Dissolution Of Secondary Phases Research Articles

Mechanical behavior and deformation mechanism of 7075 aluminum alloy under solution induced dynamic strain aging

In this study the mechanical behavior and deformation mechanism of 7075 aluminum alloy sheets under dynamic strain aging induced by solution treatment were investigated. The uniaxial tensile tests were carried out at various strain rates (8.33 × 10−5 s−1, 1.67 × 10−3 s−1, 3.33 × 10−2 s−1 and 1.33 × 10−1 s−1) for samples treated by different solution processes (solution for 30 min at temperatures of 573 K, 623 K, 673 K and 748 K, and then quenching). The Portevin-Le Chatelier (PLC) effect (i.e., serration types of stress-strain curves, strain rate sensitivity (SRS) and critical strain) were analyzed, causing changes in tensile strength, work hardening rate and elongation. The morphology, sizes and quantities of precipitates under different solution-quenching conditions were observed by scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that the dissolution of secondary phase occurs during heat treatment, accompanied by coarsening behavior at lower temperatures, which leads to the transition characteristics of mechanical properties of the aluminum alloy in solid solution state. There exists a critical point of solution temperature (623K), namely, when the solution temperature is lower than this critical point, the microstructure after quenching is mainly composed of the matrix and secondary phase; otherwise it is mainly composed of solid solution. Therefore, the samples heated at different solution temperatures have different types of PLC effects. It is worth noting that the strong effect of dynamic strain aging induced at high solution temperature and low strain rate could simultaneously enhance the strength and toughness.

Dissolution of Ag Precipitates in the Cu⁻8wt.%Ag Alloy Deformed by High Pressure Torsion.

The aim of this work was to study the influence of severe plastic deformation (SPD) on the dissolution of silver particles in Cu–8wt.%Ag alloys. In order to obtain different morphologies of silver particles, samples were annealed at 400, 500 and 600 °C. Subsequently, the material was subjected to high pressure torsion (HPT) at room temperature. By means of scanning and transmission electron microscopy, as well as X-ray diffraction techniques, it was found that during SPD, the dissolution of second phase was strongly affected by the morphology and volume fraction of the precipitates in the initial state. Small, heterogeneous precipitates of irregular shape dissolved more easily than those of large size, round-shaped and uniform composition. It was also found that HPT led to the increase of solubility limit of silver in the copper matrix as the result of dissolution of the second phase. This unusual phase transition is discussed with respect to diffusion activation energy and mixing enthalpy of the alloying elements.

Organic acids and high soil CO2 drive intense chemical weathering of Hawaiian basalts: Insights from reactive transport models

We have investigated how biota contributes to rapid chemical weathering of Hawaiian basalts using a reactive transport model and chemical data from a soil chronosequence. These Hawaiian soils have developed under a tropical forest with rainfall >200 cm/yr and exhibit extensive weathering on timescales of 104 years. We developed a series of multicomponent reactive transport models to examine the role of soil respiration and low molecular weight organic acids in generating these intense weathering patterns. The base model starts with a 1-m basaltic porous media reacting with a fluid of rainwater composition in equilibrium with atmospheric CO2. Subsequent simulations incorporate soil respiration modeled as a constant flux of CO2 at 10× atmospheric and continuous input of organic ligands – oxalate and citrate – at 10−4 molar. After 20 kyr of weathering, the base model shows limited elemental losses, high soil pH and is overall CO2(acid)-limited. Soil respiration lowers soil pH to circumneutral values, leaches all Mg and Ca from the basalt and allows precipitation of Fe(III)-oxyhydroxides, while Al stays immobile as secondary clays accumulate. After adding organic ligands, soil pH is reduced to values similar to the Hawaiian soils and Si, Al and Fe are exported from the system by dissolution of secondary phases, resulting in mass depletion patterns similar to the ones observed in Hawai’i. Dissolution of secondary minerals is generated by low pH and relatively low free activities of Al3+ and Fe3+ when organic ligands are added. These results suggest that organic acids in basalt weathering in tropical environments can sustain far-from-equilibrium conditions that drive fast elemental losses and that biologic activity contributes to weathering processes both by generating high soil PCO2 and organic acids.

