Coherent Precipitates Research Articles

Bragg coherent imaging of nanoprecipitates: role of superstructure reflections

Coherent precipitation of ordered phases is responsible for providing exceptional high-temperature mechanical properties in a wide range of compositionally complex alloys. Ordered phases are also essential to enhance the magnetic or catalytic properties of alloyed nanoparticles. The present work aims to demonstrate the relevance of Bragg coherent diffraction imaging (BCDI) for studying bulk and thin-film samples or isolated nanoparticles containing coherent nanoprecipitates/ordered phases. The structures of crystals of a few tens of nanometres in size are modelled with realistic interatomic potentials and are relaxed after introduction of coherent ordered nanoprecipitates. Diffraction patterns from fundamental and superstructure reflections are calculated in the kinematic approximation and used as input to retrieve the strain fields using algorithmic inversion. First, the case of single nanoprecipitates is tackled and it is shown that the strain field distribution from the ordered phase is retrieved very accurately. Then, the influence of the order parameter S on the strain field retrieved from the superstructure reflections is investigated. A very accurate strain distribution can be retrieved for partially ordered phases with large and inhomogeneous strains. Subsequently, the relevance of BCDI is evaluated for the study of systems containing many precipitates, and it is demonstrated that the technique is relevant for such systems. Finally, the experimental feasibility of using BCDI to image ordered phases is discussed in the light of the new possibilities offered by fourth-generation synchrotron sources.

Influence of Interface Proximity on Precipitation Thermodynamics

The formation of coherent precipitates is often accompanied by large elastic mismatch stresses, which suppress phase separation. We discuss the presence of interfaces as a mechanism for stress relaxation, which can lead to preferred zones of precipitation. In particular, we discuss the proximity of free surfaces and shear-coupled grain boundaries, for which we can obtain a substantial local energy reduction and predict the influence on the local precipitation thermodynamics. The latter case is accompanied by morphological changes of the grain boundary, which are less suitable for large-scale descriptions. For that purpose, we develop an effective description through an elastic softening inside the grain boundary and map the microscopic grain boundary relaxation to a mesoscopic elastic and phase field model, which also allows generalizing the description to multi-phase situations.

Enhancing performance of the CuCrZrTiV alloys through increasing recrystallization resistance and two-step thermomechanical treatments

Cu-0.75Cr-0.1Zr-0.3Ti-0.1V (wt.%) alloys were fabricated by vacuum induction melting, and the effects of joint Ti and V addition on the recrystallization behavior, mechanical property and corrosion resistance of the alloys were investigated. Compared with International Thermonuclear Energy Reactor (ITER) grade CuCrZr (IG-CuCrZr) alloys, the recrystallization resistance of the CuCrZrTiV alloys is highly improved by the addition of Ti and V. The outstanding recrystallization resistance of the CuCrZrTiV alloys is attributed to the reduced stacking-fault energy and the presence of high-density coherent precipitates. The corrosion resistance of the CuCrZrTiV alloys is much higher than the IG-CuCrZr alloys. Two-step thermomechanical treatments are applied to further improve the performance of the CuCrZrTiV alloys, and the tensile strength and ductility of the alloys are improved simultaneously. Enhancement of the strength is ascribed to high-density precipitates, dislocations and subgrains in the CuCrZrTiV alloys after two-step thermomechanical treatments. The texture changes from {112}<110> α texture to {110}<112> brass texture after two-step thermomechanical treatments, and deformation twins form during the development of the brass texture, which improves the ductility of the alloys. This work provides an effective way to improve the performance of the CuCrZrTiV alloys by increasing recrystallization resistance and two-step thermomechanical treatments.

Coherent precipitation and stability of cuboidal nanoparticles in body-centered-cubic Al0.4Nb0.5Ta0.5TiZr0.8 refractory high entropy alloy

The present work developed a new body-centered-cubic (BCC)-based Al0.4Nb0.5Ta0.5TiZr0.8 refractory high entropy alloy with coherent precipitation. The formation of cuboidal nanoprecipitates with a size of 25~50 nm in an 873 K-aged alloy is ascribed to a moderate lattice misfit. A Zr5Al3 phase also coexists with B2 in a same particle shape due to similar chemical compositions, in which Ti is enriched in B2 but not in Zr5Al3. The BCC/B2 coherency would be deteriorated with increasing the aging temperature, leaving coarse Zr5Al3 alone. Besides, there exist two kinds of BCC phases due to the segregations of Nb/Ta and Ti/Zr.

