Evolution of Grain Boundary Precipitates in an Al-Cu-Li Alloy During Aging

The grain boundary microstructure of Al-Cu-Li alloy AA2050 was investigated for different isothermal aging times to rationalize intergranular corrosion (IGC) characteristics. In the underaged condition, the dominant grain boundary precipitates are fine T₁ (Al₂CuLi). Extended aging revealed that grain boundaries were decorated by large T₁ precipitates and S′ phase (Al₂CuMg), with S′ growth not dimensionally constrained. Such a transition in the precipitate type at grain boundaries is a unique feature of the Al-Cu-Li system.     

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.     

Structural states of the material of compacts and sheets made from an Al-Cu-Li alloy with silver

The structure of hot-rolled sheets of V-1469 alloy of the Al-Cu-Li system with silver is studied. The sheets are prepared from ingots 70 mm in diameter by the ingot-pressed strip-hot-rolled sheet scheme. The texture of the pressed strips is characterized by the set of orientations (Bs, S, Cu) typical of thin pressed strips of aluminum alloys. During subsequent hot rolling, the Bs orientation weakens and the Cu and S orientations become more intense. This behavior indicates that a change in the crystallite orientations in the material of the sheets is controlled by a β-skeleton line in the Euler rectangle. According to the data of electron microscopic study, the main contribution to hardening during aging is made by the T₁ and Θ′ phases and the role of δ′-phase precipitates is insignificant. No precipitates of the T₂ phase are observed. The significant anisotropy of the yield stress in the 45°-direction with respect to the rolling direction is associated with T₁ and Θ′-phase precipitates.     

On stress-relaxation experiments and their significance under strain-aging conditions

Two simple tests are presented to verify whether the mechanical response of the substructure remains constant during stress relaxation. They consist of a) subjecting the sample to repeated relaxation cycles from the same reference load and b) reloading the sample after the last cycle. Either exhaustion of relaxation after repeated cycles or yield-point formation on reloading are indicative of a decrease in the mobile dislocation density. Accordingly, a method is developed for the determination of the time dependence of the mobile dislocation density, using the decrease in relaxation rates in repeated cycling. The exhaustion of relaxation in Armco iron is found to be in good agreement with predictions for the decrease of mobile dislocation density by a pinning mechanism.     

Trace element effects on precipitation processes and mechanical properties in an Al-Cu-Li alloy

A study has been made of how impurities (Na and K) and trace additions of indium, magnesium, and silicon affect the microstructure and related mechanical properties of an Al-Cu-Li alloy. Transmission electron microscopy (TEM) was used to determine the size and distribution of particles in four alloys. Indium and magnesium are both seen to stimulate T ₁ precipitation. Indium also modifies ϑ″ morphology, and magnesium greatly increases the number density of ϑ″ precipitates. Strain localization was observed in underaged Al-Cu-Li-In tensile samples, consistent with observed changes in precipitate structure. No superposition of the effects of indium and magnesium was seen. High-resolution analytical microscopy was used to inspect precipitates for segregation of trace elements during early stages of aging, but no segregation was found within the detection limits of the system. Variations in heat treatment were made in order to study nucleation kinetics and trace element interactions with vacancies. Indium, with a binding energy less than that of lithium, was not seen to interact with quenched-in vacancies, while magnesium, with a binding energy greater than that of lithium, had a strong interaction. Yield anisotropies and fracture toughnesses were measured. Removal of trace impurities of sodium and potassium correlated with improved fracture properties. Magnesium was observed to increase anisotropy, especially in the T8 temper. A model was used to explain the anisotropy data in terms of texture and precipitate distribution.     

