Formation Of Equilibrium Phases Research Articles

Mechanical Alloying to Achieve Accelerated Formation of Equilibrium Phases in Medium Entropy Alloys

Mechanical Alloying to Achieve Accelerated Formation of Equilibrium Phases in Medium Entropy Alloys

Features of the welded seam material crystalliza-tion in Ti-TiB alloy under electron-beam welding conditions

Natural metal composite materials represent a promising class of modern structural materials that need to be welded. Such materials can be welded by fusion, as has been established with the Ti-TiB alloy as an example. The enhanced operational properties of such materials are determined by the microstructure, which is characterized by the presence of microfibers of borides, carbides, or silicides in the metal matrix. To preserve the mechanical properties of materials in a welded joint, it is necessary to ensure the formation of reinforcing microfibers in the welded seam material. Determination of formation mechanism of boride microfibers, originated in the welded seam material, will become the basis for optimizing of fusion welding modes, in particular, electron beam welding mode.
 The purpose of this study is the determination of formation mechanism of boride microfibers originated in the welded seam material. Two most probable variants of the formation mechanism are analyzed, which involve eutectic decomposition during crystallization from a liquid melt or eutectoid decomposition from a metastable crystallized alloy. The third version is a mixed variant of the two above-mentioned mechanisms.
 In the article the results of metallographic analysis of features of boride phase distribution and an analysis of elemental composition of boride fibers based on local Auger electron spectroscopy are presented. The object of study was a Ti-TiB alloy joint obtained by electron-beam welding. The analysis factors were the features of size, orientation, and nature of the distribution of boride phase microfibers in different areas of the welded seam. The characteristic elemental composition of boride microfibers, which characterizes the correspondence to equilibrium phases, is also studied.
 The degree of deviation of the ratio of boron and titanium in such a phase from the thermodynamically equilibrium in different layers of the material of the welded seam, formed by an electron beam in vacuum, is determined. The dependence of boride phase distribution under various conditions of heat exchange in the welded seam material on the side surfaces and in the central regions is established. It is shown that some of boride microfibers formed in the material of the welded seam are characterized by a deviation from the thermodynamically stable composition ТіВn (n = 1) to ТіВn (n = 0.85). The dendritic nature of boride microfibers distribution and the presence of meta-stable phase formations on Ti and B basis provide the grounds for proposing the predominant mechanism for the formation of structure of the welded seam material in the Ti-TiB alloy during crystallization.
 An analysis of hypothetical variants of the formation mechanism of boride microfibers originated in the welded seam material showed that the formation of a dendritic type of structure is characteristic for the growth of crystals of a new phase in the liquid phase. Such growth is characterized by the formation of equilibrium phases. The presence of a significant amount of non-equilibrium boride phase in the welded seam indicates the residue of non-equilibrium boron in the titanium matrix and continuation of growing of boride fibers in the crystallized welded seam. A determined mechanism for formation of boride microfibers originated in the welded seam material is eutectic decomposition during crystallization from a liquid melt with the formation of TiB microfibers and further growth of such crystals due to eutectoid decomposition from a metastable crystallized Ti-TiB alloy. The results obtained make it possible to understand the mechanism of formation of a welded seam in welded natural-composite metal materials, which permits to develop the recommendations for optimizing the welding technology for such materials.

Exceptional thermal stability and enhanced hardness in a nanostructured Mg-Gd-Y-Zn-Zr alloy processed by high pressure torsion

A Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr (wt.%) alloy is processed by solution treatment and high pressure torsion (HPT) at room temperature to produce a nanostructured light material with high hardness. The stability of this alloy is subsequently tested through isochronal annealing for 0.5 h at 373 K to 673 K. The results reveal a thermal stability that is vastly superior to that of conventional Mg-based alloys processed by severe plastic deformation: the grain size remains at around 50 nm on heating to 573 K, and as the temperature is increased to 673 K, grain growth is restricted to within 500 nm. The stability of grain refinement of the present alloy/processing combination allowing grain size to be limited to 55 nm after exposure at 573 K, appears to be nearly one order of magnitude better than for the other SPD processed Mg-RE type alloys, and 2 orders of magnitude better than those of SPD processed RE-free Mg alloys. This superior thermal stability is attributed to formation of co-clusters near and segregation at grain boundaries, which cause a thermodynamic stabilization of grain size, as well as formation of β-Mg5RE equilibrium phase at grain boundaries, which impede grain growth by the Zener pinning effect. The hardness of the nanostructured Mg-Gd-Y-Zn-Zr alloy increases with increasing annealing temperature up to 573 K, which is quite different from the other SPD-processed Mg-based alloys. The high hardness of 136 HV after annealing at 573 K is mainly due to solute segregation and solute clustering at or near grain boundaries.

