Coherent Precipitates Research Articles

Phase diagram of FeNiCoAlTaB ferrous shape memory alloy on aging time

Ferrous shape memory alloy, Fe41Ni28Co17Al11.5Ta2.5B0.05, has shown large superelastic strain and strength in previous study. In the fabrication of this alloy, aging process is crucial for the formation of shape memory effect/superelasticity. However, its phase evolution on aging time is not clearly known. In this study, we systematically studied the phase diagram of this alloy on aging time. It is found that the unaged alloy shows a strain glass transition. With the aging time proceeding, the martensitic transformation gradually emerges. The phase diagram can be explained by the formation of coherent precipitates induced by aging. The heterogeneous strain between coherent precipitates and matrix is the driving force responsible for the emerging martensitic transformation. The generic explanation is supposed to be useful in martensitic transformation engineering for developing novel shape memory alloys from non-transforming materials.

Ultrastrong steel via minimal lattice misfit and high-density nanoprecipitation.

Next-generation high-performance structural materials are required for lightweight design strategies and advanced energy applications. Maraging steels, combining a martensite matrix with nanoprecipitates, are a class of high-strength materials with the potential for matching these demands. Their outstanding strength originates from semi-coherent precipitates, which unavoidably exhibit a heterogeneous distribution that creates large coherency strains, which in turn may promote crack initiation under load. Here we report a counterintuitive strategy for the design of ultrastrong steel alloys by high-density nanoprecipitation with minimal lattice misfit. We found that these highly dispersed, fully coherent precipitates (that is, the crystal lattice of the precipitates is almost the same as that of the surrounding matrix), showing very low lattice misfit with the matrix and high anti-phase boundary energy, strengthen alloys without sacrificing ductility. Such low lattice misfit (0.03 ± 0.04 per cent) decreases the nucleation barrier for precipitation, thus enabling and stabilizing nanoprecipitates with an extremely high number density (more than 1024 per cubic metre) and small size (about 2.7 ± 0.2 nanometres). The minimized elastic misfit strain around the particles does not contribute much to the dislocation interaction, which is typically needed for strength increase. Instead, our strengthening mechanism exploits the chemical ordering effect that creates backstresses (the forces opposing deformation) when precipitates are cut by dislocations. We create a class of steels, strengthened by Ni(Al,Fe) precipitates, with a strength of up to 2.2 gigapascals and good ductility (about 8.2 per cent). The chemical composition of the precipitates enables a substantial reduction in cost compared to conventional maraging steels owing to the replacement of the essential but high-cost alloying elements cobalt and titanium with inexpensive and lightweight aluminium. Strengthening of this class of steel alloy is based on minimal lattice misfit to achieve maximal precipitate dispersion and high cutting stress (the stress required for dislocations to cut through coherent precipitates and thus produce plastic deformation), and we envisage that this lattice misfit design concept may be applied to many other metallic alloys.

Moisture sources and pathways associated with the spatial variability of seasonal extreme precipitation over Canada

Nine regions with spatially coherent seasonal 3-day total precipitation extremes across Canada were identified using a clustering method that is compliant to the extreme value theory. Using storm back-trajectory analyses, we then identified possible moisture sources and pathways that are conducive to occurrences of seasonal extreme precipitation events in four seasons for the nine regions identified. Moisture pathways for all extreme precipitation events were clustered to nine dominant moisture pathway patterns using the self-organizing map method. Results show that horizontal moisture pathway patterns and their occurrences were not evidently different between seasons. However, warm (summer and fall) and cold (winter and spring) seasons show considerable differences in the spreading of moisture sources in all nine regions, even though many sources do not frequently contribute to extreme precipitation events. In all four seasons, terrestrial evapotranspiration had provided major moisture sources to many extreme precipitation events occurred in inland regions. Central Canada had received more widespread moisture sources over surrounding oceans of North America than western and eastern Canada, because of more diverse moisture pathway patterns for central Canada that transport moisture from all surrounding oceans to central Canada. Extreme precipitation in southwestern Canada mainly resulted from atmospheric rivers over the North Pacific Ocean. For northwestern Canada, moisture pathway patterns were from the northern Pacific, Arctic and northern Atlantic oceans, even though more than 78% of trajectories for northwestern Canada were from the North Pacific. Westerlies from the North Pacific Ocean and northern polar jet streams controlled dominant pathways to central and eastern Canada. More extreme precipitation events over Canada were fed by the Arctic Ocean in warm than in cold seasons.