Effect of microadditions of Mn and Mo on dual L12- and α-precipitation in a dilute Al-Zr-Sc-Er-Si alloy

The effects of small additions of 0.25 at.% Mn and 0.11 at.% Mo to a cast, dilute Al-0.08Zr-0.02Sc-0.01Er-0.10Si (at.%) alloy were investigated by Vickers microhardness, electrical conductivity, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Isochronal aging experiments revealed that the Mn/Mo-modifications allowed the alloy to achieve a 30 MPa higher peak microhardness, which was maintained at higher temperatures (475 °C vs. 400 °C). This is attributed to α-Al(Mn,Mo)Si submicron precipitates forming at 450–475 °C, following precipitation of L12-Al3(Zr,Sc,Er) nanoprecipitates at 350–425 °C. Isothermal aging experiments at 400 °C indicated that the Mn/Mo addition improved the coarsening resistance of the L12-precipitates to yield a coarsening rate about four times slower than that found in Mn/Mo-free alloys. Compared to the base alloy, this results in a 30% reduction in the nanoprecipitate radii after six months of aging. The α-phase consists mainly of coherent plate-like precipitates, with a minority of cuboidal semi-coherent precipitates, displaying well-defined crystallographic orientation relationships with the matrix. Increasing the Mn concentration to 0.40 at.%, led to the formation of primary Al12(Mn,Mo) precipitates, which are too coarse to provide any strength benefit.

Investigation of the retained austenite orientation in iron precipitates of martensitic transformation

The retained austenite orientation in iron precipitates of martensitic transformation was studied in a Cu-Fe-P alloy. The alloy was isochronally aged at 723 K with tensile stresses of 0 and 16 MPa after solution treatment at 1143 K for 2 h respectively. The results showed that [220]γ-Fe and [200]γ-Fe of austenite (γ-Fe) in iron precipitates was parallel to [220]Cu and [200]Cu of copper matrix during ageing without applied stress, respectively. When the ageing treatment was carried out with an applied stress of 16 MPa, the martensitic transformation occurred in the iron precipitates. For the first time, it was found that not crystal orientation of [220] but [200] of retained γ-Fe in the iron precipitate was parallel to [220] of copper matrix, which was attributed to partial dislocations slipping and shearing coherent precipitates. The results suggest that the crystal orientation of the retained austenite in iron precipitates changes during martensitic transformation.

Plastic yielding and deformation bursts in the presence of disorder from coherent precipitates

Alloying metals with other elements is often done to improve the material strength or hardness. A key microscopic mechanism is precipitation hardening, where precipitates impede dislocation motion, but the role of such obstacles in determining the nature of collective dislocation dynamics remains to be understood. Here, three-dimensional discrete dislocation dynamics simulations of FCC single crystals are performed with fully coherent spherical precipitates from zero precipitate density upto $\rho_p = 10^{21}\,\text{m}^{-3}$ and at various dislocation-precipitate interaction strengths. When the dislocation-precipitate interactions do not play a major role the yielding is qualitatively as for pure crystals, i.e., dominated by "dislocation jamming", resulting in glassy dislocation dynamics exhibiting critical features at any stress value. We demonstrate that increasing the precipitate density and/or the dislocation-precipitate interaction strength creates a true yield or dislocation assembly depinning transition, with a critical yield stress. This is clearly visible in the statistics of dislocation avalanches observed when quasistatically ramping up the external stress, and is also manifested in the response of the system to constant applied stresses. The scaling of the yielding with precipitates is discussed in terms of the Bacon-Kocks-Scattergood relation.

