Spark plasma sintering of a nanocrystalline Al-Cu-Mg-Fe-Ni-Sc alloy

The microstructure and aging behavior of a nanocrystalline Al-Cu-Mg-Fe-Ni-Sc alloy was studied. The nanocrystalline powders were produced by milling at liquid nitrogen temperature and then consolidated using spark plasma sintering (SPS). The microstructure after SPS consisted of a bimodal aluminum grain structure (coarse-grained and fine-grained regions), along with Al₉FeNi and Al₂CuMg particles dispersed throughout. The microstructure observed in the as-consolidated sample is rationalized on the basis of high current densities that are generated during sintering. Solution treatment and aging of the SPS Al-Cu-Mg-Fe-Ni-Sc sample resulted in softening instead of hardening. This observation can be explained by the reduced amount of Cu, Mg, and Si in solid solution available to form S′ Al₂CuMg due to the precipitation of Al₇FeCu₂ and Si-rich particles, and by the fact that rodlike S′ Al₂CuMg particles could only precipitate out in the coarse-grained regions, greatly decreasing their influence on the hardness. This lack of precipitation in the fine-grained region is argued to represent a new physical observation and is rationalized on the basis of physical and thermodynamic effects. The nanocrystalline SPS Al-Cu-Mg-Fe-Ni-Sc sample was also extremely thermally stable, retaining a fine-grained structure even after solution treatment at 530°C for 5 h. The observed thermal stability is rationalized on the basis of solute drag and Zener pinning caused by the impurities introduced during the cryomilling process.     

Microstructure and mechanical behavior of Al-Li-Cu-Mg alloy 8090 microalloyed with V and Be

The effects of microadditions of V and Be on age-hardening, microstructure, and mechanical behavior of Al-Li-Cu-Mg alloy 8090 are investigated. Small additions of V and Be significantly increase the peak hardness and mechanical properties of the alloy aged at 190 °C. A transmission electron microscopy (TEM), scanning electron microscopy/energy dispersive spectroscopy (SEM/ EDS), optical, and X-ray diffraction investigation shows a higherS′ (Al₂CuMg) precipitate density for alloys containing V and Be, indicating an enhanced nucleation rate forS’ precipitation. The V and Be additions are more effective than prior-aging deformation for increasing the yield strength of age-hardened 8090 alloy. The V addition increases ductility significantly, which is attributed to the homogeneous distribution of slip lines and extensive cross slip that occurs during deformation of the V-containing alloy.     

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.     

High-Performance dispersion-strengthened Cu-8 Cr-4 Nb alloy

A new high-temperature-strength, high-conductivity Cu-Cr-Nb alloy with a Cr:Nb ratio of 2:1 was developed to achieve improved performance and durability. The Cu-8 Cr-4 Nb alloy studied has demonstrated remarkable thermal and microstructural stability after long exposures at temperatures up to 0.98 T_m. This stability was mainly attributed to the slow coarsening kinetics of the Cr₂Nb precipitates present in the alloy. At all temperatures, the microstructure consists of a bimodal and sometimes trimodal distribution of strengthening Cr₂Nb precipitates, depending on precipitation condition, i.e., from liquid or solid solution, and cooling rates. These precipitates remain in the same size range, i.e., large precipitates of approximately 1 μm and small precipitates less than 300 nm, and effectively pin the grain boundaries, thus retaining a fine grain size of 2.7 μm after 100 hours at 1323 K. This grain-boundary pinning and sluggish coarsening of Cr₂Nb particles explain the retention of good mechanical properties after prolonged holding at very high temperatures, e.g., twothirds of the original yield strength after aging for 100 hours at 1273 K. The main sources of strengthening are the Hall-Petch and Orowan mechanisms due mostly to small particles. The coarsening kinetics of the large precipitates are most likely governed by grain-boundary diffusion and, to a lesser extent, volume diffusion mechanisms.     

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.     

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.     

The effect of processing and microstructure development on the slip and fracture behavior of the 2.1 wt pct Li AF/C-489 and 1.8 wt pct Li AF/C-458 Al-Li-Cu-X alloys

Although Al-Li-Cu alloys showed initial promise as lightweight structural materials, implementation into primary aerospace applications has been hindered due in part to their characteristic anisotropic mechanical and fracture behaviors. The Air Force recently developed two isotropic Al-Li-Cu-X alloys with 2.1 wt pct Li and 1.8 wt pct Li designated AF/C-489 and AF/C-458, respectively. The elongation at peak strength was less than the required 5 pct for the 2.1 wt pct Li variant but greater than 10 pct for the 1.8 wt pct Li alloy. The objectives of our investigations were to first identify the mechanisms for the large difference in ductility between the AF/C-489 and AF/C-458 alloys and then to develop an aging schedule to optimize the microstructure for high ductility and strength levels. Duplex and triple aging practices were designed to minimize grain boundary precipitation while encouraging matrix precipitation of the T₁ (Al₂CuLi) strengthening phase. Certain duplex aged conditions for the AF/C-489 alloy showed significant increases in ductility by as much as 85 pct with a small decrease of only 6.5 and 2.5 pct in yield and ultimate tensile strength, respectively. However, no significant variations were found through either duplex or triple aging practices for the AF/C-458 alloys, thus, indicating a very large processing window. Grain size and δ′ (Al₃Li) volume fraction were determined to be the major cause for the differences in the mechanical properties of the two alloys.     