Microstructure-property relationships of two AI-3Li-2Cu-0.2Zr-XCd alloys

The microstructure and tensile properties of two A1-3 wt pct Li-2 wt pct Cu-0.2 wt pct Zr alloys, one Cd-free and one containing 0.2 wt pct Cd, have been investigated. The Cd-free alloy remained unrecrystallized for all solutionizing treatments studied, whereas a special treatment had to be developed to prevent recrystallization during solutionizing of the 0.2 wt pct Cd alloy. In combination with cadmium, zirconium either enters into, or nucleates on, the course Al₇Cu₂Fe and T₂ phases during high temperature annealing. This reduces the volume fraction of small coherent Al₃Zr particles in the matrix which normally inhibits recrystallization. Consequently, a low temperature anneal to precipitate Al₃Zr is necessary prior to high temperature solutionizing in order to prevent recrystallization in the Cd-containing alloy. Unlike its effect in lower lithium, higher copper content aluminum alloys, cadmium does not significantly affect the nucleation of the strengthening precipitates. If anything, cadmium has a detrimental effect on the age hardening response of this alloy, since it increases the formation of coarse Al-Cu-Li equilibrium phases at grain and subgrain boundaries and thus removes some of the copper and lithium from participating in the formation of the strengthening precipitates T₁ and δ′. Subgrain boundary fracture occurred during tensile tests of both alloys in the unrecrystallized condition; however, transgranular fracture occurred in tests of the partially recrystallized 0.2 wt pct Cd alloy. Both types of fractures are believed due to a form of strain localization associated with precipitate free zones and shearable precipitates. Formerly with the Fracture and Fatigue Research Laboratory, Georgia Institute of Technology, Atlanta, GA     

Comparison of the Effect of Individual and Combined Zr and Mn Additions on the Fracture Behavior of Al-Cu-Li Alloy AA2198 Rolled Sheet

The effect of individual and combined addition of dispersoid-forming alloying elements Zr and Mn on the fracture behavior of the Al-Cu-Li alloy 2198 has been investigated by the Kahn tear test. Overall, the standard baseline 2198 alloy containing only Zr exhibited the best performance, while the alloy with the combined presence of Zr and Mn was slightly inferior. The lowest properties were seen for a Zr-free 2198-0.4Mn alloy variant. In the T351 temper fracture initiated at coarse constituent particles that formed large cavities and microvoid sheets linked the initial sites of void growth. In the Mn-containing alloys microvoids clearly nucleated at the coarser Al₂₀Cu₂Mn₃ dispersoids within the microstructure, while this was not identifiable for the finer coherent Al₃Zr dispersoids. However, this difference in the mechanism of cavity linkage had little effect on the overall toughness of the materials, which was more closely related to the effect of Mn and Zr on the level of recrystallization. Extended artificial aging promoted grain boundary decohesion due to the precipitation of high densities of T₁ particles on GBs and favored a cleavage fracture mode. Particle decohesive fracture was also promoted by T₁ precipitation on the Mn dispersoids.     

Stress relaxation and solid solution hardening of cubic ZrO2 single crystals

Solid solution hardening in cubic ZrO₂ single crystals of varying Y₂O₃ contents (12.7, 15.2, 17.7, and 20.5 mol %) oriented for easy 100 〈011〉 slip has been studied at 1400°C. Strain rate cycling and stress relaxation experiments have been performed to characterize the thermally-activated deformation processes. The strain rate sensitivity is very low at small strains but increases with increasing strain; the values measured by stress relaxation are greater than those derived from the strain rate cycling experiments, and the relaxation curves show “inverse” curvature at small strains. The athermal component of the flow stress originating from long-range dislocation interactions was estimated from dislocation densities obtained from etch pit micrographs. The dislocation density increases with increasing Y₂O₃ concentration, but the densities are too small to cause the appreciable athermal component of the flow stress; we believe that significant recovery must have occurred during cooling. The stress relaxation data can be interpreted by assuming that the deformation itself is mainly athermal, but that thermally-activated recovery takes place during the deformation; the Y₂O₃ solute may cause hardening by decreasing the diffusion kinetics. Alternatively, it is possible that the flow stress is controlled by the intrinsic lattice resistance of secondary slip systems.     

