Resistance to shape refinement of precipitates in Al-(Si,Ge) alloys during thermal cycling

The present work addresses the response of Si-Ge precipitates in Al-0.5Si-0.5Ge (at. pct) to thermalcycling treatments of the sort known to refine the shapes of Ge precipitates in binary Al-Ge alloys. Alloys aged at 250 °C contained both small, platelet precipitates on {111} planes and larger, equiaxed precipitates that were heavily twinned. Thermal cycling between 250 °C and 360 °C led to partial or complete dissolution of the platelets. However, the equiaxed particles coarsened at an essentially constant shape; the shape refinement that led to untwinned, single-variant octahedral precipitates in binary Al-Ge did not occur. The apparent reason is the heavy twinning of the Si-Ge precipitates, which produces particles with a nearly spherical shape and rounded, incoherent interfaces and is, hence, a viable mechanism for relaxing the large misfit strain of the precipitate structure. The twinned particles undergo normal coarsening at an essentially constant shape. After thermal cycling, the precipitates contain Si and Ge in the approximate ratio of 70Si-30Ge, which is in the composition range expected for the cycling temperature.     

Silicide precipitation in the Ti−Zr−Al−Si system

Silicide precipitation has been studied in a number of martensitic Ti−Si alloys and ternary and more complex alloys containing zirconium and aluminum. The precipitation has been shown to be strongly influenced by composition. Heterogeneous nucleation of Ti₅Si₃ occurs at, and above, 500°C in binary Ti−Si alloys containing up to 2.4 pct Si and little conventional precipitation hardening can be achieved. Zirconium additions are incorporated into the silicide and produces finer, more stable particle dispersions accompanied by an improved aging response. If critical concentrations of zirconium (approximately 5 pct) and silicon (approximately 1 pct) are present, matrix nucleation of G.P. zones occurs at, and below 550°C. This is accompanied by a significant aging response. Additions of aluminum reduce the rate of precipitate growth while having little effect on the nature of the precipitates produced. It has been shown that typical commercial heat treatments of “near α” silicon-bearing alloys (0.5 pct Si max) do not produce silicide dispersions. This paper is based upon a thesis submitted by H. M. FLOWER in partial fulfillment of the requirements of the degree of Doctor of Philosophy at the University of London.     

Precipitation in Two Al-Mg-Ge Alloys

Two Al-Mg-Ge alloys with compositions Al-0.87Mg-0.43Ge (at. pct) and Al-0.59Mg-0.71Ge (at. pct) were investigated and compared using high-resolution transmission electron microscopy, annular dark-field scanning transmission electron microscopy, and nano-beam electron diffraction. The alloys contained fine needle- and lath-shaped precipitates after aging at 473 K (200 °C) for 16 hours, which produced hardnesses similar to those measured in comparable Al-Mg-Si alloys. The β″ phase was not observed. Instead, hardness was achieved by β′-like and disordered precipitates in the Mg-rich alloy, and U1-like and disordered precipitates in the Ge-rich alloy. In all cases, the fine precipitates had structures containing an ordered near-hexagonal network of Ge atoms with a = b ≈ 0.4 nm, which could be visualized directly in annular dark-field mode. The network is very similar to the recently discovered Si network that relates all precipitate structures in the Al-Mg-Si alloys. The orientation of the precipitate unit cells and the Ge network relative to the Al matrix differed from what has been observed for β′ and U1 in the Al-Mg-Si system.     

Role of Mo and W during sensitization of superaustenitic stainless steel—crystallography and composition of precipitates

This study concerns the crystallographic identification and compositions of precipitates formed in superaustenitic stainless steel. Three experimental alloys, all containing 24 wt pct Cr, 22 wt pct Ni, and 0.5 wt pct N but with varying amounts of Mo and W, were investigated after sensitization heat treatment (aging) at 900 °C. The contents of Mo and W in the three alloys were 7 wt pct Mo, (6 wt pct Mo + 2 wt pct W) and (5 wt pct Mo + 5 wt pct W), respectively. While σ and x were the main secondary phases found in the W-free alloy, replacement of Mo by W was found to promote the formation of Laves-related phases with high Mo + W content. The complex crystallographic nature of Laves-related precipitates was exemplified through the formation of intergrowing C14 Laves, μ, and C phases, all with closely related crystal structures. There was no difference in chemical composition between the three phases. Prolonged aging resulted in intragranular precipitation of different intermetallic phases, as well as formation of nitrogen bearing phases, π and Cr₂N, adjacent to previously formed intermetallic precipitates. The content of Mo + W was found to decrease with increasing aging time for all secondary phases.     

