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Deformation-induced grain-growth in nanocrystalline materials is a widely-reported phenomenon that has been attributed to grain boundary (GB) processes. In this paper, we report on the opposite phenomenon, wherein a stable nanocrystalline (NC) Cu-Ta alloy undergoes a further refinement of the nano-grains during severe plastic deformation (SPD). SPD up to 250% results in a significant grain-size reduction despite the 350°C increase in temperature caused by the deformation process. Experiments and atomistic-simulations show that this unexpected grain-refinement is a direct result of well-dispersed Ta-nanoclusters throughout grain centers and along GBs acting as kinetic-pinning agents and suppressing GB processes that occur during recrystallization.

Non-metallic contamination is a practically unavoidable byproduct of commercial synthesis processes; however, non-metallic impurities are often overlooked when considering material design and performance of nanostructured materials. Importantly, there is disputing evidence as to whether non-metallic contamination stabilize or destabilize nanocrystalline materials against grain growth. Furthermore, it is unclear if non-metallic contamination has a significant impact on hardness of nanocrystalline systems. In this work, two nanocrystalline Ni-28at%W alloys with different impurity concentrations were produced via mechanical alloying to directly evaluate the effect of contamination on thermal stability and hardness of nanostructured materials. The alloys were isothermally annealed at several temperatures, and the microstructures were compared by applying atom probe tomography and aberration-corrected scanning transmission electron microscopy. It was determined that impurity concentrations can be substantially reduced by using specific milling media and pre-milling reduction processes. Furthermore, the clean and contaminated alloys exhibited similar thermal stabilities against grain growth, but the clean alloy displayed a 100% improvement in hardness despite a similar grain size and distribution of second phases. Impurity phases including CrOx, Ni6W6C, and trapped Ar pores were also identified and likely contributed to the thermal stability and mechanical properties observed in this study. Overall, this work suggests that impurities my not always be detrimental to thermomechanical behavior of nanocrystalline systems.

Nanocrystalline Cu-Ta is among the most successfully developed nanostructured alloys involving microstructural design, synthesis, characterization, and testing. However, the interplay of direct and indirect thermal stability mechanisms is still unclear. In this work, mechanically alloyed Cu-10at.%Ta was characterized with atomic-resolution electron microscopy to identify thermal stability mechanisms. A new mechanism involving athermal core-shell particles that were coarsening resistant was discovered after annealing between 100–800 °C up to 1000 h. Evidence suggests that oxide amorphous shells decreased the phase boundary energy thereby reducing the thermodynamic driving force for particle growth and indirectly maximizing thermal stability of Cu(Ta) grains.

Nanocrystalline (NC) materials possess excellent room temperature properties, such as high strength, wear resistance, and toughness as compared to their coarse-grained counterparts. However, due to the excess free energy, NC microstructures are unstable at higher temperatures. Significant grain growth is observed already at moderately low temperatures, limiting the broader applicability of NC materials. Here, we present a design approach that leads to a significant improvement in the high temperature tensile creep resistance (up to 0.64 of the melting temperature) of a NC Cu-Ta alloy. The design approach involves alloying of pure elements to create a distribution of nanometer sized solute clusters within the grains and along the grain boundaries. We demonstrate that the addition of Ta nanoclusters inhibits the migration of grain boundaries at high temperatures and reduces the dislocation motion. This leads to a highly unusual tensile creep behavior, including the absence of any appreciable steady-state creep deformation normally observed in almost all materials. This design strategy can be readily scaled-up for bulk manufacturing of creep-resistant NC parts and transferred to other multicomponent systems such as Ni-based alloys.

