Kinetics Of Discontinuous Precipitation Research Articles

The effects of Nb additions on the microstructure evolution in Cu–9Ni–6Sn alloy

Cu–9Ni–6Sn specimens with different Nb contents ranging from 0 to 0.22 wt% were prepared by a vacuum induction furnace, and needle-like NbNi3 particles were found at the grain boundaries and interior after solution treatment. The results indicated that these NbNi3 particles suppressed the discontinuous precipitation (DP) growth during aging treatment. By using Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation and Arrhenius equation, the kinetics of DP is also figured out and the activation energy (Q) of the DP in Cu–9Ni–6Sn-xNb (x = 0, 0.05, 0.22 wt%) alloys are 75.8 kJ/mol, 80.9 kJ/mol and 88.5 kJ/mol, respectively, indicating that the atom diffusion for producing DP is more difficult in Nb-bearing alloys, and the formation of DP is inhibited correspondingly. What's more, the lamellar spacing (S) of DP increased and the growth rate of DP decreased in the with the Nb-bearing Cu–9Ni–6Sn alloy.

Effect of vanadium on the microstructure and kinetics of discontinuous precipitation in Cu–3.2Ti–0.2Fe alloy

The microstructure and kinetics of discontinuous precipitation in Cu–3.2Ti–0.2Fe– x V ( x = 0, 0.1, 0.2 and 0.3 wt.%) alloys aged at 723, 773, 823 and 873 K were studied by using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectrometer (EDS), with the help of CALculation of PHAse Diagram (CALPHAD) method and Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation. The volume fraction of Laves_C14 was increased with raising V content, leading to the effective grain refinement during casting and the inhibiting grain growth in solution treatment process. The activation energies of Cu–3.2Ti–0.2Fe– x V ( x = 0, 0.1, 0.2 and 0.3 wt.%) alloys were 196.4, 172.9, 147.4, 154.1 kJ mol −1 , respectively. The discontinuous precipitation was promoted and controlled by grain boundary diffusion in the early stages at low temperature, while it was inhibited by bulk diffusion at high temperature or in the later stages at low temperature. In addition, the peak-aged hardness was improved with proper amount of V addition due to the strengthening of Laves_C14 and grain refinement, as well as the inhibition of discontinuous precipitation.

On the Cr-rich continuous and discontinuous precipitation kinetics in Ni-Cr alloys by microstructural characterization and differential scanning calorimetry analysis

Microstructural dependence of continuous and discontinuous Cr-rich precipitation kinetics in Ni-45%Cr alloys on composition and thermal history was investigated by microstructure characterization and differential scanning calorimetry (DSC). The results indicated that discontinuous precipitation was favored at low temperatures and high Cr contents, while continuous precipitation was favored at high temperatures. Activation energies estimated by Kissinger equation using DSC and microstructural evolution showed that Nb and B suppressed, but Zr promoted discontinuous precipitation. It provided a simple effective method to figure out the effect of additions on precipitation kinetics.

Effect of Prior Plastic Deformation on the α-Cr Precipitation in a Binary Ni-Cr Alloy

The effect of prior plastic deformation on the discontinuous precipitation in a supersaturated binary Ni-Cr alloy (42 wt pct Cr) was investigated. Solutionised samples were cold rolled to different extents of reduction (20, 50 and 70 pct) and then subjected to aging heat treatments in the temperature range of 923 K to 1123 K (650 °C to 850 °C) for different time periods, resulting in the precipitation of α-Cr in various morphologies. The α-Cr precipitates were in the form of lamellar discontinuous precipitation, acicular continuous precipitation along the deformation bands, globularization and equiaxed precipitation concurrently with matrix recrystallization. The volume fraction of each morphology varied according to the heat treatments. Overall, the prior plastic deformation had a retarding effect on the kinetics of discontinuous precipitation and accelerated the kinetics of continuous precipitation. The retardation of discontinuous precipitation was attributed to the loss of supersaturation of the matrix due to the other competing modes of precipitation supported by deformation. Plastic deformation prior to aging is shown to be a possible route for engineering the microstructure in this alloy.

