<Emphasis Type="Italic">In-situ</Emphasis> reactive processing of nickel aluminides by laser-engineered net shaping

Nickel aluminide intermetallics (e.g., Ni₃Al and NiAl) are considered to be attractive materials for high-temperature structural applications. Laser-engineered net shaping (LENS) is a rapid prototyping process, which involves laser processing fine metal powders into three-dimensional shapes directly from a computer-aided design (CAD) model. In this work, an attempt has been made to fabricate aluminide intermetallic compounds via reactive in-situ alloying from elemental powders using the LENS process. In-situ reactive alloying was achieved by delivering elemental Ni and Al powders from two different powder feeders, eliminating segregation observed in the samples deposited by using the premixed elemental powders. Nickel aluminides of various compositions were obtained easily by regulating the ratio of their feed rates. The aluminide deposits exhibited a high solidification and subsolidus cracking susceptibility and porosity formation. The observed porosity resulted from a water-atomized Ni powder and can be minimized or eliminated by the use of a N₂-gas-atomized Ni powder of improved quality. Cracking was due to the combined effect of the high thermal stresses generated from the LENS processing and the brittleness of the intermetallics. Crack-free deposits were fabricated by preheating the substrate to a temperature of 450 °C to 500 °C during LENS processing. Compositionally graded Ni-Al deposits with a gradient microstructure were also produced by the in-situ reactive processing.     

Copper coatings for minimization of retention and permeation of implanted tritium in aluminum alloy 6061

Copper coatings deposited on Al-6061 substrates by radio frequency magnetron sputtering, to prevent the retention and permeation of energetically implanted tritium in Al-6061, were evaluated by a variety of characterization techniques. The coatings, weighing in the 0.03 to 0.088 kg/m² range, were smooth and had a fine grain structure. They contained the intermetallic phases Cu₉Al₄ and CuAl₂ as well as copper. The fractions of Al and Cu in any coating increased and decreased, respectively, with increasing depth below the surface. Furthermore, the fractions of Al and Cu on the coating surface decreased and increased, respectively, with increasing coating weight. There was no texture or preferred orientation in the Cu phase of the coatings. A significant amount of oxygen was also detected at the original substrate surface. Residual stress measurements revealed that, in both Cu and CuAl₂, the stresses in the coating plane were compressive, while the stress normal to the coating plane was zero in Cu but tensile in CuAl₂. The shear-stress components were, however, negligible in both the Cu and CuAl₂ phases. In the coating plane, the residual stress in Cu was always much smaller than that in the CuAl₂ phase. Bond-strength measurements using tensile-pull testing provided a lower limit of the bond strength of about 2 MPa.     

Microstructure and phase constituents in the interface zone of Mg/Al diffusion bonding

The microstructure and phase constituent for the Mg/Al diffusion-bonded joint were studied via scanning electron microscope (SEM), microhardness test, electron probe microanalyzer (EPMA), and X-ray diffraction (XRD). The test results indicated that the new compact phase was formed near the transition region of the Mg/Al diffusion interface. There are three new phase layers in the transition region. The microhardness of the diffusion zone is higher than that of the Mg and Al substrate. The fracture morphology mainly consists of a coarse and gray fracture, and the fracture is mainly the mixed fracture of cleavage and intergranular. X-ray diffraction tests indicate that the diffusion zone of the Mg/Al diffusion-bonded joint consists of intermetallic compounds MgAl, Mg₃Al₂, and Mg₂Al₃. With the increase of temperature, the content of Mg₃Al₂ and Mg₂Al₃ phases with good stability was increased.     

Dense CoAl-based alloys with improved ductility: Solid-state synthesis and microstructure control

Dense single- and multiphase B₂ CoAl-based intermetallics were synthesized from fine elemental Co-Al blends by a completely solid-state processing route at temperatures as low as 800 °C. To ensure full density of the final product, a moderately high external pressure (≤500 MPa) was applied during the solid-state reactive synthesis. Microstructure and mechanical properties of the materials obtained were investigated employing X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, and microhardness and compression testing. To improve the room-temperature ductility, an attempt was made to control the microstructure of hyperstoichiometric CoAl alloysvia solution treatment with subsequent aging. At the early stages of aging, fine Widmanstätten precipitates of Co were formed; the coarsening of Co dispersions during long or higher temperature anneals resulted in the pronounced compressive ductility of the overaged samples. In addition to general precipitation, cellular precipitation was observed in CoAl alloys with a high Co content. The relatively coarse equilibrium cellular precipitates were found to rapidly overgrow the fine metastable Widmanstätten Co plates, causing rapid overaging of these alloys. The lattice parameter of hyper-stoichiometric CoAl was found to decrease linearly with increasing Co content.     

