Discontinuous Precipitation Cells Research Articles

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.

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.

Mg–Al系合金の不連続析出挙動とミクロ組織

The effects of addition of Zn on the discontinuous precipitation (DP) behavior have been metallographically examined in AM90 (AM) and AZ91Mg (AZ) alloys aged at 423, 448 and 473 K. Prior to the occurrence of a significant age-hardening within grains, DP cells in the alloys nucleate at grain boundaries and grow into the grains ahead of reaction fronts. The boundaries migrate at a constant rate at the early stage of aging. For both alloys, the cell growth rate and average inter-lamellar spacing increase with aging temperature. The cell growth rates for the AM alloy are slightly faster than those for the AZ alloy. The incubation periods to initiate DP are identical for the alloys. Kinetics analyses of DP using the model of Turnbull, and that of Petermann and Hornbogen have yielded grain-boundary diffusion data. The activation energies of boundary diffusion are 76 to 98 kJ/mol, depending on the models and the alloys. The area fraction of DP cells in the AZ alloy at the over-aging stage is smaller than that in the AM alloy at the same stage. The smaller area fraction in the AZ alloy is ascribed to the higher number density of continuous β-Mg17Al12 precipitates which successively suppress the boundary migration.

Bend formability and strength of Cu–Be–Co alloys

The effects of addition of 0.01 and 0.03 wt% Mg on the bend formability and strength of a Cu–1.8 wt% Be–0.21 wt% Co alloy aged at 320 °C for 30 min have been investigated metallographically. The addition of Mg to the Cu–Be–Co alloy enhances the bend formability and strength of the alloy. The enhancement of strength is caused by the increase in volume fraction of \( \gamma_{\text{I}}^{\prime } \) precipitates in the Cu matrix. In bending of the alloys with and without 0.01 and 0.03 wt% Mg, a number of micro necks first arise along grain boundaries, and part of them grows, resulting in surface wrinkles, which finally lead to surface cracking. The cracking is initiated from voids formed by destruction of bar-like γ precipitates in discontinuous precipitation (DP) cells and propagates along grain boundaries. The addition of Mg decreases the width of DP cells, resulting in better bend formability. This arises because smaller stress concentration due to less inhomogeneous deformation develops in cells and, as a result, destruction of the γ precipitates in cells occurs less easily as the cell width decreases.

Deformation of Cu-Be-Co Alloys by Aging at 593 K

The warping deformation along grain boundaries has been found for thin plates of Cu-1.8 mass%Be-0.2 mass%Co and Cu-1.8 mass%Be-0.2 mass%Co-0.03 mass%Mg alloys aged at 593 K for 3 h after cold rolling to 0% and 20% reduction, but not been detected for the two alloys aged at the same condition after cold rolling to 90% reduction, by surface roughness measurements. The aging produces γ′ precipitates in grains and discontinuous precipitation (DP) cells at grain boundaries in the two alloys after 0% and 20% cold-rolling, but no DP cells in the alloys after 90% cold-rolling. The addition of Mg or the increase in cold-rolling rate decreases the width of DP cells, resulting in reduction in the degree of warping deformation. The warping deformation along grain boundaries can be attributed to the difference between the length changes of the DP cell and the grain interior by the aging.

Effect of an applied stress on discontinuous precipitation in a Cu-Ag alloy

The effects of an applied tensile stress on the growth rate and morphology of discontinuous precipitation (DP) product have been studied for a Cu-5wt%Ag alloy aged at 300 °C. The DP cell consists of lamellae of the rod-shaped Ag precipitates and solute-depleted Cu matrix. The tensile stress accelerates the growth of DP cells along both the loading direction (LD) and transverse direction (TD) but the cell growth rate along the TD is faster than that along the LD. Transmission electron microscopy has revealed that the tensile stress is apt to produce the Ag precipitates elongated in a <110> direction nearly perpendicular to the LD in a cell, irrespective of the cell growth direction. The observed morphology of the Ag precipitates and the promotion of the cell growth, namely precipitate growth under tension can be understood through the interaction energy between the external stress and the misfit strains of precipitate. The growth of Ag precipitates toward the direction perpendicular to the LD explains the faster cell growth along the TD.

