Nucleation Of Precipitates Research Articles

Formation of a new intermediate phase and its evolution toward θ' during aging of pre-deformed Al-Cu alloys

The frequent occurrence of hitherto unknown phase Pre-θ'-2 and unusual 1.5 cθ' thick θ' precipitate was observed by atomic-resolution scanning transmission electron microscopy in the well-studied Al-Cu alloys. This phenomenon is associated with heterogeneous precipitate nucleation and growth on pre-existing dislocations introduced by slight deformation prior to aging. In this study, the precise structure details of Pre-θ'-2 was determined by atomic scale imaging, image simulation based on image forming theories and first principle calculations. Pre-θ'-2 has a well-defined ordered structure sandwiched between two 2aAl (∼1.5cθ') spaced Cu layers on {200}Al planes. The strong structural similarities between Pre-θ'-2 and 1.5 cθ' thick θ' in terms of interfacial structure and thickness, coupled with energetic calculations and preliminary in-situ observations, lead us to propose a new precipitation path toward key strengthening phase θ'.

Intragranular nucleation of tetrahedral precipitates and discontinuous precipitation in Cu-5wt%Ag

Both continuous and discontinuous precipitation is known to occur in CuAg alloys. The precipitation of Ag-rich phase has been experimentally investigated by atom probe tomography and transmission electron microscopy after ageing treatment of Cu-5%wtAg at 440 °C during 30’. Both continuously and discontinuously formed precipitates have been observed. The precipitates located inside the grains exhibit two different faceted shapes: tetrahedral and platelet-shaped precipitates. Dislocations accommodating the high misfit at the interface between the two phases have also been evidenced. Based on these experimental observations, we examine the thermodynamic effect of these dislocations on the nucleation barrier and show that the peculiar shapes are due to the interfacial anisotropy. The appropriate number of misfit dislocations relaxes the elastic stress and lead to energetically favorable precipitates. However, due to the large misfit between the parent and precipitate phases, discontinuous precipitation that is often reported for CuAg alloys can be a lower energetic path to transform the supersaturated solid solution. We suggest that the presence of vacancy clusters may assist intragranular nucleation and decrease the continuous nucleation barrier. We eventually propose qualitative thermodynamic and kinetic justifications accounting for the relative importance of homogeneous and discontinuous precipitation modes as a function of temperature.

Thermodynamics and kinetics of nucleation in binary solutions

A new approach that is a combination of classical thermodynamics and macroscopic kinetics is offered for studying the nucleation kinetics in condensed binary solutions. The theory covers the separation of liquid and solid solutions proceeding along the nucleation mechanism, as well as liquid-solid transformations, e.g., the crystallization of molten alloys. The cases of nucleation of both unary and binary precipitates are considered. Equations of equilibrium for a critical nucleus are derived and then employed in the macroscopic equations of nucleus growth; the steady state nucleation rate is calculated with the use of these equations. The present approach can be applied to the general case of non-ideal solution; the calculations are performed on the model of regular solution within the classical nucleation theory (CNT) approximation implying the bulk properties of a nucleus and constant surface tension. The way of extending the theory beyond the CNT approximation is shown in the framework of the finite-thickness layer method. From equations of equilibrium of a surface layer with coexisting bulk phases, equations for adsorption and the dependences of surface tension on temperature, radius, and composition are derived. Surface effects on the thermodynamics and kinetics of nucleation are discussed.

Effects of Si contents and pre-strain on aging behavior in Al–Mn–Mg alloys containing a small amount of Cu

The aim of the present paper is to monitor the precipitation hardening potentials during aging at 150°C in Al–Mn–Mg alloys containing a small amount of Cu. The age-hardening response and phase transformation are significantly affected by the Si contents of the order of 0.1%. The hardening response increases during aging with increasing Si content. The hardening potential is strongly deteriorated by tensile deformation prior to the heat treatment. The pre-strain causes an inhomogeneous nucleation of precipitation on dislocations and cell walls and restricts the transgranular precipitation during aging. In the 10% pre-strained alloys, a short-term aging leads to a dislocation recovery, resulting in increase in n-value. The further decrease of dislocation density and increase in n-value occur due to a small degree of precipitation hardening in a 0.15% Si added alloy, whereas the dislocation recovery is suppressed by the competitive precipitation in a 0.36% Si added alloy during the prolonged aging.

