The effect of cold work on the precipitation of Ω and θ′ in a ternary Al-Cu-Mg alloy

The effect of plastic deformation on the microstructural evolution of an Al-5.0Cu-0.5Mg (wt pct) ternary alloy was investigated. Hardness measurements and quantitative precipitate analysis were performed on specimens that were water quenched from a solution heat treatment, stretched either 0 or 6 pct and immediately aged at ambient temperature or artificially aged at 200 °C or 250 °C for times up to 3000 hours. Quantitative transmission electron microscopy (TEM) was used to characterize Ω and θ′ number density, diameter, and thickness as a function of preage mechanical stretch and artificial aging condition. Age hardening curves for naturally and artificially aged specimens revealed an increase in hardness corresponding with a preage stretch. Quantitative TEM verified an increase in the number density and a refinement of precipitates for both Ω and θ′ between the 0 and 6 pct stretch condition for those samples artificially aged. When aged at 200 °C, θ′ exhibited superior coarsening resistance relative to the Ω phase. The quantified Ω coarsening kinetics were greater than similar Ag-containing alloys. To investigate the effects of trace Si additions on subsequent microstructural evolution, a series of Al-Cu-Mg-Si quaternary alloys were produced. The addition of 0.1Si (wt pct) was found to suppress Ω precipitation in most Al-4.0Cu-xMg alloys investigated. These initial results indicate that Ω precipitation may be related to the Mg/Si ratio.     

Characterisation of solution annealed VT-14 titanium alloy using eddy current based electrical resistivity measurements

This paper discusses characterisation of microstructures in Ti-4.5Al-3Mo-1V (VT-14) alloy specimens solutionized in the temperature range of 923K–1373K, using room temperature electrical resistivity measurements. Using phase angle of impedance change in eddy current coil as a parameter, calibration curve between resistivity and phase angle has been established using three reference specimens of known resistivity. The changes in the electrical resistivity of the specimens have been correlated with optical microscopy investigations. The electrical resistivity of unstable β phase has been compared with α and α′ phases and possible reasons for higher resistivity of β phase and lower resistivity of α′ have been given based on the rule of mixtures and scattering and mobility of electrons. The electrical resistivity of α′ martensite phase of the chosen VT-14 alloy has been determined as 1.04 αΩ-m. The studies reveal that it may be possible to identify the β transus temperature of α+β titanium alloys from the electrical resistivity data.     

The effect of ternary trace additions on the nucleation and growth of γ′ precipitates in an Al-4.2 At. Pct Ag alloy

The role of trace elements on the nucleation and growth kinetics of γ′ precipitates in an aged Al-4.2 at. pct Ag alloy was determined. Comparing the characteristic properties of different trace elements on the precipitate dimensions and density after aging showed that a combination of high solute/vacancy binding energy and diffusivity was required to significantly affect the length, thickness, and density of γ′ precipitates. The elements which had the most pronounced effect were In and Sn, and the high solute/vacancy binding energy of In and Sn appears to be largely electronic in origin. The mechanism by which these trace elements affect the micro-structure is to reduce the number of quenched-in dislocation loops for heterogeneous nucleation of γ′ precipitates. The decreased number of nucleation sites leads to a prolonged matrix super-saturation, which causes an increase in the length, thickness, and ledge density of γ′ precipitates. Formerly with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University Formerly with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA     

Coprecipitation of Ω and σ phases in Al-Cu-Mg-Mn alloys containing Ag and Si

The individual influence of small additions of Ag and Si on the nucleation of the Ω (i.e., a chemically modified coherent form of ϑ-Al₂Cu) and σ (Al₅Cu₆Mg₂) phases, respectively, in Al-Cu-Mg alloys has been known. These phases nucleate directly and exhibit reduced rates of coarsening at the commercial aging temperatures. Alloys containing a uniform and fine distribution of these phases may, therefore, be of interest for further investigation for applications at temperatures below 200 °C. In a recent study, using an Al-Cu-Mg-Mn-Ag-Si alloy aged at 180 °C, it was shown that the Ω phase formed as a major precipitate nucleated predominantly upon the Mn-bearing dispersoids, while the σ phase was present as a minor one. This article describes the conditions under which widespread nucleation of σ phase may occur at unidentified sites in the matrix as well as upon the Mn-bearing dispersoids in the alloy. Widespread nucleation of σ phase begins in the alloy following the onset of dissolution of Guinier-Preston-Bagaryatski (G-P-B) zones that form early in the aging cycle as the major precipitate. It is established that nucleation of σ as a major phase precipitate requires a critical minimum supersaturation of Si in the solid solution. This article further points out that several constituent phases (implying those which form as solidification products and survive the homogenization treatment) together with the Mn-bearing dispersoids dissolve Si, thereby considerably reducing the Si supersaturation in the solid solution. The implications of these observations are discussed in view of the available data on the nucleation of σ phase in such alloys.     

