Grain boundary segregation can reduce the driving force for grain growth in nanocrystalline materials and help retain fine grain sizes. However, grain boundary segregation is enthalpically driven, and so a stabilized nanocrystalline state should undergo a disordering process as temperature is increased. Here we develop a Monte Carlo-based simulation that determines the minimum free energy state of an alloy with a strong tendency for grain boundary segregation that considers both different grain sizes and a large solute configuration space. We find that a stable nanocrystalline alloy undergoes a disordering process where grain boundary segregated atoms dissolve into the adjacent grains and increase the grain size as a function of temperature. At a critical temperature, the single crystal state becomes the most preferred. Using this method, we are able to determine how the grain size changes as a function of temperature and produce equilibrium phase diagrams for nanocrystalline alloys.

In the microsize regime, all crystalline metals studied to-date exhibit a “smaller-is-stronger” size effect. Here, we report an unusual weakest-size phenomenon in the precipitated alloy duralumin 2025, i.e., below a critical size of ∼7 μm, the strength increases as the size decreases, while above this size, the strength increases toward the bulk value with increasing size. At the critical size, strain-hardening is also slowest and the room-temperature creep is fastest. Interestingly, the reduction of strength at the weakest size is more significant for the peak-aged state of duralumin 2025 than its naturally aged state. Theoretical modeling shows that at the weakest size, both strengthening mechanisms of precipitation hardening and dislocation starvation are ineffective. The present results indicate that the conventional wisdom of precipitation hardening is not applicable in the micro-regime, and the common “smaller-is-stronger” understanding is incorrect when material microstructures impose internal length scales that can affect strength.

Several recent X-ray photon correlation spectroscopy works have reported an anomalous atomic dynamics in hyperquenched metallic glasses. Here, we compare and contrast these microscopic dynamics with that found in a Zr44Ti11Ni10Cu10Be25 bulk metallic glass, prepared with a cooling rate some 6 orders of magnitude lower. In both cases, structural relaxation in the glass is governed by internal stresses, giving rise to highly compressed density correlation functions. Differently from the fast aging reported in previous studies, here the atomic dynamics displays a slow linear atomic-level aging, while not affecting the shape parameter. Traditional macroscopic phenomenological models fail to capture the temperature dependence of the microscopic structural relaxation time, suggesting a length scale dependence of the aging. Interestingly, the dynamics does not seem to be affected by the presence of a low percentage of frozen nanocrystals and displays a temperature dependence similar to that observed in macroscopic viscosity measurements.

With increasing performance requirements in power electronics, the necessity has emerged to investigate the thermo-mechanical behavior of thick Cu metallizations (≥5 µm). Cu films on rigid substrates in the range of 5–20 µm were thermally cycled between 170 and 400 °C by a fast laser device. Compared to the initial microstructures, a texture transition toward the {100} out-of-plane orientation with increasing film thickness was observed during thermo-mechanical cycling, along with an abnormal grain growth in the {100}-oriented grains and a gradual development of substructures in a crystallographic arrangement. Compared to the well-studied thin Cu film counterparts (≤5 µm), the surface damage showed a 1/hf dependency. Transition from an orientation independent (hf = 5 µm) to an orientation specific thermo-mechanical fatigue damage (hf = 10, 20 µm) was observed following a higher damager tolerance in {100} oriented grains.

The effects of high magnetic fields on the solidification structures of ternary Al–Fe–Zr alloy were investigated. The results showed that the primary Al3Fe crystals mainly show bar-like form, whereas the unmelted Al3Zr crystals reveal tabular and the newly crystallized primary Al3Zr crystals have fine/coarse needle-like forms. When a 12 T magnetic field is applied, the primary Al3Fe crystals are distributed homogenously and the fine needle-like primary Al3Zr levitated. Moreover, the primary Al3Fe crystals align horizontally in the upper but vertically in the lower part of the specimen. The needle-like primary Al3Zr crystals align vertically, whereas the tabular ones have their two opposite corners on the large surfaces toward the positive and negative magnetic field direction. Crystallographic analysis indicates that 〈100〉 and 〈110〉 are the preferred axes of the primary Al3Fe and the Al3Zr crystals with respect to the magnetic field, respectively. The redistribution and realignments of the crystals are discussed.

