Steels with the body-centered cubic (bcc) structure suffer low-temperature brittleness due to an inherent ductileto-brittle transition that inhibits plastic deformation. Strategies to improve the cryogenic toughness generally involve stabilizing a face-centered cubic (fcc) phase to prevent this transition; however, this involves alloying with high concentrations of nickel, cobalt, and chromium, which are expensive and unsustainable due to their high environmental impact, energy-intensive extraction processes, and limited global reserves. Here, we engineered a low-carbon, micro-alloyed steel to possess a dual-phase, ultrafine-grained ferrite/martensite lamellar microstructure. This structure confers an unusual inverse-temperature dependence of impact toughness across a broad temperature range (383 K to 77 K) and exceptional resistance to fracture under both impact and quasistatic loading conditions at cryogenic temperatures (77 K). These properties are achieved through a combination of extrinsic toughening from delamination and crack bridging, as well as intrinsic toughening by interface dislocation-mediated plastic deformation within ferrite and activation of multiscale substructure sliding in martensite. This microstructural design strategy offers a pathway to engineer plain bcc steels with exceptional cryogenic damage tolerance without the addition of expensive and critical elements.

Understanding how grain boundaries mediate fracture remains a critical challenge in designing ductile, high-performance refractory alloys. Here, we extend the Rice-Thomson criterion to account for the angle between cracks and the impinging grain boundaries (GBs), capturing the competition between intergranular fracture and dislocation-mediated plasticity. Using machine learning interatomic potentials, we performed molecular statics simulations to probe fracture mechanisms in nanocrystalline NbMoTaW and Nb45Ta25Ti15Hf15, each with two different grain sizes, revealing trends consistent with experimental observations and the extended Rice model. Comparison with averaged R-curves for bulk samples demonstrates that GBs enhance ductility in Nb45Ta25Ti15Hf15 in both grain sizes investigated. In contrast, GBs only locally improve fracture resistance in NbMoTaW when cracks are temporarily pinned at GBs inclined at high angles from the crack, but generally promote brittle intergranular fracture. These contrasting behaviors are attributed to differences in GB cohesion, reflecting clear alloying trends that align with ab-initio calculations and trends observed experimentally. Our results bridge classical fracture theory, atomistic simulations, and experimental observations, providing a comprehensive understanding of the fracture mechanisms in nanocrystalline refractory complex concentrated alloys.

A simple Rice-Ashby type model for ductile–brittle transition temperature (DBTT) of body-centered cubic (bcc) complex concentrated alloys (structures) is presented. The effect of accumulation of dislocation density on DBTT is also analyzed. The model results are compared with experimental yield stress vs. temperature data for four complex concentrated alloys: Nb45Ta25Ti15Hf15 (NTTH), MoNbTaW, HfNbTaTiZr, NbTiZr and two pure bcc metals, Fe and W. It is shown that the DBTT behavior of these alloys and pure metals are in agreement with the simple ductility model presented in this manuscript. The DBTT model presented in this manuscript along with yield strength models for bcc complex concentrated alloys described in the literature should serve as a useful guide for designing such alloys with good high temperature strength and significant room temperature ductility.

The effect of microstructural evolution induced by heat treatment on the strength, fracture toughness, and crack resistance curve of a laser powder bed fused (LPBF) Al-Mg-Sc-Zr alloy in three conditions (as-built, peak-aged, and over-aged) and along two orthogonal directions was examined. The as-built samples exhibit a bimodal grain structure comprising ultrafine equiaxed grains and coarse columnar grains, displaying isotropic crack initiation toughness (K-JIC) but anisotropic crack growth resistance, due to preferential crack propagation along ultrafine equiaxed grain regions. Peak-aged samples show increased strength due to densely-distributed Al-3(Sc, Zr) precipitates but reduced toughness and unstable crack growth. In contrast, the over-aged samples exhibit the highest K-JIC values, driven by extensive crack-tip plasticity and larger plastic zone size, leading to pronounced anisotropy. These results emphasize the critical role of microstructure and plastic zone size in governing the fracture behavior of LPBF Al alloys.

