Aditya Ayyagari

To enhance the durability and reliability of high-pressure fuel system components operating with low-viscosity fuels, an analysis of the tribological performance of potential coating material candidates was conducted. This study focuses on evaluating the friction and wear characteristics of various coatings widely used by industry for surface protection, including carbides, nitrides, diamond-like carbon (DLC), and solid lubricants, under accelerated reciprocating ball on flat conditions intended to provide information on longer-term operation in fuel pumps. The tribological performance of these coatings in fuels of varying chemistry and viscosity, including ethanol, decane, dodecane, and aviation fuel (F-24), was compared to the mechanical properties of materials, such as hardness and elastic modulus. Results indicate that carbides show the lowest friction and wear values across different fuel environments. CrN-based coatings demonstrate a decrease in friction and wear in comparison to other nitrides. This comprehensive investigation lays the groundwork for informed design decisions in developing high-performance coatings tailored to withstand the challenges of low-viscosity fuel environments.

High-performance energy storage devices (HPEDs) play a critical role in the realization of clean energy and thus enable the overarching pursuit of nonpolluting, green technologies. Supercapacitors are one class of such lucrative HPEDs; however, a serious limiting factor of supercapacitor technology is its sub-par energy density. This report presents hitherto unchartered pathway of physical deformation, chemical dealloying, and microstructure engineering to produce ultrahigh-capacitance, energy-dense NiMn alloy electrodes. The activated electrode delivered an ultrahigh specific-capacitance of 2700 F/cm³ at 0.5 A/cm³. The symmetric device showcased an excellent energy density of 96.94 Wh/L and a remarkable cycle life of 95% retention after 10,000 cycles. Transmission electron microscopy and atom probe tomography studies revealed the evolution of a unique hierarchical microstructure comprising fine Ni/NiMnO₃ nanoligaments within MnO₂-rich nanoflakes. Theoretical analysis using density functional theory showed semimetallic nature of the nanoscaled oxygen-vacancy-rich NiMnO₃ structure, highlighting enhanced carrier concentration and electronic conductivity of the active region. Furthermore, the geometrical model of NiMnO₃ crystals revealed relatively large voids, likely providing channels for the ion intercalation/de-intercalation. The current processing approach is highly adaptable and can be applied to a wide range of material systems for designing highly efficient electrodes for energy-storage devices.

There has been tremendous interest in recent years in a new class of multi-component metallic alloys that are referred to as high entropy alloys, or more generally, as complex concentrated alloys. These multi-principal element alloys represent a new paradigm in structural material design, where numerous desirable attributes are achieved simultaneously from multiple elements in equimolar (or near equimolar) proportions. While there are several review articles on alloy development, microstructure, mechanical behavior, and other bulk properties of these alloys, then there is a pressing need for an overview that is focused on their surface properties and surface degradation mechanisms. In this paper, we present a comprehensive view on corrosion, erosion and wear behavior of complex concentrated alloys. The effect of alloying elements, microstructure, and processing methods on the surface degradation behavior are analyzed and discussed in detail. We identify critical knowledge gaps in individual reports and highlight the underlying mechanisms and synergy between the different degradation routes.

Metallic glass composites represent a unique alloy design strategy comprising of in situ crystalline dendrites in an amorphous matrix to achieve damage tolerance unseen in conventional structural materials. They are promising for a range of advanced applications including spacecraft gears, high-performance sporting goods and bio-implants, all of which demand high surface degradation resistance. Here, we evaluated the phase-specific electrochemical and friction characteristics of a Zr-based metallic glass composite, Zr_56.2Ti_13.8Nb_5.0Cu_6.9Ni_5.6Be_12.5, which comprised roughly of 40% by volume crystalline dendrites in an amorphous matrix. The amorphous matrix showed higher hardness and friction coefficient compared to the crystalline dendrites. But sliding reciprocating tests for the composite revealed inter-phase delamination rather than preferred wearing of one phase. Pitting during potentiodynamic polarization in NaCl solution was prevalent at the inter-phase boundary, confirming that galvanic coupling was the predominant corrosion mechanism. Scanning vibration electrode technique demonstrated that the amorphous matrix corroded much faster than the crystalline dendrites due to its unfavorable chemistry. Relative work function values measured using scanning kelvin probe showed the amorphous matrix to be more electropositive, which explain its preferred corrosion over the crystalline dendrites as well as its characteristic friction behavior. This study paves the way for careful partitioning of elements between the two phases in a metallic glass composite to tune its surface degradation behavior for a range of advanced applications.

Activation energy and diffusion kinetics are important in modulating the high temperature oxidation behavior of metals. Recently developed multi‐principal element alloys, also called high entropy alloys (HEAs), are promising candidate material for high temperature applications. However, the activation energies and diffusion kinetics of HEAs have been limitedly explored. We investigate the diffusional activation energy for oxidation of Al_0.1CoCrFeNi HEA. Compared to conventional steels and Ni‐based super alloys, the HEA showed a significantly higher diffusion activation energy. This behavior is explained based on low potential energy of the lattice and interstitial sites which effectively trap the atoms, limiting their diffusion. The atomic mean jump frequency for interstitial diffusion of oxygen in the HEA is four‐orders of magnitude lower than T22 and T91 steels and seven‐orders of magnitude lower compared to pure iron. Al_0.1CoCrFeNi HEA showed the lowest oxidation rate compared to conventionally used steels, super‐alloys, and coatings.