Influence of tempering temperatures on the microstructure, secondary carbides and mechanical properties of spray-deposited AISI M3:2 high-speed steel

The evolution of microstructure and mechanical properties of a spray-deposited AISI M3:2 high-speed steel (HSS) are investigated using a series of tempering temperatures. The results demonstrate a strong correlation between secondary precipitates and tempering. With increasing tempering temperature, secondary precipitates exhibit a transformation from coarse strips of M3C type to a refined and dense coherent precipitates of types MC and M2C. Highly dispersed fine secondary precipitates contribute to an enhanced bending strength and the secondary hardening peak without reduction of its impact toughness. The dimension of MC and M2C precipitates increased while the volume fraction decreased with the increasing temperature. Tempering at 650 °C can result in the generation of ellipsoidal M3C carbides at the cost of the fine MC and M2C precipitates. Overaging at 700 °C lead to the transformation of the fine precipitates to coarse MC, M3C and M7C3, and finally to M23C6 and M6C, along with the matrix structure change from plate martensite to recrystallized ferrite, which enables the toughness to increase significantly. The strengthening effects of secondary precipitates are also quantitatively analyzed and assessed.

Unraveling the dislocation–precipitate interactions in high-entropy alloys

The precipitates play a significant role in not only enhancing the strength, but also maintaining the high toughness in alloys. However, the interactions of the nanoscale precipitates with dislocations in the high entropy alloys (HEAs) are difficult to observe directly by in-situ TEM experiments due to the limits of the resolution and time. Here, using atomic simulations we report the synergistic strengthening of the coherent precipitate and atomic-scale lattice distortion in the HEAs at cryogenic/elevated temperatures. The effects of temperature, chemical disorder, precipitate spacing, precipitate size, elemental segregation, and dislocation-cutting number on the critical stress for the dislocation to overcome a row of precipitates are studied. A random stacking fault energy landscape along the slip plane, the lattice distortion at different temperatures, and the interface/surface energy at various crystallographic orientations are obtained. Compared with the traditional metals and alloys, HEAs have the severe atomic-scale lattice distortions to generate the local high tensile/compressive stress fields. This complex stress causes the dislocation line to bend, and thus improves the dislocation slip resistance, resulting in the strong solid-solution strengthening. The stacking fault strengthening induced by the obvious difference of the stacking fault energies between the HEA matrix and precipitate (within the inner of the HEA matrix), and the formation of the antiphase domain boundary contribute to the high strength. The precipitate embedded by the solute atoms produces the strong lattice distortion to enhance the dislocation slip resistance at high temperatures. Hence, the current results provide the mechanistic insight into the phenomenon that the coherent precipitate combined with the severe atomic-scale lattice distortion can enhance the strength at cryogenic/elevated temperatures to further broaden the scope of applications of advanced HEAs.

Induction of uniaxial anisotropy by controlled phase separation in Y-Co thin films

In this study, molecular beam epitaxy is utilized to stabilize a nanostructured thin-film magnet consisting of a soft magnetic ${\mathrm{Y}}_{2}{\mathrm{Co}}_{17}$ exchange coupled to hard magnetic ${\mathrm{YCo}}_{5}$. While, typically, a phase decomposition can be obtained in rare-earth cobalt systems only by the addition of further elements like Cu, Fe, and Zr and complex heat treatments, here we directly induce phase separation by growth kinetics. The resulting nanoscale architecture, as revealed by cross-sectional transmission electron microscopy, is composed of a network of coherently interlinked and aligned ${\mathrm{Y}}_{2}{\mathrm{Co}}_{17}$ and ${\mathrm{YCo}}_{5}$ building blocks. The formation of coherent precipitations is facilitated by the perfectly matching lattice constants, atomic species, and crystal symmetry of the two phases with vastly different magnetocrystalline anisotropies. The hard magnetic phase induces an aligned uniaxial anisotropy in ${\mathrm{Y}}_{2}{\mathrm{Co}}_{17}$, resulting in substantial coercivity associated with enhanced energy products. This work highlights the importance of thin-film epitaxy in understanding magnetic hardening mechanisms and suggests strategies for a rational design of future sustainable magnetic systems.