Effect of Solution Treatment Temperature on The Microstructure and Mechanical Properties of Al-5.1Zn-1.9Mg Alloy Produced by Squeeze Casting

Aluminum alloys are widely used in aviation industries, especially for the body and wings of aircraft. The heat treatment for 7xxx series aluminum alloys is precipitation hardening which consists of solution treatment, quenching, and ageing. One key for successful ageing process is the amount of solute elements and vacancies dissolve in the matrix during solution treatment. Therefore, this research is aimed to study the effects of solution treatment temperature on the hardness and microstructure of Al-5.1Zn-1.9Mg (wt. %) alloy which produced by squeeze casting. Solution treatment temperatures were varied to 220, 420, and 490 °C for 60 minutes, followed by quenching. The samples were then stored at -10 °C to prevent natural ageing. Characterization included hardness testing and microstructural observation by using OM and SEM-EDS, XRD, and STA. Ageing was conducted at 130 °C for 48 h followed by hardness testing and microstructural observation.The results showed that increasing solution treatment temperature induced enhancement of second phase dissolution and the amount of trapped vacancies in the matrix. However dissolution of second phase was hardly detected at solution treatment temperature of 220 °C. It was shown by the volume fraction of the second phase found after homogenizing was 7.13 % and decreased to 7.06, 4.80, and 4.19 % after solution treatment at temperatures 220, 420, and 490 °C respectively. Therefore the increase in hardness after ageing at 130 °C for 48 hours was 11.54, 42,1, and 66.7 HRB for solution treatment temperatures of 220, 420, and 490 °C respectively. This can be concluded that inceasing solution treatment temperature enhanced the hardness of alloy after ageing.

Influence of FSP on the microstructure, microhardness, intergranular corrosion susceptibility and wear resistance of AA5083 alloy

ABSTRACTThis article investigates the role of friction stir processing (FSP) process parameters on the evolution of microstructure, hardness, intergranular corrosion resistance and wear resistance of aluminium alloy AA5083. The FSP trials are performed by changing the process parameters as per face-centered central composite design. The friction stir processed (FSPed) specimens subjected to intergranular corrosion test and wear test are characterized using field emission scanning electron microscope, energy dispersive x-ray spectroscopy and X-ray diffraction. Outcomes suggest that grain refinement, dispersion and partial dissolution of secondary phase has simultaneously increased the hardness, intergranular corrosion resistance and wear resistance of the FSPed specimens. The study found that tool rotation speed of 700 rpm, tool traverse speed of 60 mm min−1 and shoulder diameter of 15 mm results in maximum hardness, wear resistance and intergranular corrosion resistance.

Absence of Nanostructuring in NaPb mSbTe m+2: Solid Solutions with High Thermoelectric Performance in the Intermediate Temperature Regime.

Thermoelectric devices directly convert heat into electrical energy and are highly desired for emerging applications in waste heat recovery. Currently, PbTe based compounds are the leading thermoelectric materials in the intermediate temperature regime (∼800 K); however, integration into commercial devices has been limited. This is largely because the performance of PbTe, which is maximized ∼900 K, is too low over the temperatures of interest for most potential commercial applications (generally under 600 K). Improving the low temperature performance of PbTe based materials is therefore critical to achieve usage outside of existing niche applications. Here, we provide an in-depth study of the cubic NaPb mSbTe m+2 system of compounds ( m = 1-20) and report that it is an excellent class of low- to medium-temperature thermoelectrics when m = 10-20. We show that the as-cast polycrystalline ingots exhibit degenerate p-type conduction and high maximum ZTs of 1.2-1.4 at 650 K when m = 6-20. Because the ingots are found to be extremely brittle, we utilize spark plasma sintering (SPS) to prepare more mechanically robust samples, and surprisingly, find that SPS results in an undesired change in charge transport toward n-type behavior. We show this unanticipated transition from p-type behavior as ingots to n-type after SPS is due to dissolution of secondary phases that are present in the ingots into the primary matrix during the SPS process, resulting in a transformation from an inhomogeneous state to a solid solution without any observable evidence of nanoscale precipitation. This is in sharp contrast to the seemingly similar AgPbmSbTe m+2 (LAST) system, which is heavily nanostructured. The SPSed NaPb mSbTe m+2 is doped p-type by tuning the cation stoichiometry, i.e., Na1+ xPb m- xSb1- yTe m+2. The optimized compounds have very low lattice thermal conductivities of 1.1-0.55 W·m-1·K-1 over 300-650 K, which enhances the low-intermediate temperature performance and gives rise to maximum ZT values up to 1.6 at 673 K as well as an excellent ZTavg of 1.1 over 323-673 K for m = 10, 20, making Na1+ xPb m- xSb1- yTe m+2 among the highest performing PbTe-based thermoelectrics under 650 K.