The very-high-cycle fatigue (VHCF) behavior of Ti-6Al-4V alloy produced by selective laser melting (SLM) was systematically investigated, with particular emphasis on the microstructural features at crack initiation sites. Fatigue cracks predominantly originated from internal pores, accompanied by the formation of a fine granular area (FGA) and characteristic fisheye (FiE) morphologies. For the first time in the selectively laser melted (SLMed) Ti-6Al-4V, a nanocrystalline layer at the immediate crack surface within the FGA was characterized in detail, providing direct experimental evidence in support of the numerous cyclic pressing (NCP) model for crack initiation and early-stage propagation. The interrelation among pore size, applied stress amplitude, and the degree of microstructural refinement within the FGA was clarified. These findings offer new insights into the fatigue behavior of additively manufactured (AM) alloys and underscore the critical influence of process-induced defects and post-processing treatments on VHCF performance.

Surface and subsurface defects in laser powder bed fusion affect fatigue performance, limiting its application in safety-critical components. Applying contour scanning potentially improves side surface quality and near-surface features, but the involved multi-scale physical mechanisms for defect formation and suppression remain unclear. In this study, we develop a comprehensive and high-fidelity numerical simulation model that simultaneously captures melt pool dynamics, surface roughness formation, and subsurface defect evolution during contour scanning. Model predictions show good agreement with experimental results of melt pool geometry, surface roughness, and near-surface defects. The results demonstrate that controlling energy density near the keyhole regime improves melt pool stability and track uniformity, thereby reducing surface roughness and suppressing the formation of lack-of-fusion and keyhole-induced pores. A mechanism is identified where gas bubbles undergo expansion, shrinkage, and convection-driven motion, influenced by vapor condensation, Marangoni flow, and buoyancy. These dynamics determine whether bubbles coalesce, migrate, or become entrapped within the melt pool. Optimized contour parameters reduce surface roughness by more than 50 % (Sa from similar to 10 to similar to 4 mu m; Sq from similar to 10 to similar to 5 mu m), and extend fatigue life by over threefold (from similar to 7000 to similar to 25,000 cycles) relative to suboptimal conditions. Moreover, the remelting effect of multi-layer scanning promotes pore closure at intermediate depths but increases porosity near the top layers. A gradient-decreasing laser energy strategy is proposed to balance these effects. The identified contour mechanisms provide a scientific basis for developing in-situ laser control strategies aimed at enhancing surface quality and fatigue resistance in additively manufactured metallic components.

The striking variation in damage tolerance among refractory complex concentrated alloys is examined through the analysis of atomistic fracture simulations, contrasting behavior in elemental Nb with that in brittle NbMoTaW and ductile Nb45Ta25Ti15Hf15. We employ machine-learning interatomic potentials (MLIPs), including a new MLIP developed for NbTaTiHf, in atomistic simulations of crack tip extension mechanisms based on analyses of atomistic fracture resistance curves. While the initial behavior of sharp cracks shows good correspondence with the Rice theory, fracture resistance curves reveal marked changes in fracture modes for the complex alloys as crack extension proceeds. In NbMoTaW, compositional complexity appears to promote dislocation nucleation relative to pure Nb, despite theoretical predictions that the alloy should be relatively more brittle. In Nb45Ta25Ti15Hf15, alloying alters the fracture mode compared to elemental Nb, promoting crack tip blunting and enhancing resistance to crack propagation.

Atomic structure and electronic state influence deformation mechanisms in traditional and high-entropy alloys (HEAs). In HEAs, nature and scale differ from those of traditional alloys due to lattice distortion and variations in local chemistry resulting from large concentrations of multiple principal elements. In CrCoNi, a face-centered cubic (fcc) HEA, dislocation dissociation, nanotwinning, and transformation-induced plasticity are promoted at cryogenic temperatures (<77 K). In Nb45Ta25Ti15Hf15, a body-centered cubic (bcc) HEA, screw dislocations, twinning, and kink band formation are activated at temperatures ranging from 77 to 1,473 K. These deformation mechanisms impart exceptionally high fracture resistance in CrCoNi and Nb45Ta25Ti15Hf15. However, their tensile stress-strain curves differ significantly at these temperatures; while CrCoNi exhibits extensive strain hardening, Nb45Ta25Ti15Hf15 demonstrates nearly elastic, perfectly plastic stress-strain behavior. Understanding the origin of the high fracture resistance of these alloys, despite their contrasting stress-strain behavior, would enable the discovery of HEAs suitable for applications in extreme environments.

