Solid-state Precipitation Research Articles

Influence of fine-scale B2 precipitation on dynamic compression and wear properties in hypo-eutectic Al0.5CoCrFeNi high-entropy alloy

The near equi-atomic high-entropy alloys based on Al, Co, Cr, Fe, Ni have provided a novel microstructural template for alloy design. The face centered cubic (FCC) matrix strengthened by body centered cubic (BCC) based ordered B2 precipitates can be used to develop new generation of precipitation strengthened alloys. The present study focuses on an Al0.5CoCrFeNi hypo-eutectic alloy consisting of an as-solidified FCC + B2 microstructure. Guided by solution thermodynamic modeling, this alloy has been isothermally aged for solid-state precipitation of a high fraction of fine scale intragranular B2 particles. While, the positive influence of B2 on tensile strength, and work hardenability have been published earlier, the current work presents effect of fine scale B2 phase on sliding wear and dynamic compression properties. The flow stress at 0.02% true strain increased from ∼670 MPa to ∼1350 (102%) MPa, the ultimate compressive strength increased from ∼1160 MPa to 1500 MPa (20%), and the wear resistance increased more than 5 times with progressively increasing phase fraction of B2 precipitates.

Equilibrium Multi-precipitate Configurations

In this paper, we formulate a phase-field model for the computation of equilibrium configurations of multiple phases that arise out of a solid-state precipitate reaction, in the presence of coherency stresses. Here, we utilize the phase-field framework to minimize the sum of the elastic and the interfacial energies for a given volume of the precipitates using an extension of the volume-preserved Allen–Cahn algorithm (Garcke et al. in Math Models Methods Appl Sci 18(08):1347–1381, 2008; Bhadak et al. in Metall Mater Trans A 49A(11):5705–5726, 2018). Using this technique, we investigate the precipitate organization for three solid-state reactions. The first is the classical two-phase precipitate reaction that leads to the formation of core–shell microstructures, where we clarify the influence of elasticity on the formation of such clusters. Following this, we investigate two symmetry-breaking transitions (cubic to tetragonal) and (hexagonal to orthorhombic), that lead to the formation of multi-variant clusters where we study the organization of the precipitates as a function of the elastic properties of the precipitate and the matrix.

Si-mediated reassembly of interfacially segregated Sc atoms in an Al–Cu–Sc alloy exposed to high-temperature creep

The formation of second-phase particles by solid-state precipitation is well-known as the most effective strengthening method for the light-weight aluminum (Al) alloys. Here, the interfacial evolutions from Sc solute segregation to Al3Sc precipitation at θ′-Al2Cu/matrix interfaces were investigated in an Al–Cu–Sc alloy crept at beyond 300 °C. By considering the trace Si impurity, it is proposed that Al3Sc precipitation preferentially occurs at the Si-concentrated interfacial locations, i.e. growth ledges. Two effects of interfacial Al3Sc precipitation on θ′-Al2Cu were demonstrated: (1) The Al3Sc precipitation was processed at the expense of the adjacent Sc segregation and allows localized θ′-Al2Cu decomposition, causing “precipitate splitting”. (2) The Al3Sc-enclosed growth ledges resultantly prevented θ′-Al2Cu from coarsening and equilibrium transformation to θ-phase at temperature as high as 400 °C. This work is expected to provide a precipitate-stabilizing strategy by interfacial architecture to maintain the closely spaced dispersion of precipitates and its strengthening contribution at elevated temperature.