Some aspects of the precipitation of metastable intermetallic phases in INCONEL 718

Some aspects of the precipitation of the metastable intermetallic phases —γ″ and γ″—in the commercial nickel base superalloy, INCONEL 718, have been investigated over a wide range of aging temperatures. It has been confirmed that the spherical γ″ particles and the ellipsoidal γ″ particles evolve predominantly through homogeneous nucleation. Precipitation of the former does not appear to precede that of the latter in this alloy. The tetragonal distortion associated with the γ″ particles has been found to increase with increasing precipitate size. It has been observed that at certain temperatures, physical association between precipitates of the two types occurs frequently, leading to the development of different composite precipitate morphologies. During coarsening, the precipitate size has been found to depend linearly on the cube root of the aging time for γ′ as well as γ″ particles.     

Effect of silicide precipitation on tensile properties and fracture of alloy Ti-6Al-5Zr-0.5Mo-0.25Si

The effect of silicon in solid solution and in the form of suicides has been studied on the tensile properties and fracture behavior of alloy Ti-6Al-5Zr-0.5Mo-0.25Si (alloy 685). The heat treatment to hold silicon in solid solution consists of solutionizing at 1323 K for 0.5 hour under vacuum (∼10^-5 MPa), followed by water quenching, and the treatment to precipitate suicides involves subsequent aging of the solutionized and water quenched specimens at 1073 K for 24 hours. There is only marginal effect of the aging treatment on strength values; however, the ductility parameters are found to be drastically reduced. There are marked differences in the fracture behavior of the alloy in the as-quenched and the quenched and aged conditions. While the fracture surface of the unaged specimen shows characteristic dimples, there is a large number of facets on the fracture surface of the aged specimen. The facets in the central region are relatively smaller in size than those in the peripheral zone. The central facets show fluted features at higher magnifications; however, the peripheral facets are usually featureless. The faceted fracture in the aged condition is attributed to enhanced tendency for heterogeneous planar slip. The fracture characteristics correlate with the observed differences in the ductility in the two conditions.     

Experimental observations on the nucleation and growth of δ′ (Al3Li) in dilute Al-Li alloys

The influence of nucleation temperature and pre-aging thermal history on δ′ (Al₃Li) precipitate distributions was studied in a binary Al-8.2 at. pct Li alloy. The distribution of δ′ in the microstructure is found to be homogeneous and insensitive to variations in pre-aging thermal history when nucleation occurs at large undercoolings (ΔT > 90 °C). When nucleation occurs at low undercoolings (90 °C > gDT > 40 °C), nucleation is homogeneous but the precipitate number density is influenced by preaging thermal history. At very small undercoolings (ΔT < 40 °C) nucleation next to dislocations is favored unless large numbers of excess vacancies are present, in which case homogeneous nucleation can be more prevalent than nucleation next to dislocations. The early stages of δ′ growth are seen to be consistent with bulk diffusion control. Enhanced growth rates are observed when the material contains a higher concentration of excess vacancies. The retention of this enhanced growth for long periods of time suggests that vacancies can be trapped within the structure for long times despite their mobility. Morphological instabilities are found to develop during δ′ particle growth. These instabilities occur at very small particle sizes and cause the normally spherical particles to tend toward a dendritic morphology.     

Effect of silicide precipitation on tensile properties and fracture of alloy Ti-6Al-5Zr-0.5Mo-0.25Si

The effect of silicon in solid solution and in the form of suicides has been studied on the tensile properties and fracture behavior of alloy Ti-6Al-5Zr-0.5Mo-0.25Si (alloy 685). The heat treatment to hold silicon in solid solution consists of solutionizing at 1323 K for 0.5 hour under vacuum (∼10^-5 MPa), followed by water quenching, and the treatment to precipitate suicides involves subsequent aging of the solutionized and water quenched specimens at 1073 K for 24 hours. There is only marginal effect of the aging treatment on strength values; however, the ductility parameters are found to be drastically reduced. There are marked differences in the fracture behavior of the alloy in the as-quenched and the quenched and aged conditions. While the fracture surface of the unaged specimen shows characteristic dimples, there is a large number of facets on the fracture surface of the aged specimen. The facets in the central region are relatively smaller in size than those in the peripheral zone. The central facets show fluted features at higher magnifications; however, the peripheral facets are usually featureless. The faceted fracture in the aged condition is attributed to enhanced tendency for heterogeneous planar slip. The fracture characteristics correlate with the observed differences in the ductility in the two conditions.     