Thermodynamics and kinetics of discontinuous precipitation in Cu–9Ni–xSn alloy

In this work, the hardness during aging, the, morphology and the thermodynamics and growth kinetics of discontinuous precipitation (DP) in Cu–9Ni–xSn alloy were investigated by mechanical testing, metallographic optical microscopy, scanning electron microscopy, and transmission electron microscopy. The results show that Cu–9Ni–xSn alloy overages faster with increasing Sn content and aging temperature. Also, thermodynamic analysis shows that the γ-DO3 phase precipitates more easily with increasing Sn content. The growth kinetics of DP were described by using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation. The diffusion activation energy (Q) values of DP in Cu–9Ni–xSn (x = 3,4,6) alloy are calculated to be 45.5 kJ/mol, 54.6 kJ/mol, and 97.7 kJ/mol, respectively, indicating that the DP reaction was controlled by grain boundary diffusion. The thermodynamics and growth kinetics analyses of DP support the concept that the diffusion of Cu–9Ni–xSn alloy becomes easier with increasing Sn content. Moreover, the results show that the aging temperature and Sn content greatly affect the Interlamellar spacing of DP in Cu–9Ni–xSn alloy.

Suppression of discontinuous precipitation in age-hardening Cu-15Ni-8Sn alloy by addition of V

In the present study, the morphology and growth kinetics of discontinuous precipitation (DP) in Cu-15Ni-8Sn-xV alloys were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicate that the DP colony presents a lamellar structure consisting of alternating lamellae of α-Cu matrix and γ-DO3 phase ((CuxNi1-x)3Sn). In addition, the growth kinetics of DP in Cu-15Ni-8Sn alloy was evaluated using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation and the time exponent can be ascertained between n = 2 and n = 1. It also demonstrates that the initiation of DP in Cu-15Ni-8Sn alloy occurs at grain edges and boundaries. Moreover, the activation energy (Q) of DP is determined to be about 75 kJ/mol by using the Arrhenius equation, thus DP reaction is believed to be controlled by grain boundary diffusion. Interestingly, the addition of V has an obvious effect on the suppression of DP in Cu-15Ni-8Sn alloy. It can be determined that the activation energy (Q) of DP in Cu-15Ni-8Sn-0.2V alloy is about 350 kJ/mol, which is much larger than that of in Cu-15Ni-8Sn alloy. The increasing activation energy of DP after adding V can certainly suppress the formation of DP greatly. Additionally, it is found that a larger number of Ni3V particles generate accompanied by addition of V in Cu-15Ni-8Sn-0.4V alloy. Therefore, the considerable precipitated Ni3V particles impose a strong pinning effect on grain boundary, which also should be responsible for the resultant suppressing effect. Besides, the interlamellar spacing of DP increases and the growth rate of lamellae decreases with increasing V content when aging at 673 K.

Dilatometric and DSC Study of the Kinetics of Discontinuous Precipitation of Ag2Al Intermetallic in Al – 10% Ag Alloy

The kinetics of discontinuous precipitation of Ag2Al intermetallic in alloy Al – 10% Ag is studied after 10-h holding in vacuum at 530°C and subsequent water quenching. The DSC and dilatometric curves are plotted for heating rates ranging from 5 to 20 K/min. The activation energy of formation of the Ag2Al γ-phase is computed from the DSC data with the help of the Boswell equation and by the Kissinger method.

