Decomposition Of Supersaturated Solid Solution Research Articles

Study of supersaturated solid solution decomposition during quenching of sheets from Al-Mg-Si aluminum alloy

This paper presents the results of a study of the stability of a supersaturated solid solution (SSS) of sheets made of thermally hardened aluminum alloy of the Al-Mg-Si system with a small addition of copper (Al-0.6Mg-1.0Si-0.2Cu) under various quenching modes. The samples were subjected to isothermal or continuous quenching with different quenching cooling rates, after which artificial aging was carried out at a temperature of 170°C. From the results of thermodynamic modeling of the equilibrium phase composition of the alloy, it was found that for the temperature range from 300 to 530°C, the presence of the β-phase (Mg2Si) is most likely. With the use of transmission electron microscopy and X-ray spectral microanalysis, it was found that during quenching, the decomposition of SSS leads to the precipitation of undesirable large particles of metastable β-type phases or equilibrium β-phase. The nucleation of the secretions is realized in the form of rod-shaped particles by a heterogeneous mechanism mainly on the surface of the α-phase dispersoids (Al15(Mn,Fe)3Si2), which thus significantly increase the quenching sensitivity of the alloy. The formation of these secretions at a low quenching rate causes, during subsequent aging, a decrease in the proportion and density of formation of strengthening particles of the β" phase, and also leads to an increase in their size and heterogeneity of distribution in the aluminum matrix, which reduces the potential of dispersion hardening during aging and corrosion resistance of the material.

Enhancement in dispersoid precipitation and dispersion strengthening by prior deformation in an Al–Mg–Mn alloy

Dispersion strengthening effect of Mn-containing dispersoids in the non-age hardenable Al–Mg–Mn alloys is generally limited since it is difficult to achieve the high-density and fine sized dispersoids through conventional homogenization treatment. In this work, inspired by the heterogeneous precipitation along dislocation for Mn-containing dispersoids, strain induced dislocations was introduced by prior deformation to increase the nucleation sites in the matrix. Based on the classical nucleation theory, the nucleation rate in the pre-strained specimen is improved by an order of magnitude compared with that of the as-cast specimen. Detailed electrical conductivity tests and transmission electron microscopy observations showed that prior deformation can promote the decomposition of supersaturated solid solution and successfully increase the number density of Mn-containing dispersoids, but decrease their sizes from 323 ± 83 nm to 118 ± 43 nm in long axis dimension after homogenization treatment. An improvement of 36.5 MPa in yield strength can be achieved through the enhanced dispersion strengthening mechanism. Simultaneously, the diffusion distance of Mn solute atoms dramatically increases from 0.85 μm to 27 μm due to the enhancement in diffusivity, which can be mostly attributed to the increase in diffusion paths caused by the higher density of strain-induced dislocations. Thus, the volume fraction of dispersoid free zones was decreased by 51% in the pre-strained specimen, indicating that a more uniform distribution of dispersoids in the Al matrix can be obtained by prior deformation. This work provides some insight into the enhanced dispersion strengthening effect of Mn-containing dispersoids by prior deformation, which is a promising approach to further improve mechanical properties of non-age hardenable Al alloy.

Freezing solute atoms in nanograined aluminum alloys via high-density vacancies

Low-temperature decomposition of supersaturated solid solution into unfavorable intergranular precipitates is a long-standing bottleneck limiting the practical applications of nanograined aluminum alloys that are prepared by severe plastic deformation. Minimizing the vacancy concentration is generally regarded as an effective approach in suppressing the decomposition process. Here we report a counterintuitive strategy to stabilize supersaturated solid solution in nanograined Al-Cu alloys via high-density vacancies in combination with Sc microalloying. By generating a two orders of magnitude higher concentration of vacancies bonded in strong (Cu, Sc, vacancy)-rich atomic complexes, a high thermal stability is achieved in an Al-Cu-Sc alloy that precipitation is nearly suppressed up to ~230 °C. The solute-vacancy complexes also enable the nanograined Al-Cu alloys with higher strength, greater strain hardening capability and ductility. These findings provide perspectives towards the great potentials of solute-vacancy interaction and the development of nanograined alloys with high stability and well-performed mechanical properties.