Influence of Cu additions on the morphology of GeSi precipitates in an Al-Ge-Si alloy

Combined additions of Ge and Si to Al are known to produce higher precipitation hardening than that which occurs in the constituent binaries, when the total amounts of alloying atoms are the same for all the alloys investigated. In the resultant Al-Ge-Si alloys, the diamond cubic precipitates contain both Ge and Si and are designated as GeSi. During artificial aging at 160 °C, the GeSi precipitates are commonly present in three forms, i.e., equiaxed, 〈100〉_Al lath, and triangular plate. The equiaxed form is the dominant one of the three. This article examines the influence of varying amounts (i.e., 2 to 4 wt pct) of Cu additions on the morphology of GeSi precipitates formed in an Al-2.6 wt pct Ge-1.04 wt pct Si alloy during artificial aging at 160 °C. It is shown that Cu additions have the remarkable effect of maximizing the nucleation frequency of the 〈100〉_Al lath form and simultaneously suppressing the nucleation of the equiaxed and the plate forms of the GeSi precipitates. Increasing Cu additions also increase the homogeneity and cause refinement of the 〈100〉_Al laths. These results are discussed in light of (1) the critical requirement of vacancies for the nucleation and growth of GeSi precipitates having an atomic volume larger than Al and (2) the crystallographic nature of the negative dilation strains that develop locally in the Cu-rich regions of the Al matrix. It is further shown that, in the alloys containing increased levels (i.e., exceeding about 2.5 wt pct) of Cu, the precipitation of ϑ′ (metastable ϑ-Al₂Cu) phase occurs, and that the nucleation of Cu-rich ϑ′ precipitates occurs upon the 〈100〉_Al laths of GeSi. The latter effect is discussed in terms of the attainment of both the nucleation site and the necessary solute supersaturation at the 〈100〉_Al GeSi/α-Al interfaces.     

Decomposition of Fe-Ni martensite: Implications for the low-temperature (≤500 °C) Fe-Ni phase diagram

The low-temperature (<500 °C) decomposition of Fe-Ni martensite was studied by aging martensitic Fe-Ni alloys at temperatures between 300 °C and 450 °C and by measuring the composition of the matrix and precipitate phases using the analytical electron microscope (AEM). For aging treatments between 300 °C and 450 °C, lath martensite in 15 and 25 wt pct Ni alloys decomposed with γ [face-centered cubic (fcc)] precipitates forming intergranularly, and plate martensite in 30 wt pct Ni alloys decomposed with γ (fcc) precipitates forming intragranularly. The habit plane for the intragranular precipitates is {111}_fcc parallel to one of the {110}_bcc planes in the martensite. The compositions of the γ intergranular and intragranular precipitates lie between 48 and 58 wt pct Ni and generally increase in Ni content with decreasing aging temperature. Diffusion gradients are observed in the matrix α [body-centered cubic (bcc)] with decreasing Ni contents close to the martensite grain boundaries and matrix/precipitate boundaries. The Ni composition of the matrix α phase in decomposed martensite is significantly higher than the equilibrium value of 4 to 5 wt pct Ni, suggesting that precipitate growth in Fe-Ni martensite is partially interface reaction controlled at low temperatures (<500 °C). The results of the experimental studies modify the γ/α + γ phase boundary in the present low-temperature Fe-Ni phase diagram and establish the eutectoid reaction in the temperature range between 400 °C and 450 °C. Formerly Research Assistant, Department of Materials Science and Engineering, Lehigh University     

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.     