Laser surface modification of Mg-Gd-Ca alloy for corrosion resistance and biocompatibility enhancement

Magnesium alloy has attracted much attention as biodegradable material for medical devices including implants and stents in recent years. However, poor corrosion resistance is the main problem to limit practical applications because high degradation rate will cause fast increase in pH, while the large release of ions into the medium can lead to cytotoxicity. In this work, we demonstrate the capability of laser surface modification for better corrosion resistance of Mg-Gd-Ca alloy to avoid rapid degradation, which helps to enhance the biocompatibility. Microstructure of both as-received surface and laser-modified surface were analyzed carefully by Scanning Electron Microscopy and X-ray Diffraction. Electrochemical corrosion behavior and immersion test was investigated in order to examine the degradable behavior of laser-modified Mg-Gd-Ca alloy. The enhanced corrosion resistance was found to be caused by the combined effect of the dissolution of secondary phase and the decrease of galvanic corrosion in the laser-modified zone. Moreover, results of direct cell culturing suggested that the laser-modified surface exhibited good cell adhesion property, spreading performance and proliferation capacity.

Analysing the influence of FSP process parameters on IGC susceptibility of AA5083 using Sugeno – Fuzzy model

Aluminium alloy AA5083 was friction stir processed to improve the intergranular corrosion (IGC) resistance. FSP trials were performed by varying the process parameters as per Taguchi’s L18 orthogonal array. IGC resistance of the friction stir processed specimens were found by immersing them in concentrated nitric acid and measuring the mass loss per unit area. Results indicate that dispersion and partial dissolution of secondary phase increased IGC resistance of the friction stir processed specimens. A Sugeno fuzzy model was developed to study the effect of FSP process parameters on the IGC susceptibility of friction stir processed specimens. Tool Rotation Speed, Tool Traverse Speed and Shoulder Diameter have a significant effect on the IGC susceptibility of the friction stir processed specimens.

Microstructure and tensile properties of rolled Mg-4Al-2Sn-1Zn alloy with pre-rolling deformation

The coarse segregative eutectic phases, emerged in the twin roll casting (TRC) process of Mg strips, which cannot be resolved solely by controlling the processing variables or post heat treatment, are detrimental to the subsequent rolling deformation. Considering it, we introduce the pre-rolling steps on the TRC Mg-4Al-2Sn-1Zn (ATZ421) alloys, which is expected to facilitate the dissolution of these segregative phases by increasing distortion of crystal lattices and rolling stress, and further investigate the effect of a single pass or two passes pre-rolling on the microstructure evolution and tensile properties. It indicates that the pre-rolled alloy with two passes presents more uniform precipitates and higher twinning density in comparison with that using a single pass. The uniform distribution of precipitates in two passes pre-rolling is due to the increase in density of dislocations and deformed twins, which results in more distortion of crystal lattices with more stored energy, facilitating the diffusion of solutes. Moreover, the pre-rolling of two passes introduces more rolling stress into Mg matrix, which is beneficial for the crush of coarse eutectic phases, and further accelerates the sufficient dissolution of second phases. After the subsequent rolling and annealing processes, the sheet with pre-rolling deformation of two passes shows finer microstructure, which mainly benefits from the pinning effects of uniform nano-sized precipitates. As a result, the annealed sheet with two-pass pre-rolling deformation exhibits an advantageous combination of high strength and high ductility with fine microstructure, showing an ultimate tensile strength of ~297 MPa, a yield strength of ~ 205 MPa and an elongation of 25.0%. This work provides an effective method of dissolving the coarse segregative phases in the TRC Mg alloys with significant potential for industrial production.

