Amorphous Intergranular Films Research Articles

Achieving excellent mechanical and electrical properties in transition metal oxides and rare earth oxide‐doped KNN‐based piezoceramics

AbstractPotassium sodium niobate (KNN)‐based piezoelectric ceramics have emerged as a promising alternative to lead‐based systems due to their exceptional properties. While extensive research has focused on improving the electrical properties of KNN‐based ceramics through doping and processing optimization, the concurrent investigation of their mechanical properties has been lacking. This study presents a comprehensive analysis of the mechanical and electrical properties of KNN‐based lead‐free piezoceramics doped with various transition metal oxides and rare earth oxides, based on substantial experimental data. Our findings reveal that the as‐sintered KNN‐based ceramics exhibit not only outstanding electrical properties but also remarkable mechanical robustness compared to conventional toughened lead zirconate titanate (PZT)‐based ceramics. These exceptional electrical and mechanical properties are attributed to the micro‐scale and atomic‐scale structure of the modified KNN‐based ceramics, characterized by a highly condensed structure, an inhomogeneous distribution of nano‐domain structure, and the presence of amorphous intergranular films at grain boundaries.

Intergranular amorphous film in GeO2-enriched Li1.5Al0.5Ti1.5(PO4)3 composite electrolytes for high-performance solid-state lithium-ion batteries

Solid-state electrolytes have emerged as a key area of development in the field of Li-ion batteries owing to safety concerns surrounding liquid electrolytes. Among solid-state electrolytes, Li1.5Al0.5Ti1.5(PO4)3 (LATP), a NASICON-type material, is a leading candidate owing to its promising ionic conductivity, chemical and environmental stability, and cost-effectiveness. However, its ionic conductivity is limited by grain-boundary scattering, which hinders its broader adoption. Herein, we introduce a novel grain-boundary engineering strategy for the LATP electrolyte system using typical solid-state method, wherein a Ge-rich liquid phase spontaneously forms at the grain boundaries of GeO2-enriched LATP during synthesis, producing an intergranular amorphous film in the final material that significantly enhances Li-ion transport at the grain boundaries. With an optimal content of 4 wt% GeO2, the ionic conductivity reaches 8.92 × 10−4 S cm−1—an eightfold increase compared to that of pristine LATP. This high ionic conductivity also bestows 4 wt% GeO2-LATP with excellent cell performance, with a symmetric Li/4 wt% GeO2-LATP/Li cell exhibiting stable operation for over 500 h with low overpotentials. Our findings underscore the importance of grain-boundary engineering in advancing solid-state electrolytes and pave the way for the commercialization of next-generation all-solid-state Li-ion batteries.

Substantial toughening by thick nanoscale amorphous intergranular films in nanocrystalline materials

Amorphous intergranular films (AIFs) have been proven in experiments to improve the damage tolerance of nanocrystalline materials. However, a quantitative study is still lacking. Molecular dynamics simulations were performed here to investigate the effect of CuNb AIFs on the fracture toughness of nanocrystalline Nb. In order to clarify the role of AIFs, a bicrystal Nb model with one straight symmetrical tilt grain boundary and a mode-I crack in one of the grains was constructed, in which the AIF effect was introduced by replacing the normal grain boundary with a CuNb AIF. Then, AIF thickness-dependent tensile deformation of the bicrystal Nb samples was simulated. The work-of-fracture, which is defined as the released strain energy due to the newly generated unit area in the crack during stretching, was employed to quantify the fracture toughness of the bicrystal systems. The results show that the fracture toughness of the AIF sample can be tripled due to the blunted crack tip and the relieved stress concentration at the crack tip as compared to the AIF-free one that exhibits a brittle crack propagation behavior. Also, the thicker the AIFs, the more pronounced this reinforcing effect. More importantly, it is found that there exists a critical AIF width of 1.7 nm, below which the crack will eventually break through the AIF, and above which the crack failed to do this. It is revealed that the enhanced fracture toughness originated from the transformation of brittle crack propagation to abundant dislocation emission from AIFs.

