Decomposition Of Supersaturated Solid Solution Research Articles

Study of the decomposition of the magnesium-based solid solutions in Mg-Sm-Tb alloys

Kinetics and structural transformations upon the decomposition of a magnesium-based supersaturated solid solution in Mg-Sm-Tb alloys have been studied at various relations between the concentrations of terbium and samarium. It has been established that with increasing terbium content in the alloys the strengthening upon the decomposition of the supersaturated magnesium-based solid solution increases. The decomposition of the supersaturated magnesium-based solid solution in the Mg-Sm-Tb alloys with a percentage ratio (wt %) Tb: Sm of about 2.5 exhibits signs characteristic of the decomposition of the supersaturated solid solution in binary Mg-Tb alloys. With allowance for the equilibrium Mg-Sm-Tb phase diagram, this gives grounds to suppose that samarium is mainly dissolved in the products of the decomposition of the magnesium solid solution that are characteristic of Mg-Tb alloys.

A new way of phase identification, of AgGaGeS4∙nGeS2 crystals

The phase identification of AgGaGeS4·nGeS2 (n=0–4) crystals grown by vertical Bridgman–Stockbarger technique was carried out to find the boundary value n between a homogeneous solid solution and its mixture with GeS2. To obtain reliable results, the conventional methods of X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX) were completed by less common vapor pressure measurement in a closed volume and precise density measurements, which are very sensitive to the detection of small amounts of crystalline and glassy GeS2 and heterogeneous state of the crystals. The boundary value n=1.5 at 1045 K and the coexistence of the solid solution AgGaGeS4·1.5GeS2 with the β-GeS2 phase for n>1.5 was found. Glassy GeS2 (~2 mol%) was revealed by vapor pressure measurement and XRD studies in all the crystals. This is discussed in terms of the supersaturated solid solution decomposition upon temperature decreasing and the microphase separation of overcooled melt near the melting point under non-equilibrium crystallization. For the first time, the p–T dependence for glassy GeS2 was measured by the vapor pressure measurements.

Early decomposition of supersaturated solid solutions of Al–Zn–Mg casting alloys

This study aimed at investigating the structural, electrical resistivity and hardness changes of aged Al–Zn–Mg alloys. It was observed that the isothermal annealing in temperature range of 25–200°C leads to the following precipitation sequence: Guinier–Preston zones→η′-phase→η-phase. The significant increase of electrical resistivity (ρ) of the considered alloys was associated with the stage of GP zone decomposition. The formation of the intermediate η′-phase was accompanied with a decrease of electrical resistivity and an increase of hardness. Non-continuous quenching was found to suppress the η′-phase formation; while elevated quenching rates led to the formation of uniformly distributed fine precipitates in the alloy bulk.

Evolution of microstructure and mechanical properties in Cu–14%Fe alloy during severe cold rolling

Cu–14%Fe alloy was produced by vacuum arc remelting and casting in water-cooled mold with subsequent hot extrusion. It had fine grained microstructure with supersaturated solid solutions based on Cu and Fe phases. The alloy was subjected to cold sheet rolling with true strains up to 6.6. Microstructure was studied by SEM and TEM, specific attention was paid to chemical composition of individual phases—copper matrix and iron particles. After rolling very fine lamellar structure formed in copper matrix. Rolling also has resulted in decrease of iron content in matrix from 3.8% in initial condition to 1% after rolling with strain of 5.1 due to decomposition of supersaturated solid solution. At higher strain some deformation-induced mechanical alloying was observed, leading to local increase of iron content. Iron particles also were heavily refined; their average thickness after rolling with strain of 6.6 is about 30nm (according to TEM data). Several possible mechanisms of their refinement in addition to homogeneous plastic deformation are proposed. Strength of the alloy measured by tensile testing increased significantly after rolling. For example, ultimate tensile strength increased from 325MPa in initial condition to almost 1000MPa after rolling with strain of 6.6. Good correlation between mechanical strength and spacing between iron particles in accordance with Hall–Petch relationship was found.

Stress corrosion crack of ZnAl15 alloys in hot and humid environment

Corrosion behavior of cold-drawn, stress-relieved and as-annealed ZnAl15 alloys in hot and humid environment was investigated. The results show that stress corrosion crack of ZnAl15 alloys occurred in hot and humid environment and the main corrosion products was ZnO with a small amount of Al(OH)3. The as-annealed and stress-relieved ZnAl15 alloys resulted in better corrosion resistance than the cold-drawn, especially the as-annealed. It implies that the residual stress from cold-drawing and the phase transformation stress from decomposition of supersaturated solid solution accelerated corrosion together during corrosion process.