Study of Quench Sensitivity and Microstructure Evolution of Al-Mg-Si Alloy Sheets

In the actual industrial production process, it is usually appropriate to reduce the cooling rate and control the residual stress. In this study, the Time-Temperature-Property curve of Al-Mg-Si alloy sheets was measured by interruption quenching and subsequent artificial aging method. The microstructure evolution of Al-Mg-Si alloy was carefully characterized using optical microscopy (OM) and transmission electron microscopy (TEM). It was found that the nose temperature of the TTP curve drawn by experiment was ~360°C, closing to the nose temperature of ~365°C obtained from the simulated TTT curves. The number of equilibrium phase rapidly increased with the increasing of holding time, while no obvious equilibrium phase formation at the low temperature region and high temperature region. The critical cooling rate is 14.3°C/s, the determination of the critical cooling rate has important reference value for the control of alloy sheet during quenching process in the actual industrial production. The quenching sensitive region of the Al-Mg-Si alloy sheet is between 290°C and 440°C.

Optimising the Al and Ti compositional window for the design of γ’ (L12)-strengthened Al–Co–Cr–Fe–Ni–Ti high entropy alloys

The effect of the relative contents of Al and Ti on the equilibrium phase formation in a novel Ni51Co18Fe5Cr10Al16-XTiX high entropy superalloy (HESA) system was analysed using Thermo-Calc software (TCHEA-3 database). The proposed HESAs showed a broad γ+γ′ equilibrium phase field over an extended temperature range (from 660 °C to a γ′-dissolution temperature >1184 °C) without the presence of brittle intermetallic phases. The effect of Al/Ti ratio on the microstructure, mechanical properties and precipitation strengthening of the HESAs was determined. The proposed alloys showed a high volume fraction (∼60%) of a (Ni, Co)3(Al, Ti)-type ordered (γ′) phase in a disordered face-centered cubic matrix phase (γ). The microstructure and the thermal properties of the HESAs showed excellent correlation to the theoretical thermodynamic predictions. A lower Al/Ti ratio improved the mechanical properties of the HESA system due to the high lattice misfit between the γ and γ′ phases. The homogenised and aged Ni51Co18Fe5Cr10Al8Ti8 HESA exhibited a γ′-volume fraction of 63%, γ′-dissolution temperature of 1200 °C and yield strengths of 1056 MPa and 852 MPa at room-temperature and 800 °C, respectively. The present work shows the potential to design Al–Co–Cr–Fe–Ni–Ti HESAs with a high volume fraction of γ′-phase and excellent mechanical properties.

Formation and characterization of an Al-rich metastable phase in the Al–B phase diagram

Vacuum heat treatment of mechanically alloyed powders of boron and aluminium leads to the formation of a metastable Al-rich phase, which can be quenched. Its structure, composition and thermal stability are established. With the chemical formula Al1.28B the rhombohedral phase is unusually rich in Al. The parameters of the unit cell determined from X-ray powder diffraction are a = 18.3464 (19), c = 8.9241 (9) Å, V = 2601.3 (6) Å3, space group R 3. It is stable on heating to 630°C. It is suggested that this phase is an important intermediate step in the formation of AlB2 and, eventually, of other borides; its nucleation and thermal stability are explained by high elastic energy hindering the formation of equilibrium phases at low temperatures.

Phase Formation, Microstructure and Mechanical Properties of Mg67Ag33 as Potential Biomaterial

The phase and microstructure formation as well as mechanical properties of the rapidly solidified Mg67Ag33 (at. %) alloy were investigated. Owing to kinetic constraints effective during rapid cooling, the formation of equilibrium phases is suppressed. Instead, the microstructure is mainly composed of oversaturated hexagonal closest packed Mg-based dendrites surrounded by a mixture of phases, as probed by X-ray diffraction, electron microscopy and energy dispersive X-ray spectroscopy. A possible non-equilibrium phase diagram is suggested. Mainly because of the fine-grained dendritic and interdendritic microstructure, the material shows appreciable mechanical properties, such as a compressive yield strength and Young’s modulus of 245 ± 5 MPa and 63 ± 2 GPa, respectively. Due to this low Young’s modulus, the Mg67Ag33 alloy has potential for usage as biomaterial and challenges ahead, such as biomechanical compatibility, biodegradability and antibacterial properties are outlined.

