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While conventional Gibson–Ashby models provide a general insight into how elastic modulus and yield strength degrade with increasing overall porosity in materials, very limited work has investigated the effects of pore size and distribution on the mechanical properties of metals. One key question is whether and how pores can be utilized for improved mechanical properties rather than being eliminated or minimized for full densification. To fill in this gap, austenitic stainless steel 316L samples with intentional pores of varying diameters and distributions were fabricated by spark plasma sintering using starting powders with different morphologies. Characterization of pore features was not limited to the total volume percentage but also addressed the pore size, shape, interpore spacing, and pore surface area. The mechanical properties of those samples were investigated at multiple length scales to investigate the effect of pore characteristics, including macro-scale compression testing, Vickers micro-indentation, nanoindentation, and nanoscratch. Results suggested incorporating submicron pores improved both the yield strength and strength to weight ratio. The sample containing submicron pores represented an outlier in the classical Hall–Petch relation between yield strength and grain size, and it achieved a yield strength of 482 MPa, compressive strength of ~ 1.4GPa at a strain of 0.3 without fracture, and a specific yield strength of 67.7 MPa cm3/g. The mechanism was attributed to local stiffening and (Cr, Mn)-rich precipitates surrounding the submicron pores. It was discovered, for the first time, the specific yield strength and the pore diameter followed a Hall–Petch type correlation.

Spark plasma sintering (SPS) utilizes joule heating to achieve rapid heating to densify a powder compact. Thermal gradients during SPS lead to microstructural and property gradients in the sintered parts. To elucidate the effect of starting powder morphology on these gradients, the present work investigated location-specific microstructure and mechanical properties of austenitic stainless steels fabricated via SPS of unsymmetrical powder compacts by paring three types of powders: spherical, plate-like and flakey. Local mechanical properties as a function of radial distance from the center of the sample were investigated via microhardness tests, nanoindentation, and nanoscratch. Radially graded microstructure (porosity and grain size) resulting from the thermal gradients in SPS led to gradients in local microhardness (ΔH = 120–150 MPa) and Young’s modulus (ΔE = 30–65 GPa), yielding higher wear resistance at the sample surface and higher elastic modulus at the sample center, suggesting that higher powder morphology aspect ratios lead to larger thermal gradients.

Mayenite electride has attracted increasing research interests because of its unique electronic properties. The thermionic emission behavior of the mayenite electride is relatively unknown. Previous studies revealed that mayenite electride exhibited a bare work function ranged from 2.1–2.6 eV when the thermionic emission was tested in vacuum, and enhanced emission currents could be achieved by applying a super-high external electric field. In this paper, the thermionic emission behavior and the corresponding effective work function of two types of mayenite electride based composites, mayenite electride-titanium and mayenite electride-carbon, were investigated in an Ar glow discharge plasma at elevated temperatures (400–1000 K) without applying a high external electric field, which is critical for the application in electric propulsion and other aerospace apparatus, yet never had been done before. During the testing, the thermal equilibrium process and plasma sheath expansion were observed. The effective work function of the two mayenite electride composites were determined as a function of temperature. The Rasor-Warner model was applied to determine the bare work function and adsorption-site density of the mayenite electride based composites. Results suggested that the adsorption of Ar ions led to the enhanced thermionic emission (~30 A/m2 at 985 K) and low effective work functions (0.9–2.2 eV) of mayenite electride based composites, without the need of applying high electric fields. Our findings will pave the ways for the application of mayenite electride and its composites as the thrust cathode materials for electric propulsion where plasma is present.

The influence of nanoparticle surface facets on electrochromic properties remains largely unexplored in nanostructured “smart” materials. Here, we explore how surface facets influence the coloratio...

Precipitates have been utilized to strengthen various alloys since early last century. Nanostructured (NS) or ultrafine grain (UFG) structured alloys were extensively developed in recent decades, facing a common challenge of limited ductility. While introducing precipitates emerged as an effective strategy to solve the strength-ductility tradeoff issue, the precipitation behavior in NS or UFG alloys differs from that in conventional coarse-grained counterparts. This article provides a review on the scientific discovery of precipitation in NS or UFG alloys in the past 20 years, including the formation mechanism and growth of precipitates, the interplay between precipitates and solid solutes, dislocations, or grain boundaries, as well as the consequent mechanical behavior. Specifically, this article covers Al alloys, Mg alloys, Ti alloys, and high-entropy alloys. Modern research trends in this field are highlighted.