Characterization of retrogression and reaging behavior of 8090 Al-Li-Cu-Mg-Zr alloy

An 8090 Al-Li-Cu-Mg-Zr alloy in the peak-aged (T8) temper was subjected to retrogression treatment at temperatures above and below the δ′ (Al₃Li) solvus line and immediately reaged to various tempers. Retrogression and reaging (RRA) behavior is characterized by hardness testing, tensile testing, transmission electron microscopy (TEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and electrochemical polarization studies. Retrogression of the T8 temper alloy causes dissolution primarily of δ′ (Al₃Li) precipitates into solid solution that results in a decrease of hardness and tensile strength and an increase of ductility of the alloy. Reaging of the retrogressed state causes reprecipitation of the δ′ precipitates in the matrix resulting in the restoration of strength and ductility properties. Retrogression and reaging to the peak-aged temper, designated at T77 temper, has been found to retain the strength of the conventional T8 temper, but with the gross aging time in the RRA temper almost twice that of the conventional T8 temper, the microstructure of the RRA temper approaches that of the overaged (T7) temper. Thus, RRA treatment contributes to an improvement of stress corrosion cracking (SCC) resistance over the conventional T8 temper while retaining the mechanical properties of T8 temper.     

Tension creep of wrought single phase γ TiAl

The minimum strain rate, tertiary creep and damage behavior of a single phase gamma (γ) TiAl alloy over the temperature range 760–900°C at initial applied stress levels ranging from 32 to 345 MPa are reported. Two regions of creep deformation are identified. These consist of a region having a stress exponent of 6 and an activation energy of 560 kJ/mol and a region having a stress exponent of 1 and an activation energy of 192 kJ/mol. These are postulated to represent dislocation and boundary diffusion dominated creep respectively. The activation energy for dislocation creep is suggested to represent the energy to generate an appreciable density of dislocations in the minimum strain rate region. In the diffusional regime the minimum strain rates at 760°C lie well below the predicted minimum strain rates when compared to the Coble creep equation. In addition, a natural transition from diffusional creep to glide dominated deformation occurs at 760°C with increasing stress level. Tertiary creep of this material is found to correlate well with a two state variable approach. The initial stage of tertiary creep is dominated by an increase in the mobile dislocation density with increasing creep strain. Tertiary creep is found to obey a power law relationship with a stress exponent of 3 and an activation energy of 304 kJ/mol and is explained by the coupling of an increasing mobile dislocation density in the early stage of tertiary with constrained cavity growth in the late stage which leads to specimen failure.     

Fatigue behavior of a nial precipitation hardening medium carbon steel

The fatigue behavior of an Fe-0.30C-4.48Ni-l.32Al steel tempered to give three different microstructures of the same ultimate tensile strength has been investigated by light and electron microscopy, low and high cycle fatigue tests, X-ray line broadening and stress relaxation measurements. The three different heat treatments produced the following structures: I) a conventional quenched and tempered microstructure with a high density of dislocations and elongated carbides, II) a microstructure of high dislocation density, coarse carbides and fine coherent NiAl precipitates and III) a highly tempered micro-structure with a recovered dislocation substructure, coarse carbides and fine coherent NiAl precipitates. In low cycle, strain controlled fatigue cyclic softening in Treatment I was accompanied by a rearrangement of the dislocation substructure and a reduction in both the internal stress and lattice microstrain. Treatment II, which remained cyclically stable during the initial portion of the fatigue life, showed little change in the internal stress and dislocation density and showed a slight increase in lattice microstrain. Treat-ment III, which initially cyclically hardened, exhibited a rise in internal stress, lattice microstrain and dislocation density. The behavior of Treatments II and III is attributed in part to the presence of the fine NiAl precipitates which appear to reduce the tendency of the transformation induced dislocation substructure to rearrange itself into a cell structure during fatigue. In high cycle, stress controlled fatigue Treatment II showed the best fatigue resistance and Treatment I the worst. Improvement in life was attributed to improved resistance to crack initiation. Formerly Graduate Student, Marquette University,     

2195铝锂合金力学性能和组织与冷热变形过程的相关性

采用力学性能测试、光学显微镜(OM)、X射线衍射(XRD)及透射电镜(TEM),研究了 5 mm厚度2195铝锂合金挤压板材和后续2 mm厚度冷轧薄板T8时效态力学性能及微观组织的演化.研究结果表明:相同T8时效(3.8％预变形,148℃,38 h)条件下,5 mm厚度挤压板材抗拉强度及屈服强度比后续2mm厚度冷轧薄板...     