Modeling homogenization behavior of Al-Si-Cu-Mg aluminum alloy

The effect of the as-cast secondary dendrite arm spacing and homogenization temperature on the secondary phase dissolution and solute redistribution during homogenization of cast Al-Si-Cu-Mg aluminum alloy was investigated. Particularly, the concurrent dissolution of θ-Al2Cu and Q-Al5Cu2Mg8Si6 phases was studied for the first time. Two modeling approaches were developed using finite-difference analysis to predict the dissolution kinetics; (i) a stationary interface modeling approach in which the interface is considered to be stationary and (ii) a moving boundary modeling approach in which the particle-matrix interface is moving. The dissolving particle shape was considered to be spherical and the dissolution process to be diffusion controlled. The two model predictions were compared against the experimental data obtained from microstructural characterizations. It was shown that although the stationary interface approach provides reasonable results in predicting the dissolution behavior at higher volume fractions of the secondary phase, the results deviate from the experimental measurements at lower volume fractions. On the other hand, the moving boundary approach was capable of predicting the final dissolution time of the secondary phase. It was also shown that the Q-phase does not dissolve completely as opposed to the θ-phase.

A comprehensive evaluation between the efficiency of different treatments in modifying the properties and behavior of magnesium alloys as degradable biomaterials

The addition of alloying elements to magnesium leads to microstructural refinement and improves its properties. However, this strategy is accompanied by another concurrent phenomenon − that is − intergranular segregation and the formation of secondary phases in grain boundaries, deteriorating the properties. In this work, the efficacy of two main factors on modifying the mechanical properties and corrosion behavior of Mg‐4Zn‐1Al‐0.2Ca alloy was investigated separately: 1) dissolution of secondary phases; and 2) grain refinement. Based on the results, heat treatment of the as‐cast alloy can increase the UTS from 174.4 to 213.2 MPa, decrease the corrosion current density from 81 to 49 μA/cm2, and enhance the polarization resistance from 1410 to 1780 Ω cm2. While, the UTS of the as‐rolled sample increased to 355 MPa, its corrosion current density decreased to 11 μA/cm2, and its polarization resistance improved to 2078 Ω cm2, indicating that compared to dissolution heat treatment, microstructural refinement plays a considerably more effective role in modifying the alloy's behavior. Cytotoxicity and cell adhesion studies also implied on the biosafety of this material in vitro.

Transformation and dissolution of second phases during solution treatment of an Al–Zn–Mg–Cu alloy containing high zinc

The transformation and dissolution of Mg(Zn, Cu, Al)2 phase during solution treatment of an Al–Zn–Mg–Cu alloy containing high zinc were investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDX) and X-ray diffraction (XRD). The results show that solution temperature is the main factor influencing phase dissolution. With solution temperature increasing, the content of residual phases decreases. Phase transformation from Mg(Zn, Cu, Al)2 to S(Al2CuMg) occurs under solution temperature of 450, 460 and 465 °C. Mg(Zn, Cu, Al)2 phase is directly dissolved into the matrix under solution temperature of 470 and 475 °C, and no S(Al2CuMg) phase transformed from Mg(Zn, Cu, Al)2 phase is observed. The formation of S(Al2CuMg) phase is mainly controlled by Zn elemental diffusion. The mechanism of transformation and dissolution of second phases was investigated. At low temperature, the dissolution of Zn is faster than that of Mg and Cu, resulting in an appropriate condition to form S(Al2CuMg) phase. At high temperature, the dissolution of main alloying elements has no significant barrier among them to form S(Al2CuMg) phase.

The solution treatment of in-situ sub-micron TiB2/2024 Al composite

In this work, we studied the solution treatment of in-situ TiB2/2024 Al composite. In the composite, TiB2 particles have ranged from 30 to 500nm, and the second phases (SPs) of θ and S were identified to coexist with TiB2. Furthermore, mechanical properties subjected to different solution temperatures (STs) for T4 state were investigated. It was found that the increase of ST can effectively improve mechanical properties of composite. The optimum ST for the composite was 505°C, which was higher than ST (500°C) typically used in 2024 Al alloy. To understand this phenomenon, a modified diffusion-controlled dissolution model for the composite has been proposed. In the classical model for the alloy, the dissolution process should depend on the initial radius of SP, solution time and ST. However, the TiB2 aggregates around SPs should act as particle shells to reduce the effective contact area between the alloy matrix and SPs. This covering effect has hindered the dissolution of SPs in the composite. In consideration of this effect, a modified diffusion-controlled dissolution model was proposed for the composite. The increase of the ST that was more approaching to the eutectic temperature was a required strategy to enhance the SP dissolution.