Исследование распада пересыщенного твердого раствора в высокомагниевых алюминиевых сплавах со скандием, легированных гафнием

Magnesium-rich aluminum alloys with small scandium additives are widely used in many branches of modern industry due to the high level of their mechanical properties. However, the issue of low thermal stability of Al3Sc particles, which does not allow performing deformation processing of this group of alloys at a temperature above 400 °С, continues to be relevant. Hafnium addition can become one of the ways to solve this problem as hafnium forms a shell around the Al3Sc particles and, due to the low diffusion coefficient in the aluminum matrix, reduces their coagulation rate. The paper studies the influence of addition of 0.2 % and 0.5 % Hf on the electrical conductivity and the process of supersaturated solid solution decomposition, as well as on the size and quantity of nanoparticles in the 1570 magnesium-rich aluminum alloy at its thermal treatment. The authors studied the kinetics of supersaturated solid solution decomposition in the 1570, 1570–0.2Hf, and 1570–0.5Hf alloys by the electrical conductivity measuring and constructed C-curves describing the supersaturated solid solution decomposition in the studied alloys in the temperature range of 260–440 °С. Besides, using transmission electron microscopy, the strengthening nanoparticles of the 1570 and 1570–0.5Hf alloys were studied during heating to 370 °C and 4-hour soaking. The study showed that hafnium addition significantly slows down the supersaturated solid solution decomposition in the 1570 alloy. The authors identified that in the alloys with hafnium additives, the supersaturated solid solution decomposition is the most intense at a temperature of 350 °С, and in the alloys without hafnium – at a temperature of 430 °С. The transmission microscopy data confirm that the 1570 alloy without hafnium contains 3–4.5 times more nanoparticles than the 1570–0.5Hf alloy.

Formation of catalyst nanoparticles for the growth of carbon nanotubes during annealing of amorphous Co-Zr-O films

The formation of nanosized catalyst particles for the growth of carbon nanotubes can be carried out during the crystallization of amorphous films consisting of two metals, one of which has higher free energy of the oxide. During annealing in the presence of oxygen, this metal is oxidized with the reduction of the second metal, which leads to the formation of nanoparticles embedded in the oxide of the first metal. This the process has been experimentally and theoretically studied by the example of the formation of cobalt nanoparticles on the surface of amorphous Co-Zr-O films as a result of the decomposition of a supersaturated solid solution and mechanical stresses arising during the oxidation of zirconium. We have proposed a mechanism for the formation of catalyst nanoparticles and phenomenological model of this process developed on the basis of the phase-field theory. Keywords: carbon nanotubes, nanoparticles of catalyst, phase transitions, decomposition of supersaturated solid solutions.

Условия кристаллизации слитков высокопрочных алюминиевых сплавов с повышенным содержанием циркония

The structure of model alloys of the aluminum-zirconium system and industrial 1973-type alloy with an increased content of zirconium after rapid cooling to temperatures of the liquid-solid region and isothermal holding at these temperatures has been studied. It is shown that during primary crystallization of intermetallic precipitates with low crystallization rate, it is advisable to use the model of decomposition of supersaturated solid solutions. It was determined that formation of Al3Zr particles leads to zirconium decrease in the solid solution in center and grain boundary. Under certain crystallization conditions, there is an incubation period for the zirconium intermetallic compounds precipitation. It is shown that the incubation period for aluminum with 0.25 % zirconium is more than 2 minutes, and for 0.5 % zirconium — less than 0.5 minutes. The value of the dendritic parameter of the 1973-type alloy with an increased zirconium content during two-stage crystallization corresponds to the known regularity of the change in the dendritic parameter depending on the cooling rate. Casting of alloy ingots of 1960-type alloy with 0.27 % Zr by the two-stage cooling with the use of a water-cooling tray has been tested. This made possible to produce ingots with 84 mm diameter without the formation of primary intermetallic compounds. The mechanical properties of forgings from such ingots showed an increase in ductility by 20 % while maintaining the strength characteristics.

