Self-Similar Random Process and Chaotic Behavior In Serrated Flow of High Entropy Alloys

The statistical and dynamic analyses of the serrated-flow behavior in the nanoindentation of a high-entropy alloy, Al0.5CoCrCuFeNi, at various holding times and temperatures, are performed to reveal the hidden order associated with the seemingly-irregular intermittent flow. Two distinct types of dynamics are identified in the high-entropy alloy, which are based on the chaotic time-series, approximate entropy, fractal dimension, and Hurst exponent. The dynamic plastic behavior at both room temperature and 200 °C exhibits a positive Lyapunov exponent, suggesting that the underlying dynamics is chaotic. The fractal dimension of the indentation depth increases with the increase of temperature, and there is an inflection at the holding time of 10 s at the same temperature. A large fractal dimension suggests the concurrent nucleation of a large number of slip bands. In particular, for the indentation with the holding time of 10 s at room temperature, the slip process evolves as a self-similar random process with a weak negative correlation similar to a random walk

Fabrication and characterization of advanced materials using laser metal deposition from elemental powder mixture

Over the past decades of years, a great deal of money has been spent to machine large and complex parts from high-performance metals (i.e., titanium components for aerospace applications), so users attempt to circumvent the high cost of materials. Laser metal deposition (LMD) is an additive manufacturing technique capable of fabricating complicated structures with superior properties. This dissertation aims to improve the applications of LMD technique for manufacturing metallic components by using various elemental powder mixture according to the following three categories of research topics. The first research topic is to investigate and develop a cost-effective possibility by using elemental powder mixture for metallic components fabrication. Based on the studies of fabricating thin-wall Ti-6Al-4V using elemental powder mixture, comparative close particle number for Ti, Al and V powder could easily get industry qualified Ti-6Al-4V components. The particle number for each element in powder blends has been proved to be a key factor for composition control in the final deposit part. The second research topic focuses on the application improvements of elemental powder manufacturing. By fabricating AlxCoFeNiCu1-x (x = 0.25, 0.5, 0.75) high entropy alloys from elemental powder based feedstocks, it enhances the usage of elemental powder to fabricate novel materials with complex compositions. The third research topic extends the applications of using elemental powder mixture to the broader area. A functionally gradient material (FGM) path is developed to successfully join titanium alloy with γ-TiAl. This dissertation leads to new knowledge for the effective fabrication of unique and complex metallic components. Moreover, the research results of the dissertation could benefit a wide range of industries --Abstract, page iv

Mechanistic Understanding Of Phase Stability, Transformation, And Strengthening Mechanisms In Lightweight High Entropy Alloys And High Entropy Ceramics

High-entropy alloys (HEAs) are a novel family of solid-solution alloys that have gained international interest due to their exceptional characteristics. Because of the need from the transportation and defense sectors, lightweight HEAs have attracted researcher’s curiosity as prospective advanced materials. Low-weight high entropy alloy synthesizes using arc melting with a mass ratio of AlCrFeMnTix(0.1,0.15,0.2). The synthesized HEA is comprised of a mixture of body center cubic (bcc) and ordered bcc (L21) solid solution phases. The synthesized HEAs have heat treated at 650C, 800C, and 1150C for 1hr, 4hr after solutionized at 1150C for 2 hr to understand the effect of temperature evaluation in these HEAs. To investigate the role of titanium in improving the strengthening of Al1.5CrFeMnTix(x=0.1,0.15,0.2) and tailor the mechanical properties via studying strengthening precipitation mechanisms. The results show that the density of the alloy is 5.88 g cm-3, which fulfills the criteria of low-weight HEA; also, from the XRD, the BCC+ L21 phases are available inside the HEA Composition matrix. Ultra-high temperature ceramics (UHTCs) consist of different ceramics like boride, carbide, diboride, and nitride of the transition metals. The UHTCs show prominent characteristics such as higher melting point, higher oxidation resistance, lower thermal conductivity, higher yield strength, and microhardness. High entropy ceramic (HEC) is a multi-component alloy similar to the high entropy alloy (HEA) specifically developed for hypersonic vehicles, nuclear reactors, and high temperature applications. In the current study, mechanical alloying(MA) and spark plasma sintering(SPS) were used to develop equiatomic HEC (Hf0.2Nb0.2Ta0.2Ti0.2Zr0.2)N. The HECs were mechanically alloyed using the high-energy ball mill at 500 RPM for 6 hours. The ball to powder (BPR) ratio of 10:1 was maintained during the milling. This mechanically alloyed powder was sintered using SPS for 1800°C, 1900°C, and 2000°C to investigate the effect of temperature on the densification of these HECs. The consolidated samples were further analyzed using SEM, microhardness, XRD, and wear testing to understand the effect of different sintering temperatures on microstructure, phase transformation, and mechanical and tribological behavior of these high entropy nitride ceramics. Oxidation treatments have been performed at 1000°C-1200°C for 2 hours duration