The Microstructure and tensile properties of a splat-quenched Al-Cu-Li-Mg-Zr alloy

The microstructure and tensile behavior of an Al-3Cu-l.6Li-0.8Mg-0.2Zr alloy, produced by splatquenched powder metallurgy processing, were studied. The alloy exhibited homogeneous deformation, both in bulk samples and duringin situ TEM studies. This is in contrast to the strain localization that is frequently observed in Mg-free Al-Cu-Li-X alloys. The difference in deformation mode is attributed to a fine distribution of Ś (Al₂CuMg) which precipitates up to the grain boundaries. A processing treatment involving 2 pct stretch prior to aging resulted in a yield strength of 555 MPa, a reduction in area of 29 pct, and a strain to fracture of 8.8 pct. This represents an attractive improvement in specific properties compared with 7075-T76 having a similar texture.     

Microstructure and Mechanical Properties of Mg-7Al-2Sn Alloy Processed by Super Vacuum Die-Casting

The microstructural evolution of Mg-7Al-2Sn (AT72) alloy processed by super vacuum die-casting and heat treated at various conditions was studied. The results showed that the dendritic microstructure in the as-cast AT72 alloy consisted of α-Mg, Mg₂Sn, and Mg₁₇Al₁₂ phases. After solution treatment at temperatures ranging from 663 K to 703 K (390 °C to 430 °C), the Mg₁₇Al₁₂ phase dissolved into the Mg matrix entirely, while the Mg₂Sn phase partially dissolved into matrix. An average grain size of about 40 μm in the alloy could be achieved after solution treatment at 683 K (410 °C) for 16 hours. A large amount of lath-shaped precipitates of Mg₂Sn and Mg₁₇Al₁₂ was observed in the aged AT72 alloy. The results of tensile property evaluation at room temperature showed that the ductility of the solution-treated alloy was dramatically improved, in comparison with the as-cast alloy. In the peak aged condition, the tensile strength of the alloy was increased, which was attributed to the deposition of fine Mg₁₇Al₁₂ and Mg₂Sn precipitates during the aging treatment.     

Microstructure and tensile properties of Fe-40 At. pct Al alloys with C,Zr, Hf,and B additions

The influence of small additions of C, Zr, and Hf, alone or in combination with B, on the microstructure and tensile behavior of substoichiometric FeAl was investigated. Tensile prop-erties were determined from 300 to 1100 K on powder which was consolidated by hot extrusion. All materials possessed some ductility at room temperature, although ternary additions generally reduced ductility compared to the binary alloy. Adding B to the C- and Zr-containing alloys changed the fracture mode from intergranular to transgranular and restored the ductility to ap-proximately 5 pct elongation. Additions of Zr and Hf increased strength up to about 900 K, which was related to a combination of grain refinement and precipitation hardening. Fe₆Al₆Zr and Fe₆Al₆Hf precipitates, both with identical body-centered tetragonal structures, were iden-tified as the principal second phases in these alloys. Strength decreased steadily as temperature increased above 700 K, as diffusion-assisted mechanisms, including grain boundary sliding and cavitation, became operative. Although all alloys had similar strengths at 1100 K, Hf additions significantly improved high-temperature ductility by suppressing cavitation.     

Effects of Ca and Ce Addition on Tensile and Fracture Properties in Squeeze Cast AT42(Mg-4Al-2Sn) Magnesium Alloys

The effects of the addition of Ca and Ce to the AT42(Mg-4Al-2Sn) alloy on the microstructural modification and deformation, as well as the fracture mechanisms of squeeze cast magnesium alloys, were investigated in this study. Microstructural analyses indicated that the AT42 alloy contained Mg₁₇Al₁₂ and Mg₂Sn particles precipitated along cell boundaries, whereas long, needle-shaped CaMgSn particles were precipitated additionally in the AT42-0.5Ca and AT42-1Ca alloys. In the AT42-1Ca-0.5Ce and AT42-1Ca-1Ce alloys containing Al₁₁Ce₃ particles as well as Mg₁₇Al₁₂, Mg₂Sn, and CaMgSn particles, the overall distribution of precipitates was homogeneously modified considerably as the solidification cell size was refined. According to the observation of deformation and the fracture processes of the AT42-1Ca alloys, the fracture proceeded mainly along cracked, needle-shaped CaMgSn particles at a relatively low stress-intensity factor level. However, in the AT42-1Ca-1Ce alloys, the deformation and fracture proceeded into cells rather than into cell boundaries as twins were developed actively inside cells, although few microcracks were initiated at the precipitates.?Thus, the AT42-1Ca-1Ce alloy had the highest strength, ductility, and fracture toughness simultaneously because of the increase in the volume fraction of hard precipitates and the development of many twins in the Mg matrix.     