The discontinuous nature of the Bingham model poses significant challenges in the implementation and analysis of elastohydrodynamic lubrication (EHL). To address this problem, a modified Bingham model is developed and incorporated in the hard elastohydrodynamic lubrication (hard-EHL) analysis of ‘smart’ lubricants using the generalized Newtonian approach. The proposed model is validated through the observed formation of adherent and floating ‘cores’. A comprehensive parametric analysis is also conducted to examine the impact of the slide/roll ratio, dimensionless speed, dimensionless load, and dimensionless material parameter on EHL characteristics. The numerical findings are found to be consistent with the experimental measurements of EHL film thickness pertaining to electrorheological fluids. The findings are highly encouraging, demonstrating that by appropriately modifying the yield stress, ‘smart’ lubricants can be tailored to meet specific operational requirements.

Unnotched and notched tensile properties of Ti-6Al-4V (Ti64) alloy, additively manufactured using the laser powder bed fusion (LPBF) technique, at cryogenic temperatures of 90, 77 and 20 K were investigated. The LPBF process parameter combination was chosen such that the investigated alloy has not only a minimum in porosity, but also an equiaxed prior beta microstructure, which predominantly contains acicular alpha/alpha' lath structure in basket-weave morphology, that results in a high strength and ductility combination as well as insignificant mechanical anisotropy at room temperature (300 K). Tensile tests revealed that the yield (sigma(y)) and tensile (sigma(u)) strengths of LPBF Ti64 are superior while elongation to failure (e(f)) is comparable to those of the conventionally manufactured (CM) Ti64 down to 77 K, owing to the fine alpha/alpha' lath microstructure in the former. While the progressive reduction of -basal and prismatic dislocation slip activity with decreasing deformation temperature enables a steep rise in sigma(y), the unnotched specimens fractured catastrophically without macroscopic yielding at 20 K. Detailed postmortem analyses revealed the absence of significant twinning-based deformation in LPBF Ti64, in stark contrast to the CM ones at cryogenic temperatures. Instead, deformation kink bands with varying sizes and misorientation were found to be increasingly active within the prior beta grains. A large misorientation at the kink band boundary within the beta grain leads to void nucleation followed by crack growth, which eventually results in brittle failure at 20 K. While relatively notch insensitive from 300 to 77 K, LPBF Ti64 was notch brittle at 20 K. These results highlight the importance of the unique hierarchical micro- and meso-structural features of LPBF Ti64 on the tensile mechanical behavior, especially at cryogenic temperatures.

In this inceptive study, the effectiveness of the lateral sliding mode controller (LSMC) in controlling the Active Magnetic Bearings (AMBs) states in a flexible rotor system subjected to synchronous mass unbalance has been demonstrated. For accomplishing this task, a flexible rotor system has been modeled using the finite element method (FEM) considering Timoshenko beam elements for the discretization of the rotor, resulting in a 28-DOFs AMB-rotor system, followed by the implementation of second-order LSMC. It has been reported that during the levitation stage of the rotor, the control current fluctuates between +/- 2 A. Further, it has been shown that the transient response at unbalanced disk location is within the permissible limit of 1.5 mm for the operational speed range of 0-10,000 rev/min. Additionally, the performance of the controller and the system transient response at constant rotational speeds of 2000 and 8000 rev/min are also evaluated. The sigmoid and saturation functions are shown to minimize the problem of chattering due to the inherent discontinuity in the signum-function based switching.