Transforming solid-state precipitates via excess vacancies

Many phase transformations associated with solid-state precipitation look structurally simple, yet, inexplicably, take place with great difficulty. A classic case of difficult phase transformations is the nucleation of strengthening precipitates in high-strength lightweight aluminium alloys. Here, using a combination of atomic-scale imaging, simulations and classical nucleation theory calculations, we investigate the nucleation of the strengthening phase θ′ onto a template structure in the aluminium-copper alloy system. We show that this transformation can be promoted in samples exhibiting at least one nanoscale dimension, with extremely high nucleation rates for the strengthening phase as well as for an unexpected phase. This template-directed solid-state nucleation pathway is enabled by the large influx of surface vacancies that results from heating a nanoscale solid. Template-directed nucleation is replicated in a bulk alloy as well as under electron irradiation, implying that this difficult transformation can be facilitated under the general condition of sustained excess vacancy concentrations.

Phase formation during dissimilar transient liquid phase (TLP) bonding of IN939 to IN625 using a Ni-Cr-Fe-Si-B interlayer

An effort was made to assess the applicability of using transient liquid phase bonding technique to join a nickel-based precipitation-hardened superalloy (Inconel 939) to a solid-solution-hardened one (Inconel 625) to avoid the limitations of other joining techniques such as fusion welding. Microstructural development and phase formation mechanism of the joint region were studied in Inconel 939/Ni-Cr-B-Fe-Si/Inconel 625 system. The chemical composition of the isothermally solidified zone in each side of the joint was governed by the chemical composition of the adjacent base alloy. Eutectic-γ solid solution, chromium-rich boride and nickel-rich boride phases were formed at the centerline of the joints with insufficient holding times due to the segregation of chromium and boron elements during the athermal solidification. Increasing the holding time resulted in the decrement of athermally solidified zone width. In the holding time of 30 min, the isothermal solidification was finished and a bond without detrimental centerline-eutectics was produced. Hard and brittle chromium-rich and chromium-molybdenum-rich boride particles were formed by a solid-state precipitation phenomenon in the diffusion affected zone of Inconel 939 and Inconel 625, respectively. Chromium depletion was observed in the vicinity of these precipitates. The holding time had almost no influence on the width of the joint area. However, higher concentrations of solute elements were detected at the joints’ centerline in longer holding times.

Intermetallic phase formation during brazing of a nickel alloy using a Ni–Cr–Si–Fe–B quinary filler alloy

ABSTRACTFundamental understanding of the intermetallic phase formation is the key for enhancing the robustness and reliability of the brazed joints. The paper addresses the phase transformations during brazing of the Hastelloy X nickel-base superalloy using the quinary Ni–13Cr–4.5Si–4.2Fe–2.8B (wt-%) braze alloy. The mechanisms of intermetallic formation via solidification and solid-state precipitation are discussed. The athermal solidification zone (ASZ) is featured by the formation of brittle and hard borides and boro-silicides that are formed via eutectic reactions. However, in contrast to other commercial B-bearing Ni-based filler alloys, it was identified that the presence of a high-volume fraction of eutectic gamma solid solution between boride phases within the joint centreline can alleviate the deleterious effect of the intermetallic phases on the joint toughness.

Predicting Precipitation Kinetics During the Annealing of Additive Manufactured Inconel 625 Components

The prediction of solidification microstructures associated with additive manufacture of metallic components is fundamental in the identification scanning strategies, process parameters and subsequent heat treatments for optimised component properties. Interactions between the powder particles and the laser heat source result in complex thermal fields in and around the metal melt pool, which will influence the spatial distribution of chemical species as well as solid-state precipitation reactions. This paper demonstrates that a multi-component, multi-phase precipitation model can successfully predict the observed precipitation kinetics in Inconel 625, capturing the anomalous precipitation behaviour exhibited in additively manufactured components. A computer coupling of phase diagrams and thermochemistry (CALPHAD)-based approach captures the impact of dendritic segregation of alloying elements upon precipitation behaviour. The model was successful in capturing the precipitation kinetics during annealing considering the Nb-rich and Nb-depleted regions that are formed during additive manufacturing.

Precipitation strengthening of aluminum alloys by room-temperature cyclic plasticity.