Calculation of the solvus temperature of metastable phases in the Al-Mg-Si alloys

A procedure has been proposed for the self-consistent calculation of the solvus temperatures of metastable phase precipitates in Al-Mg-Si alloys and the specific energy of their interface with the aluminum matrix. The procedure is based on the results of experimental studies on the kinetics of formation of these precipitates during decomposition of supersaturated solid solutions of quenched Al-Mg-Si alloys, which were carried out by measuring the Young’s modulus and electrical resistivity. On the basis of the obtained set of solvus temperatures of the β″-phase, an empirical formula has been proposed for calculating this temperature as a function of the chemical composition of the initial solid solution.

Deformation-driven formation of equilibrium phases in the Cu–Ni alloys

The homogeneous coarse-grained (CG) Cu–Ni alloys with nickel concentrations of 9, 26, 42, and 77 wt% were produced from as-cast ingots by homogenization at 850 °C followed by quenching. The subsequent high-pressure torsion (5 torsions at 5 GPa) leads to the grain refinement (grain size about 100 nm) and to the decomposition of the supersaturated solid solution in the alloys containing 42 and 77 wt% Ni. The lattice spacing of the fine Cu-rich regions in the Cu–77 wt% Ni alloy was measured by the X-ray diffraction (XRD). They contain 28 ± 5 wt% Ni. The amount of the fine Ni-rich ferromagnetic regions in the paramagnetic Cu–42 wt% Ni alloy was estimated by comparing its magnetization with that of fully ferromagnetic Cu–77 wt% Ni alloy. According to the lever rule, these Ni-rich ferromagnetic regions contain about 88 wt% Ni. It means that the high-pressure torsion of the supersaturated Cu–Ni solid solutions produces phases which correspond to the equilibrium solubility limit at 200 ± 40 °C (Cu–77 wt% Ni alloy) and 270 ± 20 °C (Cu–42 wt% Ni alloy). To explain this phenomenon, the concept of the effective temperature proposed by Martin (Phys Rev B 30:1424, 1984) for the irradiation-driven decomposition of supersaturated solid solutions was employed. It follows from this concept that the deformation-driven decomposition of supersaturated Cu–Ni solid solutions proceeds at the mean effective temperature T eff = 235 ± 30 °C. The elevated effective temperature for the high-pressure torsion-driven decomposition of a supersaturated solid solution has been observed for the first time. Previously, only the T eff equal to the room temperature was observed in the Al–Zn alloys.

Study of the formation kinetics of Metastable phases in quenched Al-Mg-Si Alloys

The results of the experimental study of the formation kinetics of metastable phases during decomposition of supersaturated solid solutions of quenched Al-Mg-Si alloys are presented. The process has been studied by measuring the electrical conductivity at low temperatures (18–85°C) and by measuring the Young’s modulus using the acoustic method in the temperature range 120–220°C. The method of measuring the Young’s modulus is characterized by a high precision and has made it possible to distinguish between the successive stages of the decomposition due to the formation of Guinier-Preston zones, particles of the pre-β″/β″ and β′-phases. The effective activation energies have been calculated using the obtained data on the characteristic durations of the stages of the process at different temperatures. It has been shown that the activation energy of the formation and evolution of particles in the β″-phase is considerably lower than the activation energy of diffusion of alloying element atoms at equilibrium conditions, which is caused by the effect of long-lived quenching vacancies. This energy is close to the activation energy of migration of the ν + Mg complex and, according to the obtained results, is equal to 0.58 eV.

Thermodynamic properties of solid and liquid neodymium-magnesium alloys

The thermodynamic properties of liquid and solid magnesium-rich magnesium-neodymium alloys are determined by the electromotive force method using its classic equilibrium (I) and dynamic (II) versions. The results obtained by both versions agree within the limits of experimental error and clearly reveal phase transformations in the alloys, including the steplike decomposition of supersaturated solid solutions.

Study of alloy V91 of the system Al – Zn – Mg – Cu system supercooled solid solution decomposition

Isothermal and thermokinetic diagrams are worked out for the decomposition of supersaturated solid solution of for heat treated alloy V91 of the Al – Zn – Mg – Cu system. Curves for these diagrams correspond to the start of forming GP zones, metastable T′ and stable phases (T, η). An isothermal diagram for the alloy is plotted from the results of hardness and corrosion rate determination. Critical cooling rates are determined for alloy V91 from the diagrams obtained.