To explore alternative and sustainable feedstock production for metal additive manufacturing, stainless steel powders were fabricated from recycled machining chips by ball milling. To achieve the desirable powder characteristics for additive manufacturing, the effect of ball size on the powder morphology change and particle size reduction was investigated through both theoretical analysis on the maximum deformation depth in the powder during ball milling and extensive experimental work. The modeling results suggest that large balls (diameter = 20 mm) efficiently break up machining chips to coarse powder particles while small balls (diameter = 6 mm) effectively modify the powder morphology to near-spherical. Therefore, a two-stage ball milling approach, i.e., using large balls at the initial stage of milling followed by a second stage of milling with small balls, was proposed in experiments and successfully converted machining chips to powders with near-spherical shape and particle sizes ranged from 38 to 150 μm. In addition, the nanoindentation hardness of the ball milled powder created from machining chips is 56% higher than the gas atomized powder. To verify the usability of the ball milled powder created from recycled machining chips in additive manufacturing, single tracks have been successfully deposited via laser engineered net shaping.

This work investigates the feasibility of a binderless, extrusion-based additive manufacturing approach to fabricate alumina ( A l 2 O 3 ) parts from nanopowder. Traditional manufacture of ceramics with subtractive methods is limited due to their inherent hardness and brittleness, inevitably leading to ceramic parts with less-than-optimal geometries for the specific application. With an additive manufacturing approach, ceramic parts with complex 3D geometries, including overhangs or hollow enclosures, become possible. These complex ceramic parts are highly valuable in heat exchanger, condenser, biomedical implant, chemical reactant vessel, and electrical isolation applications. This research employed direct coagulation of alumina nanopowder slurries with the polyvalent salt tri-ammonium citrate providing the solidification mechanism in an extrusion-based printing process. The viscosity of the slurries was adjusted from ∼35 Pa-s to ∼1000 Pa-s by adjusting pH from ∼9 to ∼4, resulting in a paste that is suitable for extrusion, which retains near-net geometry. It was shown that the direct coagulation approach can be used to create a suspension with tuneable flow characteristics and coagulation rate, and a mechanism describing the process was proposed. The direct coagulation printing (DCP) method is described in detail, including how slurry is extruded, solidified, and printed in complex geometries, and sintered to full density. Parts were printed with a sintered resolution of 450 μm and green densities as high as 65%. After sintering at 1550 °C for up to 2 h, parts were shown to be fully dense (>97%) with an average grain size of ∼2 μm. Mechanical properties were characterized with a comparison to different materials and methods from literature, showing hardness and flexural modulus up to ∼1800 HV and 400 GPa, respectively.

Direct energy conversion using thermoelectric generators (TEGs) is a research area of growing interest because of its potential for increasing energy efficiency. Bulk thermoelectric modules are used widely in industry as Peltier cooling devices. Currently, only bismuth telluride modules are commercially available for power generation. Significant efforts have been put into exploring promising materials and techniques to improve the figure of merit (zT) at laboratory scale (5–20 g). A variety of techniques have been investigated to improve the output and useful temperature range for common industrial TEGs made from bulk polycrystalline materials including segmentation, geometric pinning, and property gradients. However, the improvement in zT at device level (500–1000 g and up) is exceptionally limited. In addition, the thermal degradation of TEGs occurs when cracks form due to thermal stresses that arise from transient heat sources, which consequently lead to a decreased lifetime. Functionally graded material (FGM) thermoelectrics in bulk and polycrystalline form have been developed to mitigate some of these issues by improving the temperature bandwidth, current output range, and lifetime. The present work provides a review of functionally graded TEGs, including their manufacturing, usage and current techniques for improving their performance. This article also provides a pathway to additional research and approaches for improving the efficiency and temperature range, as well as reducing the property degradation of bulk polycrystalline TEGs.

A functionally graded material (FGM) in terms of grain size gradation is fabricated using zinc oxide (ZnO) with spark plasma sintering and an additive manufacturing technique by diffusion bonding layers of material sintered at different temperatures to achieve a thermoelectric generator (TEG) material that can dissipate heat well and retain high energy conversion efficiency for longer-lasting and comparably efficient TEGs. This FGM is compared to a previously made FGM with continuous grain size gradation. Uniform and graded grain size conditions are modeled for thermoelectric output by using thermoelectric properties of the uniform grain size as well as the varying properties seen in the FGMs. The actual thermoelectric output of the samples is measured and compared to the simulations. The grain size has a large effect on the efficiency and efficiency range. The samples are thermally cycled with a fast heating rate to test the thermal stress robustness and degradation, and the resistance at the highest temperature is measured to indicate degradation from thermal stress. The measured efficiency after cycling shows that the FGMs survive longer lifetime than that with uniform small grains.