Microstructure and hydrogen performance of alloy 903

Transmission electron microscopy has been used to determine that the strengthening precipitate in Incoloy 903 is γ′ (Ni₃ (Al, Ti)), with no evidence of γ″ (Ni_xNb), although the latter has been reported. In addition, the alloy does not undergo a martensitic transformation when quenched to 4 K in the solution treated or aged conditions. Aged tensile specimens charged with 1200 at. ppm of hydrogen exhibited ductility losses when compared to uncharged specimens. The hydrogen-charged specimens failed in a ductile manner, the fracture being characterized by a finer dimple size than observed in uncharged specimens. The ductility loss is shown to be a function of the dimple size reduction (expressed as the ratio of dimple size with hydrogen to that without hydrogen) and can be explained in terms of accelerated dimple nucleation due to hydrogen accumulation at precipitate particles.     

The effect of orientation and sense of applied uniaxial stress on the morphology of coherent gamma prime precipitates in stress annealed nickel-base superalloy crystals

The coarsening of coherent γ′[Ni₃(Al, Ti)] precipitates in single crystals of a representative nickel-base superalloy, Udimet-700, is shown to be affected by a uniaxial stress applied during annealing. Depending on the sense of the applied stress and its crystallographic orientation, stress annealing results in oriented cuboidal, plate, or parallelepiped shaped γ′ precipitates. A general thermodynamic analysis of the effect of stress annealing on precipitate morphology is presented that takes into account free energy changes due to changes in bulk precipitation strain, effective modulus, coherency strain energy, and the total interphase boundary area. The analysis correctly predicts the observed γ′ precipitate morphologies as a function of stress axis orientation, stress sense, the lattice misfit of the precipitate phase, and the elastic constants of the matrix and precipitate phases. The analysis also shows that stress induced morphological changes can be completely precluded, as may be desired to optimize mechanical behavior, only if the elastic constants of the matrix and precipitate phases are equal. Changes in morphology due to changes in bulk precipitation strain, which in the case of Udimet-700 is shown to be the dominant effect, can be eliminated by alloying for zero lattice misfit or, in single crystals, by stressing parallel to < 111> . Applications to long-term creep behavior and to the fabrication of composite structures are discussed. Formerly with Advanced Materials Research and Development Laboratory, Pratt & Whitney Aircraft     

The effect of potential and aging on the Pb-assisted stress corrosion cracking susceptibility of alloy 22 gas tungsten arc-welded weldments

The susceptibility of as-received, solutionized, and short-term thermally aged mill-annealed (MA) and gas tungsten arc-welded (GTAW) alloy 22 to Pb-assisted stress corrosion cracking (PbSCC) was evaluated in supersaturated, deaerated, acidic PbCl₂ solutions at 95 °C. Anodic polarization tests in acidic PbCl₂ solutions showed that 16,000 ppm of Pb produced a strong anodic peak and an order of magnitude greater passive current density for both MA and GTAW alloy 22 as compared to pure NaCl solutions. Current spikes were also observed in the anodic polarization plots for the PbCl₂ solutions, suggesting periodic events of passivity breakdown and repassivation. Constant deformation SCC tests were conducted using double U-bend samples of as-received, solutionized, and thermally aged MA and double U-groove welded alloy 22 plates. The results indicate that as-received, solutionized, and thermally aged MA and GTAW alloy 22 were resistant to PbSCC in supersaturated PbCl₂ solutions at 95 °C, pH 0.5, and applied potentials near the anodic peak ranging from −100 to 50 mV_SCE. Enhanced dissolution of alloy 22 was also observed in the crevice region of the double U-bend samples tested in the 16,000 ppm PbCl₂ solutions. This Pb concentration is seven orders of magnitude greater than that found in the anticipated repository environments, and chemical speciation modeling showed that Pb²⁺ is strongly immobilized in J-13 Yucca Mountain waters through the precipitation of PbCO₃ solids. Therefore, although enhanced dissolution of the inner U-bend did occur in our tests, the overall results from this PbSCC investigation suggest that as-fabricated, solutionized, and aged MA and GTAW alloy 22 are resistant to SCC in extremely aggressive, acidic, and supersaturated PbCl₂ solutions at 95 °C. Provided that these high Pb concentrations are not attainable in the anticipated repository environments, alloy 22 is unlikely to be susceptible to SCC, localized corrosion, and enhanced dissolution by the presence of Pb. This article is based on a presentation made in the symposium “Effect of Processing on Materials Properties for Nuclear Waste Disposition,” November 10–11, 2003, at the TMS Fall meeting in Chicago, Illinois, under the joint auspices of the TMS Corrosion and Environmental Effects and Nuclear Materials Committees.     