Plastic deformation and fracture of binary TiAl-base alloys

The mechanical behavior of binary TiAl alloys containing 46 to 60 at. pct Al has been studied in bulk materials preparedvia rapid solidification processing. Bending and tensile tests were carried out at room temperature as a function of Al concentration. A few alloys were also tested from liquid nitrogen temperature to ∼ 1000°C. Deformation substructures were studied by analytical transmission electron microscopy and fracture modes by scanning electron microscopy (SEM). It was found that both microstructure and composition strongly affect the mechanical behavior of TiAl-base alloys. A duplex structure, which contains both primary y grains and transformedγ/α ₂ lamellar grains, is more deformable than a single-phase or a fully transformed structure. The highest plasticities are observed in duplex alloys containing 48–50 at. pct Al after heat treatment in the center of theγ + α phase field. The deformation of these duplex alloys is facilitated by 1/2[110] slip and {111} twinning, but very limited superdislocation slip occurs. The twin deformation is suggested to result from a lowered stacking fault energy due to oxygen depletion or an intrinsic change in chemical bonding. Other factors, such as grain size and grain boundary chemistry and structure, are important from a fracture point of view. The results on the deformation and fracture modes as a function of test temperature are also discussed.     

Interfacial Microstructure and Bonding Strength of Copper Cladding Aluminum Rods Fabricated by Horizontal Core-Filling Continuous Casting

Copper cladding aluminum (CCA) rods with a diameter of 30 mm and a sheath thickness of 3 mm were fabricated by horizontal core-filling continuous casting (HCFC) technology. The microstructure and morphology, distribution of chemical components, and phase composition of the interface between Cu and Al were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), and energy dispersive spectrometer (EDS). The formation mechanism of the interface and the effects of key processing parameters, e.g., aluminum casting temperature, secondary cooling intensity, and mean withdrawing speed on the interfacial microstructure and bonding strength were investigated. The results show that the CCA rod has a multilayered interface, which is composed of three sublayers—sublayer I is Cu₉Al₄ layer, sublayer II is CuAl₂ layer, and sublayer III is composed of α-Al/CuAl₂ pseudo eutectic. The thickness of sublayer III, which occupies 92 to 99 pct of the total thickness of the interface, is much larger than the thicknesses of sublayers I and II. However, the interfacial bonding strength is dominated by the thicknesses of sublayers I and II; i.e., the bonding strength decreases with the rise of the thicknesses of sublayers I and II. When raising the aluminum casting temperature, the total thickness of the interface increases while the thicknesses of sublayers I and II decrease and the bonding strength increases. Either augmenting the secondary cooling intensity or increasing the mean withdrawing speed results in the decrease in both total thickness of the interface and the thicknesses of sublayers I and II, and an increase in the interfacial bonding strength. The CCA rod with the largest interfacial bonding strength of 67.9 ± 0.5 MPa was fabricated under such processing parameters as copper casting temperature 1503 K (1230 °C), aluminum casting temperature 1063 K (790 °C), primary cooling water flux 600 L/h, secondary cooling water flux 700 L/h, and mean withdrawing speed 87 mm/min. The total thickness of the interface of the CCA rod fabricated under the preceding processing parameters is about 75 μm, while the thicknesses of sublayers I and II are about 1.1 and 0.1 μm, respectively.     

On the influence of in-situ reactions on grain size during reactive atomization and deposition

A fundamental study of the factors that govern grain size of 5083 Al processed via reactive atomization and deposition (RAD) is reported. Microstructural observation shows that the average grain size in RAD 5083 Al is slightly smaller than that in the material processed via N₂ spray deposition (SDN). A numerical approach, together with measurements of the temperature histories inside the deposited materials, is implemented to analyze the influence of in-situ reactions during RAD process on the evolution of grain size. The numerical results show that RAD 5083 Al possesses a slightly higher density of nuclei relative to that present in SDN 5083 Al on a per unit volume of deposited material basis at the beginning of the slow solidification of remaining liquid phase. Furthermore, the RAD material exhibits a slightly lower coarsening extent during the slow solidification. Grain growth is negligible during the solid-phase cooling. Accordingly, the calculated grain size in RAD 5083 Al is slightly smaller than that in SDN 5083 Al, consistent with the observed results.     