In the present study a novel bilayer composite consisting of a tough layer (AA7075) and hard layer (AA7075/B 4 C composite), was successfully synthesized using plasma activated sintering. No obvious defects such as porosity, cracks, or delamination were observed in the as-processed bilayer composite. Transmission electron microscopy results suggest that dislocation density in the vicinity of B 4 C particles is higher relative to that of the unreinforced layer, as imaged with the [011] zone axis. The precipitation behavior is significantly different in the two layers, despite identical process conditions. Moreover, no obvious precipitates free zones were observed in the interfacial region between the two layers. To provide insight into the mechanical behavior of the bilayer composites, we measured the bending strength on two different load-bearing surfaces. Our results indicate that when the AA7075/B 4 C layer was loaded under tensile conditions, the AA7075/B 4 C composite was prone to crack formation and catastrophic failure. In contrast, when the AA7075 layer was loaded in tensile, the bilayer composite achieved a notable bending strength value of 1234 ± 72 MPa, which is higher relative to that of aforementioned case (938 ± 85 MPa). Interestingly, no interfacial cracks or delamination were observed under these testing conditions. In addition, the fracture mechanisms that are active in the bilayer composites for the load bearing conditions studied were discussed in detail.

Metal additive manufacturing (AM) is commonly promoted as a sustainable technology because of its capability to produce engineering components of complex geometries in a single step. Prior studies on environmental impact assessment of metal AM have underestimated the resource consumption due to the overestimation of the material utilization factor and neglecting the energy consumption needed to produce the desirable feedstock powder. This paper aims to address the role of feedstock powder and the actual material utilization factor in environmental impact assessment of laser metal deposition. Recycling and reuse of the unfused powder as the feedstock for subsequent depositions are proposed.

We report on an investigation of the influence of reinforcement particle size on the microstructure and mechanical behavior of Al metal matrix composites. In our work, Al 7075/B 4 C composites containing three types of B 4 C particle sizes (56.9 µm, 4.2 µm and 2.0 µm) were synthesized and studied. For a constant value of volume fraction of B 4 C, the composite with coarse reinforcement particles exhibited a relatively homogeneous and discrete distribution of the B 4 C particles while the composites with fine reinforcement exhibited agglomeration of the B 4 C particles. The composite with the smallest B 4 C particles possessed the highest yield strength and fracture strength. Quantitative analysis of the strengthening mechanisms revealed that smaller B 4 C particles lead to larger values in strain gradient strengthening as well as CTE mismatch strengthening, which are significantly correlated to the geometrically necessary dislocations caused by the presence of B 4 C. In addition, the different spatial distributions of the B 4 C particles contributed to different fracture mechanisms in the composites.

Bending tests of submicrometer Al pillars were performed in-situ in a transmission electron microscope (TEM). The Al crystal in the bent region experienced substantial lattice distortion, as well as grain refinement resulting in a disordered dislocation configuration arising from a random distribution of low angle grain boundaries (LAGBs). This observed disordered dislocation configuration is in contrast to formation of a low energy dislocation configuration, as predicted on the basis of the theory of strain gradient plasticity, for bulk materials subjected to plane strain bending.

High temperature oxidation behavior of the bond coat layer is a critical factor that controls the failure mechanism of thermal barrier coatings (TBCs). Previous work reveald that TBCs with cryomilled NiCrAlY bond coats exhibited an improved oxidation behavior compared to equivalent TBCs with conventional bond coats. The cryomilled NiCrAlY bond coats contributed to a slower growth rate of thermally grown oxides (TGO) with a final thinner thickness and enhanced homogeneity in TGO composition. To better understand the improved oxidation behavior, a mechanistic investigation based on diffusion theory and quantum mechanics is performed to elucidate the role of aluminum diffusion in the oxidation behavior and how the microstructural features of the cryomilled NiCrAlY bond coats, i e, the creation of a thermally stable, uniform distribution of ultrafine Al-rich oxide dispersoids, affect the diffusion kinetics of Al and the migration of free electrons. It is revealed that these Al-rich oxide dispersoids result in a uniform diffusion of Al and slow migration of free electrons within the NiCrAlY bond coat, consequently leading to the improved oxidation behavior.

Nanostructured Al 5083-based composites with nano-TiB 2 reinforcement particles were fabricated via cryomilling and spark plasma sintering (SPS). TEM observation revealed that the Al matrix consists of equiaxed nano-grains (average size, ∼74 nm), and the reinforcement, TiB 2 nanoparticles (n-TiB 2 ), was distributed discretely and homogeneously in the Al matrix. The interface between the Al-matrix and n-TiB 2 appears to be free of defects, and no obvious discontinuities were observed. The composite exhibits a compressive strength of 817 MPa with 6.0% strain-to-failure. The strength is 20% higher than that of an equivalent SPS consolidated Al 5083 without reinforcement. Nanoindentation was used in our study to provide fundamental insight into the local microscopic mechanical properties. The strengthening mechanisms of the composites are analyzed taking into account the grain boundaries, the Orowan strengthening from the n-TiB 2 particles and dispersoids such as Al 2 O 3 , AlN and Al 6 Mn, as well as geometrically necessary dislocations induced in the matrix by the nano-TiB 2 particles.