Effects of traveling magnetic field (TMF) on freckle formation of directionally solidified Pb–Sn alloys were investigated experimentally and numerically. The experimental results demonstrated that freckles could form without any magnetic field and with the upward TMF. In the former case, the formation location is the center. In the latter case, it diverted from the center to the surface with the increasing intensity of TMF. However, freckle could not form under the downward TMF. Numerical results indicated that the change of the formation location under upward TMF should be attributed to the different solid/liquid interface shape, and no formation under downward TMF should be attributed to the convection intensity driven by TMF. These magnetic fields are used to modulate melt flow, which is similar to the effects of the gravity. For eliminating freckle, a microgravity environment can be established under the suitable downward TMF.

Aluminum matrix composites were prepared by powder processing route containing three different loadings of graphene nanoplatelets, i.e., 1 wt%, 3 wt%, and 5 wt%. Ball milling of composite powders was performed to ensure the uniform dispersion of nanoplatelets in aluminum powder, followed by their consolidation to near theoretical densities. Microstructural evolution after composite preparation was witnessed by X-ray diffraction, optical microscopy, and scanning electron microscopy, while the mechanical property profile was evaluated by hardness, compression, and flexural tests. The mechanical properties of composites containing 5 wt% nanoplatelets were found with maximum improvements in hardness, compression, and flexural strengths of 35%, 433%, and 283%, respectively. This increase in mechanical performance is related to uniform dispersion and microstructural development in composites by incorporating nanoplatelets. Fractographic characterization indicated a change in fracture morphology from matrix-dominant in pure aluminum to nanoplatelet-dominant in composites. In particular, shearing and pull out of nanoplatelets were observed during the fracture of composites with simultaneous restricted plastic deformation of the surrounding aluminum matrix.

In this study, SiC particle reinforced Al2024 matrix composites were fabricated by powder thixoforming (PT). Meanwhile, 2024 alloys were fabricated by permanent mold cast (PMC) and PT, respectively, to reveal superiorities of PT technology over the traditional processing technologies and the resulting composite over the matrix alloy. The microstructures and mechanical properties of the three materials were comparatively investigated. The results indicated that both the PT materials possessed finer spheroidal primary particles and smaller eutectic concentration, but the PMC alloy comprised large equiaxed grains, continuously net-shaped eutectic structures, and many porosities. The mechanical properties of the PT alloy were significantly higher than those of the PMC alloy because of the enhanced compactness and work hardening, decreased eutectic concentration, and fine primary particles. The incorporation of SiCp to the PT alloy further brought improvements, the ultimate tensile strength (UTS), yield strength (YS), and hardness were increased by 29.3% (UTS = 388 MPa), 35% (YS = 297 MPa), and 46.8% (hardness = 122.6 HV), respectively. A strengthening model considering different strengthening mechanisms and SiCp failure was proposed and YS of composite could be exactly predicted.

The orientation relationship (OR) between Al matrix and several typical inclusions, Al2O3, MgO, AlN, TiB2, and AlB2, in Al alloy have been studied by edge-to-edge matching model and refined with Δg theory. Based on the calculation of interatomic spacing misfit and interplanar spacing misfit, the number of ORs between Al and Al2O3, MgO, AlN, TiB2, AlB2 were predicted to be 1, 7, 2, 2, and 2, respectively. The result reveals that the wettability of Al to the studied inclusions could rank as MgO, AlB2, TiB2, AlN, Al2O3 in the order of decreasing, and the removability of those particles from aluminum melt rank in an opposite way from the perspectives of crystallography features of interfacial energy. Moreover, Al2O3 have a higher sensitivity to the performance of a processed aluminum alloy component than other inclusions, and MgO has the minimal impact, when the studied inclusions were residual in aluminum alloy.

Phase stability, elastic, and thermodynamic properties of (Co,Ni)3(Al,Mo,Nb) with the L12 structure have been investigated by first-principles calculations. Calculated phonon density of states show that (Co,Ni)3(Al,Mo,Nb) is dynamically stable, and calculated elastic constants indicate that (Co,Ni)3(Al,Mo,Nb) possesses intrinsic ductility. Young’s and shear moduli of the simulated polycrystalline (Co,Ni)3(Al,Mo,Nb) phase are calculated using the Voigt–Reuss–Hill approach and are found to be smaller than those of Co3(Al,W). Calculated electronic density of states depicts covalent-like bonding existing in (Co,Ni)3(Al,Mo,Nb). Temperature-dependent thermodynamic properties of (Co,Ni)3(Al,Mo,Nb) can be described satisfactorily using the Debye–Grüneisen approach, including heat capacity, entropy, enthalpy, and linear thermal expansion coefficient. Predicted heat capacity, entropy, and linear thermal expansion coefficient of (Co,Ni)3(Al,Mo,Nb) show significant change as a function of temperature. Furthermore the obtained data can be used in the modeling of thermodynamic and mechanical properties of Co-based alloys to enable the design of high temperature alloys.