An investigation into the kinetics of formation of inclusions during the decomposition of supersaturated solid solutions

The features of the formation of a system of microinclusions during the decomposition of supersaturated solid solutions are analytically described taking into account the size factor of interfacial surface energy. The suggested computation discloses the physical meaning of phenomena corresponding to phase transformation during the homogeneous and heterogeneous modes by which it proceeds. The supersaturation of the matrix phase is the determining characteristic of such a possibility at the stage of nanostructured nucleation. Unimodal and bimodal systems of microparticles can form at definite values of this quantity. Reliably established experimental facts confirm the conclusions of this theoretical analysis. The obtained solutions can be spread to the various bulk and surface systems of dispersed particles.

A study of retrogression in aging Invars

N30K10T3 Invar with ferromagnetic austenite represented by a supersaturated solid solution capable of aging in subsequent heating is studied. The possibility of the use of the “method of higher harmonics” for studying the decomposition of supersaturated solid solutions in Invars is considered. This method is used for studying modes of double aging including low-temperature and high-temperature aging stages. The temperature regimes of double aging in which the structure of the Invar undergoes “retrogression” are determine.

Structure and properties of precipitation-hardening ceramic Ti-Zr-C and Ti-Ta-C materials

Precipitation hardening alloys of the Ti-Ta-C and Ti-Zr-C systems produced by self-propagating high-temperature synthesis (SHS) have been studied. Optimum compositions of the alloys and heat-treatment conditions, under which the decomposition of supersaturated solid solution accompanied by the precipitation of fine particles both inside carbide grains and at their boundaries occurs, have been determined. The precipitation hardening ceramic materials exhibit high hardness (∼15–23 GPa) and heat resistance (the mass increase of the Ti-9.4% Ta-10.5% C alloy is less than 8 g/m2 during a 50-h exposure in air at 800°C) and can be recommended for the application as materials for deposited multifunctional coatings, high-temperature contacts, evaporation crucibles, and abrasion-resistive tools.

The Development of “Macroscopic Composition Gradient Method” and its Application to the Precipitate-Nucleation Near the Edge of Miscibility Gap

A new characterization method, "Macroscopic Composition Gradient (MCG) Method" is proposed to investigate the phase transformations near the phase boundaries, such as the solubility limit, order/disorder line and so on. Since the macroscopic composition gradient in the alloy is prepared so as to step over the phase boundary, the morphological transition of critical phenomena at the phase boundary can be observed by means of analytical transmission electron microscopy. By utilizing this method, the critical minimum size of stable precipitate in the vicinity of edge of miscibility gap is experimentally determined for the Ni3Si in Ni-Si, Ni3Al in Ni-Al, Cu4Ti in Cu-Ti and Co in Cu-Co binary alloy systems. The results are as follows: The critical nucleus size shows a steep increase up to several tens of nm in a very narrow composition range less than 0.3at% from the phase boundary. The Gibbs-Thomson relation and the conventional nucleation theory statistically rationalize such the composition dependence of nucleus size change. However, the nucleus formation is kinetically never rationalized by the conventional nucleation theories. The phase decomposition of supersaturated solid solution progresses by a mechanism of spinodal phase decomposition, even in the composition range near the solubility limit, i.e. a so-called Nucleation- Growth region. Such the phase decomposition behavior is never rationalized by the Boltzmann- Gibbs free energy, which is based on the extensive entropy. The experimental facts obtained here are explained by Tsallis's non-extensive entropy. It should be noted that the present experiments can systematically be conducted in the composition range very near the solubility limit where they has hardly been examined in the past. The MCG method proposed here is considered to open a new way to investigate the microstructure evaluation, particularly for the critical phenomena near the phase boundary.

Influence of preliminary deformation on the second-phase-particle size distribution in aging alloys

On the basis of a theoretical analysis of asymptotic states which are formed under the conditions of an enhanced density of matrix dislocations in different stages of decomposition of supersaturated solid solutions in concrete diffusion regimes, it is shown how the universal function of relative particle-size distribution is connected with the functional parameter m which is the only carrier of information on the amount of preliminary deformation e of an aging alloy. This parameter as a function of e is approximated by a step. The smearing and height of this step are determined from the found conditions of the realization of dislocation diffusion as the mechanism which controls coalescence in the initial and final (in the case of smallness of e, very remote in time) stages of the evolution of a heterophase structure. Thereby, an explicit dependence of the universal distribution function on the deformation and the temperature and duration of aging is established.