Solute clustering in Al-Mg-Si-Cu-(Zn) alloys during aging

We report on a study of the role of solute clustering and partitioning in an Al-Mg-Si-Cu alloy containing 1.0 wt % Zn. Our results demonstrate that the addition of 1.0 wt % Zn enhances the hardening response as evidenced by an increase of 65 and 135 MPa following natural aging (T4) and pre-aging (T4P) treatments, respectively. Using results obtained from atom probe tomography we report, for the first time, that co-clusters comprised of Mg, Si, Cu and Zn atoms were formed in the Zn-containing alloy. Moreover, the composition, number density, distribution and clustering kinetics of these co-clusters are all different from those associated with Al-Mg-Si-Cu. During natural aging, the aggregation of Mg and Si atoms in the Al-Mg-Si-Cu alloy can be reduced by the addition of Zn, but increasing the Mg/Si ratios in the co-clusters facilitates the nucleation of precipitates. During pre-aging, a higher Si concentration appears in the co-clusters in the two alloys, especially for the Zn-containing alloy, while Zn concentration in the solute-rich features decreases slightly with increasing their sizes. Our findings demonstrate that Zn additions facilitate the formation of fine co-clusters containing Mg, Si, Cu and Zn atoms, which are believed to be responsible for the enhanced hardening response documented during the T4 and T4P aging treatments. The partitioning of solute atoms and precipitation kinetics of the alloys are discussed in detail.

Influence of Cryorolling on the Precipitation of Cu–Ni–Si Alloys: An In Situ X-ray Diffraction Study

The effect of cryorolling on the precipitation process of deformed Cu–Ni–Si alloys was investigated through in situ synchrotron X-ray diffraction technique. The results demonstrate that the precipitation process is significantly accelerated by cryorolling. Cryorolling produces higher dislocation density, which provides more heterogeneous nucleation sites for Ni2Si precipitates, hence promotes precipitation. In the early stage of aging, the enhanced nucleation of precipitates accelerates the depletion of supersaturation, and finer precipitates are obtained. In addition, recrystallization is promoted as a result of high stored energy in the cryorolled Cu–Ni–Si alloys, which facilitates the formation of discontinuous precipitation in the late stage of aging.

Characterization of the Isothermal Precipitation Kinetics of an Al-Zn-Mg-Cu Alloy

The isothermal precipitation kinetics of an Al-Zn-Mg-Cu alloy was characterized over a wide range of temperatures by means of in situ electrical resistivity measurements, hardness testing, scanning, and transmission electron microscopy. During low temperature isothermal holding, behavior consistent with classical precipitate nucleation, growth, and coarsening was observed. At higher temperatures, the Ostwald coarsening process was characterized and was shown ultimately to result in widely dispersed, incoherent coarse particles. Isothermal temperature–time–precipitate–fraction curves were estimated based on evaluations of in situ electrical resistivity results. Isothermal treatments were observed to influence precipitation during subsequent heating but had little effect on precipitates dissolution.

Mean-field modelling of the intermetallic precipitate phases during heat treatment and additive manufacture of Inconel 718

A multi-phase, multi-component mean-field model has been developed for simulating the intermetallic precipitation kinetics in Inconel 718. The aim of this work is to develop predictive capability to aid in process optimisation and explore precipitation kinetics during additive manufacturing (AM). The model has been calibrated to available experimental data, and then applied to predict precipitation kinetics during typical solid solution treatment and aging operations, and during AM. It is shown that a Computer Coupling of Phase Diagrams and Thermochemistry (CALPHAD) based modelling approach provides a unified particle growth rate which can capture the growth, coarsening and dissolution of γ′, γ* and δ precipitates under relevant heat treatment conditions. To apply the model to AM, finite element simulations of a simple rectangular build have been carried out, using a property switching method to simulate the material deposition. The component level simulation provides the thermal fields to calculate precipitation kinetics during deposition, also allowing for the examination of the heat affected zone in the substrate. The modelling approach can capture the repeated nucleation and dissolution of precipitates that occurs during AM. The model shows good agreement with experimental data when applied to predicting precipitation kinetics during heat treatment.