A critical assessment of the mechanistic aspects in HAYNES 188 during low-cycle fatigue in the range 25 °C to 1000 °C

The low-cycle fatigue (LCF) behavior of a wrought cobalt-base superalloy, Haynes 188, has been investigated over a range of temperatures between 25 °C and 1000 °C employing a triangular waveform and a constant strain amplitude of ±0.4 pct. Correlations between macroscopic cyclic deformation and fatigue life with the various microstructural phenomena were enabled through scanning electron microscopy (SEM) and transmission electron microscopy (TEM), detailing the crack initiation and propagation modes, deformation substructure, and carbide precipitation. Cyclic stress response varied as a complex function of temperature. Dynamic strain aging (DSA) was found to occur over a wide temperature range between 300 °C and 750 °C. In the DSA domain, the alloy exhibited marked cyclic hardening with a pronounced maximum at 650 °C. Dynamic strain aging has been documented through the occurrence of serrated yielding, inverse temperature dependence of maximum cyclic stress, and cyclic inelastic strain developed at half of the fatigue life. Additionally, the alloy also displayed a negative strain rate sensitivity of cyclic stress in the DSA regime. These macroscopic features in the DSA domain were accompanied by the substructure comprised of coplanar distribution of dislocations associated with the formation of pileups, stacking faults, and very high dislocation density. Toward the end of the DSA domain, dislocation pinning by M₂₃C₆ precipitates occurred predominantly. The deformation behavior below and above the DSA domain has also been investigated in detail. The temperature dependence of LCF life showed a maximum at ≈300 °C. The drastic reduction in life between 300 °C and 850 °C has been ascribed primarily to the deleterious effects of DSA on crack initiation and propagation, while the lower life at temperatures less than 200 °C has been attributed to the combined influence of low ductility and larger cyclic response stress.     

Plastic deformation of an iron-modified titanium trialuminide alloy

Mechanical properties and dislocation structures have been studied from room temperature to 700°C in a titanium trialuminide alloy modified to the composition Al₅Ti₂Fe. In compression the material shows extensive ductility with the yield stress increasing above 300–400°C, but in bend testing it is very brittle. At low temperatures undissociated 〈110〉 dislocations are mobile and deformation is controlled by Peierls effects and by extensive work hardening. At intermediate temperatures many dislocations dissociate into mobile superdislocations giving rise to serrations in the stress-strain curve; undissociated segments appear immobile because of a solute-associated core relaxation. At high temperatures dislocations are dissociated as superdislocations, mobile on both octahedral and cube planes, with cross slip between these planes bringing about high temperature strengthening. The low ductility in bend testing may be related to the high work hardening as well as the intrinsic cleavage weakness of the material.     

High-temperature creep in an Al-8.5Fe-1.3V-1.7Si alloy processed by rapid solidification

The creep behavior of an Al-8.5Fe-1.3V-1.7Si alloy processed by rapid solidification is investigated at three temperatures ranging from 623 to 723 K. The measured minimum creep strain rates cover seven orders of magnitude. The creep behavior is associated with the true threshold stress, decreasing with increasing temperature more strongly than the shear modulus of aluminum. The minimum creep strain rate is controlled by the lattice diffusion in the alloy matrix, and the true stress exponent is close to 5. The apparent activation energy of creep depends strongly on both applied stress and temperature and is generally much higher than the activation enthalpy of lattice self-diffusion in aluminum. Also, the apparent stress exponent of minimum creep strain rate depends on applied stress as well as on temperature and is generally much higher than the true stress exponent. This behavior of both the apparent activation energy and apparent stress exponent is accounted for by the strong temperature dependence of the threshold stress-to-shear modulus ratio. The true threshold creep behavior of the alloy is interpreted in terms of athermal detachment of dislocations from fine incoherent Al₁₂(Fe, V)₃Si phase particles, admitting a temperature dependence of the relaxation factor characterizing the strength of the attractive dislocation/particle interaction.     

