Metastable phases in the early stage of precipitation in Al-Mg alloys

The growth kinetics and thermal stability of metastable phases in Al-Mg alloys have been investigated by means of electrical resistivity measurements and calorimetry. From the observation of two distinct endothermal peaks in DSC thermograms, the existence of two types of metastable phases named as G.P. zones and β" phase was recognized. G.P. zones were defined as a metastable phase precipitating directly from the supersaturated solid solution. By analogy with other systems, the β" phase could correspond to an ordered phase with an Ll₂ structure. Aging behavior was divided principally into two stages. In the first stage, mainly G.P. zones precipitated, although both metastable phases coexisted over all the aging process. In the later stage, the β" phase dominated after aging beyond 100 ks at room temperature. The phase diagram for these two metastable phases was proposed, based on both the reversion experiments and the thermal analysis. Formerly Graduate Student with Kyoto University.     

A high-resolution transmission electron microscopy study of the precipitation process in a dilute Ti-N alloy

The precipitation processes in dilute nitrogen alloys of titanium have been examined in detail by conventional transmission electron microscopy (CTEM) and high-resolution electron microscopy (HREM). The alloy Ti-2 at. pct N on quenching from its high-temperatureβ phase field has been found to undergo early stages of decomposition. The supersaturated solid solution (α″-hcp) on decomposition gives rise to an intimately mixed, irresolvable product microstructure. The associated strong tweed contrast presents difficulties in understanding the characteristic features of the process. Therefore, HREM has been carried out with a view to getting a clear picture of the decomposition process. Studies on the quenched samples of the alloy suggest the formation of solute-rich zones of a few atom layers thick, randomly distributed throughout the matrix. On aging, these zones grow to a size beyond which the precipitate/matrix interfaces appear to become incoherent and theα′ (tetragonal) product phase is seen distinctly. The structural details, the crystallography of the precipitation process, and the sequence of precipitation reaction in the system are illustrated.     

The mechanism of nucleation and precipitation in 7075-0.7 Li alloy

The effect of 0.7 wt% Li on the nucleation mechanism and precipitation behaviour of 7075 has been studied. A nucleation mechanism has been suggested to explain the observed precipitation scheme: vacancy-rich G.P. zone→T′→T (Al,Zn)₄₉Mg₃₂ in 7075-0.7 Li which replaces the precipitation of solute-rich G.P. zone→ν′→ν MgZn₂ in 7075. According to this mechanism, the lithium-vacancies (Li-v) aggregates act as nuclei for subsequent clustering of Zn and Mg atoms, and lead to limited formation of vacancy-rich G.P. zones. The vacancy concentration, the ratio of Mg/Zn and the crystallographic structure are all modified inside zones because trapping of vacancies by Li inhibits the diffusion of Zn and Mg into zones. Therefore, the morphology, size, distribution, ageing kinetics and age-hardening are all changed. Moreover, narrow precipitation free zones (PFZs) are also produced. In contrast to the early inhibition of nucleation, promotion of T phase precipitation occurs on prolonged ageing at 120°C with the aid of crystal defects such as dislocations and voids. The inhibited nucleation and reduced hardening can be improved either by increasing the solution heat treatment (SHT) temperature or by applying two-stage ageing.     

A TEM study of microstructural changes during retrogression and reaging in 7075 aluminum

Microstructural changes occurring during retrogression, and during retrogression plus reaging in 7075-T6 aluminum alloy have been investigated by means of transmission electron microscopy, and related to mechanical properties. TEM results indicate that the drop in strength during the initial stage of retrogression was due to the partial dissolution of G.P. zones while the growth of the semi-coherentη ′ was responsible for the rapid recovery of strength. It is suggested that the retrogression and reaging treatment resulted in the increase in volume fraction of G.P. zones and especially η′ precipitates over both the T6 and retrogressed conditions, therefore significantly improving the strength of the alloy.     

The aging and tempering of iron-nickel-carbon martensites

The aging and tempering of freshly quenched (M_s > RT) and virgin (M < RT) martensites with lath and plate morphologies in Fe-Ni-C alloys have been studied to obtain kinetic and structural information. At subambient temperatures, the first change is attributed to isothermal conversion of a small amount of retained austenite or to slight relaxations in the martensite, but this is not a significant part of the martensite aging process. Aging above -40 °C to about 70 °C is accompanied by the diffusion-controlled clustering of carbon atoms, resulting in an increase in electrical resistivity proportional to the carbon content but independent of the martensitic morphology. This regime is followed above 100 °C by the precipitation of ε-carbide (i.e., the conventional first stage of tempering), which may emerge directly from the carbon-rich clusters. At still higher temperatures, cementite forms separately (i.e., the conventional third stage of tempering) in competition with the ε-carbide. These two precipitation processes overlap, and their kinetics appear to be controlled by iron-atom diffusion away from the growing carbide particles along dislocation paths. No evidence was found in this investigation for a regime reflecting carbon migration to dislocations or other defects, but this possibility is not ruled out by the experimental methods employed. This paper is based on a presentation made at the “Peter G. Winchell Symposium on Tempering of Steel” held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TM-AIME Ferrous Metallurgy and Heat Treatment Committees.     

