure. While in case of conventional alloy systems (here aluminum alloy AA5024), the effect of three different grain sizes was investigated. Improvement in fatigue properties was observed with decreasing grain size. The unique microstructure produced via Friction stir processing was responsible for the improved fatigue response. Additionally, microstructures consisting of a high fraction of special boundaries within the fine and ultrafine-grained regime were also subjected to cyclic loading. The hierarchical features introduced in the eutectic high entropy alloy deflected the persistent slip bands, responsible for fatigue cracking, thus resulted in delayed crack initiation and improved fatigue life. The selective nature of fatigue was learnt in the fine grain Al0.5CoCrFeNi, where the introduction of hierarchical features did not result in improved fatigue properties. The weak links in the microstructure, while not affecting the tensile properties, got exposed during cyclic loading. Further study on the medium entropy alloy revealed the inherent reason for the improved fatigue properties. The medium entropy alloys utilized the benefit of UFG single-phase FCC matrix. The UFG matrix showed signs of transformation of FCC phase into the HCP phase during fatigue deformation and hence exhibited improved work-hardening. Alongside atomic scale transformation, stacking faults and nano-twins can also be attributed for obtained cyclic properties.

No description available.

This dissertation presents an assortment of research aimed at understanding the composition-dependence of deformation behavior and the response to thermomechanical processing, to enable efficient design and processing of low stacking fault energy (SFE) high entropy alloy (HEAs). The deformation behavior and SFE of four low SFE HEAs were predicted and experimentally verified using electron microscopy and in-situ neutron diffraction. A new approach of employing a minimization function to refine and improve the accuracy of a semi-empirically derived expression relating composition with SFE is demonstrated. Ultimately, by employing the minimization function, the average difference between experimental and predicted SFE was found to be 2.64 mJ m-2. Benchmarking with currently available approaches suggests that integrating minimization functions can substantially improve prediction accuracy and promote efficient HEA design with expansion of databases. Additionally, in-situ neutron diffraction was used to present the first in-situ measurement of the interspacing between stacking faults (SFs) which were correlated with work hardening behavior. Electron transparent specimens (< ~100 nm thick) were used in order to resolve nanoscale planar faults instead of the thicker sub-sized specimens (on the order of millimeters in thickness) which exhibit the classical stages III work hardening behavior characteristic of low SFE metals and alloys. The present study demonstrates these characteristic dimensions of SFs can be tracked in real-time using neutrons or high-energy x-rays. SFs have also been shown to act as barriers to dislocation motion and thus contribute to strengthening and sustained work hardening during deformation.

No description available.