Nanocrystalline materials are known to possess excellent radiation resistance due to high fraction of grain boundaries that act as defect sinks, provided they are microstructurally stable at such extreme conditions. In this work, radiation response of a stable nanocrystalline Cu-Ta alloy is studied by irradiating with 4 MeV copper ions to doses (close to the surface) of 1 displacements per atom (dpa) at room temperature (RT); 10 dpa at RT, 573 and 723 K; 100 and 200 dpa at RT and 573 K. Nanoindentation results carried out for samples irradiated till 100 dpa at RT and 573 K show exceptionally low radiation hardening behavior compared to various candidate materials from literature. Results from microstructural characterization, using atom probe analysis and transmission electron microscopy, show a stable nanocrystalline microstructure with minimal grain growth and a meagre swelling in samples irradiated to 100 dpa (~0.2%) and 200 dpa at RT, while no voids in those at 573 K. This radiation tolerance is partly attributed to the stability of Ta nanoclusters due to phase separating nature of the alloy. Additionally, the larger tantalum particles are observed to undergo ballistic dissolution at doses greater than 100 dpa and are eventually precipitated as nanoclusters, replenishing the sink strength, which enhanced material's radiation tolerance when exposed to high irradiation doses and elevated temperatures.

The microstructures of materials typically undergo significant changes during shock loading, causing failure when higher shock pressures are reached. However, preservation of microstructural and mechanical integrity during shock loading are essential in situations such as space travel, nuclear energy, protection systems, extreme geological events, and transportation. Here, we report ex situ shock behavior of a chemically optimized and microstructurally stable, bulk nanocrystalline copper–tantalum alloy that shows a relatively unchanged microstructure or properties when shock compressed up to 15 GPa. The absence of shock-hardening indicates that the grains and grain boundaries that make up the stabilized nanocrystalline microstructure act as stable sinks, thereby annihilating deformation-induced defects during shock loading. This study helps to advance the possibility of developing advanced structural materials for extreme applications where shock loading occurs. Shock loading of materials alters the microstructure and considerably degrades mechanical performance. Here, shock loading of a nanocrystalline Cu–Ta alloy is found to induce minor changes to microstructure and mechanical performance, attributed to the annihilation of defects during deformation.

The potential benefits of nanocrystalline (NC) alloys for use in various structural applications stem from their enhanced mechanical strengths. However, deformation-induced grain growth in NC materials reduces the strength and is a widely reported phenomenon occurring even at low-temperatures. Controlling such behavior is critical for the maturation of bulk nanocrystalline metals in various advanced engineering applications. Here, we disclose the mechanism by which grain boundary sliding and rotation are suppressed when a NC material is truly thermo-mechanically stabilized against grain growth. Unlike in any other known nanocrystalline metals, the absence of sliding and rotation during loading, at extreme temperatures, is related to short-circuit solute diffusion along the grain boundaries causing the formation of solute clusters and thus a significant change of the grain boundary structures. The departure of this unusual behavior from the well-established norm leads to a strong enhancement of many mutually exclusive properties, such as thermo-mechanical strength, creep resistance, and exceptionally high electrical/thermal conductivity. This work demonstrates that Cu-based nanocrystalline alloys can be used in applications where conventional Cu-based polycrystalline materials are not viable.

Several miniature Kolsky bars are used to obtain stress–strain curves for nanocrystalline Cu–10Ta over a range of high strain-rates. The smallest bar (steel) has a 305 μm diameter, and achieved rates up to 700 × 103/s. Different sample sizes are needed to obtain different strain-rates, and it is shown that there is no appreciable sample size effect when different sizes are tested at similar strain-rates, even though the sample sizes vary by over an order of magnitude. No significant increase in strain-rate sensitivity is noted over the strain-rate range studied, i.e., the strength increases linearly with the logarithm of strain-rate from 0.001/s to 700 × 103/s.

A ternary immiscible nanostructured Cu-Al-Nb alloy was fabricated by friction stir processing of mechanically compacted pellets. Subsequently, aging was carried out at 563 K for various times. The material exhibited hardness of ~4.3 ± 0.1 GPa in the peak-aged condition (aging for 6 h), which is remarkable among Cu-based ternary immiscible alloys. The excellent strength of the material is attributed to Hall-Petch strengthening due to the extremely refined nanocrystalline structure, coupled with precipitation strengthening due to a uniform distribution of nano-scale Al- and Nb-rich precipitates or clusters, as confirmed by X-ray diffraction and microstructural characterization.

Here, the Cottrell–Stokes ratio in a microstructurally-stable Cu-3Ta (at.%) nanocrystalline alloy is examined from the standpoint of changes in deformation mechanisms. Toward this, uniaxial compression experiments were performed in the temperature range of 113 K – 273 K. The Cottrell-Stokes ratio at the lowest temperature tested was ~1.3, and the material exhibited a very low strain-rate sensitivity at cryogenic-temperatures. Transmission electron microscopy (TEM) characterization showed negligible average grain size coarsening and a transition in the deformation mechanism toward athermal activation processes such as twinning with the reduction in the testing temperature.