In situ characterisation of nanostructured multiphase thermoelectric materials at elevated temperatures.

Multiphase thermoelectric materials have recently attracted considerable attention due to the high thermoelectric efficiencies which can be achieved in these compounds compared to their single-phase counterparts. However, there is very little known on the structural evolution of these phases as a function of temperature. In this work we performed an in situ high temperature structural characterisation of recently reported high efficiency p-type multiphase (PbTe)0.65(PbS)0.25(PbSe)0.1 compounds by hot stage transmission electron microscopy and high-resolution neutron powder diffraction. We observed the microstructural evolution of precipitates and determined the lattice parameters of phases as a function of temperature for materials, which have been heavily and lightly doped with sodium. The role of the sodium is to optimize the concentration of charge carriers. It has been shown to distribute heterogeneously between the phases in multiphase compounds. The dissolution of secondary phases is found to occur at elevated temperatures. Although sodium concentration produces no significant differences between the lattice constants of the phases and the dissolution sequence of precipitates, it affects quite significantly the kinetics of precipitation. The heavily doped samples reach structural thermodynamic equilibrium more quickly than the lightly doped compound. These results are a step forward in designing high performance multiphase thermoelectric materials.

Homogenizing annealing heat treatment effects on the microstructure, hardness and thermal behavior of Al12Mg17 complex metallic alloy

Abstract Microstructural evolutions, hardness and thermal behavior of Al 12 Mg 17 complex metallic alloy (CMA) were studied before and after homogenizing annealing. This heat treatment was conducted on the as-cast alloy at high temperature of 400 °C in different times in order to clarify the mechanism and also determining the optimum annealing time for achieving a single phase intermetallic compound. The as-cast microstructures involve γ-Al 12 Mg 17 and small amount of Mg-rich microsegregated phase which forms during eutectic solidification. Phase and microstructural evolutions during homogenization were comprehensively investigated by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray diffraction patterns (XRD). The thermal behavior and hardness of the as-cast and homogenized intermetallics have been investigated by differential scanning calorimetry (DSC) and Vickers microhardness measurements, respectively. The results of these investigations indicated that homogenization treatment resulted in a diffusion of Mg from Mg-rich domains in eutectic structure to the intermetallic matrix, leading to evolution and dissolution of second phase along with the formation of Kirkendall porosities according to proposed mechanism. Finally a single phase of Al 12 Mg 17 CMA with higher lattice parameter, hardness and melting point formed after 32 h of homogenizing annealing heat treatment.

On the development of a new pre-weld thermal treatment procedure for preventing heat-affected zone (HAZ) liquation cracking in nickel-base IN 738 superalloy

Hot cracking in the heat-affected zone (HAZ) of precipitation strengthened nickel-base superalloys, such as IN 738, during fusion welding remains a major factor limiting reparability of nickel-base gas turbine components. The problem of HAZ intergranular cracking can be addressed by modifying the microstructure of the pre-weld material through thermal treatment, which requires significant understanding of the critical factors controlling cracking behaviour. The decomposition of Mo-Cr-W-and Cr-rich borides in the alloy, among other factors, has been observed to contribute significantly to non-equilibrium intergranular liquation and, hence, intergranular liquation cracking during welding. Gleeble physical simulation of HAZ microstructure has also shown that non-equilibrium liquation is more severe in the vicinity of decomposed borides in the alloy and can occur at temperatures as low as 1,150 °C. Although currently existing pre-weld heat treatments for IN 738 superalloy minimize the contributions of dissolution of second phases, including borides, to HAZ intergranular liquation, these heat treatments are not industrially feasible due to process-related difficulties. Therefore, a new industrially feasible and effective pre-weld thermal treatment process, designated as FUMT, was developed during the present research by controlling both the formation of borides and the segregation of boron at the grain boundaries in the pre-weld heat-treated material. This thermal treatment was observed to very significantly reduce intergranular HAZ cracking in welded IN 738 superalloy. The details of the development process and developed procedure are presented in this paper.