The phase transformation and strengthening of a Cu-0.71 wt% Cr alloy

Abstract The phase transformation of a Cu-0.71 wt% Cr alloy during the aging process at 450 °C was investigated by Transmission electron microscopy (TEM) and High-resolution transmission electron microscopy (HRTEM) in the study. The strengthening mechanism was studied as well. The precipitation sequence of Cu-0.71 wt% Cr aged at 450 °C is supersaturated solid sloution→G.P zones (fcc Cr-rich phase)→fcc Cr phase→order fcc Cr phase→bcc Cr phase. This transformation can be achieved by the nucleation of coherent fcc spherical precipitates and growth and transformation of fcc precipitates into the bcc structure. When the specimen is aged aging for 1 h–24 h, both fcc and bcc precipitates exist in the matrix simultaneously. Ordering process is found during the aging process, which is in favor of the precipitation of Cr-rich phases. The orientation relationship between bcc Cr precipitates and the matrix exhibit NW-OR. The peak strength effect occurs after aging for about 8 h and the strength continues to decrease with prolonging aging time, indicating overaging. The strengthening effect of Cr-rich phase precipitated in aging at 450 °C for 8 h is calculated to be 146 MPa on the basic of the Orowan strengthening mechanism, which is in quite accordance with the experimental data (144.5 MPa).

Anisotropic layered Bi2Te3-In2Te3 composites: control of interface density for tuning of thermoelectric properties

Layered (Bi1−xInx)2Te3-In2Te3 (x = 0.075) composites of pronounced anisotropy in structure and thermoelectric properties were produced by zone melting and subsequent coherent precipitation of In2Te3 from a (Bi1−xInx)2Te3 (x > 0.075) matrix. Employing solid state phase transformation, the Bi2Te3/In2Te3 interface density was tuned by modifying the driving force for In2Te3 precipitation. The structure-property relationship in this strongly anisotropic material is characterized thoroughly and systematically for the first time. Unexpectedly, with increasing Bi2Te3/In2Te3 interface density, an increase in electrical conductivity and a decrease in the absolute Seebeck coefficient were found. This is likely to be due to electron accumulation layers at the Bi2Te3/In2Te3 interfaces and the interplay of bipolar transport in Bi2Te3. Significantly improved thermoelectric properties of Bi2Te3-In2Te3 composites as compared to the single phase (Bi1−xInx)2Te3 solid solution are obtained.

Structure and Properties of CuCr0.6 Alloy after Severe Plastic Deformation

This paper focuses on the effect of rolling with cyclic movement of rolls (RCMR) on microstructure refinement, mechanical properties and electrical conductivity of CuCr0.6 alloy after applying different heat treatments (quenching and aging). It was found that the presence of second phase particles obtained during aging treatment has a significant effect on the formation of ultrafine grain (UFG) structure during the RCMR processing. The presence of high dislocation density inside subgrains and presence microshear bands are the marked features of the microstructure after aging at 500°C/2h and RCMR deformation. Whereas after aging at 700°C/24h and RCMR processing, fine precipitates were effective in inhibiting the grain/subgrain boundary motion. The RCMR processed alloy after aging at 500°C/2h shows high mechanical strength attributed to the high density of coherent precipitates and ultrafine grained structure. The RCMR processing induces a significant reduction of the electrical conductivity for samples at quenching state but for samples at aging state electrical conductivity was restored thanks to precipitation process.

Nanoprecipitation effects on phase stability of Fe-Mn-Al-Ni alloys

Abstract Early stages of coherent precipitation have been studied in several Fe-Mn-Al-Ni samples aged at 200 °C for different time intervals in order to analyze the effect of nanoprecipitation on the phase stability during thermally induced martensitic transformations. The α-γ′ martensitic transformation was studied by means of dilatometry measurements. Transmission electron microscopy (TEM) observations were performed in order to characterize the microstructure of the samples. The size and volume fraction of nanoprecipitates were evaluated for the as-quenched material and samples aged for 10, 20 and 180 min. Considering these data and additional high resolution TEM results reported in the literature a phenomenological model is presented that enables understanding the effects of B2 precipitates on the relative phase stability between austenite and martensite, and their influence on the observed pseudoelasticity of this metallic system. With this model, the changes of the measured transformation temperatures can be predicted with considerable accuracy.