Simulation study on the nano-crystalline Cu-Ag alloys with gradient segregation

Via equidistance scaling method, several parts of the segregated domains from granular interior to grain boundary are identified. Based on this method, the model of nano-crystalline (NC) Cu-Ag alloy with gradient segregation is constructed. The issue how the solute concentration (SC) distribution is more conducive to the strength enhancement is explored by molecular dynamic (MD) simulation. The simulation results reveal that sample with higher intra-granular SC possess more dislocation activities, thinner amorphous inter-granular films (AIFs), more homogeneous deformation and higher flow stress at plastic stage. The results turn the opposite way in sample with higher SC nearby grain boundary. Meanwhile, the observed phase transition is analyzed during the deformation progress.

Ni solute segregation and associated plastic deformation mechanisms into random FCC Ag, BCC Nb and HCP Zr polycrystals

Different types of grain-boundary (GB) solute segregation in nanocrystalline alloys have been described in the literature as being either homogeneous or heterogeneous, which differently impacts their mechanical properties. In heterogeneous segregation, solute atoms tend to cluster along certain GB areas, while other GB regions remain solute-free. This phenomenon is evidenced with the segregation of Ni solute in nanocrystalline FCC Ag. Yet the physical origin for this segregation behavior and its existence in non-FCC alloys are not fully determined. Here, hybrid Monte-Carlo and molecular dynamics simulations were used to study the segregation of Ni solutes (4 at.%) into nanocrystalline FCC Ag, BCC Nb and HCP Zr metals at their solubility limit temperature and the same homologous temperature (0.405·Tm), respectively. A range of segregation configurations was found: Fully heterogeneous segregation in Ag96Ni4 at 500 K, homogeneous segregation with second-phase precipitates in Nb96Ni4 at 1110 K, homogeneous segregation with dispersion of small-scale Ni clusters in Nb96Ni4 at 1564 K and Zr96Ni4 at 464 K, and fully homogeneous segregation with amorphous intergranular films in Zr96Ni4 at 1118 K. We rationalize these changes by quantifying the Ni solute interactions at GBs. Furthermore, significant variations in mechanical behavior and associated plastic deformation mechanisms are shown for each alloy due to their different segregation behaviors. It is found that strain-localized shear banding is the most significant at interfaces with homogeneous solute segregation containing second-phase precipitates and absent with segregation leading to amorphous intergranular films. These findings underscore the importance of solute interactions in profoundly altering segregation and mechanical behavior in stable nanocrystalline alloys.

Amorphous Intergranular Film Effect on the Texture and Structural Evolution During Cold-Rolling of Nanocrystalline Ni–Zr Alloys

Amorphous Intergranular Film Effect on the Texture and Structural Evolution During Cold-Rolling of Nanocrystalline Ni–Zr Alloys

Amorphous complexions alter the tensile failure of nanocrystalline Cu-Zr alloys

Grain boundary-based mechanisms are known to control the plastic deformation and failure of nanocrystalline metals, with manipulation of the boundary structure a promising path for tuning this response. In this study, the role of interfacial structural disorder on plasticity and failure of nanocrystalline Cu-Zr alloys is investigated with in situ scanning electron microscopy tensile deformation experiments. Two model materials are created, one with only the typical ordered grain boundaries and another with amorphous intergranular films interspersed into the boundary network, while the microstructures are otherwise identical. Hence, the importance of complexion type on plasticity and failure is isolated by only varying complexion structure. The tensile experiments show that failure of the samples containing amorphous films is significantly retarded, as evidenced by an increase in the cross-sectional area reduction, a decrease in the occurrence of shear-dominated failure, a decrease in strain localization, and fracture surfaces with more elongated dimple features. As a whole, this study provides direct evidence that structural disorder at the grain boundaries can be beneficial for improving the ductility of nanocrystalline metals.

Disordered interfaces enable high temperature thermal stability and strength in a nanocrystalline aluminum alloy

Lightweighting of structural materials has proven indispensable in the energy economy, predicated on alloy design with high strength-to-weight ratios. Modern aluminum alloys have made great strides in ambient temperature performance and are amenable to advanced manufacturing routes such as additive manufacturing, but lack elevated temperature robustness where gains in efficiency can be obtained. Here, we demonstrate the intentional design of disorder at interfaces, a notion generally associated with thermal runaway in traditional materials, in a segregation-engineered ternary nanocrystalline Al–Ni–Ce alloy that exhibits exceptional thermal stability and elevated temperature strength. In-situ transmission electron microscopy in concert with ultrafast calorimetry and X-ray total scattering point to synergistic co-segregation of Ce and Ni driving the evolution of amorphous intergranular films separating sub-10 nm Al-rich grains, which gives rise to emergent thermal stability. We ascribe this intriguing behavior to near-equilibrium interface conditions followed by kinetically sluggish intermetallic precipitation in the confined disordered region. The resulting outstanding mechanical performance at high homologous temperatures lends credence to the efficacy of promoting disorder in alloy design and discovery.