To fulfill the growing demand for alternative and sustainable feedstock production for metal additive manufacturing, a novel dual-stage ball milling strategy was proposed to effectively convert recycled stainless-steel machining chips to powder with desirable characteristics for metal additive manufacturing. A theoretical analysis was performed to evaluate the impact of ball size on the chips-to-powder evolution and the consequent powder morphology. To verify the viability of using the ball milled powder created from machining chips in metal additive manufacturing, single tracks have been successfully deposited via laser engineered net shaping deposition and compared to the single tracks made from gas atomized powder using identical deposition conditions. The microstructures of these single tracks exhibited adequate adhesion to the substrate, a uniform melt pool geometry, continuity, and minimal splatter. Minimal differences in grain structure were observed between the single tracks made from ball milled powder and those made from gas atomized powder.

High-density WC-FeNi ceramic-metal (cermet) composites were fabricated using liquid-phase spark plasma sintering/field-assisted sintering technology (SPS/FAST) with in-situ formation of metal binder phase. The precursor materials were micron-sized powders of WC, Fe, Ni, and C. A low melting point from a eutectic reaction of the powders enabled the in-situ formation of FeNi alloy and facilitates liquid-phase sintering of the WC. The carbon powder was added to stabilize the formation of the binder phase. Electron backscatter diffraction (EBSD) was performed to measure grain size and orientation. The composite exhibited a 99% theoretical density and a microstructure consisting of rounded and contiguous WC grains. The average grain size is 10.5 μm. The composite has a maximum hardness of 16.1 GPa. This research provides a fast and cost-effective approach to fabricate hard metals.

We report on an investigation of the quasi-static and dynamic mechanical behavior of an ultrafine-grained (UFG) aluminum composite reinforced with nanoscale boron carbide under uniaxial compression. Aluminum composites reinforced with either nanometric or half-micron sized B 4 C exhibit high strength ~900 MPa and considerable strain-to-failure under compression (>15 pct.) at quasi-static (10 −3 s −1 ) and dynamic (10 3 s −1 ) strain rates. Microstructural analyses of the composites and unreinforced aluminum alloy were performed to determine the strengthening mechanisms and plastic behavior that govern the structure–property relationships for this class of materials. Our results reveal that the flow stress of the composites does not depend strongly on strain rate if high rate data are included, while strain rate jump tests via instrumented nanoindentation indicate that the strain rate sensitivity exhibits similar behavior as the unreinforced matrix. The unreinforced alloy undergoes strain hardening at both quasi-static and dynamic strain rates, while the composites show strain softening at dynamic strain rates and elastic nearly-perfectly plastic behavior at quasi-static rates. Adiabatic shear band formation was evident in the composite samples during dynamic loading, whereas no shear banding was observed in the unreinforced alloy for the strain rates studied herein. The notion of adiabatic shear band toughness was adopted to identify the increased propensity to shear localization in the composites vis-a-vis the unreinforced material. An attempt is made to account for the effect of the boron carbide reinforcement size on the strengthening mechanism and overall plastic response for these metal matrix composites.

Intragranular coupling of dislocations and precipitates is accomplished in an ultrafine grained aluminum 7000 series alloy through a unique thermo-mechanical processing route that involves high strain rate extrusion at ambient temperature as the last step. The as-extruded materials also exhibited a unique bimodal microstructure consisting of: (1) elongated lamellar grains with dimensions of ∼1 μm consisting of sub-grains via low angle grain boundaries, and (2) ultrafine grains approximately ∼100 nm in size with high angle grain boundaries. Our investigation shows that coupling of dislocations and precipitates within the ultrafine grains has a beneficial impact on the mechanical behavior, and results in an extremely high strength, i.e., ultimate tensile strength ∼878 MPa, with uniform elongation of 4.1% strain at fracture. Interestingly, the T6 temper leads to a decrease in strength for the ultrafine grained material with intragranular dislocations while it enhances ductility, which is opposite the behavior observed in the ultrafine grained material that does not contain a high density of intragranular dislocations. This phenomenon is attributed to the loss in dislocation strengthening and grain boundary strengthening, which could not be compensated for by the strength increase due to precipitation. The underlying mechanisms are discussed on the basis of in-situ heating in a transmission electron microscope, theoretical analysis of diffusion controlled precipitation and microstructure characterization, including transmission Kikuchi diffraction.

The wide variety of diatom frustule shapes and intricate architectures provide viable prototypes to guide the design and fabrication of nanodevices and nanostructured materials for applications ranging from sensors to nanotemplates. In this study, a combined experimental–simulation method was developed to probe the porous structure and mechanical behavior of two distinct marine diatom species, Coscinodiscus sp. (centric) and Synedra sp. (pennate), through ambient nanoindentation and finite element method analysis. These diatom frustule dimensions differed largely depending on diatom species with pore diameters d ranging from 0.3 to 3.0 μm. Young’s modulus E and hardness H measurements of the diatom frustules were obtained via nanoindentation experiments. These values varied depending on diatom species (E between 1.1–10.6 GPa, H between 0.10–1.03 GPa for the Coscinodiscus sp.; and E between 13.7–18.6 GPa, H between 0.85–1.41 GPa for the Synedra sp.). Additionally, the mechanical response of diatom structures to uniform compression was examined. Predictive simulations were performed on the aforementioned diatom frustules, as well as another diatom structure (pennate Fragilariopsis kerguelensis), to correlate the mechanical response with specific morphology variables (e.g., pore or slit sizes). Results from calculated von Mises stress and displacement distributions unveil unique information on the effect that uniform loads have on these frustules, which can aid the design of tailored nanotemplates. A correlation between mechanical properties and porosity was established for selected frustules, and reported for the first time in this study.