Effect of B on the microstructure and mechanical properties of mechanically milled TiAl alloys

The present study is concerned with γ-(Ti₅₂Al₄₈)_100−x B _x (x=0, 0.5, 2, 5) alloys produced by mechanical milling/vacuum hot pressing (VHPing) using melt-extracted powders. Microstructure of the as-vacuum hot pressed (VHPed) alloys exhibits a duplex equiaxed microstructure of α₂ and γ with a mean grain size of 200 nm. Besides α₂ and γ phases, binary and 0.5 pct B alloys contain Ti₂AlN and Al₂O₃ phases located along the grain boundaries and show appreciable coarsening in grain and dispersoid sizes during annealing treatment at 1300 °C for 5 hours. On the other hand, 2 pct B and 5 pct B alloys contain fine boride particles within the γ grains and show minimal coarsening during annealing. Room-temperature compressing tests of the as-VHPed alloys show low ductility, but very high yield strength >2100 MPa. After annealing treatment, mechanically milled alloys show much higher yield strength than conventional powder metallurgy and ingot metallurgy processed alloys, with equivalent ductility to ingot metallurgy processed alloys. The 5 pct B alloy with the smallest grain size shows higher yield strength than binary alloy up to the test temperature of 700 °C. At 850 °C, 5 pct B alloy shows much lower strength than the binary alloy, indicating that the deformation of fine 5 pct B alloy is dominated by the grain boundary sliding mechanism. This article is based on a presentation made in the symposium “Mechanical Behavior of Bulk Nanocrystalline Solids,” presented at the 1997 Fall TMS Meeting and Materials Week, September 14–18, 1997, in Indianapolis, Indiana, under the auspices of the Mechanical Metallurgy (SMD), Powder Materials (MDMD), and Chemistry and Physics of Materials (EMPMD/SMD) Committees.     

Recovery of Ductility in Ultrafine-Grained Low-Carbon Steel Processed Through Equal-Channel Angular Pressing Followed by Cold Rolling and Flash Annealing

The low-carbon steel workpieces are deformed by equal-channel angular pressing at 293 K (20 °C) up to an equivalent strain of ~12 using route B _c, which results in the bulk ultrafine-grained (UFG) structure with high dislocation density and partial dissolution of cementite. The yield strength (YS) is enhanced from 208 (as-received) to 872 MPa and the tensile strength is increased from 362 to 996 MPa, but the material loses total elongation (TE) from 36.2 to 2.9 pct. Cold rolling of equal-channel angular pressed steel produces the refined structure of grain size 0.11 μm. The YS increases further to 924 MPa with a marginal gain in ductility due to the reappearance of the γ fiber component. Flash annealing the samples, which were equal-channel angular pressed followed by cold rolling, at 873 K (600 °C) results in 27 pct of micron-sized (9 µm) ferrite grains in submicron-sized (<1 µm) matrix with a reduced defect density and small amount of precipitation of cementite. TE increases from 2.9 to 23.3 pct. The material retains a YS of 484 MPa and tensile strength of 517 MPa, which are higher than those of the as-received material. The UFG grains are failed by cleavage, but the micron-sized grains display ductile fracture. The ductility of the flash-annealed material is recovered significantly due to bimodal grain size distribution in ferrite and the development of a good amount of γ fiber texture components. The major contribution toward recovery of ductility comes from the bimodal grain size distribution in ferrite rather the precipitation of cementite.

     