Ab initio study of energetics and structures of heterophase interfaces: From coherent to semicoherent interfaces

Density functional theory calculations have been performed to study the structures and energetics of coherent and semicoherent TiC/Fe interfaces. A systematic method for determining the interfacial energy of the semicoherent interface with misfit dislocation network has been developed. The obtained interfacial energies are used to evaluate the aspect ratio for the plate-like precipitate and a quantitative agreement with the experimental results is reached. Based on the obtained interfacial energies and atomic structure details, we propose two scenarios for heterogeneous nucleation on an edge dislocation, shedding light on the thermodynamics of precipitate nucleation and growth. The present method can be easily applied to any heterophase interfaces between metals and oxides/carbides/nitrides.

The Microstructure Evolution and Deformation Behavior of AZ80 During Gradient Increment Cyclic Loading

Cyclic loading–unloading uniaxial tension experiments were conducted at temperatures ranging between 293 K and 623 K and a strain rate of 10−3 s−1 to study the cyclic accumulated plastic deformation (CAP) behavior of extruded AZ80. The 673 K/4-h heat treatment to the as-extruded AZ80 led to a noticeable decrease in yield strength which was associated with both dissolution of the β-Mg17Al12 phase and growth of the matrix grain size. The critical number of cycles needed to soften the material (Nc) decreased from 5 to 4 when the cyclic strain amplitude (ea) increased from 3.3 to 5.0 pct for the as-extruded AZ80. The average cyclic hardening rate (Θ) increased from 11 to 23 MPa/cycle after heat treatment, and this was attributed to the more pronounced twinning process in the coarse-grained microstructure. During the 293 K to 473 K CAP deformation, the increasing accumulated cyclic tension strain may have accelerated the propagation of secondary twinning leading to the Luders-like post-yield softening. Twinning was prevalent at low temperature (293 K to 473 K) in the ea = 3.0 pct CAP deformation for the heat-treated alloy, and twin-assisted precipitation occurred during the 523 K CAP deformation, which implied that the high diffusivity in the twin boundary accelerated the heterogeneous nucleation of precipitates. The preferred cracking locations changed from twin boundaries to grain boundaries when the CAP deformation temperature increased from 473 K to 523 K. As for the 623 K CAP deformation, cavities initiated at the grain boundaries, and the volume fraction of the cracks/cavities increased from 0.01 to 0.05 with increasing temperature.

Aging behavior of a copper-bearing high-strength low-carbon steel

The effects of aging temperature and time on the hardness and impact toughness of a copper-bearing high-strength low-carbon steel were investigated. The hardness of the aged samples reached maxima after 1 h and 5 h of aging at 500 and 450°C, respectively; this increase in hardness was followed by a decrease in hardness until a temperature of 700°C, at which secondary hardening was observed. The impact toughness of the aged steel was found to be higher for 5 h of aging. Transmission electron microscopy confirmed the presence of carbide and copper precipitates; also, the secondary hardening could be the result of the transformation of austenite (formed in the aging treatment) to martensite. Differential scanning calorimetry of the steel was performed to better understand the precipitation behavior. The results revealed that the precipitation of the steel exhibited two significant stages of copper precipitate nucleation and coarsening of the precipitates, with corresponding activation energies of 49 and 238 kJ·mol−1, respectively.

On the electrostatic potential assisted nucleation and growth of precipitates in Al-Cu alloy

The effect of electrostatic potential (EP), specifically without current and Joule heating effect, on the precipitation in aluminum alloy during artificial aging (180°C) was investigated via microhardness testing, transmission electron microscopy (TEM), positron annihilation spectroscopy (PAS), and measurements of vacancy formation enthalpy and vacancy migration enthalpy. The results elaborated that EP promoted the nucleation of solute clusters/GP zones in the early stage during aging. After extended aging, highly dense fine precipitates were retained in Al matrix leading to an improved strength. EP-assisted artificial aging provides a new pathway to tailor the precipitation behavior in Al alloys.

