Stability Of Precipitated Phases Research Articles

Carbonate Coprecipitation for Cd and Zn Treatment and Evaluation of Heavy Metal Stability Under Acidic Conditions.

Mining wastes or combustion ash are materials of high carbon sequestration potential but are also known for their toxicity in terms of heavy metal content. To utilize such waste materials for engineered carbon mineralization purposes, there is a need to investigate the fate and mobility of toxic metals. This is a study of the coprecipitation of metals with calcium carbonate for environmental heavy metal mitigation. The study also examines the stability of precipitated phases under environmentally relevant acid conditions. For a wide range of cadmium (Cd) and zinc (Zn) concentrations (10 to 5000 mg/L), induced coprecipitation led to greater than 99% uptake from water. The calcium carbonate phases were found to contain amounts as high as 9.9 wt % (Cd) and 17 wt % (Zn), as determined by novel synchrotron techniques, including X-ray fluorescence element mapping and three-dimensional (3D) nanotransmission X-ray microscopy (TXM). TXM imaging revealed first-of-a-kind observations of chemical gradients and internal nanoporosity within particles. These observations provided new insights into the mechanisms leading to the retention of coprecipitated heavy metals during the dissolution of calcite in acidic (pH 4) solutions. These observations highlight the feasibility of utilizing carbonate coprecipitation as an engineered approach to the durable sequestration of toxic metals.

Long-term stability of precipitated phases in CLAM steel during thermal aging

China Low Activation Martensitic (CLAM) steel as one of 9%Cr ferritic-martensitic steels was designed as the structural materials for fusion reactors and lead-based Reactor in China. The long-term thermal stability of precipitated phases in CLAM steel at 550 °C was systematically studied in this paper, including coarsening kinetics, number density and shape stability. Based on large data statistics, the coarsening kinetics of M23C6 carbide, MX carbonitride and Laves phase were analyzed to develop a coarsening model of each precipitated phase, respectively. The results showed that the coarsening rate of Laves phase was about 100 times than that of M23C6 carbide after aging for 20,000 h at 550 °C. The number density of precipitated phases increased firstly and then decreased with increasing of aging time due to a dynamic equilibrium process of the precipitation and dissolution of precipitated phase. The results will help to deep understand degradation mechanisms after thermal aging.

On the formation and stability of precipitate phases in a near lamellar γ-TiAl based alloy during creep

The formation, evolution and stability of metastable phases observed in the γ-TiAl based alloy Ti-47Al-2Cr-2Nb was studied under creep deformation with stress applied at two different hard orientations in a highly textured as-cast + HIPed material. Previously we have reported that the metastable phase Ti(Al,Cr)2 with C14 Laves phase structure forms at the γ-α2 interface which acts as sink for the alloying elements ejected from the dissolving α2 phase and also expected to effectively control the interface stresses through short range diffusion and modifications in the chemical composition [1]. Ab initio density functional theory based calculations were carried out to evaluate the effect of choice of lattice position and site occupancy of aluminium atoms in the Ti(Al,Cr)2 structure on the lattice parameter variation and thermodynamic stability. C14 with the composition 25 at. % Al was found to have lattice parameter values close to the inter-planar spacing of <110>γ and <10-10>α2 which would have a lower misfit with C14 across the interface. From the cohesive energy calculations, Laves phase C14 with a constrained lattice parameter due to the adjoining phases, exhibits higher stability than the B2 and L10 structures across a range of compositions studied. Electron diffraction simulations of C14 with a composition of 25% Al compared with the experimental data suggest that the structure C14 has taken up either a random site occupancy compared to a specific choice of ordering to minimize the interfacial stress. Though the experimental evidences do not strongly support a long-range ordering theory in C14, short-range ordering could be a tangible choice for alleviating interface misfits. The ability of C14 to assume different lattice parameters at and far from the α2-γ interface also suggest that the C14 acts as buffer layer between α2 and γ phases in the presence of local stresses, although this is not the thermodynamically expected phase at the temperature of creep experiment.

Thermochemical Analysis of Phases Formed at the Interface of a Mg alloy-Ni-plated Steel Joint during Laser Brazing

The thermodynamic stability of precipitated phases at the steel-Ni-Mg alloy interface during laser brazing of Ni-plated steel to AZ31B magnesium sheet using AZ92 magnesium alloy filler wire has been evaluated using FactSage thermochemical software. Assuming local chemical equilibrium at the interface, the chemical activity–temperature–composition relationships of intermetallic compounds that might form in the steel-Ni interlayer-AZ92 magnesium alloy system in the temperature range of 873 K to 1373 K (600 °C to 1100 °C) were estimated using the Equilib module of FactSage. The results provided better understanding of the phases that might form at the interface of the dissimilar metal joints during the laser brazing process. The addition of a Ni interlayer between the steel and the Mg brazing alloy was predicted to result in the formation of the AlNi, Mg2Ni, and Al3Ni2 intermetallic compounds at the interface, depending on the local maximum temperature. This was confirmed experimentally by laser brazing of Ni electro-plated steel to AZ31B-H24 magnesium alloy using AZ92 magnesium alloy filler wire. As predicted, the formation of just AlNi and Mg2Ni from a monotectic and eutectic reaction, respectively, was observed near the interface.