Aluminum alloys that are additively manufactured using the laser powder bed fusion (LPBF) suffer from relatively poor high cycle fatigue (HCF) resistance. In an effort to alleviate this, a high-strength Al alloy, Al-Mn-Mg-Sc-Zr, with columnar, equiaxed, and bi-modal microstructures was produced by varying the scanning velocity and the substrate temperature during the LPBF process. The tensile strength of LPBF Al-Mn-Mg-Sc-Zr alloy is 475 ± 5 – 516 ± 6 MPa with favorable elongation of approximately 11 %, higher than that of most of the other Al alloys, including conventional high-strength rolled/ECAP Al alloys and AM Al-Mg-Sc-Zr alloys. Specimens with bimodal microstructure and specimens with fully equiaxed microstructure both show a fatigue strength of 230 MPa (at 107 loading cycles), which is the highest among those reported for the LPBF Al alloys. The deformation synergy in the bimodal microstructure also improves the fatigue resistance in the strain-controlled low cycle fatigue (LCF) regime. The equiaxed microstructure restricts the to-and-fro dislocation motion during cyclic loading, which, in turn, minimizes the strain localization. At the later stages of strain accumulation, microcracks form at the grain boundaries, limiting the further improvement of the alloy's fatigue strength. This study demonstrates microstructural tailoring through AM enables improvement of the fatigue resistance of aluminum alloys.

This is an inceptive attempt to replace the classical Bingham fluid model with a continuous double Newtonian power law-based constitutive equation for smart lubricants like magneto-rheological, electro-rheological, and ferro-fluids. The implementation of Bingham model in hydrodynamic (HD) and elastohydrodynamic (EHD) lubrication analyses is highly challenging and inconvenient due to its inherent discontinuity. Therefore, the present work demonstrates the use of an already existing rheological model with an appropriate set of parameters to describe the flow behavior of smart lubricants in a soft-EHD lubrication algorithm based on the generalized Newtonian approach. The formation of both floating and adherent cores validates the proposed model. An extensive parametric study is also performed to explore the effects of operating speed, load, and slide-to-roll ratio on the soft-EHL characteristics. The results are very promising, showing that it's possible to customize smart lubricants to match specific operating conditions. This is achieved by adjusting the yield stress value accordingly, allowing for the desired variation in lubrication characteristics.

The present work investigates the transient and steady state behavior of an internally damped coaxial-rotor system (CRS) subjected to synchronous mass-imbalance. A mathematical model has been developed using a finite-element method incorporating rotary and shear inertia effects with detailed intershaft bearing modeling. The system equations expressed in state-space form are solved using the Modified Euler’s method for transient analysis. Notable findings include the identification of two backward and two forward whirl frequencies within the rotational speed range of 0–12000 RPM. Further, it is concluded that internal damping significantly affects the CRS stability as destabilization was found to occurs above 8371 RPM. Maximum steady-state amplitudes of vibration are obtained at Bearing-4 and Disk-3, with magnitudes of 1.6 mm and 1.3 mm, respectively. Besides, during run-ups, changes in angular acceleration from 150 to 300 rads−2 lead to a substantial reduction of 22.23

Strengthening materials via conventional “top-down” processes generally involves restricting dislocation movement by precipitation or grain refinement, which invariably restricts the movement of dislocations away from, or towards, a crack tip, thereby severely compromising their fracture resistance. In the present study, a high-entropy alloy Al 0.5 CrCoFeNi is produced by the laser powder-bed fusion process, a “bottom-up” additive manufacturing process similar to how nature builds structures, with the microstructure resembling a nano-bridged honeycomb structure consisting of a face-centered cubic ( fcc ) matrix and an interwoven hexagonal net of an ordered body-centered cubic B2 phase. While the B2 phase, combined with high-dislocation density and solid-solution strengthening, provides strength to the material, the nano-bridges of dislocations connecting the fcc cells, i.e ., the channels between the B2 phase on the cell boundaries, provide highways for dislocation movement away from the crack tip. Consequently, the nature-inspired microstructure imparts the material with an excellent combination of strength and toughness.