Threshold Stress Creep Behavior of Alloy 617 at Intermediate Temperatures

Creep of Alloy 617, a solid solution Ni-Cr-Mo alloy, was studied in the temperature range of 1023 K to 1273 K (750 °C to 1000 °C). Typical power-law creep behavior with a stress exponent of approximately 5 is observed at temperatures from 1073 K to 1273 K (800 °C to 1000 °C). Creep at 1023 K (750 °C), however, exhibits threshold stress behavior coinciding with the temperature at which a low volume fraction of ordered coherent γ′ precipitates forms. The threshold stress is determined experimentally to be around 70 MPa at 1023 K (750 °C) and is verified to be near zero at 1173 K (900 °C)—temperatures directly correlating to the formation and dissolution of γ′ precipitates, respectively. The γ′ precipitates provide an obstacle to continued dislocation motion and result in the presence of a threshold stress. TEM analysis of specimens crept at 1023 K (750 °C) to various strains, and modeling of stresses necessary for γ′ precipitate dislocation bypass, suggests that the climb of dislocations around the γ′ precipitates is the controlling factor for continued deformation at the end of primary creep and into the tertiary creep regime. As creep deformation proceeds at an applied stress of 121 MPa and the precipitates coarsen, the stress required for Orowan bowing is reached and this mechanism becomes active. At the minimum creep rate at an applied stress of 145 MPa, the finer precipitate size results in higher Orowan bowing stresses and the creep deformation is dominated by the climb of dislocations around the γ′ precipitates.     

The creep behavior of W-5 re

Polycrystalline W-5 wt pct Re was creep-tested in tension from 1500° to 1900°C at stresses from 2500 to 10,000 psi in a vacuum of 10^?8 torr. The steady-state strain rate was directly proportional to stress to the 5.5 power, and the apparent activation energy for creep was 104 kcal per mole. Dislocation substructure that developed during high-temperature deformation was studied by transmission electron microscopy. The total dislocation density was dependent on stress to the 2.1 power and was insensitive to temperature and strain. No subgrains were found in creep tested specimens. The rate-controlling deformation mechanism was ascribed to dislocation climb where the governing diffusion process was dislocation core diffusion. Comparison of creep data for tungsten, W-5 wt pct Re, and W-25 wt pct Re showed that W-5 wt pct Re alloy has significantly better creep properties than the other two materials.     

Microstructure stabilization in a rapidly solidified

Tensile properties and their relationship with microstructural features were investigated for a rapid solidification processed (RSP) type 304 stainless steel (SS) extruded powder material and compared with those of a conventionally processed type 304 SS. Significant improvements in tensile strength were observed up to 800 °C (maximum test temperature) for the RSP alloy. Stable and fine microstructural features, including grain size, small matrix precipitates, high residual dislocation density, and a high population of nanosized void/cavities, were observed in the RSP specimens after heat treatments to0.9T _m . The microstructural features directly re- sponsible for strengthening the RSP alloy were small grain size and the residual dislocation density. Formerly with Formerly with     

Influence of the temperature on the plastic deformation in TiAl

A new alloy, Ti-48.6Al-1.9Cr-1.9Nb-1B, with a near-equiaxed γ microstructure and with a lamellar microstructure is investigated by compression tests between 20 and 800 °C and transmission electron microscopy (TEM). The yield stress exhibits no anomaly as a function of the temperature, while an anomaly, related to strain hardening, is found at 400 °C in the hardening rate and the activation volume for both the lamellar and nonlamellar structure. Above 700 °C, a change in the deformation mechanism occurs and the material becomes remarkably softer. The TEM micrographs highlight the importance of ordinary dislocation motion for both structures at all temperatures. The comparison with previously reported TEM observations on single-phase TiAl alloys shows definitely that the density of ordinary dislocations is higher in the investigated two-phase TiAl alloy deformed at room temperature. Also, the presence of the lamellar interfaces drastically changes the mechanical properties of the alloy and the deformation mechanism. In contrast to the nonlamellar samples, superdislocations are rare, and twinning is very frequent in the lamellar structure.