Effect of titanium additions upon microstructure and properties of precipitation-strengthened Fe-Ni-Al-Cr ferritic alloys

Various amounts of Ti (0, 2, 4 and 6 wt%) is added to a ferritic alloy with a nominal composition of Fe–10Cr–10Ni–6.5Al–3.4Mo–0.25Zr–0.005B (wt%) (FBB8). The microstructure and composition of the matrix and precipitate phases are characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). For the Ti-modified steels, the bcc ferritic matrix is strengthened by submicron L21-Ni2TiAl-type precipitates which contain (i) Fe-inclusions with the precipitates' overall diameters ranging from 100 to 500 nm for both FBB8-4 wt%Ti and FBB8-6 wt%Ti, or (ii) B2-NiAl sub-precipitates with an average diameter of 50–100 nm for FBB8-2 wt%Ti. By contrast, the Ti-free FBB8 alloy contains B2-NiAl precipitates with Fe-inclusions. The four FBB8-Ti alloys were subjected to creep experiments at 700 °C in the stress range of 60–300 MPa. Threshold stresses for all studied compositions were observed, ranging from 69 to 179 MPa, with the most creep-resistant alloy being FBB8-2Ti with L21/B2 precipitates. Based on these mechanical results and detailed electron microscopy observations, the creep mechanism is rationalized to be general dislocation climb with repulsive elastic interaction between coherent precipitates and the matrix dislocations.

Interfacial structures and energetics of the strengthening precipitate phase in creep-resistant Mg-Nd-based alloys

The extraordinary creep-resistance of Mg-Nd-based alloys can be correlated to the formation of nanoscale-platelets of β1-Mg3Nd precipitates, that grow along 〈110〉Mg in bulk hcp-Mg and on dislocation lines. The growth kinetics of β1 is sluggish even at high temperatures, and presumably occurs via vacancy migration. However, the rationale for the high-temperature stability of precipitate-matrix interfaces and observed growth direction is unknown, and may likely be related to the interfacial structure and excess energy. Therefore, we study two interfaces– {112}β1/{100}Mg and {111}β1/{110}Mg– that are commensurate with β1/hcp-Mg orientation relationship via first principles calculations. We find that β1 acquires plate-like morphology to reduce small lattice strain via the formation of energetically favorable {112}β1/{100}Mg interfaces, and predict that β1 grows along 〈110〉Mg on dislocation lines due to the migration of metastable {111}β1/{110}Mg. Furthermore, electronic charge distribution of the two interfaces studied here indicated that interfacial-energy of coherent precipitates is sensitive to the population of distorted lattice sites, and their spatial extent in the vicinity of interfaces. Our results have implications for alloy design as they suggest that formation of β1-like precipitates in the hcp-Mg matrix will require well-bonded coherent interface along precipitate broad-faces, while simultaneously destabilizing other interfaces.

Coupling of Computational Thermodynamics with Kinetic Models for Predictive Simulations of Materials Properties

We present successful examples of CALPHAD thermodynamics-based precipitation simulations for three important alloy groups: Single-crystal Ni-base superalloy, austenitic stainless steel and hardenable Al-alloy. Underlying physical models for special features, such as, energies of diffuse interfaces between coherent precipitates and matrix, precipitation of incoherent particles at grain boundaries, evolution of excess vacancies during quenching and continuous aging and their role for metastable precipitate nucleation, are discussed.