The Equilibrium Phase Formation and Thermodynamic Properties of Functional Tellurides in the Ag–Fe–Ge–Te System

Equilibrium phase formations below 600 K in the parts Ag2Te–FeTe2–F1.12Te–Ag2Te and Ag8GeTe6–GeTe–FeTe2–AgFeTe2–Ag8GeTe6 of the Fe–Ag–Ge–Te system were established by the electromotive force (EMF) method. The positions of 3- and 4-phase regions relative to the composition of silver were applied to express the potential reactions involving the AgFeTe2, Ag2FeTe2, and Ag2FeGeTe4 compounds. The equilibrium synthesis of the set of phases was performed inside positive electrodes (PE) of the electrochemical cells: (−)Graphite ‖LE‖ Fast Ag+ conducting solid-electrolyte ‖R[Ag+]‖PE‖ Graphite(+), where LE is the left (negative) electrode, and R[Ag+] is the buffer region for the diffusion of Ag+ ions into the PE. From the observed results, thermodynamic quantities of AgFeTe2, Ag2FeTe2, and Ag2FeGeTe4 were experimentally determined for the first time. The reliability of the division of the Ag2Te–FeTe2–F1.12Te–Ag2Te and Ag8GeTe6–GeTe–FeTe2–AgFeTe2–Ag8GeTe6 phase regions was confirmed by the calculated thermodynamic quantities of AgFeTe2, Ag2FeTe2, and Ag2FeGeTe4 in equilibrium with phases in the adjacent phase regions. Particularly, the calculated Gibbs energies of Ag2FeGeTe4 in two different adjacent 4-phase regions are consistent, which also indicates that it has stoichiometric composition.

Influence of Ti and Zr alloying elements on microstructure and micromechanical properties of near-eutectic Nb-18.7Si alloy

The present study investigates the effect of addition of Ti and Zr alloying elements on the microstructure, phase formation and micro mechanical properties of the phases of a near eutectic Nb-18.7Si cast alloy. The alloys were chracterized using x-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalyzer (EPMA), electron back scattered diffraction (EBSD) and transmission electron microscopy (TEM). Both the alloys Nb-18.7Si and Nb-18.7Si-5Ti exhibited a two phase microstructure consisting of Nbss and Nb3Si. The Nb3Si is not a stable phase and require further long term heat treatments to form stable and high creep resistant α–Nb5Si3 phase. Interestingly, the alloys added with Zr (Nb-18.7Si-5Zr), and Ti, Zr (Nb-18.7Si-5Ti-5Zr) exhibited Nbss, α–Nb5Si3 phases. This indicates that the addition of Zr alone or together with Ti resulted in the formation of equilibrium α–Nb5Si3 phase in the as cast condition itself. The formation of such phases in these alloys is beneficial in improving the properties and being cost effective. The volume fraction of the Nbss (~22–23%) is almost same in Nb-18.7Si and Nb-18.7Si-5Ti alloys. Whereas, the volume fraction is increased more than double (~53–55%) in Nb-18.7Si-5Zr and Nb-18.7Si-5Ti-5Zr alloys. The coarsening of the Nbss phase is also observed with alloying and is more prominant in Nb-18.7Si-5Zr and Nb-18.7Si-5Ti-5Zr alloys. The nano indentation results showed that the hardness of Nbss phase increased with Ti and decreased with Zr additions. Overall, a drastic increase in the elastic modulus of the silicide phase is observed in Nb-18.7Si-5Zr and Nb-18.7Si-5Ti-5Zr alloys as compared to alloys Nb-18.7Si, Nb-18.7Si-5Ti and is due to the formation of α–Nb5Si3 phase in the former alloys compared to Nb3Si phase in the later alloys. The higher volume fraction of Nbss phase, coarsening and decrease in hardness of Nbss in Nb-18.7Si-5Zr and Nb-18.7Si-5Ti-5Zr alloys resulted in improvement in fracture toughness (10.5 ± 0.35 MPa. m1/2). The formation of equilibrium phase α–Nb5Si3 directly in the cast condition is a significant breakthrough since it eliminates the need for long term vacuum heat treatment at higher temperatures.