Composition and phase structure dependence of magnetic properties for Co2FeCr0.5Alx (x=0.9, 1.2) multi principal component alloys

This article investigates the microstructure evolution, phase formation, and magnetic properties of Co2FeCr0.5Alx (x = 0.9, 1.2) complex component alloys, as a function of heat treatment temperatures (at 500, 600, 700, and 1150°C), using XRD, optical microscopy, electron microscopy and vibrating sample magnetometry (VSM). The alloy with 20.4 at% Al (x= 0.9), identified here as C1, consisted of microscale BCC1 phase and BCC nanoscale particles containing mainly Fe and Cr, and B2 matrix with mainly Al and Co. Partial transformation of the BCC1 phase to an FCC phase was observed at 700°C and full transformation at 1150°C, through twinning. For the sample with 25.5 at% Al (x= 1.2), identified as sample C2, there were only nanoscale BCC particles in the B2 matrix with the same element segregation between the phases as C1. This increase in Al (from x= 0.9 to 1.2) content stabilised the B2 matrix phase, reduced the grain size, and increased both saturation magnetisation (Ms) and coercivity (Hc). Moreover, increasing the heat treatment temperature resulted in an increase in grain size of the B2 matrix, volume fraction and average size of the micro BCC 1 and nanoscale BCC phases for both C1 and C2, which also modified the soft magnetic properties, with Ms and Hc increasing up to 600°C followed by a decrease until 1150°C. Using the structural information as inputs for density functional theory calculations of Hc and Ms, it has been found that the Hc is influenced by the grain size of the matrix, and the volume fraction and size of the BCC1 phase at temperature higher than 600°C for C1 and 700°C for C2, but is controlled by nanoscale BCC particles below these temperatures. The Ms is controlled by the elemental diffusion and segregation. Thus, the best combination of Hc and Ms was seen with antiferromagnetic Cr segregated and partitioning in the microscale BCC1 phase, and a B2 matrix with less Cr rich precipitation, formed at 500°C, where the misfit strain between B2 matrix and nanoscale BCC was low

A ductility metric for refractory-based multi-principal-element alloys

We propose a quantum-mechanical dimensionless metric, the local$-$lattice distortion (LLD), as a reliable predictor of ductility in refractory multi-principal-element alloys (RMPEAs). The LLD metric is based on electronegativity differences in localized chemical environments and combines atomic$-$scale displacements due to local lattice distortions with a weighted average of valence$-$electron count. To evaluate the effectiveness of this metric, we examined body$-$centered cubic (bcc) refractory alloys that exhibit ductile$-$to$-$brittle behavior. Our findings demonstrate that local$-$charge behavior can be tuned via composition to enhance ductility in RMPEAs. With finite$-$sized cell effects eliminated, the LLD metric accurately predicted the ductility of arbitrary alloys based on tensile$-$elongation experiments. To validate further, we qualitatively evaluated the ductility of two refractory RMPEAs, i.e., NbTaMoW and Mo$_{72}$W$_{13}Ta$_{10}Ti$_{2.5}Zr$_{2.5}, through the observation of crack formation under indentation, again showing excellent agreement with LLD predictions. A comparative study of three refractory alloys provides further insights into the electronic-structure origin of ductility in refractory RMPEAs. This proposed metric enables rapid and accurate assessment of ductility behavior in the vast RMPEA composition space.Comment: 36 pages, 12 figures, 5 Tabl

Computation Aided Design Of Multicomponent Refractory Alloys With A Focus On Mechanical Properties

Quantum mechanical calculations paired with exponential growth in computer processing speed has created a paradigm shift in materials discovery. Simulations can be carried out to accurately predict structure-composition-property relationships of novel systems. This work focuses on calculating elastic properties of high entropy alloys, a new class of alloys that are built from 4+ elements in equi-atomic proportion. These alloys often exhibit simple microstructures and each constituent element contributes its properties to the overall bulk properties of the amalgamated material. This cocktail effect has led to the discovery of many alloys which could drive technical advances in the future. Elastic properties of a solid are important because they relate to various fundamental solid-state properties and are thermodynamically linked to the specific heat, thermal expansion, Debye temperature, and melting point. The refractory based system, Monbtaw, studied in this research, was found to have a young\u27s modulus of approximately 300 GPA. The elastic modulus decreased with addition of titanium over 11- 33 atomic percent. The elastic modulus however, was unchanged when adding vanadium at 11%, but saw a decrease in the range of 20% to 25%. The calculations also helped in predicting alloy compositions in which a single-phase solid solution exists, which is vital for capturing the cocktail effect of these alloys