Event generation for probabilistic flood risk modelling: multi-site peak flow dependence model vs. weather-generator-based approach

Abstract. Flood risk assessment is an important prerequisite for risk management decisions. To estimate the risk, i.e. the probability of damage, flood damage needs to be either systematically recorded over a long period or modelled for a series of synthetically generated flood events. Since damage records are typically rare, time series of plausible, spatially coherent event precipitation or peak discharges need to be generated to drive the chain of process models. In the present study, synthetic flood events are generated by two different approaches to modelling flood risk in a meso-scale alpine study area (Vorarlberg, Austria). The first approach is based on the semi-conditional multi-variate dependence model applied to discharge series. The second approach relies on the continuous hydrological modelling of synthetic meteorological fields generated by a multi-site weather generator and using an hourly disaggregation scheme. The results of the two approaches are compared in terms of simulated spatial patterns of peak discharges and overall flood risk estimates. It could be demonstrated that both methods are valid approaches for risk assessment with specific advantages and disadvantages. Both methods are superior to the traditional assumption of a uniform return period, where risk is computed by assuming a homogeneous return period (e.g. 100-year flood) across the entire study area.

Element segregation and α2 formation in primary α of a near-α Ti-alloy

Alloy TIMETAL®834 is a near-α Ti-alloy typically processed to have a complex bimodal microstructure that provides a good combination of mechanical properties at temperatures in excess of 450 °C. Due to the high Al content, typical ageing procedures result in the formation of intragranular and coherent nano-scale Ti3Al precipitation (α2), which increase strength but also promotes slip planarity. The present study focuses on chemical partitioning as a result of sub-β-transus heat treatment and the consequences for the two different constituents in the bimodal microstructure. The detailed chemical and structural analysis were carried out by combining Electron Probe Micro Analysis (EPMA), Wavelength Dispersive Spectroscopy (WDS), Transmission Electron Microscopy (TEM) and Atom Probe Tomography (APT) for investigating local compositional variations and their effect on the formation of α2 precipitates. Detailed microchemical analysis shows a core-shell composition arrangement of α-stabilisers with the shell composition similar to the one of secondary α. Selected area electron diffraction in the TEM and APT analysis demonstrates that those local variations in α stabilisers affect the level of α2 precipitation. In addition, EPMA maps show that while Zr and Sn are often considered to be neutral alloying elements in Ti-alloys, they do segregate to the β-phase during sub-β-transus heat treatment.

Control of nanoscale precipitation and elimination of intermediate-temperature embrittlement in multicomponent high-entropy alloys

Thermally stable high-entropy alloys (HEAs) consisting of a high density of coherent precipitates show a great potential for high-temperature applications. In this work, we systematically investigated the phase stability and coarsening kinetics of L12-type coherent precipitates in a Ni-30Co-13Fe-15Cr-6Al-6Ti-0.1B (at.%) HEA isothermally aged at 800, 900 and 1000 °C. Aged microstructures in the grain interiors under this temperature range were essentially dominated by the uniform precipitation of multicomponent L12 (Ni, Co, Fe, Cr)3(Ti, Al)-type precipitates. The coarsening kinetics of these intragranular L12 precipitates were quantitatively determined, which were adequately characterized by the classical Lifshitz-Slyozov-Wagner model. The activation energy for coarsening was determined to be 378 kJ/mol, which is relatively higher than that of conventional Ni or Co-based superalloys, suggesting a slow elemental diffusion in the HEA matrix. More importantly, the heterogeneous precipitation and the associated metastable phase transformation mechanism along grain boundaries (GBs) were carefully analyzed. Localized chemical heterogeneity was identified within the discontinuous L12 phase at the GBs, which thermodynamically destabilizes the L12 structure and encourages the formation of brittle Heusler phase. Finally, we establish a unique duplex-aging strategy that can be efficiently utilized for GB stabilization, by which these detrimental intergranular heterostructures can be greatly eliminated, leading to an exceptional resistance to intermediate-temperature embrittlement, along with enhanced tensile strengths. These findings will not only shed light on the precipitation mechanisms in compositionally complex HEAs but also generate new opportunities to the interfacial design of HEAs for advanced high-temperature applications with superior properties.