Grain boundary engineering to control the discontinuous precipitation in multicomponent U10Mo alloy

We demonstrate here that locally stabilized structure and compositional segregation at grain boundaries in a complex multicomponent alloy can be modified using high temperature homogenization treatment to influence the kinetics of phase transformations initiating from grain boundaries during subsequent low temperature annealing. Using aberration-corrected scanning transmission electron microscopy and atom probe tomography of a model multicomponent metallic alloy —uranium-10 wt% molybdenum (U-10Mo) a nuclear fuel, that is highly relevant to worldwide nuclear non-proliferation efforts, we demonstrate the ability to change the structure and compositional segregation at grain boundary, which then controls the subsequent discontinuous precipitation kinetics during sub-eutectoid annealing. A change in grain boundary from one characterized by segregation of Mo and impurities at grain boundary to a phase boundary with a distinct U2MoSi2C wetting phase precipitates introducing Ni and Al rich interphase complexions caused a pronounced reduction in area fraction of subsequent discontinuous precipitation. The broader implication of this work is in highlighting the role of grain boundary structure and composition in metallic alloys on dictating the fate of grain boundary initiated phase transformations like discontinuous precipitation or cellular transformation. This work highlights a new pathway to tune the grain boundary structure and composition to tailor the final microstructure of multicomponent metallic alloys.

Kinetics and Grain Boundary Selectivity of Discontinuous Precipitation in Binary Ni-Cr Alloy

A supersaturated Ni-Cr alloy (42 wt pct Cr) was subjected to a series of aging heat treatments in the two-phase region in the temperature range of 923 K to 1123 K (650 °C to 850 °C) for different time periods. The resultant microstructures were seen to be composed of varying volume fractions of continuous (CP) and discontinuous precipitation (DP). The DP dominated at lower temperatures, while CP dominated at higher temperatures and the expected DP termination temperature was estimated to be 1138 K (865 °C). The kinetics of DP followed the Turnbull model at lower temperatures and the Aaronson–Liu model at higher temperatures. The nucleation and growth of DP cells, which occurred via the ‘precipitate driven grain boundary migration,’ was seen to be a strong function of the nature of the participating grain boundaries.

Kinetics of discontinuous precipitation in Cu–20Ni–20Mn alloy

The morphology and growth kinetics of discontinuous precipitation (DP) in a Cu–20Ni–20Mn alloy were investigated in the temperature range of 523–673 K by optical microscopy, scanning electron microscopy, and transmission electron microscopy. A lamellar mixed structure consisting of alternating lamellae of a matrix and NiMn phase was observed in DP colonies. The volume fraction of regions formed by a DP reaction was determined by quantitative metallographic measurements. The kinetics of DP was evaluated on the basis of the Johnson–Mehl–Avrami–Kolmogorov equation, which resulted in a time exponent of approximately 1.5. We confirmed that the nucleation of the discontinuous precipitate was confined to grain edges or boundaries at an early stage of the reaction. The activation energy of DP process was determined to be approximately (72.7 ± 7.2) kJ/mol based on the Arrhenius equation; this result suggests that DP is controlled by grain boundary diffusion. The hardness values exhibited good correlation with the volume fraction of DP; this correlation was attributed to the presence of the ordered NiMn phase.

Advanced intermetallic γ-TiAl based alloys with improved microstructural stability during creep

ABSTRACTEnsuring microstructural stability under technical relevant conditions is a determining criterion for the development of innovative high-temperature materials. In this work, the influ-ence of C and Si on the microstructural stability during creep exposure was investigated for a β-solidifying γ-TiAl based alloy with a nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (in at.%), named TNM. With a two-step heat treatment a microstructure consisting of fine lamellar α2/γ-colonies, surrounded by βo-phase and areas of discontinuous precipitation, starting from the boundaries of the lamellar colonies, was adjusted. Creep tests were carried out to examine the potential of C and Si to prevent microstructural instability during creep and hence improving the creep properties. At 815 °C the discontinuous precipitation process of the TNM alloy continues during ensuing creep testing leading to a reduced creep resistance. In comparison, the minimum creep rate of the TNM-0.3C-0.3Si alloy was significantly decreased caused by the lower βo-phase content and average lamellar spacing within the α2/γ-colonies, the precipitation of p-Ti3AlC carbides and the retarded kinetics of discontinuous precipitation.