A fine γ′+α cellular structure in Fe-37.3 wt pct Ni-3.6 wt pct Al-3.3 wt pct Ti-0.2 wt pct C and its influence on high-temperature tensile properties

Fe-37.3 wt pct Ni-3.6 wt pct Al-3.3 wt pct Ti-0.2 wt pct C alloy, which reveals an excellent combination of high strength and good elongation endowed by formation of homogeneously dispersed fine γ′ precipitates in the matrix during aging at 823 K, has been investigated by means of transmission electron and optical microscopies, electron diffractions, and tensile tests. The influence of unique γ′+α cellular products on the mechanical properties has also been studied. Because of low elastic mismatch between the austenitic γ matrix and isomorphic γ′ precipitate phases, the homogeneously distributed precipitate particles, which formed at the early stage of aging, were observed to persist even after long-term aging. After very lengthy aging, the fine γ′ phase particles were changed to coarser γ′ lamellae at the grain boundary reaction front, which were alternately arranged with fine α lamellae that were estimated to have been transformed from the austenite-stabilizing-solute(Ni, C)-depleted γ lamellae. The fine duplex γ′+α cellular product did not affect deleteriously the room-temperature tensile properties of the alloy. However, the cellular structure was observed to cause the grain boundary embrittlement of the aged alloy at elevated temperatures higher than 681 K.     

The development of γ-γ′ lamellar structures in a nickel-base superalloy during elevated temperature mechanical testing

Changes in the morphology of the γ′ precipitate were examined during creep and tensile testing at temperatures between 927 and 1038 °C in [001]-oriented single crystals of a model Ni-Al-Mo-Ta superalloy. In this alloy, the γ′ particles link together to form lamellae, or rafts, which are aligned with their broad faces perpendicular to the applied tensile axis. The dimensions of the γ and γ′ phases were measured as the lamellar structure developed and were related to time and strain in an attempt to trace the changing γ-γ′ morphology. The results showed that directional coarsening of γ′ began during primary creep, and the attainment of a fully developed lamellar structure did not appear to be directly related to the onset of steady-state creep. The rate of directional coarsening during creep increased as the temperature was raised and also increased as the stress level was raised at a given testing temperature. The raft thickness remained equal to the initial γ′ size from the start of the creep test up through the onset of tertiary creep for all testing conditions. It was found that extensive rafts did not develop during the shorter testing times of the tensile tests, and that tensile testing of pre-rafted structures did not alter the morphology of the rafts. The overall behavior of the alloy was a clear indication of the stability of the finely-spaced γ-γ′ lamellar structure.     

The morphological changes ofγ′ precipitates in a Ni-8Al (wt pct) alloy during their coarsening

When a Ni-8A1 (wt pct) alloy is aged at 1020 °C (30 °C below the solvus temperature of about 1050 °C) by cooling directly from the solution-treatment temperature, theγ′ precipitates undergo Ostwald ripening from the early stage of the aging treatment because the nucleation rate is relatively high. Theγ′ precipitates are initially spheres and, as they coarsen, become cubes which then split into eight smaller cubes (octets) or clusters with square and rectangular faces. The observed critical sizes for these shape transformations are fairly close to those predicted by Khachaturyanet al. on the basis of the elastic strain effect arising from lattice misfit if the lattice parameters measured by Hornbogen and Roth are used. As the octets coarsen, they appear to transform into doublets of plates. Because shapes other than those considered by Khachaturyanet al. are also observed, only the sphere-cube and cube-octet transformations can be compared with the theoretical prediction without ambiguity. At temperatures above 1020 °C, dendritic growth occurs, and at low temperatures, the precipitate number density is so high that apparently the elastic fields of the precipitates overlap, rearranging the precipitates into aligned cubes or plates. These observations show that the shape evolution of the isolated precipitates can be observed only in a narrow aging temperature range where the nucleation rate is moderately high but where the precipitate volume fraction is relatively low.     