Grain boundary segregation is an important phenomenon for nanocrystalline materials as it influences thermal stability and mechanical properties. While several studies have considered effects of single, intentional segregants, co-segregation of intentional and unintentional segregants to general grain boundaries is not commonly investigated using experimental techniques on the atomic scale. This study utilized aberration-corrected scanning transmission electron microscopy and atom probe tomography to evaluate the grain boundary structure and chemistry of an electroplated and annealed electrodeposited Ni–W alloy. Several phases were observed in the annealed microstructure including elongated nanoscale oxide particles and relatively large impurity carbide phases. Furthermore, grain boundaries regularly exhibited ordered structures, minimal elemental tungsten segregation (intended solute) and impurity carbon segregation (unintentional solute), but moderately high-impurity oxygen segregation (unintentional solute). The unintentional segregated impurities (oxygen and carbon) resulted in a total average grain boundary composition of ~ 10 at.%. The consequence of impurity segregation is discussed in terms of thermal stability and potential mechanical properties.

Fundamentally, material flow stress increases exponentially at deformation rates exceeding, typically, ~103 s−1, resulting in brittle failure. The origin of such behavior derives from the dislocation motion causing non-Arrhenius deformation at higher strain rates due to drag forces from phonon interactions. Here, we discover that this assumption is prevented from manifesting when microstructural length is stabilized at an extremely fine size (nanoscale regime). This divergent strain-rate-insensitive behavior is attributed to a unique microstructure that alters the average dislocation velocity, and distance traveled, preventing/delaying dislocation interaction with phonons until higher strain rates than observed in known systems; thus enabling constant flow-stress response even at extreme conditions. Previously, these extreme loading conditions were unattainable in nanocrystalline materials due to thermal and mechanical instability of their microstructures; thus, these anomalies have never been observed in any other material. Finally, the unique stability leads to high-temperature strength maintained up to 80% of the melting point (~1356 K)., Metals deformed at very high rates experience a rapid increase in flow stress due to dislocation drag. Here, the authors stabilise a nanocrystalline microstructure to suppress dislocation velocity and limit drag effects, conserving low strain-rate deformation mechanisms up to higher strain rates and temperatures.

Nanocrystalline Cu-Ta alloys are emerging as a new class of structural materials preserving the nano-scale grain size up to the melting point of Cu. This extraordinary structural stability is caused by the strong pinning of grain boundaries (GBs) by Ta nano-clusters precipitating from the unstable solid solution after mechanical alloying. Many aspects of the Ta stabilization effect remain elusive and call for further experimental and simulation work. In previous atomistic computer simulations of stress-driven GB migration [JOM 68, 1596 (2016)], the GB–cluster interactions in Cu-Ta alloys have been studied for several different compositions and GB velocities. The results have pointed to the Zener pinning as the main mechanism responsible for the grain stabilization. This paper extends the previous work to the motion of individual GBs driven by capillary forces whose magnitude is similar to that in real nanocrystalline materials. Both the impingement of a moving GB on a set of Ta clusters and the GB unpinning from the clusters are studied as a function of temperature and alloy composition. The results demonstrate a quantitative agreement with the Zener pinning model and confirm the “unzip” mechanism of unpinning found in the previous work. In the random Cu-Ta solid solution, short-circuit Ta diffusion along stationary and moving GBs leads to the nucleation and growth of new GB clusters, which eventually stop the GB motion.

In this work, a nanocrystalline (NC) Cu-10at.%Ta alloy is irradiated with helium at different temperatures to assess the stability and effectiveness of Ta nanoclusters in trapping helium and suppressing swelling. Advanced microstructural characterization of the room-temperature irradiated specimens indicated the presence of small He-bubbles (∼1-2 nm) at the peak damage depth mainly at the core and along the interface of Ta nanoclusters with Cu matrix. Few bubbles were found along grain boundaries, with much smaller bubbles homogenously distributed within the copper lattice. High-temperature irradiation exhibited bubbles of ∼3-5 nm, which were primarily associated with nanoclusters as compared to other locations, with no observed faceting of the bubbles. Atom probe analysis confirmed helium partitioning to the Ta nanoclusters indicating the effective entrapment of these He atoms.