High-strength aluminum alloys are important for lightweighting vehicles and are extensively used in aircraft and, increasingly, in automobiles. The highest-strength aluminum alloys require a series of high-temperature "bakes" (120° to 200°C) to form a high number density of nanoparticles by solid-state precipitation. We found that a controlled, room-temperature cyclic deformation is sufficient to continuously inject vacancies into the material and to mediate the dynamic precipitation of a very fine (1- to 2-nanometer) distribution of solute clusters. This results in better material strength and elongation properties relative to traditional thermal treatments, despite a much shorter processing time. The microstructures formed are much more uniform than those characteristic of traditional thermal treatments and do not exhibit precipitate-free zones. These alloys are therefore likely to be more resistant to damage.

The joint properties of a high-chromium Ni-based superalloy made by diffusion brazing: Microstructural evolution, corrosion resistance and mechanical behavior

The phase transformation during diffusion brazing of high chromium Ni-based Inconel 939 superalloy, using a ternary Ni-14.9 Cr-3.7 B (wt.%) interlayer was studied. The microstructural evolution during the bonding process was governed by isothermal solidification, athermal solidification and solid-state precipitation. A single-phase gamma solid solution was the only phase detected in the isothermally solidified zone (ISZ). However, the cooling-induced solidification led to the formation of proeutectic-gamma, Ni-rich boride, Cr-rich boride and eutectic-gamma at the athermally solidified zone (ASZ) which deteriorated the corrosion resistance of the joints. The ASZ width decreased by increasing the bonding time. A reverse relation between the corrosion resistance and the ASZ width was perceived. The complete isothermal solidification, as well as the highest corrosion resistance, were obtained at the bonding time of 90 min. The solid-state precipitation resulted in the formation of Cr-rich boride particles with two blocky and Widmanstatten morphologies in the diffusion affected zone (DAZ). The hardest structure compared to the other microstructural zones belonged to the ASZ in all the bonds. The shear strength of the joints increased in longer bonding times and a reverse relation between the ASZ width and the shear strength was witnessed. The highest shear strength (∼625 MPa) which was approximately 80% of that of the base metal was achieved in the 90-min sample.

Effect of V and Zr microalloying, and heat treatment on microstructure and mechanical properties of secondary Al-7Si-3Cu-0.3Mg alloy

AbstractThe Al–Si–Cu alloys, which are widely used in automotive powertrains, exhibit limited high-temperature strength properties; the high diffusivity of the main strengthening elements Cu and Mg in α-Al at temperatures between 200 and 300 °C is a dominating factor in alloy softening. In this study, effects of slow-diffusing elements (Zr and V) and heat treatment on the microstructure and mechanical properties of secondary Al-7Si-3Cu-0.3Mg alloy were investigated. Majorities of both Zr and V were retained inside the α-Al matrix during solidification. T6 heat treatment induced the solid-state precipitation of multiple, nano-sized particles in α-Al grain interiors. Unlike Cu/Mg-rich strengthening precipitates that form during aging, the Zr/V-rich precipitates formed during solution heat treatment, which indicates high potential for high-temperature strengthening in Al–Si alloys via transition metal addition. Other transition metals, such as Mn, Fe, Cr and Ti, which were present as impurities in the base alloy significantly promoted the formation of nano-sized Zr/V-rich precipitates inside α-Al grains. These thermally more stable precipitates were credited for the enhanced high-temperature strength properties of Al-7Si-3Cu-0.3Mg alloy by ∼20 %.