Microstructure and high-temperature tensile deformation of tiai(si) alloys made from elemental powders

Two ternary TiAl-based alloys with chemical compositions of Ti-46.4 at. pct Al-1.4 at. pct Si (Si poor) and Ti-45 at. pct Al-2.7 at. pct Si (Si rich), which were prepared by reaction powder processing, have been investigated. Both alloys consist of the intermetallic compounds y-TiAl, α₂-Ti₃Al, and ξ-Ti₅(Si, Al)₃. The microstructure can be described as a duplex structure(i.e., lamellar γ/α₂ regions distributed in γ matrix) containing ξ precipitates. The higher Si content leads to a larger amount of ξ precipitates and a finer y grain size in the Si-rich alloy. The tensile properties of both alloys depend on test temperature. At room temperature and 700 °C, the tensile properties of the Si-poor alloy are better than those of the Si-rich alloy. At 900 °C, the opposite is true. Examinations of tensile deformed specimens reveal ξ-Ti₅(Si, Al)₃ particle debonding and particle cracking at lower test temperatures. At 900 °C, nucleation of voids and microcracks along lamellar grain boundaries and evidence for recovery and dynamic recrystallization were observed. Due to these processes, the alloys can tolerate ξ-Ti₅(Si, Al)₃ particles at high temperature, where the positive effect of grain refinement on both strength and ductility can be utilized.     

Metastable precipitate in a duplex martensite +

The crystal structure and chemical composition of the strengthening precipitates in an Fe- 11.8 pct Cr-8.5 pct Ni-1.8 pct Ti-3.8 pct Mo martensite + ferrite duplex steel were determined by transmission electron microscopy (TEM). The study revealed that a metastable precipitate phase formed in the ferrite and martensite phases during the initial stages of aging. The meta- stable phase was proposed to have a hexagonal crystal structure similar to that reported for the omega phase in Ti alloys. Prolonged aging formed an equilibrium orthorhombic precipitate phase with a Ni₃(Mo, Ti) stoichiometry. The higher dislocation density in martensite accelerated the precipitation reaction, and the transition to the equilibrium precipitate occurred with shorter aging times compared to ferrite. The details of the electron diffraction and analytical electron microscopy studies of the precipitates are described.     

Influence of Aging Parameters on the Tensile Properties and Quality Index of Al-9?Pct Si-1.8?Pct Cu-0.5?Pct Mg 354-Type Casting Alloys

The current study was carried out with a view to investigating the influence of age-hardening parameters, aging temperature and time, on the tensile properties and quality indices of a high-strength Al-9 pct Si casting alloy, namely, 354-Al-9 pct Si-1.8 pct Cu-0.5 pct Mg. Quality charts were used as an evaluation tool for selecting the optimum conditions to be applied, in practice, in order to develop high strength and optimum quality in 354 casting alloy. Aging at a low temperature of 428 K (155 °C) was observed to produce the greatest strength and optimum quality in the 354-type castings compared to aging at higher temperatures. The peak strength observed for 354 alloy may be attained after shorter aging times on the condition that the aging temperature is increased. The aging times required for reaching peak strength in 354 alloys are 72 hours, 40 hours, 8 hours, 1 hour, and 15 minutes at aging temperatures of 428 K, 443 K, 468 K, 493 K, and 518 K (155 °C, 170 °C, 195 °C, 220 °C, and 245 °C), respectively. Aging treatment at higher temperatures is accompanied by a reduction in the tensile properties and quality index values of the castings; however, it also introduces the possibility of a significant economical strategy for minimizing the time and the cost of this same treatment. Aging treatment at a lower temperature of 428 K (155 °C) produces fine and dense precipitates displaying smaller interparticle spacing, while at higher aging temperatures, such as 518 K (245 °C), the precipitates are coarser in size, less dense, and more widely dispersed. The quality charts developed in the course of the current research facilitate the interpretation and evaluation of the tensile properties of the 354 alloy. Such charts provide a logical evaluation tool, from the metallurgical point of view, for an accurate prediction of the influence of aging parameters studied on the properties of the alloys. Depending on the required level of tensile properties and based on the quality charts developed, it is possible to make a rigorous selection as to the most suitable aging parameters to be applied to the 354 alloy so as to obtain the best possible cost-effective compromise between alloy strength and quality.     