Influences of Co, Cu and V on Kinetics of Discontinuous Precipitation in the Ni^|^ndash;Cr System

Influences of Co, Cu and V on Kinetics of Discontinuous Precipitation in the Ni^|^ndash;Cr System

Effects of Fe, W and Mo on Kinetics of Discontinuous Precipitation in the Ni–Cr System

The effects of Fe, W and Mo on the kinetics of discontinuous precipitation in the Ni­Cr system were experimentally examined using a binary Ni­38Cr alloy and ternary Ni­38Cr­0.9Fe, Ni­38Cr­3.2W and Ni­38Cr­1.7Mo alloys. These alloys were homogenized at 1423K for 3h, solution treated at 1423K for 1h, and then isothermally annealed in the temperature range of 873­1023K for various times up to 2300h. Due to the solution heat treatment, all the alloys show the polycrystalline single-phase microstructure of the Ni-rich solid-solution (£) phase with the face-centered cubic structure. During isothermal annealing, however, the cell of the lamellar microstructure consisting of the £ phase and the Cr-rich solid-solution (i) phase with the body-centered cubic structure is formed along the grain boundary of the £ matrix and then grows into the £ matrix. At each annealing temperature, the migration distance of the moving cell boundary is proportional to the annealing time. Hence, the growth rate of the cell is constant independent of the annealing time. The growth rate and the interlamellar spacing of the cell almost monotonically increase with increasing annealing temperature at 873­1023K. The cell growth is decelerated by W and Mo but not by Fe. The deceleration is more remarkable for Mo than for W. A kinetic model for the binary discontinuous precipitation controlled by boundary diffusion was used to analyze quantitatively the experimental result. According to the analysis, W and Mo retard the boundary diffusion of Cr along the moving cell boundary, but Fe slightly hastens that. Such influence of the alloying component on the boundary diffusion is ascribable to the thermodynamic interaction between Cr and the alloying component in the moving cell boundary. As a consequence, it is concluded that the retardation of the Cr boundary diffusion causes the deceleration of the cell growth. [doi:10.2320/matertrans.M2012132]

Precipitation Kinetics and Mechanism in Cu-7 wt% Ag Alloy

The discontinuous precipitation kinetics and mechanism of the α (Ag-rich) phase in Cu-7 wt% Ag alloy has been investigated using dilatometric and calorimetric anisothermal analysis, optical microscopy, scanning and transmission electron microscopy and X-ray diffraction. Dilatometric and calorimetric curves present at ~ 500°C an important effect related to the ? (Ag-rich) phase formation and consequently the matrix β (Cu-rich) depletion. The nucleation and growth of the precipitated phase show cells formation at initial grain boundaries; a fine lamellar structure is detected by SEM and TEM and consists of alternate lamellar of the α (Ag-rich) and β (Cu-rich)-solid solutions. Cellular precipitation leads to the simultaneous appearance of two diffraction peaks and occurs apparently according to the Fournelle and Clark’s mechanism. Obtained results give an Avrami exponent n = 2.0 ± 0.2 in agreement with an interfacial controlled process having an activation energy Ea equals to 99 ± 7 kJ/mol obtained from anisothermal analysis by using different isoconversion methods. This activation energy expresses the discrepancy between isoconversion methods and the analytical diffusive model. Moreover, the supersaturation rate has an effect on the lamella spacing of the precipitated cells.

