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There is a strong surge in interest for hydrogen and direct alcohol fuel cells as zero-emission energy conversion and storage devices due to their high energy density compared to that for batteries...

Experimental results reveal that the apparent activation-energy for grain-growth in an fcc-based AlxCoCrFeNi high entropy alloy (HEA) system increases from 179 to 486 kJ/mol when the Al content increases from x = 0.1 to 0.3. These unexpectedly high apparent activation-energy values can be potentially attributed to solute clustering within the fcc solid-solution phase that develops with increasing Al content in this HEA. Detailed microstructural analysis using atom-probe tomography and density functional theory (DFT) calculations strongly indicate the presence of such nanoscale clusters. This phenomenon can change grain-growth from a classical solute-drag regime to a much more sluggish cluster-drag based mechanism in these HEAs.

In this study, we demonstrate a novel pathway to engineer the properties of metallic alloys for limiting their cavitation erosion-corrosion. A facile single-step processing technique was used to develop bimodal grain structure in stainless steel. The bimodal steel was tested in cavitation erosion and erosion-corrosion conditions. In both the cases, bimodal steel showed exceptionally high degradation resistance, nearly 7 times higher compared to as-received steel. The remarkable cavitation erosion resistance demonstrated by bimodal steel is attributed to its high yield strength along with high work-hardening rate. In addition, the bimodal steel showed significantly low corrosion rate of 0.001 mm/year in 3.5 wt % NaCl solution compared to 0.088 mm/year for as-received stainless steel.

High entropy alloys represent a new paradigm of structural alloy design consisting of (near) equal proportions of constituent elements resulting in a number of attractive properties. In particular, eutectic high entropy alloys offer a remarkable combination of high strength and good ductility from the synergistic contribution of each phase in the eutectic, thereby circumventing the strength-ductility trade-off in conventional structural materials. In the present study, wear and corrosion behavior were evaluated for the AlCoCrFeNi2.1 eutectic high entropy alloy consisting of BCC (B2), and FCC (L12) lamellae. A transition from adhesive to oxidative wear was observed in reciprocating wear analysis. The L12 phase with lower hardness preferentially deformed during the wear test. The ratio of hardness to modulus was almost two times higher for the B2 phase as compared to L12. The overall corrosion resistance of the eutectic high entropy alloy was comparable to 304 stainless steel in 3.5 wt% NaCl solution. However, detailed microscopy revealed preferential dissolution of the B2 phase. Phase-specific scanning kelvin probe analysis showed relatively higher electropositivity for the B2 phase as compared with L12, supporting the selective corrosion and higher coefficient of friction of B2.

While the elastic properties of cementite are known, there is no corresponding experimental or theoretical data available for the well-known hexagonal Ɛ-Fe2.4C carbide. In this study we investigated the dynamical and mechanical stability of different configurational structures of the hexagonal Ɛ-Fe2.4C carbide. A reliable structural model was established through the ground state energy and phonon calculations, and was used to calculate the elastic constants. The DOS calculations of that structure revealed a strong covalent bonding between Iron and C as shown by low occupation of states at the Fermi level. The calculated mechanical properties (i.e. Young's modulus Y and shear modulus G) of Ɛ-Fe2.4C carbide surpass those of the ubiquitous θ-Fe3C cementite. The calculated elastic properties are predictions, which can be used as an input to establish reliable models for steel development.

Hypereutectoid SAE 52100 steel is extensively used in bearing applications. Microstructure modification in terms of dislocation martensite and carbide refinement was achieved for SAE 52100 steel through a simple duplex heat treatment. Refinement of prior austenite grains to less than 5 μm resulted in the conversion of conventional high-carbon twinned martensite to dislocation martensite. The concurrent refinement of austenite grains and carbide precipitates was accomplished by high-temperature austenitization followed by low-temperature tempering. This resulted in nanoscale nonstoichiometric e-carbides within a heavily twinned martensitic structure. These nanoscale carbides acted as grain boundary pinning agents after their transformation to θ-carbides during the final austenitization process. The resulting microstructure was characterized by a fine dispersion of θ-carbides within dislocation martensite and showed roughly 24% increase in tensile strength and 30% better wear resistance compared to conventional structure.