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.     

Room-temperature mechanical behavior of cryomilled al alloys

Room-temperature mechanical properties of cryomilled Al-7.5 pct Mg and Al 5083 alloys are discussed in the context of a duplex microstructure, which arises during processing. After consolidation via hot isostatic pressing (“hipping”), coarse-grained regions are formed in former interparticle void volumes, and these regions become elongated during extrusion. Comparison of tensile and compression testing results on both “as-hipped” and extruded materials shows that tension-compression asymmetry is the result of these coarse-grained regions and not necessarily a fundamental property of ultrafine grained Al. The strength of the extruded materials is consistent with the Hall-Petch model of strengthening by grain size refinement, but the hipped material deviates from this trend, with a lower strength despite finer average grain size. This can also be attributed to the presence of coarse-grained regions, which substract from the strength in a predictable manner and also enhance the ability of the cryomilled material to work harden.     

Pressure-assisted reactive synthesis of titanium aluminides from dense 50Al-50Ti elemental powder blends

In the present research, dense γ-TiAl based intermetallic samples were fabricated by reactive synthesis of fully dense elemental 50 at. pct Al-50 at. pct Ti powder blends. Two different processing routes were attempted: thermal explosion under pressure (combustion consolidation) and reactive hot pressing. In both approaches, relatively low processing or preheating temperatures (900 °C) were used. The entire procedure of thermal explosion under pressure could be performed in open air without noticeable oxidation damage to the final product. The application of a moderate external pressure (≤250 MPa) during synthesis was shown to be enough to accommodate the negative volume change associated with TiAl formation from the elemental components and, thereby, to ensure full density of the final product. Microstructure and phase composition of the materials obtained were characterized employing X-ray diffraction and scanning electron microscopy with energy dispersive analysis. It was found that at elevated temperatures(e.g., 900 °C), the equiatomic 50Al-50Ti alloy lies beyond the homogeneity range of the y-TiAl phase in the Ti-Al binary and contains, in addition to γ-TiAl, Al-rich Ti₃Al. Mechanical properties of the materials synthesized were evaluated in compression tests at different temperatures and by microhardness measurements. Due to its very fine microstructure, the Ti-Al material synthesizedvia reactive hot pressing exhibited superplastic behavior at temperatures as low as 800 °C. Formarly with the Department of Materials Engineering, Drexel University. This article is based on a presentation made in the “In Situ Reactions for Synthesis of Composites, Ceramics, and Intermetallics” symposium, held February 12–16, 1995, at the TMS Annual Meeting in Las Vegas, Nevada, under the auspices of SMD and ASM-MSD (the ASM/TMS Composites and TMS Powder Materials Committees).     

Correlation of the microstructure and mechanical properties of oxide-dispersion-strengthened coppers fabricated by internal oxidation

This study was aimed at the correlation of the microstructure and mechanical properties of oxide-dispersion-strengthened (ODS) coppers fabricated by internal oxidation. Atomized copper powders mixed with Cu₂O oxidant powders were internally oxidized and then hot extruded to fabricate ODS coppers without defects. In order to sufficiently oxidize copper powders, oxidant powders should be added in amounts 30 pct in excess of the stoichiometrically calculated amount. In the extruded ODS coppers, very fine Al₂O₃ dispersoids of 10 nm in diameter were homogeneously distributed inside copper grains of 1 μm in size. The volume fraction of Al₂O₃ dispersoids increased as the Al content in atomized copper powders increased. With increasing volume fractions of Al₂O₃ dispersoids, the yield and tensile strengths increased, while the elongation and electrical conductivity decreased, and all the properties of the ODS coppers were sufficiently above the required properties of electrode materials for spot welding. To understand the mechanism responsible for the improvement of the yield strength of the ODS coppers, yield strength was interpreted using the Orowan’s strengthening model, which was fairly consistent with the experimental results.     