Influence of Al on Kinetics of Discontinuous Precipitation in Ni–38Cr Alloy

The influence of Al on the kinetics of discontinuous precipitation in the Ni–Cr system was experimentally examined using ternary Ni–38Cr–Al alloys with Al concentrations of 3.8 mass% and 4.2 mass%. The ternary alloys were homogenized at 1473 K for 16 h, solution treated at 1473–1493 K for 30 min, and then isothermally annealed in the temperature range of 773–1073 K for various times up to 50 h. Due to the solution heat treatment, both ternary alloys show the polycrystalline single-phase microstructure of the Ni-rich solid-solution (γ) phase with the face-centered cubic structure. In the early stages of isothermal annealing, however, fine particles of the Ni3Al (γ′) phase with the L12 structure are formed in the γ matrix by continuous precipitation. On the other hand, the cell of the lamellar microstructure consisting of the γ phase and the Cr-rich solid-solution (α) phase with the body-centered cubic structure is formed along the grain boundary of the γ matrix and then grows into the γ matrix. The fine particles of the γ′ phase are dispersed also in the γ phase of the cell. The migration distance of the moving cell boundary is proportional to the annealing time. Thus, the growth rate of the cell is constant independent of the annealing time. The growth rate and the interlamellar spacing of the cell monotonically increase with increasing annealing temperature. A kinetic model was used to analyze quantitatively the relationship between the growth rate and the interlamellar spacing. Although the discontinuous precipitation in the ternary Ni–38Cr–Al alloys occurs in a complicated manner, the analysis indicates that the growth of the lamellar cell is controlled by the boundary diffusion of Cr along the moving cell boundary.

Effect of heat treatments on discontinuous precipitation kinetics in Al-30 wt.% Zn alloy

The discontinuous precipitation kinetics in Al-30% wt. Zn alloy have been investigated at temperatures ranging from 348 to 503 K (75 to 230 °C) by using an optical microscopy, X-ray diffraction, differential dilatometer, differential scanning calorimetry and microhardness measurement. We have found that at all temperatures less than 180 °C the supersaturated solid solution of quenched alloy was observed to decompose completely by a cellular precipitation reaction. Quantitative metallography methods have been applied to measure the corresponding transformed volume fractions at different temperatures and times of precipitation. The variation of the heating rate and the application of different methods have allowed us to calculate two kinetic parameters of precipitation: the activation energy of the process and the Avrami exponent.

Mechanism and kinetics of discontinuous precipitation of the η phase in the 36NKhTYu alloy

A new mechanism of nucleation and growth of cells of the η phase in the deformed 36NKhTYu austenitic alloy has been found. The recrystallization nuclei are formed at the η-phase particles, which were precipitated according to the continuous mechanism. Simultaneously, they are nuclei of cells of discontinuous decomposition. Unlike the known growth mechanism, the η-phase cell grows in all directions until it collides with other cells. The Peterman-Hornbogen theory of discontinuous precipitation has been modified to describe the kinetics of the complex reaction of discontinuous precipitation of the η phase and recrystallization and also the kinetics of complex reaction of discontinuous coalescence of grains and phases. The theoretical dependences of the cell size on the aging time for these reactions are proportional to t 1/3 and t 1/4, respectively. A comparison of the theoretical and experimental results has been performed.

Morphology and kinetics of discontinuous precipitation and dissolution in an Fe-8.5Al-27Mn-1.0Si-0.92C alloy

The morphology and growth kinetics of discontinuous precipitation (DP) and discontinuous dissolution (DD) in an Fe-8.5Al-27Mn-1.0Si-0.92C alloy are investigated. The results indicate that the solid-solution-treated austenite phase decomposes into lamellar DP after aging at temperatures ranging from 900 to 1050 K. After the specimens of a preaged DP structure go through further aging at temperatures ranging from 1118 to 1173 K, the lamellar DP falls into dissolution because of the DD reaction. The lamellar spacings of the DP structure, as well as the reaction-front migration rates during DP and DD reactions, are measured. Based on Aaronson and Liu’s simplified kinetics model, the grain-boundary diffusivities are estimated. They are found to be slightly lower than the grain-boundary diffusion data of Mn in Fe and Fe in Fe as reported by Aaronson and Liu.