High entropy alloys (HEAs) have many promising properties beneficial to advanced technologies. However, their underlying deformation mechanisms are largely unclear. So, as a first step, we have developed a modified embedded atom method potential for FeCoCrMnSi alloys to study such mechanisms. We predict the phase stability, chemical short-range ordering (CSRO), and stacking fault energy (SFE) of a specific alloy system using molecular dynamics (MD) and hybrid Monte-Carlo and molecular dynamics (MC/MD) simulation techniques. Room temperature MD simulations showed that both the potential energy and free energy of the single phase ε-hcp alloy is marginally more stable than the γ-fcc phase alloy, which resulted in a large, negative SFE. However, the room temperature MC/MD simulation showed an opposite trend where the γ-fcc phase was found to be more stable than the ε-hcp phase, and this resulted in a small, positive SFE. The prediction of the lower energy γ-fcc phase and resultant SFE agreed well with the experimentally reported SFE and phase stability for the Fe40Co20Cr15Mn20Si5 HEA, illustrating the importance of CSRO. Also, the calculated basal SFE of the hcp phase was close to that of the fcc phase. Therefore, the MC/MD implementation is crucial for the proper prediction of the phase stability and structural evolution in this HEA system. Many previous studies showed the ability of hybrid MC/MD technique to obtain consistent structural and configurational information of different alloy systems. The current work illustrates the potential of accelerating HEA materials development by utilizing computational methods based on the MC/MD technique which can reduce time and cost associated with experimental methods.

For decades the poor mechanical properties of Ti alloys were attributed to the intrinsic brittleness of the hexagonal ω-phase that has fewer than 5-independent slip systems. We contradict this conventional wisdom by coupling first-principles and cluster expansion calculations with experiments. We show that the elastic properties of the ω-phase can be systematically varied as a function of its composition to enhance both the ductility and strength of the Ti-alloy. Studies with five prototypical β-stabilizer solutes (Nb, Ta, V, Mo, and W) show that increasing β-stabilizer concentration destabilizes the ω-phase, in agreement with experiments. The Young’s modulus of ω-phase also decreased at larger concentration of β-stabilizers. Within the region of ω-phase stability, addition of Nb, Ta, and V (Group-V elements) decreased Young’s modulus more steeply compared to Mo and W (Group-VI elements) additions. The higher values of Young’s modulus of Ti–W and Ti–Mo binaries is related to the stronger stabilization of ω-phase due to the higher number of valence electrons. Density of states (DOS) calculations also revealed a stronger covalent bonding in the ω-phase compared to a metallic bonding in β-phase, and indicate that alloying is a promising route to enhance the ω-phase’s ductility. Overall, the mechanical properties of ω-phase predicted by our calculations agree well with the available experiments. Importantly, our study reveals that ω precipitates are not intrinsically embrittling and detrimental, and that we can create Ti-alloys with both good ductility and strength by tailoring ω precipitates' composition instead of completely eliminating them.

There has been tremendous interest in recent years to understand and quantify tribocorrosion mechanisms operative at sliding interfaces. While a plethora of information is available on individual corrosion and wear behavior of engineering materials, it falls short of offering an accurate representation of the materials’ response when corrosion and wear act simultaneously. The complexity of understanding tribocorrosion mechanisms in sliding contacts is compounded by in situ materials’ response—such as formation of active galvanic sites due to wear, phase changes, and strain hardening. This study compiles recent reports on the tribocorrosion behavior of various engineering alloys under corrosive media and provides a comprehensive understanding of all possible scenarios of material degradation possible. It is hoped that this chapter will be equally useful to readers new to the science of tribocorrosion and as a quick reference guide for practicing professionals.