It is well known that micrometer-sized and/or submicrometer-sized metallic crystals exhibit “smaller is stronger” size effect: the yield strength σ varies with sample dimension D roughly as a power-law σ ∼ D−m. For some materials, near-theoretical strength values can be attained by reducing the dimensions of crystals to sub-micrometric or nanometric values. At these size scales, however, plastic instabilities, such as strain bursts, strain softening or low strain hardening rates, become operative due to the avalanche-like dislocation generation and escape; such instabilities contribute to disappointing flow intermittency. From a scientific standpoint, the onset of plastic instabilities has hindered fundamental study of the deformation behavior of materials near theoretical strength values. From a technological standpoint, these instabilities limit potential applications in microelectromechanical devices, for example. In this study we demonstrate that by concurrently introducing grain boundaries and secondary phase particles, plastic instabilities can be dramatically suppressed in submicron Al pillars at large strain and that this behavior is attributable to substantial dislocation storage and subsequent grain boundary (GB) mediated plasticity. Consequently the crystals possess superior strength with flow stress larger than 1.0 GPa.

Bulk nanostructured and ultrafine-grained binary Al–Fe alloys have been studied in the past for their remarkable strength, hardness, and thermal stability. These properties have been attributed to a combination of solid solution strengthening, precipitate strengthening, and grain boundary strengthening. However, to date, no systematic investigation has been performed to address the factors that govern the evolution of the various metastable and equilibrium precipitates that form as a result of thermal exposure. In this study, Al–2at.%Fe and Al–5at.%Fe powders were synthesized via helium gas atomization and argon gas atomization, respectively. Cooling rates upwards of 106 K s−1 were achieved resulting in an intermetallic-free starting structure, and a map of the structure as a function of cooling rate was established. Electron backscatter diffraction analysis revealed the presence of a larger number of low-angle grain boundaries relative to high-angle grain boundaries, which influenced nucleation and precipitation of the metastable Al6Fe phase. Cryomilling of the atomized powder was subsequently performed, which led to grain refinement into the nanometer regime, dispersion of the Fe-containing phases, and forcing of 2at.%Fe into solution within the Al matrix compared to negligible Fe in solution in the as-atomized state. Finally, differential scanning calorimetry was utilized to elucidate the metastable Al6Fe precipitation temperature (~300 °C) and subsequent phase transformation to the equilibrium Al13Fe4 phase (~400 °C). An activation energy analysis utilizing the Kissinger method revealed three important factors, in order of importance, for ease of Al6Fe precipitation: segregated regions containing iron, availability of nucleation sites, and the number of diffusion pathways.

The present work investigated the viability of using nanometric 3 mol% yttria partially stabilized zirconia (3YSZ) as a particulate reinforcement in ultrafine grained aluminum alloy matrix composites. Four types of composite materials, with variable amounts of coarse grain regions and volume fractions of 3YSZ, were fabricated through cryomilling and hot isostatic pressing; one of the materials was extruded. Al–Mg alloy (AA5083) was selected as the matrix alloy. Microstructural characterization revealed that the 3YSZ particles were well dispersed in the Al matrix. The grain sizes of the Al matrix ranged from 77 ± 41 to 362 ± 185 nm depending on the thermomechanical processing. The composite with 2.2 vol% 3YSZ exhibited an ultimate tensile strength of 795 MPa and a strain-to-failure of 1.84%. In contrast, the composite with 10 vol% 3YSZ exhibited an ultimate compressive strength of 611 MPa with 22.5% strain-to-failure; this composite also retained 30% of its room temperature strength at 673 K (400 ℃). Evaluation of strengthening mechanisms suggests that Hall-Petch strengthening is a predominant mechanism controlling the achieved strength.

To provide insight into precipitation phenomena in age-hardening Al-Zn-Mg(-Cu) matrix composites, an Al 7075 alloy composite reinforced with B4C particles was selected as a model system. The bulk composites were fabricated via plasma activated sintering and followed by a peak aged (T6) heat treatment. Two types of Al matrix zones were identified in the composite: (1) the regions in the vicinity of the matrix/reinforcement interface, defined as “matrix plastic zone” (MPZ) hereafter, and (2) the regions away from the matrix/reinforcement interface, simply defined as matrix hereafter. The precipitation behavior in the MPZ was characterized and compared to that in the matrix. The MPZ contained a high density of dislocations. The number density of GP zones in the MPZ is lower than that in the matrix while the average size of the GP zones in MPZ is coarser. In addition, semi-coherent platelet η′ precipitates were observed but only in the MPZ. The dislocations and the Al/B4C interfaces provide more heterogeneous nucleation sites for the η′ precipitates in the MPZ. The growth and coarsening of the η′ precipitates caused rapid depletion of Mg and Zn solute atoms in the MPZ.