Metastable microstructures in the solidification of undercooled high entropy alloys

High entropy alloys with multiple phases are taken for undercooling studies and established the microstructure evolution with respect to undercooling. The melt fluxing technique was used for the current study to achieve undercooling. Predictions on the equilibrium phase formation of these systems was performed using CALPHAD approach and compared with experimental observations. Metastable microstructures were observed during undercooling and the morphological changes could be correlated with the currently established mechanisms. The detailed microstructure evolution in FeCoNiCuX0.5 (X = Al, Mo, Ti, W, Zr) shows the minute addition of these elements results in variation in microstructure evolution during as cast as well as undercooled condition. The studied systems show a maximum undercooling of more than 0.18 TL. and maintained crystalline nature even in deep undercooling. The segregation nature of elements was studied and correlated with the phase field simulations obtained.

Alloyed Thermoelectric PbTe-SnTe Films Formed via Aerosol Deposition.

The discovery of new thermoelectric materials has the potential to benefit from advances in high-throughput methodologies. Traditional synthesis and characterization routes for thermoelectrics are time-consuming serial processes. In contrast, high-throughput materials discovery is commonly done by thin film growth, which may produce microstructures that are metastable or compositionally graded and, therefore, are challenging to characterize. As a middle ground between bulk synthesis and thin film deposition, we find that the aerosol deposition process can rapidly produce samples that exhibit electronic property trends consistent with those produced by traditional bulk means. We demonstrate rapid growth of discrete thermoelectric thick films of varying chemical compositions (Pb1-xSnxTe) from PbTe and SnTe polydisperse micrometer sized powder feedstocks. The high deposition rate (near 1 μm min-1) and resultant microstructures are advantageous as the diffusion length scales promote rapid thermal treatment and equilibrium phase formation. Room-temperature high-throughput measurements of the Seebeck coefficient and resistivity are compared to traditionally produced bulk materials. The Seebeck coefficient of the films follows the trends of traditional samples, but the resistivity is found to be more sensitive to microstructural effects. Ultimately, we demonstrate a framework for exploratory materials science using aerosol deposition and high-throughput characterization instrumentation.

Phase stability of single phase Al0.12CrNiFeCo high entropy alloy upon irradiation

Phase stability of NiCoFeCr and Al0.12NiCoFeCr single phase high entropy alloys (HEAs) was studied under 3 MeV Ni2+ irradiation at 500 °C to a fluence of 1 × 1017/cm2 reaching a peak dose of ~100 displacements per atom (dpa). Transmission electron microscopy (TEM) diffraction pattern and scanning transmission electron microscopy-energy dispersive X-Ray spectroscopy (STEM-EDS) were utilized to detect any second phases that formed in the irradiated regions of the specimens. While the single phase of NiCoFeCr alloy has remained stable under the irradiation, the phase in Al0.12NiCoFeCr alloy decomposed into FCC matrix phase, and Ni3Al type nanoprecipitates with L12 ordered structure. On the other hand, in Al0.12NiCoFeCr alloy, the non-irradiated region of the TEM specimen (annealed for 10 h at 500 °C) and the as-cast alloy annealed for 490 h at 500 °C exhibited no such precipitates similar to those observed in the irradiated region. Based upon the thermodynamic stability of these alloys, we propose that radiation-enhanced diffusion under the irradiation accelerated the formation of the Ni3Al type equilibrium phase. Our study clearly demonstrates that the phase stability of HEAs under irradiation should be considered as one of the deciding factors in the materials design and selection for nuclear applications.

Microstructure Evolution and Mechanical Properties of Keyhole Repair Welds in AA 2219-T851 Using Refill Friction Stir Spot Welding