Laser surface engineering was successfully used to engineer a nano carbide composite coating on a low carbon steel substrate. Two powder precursors of B4C and B4C+ 4 wt% Fe were utilized with different laser powers and processing speeds. A smooth crack free coating layer was achieved with B4C+ 4 wt% Fe powder precursor. The phase evolution and microstructure characterization of the coating layer were conducted using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Microscopy observations revealed a fine dendritic microstructure consisting of α-Fe phase dendrites with nanosized B4C precipitates, while the interdendritic regions contained mainly Fe2B and Fe3C phases. The retention of B4C in the coating layer was observed only at high laser power with high scanning speed. This is due to the lower stability of B4C with respect to Fe2B and Fe3C phases as confirmed with density functional theory based thermodynamic calculations. The hardness increased nearly seven fold for the nano-composite coating compared to the steel substrate due to evolution of high hardness ceramic phases, in addition to the refinement of the microstructure. This laser coating has a high potential in advance engineering applications where a thick coating layer with high hardness is required. Keywords: Laser surface processing, Composite coatings, Boron carbide, Thermodynamic modeling

Eutectic high entropy alloys, with lamellar arrangement of solid solution phases, represent a new paradigm for simultaneously achieving high strength and ductility, thereby circumventing this well-known trade-off in conventional alloys. However, dynamic strengthening mechanisms and phase-boundary interactions during external loading remain unclear for these eutectic systems. In this study, small-scale mechanical behavior was evaluated for AlCoCrFeNi2.1 eutectic high entropy alloy, consisting of a lamellar arrangement of L12 and B2 solid-solution phases. The ultimate tensile strength was 1165 MPa with ductility of ~18% and ultimate compressive strength was 1863 MPa with a total compressive fracture strain of ~34%. Dual mode fracture was observed with ductile failure for L12 phase and brittle mode for B2 phase. Phase-specific mechanical tests using nano-indentation and micro-pillar compression showed higher hardness and strength and larger strain rate sensitivity for B2 compared with L12. Micro-pillars on B2 phase deformed by plastic barreling while L12 micro-pillars showed high density of slip steps due to activation of more slip systems and homogenous plastic flow. Mixed micro-pillars containing both the phases exhibited dual yielding behavior while the interface between L12 and B2 was well preserved without any sign of separation or cracking. Phase-specific friction analysis revealed higher coefficient of friction for B2 compared to L12. These results will pave the way for fundamental understanding of phase-specific contribution to bulk mechanical response of concentrated alloys and help in designing structural materials with high fracture toughness.

An integrated experimental and molecular dynamics approach was used for developing low-activation high entropy alloys, potentially attractive for next generation nuclear reactor applications. These alloys were based on the concept of high entropy phase formation from a palette of elements with intrinsic low activity, namely Ta–Ti–V–Zr–X (X = Hf or W), with constituents in equimolar proportions. Molecular dynamics predicted homogenous alloy formation for Ta–Ti–V–Zr–Hf. In contrast, a tendency for Ta–W atomic-pair clustering was seen for Ta–Ti–V–Zr–W, in agreement with microstructure after melt-casting and homogenization. Both alloys showed stable microstructure with high hardness, which was retained up to 300 °C.

The effect of oxygen on phase stability and β–α″ martensitic transformation in Ti–Nb alloys has been studied using first principles calculations. Three stable atomic configurations of Ti–Nb (Ti-12.5, 16.6, and 25 at.% Nb) systems, which can transform from β-phase to α″-phase without changing the local atomic position of Nb atoms, were first identified using the cluster expansion method. Phonon calculations indicated that these structures were stable. Next, the martensitic transformation behavior of Ti–Nb–O system was studied using these structures. We observed a significant lattice distortion around oxygen atoms occupying octahedral interstitial sites that resembles a bcc type of stacking. Our results conclusively revealed that while the oxygen interstitials can oppose the atomic shuffle required for martensitic transformation, they can also cooperatively stabilize the β-phase even at 1 at.% oxygen concentrations by inducing local elastic shear strains. Interestingly, the canceling of these fields can stabilize the β-phase by suppressing the β to α″ transformation which decreases the martensitic start temperature (Ms). Our study revealed that the reduction in Ms is higher at lower Nb concentration. The stabilization of β-phase increases with oxygen concentration.