Physical Regularities for Cellular Precipitation of Co-, Cu-, and Pb-Based Supersaturated Solid Solutions

The decomposition of supersaturated solid solutions through the cellular mechanism is considered in terms of the physical regularities of this phenomenon. The general characteristics of this process are described. The mechanisms of nucleation and subsequent cell growth, as well as kinetic parameters of processes, are partially described. The influences of some external factors on the cellular precipitation process and its stages are characterized. Particularly, the effects of annealing temperature and a third element on the cellular precipitation process are studied.

Effect of Deformation Temperature on Formation of Ultrafine-Grained Structure in the Age-Hardenable Cu–Cr–Zr Alloy

The effect of the temperature of plastic deformation performed by equal channel angular pressing on the structure and physical and mechanical properties of the age-hardenable Cu–Cr–Zr alloy has been studied. Plastic deformation results in the formation of an ultrafine-grained structure in some regions with an average grain size smaller than 1 µm, a supersaturated solid solution decomposition, and the precipitation of disperse particles. The fraction of the supersaturated solid solution which is decomposed is shown to increase with increasing deformation temperature. The density of microshear bands, the dislocation density, and the fraction of high-angle boundaries and the ultrafine-grained structure increase with increasing volume fraction of disperse particles.

Study of kinetics of the supersaturated solid solution decomposition in alloys of the Al – Mg system with transition elements addition

The objective of the study is to determine kinetics of L12 structure type particles precipitation in Al – Mg system aluminum alloys, containing scandium and zirconium. The article addresses microstructure evolution of scandium containing alloys, including two-component alloy and Al – Mg system alloys, containing multiple components, i.e., 1565ch alloy (Al – Mg – Mn – Zn – Zr system) and 01570 alloy (Al – Mg – Mn – Sc – Zr system). The samples were produced by casting in water chilled copper permanent mold. Analysis of the samples, representing various alloys, including 99.95% pure aluminum, confirmed quantitative data of solutes effect on electrical resistance. The optimal homogenization modes, enabling max alloying components dissolution in aluminum matrix, were established for the studied alloys. After that the samples were subjected to heterogenization annealing in the temperature range from 250 oC to 450 oC, with soaking time ranging from 1 to 106 s. C-curves of supersaturated solid solution decomposition were plotted based on correlation of samples electrical resistance in max saturated solution and after annealing under different modes; it was demonstrated, that decomposition curves of the alloys, containing 0.2 and more scandium, have two peaks, at 300 oC and at 425 oC. The obtained data are approximated by power functions (min determination coefficient R2 is 0.97), which are helpful for such alloys thermomechanical treatment modelling. It is demonstrated, that during cast samples heat treatment at the temperature above 350 oC decomposition of alloys, containing 0.2% and more scandium, is associated with secondary crystallization, and decomposition product distribution is fan-shaped, confirming the theory of discontinuous decomposition, observed by other researchers in similar alloys. The size of L12 structure particles, generated during discontinuous decomposition, ranged from 50 to 270 nm, which is significantly different from the optimal values of 40–50 nm.The study was supported by a grant from the Russian Science Foundation, project 18-79-10099.

Effect of hafnium transition metal additives on the microstructure of 01570 aluminum alloy

The study addresses the effect of hafnium on microstructure, mechanical properties and stability of Al3Sc type strengthening particles in Al – Mg system 01570 alloy with scandium and zirconium additions. As part of the study, 01570 alloy-based ingots with hafnium content of 0.1% and 0.2% (here and everywhere hereinafter % weight is indicated) were cast in steel chill molds with zirconium and without zirconium addition. The obtained ingots grains size was examined using optical microscope. It was established, that 0.2% Hf addition to the previously designed 01570 alloy can reduce the average nominal grain diameter by half. A number of anneals was performed at temperatures of 400 oC and 450 oC with 1, 10, 30 minutes, 1, 4, 8, 16, 32 and 56 hours soaking times, to study the rate of supersaturated solid solution decomposition and determine the moment of strengthening dispersed particles coalescence onset. Annealed samples were measured for microhardness, electrical resistance and examined using scanning electron microscopy. It was demonstrated, that samples with hafnium do not lose strengthening phase stability at traditional treatment temperature of 400 oC with considerable soaking time (up to 56 h), at 450 oC decrease in microhardness is observed only after 4 hours of soaking. The samples were pressure-formed, i.e., hot and cold rolled, to determine recrystallization onset, it was established, that even at 500 oC and short soaking period (up to 30 minutes) rolled samples with hafnium still have some not recrystallized microstructure, while the samples without hafnium have visible equiaxed recrystallized grains. Using transmission electron microscope, it was possible to establish that the dispersed particles obtained in the alloy with 0.2% Hf retain a size of about 9 nm after the treatment cycle, and the average distance between them is 62 nm.The study was supported by a grant from the Russian Science Foundation, project 18-79-10099.