Decomposition characteristics of Al-Mn-Zr alloys rapidly-quenched from the melt

The rapidly-quenched structures of liquid Al-Mn-Zr ternary alloys and Al-Mn binary alloys and their decomposition behaviors were investigated by hardness tests, X-ray diffraction analyses, and TEM observations. The solid solubility of Mn can be extended to about 10 wt pct irrespective of whether the alloys contain 1 wt pct Zr or not. The solidification structures are composed of fine dendritic cells and with increasing Mn content, interdendritic precipitates gather volume and branch out into the cell grains. The decomposition of rapidly-quenched alloys takes place during aging for 1 hour at temperatures of 300 to 350 °C, and the dependence of the decomposition temperatures on the Mn content varies in this range. Precipitation hardening of the ternary alloys is intensified by the addition of Mn up to about 7 wt pct and proceeds in a two-step manner during aging in the temperature range examined (350 to 450 °C). It is suggested that the precipitates contributing to the maximum hardening are pseudomorphous to the Al₆Mn equilibrium phase.     

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.     

Solidification in the system Al-Ge-Si: The phase diagram,coring patterns,eutectic growth,and modification

In the ternary system Al-Ge-Si, the binary eutectic reactions, L (liquid) ⇔ (Al) + (Si) and L ⇔ (Al) + (Ge) are connected by a monovariant valley, L ⇔ (Al) + (SiGe), falling from 578 °C at 12.7 wt pct Si to 424 °C at 53 wt pct Ge. The binary, solid eutectic surface extends from the Al corner of the ternary phase diagram (1.65 wt pct Si to 5.2 wt pct Ge) across to the continuous (SiGe) solid solution which contains very little Al: several tie triangles, L-(A1)-(SiGe), have been determined using thermal analysis and electron microprobe analysis (EMPA). Optical and scanning electron microscopy (SEM) examination reveals that coring in the normal (SiGe) phase is discontinuous, showing composition banding, which indicates that stationary 111 facets of the solid solution were exposed to the liquid for extended periods up to 500 seconds; details of this interrupted coring were quantified by EMPA. Similar, smaller, and more gradual variations could also be detected in the (Al) matrix, and these compositional fluctuations are considered to reflect discontinuities in the local eutectic growth rates. Modification and twin-ning, induced by Na, are observed in both Al-Si and Al-Ge but decrease progressively with Ge content; coring in the modified ternary alloys is more continuous.     

Effect of processing and heat treatment on behavior of Cu-Cr-Zr alloys to railway contact wire

A new series of Cu-Cr-Zr alloys to be used as railway contact wire, Cu-0.26 wt pct Cr-0.15 wt pct Zr, Cu-0.13 wt pct Cr-0.41 wt pct Zr, and Cu-0.34 wt pct Cr-0.41 wt pct Zr, were studied. The results indicated that processing and aging treatment had an effect on the microstructure, tensile strength, and electrical conductivity behavior of the Cu-Cr-Zr alloys. Process I (solution treatment + cold work + aging) was superior to process II (cold work + solution treatment + aging), because precipitation can occur heterogeneously at the dislocations and subcells. An appropriate processing and aging treatment may improve the properties of the alloys due to the formation of fine, dispersive, and coherent precipitates within the matrix. It is demonstrated that the best combination of tensile strength and electrical conducitivity, on the order of 599 MPa and 82 pct IACS (International Annealed Copper Standard), respectively, can be obtained in alloy Cu-0.34 wt pct Cr-0.41 wt pct Zr in the solution-heat-treated, cold-worked, and aged condition. The mechanism of tensile and conductive properties of Cu-Cr-Zr alloy is also discussed.     

Silicide Precipitation in Alloy Ti-6AI-5Zr-0.5Mo-0.25Si

Silicide precipitation in the titanium alloy containing by wt pct 6Al-5Zr-0.5Mo-0.25Si (Alloy 685) has been investigated using electron diffraction. The solutionizing temperature for this alloy is 1323 K. It is observed that no resolvable silicide precipitates are present in the alloy as received, furnace cooled from 1323 K and aged at 823 K, or oil quenched from 1323 K and aged at 823 K. Specimen solutionized at 1323 K for 30 minutes followed by water quenching and aging at 923 K for 24 hours shows fine precipitates at boundaries of α′ platelets. Diffraction analysis shows that the fine precipitates belong to two different types of identifiable hexagonal silicides similar to those observed by Floweret al in the ternary Ti-5Zr-0.5Si alloy to be (TiZr)₅Si₃ and another of unknown stoichiometry. However, aging of the water quenched specimens for 24 hours at higher temperatures, 973 K and 1073 K, results in the precipitation of a silicide similar to the latter one. It is analyzed that this silicide has lattice parameters a = 0.70₂ nm and c = 0.36₈ nm. Since aging at 973 K and 1073 K gives rise to precipitation of the same silicide, it is concluded that this is the stable silicide in Alloy 685 in the temperature range investigated.     