In this work, various alloy compositions of immiscible copper-tantalum (Cu–Ta) are systematically studied to understand the interplay between cluster stability, precipitation hardening, and the overall stability of the matrix's average grain size. An alloy composition of Cu-3at.%Ta is found to exhibit dramatic improvements relative to other neighboring Ta contents (both higher and lower) only after exposures of more than 300 h at 800 °C. An extremely low steady-state growth rate, i.e., 3.1 nm per 100 h exposure, speaks to the extreme kinetics which allow this composition to retain its high mechanical strength. The key attribute responsible for the NC Cu-3at.%Ta alloy exhibiting such behavior is a high cluster density tailored through the Ta solute content to achieve the best combination of stability without sacrificing strength by limiting Orowan coarsening of the clusters compared to other Ta concentrations. In general, the growth kinetics of this Cu-3at.%Ta alloy are so sluggish that it places it among the most thermally resistant alloys ever produced. The work demonstrates that, if designed properly, bulk nanocrystalline alloys can withstand the prolonged high-temperature exposure required for high-temperature applications, while grain growth is stagnated or halted.

Stability of nanocrystalline microstructural features allows structural materials to be synthesized and tested in ways that have heretofore been pursued only on a limited basis. Here, we demonstrate using quasi-static compression and three point bend tests that, in a stabilized nanocrystalline metal with tailored solute concentrations, i.e., NC-Cu-3 at.%Ta, extraordinary properties such as ultrahigh hardness along with anomalus modulus of resilience and springback effects can be manifested. Such effects influence a wide range of materials response including elastic energy absorption, damping, fatigue and wear. The present study, therefore, represents a pathway for designing highly resilient materials for everyday applications.

Nanocrystalline materials possess excellent room temperature properties, such as high strength, wear resistance, and toughness as compared to their coarse-grained counterparts. However, due to excess free energy, nanocrystalline microstructures are unstable at higher temperatures. Significant grain growth is observed already at moderately low temperatures, limiting their broader applicability. Here, we present a design approach that leads to a significant improvement in the high temperature tensile creep resistance (up to 0.64 of the melting temperature Tm) of a nanocrystalline Cu-Ta alloy. The design approach involves alloying of pure elements for engineering nanometer sized solute clusters within the solvent grains as well as along the grain boundaries. Using a chemically optimized nanocrystalline Cu-3at.%Ta alloy as a model material system, we demonstrate that the addition of Ta nanoclusters inhibits the migration of the planar defects at higher temperatures and reduces the dislocation motion, leading to extraordinary high temperature properties. For instance, the NC Cu-3Ta alloy tested under tensile creep conditions up to the temperature of 873 K (0.64Tm) displays highly unusual behavior, including the absence of any appreciable steady-state creep deformation which is normally observed in almost all materials. This approach can be readily scaled-up for bulk manufacturing of creep resistant nanocrystalline parts. Moreover, this design strategy can be transferred to other multicomponent systems such as Ni-based alloys for making nanocrystalline materials with tailored properties.

The effect of mechanical loading, particularly at dynamic strain rates, on nanocrystalline (NC) materials has eluded researchers owing to the inherent instability of the NC structure. However, a recently developed NC Cu-10 at.%Ta alloy has exhibited an ability to maintain a NC structure at temperatures up to 873 K. Here, NC Cu-10 at.%Ta is tested under compressive strain rates ranging from 10−3 s−1 up to 105 s−1 and at temperatures from 298 K up to 1073 K. Typical materials show a sharp increase in flow stress occurring around 103 s−1 as deformation mechanisms shift away from thermal activation mechanisms; however, at 298 K, NC Cu-10 at.%Ta observes only a limited increase in flow stress indicating that typical thermally activated mechanisms still apply up to strain rates of 105 s−1. Post deformation analyses indicate a shift from nucleation of full dislocations to increased nucleation of partial dislocations at 298 K. However, as temperature increases, thermal activation mechanisms give way to viscous effects and the high density of nucleated full dislocations leads to a dramatic increase in flow stress.