The search for a suitable friction-based keyhole repair technique that fulfills the requirements for high-quality repair welds has become an important research topic, especially for aerospace applications. In order to provide and analyze an universal keyhole repair method for lightweight metals, refill friction stir spot welding is applied to through-hole repairs in 3- and 6-mm-thick sheets of precipitate hardening AA 2219-T851. Keyholes with a diameter of 7.5 mm were repair welded achieving a defect-free microstructure. The correlation between the microstructural evolution imposed by the repair process and the resulting mechanical properties is shown. A comprehensive analysis of the precipitate evolution in peak-aged AA 2219 during RFSSW is presented. Thermal cycle measurements revealed high heating rates and peak temperatures of up to 520 °C in the weld center. The thermal cycle caused mainly dissolution and minor equilibrium phase formation in the stirred zone. In the HAZ, overaging of the strengthening precipitates dominates with minor dissolution and equilibrium phase formation only in the direct proximity of the SZ. Microstructural analysis revealed typical weld zone formation with inhomogeneous grain size distribution in the SZ. The resulting mechanical properties are dominated by an inhomogeneous hardness distribution with lowest hardness in the TMAZ at 5 mm from the center of the weld. During tensile loading main yielding and the final fracture occur in the area of lowest strength. Tensile testing showed yield strength of 40 to 46% and UTS of 28 to 25% below BM values in 3- and 6-mm-thick sheets, respectively. The sheet thickness and post-weld natural aging were found to influence the mechanical properties of the weld significantly.

Exceptional Thermal Stability and Enhanced Hardness in a Nanostructured Mg-Gd-Y-Zn-Zr Alloy Processed by High Pressure Torsion

A Mg-based alloy with high rare earth (RE) content is processed by solution treatment and high pressure torsion (HPT) at room temperature to produce a nanostructured light material with high hardness. The stability of this Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr (wt.%) alloy is subsequently tested through isochronal annealing for 0.5 h at 373 K to 673 K. The results reveal a thermal stability that is vastly superior to that of conventional Mg-based alloys processed by severe plastic deformation: the grain size remains at around 50 nm on heating to 573 K, and as the temperature is increased to 673 K, grain growth is restricted to within 500 nm. The stability of grain refinement of the present alloy/processing combination allowing grain size to be limited to 55 nm after exposure at 573K, appears to be nearly one order of magnitude better than for other SPD processed Mg-RE type alloys, and 2 orders of magnitude better than SPD processed RE-free Mg alloys. This superior thermal stability is attributed to formation of co-clusters near and segregation at grain boundaries, which cause a thermodynamic stabilization of grain size, as well as formation of β-Mg5RE equilibrium phase at grain boundaries, which impede grain growth by the Zener pinning effect. The hardness of the nanostructured Mg-Gd-Y-Zn-Zr alloy increases with increasing annealing temperature up to 573 K, which is quite different from other SPD-processed Mg-based alloys. The high hardness of 136 HV after annealing at 573 K is mainly due to solute segregation and solute clustering at or near grain boundaries.

Thermodynamic Prediction of the Effect of Excess Air Ratios and Combustion Temperatures on the Equilibrium Phases Formed during Ash Fusion from Coal and Cedar Nut Shells

An analysis of equilibrium phase formation in the ashes from Polish coal and cedar nut shells (CNS) was performed for different combustion conditions in a bubbling fluidized bed boiler. Equilibrium calculations were performed using Factsage 6.3 for five excess air ratio values (λ)—0.7, 0.9, 1.0, 1.2, and 1.6—at a temperature range of 650 ≤ T ≤ 1050 °C. The masses and compositions of ashes synergistically depend on λ and T. An increased combustion temperature results in a decrease in the mass of (i) ashes formed from coal and CNS and (ii) liquid slag (in the case of CNS), resulting from a lower content of K2CO3(l) (the lower the λ, the greater the decrease in K2CO3(l)), and the migration of potassium into the exhaust gas in the form of compounds KCl(g), KOH(g), and K2SO4(g). Ashes from both coal and CNS contain compounds present (i) throughout the whole temperature range, regardless of λ value, e.g., KAlSi2O6(s) and Mg2Al4Si5O18(s), in the case of coal; e.g., Mg2SiO4(s) and MgO(s), in the case of CNS; (ii)...