Tensile strength of thermomechanically processed Cu-9Ni-6Sn alloys

The tensile properties of Cu-9Ni-6Sn alloys with different swaging amounts of 64, 77, and 95 pct, either solutionized and aged (S/A) or directly aged (D/A), were examined as a function of aging time. It was found that the aging response of Cu-9Ni-6Sn alloys varied greatly depending on the prior solution heat treatment before aging and/or different swaging amounts. The swaged S/A Cu-9Ni-6Sn alloys showed a multistage increase in tensile strength with respect to aging time, probably due to the sequential occurrence of spinodal decomposition, formation of metastable γ· precipitates, and recrystallization. The effect of different swaging amounts, ranging from 64 to 95 pct, was minimal on the aging response of S/A specimens. The prior cold working, however, appeared to favor the spinodal strengthening, comparing unswaged and swaged S/A Cu-9Ni-6Sn alloys. In 95 pct swaged D/A Cu-9Ni-6Sn alloys, the level of hardening was much less sensitive to aging time. A complex interaction between the reduction in dislocation density, the formation of equilibrium precipitates, and the reduction of Sn content in the Sn-rich segregates during an aging process is believed to be responsible for such a lean sensitivity. The increases in tensile strength of 64 and 77 pct swaged D/A Cu-9Ni-6Sn alloys were found to be much steeper than that in the 95 pct counterparts in the early and intermediate stages of aging, which is believed to be related to the relative contribution from work hardening and precipitation hardening to the strength level of D/A specimens.     

Strengthening by ordered precipitates in a Ni−Ni4Mo system

The strength characteristics and microstructures of aged Ni−Mo alloys containing ordered (Ni₄Mo) precipitates were studied as a function of aging time and temperature. It was found that 17 at. pct Mo alloy aged at 750°C produced a uniform dispersion of cuboidal β precipitates which coarsened with time producing a gradual increase in flow stress. The flow stress increment was found to vary in qualitative agreement with both order strengthening and coherency strain models. Both these models give over-estimates of the strengthening increment. A negative dependence of flow stress on temperature is attributed to coherency strain contributions.     

气态源分子束外延生长GexSi1-x/Si异质结合金

采用气态源分子束外延法成功地生长了ＧｅｘＳｉ１－ｘ／Ｓｉ异质结合金材料，所使用的气体分别是乙硅烷和锗烷。高能电子衍射被用于原位监控生长层的表面重构状态。在一定的生长温度下，ＧｅｘＳｉ１－ｘ合金组分ｘ取决于锗烷和乙硅烷的流量比。外延层的表面形貌与锗组分的大小、生长层的厚度及生长温度有关。结果表明，较大的锗组分和较高的生长温度利于由二维模式向三维模式转变的外延生长。     

Precipitation processes in Al-Cu-Mg alloys microalloyed with Si

Microalloying additions of Si are known to increase significantly the response to age hardening of 2xxx series Al-Cu-Mg alloys, and commercial alloys such as 2618 are based on this effect. Previous work has attributed this effect to a refined dispersion of S′ or S phase (Al₂CuMg) precipitates. This work reports the results of a detailed microstructural characterization, employing transmission electron microscopy-based techniques, on the effects of Si additions to a base Al-2.5Cu-1.5Mg (wt pct) alloy. It was found that the peak hardness microstructure contains a fine and uniform dispersion of Si-modified Guinier-Preston-Bagaratsky (GPB) zones. These zones are lath shaped, possessing {100} _α facets, elongated along 〈100〉 _α directions and contain Si. The S phase was also observed at peak hardness, although it is concluded that these precipitates do not contribute significantly to hardening due to their coarse dispersion, which arises from their heterogeneous nucleation on the quenched-in defect structure. Overaging was associated with the replacement of the zones by the S phase through a process involving dissolution and reprecipitation together with heterogeneous nucleation of S at the zones. The precipitation of ϑ′ (Al₂Cu) and σ(Al₅Cu₆Mg₂) phase was also observed in alloys containing ≥0.5 wt pct Si. It is demonstrated that the total solute content of the alloy has a major influence on the precipitation reactions during aging.