Microstructural instability in traditional nanocrystalline metals limits the understanding of the fundamental effect of grain size on mechanical behavior under extreme environmental conditions such as high temperature and loading rates. In this work, the interplay between Ta concentrations and processing temperature on the resulting microstructure of a powder processed, fully dense Cu-Ta alloy along with their tensile and compressive behavior at different strain rates are investigated to probe the possibility of manipulating or tuning microstructurally dependent parameters to control the flow-stress upturn phenomenon. Consequently, the results reveal that there is a crucial length scale, i.e., small grain size and appropriate cluster spacing, below which such upturn is damped out. The observation of changes in flow stress upturn behavior is consistent with the observed changes in measured high-rate plasticity, which enhances below the critical length scale. Furthermore, tension-compression asymmetry also tends to be suppressed in nanocrystalline Cu-Ta alloys, while it becomes evident as the grain size increases to an ultrafine regime. Overall, this work presents a systematic approach to control or engineer reduced high strain rate flow stress upturn behavior in metallic alloys, for high-rate applications.

Under intense heating and/or deformation, pure nanocrystalline (NC) metals exhibit significant grain coarsening, thus preventing the study of length scale effects on their physical response under such conditions. Hence, in this study, we use in-situ TEM heating experiments, atomistic modeling along with elevated temperature compression tests on a thermally stabilized nanostructured Cu–10 at.% Ta alloy to assess the microstructural manifestations caused by changes in temperature. Results reveal the thermal stability attained in NC Cu-10 at.% Ta diverges from those observed for conventional coarse-grained metals and other NC metals. Macroscopically, the microstructure, such as Cu grain and Ta based cluster size resists evolving with temperature. However, local structural changes at the interface between the Ta based clusters and the Cu matrix have a profound effect on thermo-mechanical properties. The lattice misfit between the Ta clusters and the matrix tends to decrease at high temperatures, promoting better coherency. In other words, the misfit strain was found to decrease monotonically from 12.9% to 4.0% with increase in temperature, leading to a significant change in flow stress, despite which (strength) remains greater than all known NC metals. Overall, the evolution of such fine structures is critical for developing NC alloys with exceptional thermo-mechanical properties. Keywords: In situ TEM, Nanocrystalline, Atomistic, Misfit strain

In this work, the effects of Mn on lattice parameter, electron work function (EWF), bonding charge density, and hardness of nanocrystalline Al Mn non-equilibrium solid solutions are investigated. We show how the enhancement of the EWF contributes to the observed improvement of the hardness of Al Mn solid solutions. It is understood that the physical mechanisms responsible in our model, using the EWF coupled with a power law scaled hardness, are attributed to the redistribution of electrons caused by the presence of Mn solute atoms, supporting an atomic and electronic basis for the coupling of solid solution and grain refinement strengthening mechanisms.

An immiscible nanocrystalline (NC) copper-tantalum (Cu-Ta) alloy is shown to exhibit a stable microstructure under thermo-mechanical loading conditions with exceptional mechanical strength (i.e., 1200 MPa strength at 298 K) indicating anomalous deformation mechanisms as compared to microstructurally unstable nanocrystalline materials. Therefore, in this work, various aspects of strength partitioning in such NC Cu-Ta alloys are discussed and the role of tantalum nanoclusters on the dominant deformation mechanism is presented as a function of temperature. Toward this, initially, the mechanical responses of NC Cu-Ta alloy were measured under uniaxial compression experiments at various temperatures. Later, atomistic simulations were performed along with the high-resolution electron microscopy to identify and validate the rate limiting mechanism behind the plastic deformation in NC Cu-Ta alloys. In general, the observed trend through experiments and simulations identify a transition from a dislocation – nanocluster interaction mediated deformation mechanism to one controlled by grain boundary strengthening as the temperature increases. The former mechanism is shown here to have a crucial role in the observed strengthening behavior of microstructurally stable NC materials. Overall, the paper demonstrates that through effective nano-engineering techniques, it is expected to extend the scope of nanocrystalline materials to a number of engineering design applications. Keywords: Nanocrystalline, Deformation, Transmission electron microscopy, Atomistic

The formation of a metastable Cu–Ta solid solution in a mechanically alloyed Cu–10 at.%Ta alloy and its subsequent decomposition during annealing was investigated by atom probe tomography. During annealing, the as-milled Cu-rich alloy undergoes phase separation; Ta atoms diffuse out of the Cu lattice to form Ta clusters and particles along grain boundaries and within the Cu grains. The role of the Ta clusters and the nature of the solid solution as a potential strengthening mechanism for these alloys are discussed.