Transmission electron microscopy investigation of separated nucleation and in-situ nucleation in AA7050 aluminium alloy

High resolution transmission electron microscopy (HRTEM) with nanometer-scaled energy-dispersive X-ray (EDX) was employed to investigate the transformation mechanisms of the GP zone → η′ → η precipitation sequence of AA7050, an Al-Zn-Mg-Cu alloy. Serial in-situ HRTEM frames revealed that separated nucleation of an η′ precipitate occurred elsewhere as the adjacent GPII zone dissolved. Evidence from HRTEM coupled with EDX showed that in-situ nucleation of a new η2 precipitate (one form of η) took place, wherein it gradually developed from the original η′ precipitate via a similar hexagonal structure with different compositions. The in-situ transition product was composed of two distinctive regions; one was identified as η′, and the other, as η.

Kinetic Simulations of Diffusion-Controlled Phase Transformations in Cu-Based Alloys

In this chapter, we present computational kinetics of diffusion-controlled phase transformations in Cu-based alloys, which becomes possible only most recently due to the establishment of the first atomic mobility database (MOBCU) for copper alloys. This database consists of 29 elements including most common ones for industrial copper alloys. It contains descriptions for both the liquid and Fcc_A1 phases. The database was developed through a hybrid CALPHAD approach based on experiments, first-principles calculations, and empirical rules. We demonstrate that by coupling the created mobility database with the existing compatible thermodynamic database (TCCU), all kinds of diffusivities in both solid and liquid solution phases in Cu-based alloys can be readily calculated. Furthermore, we have applied the combination of MOBCU and TCCU to simulate diffusion-controlled phenomena, such as solidification, nucleation, growth, and coarsening of precipitates by using the kinetic modules (DICTRA and TC-PRISMA) in the Thermo-Calc software package. Many examples of simulations for different alloys are shown and compared with experimental observations. The remarkable agreements between calculation and experimental results suggest that the atomic mobilities for Cu-based alloys have been satisfactorily described. This newly developed mobility database is expected to be continuously improved and extended in future and will provide fundamental kinetic data for computer-aided design of copper base alloys.

Interactive microstructural phenomena during non-isothermal annealing of an Al-Mg-Si-Cu alloy

Interactions between precipitation, recovery and recrystallization during a non-isothermal annealing process in an Al-Mg-Si-Cu alloy have been investigated through a comprehensive experimental analysis. Non-isothermal annealing with a slow heating rate that starts from a low temperature is found to promote homogeneous nucleation of precipitates. The uniformly distributed precipitates limit the extent of recovery, which in turn facilitates extensive nucleation of new grains within the deformed microstructure. The growth of the recrystallized grains is strongly controlled through pinning of grain boundaries by precipitates. During the final stages of annealing, enhanced diffusion at the recrystallization front leads to preferential precipitate coarsening, release of grain boundaries from precipitates pinning, and completion of recrystallization. These interactive microstructural phenomena explain the effectiveness of a non-isothermal annealing route combined with an adequate level of deformation in obtaining a fine-grained recrystallized structure.

Property Enhancement During Artificial Aging of Al6061- Silicon Oxide Metal Matrix Composites

In the current work Al6061- Silicon oxide (SiO2) composites are produced by two stage stir casting method and subsequently studied various properties. Microstructure, hardness, tensile and wear properties of Al6061 based composite having 2, 4 and 6% wt.% of SiO2 is analyzed and presented. The aging behavior and tensile properties of the cast material were studied. Experimental results show homogeneous dispersal of the SiO2 particles in the matrix leading to substantial improvement in the mechanical properties. Presence of SiO2 particulate in aluminium matrix alloy hastening the aging kinetics and thus attains improvement in strength. These particulates offer short circuit path for nucleation of fine precipitates to hinder the movement of dislocation and thus increases hardness as well strength of materials. After aging at 100°C for Al6061/6% wt. SiO2 composite , highest strength is observed at longer duration of aging. Comparison of the characteristics study before and after heat treatment was done and it is revealed that there is an enhancement in hardness, ultimate tensile strength and wear resistance of both Al6061 alloy and its composites after precipitation hardening treatment

Sequestration of Antimony on Calcite Observed by Time-Resolved Nanoscale Imaging.