Influence of External Stress on Discontinuous Precipitation Behavior in a Cu-Ag Alloy

The influence of an applied stress on discontinuous precipitation (DP) has been investigated for a Cu-5 wt pct Ag alloy aged at 573 K. A tensile stress enhances the growth of DP cells in both the loading direction (LD) and transverse direction (TD), but the cell growth rate in the TD, v CT , is faster than that in the LD, v CL . A compressive stress suppresses the cell growth in the TD, but does not significantly influence it in the LD. The tensile or compressive stress tends to produce discontinuous silver precipitates elongated in a $$ \left\langle {110} \right\rangle_{\alpha } $$ direction of the copper matrix in a DP cell, nearly perpendicular to or parallel to the LD. This result, along with the dependence of the cell growth rate on the sense of the applied stress, can be understood through the interaction energy between the external stress and the misfit strains of silver precipitates. An analysis of length-change measurement results has enabled estimation of the ratio of the growth rate of silver precipitates in the TD, v PT , to that in the LD, v PL , under tension or compression. The estimated values of v PT /v PL are in good agreement with the values of v CT /v CL , experimentally obtained, indicating that the anisotropy of the cell growth rate is determined by the growth behavior of silver precipitates.

Effect of external stress on discontinuous precipitation in a Cu–2.1 wt % Be alloy

The influence of an applied stress on discontinuous precipitation (DP) in a Cu–2.1 wt % Be alloy aged at 300°C was examined. A compressive stress accelerates the growth of DP cells, which consist of lamellae of the precipitated γ phase and the solute-depleted α phase, but a tensile stress does not essentially change it. The cell growth rates along the loading direction under the compressive and tensile stress are identical to those along the direction perpendicular to the loading direction under the same stress. Both the compressive and tensile stresses have no influence on the incubation time to initiate DP. From measurements of the specimen length change and investigations of the distribution of γ variants in cells in a particular case, specific γ variants among crystallographically equivalent ones are found to be formed, depending on the sense of the applied stress. This result, together with the dependence of the cell growth rate on the sense of the applied stress, can be well understood through the interaction energy between the external stress and the misfit strains of discontinuous γ precipitates.

Effects of Solute Segregation on Precipitation Phenomena and Age Hardening Response of High-Purity and Commercial AZ91 Magnesium Alloys

In the present study, the continuous and discontinuous precipitation behavior of the commercial AZ91 alloy containing manganese and the AZ91 alloy produced from high-purity elemental components with no manganese addition has been investigated. It was found that for the commercial alloy, the solute manganese segregated within the primary α-Mg dendrites during solidification and played a major role in the initiation of non-uniform distribution of β-precipitates within the matrix grains. In addition, the solute manganese significantly suppressed the growth of discontinuous (lamellar) precipitates. The high-purity alloy also exhibited non-uniform precipitation. However, the precipitation patterns differed from those observed in the commercial alloy due to the inhomogeneous distribution of aluminum, the origin of which was rooted in the solute segregation within the interdendritic regions of the solidification structure. These differences in the precipitation mode caused by the presence or absence of manganese influenced the age hardening of the alloys. The high-purity specimens aged at lower temperatures attained higher peak-hardness owing to greater area fractions of discontinuous precipitation cells.

Transformation mechanism of lamellar microstructure of AZ80 wrought Mg alloy during warm deformation

The microstructure especially the lamellar second phase evolution by a combination of deformation and heat treatment for AZ80 alloy was investigated. The results show that there are finer lamellar Mg 17Al 12 phases after hot compression with the increasing strain, while there are coarse lamellar discontinuous precipitation cells of β-Mg 17Al 12 phase spreading from the grain boundaries into the grains after T6 treatment of the compressed samples. The lamellar morphologies especially the lamellar distance of β-Mg 17Al 12 phase precipitation of the T6 treated deformation specimen at different strains differ from each other as there are different grain boundaries in the corresponding compressed specimens.