Nominally pure nanocrystalline metals do not remain nanostructured under extreme conditions of intense heating and or deformation preventing the study of their physical response under such conditions. Here we present the coupled effect of temperature and strain rate on the mechanical response of a thermally stabilized nanocrystalline Cu alloyed with 10 at% Ta. Compressive mechanical testing was performed from 24 to 1000 °C and strain rates ranging from quasi-static (10−1 s−1) to dynamic (104 s−1) rates. The response of this material exhibits a maximum quasi-static yield stress of 1.05 GPa at room temperature and an approximate yield stress of 0.5 GPa at 600 °C, with an apparently linear temperature response. In contrast to pure coarse-grained Cu, our assessment indicates that this Cu-based composite derives its properties from a combination of very small Cu-rich grains and well-dispersed Ta clusters and nanometer (

It is a new beginning for innovative fundamental and applied science in nanocrystalline materials. Many of the processing and consolidation challenges that have haunted nanocrystalline materials are now more fully understood, opening the doors for bulk nanocrystalline materials and parts to be produced. While challenges remain, recent advances in experimental, computational, and theoretical capability have allowed for bulk specimens that have heretofore been pursued only on a limited basis. This article discusses the methodology for synthesis and consolidation of bulk nanocrystalline materials using mechanical alloying, the alloy development and synthesis process for stabilizing these materials at elevated temperatures, and the physical and mechanical properties of nanocrystalline materials with a focus throughout on nanocrystalline copper and a nanocrystalline Cu-Ta system, consolidated via equal channel angular extrusion, with properties rivaling that of nanocrystalline pure Ta. Moreover, modeling and simulation approaches as well as experimental results for grain growth, grain boundary processes, and deformation mechanisms in nanocrystalline copper are briefly reviewed and discussed. Integrating experiments and computational materials science for synthesizing bulk nanocrystalline materials can bring about the next generation of ultrahigh strength materials for defense and energy applications.

Mitigating grain growth at high temperatures in binary nanocrystalline alloys is important for processing nanocrystalline alloy systems. The objective of this research is to develop a methodical design-based approach for selecting solutes in binary nanocrystalline alloys by revisiting grain boundary thermodynamics and the internal processes of grain growth and solute segregation in a closed system. In this work, the grain boundary energy is derived and systematically studied in terms of temperature, grain size, concentration, and solute segregation for binary systems of 44 solvents and 52 solutes, using readily-available elemental data, such as moduli and liquid enthalpy of mixing. It is shown that through solute segregation, the grain boundary energies of some binary systems can be reduced, resulting in thermodynamically stable grain structures and successful prediction of solutes that inhibit grain growth in some nanocrystalline alloys. Parametric studies reveal trends between equilibrium grain size, solute distribution and temperature for various binary systems culminating in the generation of nanocrystalline thermodynamic stability maps as a tool for solute selection in binary nanocrystalline alloys.

Nanocrystalline Cu–Ta alloys belong to an emerging class of immiscible materials with potential for high-temperature applications. Differential scanning calorimetry (DSC), Vickers microhardness, transmission and scanning electron microscopy (TEM/SEM), and atomistic simulations have been applied to study the structural evolution in high-energy cryogenically alloyed nanocrystalline Cu–10 at.%Ta. The thermally induced coarsening of the as-milled microstructure was investigated and it was found that the onset of grain growth occurs at temperatures higher than that for pure nanocrystalline Cu. The total heat release associated with grain growth was 0.553 kJ/mol. Interestingly, nanocrystalline Cu–10 at.%Ta maintains a mean grain size (GS) of 167 nm after annealing at 97% of its melting point. The increased microstructural stability is attributed to a combination of thermodynamic and kinetic stabilization effects which, in turn, appear to be controlled by segregation and diffusion of Ta solute atoms along grain boundaries (GBs). The as-milled nanocrystalline Cu–10 at.%Ta exhibits Vickers microhardness values near 5 GPa surpassing the microhardness of conventional pure nanocrystalline Cu by ∼2.5 GPa.