Al1.3CrFeNi eutectic high entropy alloy was designed and prepared by arc-melting to investigate the microstructure and oxidation behaviors at 1000 °C. The XRD pattern shows that this alloy had a double bcc/B2 structure. SEM images indicates that the microstructure of the alloy is composed of two precipitates of [Cr, Fe] solid solution and NiAl intermetallic, which form the typical eutectic structure. To explore the thermal application of Al1.3CrFeNi alloy, the oxidation behavior of Al1.3CrFeNi alloy at 1000 °C was investigated. From XRD and SEM results, it could be concluded that Al2O3 and Cr2O3 were the predominant oxides during the oxidation process. In addition, spinel like FeCr2O4 was also observed in the oxide scale. According to the analysis of oxide precipitates, the whole process of oxides’ formation was discussed and a simplified oxidation dynamic model of Al1.3CrFeNi alloy at 1000 °C was obtained. This could promote the development of thermal applications in multi-component alloys field.

A series of oxidation experiments were carried out on these novel γ/γ′-strengthened cobalt-based alloys of the systems Co–9Al–10W and Co–9Al–10W–0.02X (X = La, Ce, Dy, Y) at 900 °C. The appropriate amounts’ addition of rare earth elements leads to improved oxidation properties at 900 °C, especially La elements show the best oxidation resistance (129.008 mg/cm2). However, the base Co–9Al–10W alloy shows the worst oxidation performance (151.544 mg/cm2). Multilayer oxide layers formed during the oxidation process, the outer were mainly CoO and Co3O4 oxides, and the middle layer contained complex oxides (containing Co, Al, and W). The inner layer consists of little discontinuous oxides, included few Al2O3 oxides. There existed a different crack width and the base alloy had the widest crack. Moreover, there exists a phase transformation (γ/γ′ to γ/Co3W) at the interface between oxide film and substrate.

The effect of Cr fibers on the deformation of directionally solidified NiAl–Cr eutectics, prepared at three different solidification speeds affecting the fiber diameter and fiber spacing, was studied at varying length scales using different micromechanical testing techniques. In situ tensile tests in a scanning electron microscope of individual Cr fibers showed high strength accompanied by ductile behavior. Comparative microcompression tests on single-phase NiAl pillars and NiAl pillars containing a single fiber showed that the pillars with the single fiber were marginally weaker than the single-phase pillars for similar pillar diameters. Composite pillars with multiple fibers exhibited an increase of 0.2% offset strength values with increasing solidification speed. Transmission electron microscopy of the composite pillars containing a single fiber after deformation revealed significant dislocation activity both in the fiber and the matrix. It is argued that the interface between the fiber and matrix acts as dislocation source promoting plastic deformation of the brittle NiAl matrix.

In this study, the time that molten magnesium is in contact with the aluminum insert before solidification was predicted by solving the conservation equations of mass, momentum, and energy during compound casting of dissimilar Al/Mg couples. For this purpose, a three-dimensional transient model and FLOW-3D software were utilized and distributions of temperature and velocity vectors in the fluid over time were obtained. Then, the contact time at the bottom, middle, and top of aluminum insert in its interface with the magnesium melt was calculated. The results of simulation show that the contact time decreases from about 1.7 s at bottom to 1.6 s at middle and 0.8 s at top of the interface, respectively. This is consistent with the experimental metallographic observations which indicate a decrease in the thickness of formed intermetallic compounds from bottom to middle and top of the Al/Mg interface.