In situ neutron diffraction analysis of stress-free d-spacing during solution heat treatment of modified 319 Al alloy engine blocks

Aluminium alloy engine blocks have successfully replaced ferrous materials in order to maximise weight savings and improve vehicle fuel efficiency. However, the development of an optimal heat treatment process is required to improve engine block casting integrity and prevent potential problems such as in-service cylinder distortion. Optimisation of heat treatment parameters requires an in-depth study to determine how residual stresses are relieved with time during solution heat treatment. In order to perform this analysis, however, in situ neutron diffraction must first be carried out on stress-free samples of the same composition and processing history as the engine blocks to account for factors such as thermal expansion and changes in lattice parameter due to dissolution of secondary phases. The results from this study suggest that thermal expansion caused the largest change in d0 spacing, while prolonged exposure at the solutionising temperature resulted in relatively small changes in {311} and {331} d0 spacing due to phase dissolution.

Synthesis and characterization of Ni-implanted and 200-MeV Ag ion-irradiated CeO2 thin films

CeO2 thin films were deposited on quartz substrates by using the rf-sputtering technique. The 80-keV Ni- ion-implanted and, subsequently, post-annealed films have shown the formation of Ni oxide and Ni metallic phases at 7at% of Ni concentration. Such secondary phases were dissolved by swift heavy ion irradiation with 200-MeV Ag+15 ion beams. Structural properties, surface roughness, and magnetic behavior of the samples were investigated by X-ray diffraction, atomic force microscopy, and hysteresis loop measurements, respectively. Dissolution of secondary phases has been discussed in the light of irradiation-induced local temperature rise and energy loss processes.

An analytical model for secondary phase dissolution kinetics

An analytical model for dissolution kinetics of secondary phase particles upon isothermal annealing has been proposed. Considering the interactions of solute diffusion fields in front of the secondary phase/matrix interface upon dissolution, a Johnson–Mehl–Avrami type equation, subjected to necessary modification, was derived, in combination with a classic dissolution model for single-particle system. Compared with the semiempirical dissolution models, which are used to fit the experimental results and phase-field method simulation, the current model follows an analogous form, but with the time-dependent kinetic parameters. Distinct from the model fitting work published recently, the current model is derived from the diffusion-controlled transformation theory, while the modeling quality is guaranteed by the physically realistic model parameters. On this basis, the current model calculation leads to a clear relationship between the secondary phase volume fraction and the time. Accordingly, model predictions for isothermal θ′ dissolution in Al–3.0wt%–Cu alloy and silicon dissolution in Al–0.8wt%–Si alloy were performed; good agreement with the published experimental data has been achieved.

Dissolution kinetics and morphological changes ofγ′ in AD730TMsuperalloy

Alloy AD730TM is a recently developed Nickel base superalloy for application as turbine disk in modern gas turbines with improved thermal efficiency. Ingot casting followed by open die-forging and then heat treatments are the main manufacturing steps for the production of parts made of this alloy. Solution heat treatment operations are applied at different stages of the manufacturing in order to ease the deformation processing and/or prepare the microstructure for final heat treatment. In this research, the influence of various solution heat treatment schedules on morphology and distribution of the γ′ phase are investigated and documented. The obtained results will contribute to a better understanding of microstructure evolution of AD730TM during solution heat treatments. Differential Thermal Analysis (DTA) is used for the purposes of measurement of temperatures of phase transformations of the alloy. Based on DTA results, three solutionizing temperatures and three holding times were selected for performing and assessing the solution heat treatment process. Optical and electron microscopy were used to study the morphological evolution as well as the coarsening and dissolution of secondary phases at solvus and subsolvus temperatures. The results indicated that precipitate agglomeration and Ostwald ripening are the governing mechanisms during the initial stages and splitting and partial dissolution of γ′ precipitates takes place during subsolvus solution treatments.