As cast precipitation microstructures in twin-roller melt-spun Cu90Co10 alloys

As cast Cu90Co10 ribbons rapidly solidified by twin-roller melt spinning, exhibit special microstructure features. This processing method provides scenarios where a different phase selection takes place; coherent Co precipitates form directly from solidification, with neither a spinodal-like composition oscillation nor a discontinuously precipitated laminar phase. Samples are processed at tangential wheel speeds of 10m/s (V10), 15m/s (V15), 20m/s (V20) and 30m/s (V30). Microstructures resulting from this single step process are characterized and the hysteresis properties and the magnetoresistance effects evaluated. Samples V30 have a quite uniform density of coherent precipitates, with a narrow size distribution around 4nm. On contrary, non-uniform precipitate distributions are found in samples cooled at lower rates; zones with a high density of coherent Co-rich precipitates are found forming colonies. These colonies are consistent with the extended compositional fluctuations occurring during very early stages in the cooling process. Samples may exhibit wide (V10) and even bimodal (V15) size distributions. Only samples V30 behave close to the ideal superparamagnetism. Samples V20 present relatively large coercivity and relative remanence and behave as an interacting superparamagnet, while the hysteresis loops of ribbons cooled at lower rates exhibit a ferromagnetic contribution in addition to the superparamagnetic-like one. This ferromagnetic component arises from blocked precipitates, larger than the upper bound size for superparamagnetic behavior at 300K (12nm). Room temperature magnetoresistance values associated to granular scattering units decrease as the mean precipitate size increases, but they remain below 2%, which is lower than that measured in samples annealed after rapid solidification, indicating that in this latter case contributions from the spinodally segregated matrix take place in addition to that of Co granules.

A phase-field method coupled with CALPHAD for the simulation of ordered [formula omitted]-carbide precipitates in both disordered [formula omitted] and [formula omitted] phases in low density steel

In order to simulate multi-component diffusion controlled precipitation of ordered phases in low density steels using the phase-field method, the Gibbs free energy of the γ,α and κ phases in the quaternary Fe-Mn-Al-C system was linked to the CALPHAD method using a three-sublattice model which is based on the accumulation of considerable thermodynamic data in multi-component systems and the assurance of continuous variation of the interface area. This model includes the coherent precipitation of κ phase from a disordered FCC γ phase and semi-coherent precipitation of the same κ phase from a disordered BCC α structure. The microstructure evolution of κ-carbide was simulated with three-dimensional phase-field model. The simulation was first performed for a single particle in both γ and α phases to investigate the evolution of interfacial and elastic strain energy during the precipitation process. The simulation results show that κ has a cuboidal morphology in γ and elongated plate-like morphology in α which is in agreement with the morphologies reported in the literature. The multi-particle simulations were also performed for the precipitation of κ phase from both disordered γ and α. The results also demonstrate that the size of κ precipitates in γ is remarkably smaller than that in α phase.

Delineation of precipitation regions using location and atmospheric variables in two Canadian climate regions: the role of attribute selection

ABSTRACT The identification of homogeneous precipitation regions is essential in the planning, design and management of water resources systems. Regions are identified using a technique that partitions climate sites into groups based on the similarity of their attributes; the procedure is known as regionalization. In this paper the ability of four attribute sets to form large, coherent precipitation zones is assessed in terms of the regional homogeneity of precipitation statistics and computational efficiency. The outcomes provide guidance for effective attribute selection for future studies in Canada. The attributes under consideration include location parameters (latitude, longitude), distance to major water bodies, site elevation and atmospheric variables modelled at different pressure levels. The analysis is conducted in two diverse climate regions within Canada including the Prairie and the Great Lakes–St Lawrence lowlands regions. The method consists of four main steps: (i) formation of the attribute sets; (ii) determination of the preferred number of regions (selection of the c-value) into which the sites are partitioned; (iii) regionalization of climate sites using the fuzzy c-means clustering algorithm; and (iv) validation of regional homogeneity using L-moment statistics. The results of the attribute formation, c-value selection, regionalization and validation processes are presented and discussed in a comparative analysis. Based on the results it is recommended for both regions to use location parameters including latitude, longitude and distance to water bodies (in the Great Lakes region) to form precipitation regions and to consider atmospheric variables for future (climate change) applications of the regionalization procedure.

Precipitation behavior of AlxCoCrFeNi high entropy alloys under ion irradiation.