Room-Temperature strength and deformation of Tib2-reinforced near-γ titanium aluminides

A series of TiB₂-reinforced near-γ titanium aluminide (Ti-Al) matrix composites have been produced in investment-cast form and characterized with respect to microstructure and tensile deformation. The Ti-Al matrices of the composites examined are based upon the binary composition Ti-47 Al (at. pct), with varying proportions (2 to 6 cumulative percent) of manganese, vanadium, chromium, and niobium. TiB₂ has been introduced into the microstructuresvia XD* processing at levels of 7 and 12 vol pct and compared to unreinforced (0 vol pct TiB₂), base variants. The influences of heat-treatment temperature and time have also been studied for each composition and reinforcement variant. The addition of dispersed TiB₂ leads to a fine, stable, and homogeneous as-cast matrix microstructure. The measured TiB₂ size within the composites examined ranged from 1.4 to 2.6 μm. Increasing the volume fraction of TiB₂ leads to increased elastic moduli, increased ambient temperature tensile strengths, and in general, increased strain-hardening response. In some instances, the overall ductility of the alloy increases with the addition of TiB₂ reinforcement. The flow stresses of both the monolithic and composite variants exhibit conventional power-law plasticity. The results indicate that the strengthening and the flow behavior in these composites are derived from both indirect and direct sources. Strengthening contributions are indirectly derived from the microstructural changes within the matrix of the composite that evolve due to the presence of the reinforcement during its evolution and development, for example, due to grain refinement and reinforcement-derived interstitial solid-solution strengthening. Direct contributions to strength are those that can be specifically attributed to the presence of the reinforcement during deformation,e.g., through the interaction of dislocations with the reinforcing particles. When the estimates of the indirect contributions are isolated and arithmetically removed from the magnitude of the total observed strength of the composite, the increase in flow stress correlates in all instances with the inverse square root of the planar interparticle spacing for all alloy compositions, heat treatments, and levels of strain examined.     

Development of a low-melting-point filler metal for brazing aluminum alloys

The study is concerned with developing low-melting-point filler metals for brazing aluminum alloys. For this purpose, thermal analyses of a series of Al-Si-Cu-Sn filler metals have been conducted and corresponding microstructures observed. The results showed that the liquidus temperature of Al-Si-Cu filler metals dropped from 593 °C to 534 °C, when the amount of copper was increased from 0 to 30 pct. As the copper content reached further to 40 pct, the liquidus temperature would rise to 572 °C. By adding 2 pct tin into the Al-Si-20Cu alloys, the liquidus and solidus temperature would fall from 543 °C to 526 °C and from 524 °C to 504 °C, respectively. The main microstructures of Al-Si-Cu alloys consist of the α-Al solid solution, silicon particles, the CuAl₂ (ϑ) intermetallic, and the eutectic structures of Al-Si, Al-Cu, and Al-Si-Cu. For further improvement of the brazability of this filler metal, magnesium was added as a wetting agent, which would remove the residual oxygen and moisture from the brazed aluminum surface and reduce the oxide film. Based on results gleaned from the thermal analyses, a new filler metal with the composition Al-7Si-20Cu-2Sn-1Mg is proposed, which possesses a melting temperature range of 501 °C to 522 °C and a microstructure that includes an Al-Si solid solution, silicon particles, a tin-rich phase, and CuAl₂, CuMgAl₂, and Mg₂Si intermetallic compounds. When this filler metal was used to braze the 6061-T6 aluminum alloy, an optimized bonding strength of 196 ± 19 MPa was achieved.     

Microstructure evolution through the α→γ phase transformation in a Ti-48 At. pct Al alloy

The α→γ phase transformation during rapid quenching and subsequent isothermal aging has been investigated in a Ti-48 at pct Al alloy. The microstructure changes from a completely massively transformed γ-grain structure to a mixed microstructure of the massively transformed γ grains and the untransformed (meaning massively untransformed) fine α ₂/γ lamellae with an increase in the cooling rate from the high-temperature α phase field. Fine γ grains are generated from these fine α ₂/γ lamellae by subsequent again at 1323 K. The fine γ grains contain many defects, such as dislocations, microtwins (or stacking faults), domain boundaries, and variants, which are frequently observed in the massive γ grains. This result suggests that the formation mechanism of the fine γ grains during aging is similar to that of the massive γ grains. When the fine γ/γ lamellar sample, which is formed by preliminary aging at a lower temperature (1173 K), is aged at a higher temperature (1323 K), apparent changes in microstructure could not be recognized. This result indicates that the fine γ-grain formation is closely related to the α ₂ → γ phase transformation in the fine α ₂/γ lamellae. This article is based on a presentation made in the symposium “Fundamentals of Gamma Titanium Aluminides,” presented at the TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformation Committees.     