Practical aspects of discontinuous precipitation and dissolution

A method of grain refinement which does not require plastic deformation is discussed. The refinement is obtained by a carefully designed thermal treatment in which the discontinuous precipitation (DP) reaction is followed by a dissolution process. Two different mechanisms of refinement were identified based on the investigations performed on Al–22at.% Zn and Ni–4at.% Sn alloys. The first one is associated with holding the orientation relationship of the former DP process. Such a behaviour was observed in the Al–22at.% Zn alloy in which the grains are refined by a factor of 7–8. If the small grains are created via recrystallisation within the area previously covered by a DP cell, like in Ni–4at.% Sn alloy, a refinement factor of 40 or even more can be obtained.

Diffusion Along Migrating Grain Boundaries

The paper summarizes recent experiments on diffusion at migrating grain boundaries (GBs) occurring during discontinuous reactions, like discontinuous precipitation (DP) and diffusion induced grain boundary mi- gration. Analytical electron microscopy was used for measurements of the solute concentration across individual solute-depleted α lamellae. These data combined with information on the growth velocity and the thickness of an individual lamella allowed the determination of the local values of the diffusivities of the moving reaction front of the DP cell in Al-Zn, Ni-Sn, Cu-In and Co-Al alloys. The obtained diffusivities and activation energies are very similar to the relevant parameters of stationary GBs. This allows us to conclude that there is no significant difference in the rates of diffusion along migrating and stationary GBs in the systems investigated. It is therefore believed that the diffusivity values of the moving reaction front of the DP reaction can be a source of reliable information on interfacial diffusion characteristics, especially in systems and/or at temperatures where radiotracer data are not readily available.

Correlation between grain boundary misorientation and the discontinuous precipitation reaction in Mg–10 wt.% Al alloy

The correlation between the geometry of the grain boundaries (GBs) and the kinetics of the discontinuous precipitation (DP) reaction in a Mg–10wt.% Al alloy has been studied. The GBs were categorized as special or random by using the electron back-scattered diffraction (EBSD) technique and theoretical tables of the coincidence site lattice. The analysis showed convincingly the absence of low-angle GBs and no distinguished maximum on the GB misorientation angle distribution in this hexagonal system. The DP cells appeared predominantly at nonspecial (random) orientations, confirming that the initiation and growth of the reaction products occur at high-angle GBs. Generally, some orientations which are inactive after 20min became active after 40min of ageing at 500K. For each rotation axis, there were several misorientation angles (no special ones) for which a maximum growth distance of DP reaction was observed.

Discontinuous precipitation at [001] twist boundaries in Cu-0.75 wt pct Be alloy bicrystals

The misorientation dependence of discontinuous precipitation (DP) at [001] twist boundaries in Cu-0.75 wt pct Be alloy bicrystals has been systematically investigated in the temperature range 523 to 723 K. A good correlation is found between both the incubation period, τ, to initiate DP and cell growth rate, ν, and the energy of boundaries. The maximum of τ and the minimum of ν occur where the cusps of the boundary energy exist. The formation and growth of DP are easier at higher-energy boundaries. The DP cells at [001] twist boundaries nucleate and grow less than those at [001] symmetric tilt boundaries in the same-alloy bicrystals. A kinetic analysis of DP using the models of Turnbull and Petermann and Hornbogen has yielded grain-boundary diffusion data. Although the activation energy, Q b , of boundary diffusion changes with the models, the values of Q b are smaller than the activation energy for volume diffusion of Be in Cu. The diffusivity in a boundary shows a close correlation with the energy of the boundary. A lower-energy boundary has a lower diffusivity with a larger activation energy and a larger pre-exponential factor.

Ｍｇ‐Ａｌ合金における不連続析出セルの生成および成長

Ｍｇ‐Ａｌ合金における不連続析出セルの生成および成長

Effect of continuous cooling on the decomposition processes in a Zn–1.6 wt.% Al alloy

The decomposition processes in a two-phase Zn–1.6 wt.% Al alloy, homogenized and quenched to room temperature through continuous cooling (1.5 Kmin−1), have been studied by scanning electron microscopy. At the beginning of cooling, continuous precipitates were formed at grain boundaries and in grain interiors (α0 phase, Zn-rich solid solution), with simultaneous eutectoid decomposition of non- stoichiometric intermetallic ZnAl (β phase). As the cooling proceeded, the density of continuous precipitates at grain boundaries and in grain interiors increased and at the same time the coarsening of eutectoid phase initiated. Although some precipitates at grain boundaries have grown by a `pucker mechanism', the occurrence of discontinuous precipitation was not observed during cooling. After a long period of aging, however, discontinuous precipitation cells were formed through basically two stages: (a) precipitation of a metastable Al-rich phase on the continuous precipitates formed at grain boundaries and (b) nucleation and growth of discontinuous precipitation cells within the metastable Al-rich phase.