Mitigating the Hall-Petch breakdown in nanotwinned Cu by amorphous intergranular films

In this work, we present nanotwinned (nt-) Cu with amorphous intergranular films (AIFs) as a prototype to mitigate the Hall-Petch breakdown in nt-metals. By using atomistic simulations, we show that by introducing AIFs into nt-Cu, the dominating plastic deformation mechanisms shift from typical dislocation-mediated deformation to interfacial activities in the AIFs. The reduced dislocation nucleation in nt-Cu by AIFs thus slow down the dislocation-mediated Hall-Petch breakdown prevailing in regular nt-metals. We hope that this work provides new insights into the design of ultrastrong and tough materials beyond the limit of conventional nt-metals.

Annealing-induced abnormal hardening in nanocrystalline NbMoTaW high-entropy alloy thin films

High temperature annealing of high-entropy alloys (HEA) typically results in reduced hardness due to grain coarsening, limiting its potential for applications at elevated temperatures. However, in this work, our nanocrystalline NbMoTaW HEA thin film exhibits enhanced hardness up to ~ 17.1 GPa after isothermal annealing at 800 °C for ~ 6 h (hrs). This phenomenon can be explained mainly by the formation of amorphous intergranular films (AIFs). This work does not only demonstrate a high-performance HEA film for potential high-temperature applications, but also provides a practical approach to tailor mechanical properties of nanostructured HEAs.

Suppression of shear localization in nanocrystalline Al–Ni–Ce via segregation engineering

Shear localization in nanocrystalline metals is a severely limiting factor precluding their use as practical engineering materials. While several strategies exist to enhance the thermal and mechanical behavior of these materials, there are still many outstanding questions regarding the effects of chemical segregation on shear localization in FCC nanocrystalline materials. In this paper we investigate the mechanical response of a ternary aluminum alloy with a sub-10 nm nanocrystalline microstructure subject to various thermal treatments. Contrary to previous observations, our results suggest that annealing up to 0.7 Tm reduces the propensity for shear localization and increases strength, as demonstrated by a transition in deformation morphology from pronounced strain localization to more homogeneous deformation during indentation. This behavior coincides with the formation of an amorphous intergranular film during annealing, causing intragranular dislocation plasticity to be favored over other grain boundary dominated deformation mechanisms, in turn resulting in a lower propensity for long range plastic localization.

Achieving exceptional radiation tolerance with crystalline-amorphous nanocrystalline structures

Nanostructured materials with amorphous intergranular films (AIFs) have demonstrated superior strength and ductility. Their radiation tolerance is expected to be high as the large fraction of interfacial volume efficiently sinks radiation-induced defects. Here we demonstrate how a crystalline-amorphous system (nanocrystalline Cu with Zr-doped AIFs) responds to continuous irradiation with molecular dynamics simulations. We propose a diffusion model that well characterizes the cascade-driven mixing process, and reveal that the spread of Zr distribution scales linearly with the damage level. The exceptional radiation resistance is attributed to the interfaces acting as sustainable defect sinks, Zr mixing into the bulk to enhance local defect annihilation due to solute-interstitial dragging, and Zr impeding radiation-enhanced grain growth by restraining AIFs from migration and maintaining interface stiffness. These findings suggest that AIF-engineered systems hold promise as highly radiation-tolerant materials with strong structural stability and self-healing capability under radiation damage.