Materials performance is central to the satisfactory operation of current and future nuclear energy systems due to the severe irradiation environment in reactors. Searching for structural materials with excellent irradiation tolerance is crucial for developing the next generation nuclear reactors. Here, we report the irradiation responses of a novel multi-component alloy system, high entropy alloy (HEA) AlxCoCrFeNi (x = 0.1, 0.75 and 1.5), focusing on their precipitation behavior. It is found that the single phase system, Al0.1CoCrFeNi, exhibits a great phase stability against ion irradiation. No precipitate is observed even at the highest fluence. In contrast, numerous coherent precipitates are present in both multi-phase HEAs. Based on the irradiation-induced/enhanced precipitation theory, the excellent structural stability against precipitation of Al0.1CoCrFeNi is attributed to the high configurational entropy and low atomic diffusion, which reduces the thermodynamic driving force and kinetically restrains the formation of precipitate, respectively. For the multiphase HEAs, the phase separations and formation of ordered phases reduce the system configurational entropy, resulting in the similar precipitation behavior with corresponding binary or ternary conventional alloys. This study demonstrates the structural stability of single-phase HEAs under irradiation and provides important implications for searching for HEAs with higher irradiation tolerance.

Temporal evolution of coherent precipitates in an aluminum alloy W319: A correlative anisotropic small angle X-ray scattering, transmission electron microscopy and atom-probe tomography study

Small-angle X-ray scattering (SAXS), transmission electron-microscopy (TEM), and atom-probe tomography (APT) are utilized correlatively to study the evolution of nanometer length-scale coherent precipitates in a commercial age-hardenable aluminum alloy, W319. SAXS, with its high-field-of-view and resultant large statistics, perfectly complements TEM and APT at the nanometer length-scale. The combined results provide a comprehensive description of the evolution of the fine precipitates. W319 exhibits two types of fine coherent precipitates on aging namely, θ′ and Q, whose temporal evolution has been studied using time-resolved in situ SAXS analyses at three different aging temperatures: 438, 463, and 533 K. Complementary information about the morphology, dimensions and number density of the precipitates were determined from ex situ APT and TEM analyses. Coherent precipitates from a single grain of the α-Al-matrix of W319 give rise to an anisotropic SAXS signal, which is subsequently simulated, taking into account the relative orientation of the sample and the crystallographic orientation relationships of the different precipitates. The simulated anisotropic SAXS intensity, based on the results from APT and TEM, is validated utilizing the experimentally obtained 2D SAXS patterns. This permits us to determine the dimensions and number densities of the precipitates from anisotropic SAXS signals, and a complete quantitative analysis of the in situ temporal evolution of the coherent precipitates is performed. Additionally, the present study demonstrates an alternative technique for extracting structural parameters from anisotropic SAXS analyses. The methodology presented herein will be useful for age-hardening analyses of commercial age-hardenable Al-based alloys and Ni-based super alloys.

Formation and evolution of intermetallic nanoparticles and vacancy defects under irradiation in Fe[sbnd]Ni[sbnd]Al ageing alloy characterized by resistivity measurements and positron annihilation

In this paper, we study the effects of intermetallic nanoparticles like Ni3Al on the evolution of vacancy defects in the fcc FeNiAl alloy under electron irradiation using positron annihilation spectroscopy. Electrical resistivity measurements have been used as a testing method for characterizing the evolution in the underlying precipitate microstructure due to heat treatment and irradiation. It was shown that the nanosized (∼4.5 nm) intermetallic precipitates homogeneously distributed in the alloy matrix caused a several-fold decrease in the accumulation of vacancies as compared to their accumulation in the pre-quenched alloy. This effect was enhanced with the irradiation temperature. The irradiation-induced growth of intermetallic nanoparticles was also observed in the pre-quenched FeNiAl alloy under irradiation at 573 K. Thus, resistivity measurement and positron confinement in ultrafine intermetallic particles, which we revealed earlier, provided the control over the evolution of coherent precipitates, along with vacancy defects, during irradiation and annealing.

The influence of volcanic eruptions on the climate of tropical South America during the last millennium in an isotope-enabled general circulation model