A Comparative Study of the Kinetics of Interface Diffusion Controlled Transformations In Fe-Zn Alloys

A comparative study of the kinetics of discontinuous precipitation, coarsening and dissolution, diffusion induced grain boundary migration and grain growth has been carried out in Fe-Zn alloys. Except for cells of the discontinuous coarsening reaction, rates of migration of the cell boundaries of precipitation and dissolution and diffusion induced grain boundary migration (DIGM) rates lie within an order of magnitude of each other. The rate of cell boundary migration during discontinuous coarsening is two orders of magnitude smaller than those for the four transformation processes mentioned above. The kinetics (of minimum rate) of grain growth which occurs after long annealing times is comparable to the discontinuous precipitation and coarsening in the Fe-Zn alloy. The kinetics (of maximum rate) of grain growth, which corresponds to short annealing times, are comparable to the discontinuous dissolution and DIGM. The diffusivity values lie within an order of magnitude of the boundary diffusivity during grain growth when considered at the same temperature (extrapolated). Although the driving force differs by four orders of magnitude from as low as 0.08 Jmol−1 for grain growth to 1420 Jmol−1 for DIGM at 923 °K, the small difference in the diffusivity (kDbδ) predicts that the conduit for solute transport is the same for the above transformations.On a effectué une étude comparative de la cinétique de précipitation discontinue, de grossissement et de dissolution, de migration de joints de grain induite par diffusion (DIGM) et de croissance de grain, d'alliages de Fe-Zn. À l'exception des cellules de réaction de grossissement discontinu, le taux de migration des joints de cellules de précipitation, de dissolution ainsi que le taux de migration de joints de grain induite par diffusion se situent à un ordre de grandeur l'un de l'autre. Le taux de migration du joint de la cellule lors du grossissement discontinu est de deux ordres de grandeur plus petit que celui des quatre procédés de transformation mentionnés ci-dessus. La cinétique (taux minimum) de croissance de grain qui se produit après une longue durée de recuit est comparable à la précipitation discontinue et au grossissement de l'alliage Fe-Zn. La cinétique (taux maximum) de croissance de grain qui correspond à de courtes périodes de recuit est comparable à la dissolution discontinue et à “DIGM”. Les valeurs de diffusivité se situent à un ordre de grandeur de la diffusivité du joint lors de la croissance de grain lorsque considérée à la même température (extrapolée). Bien que la force directrice diffère par quatre ordres de grandeur, allant de 0.08 Jmol−1 pour la croissance de grain jusqu'à 1420 Jmol−1 à 923 °K pour “DGIM”, la petite différence de diffusivité (kDbδ) prédit que le conduit pour le transport de soluté est le même pour les transformations ci-dessus.

Kinetics of discontinuous precipitation and type I discontinuous coarsening in Zn–4 at % Ag alloy

A detailed study of the kinetics of discontinuous precipitation (DP) and type I discontinuous coarsening (DCI) in Zn-4 at % Ag alloy is reported here for the first time. DCI succeeds DP during prolonged isothermal ageing. Both DP and DCI are characterized by a predominantly lamellar morphology of the precipitate phase, statistically constant interlamellar spacing and steady state reaction front (RF) velocity at a given temperature. The interlamellar spacing increases with temperature. The RF velocity shows a C-curve behavior for DP, but increases monotonically for DCI, as a function of temperature. DCI is distinguished from DP by a 3-5 times larger interlamellar spacing and 1-2 orders of magnitude lower RF velocity than those of DP under comparable conditions. DCI may be initiated from an interface between two DP colonies, a former DP-RF, or the free surface intersecting a DP colony. Kinetic analysis of DP using the models of Turnbull, Cahn, Hillert, and Petermann and Hornbogen, and of DCI using the modified Petermann and Hornbogen model (by Fournelle) have yielded grain boundary diffusivity data in the temperature range 353-573 K. Subsequent Arrhenius analysis shows that the activation energy of the DP and DCI processes lies between 50-66 kJ mol -1 . The latter is comparable with the activation energy of grain-boundary self-diffusion of Zn and is nearly half that of tracer impurity diffusion (volume/bulk) of Ag in Zn. Hence, it is concluded that DP and DCI are grain-boundary diffusion-controlled processes in the present alloy.