Amorphous intergranular films mitigate radiation damage in nanocrystalline Cu-Zr

Nanocrystalline metals are promising radiation tolerant materials due to their large interfacial volume fraction, but irradiation-induced grain growth can eventually degrade any improvement in radiation tolerance. Therefore, methods to limit grain growth and simultaneously improve the radiation tolerance of nanocrystalline metals are needed. Amorphous intergranular films are unique grain boundary structures that are predicted to have improved sink efficiencies due to their increased thickness and amorphous structure, while also improving grain size stability. In this study, ball milled nanocrystalline Cu-Zr alloys are heat treated to either have only ordered grain boundaries or to contain amorphous intergranular films distributed within the grain boundary network, and are then subjected to in situ transmission electron microscopy irradiation and ex situ irradiation. Differences in defect density and grain growth due to grain boundary complexion type are then investigated. When amorphous intergranular films are incorporated within the material, fewer and smaller defect clusters are observed while grain growth is also limited, leading to nanocrystalline alloys with improved radiation tolerance.

Enhanced TiAl/Ti3SiC2 diffusion bonding by introducing amorphous intergranular films in Ti3SiC2 through Al doping

The Al-enriched amorphous intergranular films were firstly found and introduced in Ti3SiC2 substrates through Al doping, for enhancing the TiAl/Ti3SiC2 diffusion bonding. The existence of the Al-enriched amorphous intergranular films changed the main diffusion element from Si in the undoped TiAl/Ti3SiC2 joint to Al in the Al-doped joints. The build-ups of the interfacial compounds were γ+α2/γ/Ti5Si3/Ti3SiC2 in the undoped TiAl/Ti3SiC2 joint and were γ+α2/γ/TiAl2/TiAl3+Ti5Si3+Ti5Si4/Ti3SiC2 in the Al-doped joints. The formation of the brittle Ti5Si3 layer was inhibited through Al doping, which enhanced the shear strength of the joint.

Revealing the Strain Effect on Radiation Response of Amorphous–Crystalline Cu-Zr Laminate

Nanocrystalline materials containing amorphous intergranular films (AIFs) exhibit excellent mechanical properties, radiation resistance, and thermal stability and may serve as promising candidate materials for use in advanced nuclear energy systems. The aim of this work is to reveal the effect of mechanical stress on the radiation damage behavior of AIF systems. Based on a bicrystal Cu system with Zr-doped AIFs, molecular dynamics is used to simulate the radiation process and examine the AIF sink efficiency, defect propensity, defect size distribution, and Zr mixing under uniaxial and hydrostatic strain conditions. The results show that the sink efficiency of the glue-like AIFs is not compromised under applied strains. The anisotropy resulting from the intrinsic microstructure and elastic deformation leads to a distinct radiation response, where extension (contraction) of the structure perpendicular to the AIFs increases (decreases) the vacancy density. The strain-dependent defect density, along with the cluster size distributions, can be interpreted based on the variations in the defect formation energy and anisotropic defect diffusion. Finally, the Zr mixing induced by collision cascades is found to be insensitive to the mechanical strains. These findings provide meaningful information towards understanding the stress effect on the radiation response of AIF systems.

Atomistic investigation of the deformation mechanisms in nanocrystalline Cu with amorphous intergranular films

Grain boundaries in nanocrystalline (NC) materials are important as they control the microstructural evolution and act as both sinks and sources for dislocation activities. In order to enhance the absorption of dislocations and restrict the crack nucleation and growth, the conventional grain boundaries can be substituted with amorphous intergranular films (AIFs). In the present atomistic study, we investigated the deformation mechanism of bicrystals and NC Cu specimens with AIF under dynamic and static loading conditions with a particular focus on the influence of grain sizes (3 nm–17 nm) and AIF thicknesses (0.5 nm–1.5 nm). We found that the presence of AIF homogenized the interfacial energy irrespective of the grain orientations and decreased its overall value, which posed a strong effect on the strength of the metallic system. In addition, we observed a shift of the deformation mechanism from that dominated by dislocations to interfacial activities due to the presence of AIF as the grain size or AIF thickness changed. Finally, results from high-temperature creep deformation showed that the NC Cu with AIF had excellent thermal stability despite its small grain size.

