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.

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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

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.

In the present study both fusion based - laser powder bed fusion (LPBF), and solid state - additive friction stir deposition (AFSD) additive manufacturing processes were employed for the manufacturing of a metastable high entropy alloy (HEA), Fe40Mn20Co20Cr15Si5 (CS-HEA). A processing window was developed for the LPBF and AFSD processings of CS-HEA. In case of LPBF, formation of solidification related defects such as lack of fusion pores (for energy density ≤ 31.24 J/mm3) and keyhole pores (for energy density ≥ 75 J/mm3) were observed. Variation in processing conditions affected the microstructural evolution of the metastable CS-HEA; correlation between processing conditions and microstructure of the alloy is developed in the current study. The tendency to transform and twin near stress concentration sites provided excellent tensile and fatigue properties of the material despite the presence of defects in the material. Moreover, solid state nature of AFSD process avoids formation of solidification related defects. Defect free builds of CS-HEA using AFSD resulted in higher work hardening in the material. In summary, the multi-processing techniques used for CS-HEA in the present study showcase the capability of the AM process in tailoring the microstructure, i.e., grain size and phase fractions, both of which are extremely critical for the mechanical property enhancement of the alloy.

The goal of this work was to evaluate the electrochemical properties of TiZrNbTa thin films deposited by magnetron sputtering using an industrial physical vapor deposition system. Samples were deposited on both Si(001) and 316L stainless steel. The samples deposited on Si(001) were either crystalline (bcc reflections) or amorphous, depending on the sputtering parameters. The crystalline films were composed of thin films with two different layers. The upper layer was nanocolumnar composed of elongated nanocolumns, while the lower was dense. The amorphous films had only one nanocolumnar layer and higher porosity. Polarization curves revealed that all samples had low corrosion current densities, in the order of 10-8 A/cm2. The samples showed an extended passive region up to 3.0 V vs Ag/AgCl due to the growth of a passivating oxide. The surface of the samples consisted of Nb2O5, ZrO2, TiO2 and Ta2O5. The chronoamperometry tests showed current oscillations, related to a break-down and reformation of the passive film. Electrochemical impedance spectroscopy revealed that all samples behaved similarly in all three electrolytes, and the simulated electrical circuits were indicating no corrosion reactions. A decrease in capacitance values after polarization was observed and was related to the formed surface oxide. Samples deposited on 316L stainless steel showed a passive regime for a shorter potential window, probably related to surface defects of the films. Heat treatments at 400 and 800 Celsius for 20 hours could not trigger the phase transformation from single bcc to hcp or dual bcc, as predicted by the Thermo-Calc software.

No description available.

Plasma centrifugal atomization is a method widely used in the production of spherical powders of metals and alloys with relatively low melting points. A novel plasma centrifugal atomization process suitable for high melting point materials (i.e. 3500 ᵒC and above) was developed by Metasphere Technology AB, currently Höganäs Sweden AB. In this process, feedstock material in the form of crushed powder with particle sizes in the range 400-1000 µm is fed into a rotating crucible and subsequently melted by the glow discharge of a plasmatron. Due to high rotational speeds, a melt film forms at the edge of the crucible and breaks into fine droplets that are ejected into the reactor chamber and solidified in a whirl of cold inert gases. Capability of the plasmatron to reach very high temperatures, combined with extremely rapid cooling of the ejected droplets, allow for the fabrication of fine powders of refractory alloys exhibiting metastable phases that cannot be obtained otherwise.  Oil drilling, ore processing and metal shaping applications, among other, require tool materials capable of withstanding harsh working conditions under heavy loads. Owing to their physical, chemical and mechanical properties, tungsten-carbon alloys are among the most suited materials for such applications. Melting followed by rapid solidification of tungsten-carbon mixtures with 3.9 wt.% C results in a biphasic structure composed of WC lamellae inserted in a W2C matrix, known as cast tungsten carbide (CTC). Due to the metastable nature of both phases present, CTC exhibits exceptional mechanical properties. CTC is mainly used as reinforcing dispersed phase in metal matrix composite hardfacing overlays, which are deposited by plasma transferred arc (PTA) welding or laser cladding onto steel tools. High-entropy alloys (HEAs) are defined as multi-component solid solutions with equimolar or near-equimolar concentration of all principal elements. Owing to their outstanding mechanical, corrosion, erosion, oxidation and radiation resistance properties compared to conventional alloys, HEAs are among the most suited materials for aerospace and nuclear applications. Several processing routes have allowed for laboratory-scale production of HEAs. Nevertheless, size and shape of bulk components that can be thus produced are largely limited. In a quest for up-scaling the processing of high-end bulk HEA components, plasma centrifugal atomization of pre-alloyed refractory HEA spherical powders suitable for additive manufacturing was envisaged. In this work, capabilities of the novel plasma centrifugal atomization for processing of refractory alloys into fine spherical powders have been evaluated based on two different material systems, namely CTC and a refractory HEA containing Ti, V, Zr, Nb, Mo, Hf, Ta, W. Challenges of local mechanical characterization of micron-sized powders have been addressed and a robust method for testing of individual particles has been developed. Mechanical properties such as hardness and fracture toughness of plasma atomized CTC powders have been extensively investigated and related to the corresponding thermal stories. Experimental results suggest significant straining of the crystal lattice in the case of as-atomized CTC, possibly due to extremely high cooling rates experienced by the solidifying particles. This has been ruled out the main reason for the outstanding mechanical properties of plasma atomized CTC compared to both spheroidized CTC and conventional cast &amp; crushed CTC. Effective stress relieve was possible upon heat treatment. Plasma atomization of the refractory HEA yielded similar results, where an extremely fine microstructure with no noticeable chemical segregation was obtained. Indentation hardness of this novel microstructure was found to be approximately 25% higher than that of similar alloys reported in literature. HEA powder thus produced was then consolidated into bulk HEAs with very simple geometries, proving that this powder can be further processed into components of more or less complexity for pre-defined applications.