Discontinuous precipitation in a Zn-1.6 wt.% Al alloy

The decomposition processes in a Zn-1.6 wt.% Al alloy, homogenized at 628K and subsequently water quenched, have been studied by scanning electron microscopy during homogenization and isothermal aging at 296K. The presence of the metastable ZnAl phases was verified during homogenization, and their dissolution has given rise to the discontinuous precipitation reaction. The discontinuous precipitation cell structure consisted of the rodlike precipitates interposed in the matrix of Zn-rich solid solution. The interrod spacing coarsened during aging because of the interaction between the discontinuous precipitation cells and the continuous matrix precipitates. From the calculated volume fractions of matrix and precipitated phases during homogenization and aging, probable mechanisms of dissolution of metastable phases and discontinuous precipitation formation have been proposed.

Cellular intergrowth between quartz and sodium-rich plagioclase (myrmekite) ? an analogue of discontinuous precipitation in metal alloys

Gneissic rocks of the contact aureole of the Traversella Intrusion (N. Italy) reveal a cellular intergrowth of quartz and oligoclase (myrmekite), originating from alkali feldspar grain boundaries. The myrmekite occurs in a temperature range of 500 to 670°C. The size of the cells and the lamellar spacing of the quartz rods increase with rising temperature. A comparison of the characteristic features of myrmekite cells with discontinuous precipitation cells of alloys indicates many similarities between the reactions. A is concluded that the formation of myrmekite is a solid-state reaction; a supersaturated alkali feldspar decomposes into a cellular structure growing behind a moving incoherent grain boundary. Micro probe analyses of the original alkali feldspar, the reaction products, and the volume relationship of the reactants (quartz, sodium-rich plagioclase) show that an ion exchange occurs during the reaction (K+ removed; Na+, Ca2+ and Si4+ added). A model describing the early stages of the formation of myrmekite is presented.

Discontinuous precipitation in Cu-Mg alloys

The relation between discontinuous precipitation and continuous precipitation and the cell growth kinetics of the discontinuous precipitation in Cu-Mg alloys containing 2.0, 2.6 and 3.2 wt % Mg have been investigated, mainly by metallographic observation. The volume fraction of cells, the cell width and the interlamellar spacing have been determined by quantitative metallographic measurements. The cell growth rate decreases progressively with ageing time after the initial linear growth of the cells. This may be attributed to the influence of continuous precipitation on the cell growth. The volume fraction of the discontinuous precipitation cells,f, can be represented by the Johnson-Mehl equation: f=1−exp (−btn). The value of the parameter n is about 2 and is independent of both the ageing temperature and alloy composition in the ageing range where the cell growth rates are constants. Mass transport of magnesium during the linear growth of cells occurs by grain boundary diffusion in a Cu-Mg solid solution.

Cu-Mg合金の不連続析出速度について

The relation between the discontinuous precipitation and the continuous precipitation, and the kinetics of the discontinuous precipitation in Cu-Mg alloys containing 2.0, 2.6 and 3.2 mass%Mg have been investigated by metallographic observations and micro-hardness measurements. Variations in cell width and volume fraction of discontinuous precipitation cells with ageing time have been determined in the specimens aged at temperatures from 573 K to 873 K.Results obtained are as follows: (1) The decomposition of Cu2Mg from supersaturated solid solutions of magnesium in copper during ageing is initiated by a discontinuous precipitation and followed by a continuous precipitation. (2) The cell growth is associated with the migration of a grain boundary. (3) The cell growth rate decreases with ageing time progressively after the linear growth rate of cells. This decrease will be attributed to the influence of the continuous precipitation on the cell growth. (4) The volume fraction of the discontinuous precipitation cells, f, can be represented by Johnson-Mehl type equation: f=1−exp(−btn), (where, t is the ageing time and b and n are constants). The value of the parameter n is 2 and independent of ageing temperature and alloy composition in the range of ageing time in which cell growth rates are constant.