Abstract. Currently, little is known on how volcanic eruptions impact large-scale climate phenomena such as South American paleo-Intertropical Convergence Zone (ITCZ) position and summer monsoon behavior. In this paper, an analysis of observations and model simulations is employed to assess the influence of large volcanic eruptions on the climate of tropical South America. This problem is first considered for historically recent volcanic episodes for which more observations are available but where fewer events exist and the confounding effects of El Niño–Southern Oscillation (ENSO) lead to inconclusive interpretation of the impact of volcanic eruptions at the continental scale. Therefore, we also examine a greater number of reconstructed volcanic events for the period 850 CE to present that are incorporated into the NASA GISS ModelE2-R simulation of the last millennium. An advantage of this model is its ability to explicitly track water isotopologues throughout the hydrologic cycle and simulating the isotopic imprint following a large eruption. This effectively removes a degree of uncertainty associated with error-prone conversion of isotopic signals into climate variables, and allows for a direct comparison between GISS simulations and paleoclimate proxy records. Our analysis reveals that both precipitation and oxygen isotope variability respond with a distinct seasonal and spatial structure across tropical South America following an eruption. During austral winter, the heavy oxygen isotope in precipitation is enriched, likely due to reduced moisture convergence in the ITCZ domain and reduced rainfall over northern South America. During austral summer, however, more negative values of the precipitation isotopic composition are simulated over Amazonia, despite reductions in rainfall, suggesting that the isotopic response is not a simple function of the "amount effect". During the South American monsoon season, the amplitude of the temperature response to volcanic forcing is larger than the rather weak and spatially less coherent precipitation signal, complicating the isotopic response to changes in the hydrologic cycle.

Annealing effects on precipitation and high-temperature properties of a Cu-containing alumina-forming austenitic steel

A novel Cu-containing alumina-forming austenitic (AFA) stainless steel was annealed at 1080°C and 1230°C following the same treatments. Annealing at different temperatures renders different microstructure and properties of the AFA steels. In comparison to alloy A1080, the grain size of alloy A1230 after annealing is larger and the high temperature properties of A1230 is better, i.e. higher strain hardening ability and longer creep/rupture life at 700°C. Coherent L12-ordered precipitates of 10–20nm appear in both alloys and the amount of nanoscale MC precipitates is obviously higher in alloy A1230. Evidence of L12-ordered precipitates cut by dislocation and dislocation pinned by MC precipitates is clearly shown. The precipitation behavior of MC carbides agrees with the calculated Time-Temperature-Transformation (TTT) curves. The results show that nanoscale MC precipitates formed during creep testing contribute to the longer creep/rupture life of alloy A1230.

Temperature dependent dislocation bypass mechanism for coherent precipitates in Cu–Co alloys

Molecular dynamics simulations of dislocation interaction with coherent cobalt precipitates embedded in Cu–Co alloys reveal a temperature dependent bypass mechanism. Below 300 K, the trailing partial dislocation clearly bypasses the coherent, face centered cubic (FCC) cobalt precipitate by Orowan looping, caused by a reversible structural transformation as the leading partial locally converts the precipitate to the lower-energy hexagonal close packed (HCP) structure. The FCC versus HCP energy difference of cobalt is temperature dependent, and the dislocation bypass mechanism becomes pure shear above 300 K. Based on a combination of inertial effects due to phonon drag and this observed bypass mechanism, we develop a temperature dependent critical resolved shear stress (CRSS) model, which is in excellent agreement with long-standing measurements of the CRSS temperature dependence of Cu–Co alloys, and those obtained from MD simulation. The model explains both the CRSS increase at low temperatures and the existence of a peak value around 200 K.

Evaluation of Composition, Microstructure Characterization and Interfacial Properties of Zn—SnO2 Metal Matrix Composite Coating

In this paper the microstructure and tribological behavior of Zn—SnO2 (Zn—Sn) alloys produced through chloride and sulphates co-deposition is presented for comparison. 7.0 wt % SnO2 was added to Zn bath and deposited at 0.3 V. The interfacial effect and microchemistry of the fabricated composite was studied by optical microscope, X-ray diffraction (XRD), scanning electron microscope (SEM) equipped with energy disperse spectrum (EDS). The tribological behavior of the metal composites with SnO2 particles as reinforcement was studied using reciprocating sliding tester. The scanning electron microscopy (SEM) and atomic force microscope (AFM) of the composite surfaces indicates that there is good interfacial interaction between the alloy formulated matrixes made from the two baths and the substrate. Reasonable uniform distribution of Sn metal phase particulates is shown for both coating alloy. Increases in hardness and wear resistance are attributed to the uniform and coherent precipitation in the metal interface especially for Zn—7Sn—S—0.3V. In general, 7 wt % Sn additions to the bath showed more hastening to improved surface properties and better mechanical characteristics.