No description available.

Planar defects such as free surfaces and grain boundaries in metals and alloys play important roles affecting many material properties such as fracture toughness, corrosion resistance, wetting, and catalysis. Their interactions with point defects and solute elements also play critical roles on governing the microstructural evolution and associated property changes in materials. This work seeks to use atomistic modeling to obtain a fundamental understanding of many surface and interface related properties and phenomena, namely: orientation-dependent surface energy of elemental metals and alloys, segregation of solute elements at grain boundaries and their impact on grain boundary cohesive strength, and the controversial sluggish diffusion in both the bulk and grain boundaries of high entropy alloys. First, an analytical formula is derived, which can predict the surface energy of any arbitrary (h k l) crystallographic orientation in both body-centered-cubic (BCC) and face-centered-cubic (FCC) pure metals, using only two or three low-index (e.g., (100), (110), (111)) surface energies as input. This analytical formula is validated against 4357 independent single element surface energies reported in literature or calculated by the present author, and it proves to be highly accurate but easy to use. This formula is then expanded to include the simple-cubic (SC) structure and tested against 4542 surface energies of metallic alloys of different cubic structures, and good agreement is achieved for most cases. Second, the effect of segregation of substitutional solute elements on grain boundary cohesive strength in BCC Fe is studied. It is found that the bulk substitution energy can be used as an effective indicator to predict the embrittlement or strengthening potency induced by the solute segregation at grain boundaries. Third, the controversial vacancy-mediated sluggish diffusion in an equiatomic FeNiCrCoCu FCC high entropy alloy is studied. Many literature studies have postulated that the compositional complexity in high entropy alloys could lead to sluggish diffusion. To test this hypothesis, this work compares the vacancy-mediated self-diffusion in this model high entropy alloy with a hypothetical single-element material (called average-atom material) that has similar average properties as the high entropy alloy but without the compositional complexity. The results show that the self-diffusivities in the two bulk systems are very similar, suggesting that the compositional complexity in the high entropy alloy may not be sufficient to induce sluggish diffusion in bulk high entropy alloys. Based on the knowledge learned from the bulk alloy, the exploration of the possible sluggish diffusion has been extended to grain boundaries, using a similar approach as in the study of self-diffusion in bulk. Interestingly, the results show that sluggish diffusion is evident at a Σ5(210) grain boundary in the high entropy alloy due to the compositional complexity, especially in the low temperature regime, which is different from the bulk diffusion. The underlying mechanisms for the sluggish diffusion at this grain boundary is discussed. / Doctor of Philosophy / Human beings have utilized metals and alloys for over ten millennia and learned much from them. Based on the accumulated knowledge, they have countless applications in our current daily life. However, there is still much to learn for improving our current technology and even opening new opportunities. Throughout most of history, our understanding of these materials was largely obtained through empirical experimentation and refining them into theories and scientific laws. Nowadays, due to the advancements in computer simulations, we can learn more by modeling the behaviors of metals and alloys at the length and time scales that are either be too arduous, costly, or currently impossible experimentally. This work aims at using computer modeling to study some important surface/interface related physical behaviors and properties in metals and alloys at the atomistic scale. First, this work intends to develop a robust surface energy model in an analytical form for any crystallographic orientation. Surface energy is an important material property for many surface-related processes such as fracturing, wetting, sintering, catalysis, and crystalline particle shape. Surface energy is different at different surface orientations, and predicting this difference is important for understanding these surface phenomena. Second, the effect of solute segregation on grain boundary cohesive strength is studied. Most commonly used metallic materials consist of many small crystalline grains and the borders between them are called grain boundaries, which are weak spots for fracture. The minimum energy required to split a boundary is called the grain boundary cohesive strength. The presence of solutes or impurities at grain boundaries can further alter the cohesive strength. A better understanding of this phenomena will eventually help us develop more fracture-resistant materials. The third project deals with the possible sluggish/retarded diffusion in high entropy alloys, which contain five or more principal alloying elements and have many unique mechanical, radiation-resistant, and corrosion-resistant properties. Many researchers attribute these unique properties to the slow species diffusion in these alloys, but its existence is still controversial. This work studies the atomic-level diffusion mechanisms in an FeNiCrCoCu high entropy alloy both in bulk (grain interior) and at grain boundaries in order to determine if sluggish diffusion is present and its causes.