The Aging and Tempering of Iron-Nickel-Carbon Martensites

The aging and tempering of freshly quenched (M_s > RT) and virgin (M_s < RT) martensites with lath and plate morphologies in Fe-Ni-C alloys have been studied to obtain kinetic and structural information. At subambient temperatures, the first change is attributed to isothermal conversion of a small amount of retained austenite or to slight relaxations in the martensite, but this is not a significant part of the martensite aging process. Aging above ?40 °C to about 70 °C is accompanied by the diffusion-controlled clustering of carbon atoms, resulting in an increase in electrical resistivity proportional to the carbon content but independent of the martensitic morphology. This regime is followed above 100 °C by the precipitation of ε-carbide (i.e., the conventional first stage of tempering), which may emerge directly from the carbon-rich clusters. At still higher temperatures, cementite forms separately (i.e., the conventional third stage of tempering) in competition with the ε-carbide. These two precipitation processes overlap, and their kinetics appear to be controlled by iron-atom diffusion away from the growing carbide particles along dislocation paths. No evidence was found in this investigation for a regime reflecting carbon migration to dislocations or other defects, but this possibility is not ruled out by the experimental methods employed.     

Effects of chromium and copper additions on precipitation in Al−Zn−Mg alloys

The effects of chromium and copper additions on precipitation in several Al?Zn?Mg alloys have been investigated. Results show that chromium additions heterogenize precipitation in aged Al?Zn?Mg alloys by creating special nucleation sites. Multirowed bands of incoherent precipitates appeared in the grain boundaries and subboundaries in an Al-5 pct Zn-2 pct Mg-0.1 pct Cr alloy. It is believed that fine nuclei associated with the existence of chromium-rich regions are formed during solidification and are retained after solution heat treatment. These nuclei would lead to the formation of incoherent precipitates during quenching and aging. Chromium is, therefore, considered to causehigh temperature nucleation. Copper additions to Al?Zn?Mg alloys accelerate precipitation at lower aging temperatures and increase the density of G. P. zones nucleated at relatively lower temperatures (20 to 90°C). In this way copper considerably strengthens Al?Zn?Mg alloys. Copper, in contrast to chromium, causes increased low-temperature nucleation of G. P. zones.     

The G.P. zones in AlCu alloys—I

Absolute measurements of the diffuse X-ray scattering from an Al-1.94 at.% Cu single crystal aged 1 hr at 353 K into the G.P.1 state have been made at a synchrotron source. It is confirmed that this state consists of a mixture of single and multilayer zones on {100} planes of the matrix, and essentially 100% copper. The multilayer zones have roughened or incomplete surfaces. Due to the high intensity and resolution of such an X-ray source, the interatomic displacements are obtained to a high precision for the first time. These displacements indicate that the interplanar strains parallel and normal to the zone faces oscillate in sign. For the single layer zones, the nearest Al plane collapses toward the zone by ~ 10%, whereas the next plane moves away by a similar amount. These displacements vanish near the fourth or fifth At plane. The strains near the multilayer zones are similar.     

Influence of thermal aging on the reactivity of duplex stainless steel surfaces

The annealing of large cast pieces in duplex stainless steel (SS) and the different heat cycles resulting from repairs involve significant structural changes characterized by carbide and intermetallic phase precipitation. This yields to lower local corrosion resistance in sea water due to changes in the local content of alloying elements. The precipitation of chromium carbide affects the resistance to the intergranular corrosion and the repassivation behavior. The eutectoidal decomposition of ferritic phase into regenerated austenite and in sigma phase (α → γ _r + σ) results in weakening the resistance to pit nucleation in synthetic sea water. In contrast, such precipitation will not have any significant effect when the treatment temperature is high enough to involve a rapid rehomogenization of depleted zones and ensure a self-healing.     

On the kinetics of discontinuous precipitation in an Al-14.6 at.% Zn alloy

The influence of the continuous precipitation on the discontinuous precipitation in an Al-14.6 at.% Zn alloy has been studied using optical microscopy, resistivity measurement, high angle X-ray study, small angle X-ray study, and transmission electron microscopy. Results showed that a considerable amount of decomposition (up to 9 ∼ 15 vol.%) takes place in the matrix through the prior continuous precipitation before the onset of the discontinuous precipitation at the grain boundary. The growth rate of the cellular structure due to the discontinuous precipitation appears not to be directly related with the available chemical free energy, nor with the available total free energy, but to be directly related with the strain energy associated with the prior continuous precipitation of coherent G.P. zones.     