Strengthening mechanisms in Al[sbnd]Ni[sbnd]Sc alloys containing Al3Ni microfibers and Al3Sc nanoprecipitates

Dilute AlSc alloys (<0.4 wt% Sc) are creep-resistant up to ∼300 °C owing to coherent, nanosize Al3Sc particles created by solid-state precipitation. By contrast, eutectic AlNi alloys (Al-6 wt.% Ni) derive their high strength at elevated temperature from Al3Ni microfibers formed during solidification. Here, we investigate ternary Al6Ni-0.2Sc and Al6Ni-0.4Sc alloys with both types of strengthening precipitates (Al3Sc and Al3Ni) and compare them to binary Al6Ni, Al-0.2Sc and Al-0.4Sc alloys with a single population of precipitates. Kinetics of Al3Sc and Al3Ni precipitation and coarsening are studied via hardness measurements during isochronal and isothermal aging; the two phases resist coarsening up to 475 and 350 °C, respectively, upon short term exposures (1 h isochronal steps). No noticeable effect of Sc is observed on the Al3Ni micro-fiber composition and hardening in the alloy. Similarly, Ni does not significantly affect the hardening provided by the Al3Sc precipitates, despite the presence of 0.14–0.17 at.% Ni in the Al3Sc nano-precipitates. The strengthening contributions of the Al3Ni and Al3Sc phases at ambient temperature are cumulative in the ternary alloys. For creep deformation at 300 °C, all alloys show a creep threshold stress, indicating that both types of precipitates impede dislocation motion. The ternary AlNiSc alloys show higher creep threshold stresses than their binary counterparts, also consistent with cumulative strengthening effects of precipitation strengthening (from Al3Sc nano-precipitates) and load transfer (from Al3Ni micro-fibers).

Microstructural Characterization and Electrochemical Behavior of AA2014 Al-Cu-Mg-Si Alloy of Various Tempers

AA2014 Al-Cu-Mg-Si alloy is subjected to aging heat treatment, and microstructural features and electrochemical behavior of the AA2014 Al-Cu-Mg-Si alloy of various tempers have been characterized. Hardness measurement of the alloy of different tempers showed typical age hardening behavior. X-ray diffraction and transmission electron microscopy studies revealed the presence of characteristic phases. Differential scanning calorimetry study displayed the sequence of solid-state precipitation and dissolution reactions in the AA2014 alloy of various tempers. Electrochemical behavior was assessed by measurement of variation of open-circuit voltage with time, potentiodynamic polarization and potentiodynamic cyclic polarization of the AA2014 alloy of various tempers in 3.5 wt.% NaCl solution at near neural (pH ~ 7) and at alkaline (pH 12) conditions. Potentiodynamic polarization studies of different tempers in 3.5 wt.% NaCl solution at pH ~ 7 showed that the corrosion potential (Ecorr) value has shifted toward more negative (active) direction with increasing aging time. The cyclic polarization in 3.5 wt.% NaCl solution at pH ~ 7 displayed active–passive behavior and also showed a large hysteresis loop indicating high pit growth damage.

Numerical Benchmark of Phase-Field Simulations with Elastic Strains: Precipitation in the Presence of Chemo-Mechanical Coupling

Phase-field studies of solid-state precipitation under strong chemo-mechanical coupling are performed and benchmarked against the existing analytical solutions. The open source software packages OpenPhase and DAMASK are used for the numerical studies. Solutions for chemical diffusion and static mechanical equilibrium are investigated individually followed by a chemo-mechanical coupling effect arising due to composition dependence of the elastic constants. The accuracy of the numerical solutions versus the analytical solutions is quantitatively discussed. For the chemical diffusion benchmark, an excellent match, with a deviation <0.1%, was obtained. For the static mechanical equilibrium benchmark Eshelby problem was considered where a deviation of 5% was observed in the normal component of the stress, while the results from the diffuse interface (OpenPhase) and sharp interface (DAMASK) models were slightly different. In the presence of the chemo-mechanical coupling, the concentration field around a static precipitate was benchmarked for different coupling coefficients. In this case, it is found that the deviation increases proportional to the coupling coefficient that represents the strength of coupling concentration and elastic constants. Finally, the interface kinetics in the presence of the considered chemo-mechanical coupling were studied using OpenPhase and a hybrid OpenPhase–DAMASK implementation, replacing the mechanical solver of OpenPhase with DAMASK’s. The observed deviations in the benchmark studies are discussed to provide guidance for the use of these results in studying further phase transformation models and implementations involving diffusion, elasticity and chemo-mechanical coupling effect.