Tailoring microstructures and tensile properties of a precipitation-strengthened (FeCoNi)94Ti6 medium-entropy alloy

The coherent precipitate L12-type Ni3Ti nanoclusters and nano-sized particles in a fcc-FeCoNi medium-entropy alloy (MEA) matrix were designed and realized through minor Ti addition. The mechanical properties were modulated by thermo-mechanical processing and various annealing treatments, which leads to different hardening mechanisms such as dislocation strengthening and precipitation strengthening. An excellent combination of yield strength (∼893 MPa), ultimate tensile strength (UTS ∼ 1263 MPa) and ductility (∼24% elongation) can be achieved because of the precipitation strengthening from the L12-type Ni3Ti nano-sized precipitates. The present study is important not only for understanding the strengthening mechanisms of metallic materials, but also provides a generic way to obtain the desirable mechanical properties of alloys via microstructural regulation.

Achieving exceptional wear resistance in a compositionally complex alloy via tuning the interfacial structure and chemistry

Titanium alloys have been widely used for medical devices and structural applications. However, conventional titanium alloys often suffer from low resistance to wear, particularly at elevated temperatures. Herein, an equiatomic TiMoNb compositionally complex alloy (CCA) is shown to exhibit wear resistance comparable to alumina at room temperature (RT). Even at 600 °C, the alloy still shows an extremely low wear rate of the order of 10−6 mm3/(N·m). The remarkable wear resistance is achieved via tuning the interfacial structure and chemistry in TiMoNb CCA, including nanostructuring, titanium segregation at the grain boundaries, and the formation of a high density of nanoscale coherent Ti-rich precipitates with cube-on-cube orientation relationship with the ultrafine-grained matrix. The present results provide significant insights into the design of novel alloys for service in harsh environments.

Shock wave characterization of precipitate strengthening of PH 13–8 Mo stainless steel

The shear stresses required for dislocations passage of highly coherent NiAl-based precipitates, formed in commercial PH 13–8 Mo steel after homogenization, quenching and aging for different duration of time at 510 °C, were determined in a series of planar impact experiments using samples with different thickness. The experiments revealed two regimes of decay of the elastic precursor waves: a fast one, at the shear stress τ greater than some threshold τ*, associated with the interaction of moving dislocations with lattice phonons, and a slow one, at τ < τ*, corresponding to the precipitates' cutting with the help of thermal fluctuations. Consequently, the stress τ* can be regarded as the stress which permits the passage of the precipitate by a dislocation without thermal support. Precipitates' geometry and size, determined based on the activation volume at thermally-activated regime, are in a reasonable agreement with precipitates dimensions estimated from High Resolution Transmission Electron Microscopy images.

From coherent to semicoherent—Evolution of precipitation crystallography in an fcc/bcc system

How are crystallographic features developed in phase transformation systems with anisotropic misfits (e.g., bcc/fcc systems)? Comprehensive knowledge about this question is still very limited, which impedes the thorough understanding and the quantitative modeling of microstructure evolution. In this work, we simulated the early stage growth of a Cr precipitate (bcc) in a Cu matrix (fcc) by combining Monte Carlo and molecular dynamics. The simulation results reveal fine details about the evolution of crystallographic features, including the orientation relationship (OR) between the two phases, the precipitate morphology, and the interfacial dislocation structures. The governing event during the evolution process is the generation of a dominant set of dislocations. The selection of the dominant dislocations is rationalized based on minimization of the interfacial energy in the major facet, which contains a single set of dislocations. The initial OR between the coherent precipitate and the matrix is close to the Nishiyama–Wassermann OR. In response to the generation of the dominant dislocations, the OR jumps towards the ideal OR corresponding to the O-line condition, which is close to the Kurdjumov–Sachs OR. This tendency reflects the experimental observations in a Cu-Cr system and provides helpful insight into the actual evolution of crystallographic features.