A Brief Introduction to Alloy Phase Chemistry Determination under EPMA/SEM-EDS Conditions and its Applications

A so called multiphase separation method (MPSM) is proposed to quantitatively separate precipitated phases from their surrounding matrix phase in chemistry for bulk alloy/steel samples under EPMA/SEM-EDS measurement conditions. Applied examples to comparisons of the results through the MPSM with the values either cited or obtained via other analytical means relevant are indicative of the feasibility, accuracy as well as applicability of the MPSM, which deal with chemistry, amount, lattice parameter, elemental partitioning, atomic-site occupancy and stability of precipitated phases of either superalloy or heat-resistant steel samples analyzed. Successful applications of the MPSM not only show a significant improvement for difficulties in accurate quantification in phase chemistry under the EPMA/SEM-EDS measurement conditions but also provide with a useful and helpful tool to determine some other important physical quantities in alloys and steels, which make it possible to quantitatively and more widely evaluate structure-property relationships of the materials investigated through analyzing their bulk samples under EPMA/SEM-EDS measurement conditions.

Precipitation processes in a Mg–Zn–Sn alloy studied by TEM and SAXS

The microstructural evolution of a Mg–Zn–Sn alloy was studied by combining X-ray diffraction (XRD), small angle X-ray scattering (SAXS), and transmission electron microscopy (TEM) in order to determine the individual phases, their size and volume fraction of the alloy. Solutionized and aged samples are analysed in detail concerning the nucleation, growth, morphology, and stability of precipitate phases. In the aged samples, firstly MgZn2 particles with a rod-like shape form, and secondly plate-like MgSn2 precipitates. The MgZn2 phase shows a well-defined orientation relationship with the Mg matrix. The formation of two types of precipitates is responsible for the occurrence of two pronounced hardness maxima. The growth behaviour of the MgZn2 phase is determined by combining TEM and SAXS measurements and the results are compared to the Lifschitz–Slyozov–Wagner (LSW) theory.

Fe–15Ni–13Cr austenitic stainless steels for fission and fusion reactor applications. I. Effects of minor alloying elements on precipitate phases in melt products and implication in alloy fabrication

In an effort to develop alloys for fission and fusion reactor applications, 28Fe–15Ni–13Cr base alloys were fabricated by adding various combinations of the minor alloying elements, Mo, Ti, C, Si, P, Nb, and B. The results showed that a significant fraction of undesirable residual oxygen was removed as oxides when Ti, C, and Si were added. Accordingly, the concentrations of the latter three essential alloying elements were reduced also. Among these elements, Ti was the strongest oxide former, but the largest oxygen removal (over 80%) was observed when carbon was added alone without Ti, since gaseous CO boiled off during melting. This paper recommends an alloy melting procedure to mitigate solute losses while reducing the undesirable residual oxygen. In this work, 14 different types of precipitate phases were identified. Compositions of precipitate phases and their crystallographic data are documented. Finally, stability of precipitate phases was examined in view of Gibbs free energy of formation.

Fe–15Ni–13Cr austenitic stainless steels for fission and fusion reactor applications. III. Phase stability during heavy ion irradiation

The phase stability in Fe–15Ni–13Cr alloys was investigated as a function of minor alloying additions after 4 MeV Ni ion irradiation at 948 K. The results showed that the stability of precipitate phases was dictated mainly by the defects produced by radiation damage and preferential segregation of Si and Ni at defects. In addition, radiation enhanced diffusion and cascade induced dissolution and mixing allowed kinetically sluggish phases to form rapidly under irradiation. These radiation effects caused an enhancement, retardation, or modification of thermal phases, and formation of new phases. The relative stability of precipitate phases varied sensitively with alloy composition. The roles of each alloying element on phase stability and the impact of radiation on the mechanisms of phase evolution were systematically studied and documented. The knowledge obtained from this work provides guidelines for designing alloys that lead to develop desired precipitate microstructures under irradiation.

Thermodynamic modelling of precious metals alloys

The alloy system Au−Ag−Pd−Pt−Sn is described with the help of the thermodynamic software ‘Chemsage’ and a specially modelled dataset. Calculations on the stability of precipitated phases show good agreement with experimental results from hardness tests and metallography and thereby provide a better understanding of the observed material properties. The use of such modelling provides an efficient tool for faster and more direct development of precious metal alloys.

Thermodynamische Modellierung der Ausscheidungshärtung in Edelmetall-Legierungssystemen

The quaternary system Au-Pt-Pd-Sn is described with the help of the thermodynamic software Chemsage and a specially modelled dataset. Calculations on the stability of precipitated phases show good agreement with experimental results from hardness tests and metallography and thereby provide a better understanding of the observed material properties. The use of such modelling provides an efficient tool for faster and directer development of precious metal alloys.

Microstructural stability in a Cu-Cr-Si alloy under 300 keV Cu + ion irradiation

The stability of precipitate phases in a Cu-base alloy under irradiation with 300 KeV Cu + ions has been investigated. The irradiations were carried out in the temperature range of 200°C to 530°C at displacement rates of 2.3 × 10 −2 and 2.3 × 10 −5 dpa/s, respectively. Of the two types of precipitates which form in this alloy under thermal ageing, the metastable. coherent precipitates of fcc chromium either do not form or dissolve, if already present, under most irradiation conditions. The second population of precipitates, i.e. incoherent particles of the stable phase Cr 3Si, grow to a temperature dependent stable size under all irradiation conditions. The results are interpreted in terms of mechanisms of cascade mixing and radiation enhanced diffusional growth.

The physical state of ion implanted solids

The destruction of the crystalline lattice during ion implantation is discussed in general terms together with its post-irradiation annealing. It is shown that the creation of mobile defects during implantation can give rise to an irradiation enhanced diffusion which can in some cases dominate thermally activated diffusion. The expected state of implanted impurity atoms is discussed in some detail with special attention to the formation and stability of precipitate phases in an irradiation environment.