Section of the isothermal tetrahedron of the Al-Cu-Mg-Zr phase diagram at 490°C

The phase equilibria in the Al-Cu-Mg-Zr system at 490°C have been studied for Al-rich alloys with 0.3% Zr and from 0 to 10% Cu or Mg. The (Al) solid solution is found to be in equilibrium with only binary θ(CuAl₂) and ZrAl₃ and ternary S (CuMgAl₂) phases of the ternary Al-Cu-Mg system. The section of the isothermal tetrahedron of the Al-Cu-Mg-Zr phase diagram at 490°C, which corresponds to 0.3% Zr and up to 10% Cu or Mg, is constructed.     

Characterization of microstructure in laser-surface-alloyed layers of aluminum on nickel

In order to obtain basic understanding of microstructure evolution in laser-surface-alloyed layers, aluminum was surface alloyed on a pure nickel substrate using a CO₂ laser. By varying the laser scanning speed, the composition of the surface layers can be systematically varied. The Ni content in the layer increases with increase in scanning speed. Detailed cross-sectional transmission electron microscopic study reveals complexities in solidification behavior with increased nickel content. It is shown that ordered B2 phase forms over a wide range of composition with subsequent precipitation of Ni₂Al, an ordered ω phase in the B2 matrix, during solid-state cooling. For nickel-rich alloys associated with higher laser scan speed, the fcc γ phase is invariably the first phase to grow from the liquid with solute trapping. The phase reorders in the solid state to yield γ′ Ni₃Al. The phase competes with β AlNi, which forms massively from the liquid. The β AlNi transforms martensitically to a 3R structure during cooling in solid state. The results can be rationalized in terms of a metastable phase diagram proposed earlier. However, the results are at variance with earlier studies of laser processing of nickel-rich alloys.     

Kinetics of low-temperature pack aluminide coating formation on alloy steels

The kinetics of pack aluminide coating formation and growth on alloy steels was studied in the temperature range of 550 °C to 700 °C. The pack Al content was varied from 1 to 10 wt pct and aluminizing time from 1 to 16 hours. The halide salts AlCl₃ and NH₄Cl were studied as activators. In the AlCl₃ activated packs, it was observed that all the coatings consisted of a single Fe₂Al₅ layer with an abrupt coating/substrate interface and varying the pack chemical compositions and processing conditions affected the growth rate of the layer thickness but not the microstructure of the coating. In these packs, it was revealed that the growth kinetics of the layer thickness (h in μm) that accounts for the effects of temperature (T in K), time (t in hours), and pack Al content (W in wt pct) can be described byh=75,141.8 exp (−8820.8/T)W^1/2 t ^1/2. In the NH₄Cl activated packs, it was shown that coating formation and dissolution took place simultaneously at 650 °C. However, increasing Al content in this type of packs will increase the coating formation rate, making it possible to form a sufficient coating layer. now retired.     

Rapid solidification processing of a Mg-Li-Si-Ag alloy

A Mg-13Li-4Si-lAg (wt pct) alloy with improved ductility and thermal stability was developedvia the rapid solidification (RS) processing technique. Silicon was added to the alloy as the third alloying element in order to form a thermally stable intermetallic dispersoid phase required for improved mechanical properties at ambient and elevated temperatures. The microstructure of the as-spun and heat-treated alloy was characterized using differential scanning calorimetry (DSC), X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Microhardness measurements were conducted on as-spun and heat-treated alloy in order to obtain qualitative prop-erty data and to investigate the extent of the degradation of properties at elevated temperatures. It was found that the melt-spun Mg-Li alloy possessed a microstructure consisting of a fine dispersion of Mg₂Si phase in a fine-grained body-centered cubic (bcc) Mg-Li solid solution, resulting in the desired improvements in thermal stability and mechanical properties. Formerly Graduate Student, Department of Materials Science and Engineering, University of California