The evolution of microstructure and properties of a Cu–Ti–Cr–Mg–Si alloy with high strength during the multi-stage thermomechanical treatment

Abstract A Cu–Ti–Cr–Mg–Si alloy with the super-high strength and high electrical conductivity was designed, and the effects of multi-stage thermomechanical treatment on the microstructure and properties of the alloy were investigated. The alloy treated by the multi-stage thermomechanical treatment had a yield strength of 1072 MPa, tensile strength of 1144 MPa, elongation of 6.1% and electrical conductivity of 24.1 %IACS. During the thermomechanical treatment, the nanoscale β′–Cu4Ti particles, Cr precipitates and submicro-particles that mainly consisted of Cu, Ti, Cr and Si elements formed, leading to the high strength of the alloy. β′–Cu4Ti phase with body-centered tetragonal structure had a classic orientation relationship with copper matrix: [001]Cu//[001]β' and [3-10]Cu//[100]β'. The super-high strength of the designed alloy was mainly attributed to the precipitation strengthening, sub-structure strengthening, solid solution strengthening and work hardening. Electrical conductivity increased during aging due to the decomposition of supersaturated solid solution. Compared with direct aging treatment, multi-stage thermomechanical treatment with the stepped temperature-reduction further improved the mechanical and electrical properties of the alloy.

Simulation of Microstructure Evolution in Metal Materials under Hot Plastic Deformation and Heat Treatment

The development of modern computational techniques and equipment enables one to perform high-precision calculations of complex processes for industry, including metallurgy. This review has classified the basic physical and mathematical models of structure formation during heat and deformation treatment. The Kocks–Mecking–Estrin model describing the dislocation structure at the initial stage of hot plastic deformation has been analyzed. The models of dynamic, metadynamic, and static recrystallization kinetics based on the Johnson–Mehl–Avrami–Kolmogorov equation have been considered. The models of the kinetics of phase transformation upon heating and cooling of steel have been reviewed. The Kampmann–Wagner model that describes the decomposition of supersaturated solid solution during the aging of aluminum alloys has also been considered. The main computational techniques to calculate microstructural evolution, such as the cellular automaton, Monte Carlo, and multiphase-field techniques have been considered. They exhibit high accuracy when calculating recrystallization processes and phase transformations. The systematization of the existing models that describe structural evolution has revealed the possibility to develop complex models for the comprehensive calculation of full cycles to process metal materials by heating and deformation. These models can be used for the optimization and development of new processing techniques.

Understanding the age-hardening mechanism of CrWN coating

A microstructure and age-hardening investigation was performed on a chromium nitride and tungsten nitride (CrWN) transition metal nitride coating for optical glass molds using electron microscopy and nano-indentation. The as-deposited coating had a non-uniformly distributed coherent solid solution matrix causing limited strengthening and unsatisfactory mechanical performance. Annealing of the coating induced spinodal decomposition of supersaturated solid solution and promoted diffusion within and homogenization of the solid solution matrix. Large areas of homogenized solid solution greatly strengthened the coherent matrix resulting in significant age-hardening of the annealed coating. These findings provide supporting data for understanding the age-hardening mechanism of transition metal nitride coatings.