It is desirable to evaluate the ratcheting behavior of biomedical magnesium under cyclic loading with and without precorrosion, due to the promising future in biomedical implant field. This study focuses on the investigation of the uniaxial ratcheting strain evolutions of ZEK100 magnesium alloy sheet under various loading conditions and different corrosion time. To illustrate the ratcheting response in detail, the effects of several factors on the ratcheting strain evolution were discussed, including mean stress, stress amplitude, specimen orientations, loading history, and precorroded duration. A series of asymmetrical multistep stress-controlled ratcheting tests were conducted. The mean stress, stress amplitude, and precorrosion duration have significant influence on the ratcheting response of material. ZEK100 magnesium alloy is sensitive to loading history. ZEK100 magnesium alloy exhibits anisotropic behavior, and it is found that the final ratcheting strain of transverse direction (TD) specimens is always larger than that of rolling direction (RD) specimens. The corrosion behavior of ZEK100 magnesium alloy in phosphate buffered solution (PBS) simulated physiological environment was also studied. The corrosion process is characterized by pitting corrosion, and the corrosion rate of material stabilizes at about 2.4 g/(m2 d) after an exponentially decrease at initial stage.

Wear and corrosion properties of AM70 magnesium alloy subjected to equal channel angular pressing (ECAP) were investigated using pin-on-disc dry sliding wear test and electrochemical impedance spectroscopy (EIS), respectively. Wear test was conducted with 30 and 40 N loads with sliding distance of 5000 m and at a constant speed of 3 m/s. Reduced coefficient of friction (COF) and wear mass loss of ECAP processed samples showed increased wear resistance. Worn surface analysis by scanning electron microscope (SEM) showed the presence of delamination, wear debris, and plowing. Energy dispersive X-ray spectrometer (EDS) revealed the occurrence of oxidation, and the wear mechanism was identified as abrasion and oxidation wear. EIS plots showed the improvement in corrosion resistance of ECAP processed magnesium alloy compared to initial condition due to grain refinement and homogeneous distribution of secondary particles.

The surfacing welding has been widely utilized in the industrial equipment manufacturing and repairs. The wear properties of surfacing alloys have an important effect on the whole performance of repaired components. The solution treatment (T4) and solution treatment followed by aging (T6) effects on the dry sliding wear behavior of surfacing AZ91 magnesium alloys with tungsten inert gas welding were investigated in this work. The results demonstrated that the surfacing alloy without treatment exhibited poor wear resistance, due to the massive intermetallic β-phases (Mg17Al12). These phases were believed to produce stress concentrations in the particle-to-matrix interface and tended to generate cracks during friction. The T4 alloy had more improved wear resistance than the as-received alloy. The T6 treatment improved the wear resistance further, resulting from the high density dispersed fine β-phase precipitation in the α-Mg matrix, which enhanced the alloy strength and hardness and decreased the subsurface metal deformation degree caused by friction.

Low volume fraction (0.5, 1, and 2 vol%) SiO2 reinforced magnesium nanocomposites were synthesized using powder metallurgy technique followed by hot extrusion. The nanocomposites were studied for physical, microstructural, ignition, and mechanical properties to study the influence of nanoparticulate addition on monolithic magnesium. The grain size of the developed nanocomposites was observed to marginally decrease with the addition of SiO2 nanoparticulates with 2 vol% SiO2 addition resulting in a grain size of ∼23 μm which is ∼32% lower than that of pure Mg. The ignition temperature of pure Mg was enhanced with the addition of SiO2 nanoparticulates with Mg 2 vol% SiO2 nanocomposite exhibiting an ignition temperature of 611 °C (∼20 °C greater than pure Mg and AZ31 alloy). Under room temperature tensile loading, Hall–Petch strengthening mechanism was the most dominant wherein the addition of SiO2 nanoparticulates to pure magnesium enhances the strength within 0–2 vol% range and ductility in 0–1 vol% range.

Cyclic deformation and low-cycle fatigue behavior of Mg–10Gd–3Y–0.5Zr alloy in sand-cast and aging treatment conditions (sand-cast-T6) were investigated by carrying out full reversed strain-controlled tension-compression tests at the strain amplitude ranging from 0.25 to 0.7%. The results show that stress–strain hysteresis loops of the studied alloys display near tension-compression symmetry, which is dominated by microstructure and strain amplitude. Both sand-cast and sand-cast-T6 alloys exhibit cyclic hardening and softening phenomenon with increasing loading cycles. Meanwhile, the fatigue life of the aged alloy is higher than that of the sand-cast alloy at all applied strain amplitudes. The theoretical strain fatigue limits (ε0) of sand-cast and sand-cast-T6 alloys are 2.1% and 2.3%, respectively. In addition, the low-cycle fatigue behavior of the studied alloy at different strain amplitudes was also investigated.