Thick amorphous complexion formation and extreme thermal stability in ternary nanocrystalline Cu-Zr-Hf alloys

Building on the recent discovery of tough nanocrystalline Cu-Zr alloys with amorphous intergranular films, this paper investigates ternary nanocrystalline Cu-Zr-Hf alloys with a focus on understanding how alloy composition affects the formation of disordered complexions. Binary Cu-Zr and Cu–Hf alloys with similar initial grain sizes were also fabricated for comparison. The thermal stability of the nanocrystalline alloys was evaluated by annealing at 950 °C (>95% of the solidus temperatures), followed by detailed characterization of the grain boundary structure. All of the ternary alloys exhibited exceptional thermal stability comparable to that of the binary Cu-Zr alloy, and remained nanocrystalline even after two weeks of annealing at this extremely high temperature. Despite carbide formation and growth in these alloys during milling and annealing, the thermal stability of the ternary alloys is mainly attributed to the formation of thick amorphous intergranular films at high temperatures. Our results show that ternary alloy compositions have thicker boundary films compared to the binary alloys with similar global dopant concentrations. While it is not required for amorphous complexion formation, this work shows that having at least three elements present at the interface can lead to thicker grain boundary films, which is expected to maximize the previously reported toughening effect.

Amorphous Intergranular Films Enable the Creation of Bulk Nanocrystalline Cu–Zr with Full Density

Nanocrystalline metal alloys show great potential as structural materials, but are often only available in small volumes, such as thin films or powders. However, recent research has suggested that dopant segregation and grain boundary structural transitions between states known as complexions can dramatically alter grain size stability and potentially enable activated sintering. Herein, strategic consolidation routes for mechanically alloyed Cu‐4 at% Zr powders are explored to capture the effects of amorphous complexion formation on the densification of bulk nanostructured metals. An increase in the density of the consolidated samples is observed, which coincides with the formation of amorphous intergranular films. At the same time, the grain size is reasonably stable after exposure to these temperatures. As a result, a bulk nanograined metal with a grain size of 57 nm and a density of 99.8% is produced, which shows an impressive balance of small grain size and high density using simple consolidation techniques.

Understanding yielding and the unusual ductile-brittle-ductile transition in Fe-based amorphous nanocrystalline alloy: A combined micromechanical and thermodynamic study

Fe-based amorphous nanocrystalline alloys have been attracting tremendous research interest because of their excellent soft magnetic properties. However, their applications are hindered because of their room-temperature brittleness and the underlying mechanisms are yet to be fully understood. In this work, we successfully fabricated a series of Fe-based amorphous nanocrystalline alloys through the controlled nanocrystallization in Fe-based amorphous ribbons, which resulted in rather thin amorphous inter-granular films with an average thickness reducing from 10 nm to less than 0.5 nm as the volume fraction (Vf) of the nanocrystals increased from 16% to 95%. According to the extensive microcompression experiments, we demonstrated that the yielding strain of the amorphous nanocrystalline alloys decreases progressively with the increasing Vf and identified three types of distinctive deformation behaviors, including (I) shear banding for Vf < 70%, (II) shear banding induced cracking for 70% < Vf < 90% and (III) distributed plasticity for Vf > 90%. As combined with the transmission electron microscopy and strain rate sensitivity analyses, we developed a micromechanical and thermodynamic model which quantitatively explains the yielding behavior of the amorphous nanocrystalline alloys and the unusual phenomenon of ductile-brittle-ductile transition, as manifested by the three deformation behaviors. Our current findings provide important insights into the design of strong-yet-ductile soft magnetic amorphous nanocrystalline alloys.

On the role of Cu-Zr amorphous intergranular films on crack growth retardation in nanocrystalline Cu during monotonic and cyclic loading conditions

In the present study, molecular dynamics simulation was used to investigate the influence of amorphous intergranular films (AIF) and its thickness on possible crack retardation in nanocrystalline Cu under tensile and cyclic loading conditions. Structural analyses were carried out based on different methods such as centro-symmetry parameter, common neighbor analysis, and atomic strain. Results showed that specimens with AIF thickness as small as 0.5 nm can restrict the propagation of both perpendicular and parallel cracks during uniaxial tensile deformation. By increasing the AIF thickness, the crack propagation was found to be further retarded due to crack tip blunting. Atomic strain analysis also revealed that thicker AIF localized the strain distribution more strongly thus inhibiting the nucleation of multiple intergranular cracks and subsequent rapid fracture. In case of cyclic loading, the strain was accumulated through the formation of twins/stacking faults when the AIF thickness was 0.5 and 1 nm, which can interact with the AIF and eventually cause the nucleation of voids/cracks. However, increasing the AIF thickness to 2 nm can restrict such mechanism as the strain was mostly accommodated by the amorphous phase.