Decomposition of Supersaturated Solid Solution in Fast-Quenched Al-Co-Zr and Al-Fe-Co-Zr Alloys

Aluminum-based alloys are the most popular structural materials. Strength and refractoriness can be improved by doping with transition metals; this effect can be amplified by fast quenching from liquid state, which will expand the solid-solution regions and disperse the intermediate phases in transition metal-aluminum alloys. The decomposition of supersaturated solid solutions (SSS) under special conditions releases the fine-dispersed hardening intermetallic phases, which boosts the strength of the alloy. The way fast-quenched Al-Co-Zr and Al-Fe-Co-Zr alloys change their microhardness after isothermic annealing at 470, 570, or 670 K indicates dispersion hardening. Durometer testing points to a considerable hardening of fast-quenched Al-Co-Zr and Al-Fe-Co-Zr alloys, which is caused by the decomposition of supersaturated solid solutions. Staging the decomposition of Co, Zr, or Fe SSS in aluminum helps predict how the doping additives as well as the temperature and timing of subsequent thermal treatment affect the strength of aluminum alloys.

Formation of Finely Dispersed Structures in Alloys of Aluminum with Cobalt and Zirconium

Rapidly quenched alloys of aluminum with cobalt and zirconium are investigated using a combination of means of physicochemical analysis to study the formation of finely dispersed structures that considerably enhance the strength characteristics of materials. Rapidly quenched alloys are studied, since it is a mode of alloy treatment that allows the maximum isolation of the strengthening phase. It is found that the transfer of the single-phase structure of a supersaturated solid solution of Alsupers(Co,Zr) to Als(Co,Zr) + ZrAl3 + Co2Al9 is accompanied by an increase in hardness, due to the sequential separation of the intermetallic phases in the finely dispersed state. It is shown that the value of the dispersion hardening effect depends largely on the temperature and time parameters of the decomposition of supersaturated solid solutions.

Influence of decomposition of oversatured liquid solutions on the structure and microhardness of quickly curing alloys of the Pb–Sn system

The results of studies of the structure and microhardness of foil alloys of the lead–tin system obtained by high-speed cooling from the liquid phase are presented. The foil sample had the following dimensions: length – up to 10 cm, width – up to 1 cm, and thickness – 30–80 microns. Melt cooling rate was not less than 105 K/s. A rapidly cooled foil is chara cterized by a dispersed structure. The size of the discharge of tin and lead does not exceed 5 μm. The specific surface of the interfacial boundaries achieve 1.7 μm–1. Due to supercooling, a microcrystalline structure forms in the foil. The average lengths of chords of random secants on lead and tin grain sections in the Pb–73 at.% Sn alloy foil are 0.8 and 1.8 μm respectively. The texture of (111) lead and (100) tin is formed in the foil of alloys of the lead – tin system under certain conditions. The formation of the structure of lead alloys containing from 20 to 95 at.% tin is due to the occurrence of spinodal decomposition of a supersaturated liquid solution, and, in other alloys, due to decay by the mechanism of formation and growth of nuclei of crystalline phases. The stratification of the liquid solution leads to the formation of areas enriched in lead and tin, which contribute to the formation of crystallization centers that are equally distributed in the volume of the foil. The microhardness of the foil alloys, whose compositions are close to eutectic, is less than the microhardness of massive alloys of the same composition, which is associated with the softening effect of grain boundaries and interphase boundaries. Exposure of these alloys at room temperature causes an increase in microhardness due to a decrease in slippage at the boundaries. The decomposition of supersaturated solid solutions of Pb–5 at.% Sn and Sn–1 at.% Pb alloys leads to a decrease in microhardness due to the weakening of the effect of the solid solution hardening mechanism. The results of the study can be used to create fusible solders, bearing alloys, alloys for cable sheaths with improved physicochemical properties.