The age-hardening response for ultrafine-grained, powder-metallurgy-consolidated aluminum 7091 was investigated for the first time. Peak hardening occurred after aging at 353 K (80 °C) for only 4 hours; further aging for up to 26 hours resulted in only slight fluctuations in hardness values. After the 4-hour aging, the precipitate population consists of a high density of nanoscale GP zones (diameter ~3 nm) and nanoscale η′ phase (

A high-strength, ultrafine-grained Al alloy with a lamellar structure was fabricated via cryomilling and consolidation (hot isostatic pressing), followed by a two-stage thermomechanical processing approach (e.g. high temperature rotary swaging and room temperature high strain rate extrusion). Each lamellar band consisted of multiple ultrafine grains connected by low misorientation angle grain boundaries. This structure allows the dislocations to glide through low misorientation angle grain boundaries and to accumulate within the band, providing a high uniform elongation while retaining high strength.

Trimodal composites, consisting of nanocrystalline or ultrafine grains (UFGs), coarse grains (CGs), and ceramic particles, were originally formulated to achieve combinations of physical and mechanical properties that are unattainable with the individual phases, such as strength, ductility, and high-strain-rate deformation. The concept of a trimodal structure is both scientifically novel as well as technologically promising because it provides multiple controllable degrees of freedom that allow for extensive microstructure design. The UFGs provide efficient obstacles for dislocation movement, such as grain boundaries and other crystalline defects. The size, distribution, and spatial arrangement of the CGs can be controlled to provide plasticity during deformation. The size, morphology, and distribution of the reinforcement particles can be tailored to attain various engineering and physical properties. Moreover, the interfaces that form among the various phases also help determine the overall behavior of the trimodal composites. In this article, a review is provided to discuss the selection and design of each component in trimodal Al composites. The toughening and strengthening mechanisms in the trimodal composite structure are discussed, paying particular attention to strategies that can be implemented to tailor microstructures for optimal mechanical behavior. Recent results obtained with high-performance trimodal Al composites that contain nanometric reinforcements are also discussed to highlight the ability to control particle–matrix interface characteristics. Finally, a perspective is provided on potential approaches that can be explored to develop the next generation of trimodal composites, and interesting scientific paradigms that evolve from the proposed design strategies are discussed.

To provide insight into the relationships between precipitation phenomena, grain size and mechanical behavior in a complex precipitation-strengthened alloy system, Al 7075 alloy, a commonly used aluminum alloy, was selected as a model system in the present study. Ultrafine-grained (UFG) bulk materials were fabricated through cryomilling, degassing, hot isostatic pressing and extrusion, followed by a subsequent heat treatment. The mechanical behavior and microstructure of the materials were analyzed and compared directly to the coarse-grained (CG) counterpart. Three-dimensional atom-probe tomography was utilized to investigate the intermetallic precipitates and oxide dispersoids formed in the as-extruded UFG material. UFG 7075 exhibits higher strength than the CG 7075 alloy for each equivalent condition. After a T6 temper, the yield strength (YS) and ultimate tensile strength (UTS) of UFG 7075 achieved 734 and 774 MPa, respectively, which are ∼120 MPa higher than those of the CG equivalent. The strength of as-extruded UFG 7075 (YS: 583 MPa, UTS: 631 MPa) is even higher than that of commercial 7075-T6. More importantly, the strengthening mechanisms in each material were established quantitatively for the first time for this complex precipitation-strengthened system, accounting for grain-boundary, dislocation, solid-solution, precipitation and oxide dispersoid strengthening contributions. Grain-boundary strengthening was the predominant mechanism in as-extruded UFG 7075, contributing a strength increment estimated to be 242 MPa, whereas Orowan precipitation strengthening was predominant in the as-extruded CG 7075 (∼102 MPa) and in the T6-tempered materials, and was estimated to contribute 472 and 414 MPa for CG-T6 and UFG-T6, respectively.