This study proposes the utilization of artificially textured surfaces to enhance film thickness in line contacts operating under soft elastohydrodynamic lubrication. Moreover, this investigation seeks to examine the effect of roughness of the surface, applied load, and average rolling speed on the efficacy of artificially textured surfaces in artificial knee joints, with the objective of enhancing the life of Total Knee Arthroplasty. The numerical analysis involves solving the elasticity and Reynolds equations. One of the two surfaces is artificially textured by superimposing a sinusoidal profile on its macro-geometry. The Newton-Raphson technique is employed to obtain the numerical solution of the governing equations discretized using the finite difference approach. For comparative analysis, both artificially textured and smooth surface conditions are individually considered in this study. The artificially textured surface exhibits a considerable enhancement in the thickness of the lubricating film compared to the smooth surface case under identical operating conditions. Furthermore, it is noted that the efficacy of the artificially textured surface becomes more evident at higher magnitudes of applied load and lower average rolling speeds. The significant enhancement up to 8 % in minimum film thickness achieved in this study represents a notable and valuable contribution to the field, underscoring its novelty and potential impact.

The present work demonstrates the effectiveness of an active magnetic damper (AMD) in improving the vibration characteristics of an overhung rotor system (ORS) considering the effects of synchronous mass-unbalance, rotor bow, and viscous internal damping. A finite element-based rotordynamic analysis has been conducted to evaluate the system response using state-space formulation. MATLAB codes are developed for the numerical implementation of the rotordynamic model pertaining to the flexible ORS integrated with an AMD consisting of three parallel feedback loops. The internal damping is found to reduce the vibration amplitudes at the bearing and disk locations. However, it leads to system instability in the absence of AMD beyond the first critical speed (670 RPM) which is indicated by the change in the real part of eigenvalues. Under these circumstances, the use of AMD causes a remarkable improvement and the system stabilizes for almost the entire speed range considered here, 320–10,000 RPM. In addition, the AMD causes a maximum reduction of 95.8%, 96.5%, and 83.3% in the vibration amplitudes at the locations of Bearing 1, Bearing 2, and the overhung disk, respectively. Using an appropriate set of AMD parameters, the system characteristics can be altered effectively as per the requirement.

Engineering materials exhibit an undesirable tradeoff between strength and resistance to crack propagation (fracture toughness). Here we demonstrate how this tradeoff can be circumvented by thermo-mechanical processing that produces a partially recrystallized, heterogeneous microstructure. An equimolar CrCoNi alloy was forged at room temperature (298 K) to produce high densities of three-dimensional crystallographic defect networks. Post-deformation heat treatments caused localized recrystallization that resulted in a bimodal microstructure with hard, non-recrystallized grains and soft, recrystallized grains. In this condition, the yield strength at 298 K is 2.75x the values previously obtained for the same alloy in the fully recrystallized state while the fracture toughness remains the same. The yield strength is further enhanced at 77 K without compromising the fracture toughness. This outstanding strength-toughness combination at 77 K exceeds those reported for other metallic materials and appears to result from the composite nature of the microstructure with non-recrystallized grains providing strength and recrystallized grains enabling plasticity that dissipates stresses during crack propagation. Our findings indicate that by tuning the degree of recrystallization through thermomechanical processing techniques, it will be possible to further expand the envelope bounding the strength and toughness of a range of structural metals at engineering component scales.

Single-phase body-centered cubic (bcc) refractory medium- or high-entropy alloys can retain compressive strength at elevated temperatures but suffer from extremely low tensile ductility and fracture toughness. We examined the strength and fracture toughness of a bcc refractory alloy, NbTaTiHf, from 77 to 1473 kelvin. This alloy's behavior differed from that of comparable systems by having fracture toughness over 253 MPa·m1/2, which we attribute to a dynamic competition between screw and edge dislocations in controlling the plasticity at a crack tip. Whereas the glide and intersection of screw and mixed dislocations promotes strain hardening controlling uniform deformation, the coordinated slip of <111> edge dislocations with {110} and {112} glide planes prolongs nonuniform strain through formation of kink bands. These bands suppress strain hardening by reorienting microscale bands of the crystal along directions of higher resolved shear stress and continually nucleate to accommodate localized strain and distribute damage away from a crack tip.