Quantitative Phase Analysis by X-ray Diffraction—Doping Methods and Applications

X-ray powder diffraction is an ideal technique for the quantitative analysis of a multiphase sample. The intensities of diffraction lines of a phase in a multiphase sample are proportional to the phase fraction and the quantitative analysis can be obtained if the correction for the absorption of X-rays in the sample is performed. Simple procedures of quantitative X-ray diffraction phase analysis of a multiphase sample are presented. The matrix-flushing method, with the application of reference intensities, yields the relationship between the intensity and phase fraction free from the absorption effect, thus, shunting calibration curves or internal standard procedures. Special attention is paid to the doping methods: (i) simultaneous determination of the fractions of several phases using a single doping and (ii) determination of the fraction of the dominant phase. The conditions to minimize systematic errors are discussed. The problem of overlapping of diffraction lines can be overcome by combining the doping method (i) and the individual profile fitting method, thus performing the quantitative phase analysis without the reference to structural models of particular phases. Recent suggestions in quantitative phase analysis are quoted, e.g., in study of the decomposition of supersaturated solid solutions—intermetallic alloys. Round Robin on Quantitative Phase Analysis, organized by the IUCr Commission on Powder Diffraction, is discussed shortly. The doping methods have been applied in various studies, e.g., phase transitions in titanium dioxide, biomineralization processes, and phases in intermetallic oxide systems and intermetallic alloys.

Effect of annealing environment on the microstructure and mechanical property of CrWN glass molding coating

The CrWN glass molding coating was synthesized by plasma enhanced magnetron sputtering (PEMS). The effect of annealing environment (e.g., vacuum, N2 and Air) on the microstructure and mechanical property of as-deposited coating was investigated by the X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), atomic force microscope (AFM) and nanoindentation tests. Results showed that the as-deposited coating exhibited dense columnar structure consisting of multilayer CrN and W2N phases. The vacuum annealed coating showed similar thickness to that of as-deposited coating, but underwent visible surface coarsening because of slight oxygen erosion. The N2 and air annealed coatings suffered from varying degree of oxidation damage accompanied by the formation of mixed WO3-CrWO4 phases, which consequently resulted in severe thickness expansion and surface coarsening. The vacuum annealing induced a spinodal decomposition of supersaturated solid solution to form nm-sized CrN and W2N domains. Strain fields originating from the lattice mismatch eventually caused a pronounced age-hardening in the vacuum annealed coating. Whereas the N2 and air annealed coatings showed significant mechanical degradation because these loose WO3-CrWO4 oxides degraded the stable matrix structure. Our results clearly demonstrated that vacuum environment effectively suppressed the oxidation damage and mechanical degradation of glass molding coating.

Decomposition of Supersaturated Solid Solution during Non-isothermal Aging and Its Effect on the Physical Properties and Microstructure of the Ag-Cu7.5 Alloy

The aim of the study was to examine the effect of supersaturated solid solution decomposition during non-isothermal aging on the microstructure and selected physical and mechanical properties of the Ag-7.5 wt.% Cu alloy. The decomposition mechanism was determined by complex research methods, i.e., physical properties testing, microstructure examinations, as well as hardness measurement. Decomposition of Ag-Cu7.5 supersaturated solid solution during heating has two stages. The first stage involves a significant increase in hardness and has no effect on electrical conductivity. During the second stage, growth of discontinuous precipitates and an increase in hardness and electrical conductivity are observed. Discontinuous precipitation cells consist of βCu rods and plates surrounded by αAg phase. Obtained activation energy of this microstructural evolution is lower than its values presented in the literature.

Effects of hot forming cold die quenching and inter-pass solution treatment on the evolution of microstructure and mechanical properties of AA2024 aluminum alloy after equal channel angular pressing

The aim of this research is to investigate the effect of hot forming cold die quenching (HFQ) equal channel angular pressing (ECAP) on the microstructural evolution and mechanical properties of AA2024 aluminum alloy. For this purpose, solution treated AA2024 aluminum alloy was initially ECAPed in HFQ and water quenched (WQ). Microstructural evolutions, dislocation density, and mechanical properties were investigated after up to 3 (WQ-ECAP) and 5 (HFQ-ECAP) passes of deformation. Intermediate solution treatment was conducted after each ECAP pass. The samples underwent aging treatment after deformation. Large amount of shear banding was observed in samples after the first ECAP pass which was reduced in the successive passes of ECAP through intermediate solution treatment. Tensile properties and microhardness enhanced by increasing number of ECAP passes. However; after the fifth pass of HFQ-ECAP, sudden decrease in mechanical properties was observed. Partial decomposition of supersaturated solid solution was found to occur at higher strains. Mechanical properties and deformability were improved in the HFQ-ECAP samples with respect to WQ-ECAP.