Selective Laser Melting of an Al–Fe–V–Si Alloy: Microstructural Evolution and Thermal Stability

Selective laser melting was used to produce an aluminum alloy Al-8.5Fe-1.3V-1.7Si (wt%). The effects of heat treatment on microstructure evolution and phase stability during long-term thermal exposure of the deposits were investigated. Results show that the microquasi-crystalline phase, Al12(Fe,V)3Si and AlmFe metastable phases coexisted with α-Al in the as-produced alloy. Annealing at 400 °C resulted in decomposition of microquasi-crystalline phase and supersaturated α-Al into Al12(Fe, V)3Si phase in the fusion zone, accompanied by the decrease in alloy hardness. The activation energy of this decomposition process was 115 kJ/mol. A more homogenous microstructure was obtained after annealing at 400 °C for 60 min, which was resistant to coarsening exposed at 425 °C up to 500 h. The Al12(Fe,V)3Si and AlmFe phases were coarsened at 475 and 525 °C with increasing the exposure time. Coarsening of Al12(Fe,V)3Si phase was attributed to a combination of volume diffusion and grain boundary diffusion mechanism of Fe. Heat treatment at 600 °C resulted in accelerated microstructure coarsening and formation of large-sized equilibrium phases, which significantly degraded the room temperature microhardness.

Microstructural Characterization and Thermodynamic Simulation of Cast Al–Zn–Mg–Cu Alloys

Wedge castings were produced using six Al–Zn–Mg–Cu aluminum alloys with zinc concentrations ranging from 8 to 15 wt%, Zn/Mg ratios ranging from 1.7 to 6.5, and copper ranging from 0.6 to 1 wt%. Thermal analysis and microstructural characterization of the samples in the as-cast, solution-treated, and aged conditions were performed and compared to Calculation of Phase Diagram simulations. The calculated phase stability in the alloys was overall remarkably accurate for both the solidification and equilibrium phase formation. Additional equilibrium calculations were made to determine the range of alloying concentrations that would not contain eutectic T or η phase. The phase stability in a total of 195 chemistries was calculated.

Phase stability of non-equiatomic CoCrFeMnNi high entropy alloys

The objective of this study is to experimentally and theoretically investigate the phase stability of non-equiatomic FexMn62−xNi30Co6Cr2 based high entropy alloys, where x ranges from 22 to 42at.%. Another aim is to systematically and critically assess the predictive capability of the CALPHAD approach for such high entropy alloy systems. We find that the CALPHAD simulations provide a very consistent assessment of phase stability yielding good agreement with experimental observations. These include the equilibrium phase formation at high temperatures, the constituent phases after non-equilibrium solidification processes, unfavorable segregation profiles inherited from solidification together with the associated nucleation and growth of low temperature phases, and undesired martensitic transformation effects. Encouraged by these consistent theoretical and experimental results, we extend our simulations to other alloy systems with equiatomic compositions reported in the literature. Using these other equiatomic model systems we demonstrate how systematic CALPHAD simulations can improve and accelerate the design of multicomponent alloy systems.

About the Formation of the Supersaturated Solid Solutions by Diffusion Process

It is generally agreed that during diffusion process between two phases e.g. matrix/diffusant layer the thermodynamic equilibrium state is fulfilled and the supersaturated solid solutions can not formed by diffusion. Nevertheless, in many cases the formation of equilibrium phases does not occur due to kinetic and other reasons, and metastable phases can be formed. The analysis of the concentration profiles for bulk diffusion obtained at low temperatures consistent with B-regime for grain boundary diffusion in systems Cu/Al, Fe and Co/Cu is performed. It is shown that in these systems the supersaturated solutions are formed and the extent of supersaturation is the more as the diffusion temperature is lower. The concentration of diffusant may be 5-10-times greater than solubility according to phase diagram.

Zn-22Al-2Cu alloy phase transformations at different homogenizing temperatures

The effect of the homogenizing temperature on phase transformations and hardening behavior was studied in a Zn-22Al-2Cu (wt%) alloy homogenized for 7 days at 315, 350, and 390 °C. The homogenized alloy was subsequently quenched and aged naturally at room temperature or artificially at 200 °C for different time periods. The β phase was unstable at room temperature and it decomposed through the β → α + η reaction during natural aging. An increase in the homogenizing temperature caused a decrease in the kinetics of the decomposition of the β phase in the alloy. Natural aging also caused an increase in the hardness due to the formation of nanometric grains with α and η phases and the presence of an intermetallic ɛ phase. This result agrees with the time exponents (n) between 1.5 and 2.6, which were obtained using the Johnson-Mehl-Avrami-Kolmogorov equation for the decomposition of the β phase. The transformed fraction curves exhibited the fastest and slowest growth kinetics of the â phase decomposition at homogenizing temperatures of 350 and 390 °C, respectively. The artificial aging induced the formation of equilibrium phases and began a decrease in hardness due to the coarsening process of the equilibrium α, η, and τ′ phases.