Effect of Sink Strength on Coherency Loss of Precipitates in Dilute Cu Base Alloys During in Situ Ion Irradiation

In situ irradiations with 1 MeV Kr ions at 50~613 K up to a fluence of 6.25 × 1014 ions/cm2 (~1.25 dpa) have been performed on pre-aged dilute Cu-0.9%Co, Cu-0.9%Fe and Cu-0.8%Cr alloys containing uniform matrix dispersions of coherent precipitates in order to study the effects of initial precipitate sink strength, damage dose and irradiation temperature on radiation-induced coherency loss of precipitates. Coherent precipitates with different sink strengths (2πNd , where N and d are the precipitate density and diameter) were used in this work to examine potential differences in atomic relaxation during absorption of point defects. At low sink strengths (~1013 m-2 ), loss of precipitate coherency could be induced for doses below ~0.1 dpa. There might be a preferential medium-range strain-induced bias for absorption of interstitial defects due to tensile strains at the undersized precipitates, which induces relatively rapid loss of coherency at low precipitate sink strengths. High sink strength (~1014 m-2 ) conditions were relatively resistant to the radiation-induced loss of coherency and this might be due to nearly equal numbers of interstitial and vacancy defects arriving at the precipitate interface for such high sink strength conditions. The precipitate coherency loss was observed to have a weak dependence on irradiation temperature. Molecular dynamics simulations confirm a strong effect of precipitate sink strength on the probability of interstitial absorption at precipitates.

Polycrystalline SnSe–Sn1–vS solid solutions: Vacancy engineering and nanostructuring leading to high thermoelectric performance

P-type polycrystalline Sn1-vS compounds and Sn0.985S1-xSex solid solutions are prepared by combining mechanical alloying (MA) and spark plasma sintering (SPS). Dislocations in the grains are successfully introduced through vacancy engineering in Sn1-vS compounds. Point defects provide high–frequency phonons scattering from the Se substitution in Sn0.985S1-xSex, in addition to the strong scattering of mid–frequency phonons by dislocations. This leads to a low lattice thermal conductivity and an enhanced thermoelectric figure of merit (ZT) of ≈1.1 for the Sn0.985S0.25Se0.75 at 823 K. The Ag dopant is selected to further enhance the electrical transport properties of the optimized composition. The power factor improved from 4.5 (Sn0.985S0.25Se0.75) to ≈5.3 μW m−1 K−2 for Sn0.978Ag0.007S0.25Se0.75 sample. Surprisingly, the Ag doping induced a nanostructured matrix with dispersed spherical coherent precipitates of AgSnSe2 inside the grains, which further strengthens the scattering of phonons. The presence of AgSnSe2 nanoscale precipitates inside the grains and dislocations–induced by Ag at the grain boundaries contributes to an impressively low lattice thermal conductivity (κL). Consequently, the maximum ZT ≈1.75 at 823 K is achieved for the Sn0.978Ag0.007S0.25Se0.75 sample.

Microstructural evolution and mechanical properties of Al0.3CoCrFeNiSix high-entropy alloys containing coherent nanometer-scaled precipitates

In this work, Al0.3CoCrFeNiSix (x = 0, 0.2, 0.5, 0.8, 1.0 in molar ratio) high-entropy alloys (HEAs) were designed by introducing a non-metallic element Si. The microstructural evolution and its effect on the mechanical properties were discussed. The results suggested that the addition of Si promoted the transition from face-centered cubic (FCC) to body-centered cubic (BCC)/ordered BCC (B2) phases, in which the spherical Cr-rich BCC nanoparticles were coherently dispersed in the Al, Ni-rich B2 matrix. The mechanical properties were improved by adding an appropriate amount of Si. When x increased from 0 to 0.2, the tensile yield strength and ultimate tensile strength increased by 63% and 60%, respectively, while the elongation remained at 56%. The Vickers hardness enhanced from 143 HV (x = 0) to 826 HV (x = 1.0) with increasing Si content, and the specific wear rate reduced by two orders of magnitude accordingly. The strengthening mechanisms were evaluated for this series of HEAs based on the correlation between microstructure and mechanical properties.