The surface mechanical attrition treatment (SMAT) process was applied to pure Cu at both cryogenic and room temperatures. The cryogenic SMAT process resulted in a 60% reduction of grain size in the polycrystalline microstructure compared to that at room temperature. The level of grain refinement is related to a transition in the dominant deformation mechanism during SMAT from a dislocation-mediated behavior at room temperature to a twinning/shear band-mediated behavior at cryogenic temperatures, which also helps to suppress thermally activated processes.

Nickel-yttrium nanocrystalline alloys with an as-milled grain size of approximately 6.5 nm were synthesized using high-energy cryogenic mechanical alloying. The microstructural changes due to annealing were characterized using x-ray line broadening, microhardness, focused ion beam channeling contrast imaging, and transmission electron microscopy. Experiments demonstrated that increasing yttrium content led to stabilization of the nanocrystalline grain size at elevated homologous annealing temperatures. Additionally, it was found that inadvertent contamination with nitrogen during the milling process caused the formation of yttrium nitride (YN) precipitates, which, in turn, resulted in an additional nonlinear hardening effect beyond the expected hardening due to grain-size reduction. Results reveal that kinetic pinning by YN particles is effective in retaining a nanostructure to relatively high temperatures.

Development of ultra-high strength and corrosion-resistant aluminum (Al) alloys is demonstrated by a combination of suitable alloying elements and processing technology able to cause extended solid solubility and nanocrystalline structure. Binary Al-transition metal (M: Cr, Ni, Mo, Si, Ti, Mn, V, Nb) alloys, produced by high-energy ball milling and subsequent cold compaction, have exhibited significantly high hardness and corrosion resistance compared to any commercial Al alloy. The cyclic potentiodynamic polarization tests revealed a significant improvement in pitting and repassivation potentials. X-ray diffraction analysis revealed the grain refinement IMPACT STATEMENT High-energy ball-milled Al alloys, owing to excellent corrosion resistance and high hardness, are expected to be a new class of Al alloys and initiate a multidisciplinary research direction.

Copper (Cu) was cryogenically milled with tungsten (W) in a high-energy ball mill. The process created W particles dispersed in a nanocrystalline Cu matrix. These “alloys” were then annealed to a maximum temperature of 800 °C. The addition of W stabilized the Cu at∼40 nm during annealing to 400 °C for a 1 at% W composition and to 600 °C for 10 at% W. As evidenced through hardness measurement, the W provided a significant increase in strength over pure Cu, and the 10 at% W material maintained a 2.6 GPa hardness after annealing at 800 °C. The stabilization and strengthening mechanisms are compared against theoretical prediction and found to be in good agreement. Although the strength and stability are significantly improved over pure Cu, the maximum benefit was hindered by an extremely broad W particle size distribution (∼5–5000 nm). For the 10 at% W alloy, only half of the added W was reduced to nanoscale where kinetic pinning and strengthening become most effective.

This paper describes the stabilization of nanocrystalline grain sizes in Pd and Fe by the addition of Zr solute atoms. The grain size as a function of annealing temperature was measured by both x-ray diffraction (XRD) line broadening analysis and microscopy methods. The latter methods showed that the XRD grain size measurements for the samples annealed at the higher temperatures were not valid. It appears that thermodynamic stabilization may still be operative in the Fe-4at.% Zr alloy but not in the Pd-19at.% Zr alloy from the experimental results and calculations of the enthalpy of segregation.

Nanocrystalline Cu–Ta alloys belong to an emerging class of immiscible high-strength materials with a significant potential for high-temperature applications. Using molecular dynamics simulations with an angular-dependent interatomic potential, we study the effect of Ta on the resistance to grain growth and mechanical strength of nanocrystalline Cu–6.5 at.% Ta alloys. Ta segregation at grain boundaries greatly increases structural stability and strength in comparison with pure copper and alloys with a uniform distribution of the same amount of Ta. At high temperatures, the segregated Ta atoms agglomerate and form a set of nanoclusters located at grain boundaries. These nanoclusters are capable of pinning grain boundaries and effectively preventing grain growth. It is suggested that the nanoclusters are precursors to the formation of larger Ta particles found in Cu–Ta alloys experimentally.