To provide insight into the influence of length scales on precipitation phenomena, a bulk Al 7075 alloy with an ultrafine grain (UFG) structure was fabricated via cryomilling, hot isostatic pressing and extrusion, followed by solution treatment and artificial aging. The precipitation behavior in the UFG material was studied by transmission electron microscopy (TEM) and compared to its coarse grain (CG) counterpart. Results indicate that there are important differences in the size, chemistry and spatial distribution of precipitates between these two bulk materials. Differences in grain size were noted to influence the dislocation substructure, which in turn governed precipitation kinetics. Specifically, a high volume of grain boundaries (GBs) leads to a low concentration of vacancies in the UFG material, which hinders homogeneous nucleation of precipitates during aging. The nucleation of Guinier–Preston (GP) zones in UFG after extrusion is influenced by the presence of dislocations, which provide heterogeneous nucleation sites. The density of precipitates in the UFG was comparatively lower than that in the CG counterpart, because of the lower dislocation density in the grain interior. Solution treatment promoted recovery of dislocations in the UFG material. Then GP zones and nanoscale platelet η′ phase precipitates nucleated homogeneously during artificial aging. In comparison, the network of dislocations that remained in the CG material after solution treatment facilitated the formation of plate-like η′ phase precipitates around dislocations during artificial aging. Moreover, copious GP zones were observed in the CG interior after artificial aging with a comparatively higher density than those in the UFG.

The presence of nitrogen in nanostructured aluminum composites milled in liquid nitrogen has been argued to be responsible for reported property improvements, such as high strength and thermal stability. However, it has been experimentally difficult to precisely determine the nature of the nitride phases that form given their small scale (e.g., nm) and limited volume fraction. Moreover, the mechanism by which the nitrogen molecules are absorbed into the aluminum lattice during the cryomilling process remains poorly understood, and experimental studies are often constrained by resolution limits. In this paper, the absorption of nitrogen in Al-alloy composites during cryomilling is investigated from a computational perspective with the aid of the molecular dynamics simulation function in Materials Studio® software. Results indicate that Mg solute atoms in the matrix tend to segregate to Al/Al2O3 interfaces, creating vacancies in the Al lattice and subsequently enhancing the absorption of nitrogen into the Al lattice. The simulations also reveal that not only free N atoms but also N2 molecules can be absorbed into the Al lattice by occupying an Al vacancy. In addition, it is demonstrated that a structural defect, i.e., the presence of vacancies regardless of their source, is a requirement for enhanced absorption of nitrogen in the Al lattice.

Isothermal oxidation behavior of thermal barrier coatings (TBCs) with cryomilled NiCrAlY bond coats, in comparison with equivalent TBCs with conventional bond coats was investigated. The growth rate and homogeneity of the thermally grown oxide (TGO) were quantified and characterized through rigorous statistical evaluation. The initial TGO growth follows parabolic growth kinetics in both systems, but the cryomilled coatings exhibit improved homogeneity in TGO thickness, thinner TGO layers after long-term exposure, slower TGO growth rate, and uniformity in TGO composition consisting primarily of α-Al 2 O 3 with a limited presence of other oxides such as Ni(Cr, Al) 2 O 4 spinel. Results indicate that the distinct bond coat structure derived from cryomilling, i.e., the creation of a thermally stable, uniform distribution of ultrafine Al- or Y- rich oxide dispersoids within the bond coat layer, may affect the Al outward diffusion from the bond coat to the bond coat/top coat interface, which ultimately influences the TGO growth behavior. In addition, computational simulation is applied using the commercially available Thermo-calc® software to provide further insight into the chemical composition of the TGO layers.

Previous work has demonstrated that the lifetime of atmospheric plasma sprayed thermal barrier coating (TBC) systems incorporating cryomilled NiCrAlY bond coats show superior reliability with up to 3 times longer lifetimes compared to conventional ones. These conventional and cryomilled NiCrAlY bond coats at end-of-life (after thermal cycle failure) were studied in detail in the present work with a focus on the mechanical behavior in the temperature range from room temperature to 1273 K (1000 °C). The investigations were carried out using microtensile samples and the DIC technique. It turns out that the low-temperature strength of the cryomilled NiCrAlY bond coat is inferior to that of conventional ones, which might be due to a more pronounced porosity. At higher temperatures (between 1173 K and 1273 K (900 °C and 1000 °C)), the cryomilled bond coat shows almost twice the strength of the conventional bond coat, despite having been exposed to almost 3 times as many thermal cycles. The thermal stability of the nitride dispersoids appears to compensate for the gamma prime dissolution that typically occurs at these elevated temperatures, allowing for strength retention.