Geometry-Induced Spatial Variation of Microstructure Evolution During Selective Electron Beam Melting of Rene-N5

High gamma prime (γ′) nickel-based alloys produced by selective electron beam melting are of interest to the turbine industry which requires control of microstructure in relation to loading conditions within complex component geometry. Welding literature predicts cracking and microstructure evolution as a function of alloy composition and process parameters in this class of alloys. In addition, Additive manufacturing causes variations in the above conditions due to the interaction of geometry on processing and heat transfer. The influence of geometry on processing conditions was explored for alloy Rene N5 by characterizing the solidification grain microstructure and solid-state precipitation. A Semi-Analytical Heat Transfer Model was employed to explain the resulting variation in solidification grain morphology that occurred due to the part geometry. The as-built precipitation structure was found to vary as a function of build height and had no correlation to the solidification grain structure or the layer geometry.

Microstructure and micro-texture evolution of additively manufactured β-Ti alloys

This study focuses on the microstructural evolution in additively manufactured (AM) β titanium alloys due to solid-state phase transformations occurring during the reheating of previously deposited layers, directly influencing the uniformity of microstructure across the entire build. During the AM of titanium alloys of a wide variety of compositions, including α + β alloys such as Ti-6Al-4 V, and β alloys, when the laser or electron beam hits the sample, the grains in the previously deposited topmost layers either re-melt or transform into the β phase. Subsequently, during the cooling cycle, depending on the alloy composition, second-phase precipitation may occur within these layers via solid-state precipitation. The present study compares two binary β -Ti alloys, Ti-12Mo and Ti-20 V, that have been processed using laser engineered net shaping (LENS™), a directed energy deposition technique for AM. Compared to Ti-V, which exhibited grains of only the β phase in the as-built condition, the less β stabilized Ti-Mo had extensive second-phase α precipitation within the build. The location within the LENS™ build played a pivotal role in determining the size scale, area fraction, and morphology of the α precipitates. These changes have been attributed to the different thermal cycles experienced during the deposition process. Irrespective of the alloy composition, columnar grains were observed in the depositions with a strong [001]β texture along the build direction. In the Ti-12Mo alloy, wherein second phase α precipitation takes place, there was no significant α texturing, with all twelve variants forming.

A Comparative Study of Photocatalytic Activity of ZnO/activated Carbon Nanocomposites Prepared by Solid-state and Conventional Precipitation Methods

ZnO/activated carbon nanocomposites (ZnO/ACns) were synthesized by applying two different solid-state and precipitation methods. Precipitation method was initiated with zinc chloride and sodium hydroxide as starting materials in the presence of activated carbon. The synthesized sample was calcined at 250 °C for 1 h. The preparation of nanocomposite by solid-state method was accomplished by heating a mixture of ZnCl2 and activated carbon at 250 °C for 1 h, which led to the formation of ZnO particles on the surface. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Photocatalytic activity of these ZnO/acnes was evaluated by the degradation of methyl orange under UV-light radiation. The SEM images of samples show that the agglomerations of particles are much less in solid-state method prepared samples. The results of EDS indicate that the ratio of Zn/C in the pure activated carbon and nanocomposites prepared by solid-state and precipitation methods were 0, 1.7 and 3.7 % (W/W), respectively. The XRD of the samples clearly shows that the sample prepared by the above routes has ZnO structures. Finally, ZnO/ACn prepared by the solid-state method demonstrated better photocatalytic efficiency against methyl orange dye than composites made by precipitation method.