Decomposition of the supersaturated solid solutions at the stage of coalescence under conditions of passage from dislocation-controlled to volume diffusion

The object of a theoretical examination is here the heterogeneous isomorphic continuous decomposition of supersaturated solid solutions for the case of binary aging alloys with a developed dislocation structure under enhanced temperatures in the unsteady stage of coalescence, whose transient nature is caused by the instability with respect to a change in the mechanism of mass transfer acting in the system, which controls coarsening of the disperse heterophase structure. For the quasi-stationary diffusion regime, a complete analytical description of the evolution of the second-phase-size distribution function is given for the stage of the transient kinetics, when the initially prevailing diffusion along dislocations, which supply the alloying component to the precipitation phase, is succeeded by volume diffusion at the last stages of decomposition.

Diffusional decomposition of supersaturated solid solutions at grain boundaries

Abstract A kinetic theory of nucleation and growth of single atom phase precipitates in a supersaturated solid solution at grain boundaries is developed. In the present theory, particular attention is devoted to the analysis of two stages of the overall transformation process: (1) – a stage of quasi-steady-state nucleation and (2) – a transient stage of coarsening. As a result of this analysis, the quasi-steady-state nucleation rate, the number of new phase particles formed via nucleation and diffusional growth, and the time-evolution of the size distribution of the precipitates are established. Moreover, estimates are given for the duration of the different stages of the decomposition process.

Softening of nanostructured Al–Zn and Al–Mg alloys after severe plastic deformation

The microstructure and hardness of binary Al–Zn and Al–Mg alloys were studied both in the as-cast state and after high-pressure torsion (HPT) with 5 torsions (shear strain about 6). The size of (Al) grains and of reinforcing second-phase precipitates decreases drastically after HPT and reaches the nanometre range. During HPT, the Zn-rich supersaturated (Al) solid solution decomposes and closely approaches the equilibrium state corresponding to room temperature. The decomposition of Mg-rich supersaturated (Al) solid solution is less pronounced. In the as-cast state the hardness of the supersaturated solid solutions increases with increasing Zn and Mg content due to solid-solution hardening. However, after HPT the work hardening and Hall–Petch hardening due to the decreasing grain size compete with softening due to the decomposition of a supersaturated solid solution. As a net effect, severe plastic deformation results in softening of the binary Al–Zn and Al–Mg alloys. Softening is more pronounced in the Al–Zn alloys where HPT leads to almost full decomposition of the supersaturated solid solution.

Specific Features of the Strengthening during Severe Plastic Deformation of Supersaturated Solid Solutions

The effect of the supersaturated solid solution decomposition occurring prior to, during, and after severe plastic deformation by torsion under high hydrostatic pressure on strengthening is examined by the examples of Al-Cu-Mg, Al-Mg-Sc, and Mg-Sm alloys and 0.12%C-0.85%Mn- 0.65%Si and 0,1%C-1.12%Mn-0.08%V-0.07%Ti low-carbon steels. The decomposition of the supersaturated solid solution was realized upon cooling from the quenching temperature (lowcarbon steels), prior to deformation (Al-Cu-Mg-, Mg-Sm alloys), during deformation (Al-Cu-Mg-, Mg-Sm alloys), and after deformation. It is shown, the decomposition of the supersaturated solid solution is effective for the grain refinement down to nanoscale and strengthening, but, for different materials, different combinations with SPD should be used.

Nanoprecipitation resulting from the decomposition of supersaturated solid solutions in the tracks of swift heavy ions

The electron subsystem of a material is strongly excited when swift heavy ions pass through the material. The subsequent relaxation of this excitation results in considerable short-term (<10−9 s) heating of the material in the nanometer vicinity of the projectile trajectory. Nanoprecipitation stimulated by such thermal spikes in supersaturated solid solutions is studied. Nanoprecipitates are shown to form when the temperature in the track reaches a point where the characteristic time of precipitation becomes shorter than the time of cooling of the track. The region of most efficient precipitation may be offset from the track axis. The initial cylindrical nonuniformity of the spatial density of nucleating clusters may cause the formation of nanodimensional tubular heterostructures extended along the trajectory of the heavy ions. The parameters of the system that are the most favorable to the tubular mode of precipitation are found.