NiCoCrAlY alloys, commonly used as the bond coat layer in thermal barrier coatings, have been widely studied through experimentation. It is generally agreed that the major phases in the NiCoCrAlY are γ-Ni(Co,Cr) matrix and β-NiAl, with occasional presence of γ′-Ni 3 Al, (Co, Cr)-rich σ, and/or α-Cr precipitates, depending on the chemical composition and the temperature. It is noted, however, that the chemical compositions of the NiCoCrAlY alloys vary from one study to another. The present work reviews the literature on the experimental descriptions of the phases present in NiCoCrAlY and the corresponding phase transformations. To complement this literature review and, more importantly, to allow for consideration of alloy compositions not yet studied experimentally, equilibrium phases within the NiCoCrAl quaternary system are investigated in terms of the change in phase fractions as a function of temperature, isotherms at elevated temperatures (700–1100 °C) with a wide and continuous variation in composition (Co, Cr or Al content), and the phase compositions, through computation by Thermo-calc ® . The results are in good general agreement with the experimental findings, thus validating the approach. The γ′ and the σ phases are emphasized in the analysis, because of their significant contributions to the thermo-mechanical properties of the NiCoCrAlY alloys. Results indicate that an increase in Co content leads to decreased stability of the γ′ phase in the temperature range of 700–1100 °C; while appropriate Co additions increase the stability of the σ phase, with the appearance of a four-phase (γ + γ′ + σ + β) co-existing region, at temperatures below 1000 °C.

As-fabricated thermal barrier coating (TBC) systems generally consist of a superalloy substrate, a MCrAlY bond coat (M = Ni, Co, Fe), and a ceramic (usually partially stabilized zirconia) top coat. The conventional methods for producing the two coating layers generally derive from thermal spray and physical vapor deposition techniques. Thermal exposure leads to the formation of an additional layer: the thermally grown oxide (TGO) between the bond coat and top coat. In the present work, a TBC system is synthesized through the application of spark plasma sintering (SPS), which provides not only the opportunity to synthesize all three layers at once, but the process is quite rapid and can produce dense layers. More specifically, this paper describes the application of this method to an yttria-stabilized ZrO 2 (YSZ) top coat and a NiCrAlY bond coat on a Ni-base Hastelloy X substrate. A one-micron thick Al 2 O 3 TGO layer is also created from the reaction between an Al foil layer inserted in the stack prior to sintering and the ZrO 2 in the top coat. The effects of select process conditions are considered. The resulting multi-layer system is characterized with optical microscopy, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), energy dispersive X-ray analysis (EDAX) and X-ray diffraction (XRD). Differential thermal analysis (DTA) is used to investigate the reaction between the Al foil and the YSZ top coat.

Previous research has revealed that thermal barrier coatings with cryomilled bond coats exhibit improved thermal cycling lifetime by growing a continuous and uniform oxide layer at a slower rate; yet the mechanism controlling the ultimate failure remains unclear. In an effort to provide a foundation for understanding the improved behavior, the influence of cryomilling on the microstructure of the NiCrAlY bond coat material is investigated in this article. Rather than focusing on the alumina scale formation, the microstructural features and their evolution within the high-velocity oxy-fuel (HVOF)-sprayed NiCrAlY bond coats themselves, prepared from conventional powder and cryomilled powder, were carefully compared through extensive scanning electron microscope/energy-dispersive X-ray spectroscopy characterization. In addition, the as-cryomilled NiCrAlY powder is characterized to provide evidence of the direct influence of cryomilling and to exclude the impact from the HVOF spraying. It is found that the es...

Improved thermal cycling lifetime has been observed in thermal barrier coatings (TBCs) with cryomilled NiCrAlY bond coat. To understand this improved behavior, a robust experimental investigation is coupled with mechanistic explanations to describe the influence of cryomilling on microstructure, phase stability and oxidation behavior of the bond coat. It is found that cryomilling results in two significant changes in the NiCrAlY bond coat: unintentional Fe additions and creation of a homogeneous distribution of ultrafine oxide/nitride dispersoids. Through extensive microstructural analysis combined with computational simulation using Thermo-Calc® software, it is determined that the presence of Fe stabilizes the high temperature γ and β phases in the NiCrAlY bond coat, corresponding to a decrease in the transformation temperature. The results are explained on the basis of the Gibbs free energy for the individual phases. Characterization of the thermally grown oxide (TGO) in TBCs after isothermal oxidation with rigorous statistical evaluation indicates that the TGOs in the TBCs with the cryomilled bond coats are more uniform in thickness and slower growing. Both behaviors are attributed to the more homogeneous distribution of oxide dispersoids, which are a direct result of the cryomilling, yet remain stable after extensive thermal exposure.

NiCrAl alloys are commonly used for the bond coat in thermal barrier coatings (TBCs). Previous work has investigated the effects of intentional chemical additions on phase stability, but there is also a need to consider the effects of unintentional additions. For instance, cryomilling of NiCrAlY powder, which has been demonstrated to increase TBC lifetime, results in unintentional Fe additions. In this study, the effects of Fe additions are evaluated both through experimentation and computational simulation. The combined results indicate that the presence of Fe stabilizes the high-temperature γ and β phases, corresponding to a decrease in the transformation temperature. These results are explained on the basis of Gibbs free energy for individual phases. Phase composition is also investigated, indicating that, even without Fe, the phases are more complex than typically assumed and that, at the low concentrations associated with cryomilling, the Fe goes into solution in the Ni-rich phases.

Precipitation-hardened alloys can be further strengthened by reducing the grain size down to the ultrafine regime (