Laser powder bed fusion (LBPF) was employed to fabricate a strain-transformable β-Ti alloy, Ti-10V-2Fe-3Al (wt%). While the alloy is known to exhibit transformation induced plasticity (TRIP), the as-fabricated alloy, under tensile loading, did not show the same TRIP effects, even though it exhibits the same β + ω microstructure. The repeated heating-cooling cycles experienced during the LBPF process leads to the early stages of rejection of solute elements (Fe, V, and Al), forming isothermal omega (ω) precipitates, which were captured via detailed investigations coupling transmission electron microscopy (TEM) and three-dimensional atom probe tomography (APT). While these homogeneously distributed isothermal ω precipitates lead to a higher yield strength, the TRIP/TWIP effects within the β matrix were suppressed, leading to very low ductility and virtually no strain-hardenability. Interestingly, after a simple β-solution heat treatment followed by quenching, leading to a β + ω (athermal) microstructure, the TRIP/TWIP effects were reactivated in the same LPBF Ti-10 V-2Fe-3Al alloy. The alloy exhibited substantial recovery of tensile ductility and a very large strain hardening (tensile strength minus yield strength ~500 MPa), with a high average strain hardening rate ~15000. Such a very high strain hardening rate in case of LBPF processed Ti-10 V-2Fe-3Al, appears to arise from a rapid strain-induced transformation from β to α" at the early stages of plastic deformation, leading to a high-volume fraction of the martensitic phase, coupled with hierarchical twinning within the martensite plates.

Here, an excellent cavitation erosion-corrosion resistance of stainless steel using a novel severe surface deformation technique, reciprocating friction processing (RFP) was demonstrated. The RFP treated stainless steel showed nano-grained, nano-twinned microstructure along with high dislocation density. The RFP treated steel significantly surpassed the performance of advanced structural materials including high entropy alloy (Al0.1CoCrFeNi) and complex concentrated alloy (SiC reinforced Al0.1CoCrFeNi), demonstrating the lowest cavitation erosion and cavitation erosion-corrosion rates for this class of materials. The RFP treated steel demonstrated outstanding corrosion behavior in 3.5% NaCl solution as well. The unusual performance of RFP treated steel is attributed to its excellent mechanical strength and strong passivating behavior.

A dense array of vertically aligned indium antimonide (InSb) nanowires with high aspect ratio (diameter 150 nm, length 20 &mu, m) were grown in the pores of a track-etched polycarbonate membrane via a one-step electrochemical method. There are several reports on InSb nanowire growth in the pores of a mechanically rigid, nano-channel alumina template (NCA), where nanowire growth occurs in the pores of the NCA. This work on InSb nanowire growth in pores of track-etched polycarbonate (PC) membrane sheds light on the various factors that affect nucleation and nanowire growth. The average length and diameter of the as-grown nanowires was about 10 &mu, m and 150 nm, respectively. Two possible mechanisms accounting for two different morphologies of the as-grown nanowires are proposed. The polycrystallinity observed in some of the nanowires is explained using the 3D &lsquo, nucleation-coalescence&rsquo, mechanism. On the other hand, single crystal nanowires with a high density of twin defects and stacking faults grow epitaxially by a two-dimensional (2D) nucleation/growth mechanism. To assess the electrical quality of the nanowires, two- and four-terminal devices were fabricated using a single InSb nanowire contacted by two Ni electrodes. It was found that, at low bias, the ohmic current is controlled by charge diffusion from the bulk contacts. On the other hand, at high bias, the effects of space charge limited current (SCLC) are evident in the current&ndash, voltage behavior, characteristic of transport through structures with reduced electrostatic screening. A cross-over from ohmic to SCLC occurs at about 0.14 V, yielding a free carrier concentration of the order of 10 14 cm &minus, 3 .

The effect of W, Mo, V, Ta, and Nb, five common β-stabilizing substitutional elements, on α-Ti stacking fault energy has been studied using first principle calculations. The generalized stacking fault energy (GSFE) curves have been determined for different concentrations of β-stabilizers at the fault plane using supercells with up to 360 atoms. Both basal and prismatic slip systems with the stable (γSF) and unstable (γUSF) stacking faults and twinning fault energies were determined. All the alloying elements reduce the stacking fault energy for Ti for both basal and prismatic slip. At higher concentration of 25 at. % of V, Ta, and Nb at the slip plane, the basal slip becomes more favorable than the prismatic slip in Ti. Ti-Mo and Ti-W systems also show a significant shift in the GSFE curve towards a higher shear deformation strain along 〈 011¯0〉 due to the change in bond character between Ti and those two elements. Using Rice criterion, which employs γS/γUSF ratio to estimate ductility, we show that all t...