Decomposition of the γ solid solution in a nickel-based cast alloy at a high uniform pressure

The formation of γ′ particles in an as-cast nickel alloy at barothermal treatment temperatures of 1235, 1260, and 1320°C and a pressure of 175 MPa for an action time of 2 h (isobar-isothermal holding (IIH)) is studied by quantitative metallography using optical and electron microscopy and image processing computer programs. An analysis of images of the dendritic structure of the alloy, which is formed by the morphology and sizes of γ′ particles, demonstrates an increase in the degree of homogeneity of the alloy with the IIH temperature. The concentration of γ′ particles is determined in the initial alloy and after barothermal treatment (BTT) at temperatures of 1235, 1260, and 1320°C, and the volume fraction of the γ′ phase is found to significantly decrease as the IIH temperature increases. An activation mechanism is suggested for the formation of the nucleation centers of γ′ particles in the γ solid solution, and the activation energy of the dissolution/coalescence of γ′ precipitates is determined. The fraction of nonequilibrium γ′ particles is determined in the material in the initial state and after BTT at three IIH temperatures. The precipitation of γ′ particles is characterized by a bimodal character with the formation of nanoparticles in the initial material and upon BTT at temperatures of 1235 and 1260°C.     

Rapidly solidified P/M X2020 aluminum alloys

X2020 aluminum alloys were produced with variations in the Li/Cu ratio by the ultrasonic gas atomization process. In alloy 68 (Al-4.9Cu-l.2Li) and 69 (Al-4.4Cu-l.55Li) alloys, the Θ′ and T₁, phases are dominant with evidence of the T_B phase. In the 70 (Al-3.5Cu-2.8Li) alloy, the δ′ phase is dominant with a trace of T₁. It was found that Θ′ andT ₁ are effective strengtheners whereas δ′ provides excellent fatigue crack initiation resistance. Overall results indicate that the fracture behavior of three RS-PM X2020 alloys is closely related to alloy production route as well as to the phases present in the alloys. Formerly Research Assistant, Massachusetts Institute of Technology.     

Alloy development for the enhanced stability of Ω precipitates in Al-Cu-Mg-Ag alloys

The coarsening resistance and thermal stability of several Ω plate-dominated microstructures were controlled through altering the chemistry and thermomechanical processing of various Al-Cu-Mg-Ag alloys. Quantitative comparisons of Ω nucleation density, particle size, and thermal stability were used to illustrate the effects of alloy composition and processing conditions. The long-term stability of Ω plates was found to coincide with relatively high levels of silver and moderate magnesium additions, with the latter limiting the competition for solute with S-phase precipitation. This analysis revealed that certain microstructures initially dominated by Ω precipitation were found to remain stable through long-term isothermal and double-aging heat treatments, which represents significant improvement over the previous generation of Al-Cu-Mg-Ag alloys, in which Ω plates dissolved sacrificially after long aging times. The quantitative precipitate data, in conjunction with a thermodynamic database for the aluminum-rich corner of the Al-Cu-Mg-Ag quaternary system, were used to estimate the chemistry of the α/Ω-interphase boundary. These calculations suggest that silver is the limiting species at the α/Ω interfacial layer and that Ω plates form with varying interfacial chemistries during the early stages of artificial aging, which is directly related to the overall stability of certain plates.     

The effect of trace additions of sn on precipitation in Al-Cu alloys: An atom probe field ion microscopy study

We have studied the early stages of θ′ precipitation in an Al-Cu-Sn alloy by atom probe field ion microscopy (APFIM) and transmission electron microscopy (TEM). Clusters of Sn atoms were observed in as-quenched samples, and their formation clearly precedes the formation of the θ′ phase. Transmission electron microscopy also revealed that elevated temperature aging leads to the formation of spherical particles. Atom probe analysis and microbeam electron diffraction suggested that these particles were β-Sn (I4₁/amd, α = 0.583 nm, and c = 0.318 nm). The TEM observations showed that θ′ nucleated heterogeneously on these particles. We have also examined the θ′/α interface following further precipitate growth to check for possible Sn segregation. Atom probe analysis of both the broad face and the narrow rim of the platelike θ′ precipitates has shown no evidence of Sn segregation at θ′/α interfaces. It was also found that cold work prior to aging inhibits the formation of Sn particles resulting in a lower number of these types of nucleation sites for the θ′ phase. Atom probe analysis has also revealed solute depletion at grain boundaries during the early stages of aging in the Al-Cu-Sn alloy, and this, in conjunction with vacancy depletion, explains the formation of narrow precipitate-free zones observed following further aging. Formerly Japan Society for the Promotion of Science Postdoctoral Research Fellow, Institute for Materials Research, Tohoku University Formerly Research Associate, Institute for Materials Research, Tohoku University