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Physical Factors in Phase Formation
Published in Daniel D. Pollock, PHYSICAL PROPERTIES of MATERIALS for ENGINEERS 2ND EDITION, 2020
The extent of solid solubility of an alloying element is influenced strongly by the value of its electronegativity as compared to that of the host element (Section 10.3.2). The greater the difference between these values, the greater the tendency to form compounds. This may occur even when the size and other factors are favorable for solid solutions. In general, the greater the stability of any intermediate phase, the more limited the extent of a primary solid solution, and the steeper is the slope of the solvus. Most of the alloy systems that form stable intermetallic phases have solvus curves that indicate decreasing solid solubilities with decreasing temperatures. Precipitation will occur on cooling alloys with compositions that are close to that of the solvus at elevated temperatures. The physical and mechanical properties of such alloys depend primarily on the nature of the precipitate, its precipitation mode, and the degree to which it is dispersed (Sections 6.6 and 6.6.1). For example, an alloy that forms a continuous network of a brittle, intermediate compound at the grain boundaries will demonstrate significantly lower mechanical properties than one in which the precipitate is finely divided and well dispersed within the grains as well as in the grain boundaries. Other physical properties of alloys are affected by precipitates as described in Section 9.15.2. Section 10.7.6 shows that many intermetallic phases correspond to certain electromion ratios.
Microstructure and phase transformations in alloys
Published in Ash Ahmed, John Sturges, Materials Science in Construction: An Introduction, 2014
The solubility in each of these solid phases is limited, thus at any temperature below the line BEG only a limited concentration of tin will dissolve in lead (for the a phase) and the same for lead in tin (for the b phase). The solubility limit for the a phase is given by the boundary line CBA on the left-hand side of the diagram, labelled a; it increases to a maximum of 18.3 per cent tin (Sn) at 183 °C at point B and decreases back to zero at A (327 °C). At temperatures below 183 °C, the solubility limit line separating the a and a + b phase regions is called the solvus line, as illustrated; the solvus line separates a homogeneous solid solution from a field of several phases which may form by melting. The boundary line AB is the solidus line. The solidus line is that below which the solution is completely solid (does not contain a liquid phase). The solvus and solidus lines also exist for the b phase, HG and GF, respectively. The maximum solubility of lead (Pb) in the b phase at point G is 2.2 wt% (100 – 97.8, as shown on the right-hand side of the diagram). The horizontal line BEG is also a solidus line as it represents the lowest temperature at which a liquid phase may exist for any stable lead–tin alloy.
Phase Equilibria
Published in George A. Lane, Solar Heat Storage: Latent Heat Materials, 1983
X-ray diffraction techniques, which determine the crystal lattice spacing, are valuable in phase diagram work. This is particularly useful for locating solvus curves and solid-solid transition boundaries. For partially miscible systems, as a second component is added to form a solid solution, the lattice spacing changes continuously. When the solvus boundary is reached, however, there is no further change in lattice spacing, and a second phase (pure component, terminal solid solution, intermediate compound, or solid solution, etc.) appears.
Effect of serrated grain boundary on tensile and creep properties of a precipitation strengthened high entropy alloy
Published in Science and Technology of Advanced Materials, 2023
Jhuo-Lun Lee, Pei-Te Wang, Kai-Chi Lo, Pai-Keng Shen, Nien-Ti Tsou, Koji Kakehi, Hideyuki Murakami, Che-Wei Tsai, Stéphane Gorsse, An-Chou Yeh
Based on the simulated phase diagram shown in Figure 1(a), the γ′ solvus temperature was 1080°C, and the B2 phase solvus temperature was 1100°C, so the solution heat treatment in this work was conducted at a temperature above the solvus of γ′ and B2, i.e. 1120°C for 1 h, and the aging heat treatment to induce γ′ precipitation was conducted at 750°C for 48 h. Furthermore, to study the interactions between γ′ particles and grain boundaries, interrupted cooling experiments with different terminating temperatures were conducted. The cooling rate after the solution heat treatment was set at 1°C/min. Samples were then water quenched at various temperatures, i.e. 1060°C, and 1000°C. The heat treatment history of the interrupted cooling experiments is shown in Figure 2(a). In this study, mechanical properties evaluations were based on samples subjected to two different heat treatment processes, i.e. HT-1, and HT-2. The corresponding heat treatment profiles of HT-1, and HT-2 are shown in Figure 2(b,c) respectively. The HT-1 process involved a solution heat treatment with air-cooling to room temperature followed by an aging heat treatment. The HT-2 heat treatment consisted of a solution heat treatment with slow cooling of 1°C/min to 1000°C, and then air-cooled to room temperature plus an aging heat treatment. All heat treatments were conducted by the Thermo Scientific™ (USA) Lindberg/Blue box furnace.
Recrystallisation, concurrent precipitation, and texture development in cold forged Mg-6Al-3Sn magnesium alloy
Published in Canadian Metallurgical Quarterly, 2023
Purnashis Chakraborty, Vikrant Tiwari
The influence of concurrent precipitation (Mg17Al12 and Mg2Sn) of AT63 alloy on the kinetics of recrystallisation was addressed. The precipitation results were presented through SEM micrographs, as shown in Figures 9 and 11. Mostly, all precipitates were formed at the grain and twin boundaries. The morphology of particles was seen as spherical and rod-shaped. The recrystallisation rate was slow at an annealing temperature of 200°C because more and more precipitation occurred at the grain boundaries as the recrystallisation progressed, which did not allow the grains to move smoothly. As the annealing temperature increased, the amount of concurrent precipitates significantly reduced. For instance, Figure 11 shows that the precipitation quantity decreased at 250°C compared to 200°C. Based on the phase diagram, it was expected that annealing within 300-400°C will nearly be in the single-phase region (solvus temperature ∼ 400°C) for this alloy system. Thus, no attempt was made to quantify the precipitates. There is no hindrance to the recrystallisation progress at the high annealing temperature.
Multiphase modelling of the growth kinetics of precipitates in Al-Cu alloys during artificial aging
Published in Philosophical Magazine, 2021
Tohid Naseri, Daniel Larouche, Pierre Heugue, Rémi Martinez, Francis Breton, Denis Massinon
The evolution of the θ″ phase was almost 100% controlled by the interface and its volume fraction remains very low as explained above. The interfacial mobility used for the growth of θ'’ precipitates allows a good prediction of the onset temperature for that phase during a DSC heating scan, but the correspondence between the calculated and the measured peak amplitude is not so good. This is likely due to the data retrieved from the phase diagram, knowing that there is large uncertainty about the true solvus of θ'’, the experimental data used to validate this solvus being scarce and approximate since they depend on the size of the precipitates [24]. For that reason, trying to get a better fit with the interfacial mobility during the dissolution of this phase was not attempted. Therefore, we have preferred to use the same interfacial mobility for dissolution and growth. For the 2 other types of precipitates, it is clear that the interfacial mobility between growth and dissolution must differ to obtain a good fit. Rodriguez-Veiga et al. [25] have already reported that the activation energies associated with the growth and dissolution of a precipitate are different in an Al-4 wt%Cu. In their study, the activation energy for dissolution of θ'’ and θ’ was larger than for growth.