Phase decomposition of rapidly solidified Fe-Mn-Al-C austenitic alloys

The phase decomposition of two (Fe_0.65Mn_0.35)_0.83Al_0.17-xC (x=3 and 4 at. pct orx=0.74 and 0.98 wt pct) austenitic alloys prepared by rapid solidification (RS) has been investigated on aging at 823 and 923 K by means of X-ray diffraction, transmission electron microscopy, and scanning electron microscopy. Under low bulk carbon supersaturation conditions (823 K aging of low carbon alloy and 923 K aging of high carbon alloy), zones formed preferentially at the cellular boundaries and in the bands in the {100} planes, giving rise to line broadening in the X-ray diffraction patterns. On the other hand, the initial aging under high, carbon supersaturation condition (823 K aging of high carbon alloy) resulted in the sideband formation, resulting from homogeneous structural modulation in the <100>_γ directions throughout the grain. The bulk carbon supersaturation dependence of initial decomposition modes indicates that carbon atom fluctuations are crucial in the initial state of phase decomposition, and that the observed {100} modulated structure corresponds to a structure consisting of alternate carbon-rich and carbon-poor zones. Together with the interstitial clustering process, an fcc-based substitutional ordering reaction concurrently took place. Later on these zones were replaced by a coherent metastable phase in the matrix, which was finally transformed into the cubic carbide (κ carbide) of (Fe, Mn)₃AlC_x chemical formula with the L'1₂ structure. However, at the end, a combined heterogeneous β-Mn and κ carbide precipitation seemed to finalize the decomposition process over the matrix κ carbide precipitation.     

The Structure and Kinetics of the Nanoscale Precipitation Processes in Al-1.0 wt pct Mg2Si-0.4 wt pct Mg-0.5 wt pct Ag Alloy

The influence of addition of 0.4 wt pct Mg on the precipitation sequence in the balanced Al-1.0 wt pct Mg₂Si bearing 0.5 wt pct Ag has been investigated during the continuous heating of the quenched alloy from the solid solution state. Differential scanning calorimetry (DSC) and high-resolution transmission electron microscopy techniques have been used. The DSC experiments showed that all processes occurred are thermally activated. The activation energies of the precipitation processes have been determined and hence the kinetics of these precipitates have been determined. The obtained results have shown that the existence of excess Mg inhibits the formation of the early stage clusters of solute-vacancy clusters. These clusters can be assisted by the binding energies between solute Si, Mg, and Ag atoms and the excess vacancies. On the other hand, excess Mg accelerates the precipitation of random, β′-phase and β-phase precipitates.     

Dynamical structure change during reversion in AlZn binary alloy

To give information on the dynamical property of reversion phenomena in Al-Zn alloy, time-resolved small-angle X-ray scatterings have been measured using synchroton radiation. The time-dependent dissolution rate of G.P. zones is well described by the Johnson-Mehl-Avrami equation. The experimental time-exponent and temperature-dependent relaxation time deviate largely from expectations, suggesting that the reversion process is not controlled by the diffusion of solute atoms in the matrix. It has been found that Guinier radius increases slightly during reversion.     

Clusters in carbon martensite: Thermodynamics and kinetics

An original method of evaluation of the cluster population in carbon martensite has been developed. Using this method, it is shown that Kurdjumov’s model of carbon redistribution within the different octahedral site sublattices can quantitatively account for both observed normal and abnormal tetragonality in carbon martensite. It is also shown that the existence of the internal strains in martensite constitutes a necessary and sufficient condition for the energetic preference of tetrahedral over the cubic lattice. The presence of the residual tetragonal distortion in the quasi-cubic phase of k-martensite is associated with the presence of the mixed clusters formed of the atoms belonging to O _c sublattice as well as to remaining ones. By using a computer simulation of the dynamical behavior of carbon martensite approaching the thermodynamical equilibrium, it was found that the ultimate state of this system is strongly beyond the thermal equilibrium. Even after long-term aging, the free energy is far beyond the minimum value allowed for this system. The reason for such a behavior and the possible aging processes proceeding in this system are discussed at the molecular level. All of the ordering parameters are affected by the aging process. The evolution proceeds in the distinctly different time intervals for different parameters. At first, the long-range ordering parameter that determines the tetragonality of martensite evolves and reaches the stable value. In the next stage, the formation and then disintegration of two-particle clusters occurs. Disintegration of two-particle clusters coincides with the stage when three-particle cluster formation occurs at a high rate. Threeparticle clusters also disintegrate when some time elapses. The same pattern repeats regarding four-, five-, six-, seven-, and eight-particle clusters. To simplify the calculations, the nine-particle clusters are assumed to be the largest possible and are identified with an existence of superstructure. The formation of 100 pct of nine-particle clusters with no contribution of free atoms in an alloy ceases all aging processes. The evolution of these processes is illustrated graphically in the time range from 16 seconds to 1500 years, as estimated on the basis of experimental data.     