Antimony, which has damaging effects on the human body and the ecosystem, can be released into soils, ground-, and surface waters either from ore minerals that weather in near surface environments, or due to anthropogenic releases from waste rich in antimony, a component used in batteries, electronics, ammunitions, plastics, and many other industrial applications. Here, we show that dissolved Sb can interact with calcite, a widespread carbonate mineral, through a coupled dissolution-precipitation mechanism. The process is imaged in situ, at room temperature, at the nanometer scale by using an atomic force microscope equipped with a flow-through cell. Time-resolved imaging allowed following the coupled process of calcite dissolution, nucleation of precipitates at the calcite surface and growth of these precipitates. Sb(V) forms a precipitate, whereas Sb(III) needs to be oxidized to Sb(V) before being incorporated in the new phase. Scanning-electron microscopy and Raman spectroscopy allowed identification of the precipitates as two different calcium-antimony phases (Ca2Sb2O7). This coupled dissolution-precipitation process that occurs in a boundary layer at the calcite surface can sequester Sb as a solid phase on calcite, which has environmental implications as it may reduce the mobility of this hazardous compound in soils and groundwaters.

Effects of minor Sc addition on the microstructure and mechanical properties of 7055 Al alloy during aging

This study investigated the effects of 0.25% (mass ratio) Sc addition on the microstructure and mechanical properties of 7055 alloy during aging. Sc addition induced a grain boundary effect, which refined the grains of the 7055 alloy. Sc addition also increased the precipitation nucleation efficiency and promoted the uniform precipitation of the η′ phase of 7055 alloy during aging. Compared with pure 7055 alloy, 7055 alloy with Sc exhibited improved hardness, tensile strength, ductility, and thermal stability. This improvement was attributed to the finer size, higher density, more uniformly distributed η′ phase, less η phase, and finer Al grains of 7055 alloy with Sc compared with those of 7055 alloy without Sc.

Formation mechanism of stable NbC carbide phase in Nb-1Zr-0.1C (wt.%) alloy

In the present work, several long standing issues related to the formation of various carbides and transformation of Nb2C→NbC carbide in Nb-1Zr-0.1C alloy have been addressed. For this purpose, samples were processed by two routes. In the first route, as-solidified samples were extruded and recrystallized and in the second route, as-solidified samples were directly heat treated without imparting deformation. By combining detailed diffraction analyses and ab-initio calculations, identity of the Nb2C carbides in the as-solidified and extruded samples, among the various reported Nb2C crystal structures has been established as an orthorhombic crystal structure (α-Nb2C) having Pnma space group. Chemical analyses showed that the Nb2C carbides in extruded samples had higher Zr content as compared to the carbides formed in as-solidified samples. Experimental and theoretical results revealed that Zr destabilizes Nb2C phase which was shown as a possible reason for the precipitation of a more stable (Nb,Zr)C phase in recrystallized as well as in directly heat treated as-solidified samples. Detailed precession electron diffraction analysis of (Nb,Zr)C particles showed that if the nucleation of precipitates occurs prior to recrystallization and the growth of the precipitates occurs simultaneously with recrystallization, precipitates do not exhibit specific orientation relationship (OR) with the matrix phase. In contrast, if nucleation and growth of precipitates take place after recrystallization or in samples which do not undergo recrystallization, a specific (Baker-Nutting) OR is followed. Finally, by establishing atomic interrelationship by high resolution electron microscopy, the formation mechanisms of (Nb,Zr)C carbides in different morphologies in Nb alloy have been explained.

Tailoring the mechanical properties of sputter deposited nanotwinned nickel-molybdenum-tungsten films

Advanced metallic alloys are attractive in microelectromechanical systems (MEMS) applications that require high density, electrical and thermal conductivity, strength, and dimensional stability. Here we report the mechanical behavior of direct current (DC) magnetron sputter deposited Nickel (Ni)-Molybdenum (Mo)-Tungsten (W) films annealed at various temperatures. The films deposit as single-phase nanotwinned solid solutions and possess ultra-high tensile strengths of approximately 3 GPa, but negligible ductility. Subsequent heat treatments resulted in grain growth and nucleation of Mo-rich precipitates. While films annealed at 600 °C or 800 °C for 1 h still showed brittle behavior, films annealed at 1,000 °C for 1 h were found to exhibit strength greater than 1.2 GPa and near 10% tensile ductility. In addition to the excellent mechanical properties, alloy films further exhibit remarkably improved dimensional stability – a lower coefficient of thermal expansion and greater microstructural stability. An excellent balance between mechanical properties and dimensional stability make sputter deposited Ni-Mo-W alloys promising structural materials for MEMS applications.