Aluminum matrix composites: Fabrication and properties

Aluminum alloy matrix composites containing 1 to 30 wt pct of fibrous and particulate nonmetals varying in size from 0.06 μm to 840 μm were fabricated. The composites were cast into cylindrical molds for friction and wear tests, hot extrusion and tensile tests. The distribution of the nonmetals in the cast ingots was homogeneous. Friction and wear tests were done on a pin (52100 bearing steel) and dish type machine without lubrication. It was found that composites containing ∼10 wt pct or more of SiC, TiC, Si₃N₄, Al₂O₃, glass, solid waste slag, and silica sand wear less than the pure matrix alloy, but have slightly higher average coefficients of friction. Wear in composites containing soft particles, especially MgO and boron nitride was higher than the pure matrix alloy. The average coefficient of friction of all the composites was in the range of 0.35 to 0.58. Increasing the sliding velocity reduced this range to ∼ 0.4 to 0.45. The longitudinal tensile properties of the extruded composites (with the exception of loss of ductility in some cases) are comparable to that of the matrix alloys. Improvements in strength or ductility were noted. For example, addition of 15 wt pct of 3 μm size Al₂O₃ particles raised the yield and ultimate strength of the Al-4 pct Cu-0.75 pct Mg alloy matrix from 227 to 302 MPa, and 356 to 403 MPa, respectively. The corresponding percent elongation decreased from 25.8 to 12.5. The fact that the various composites can be readily cast and hot formed suggests a variety of engineering applications. AKIRA SATO, formerly Visiting Scientist at Massachusetts Institute of Technology, Cambridge.     

The effect of strontium on the Mg2Si precipitation process in 6201 aluminum alloy

A transmission electron microscopy (TEM) study of a 6201 aluminum alloy to which controlled strontium additions were made has revealed important differences compared to the same alloy free of strontium. In the as-cast state, strontium favors the formation of α-AlFeSi (Al₈Fe₂Si) rather than β-AlFeSi (Al₅FeSi) phase, resulting in a greater quantity of excess silicon present in the strontium-treated alloy. During heat treatment, the excess silicon allows a greater density of finer β″-Mg₂Si precipitates to form, leading to increased tensile strength values and increased electrical resistivity. Strontium also retards the growth of the precipitates formed during heat treatment and inhibits formation of the equilibrium β-Mg₂Si phase. As a result, the strontium-treated alloy exhibits a resistance to overaging.     

Solidification Microstructure and Mechanical Properties of Cast Magnesium-Aluminum-Tin Alloys

The solidification microstructure and mechanical properties of as-cast Mg-Al-Sn alloys have been investigated using computational thermodynamics and experiments. The as-cast microstructure of Mg-Al-Sn alloys consists of α-Mg, Mg₁₇Al₁₂, and Mg₂Sn phases. The amount of Mg₁₇Al₁₂ and Mg₂Sn phases formed increases with increasing Al and Sn content and shows good agreement between the experimental results and the Scheil solidification calculations. Generally, the yield strength of as-cast alloys increases with Al and Sn content, whereas the ductility decreases. This study has confirmed an early development of Mg-7Al-2Sn alloy for structural applications and has led to a promising new Mg-7Al-5Sn alloy with significantly improved strength and ductility comparable with commercial AZ91 alloy.     

Mechanical and Wear Properties of Al–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Metal Matrix Composites Fabricated by the Combined Effect of Stir and Squeeze Casting Method

The effects of nano particles on double shear strength and tribological properties of A356 alloy reinforced with Al₂O₃ nano particles of size 30 nm were investigated. The percentage inclusions of Al₂O₃ were varied from 0.5 to 1.5 wt%. The particles were added with stirring at 400 rpm and squeeze casting at 750 °C and pressure of 600 MPa in a squeeze casting machine. Comparison of the performance of as cast samples of A356/Al₂O₃ nano composite was conducted. The tribological properties of the samples were also investigated by pin-on-disk tests at 10, 30 and 50 N load, sliding speed 0.534 m/s and sliding distance 1100 m in dry condition. SEM images of microstructure analysis of the composite, Al₂O₃ (0.5 and 1 %) particles were well dispersed in the A356 alloy matrix. Partial agglomeration was observed in metal matrix composite with higher (1.5 %) Al₂O₃ particle contents. The nano dispersed composites containing 0.5 and 1 wt% of Al₂O₃ nano particles exhibited the highest double shear strength, lesser wear loss and coefficient of friction.     

Toughness-strength relations in the overaged 7449 al-based alloy

This article examines the relationship between plane strain fracture toughness, K _Ic , the tensile properties, and the microstructure of the overaged 7449 aluminum-plate alloy, and compares them to the 7150 alloy. The 7449 alloy has a higher content of η′/η precipitates; and, the 7150 alloy contains a greater amount of coarse intermetallic particles, as it contains an appreciable amount of coarse S phase (Al₂CuMg), which is largely absent in the 7449 alloy. The toughness of the alloys shows an increase on overaging, and the 7449 alloy shows a reasonably linear toughness—yield strength relation on extended overaging. Several mechanisms of failure occur: coarse voiding at intermetallics and a combined intergranular/transgranular shear fracture mode, with the former becoming more important as overaging progresses. Drawbacks of existing models for toughness are discussed, and a new model for plane strain fracture toughness, based on the microstructurally dependent work-hardening factor, K _A , introduced in Ashby’s theory of work hardening, is developed. This model predicts a linear relation between K _Ic and K _A ^/0.85 /σ _ys ^/0.35 , where σ _ys is the yield strength, which is consistent with the experimental data.     

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.