The effect of multiple-step aging on the strength properties and precipitate-free zone widths in Al−Zn−Mg alloys

The effect of multiple-step aging treatments on the strength properties of Al-5 pct Zn?(2 and 4 pct)Mg alloys has been investigated. Some correlation has been made with precipitate-free zone widths and precipitation within the grains. Results show that the highest strengths are reached by multiple-step aging when the temperatures of all the aging steps are kept relatively low,i.e., less than about 120°C. The maximum strength is attained when the precipitate-free zone width is minimized and no evidence of the conversion of G.P. zones to M’ within the grains is observed in the electron microscope. A wide precipitate-free zone is indicative of a relatively low-strength, highly ductile Al?Zn?Mg alloy. Al?Zn?Mg alloys in the composition range studied when aged to maximum strength are believed to be principallychemically hardened. The high strengths exhibited by these alloys are attributed to the difficulty of dislocations to break the strong Zn?Mg bonds within the G.P. zones.     

Precipitation strengthening and thermally activated plastic flow in polycrystalline Al−Zn alloys

The influence of G. P. zone size and volume fraction on the yield strength of polycrystalline Al?Zn alloys has been determined. The data obtained at 77°K plot linearly against both R^1/3 f ^2/3 and R^1/2 f ^1/2, but the slope of the latter plot corresponds most closely to the value expected if coherency strains between the G. P. zones and the matrix are determinant. The data also show that precipitation occurs in the early stages of aging. Yield stess studies as a function of temperature for fully aged specimens indicate thatUc, the energy of thermally activated flow, is about 0.5 ev, nearly identical with the value obtained for unalloyed aluminum. This may indicate that thermally activated cutting of G. P. zones does not occur in these alloys.     

Effect of Sn Addition in Preprecipitation Stage in Al-Cu Alloys: A Correlative Transmission Electron Microscopy and Atom Probe Tomography Study

The effect of a trace addition of Sn (0.01?at. pct) in Al-1.7Cu (at. pct) alloy in the preprecipitation stage has been investigated by atom probe tomography (APT) and transmission electron microscopy (TEM). APT demonstrates that Sn clusters form independently of Cu in the as-quenched (AQ) state in Al-1.7Cu-0.01Sn alloy. The Sn clusters lead to the rapid nucleation of ??-Sn precipitates during increased temperature aging. The APT analysis also indicates that Cu tends to cluster with Sn when the increased temperature aging commences. The TEM experiments revealed that GP zones formed during the first 30?seconds of aging at 473?K (200?°C) in both alloys. These GP zones were unstable and underwent a reversion reaction such that they were not detected after 180?seconds of aging. This process is thought to arise from heating rate effects and, at these higher temperatures, supplies a flux of Cu atoms that results in the heterogeneous nucleation of the ???? (Al₂Cu) phase.     

Differential scanning calorimetry and electron diffraction investigation on low-temperature aging in Al-Zn-Mg alloys

Differential scanning calorimetry (DSC) has been combined with transmission electron microscopy (TEM) to investigate the low-temperature decomposition processes taking place in an Al-5 wt pct Zn-1 wt pct Mg alloy. It was confirmed that two types of GP zones, i.e., GP(I) (solute-rich clusters) and GP(II) (vacancy-rich clusters), formed independently during decomposition of the supersaturated solid solution. The GP(I) zones form at a relatively low aging temperature and dissolve when the aging temperature is increased. The GP(II) zones are stable over a wider range of temperatures. To investigate the nature of the zones in the Al-Zn-Mg alloy, differential scanning calorimetry and transmission electron microscopy have also been carried out on binary Al-Zn alloys containing 5 wt pct and 10 wt pct Zn. In these Al-Zn alloys, GP zones formed rapidly during quenching, and they gave rise to characteristic electron diffraction patterns identical to those from GP(II) in the Al-Zn-Mg alloy system, implying that GP(II) zones in Al-Zn-Mg alloys are very similar to the zones formed in binary Al-Zn alloys. Thus, it is likely that GP(II) zones in Al-Zn-Mg alloys are zinc-rich clusters. In the Al-5 wt pct Zn-1 wt pct Mg alloy, both GP(I) and GP(II) were found to transform to η′ and/or η particles during heating in the differential scanning calorimeter. The η′ was also observed to form after prolonged isothermal aging of the Al-Zn-Mg alloy at 75 °C or after short aging times at 125 °C.