Using a modified regular solution model for grain boundary solute segregation, the relative thermal stability of a number of Fe-based nanocrystalline binary alloys was predicted with considerable accuracy. It was found that nanocrystalline iron was strongly stabilized by zirconium, moderately stabilized by tantalum, and not significantly stabilized by nickel or chromium. These findings are fully in line with the aforementioned predictions. This success with iron based alloys highlights the utility of this practical approach to selecting stabilizing solutes for nanocrystalline alloys.

Grain growth stability in mechanically alloyed nanocrystalline Pd81Zr19 was investigated. Previous research suggested that the alloy is thermodynamically stable to very high temperatures. When X-ray diffraction (XRD) is used to estimate the grain size of annealed samples the alloy appears to have remarkable resistance to growth. Microscopy done here on the same alloy indicated that the XRD estimates are not accurate for samples annealed above 600 °C. It appears that when this alloy is annealed at high temperatures XRD peak broadening is retained for reasons that are unrelated to the grain size. The alloy still has much improved grain growth stability compared with pure Pd, but not as significant as suggested by the XRD results. A similar phenomenon was observed in Fe–Zr alloys.

Fe–Zr nanocrystalline alloys with an as-milled grain size less than 10 nm were synthesized by ball milling. The microstructure changes due to annealing were characterized using X-ray line broadening, microhardness, focused ion beam channeling contrast imaging, and transmission electron microscopy (TEM). Additions of 1/3 to 4 at.% Zr stabilized nanocrystalline grain sizes at elevated annealing temperatures compared to pure Fe. With 4 at.% Zr, a fully nanocrystalline microstructure with a TEM grain size of 52 nm was retained at temperatures in excess of 900 °C. Alloys with lower Zr contents showed less stability, but still significant compared to pure Fe. Bimodal nano–micro grain size microstructures were also observed.

Nanostructured Cu–Ta alloys show promise as high-strength materials in part due to their limited grain growth. In the present study, we elucidate the role of Ta on the transition from deformation twinning to dislocation-mediated slip mechanisms in nanocrystalline Cu through atomistic simulations and transmission electron microscopy characterization. In particular, computed generalized stacking fault energy curves show that as Ta content increases there is a shift from twinning to slip-dominated deformation mechanisms. Furthermore, heterogeneous twinnability from microstructural defects decreases with an increase in Ta. The computed effect of Ta on plasticity is consistent with the HRTEM observations. IMPACT STATEMENT We show for the first time using atomistic simulations and TEM that, similar to grain size, the Tanano-particles can be used to tailor the governing deformation mechanisms in NC-alloys.

Characterization of the mechanical properties of electrodeposited nanocrystalline Cu with an average grain size of 74 nm was carried out using two different testing techniques, shear punch tests and tensile tests. The grain size distribution was broad and the volume fraction of larger grains was appreciable. The electrodeposited Cu had a high yield strength combined with moderate ductility and strain hardening. Scatter in the ductility values was attributed to residual porosity and inhomogeneity in the microstructure. Measurements of the strain rate sensitivity showed a significant increase in the rate sensitivity and a decrease in the activation volume for the deformation of nanocrystalline Cu compared with similar tests on coarse-grained cold worked Cu.

Advanced nanocrystalline alloys have shown remarkable property improvements, particularly, order-of-magnitude strength increases, when compared to their coarse-grained counterparts. However, a major obstruction to the widespread application of such materials is the degradation of properties via rapid grain growth at even ambient temperatures. Conventional methods for circumvention of this problem at low temperatures have largely steered toward kinetically pinning the boundaries with disperoids, or through misorientation of grain boundaries, yet even these methods have limited utility at elevated temperatures needed for routine sintering and forming operations. In this work, we will present a synergistic approach to the development of thermally stable nanostructured Mg-alloys which incorporates elements of predictive modeling of suitable alloy systems, fabrication of nanostructured alloy powders by high energy ball milling and consolidation of the powders at elevated temperatures.