Lattice geometry controlled synthesis of Cu – Doped nickel oxide nanoparticles

NiO3+δ and Ni1−xCuxO3+δ (x = 5, 10, 20) were synthesized by solid state precipitation method, using precursors, nickel chloride and copper chloride with potassium hydroxide as an alkalizing agent. The present work reports the synthesis of copper doped nickel oxides. The phase formation, crystallinity index, crystal structure and microstructure with composition analysis were investigated with the help of various characterization techniques. The FTIR spectrum reveals that characteristic metal oxygen bands of the Ni–O stretching at 447cm−1 with crystallinity index 0.39. Phase pure, hexagonal NiO3+δ formation was identified from the XRD data. Nanosized particles with size as small as ~ 6 to 15nm for pure sample and ~ 11 to 50nm for the copper doped sample are observed from field emission scanning electron micrographs. Thermo chemical calculations such as enthalpy of reaction 77.44kJmole−1, Gibbs free energy − 639.23kJmole−1and entropy of the reaction 0.0024kJK−1mole−1 have been calculated. Specific surface area of NiO3+δ and Ni1−xCuxO3+δ (x = 5, 10, 20) is obtained by Sauter formula, i.e. ~ 292m2/g, ~ 656m2/g, ~ 557m2/g, and ~ 517m2/g respectively.

Microstructure and ablation mechanism of SiC-ZrC-Al2O3 coating for SiC coated C/C composites under oxyacetylene torch test

SiC-ZrC-Al2O3 ceramic coating was prepared on the SiC coated carbon/carbon (C/C) composites by in-situ reaction method to improve its ablation resistance at high temperatures. The phase compositions, microstructures, and ablation properties (carried out by an oxyacetylene torch) of the coating were investigated. During ablation process, the outer coating experienced solid state (ZrO2 and Al2O3) precipitate from the molten eutectic (Zr-O-Al-Si layer) and formed dense Zr-O-Al-Si layer on the surface of the coating. These dense layers can prevent oxygen diffuse into the specimen. After ablation, the coating forms layered structure: SiO2 glass layer distribute with some hemi-spherical particles (Zr-O-Al, dense dendrites structure), Zr-O-Al-Si, Zr-O-Al, Zr-O-Si, less affect SiC-ZrC-Al2O3 and SiC layer. The newly formed Al2(SiO4)O and ZrSiO4 are stable phases with a dense structure, which can effectively consume the internal stress and prevent the spread of cracks. With the increase of ablation time, the SiO2 evaporate, leading to the increase of hemi-spherical particles.

Coherent Power Measurements with a Compact Airborne Ka-Band Precipitation Radar

AbstractCoherent power is an alternative to the conventional noise-subtracted power technique for measuring weather radar signal power. The inherent noise-canceling feature of coherent power eliminates the need for estimating and subtracting the noise component, which is required when performing conventional signal power estimation at low signal-to-noise ratio. The coherent power technique is particularly useful when averaging a high number of samples to improve sensitivity to weak signals. In such cases, the signal power is small compared to the noise power and the required accuracy of the estimated noise power may be difficult to achieve. This paper compares conventional signal power estimation with the coherent power measurement technique by investigating bias, standard deviation, and probability of false alarm and detection rates as a function of signal-to-noise ratio and threshold level. This comparison is performed using analytical expressions, numerical simulations, and analysis of cloud and precipitation data collected with the airborne solid-state Ka-band precipitation radar (KPR) operated by the University of Wyoming.

Generalization of the binary structural phase field crystal model

Binary phase field crystal (PFC) models have been successful in describing a broad selection of phenomena in binary alloy materials: eutectic and dendritic solidification, the Kirkendall effect, clustering, solid state precipitation, and many more. In this paper, authors present improvements to the binary structural PFC model and show that these improvements allow us to model new material phase diagrams. They apply their improved model to study the kinetics of precipitation from a liquid solution where they observe a two-step nucleation pathway. This agrees with recent experimental observations in which spinodal decomposition precedes nucleation in solute-rich domains. They also find a phenomenon not previously described in the literature in which precipitate growth is accelerated in the presence of uncrystallized, solute-rich liquid domains.