Design of quaternary Ir-Nb-Ni-Al refractory superalloys

We propose a method for developing new quaternary Ir-Nb-Ni-Al refractory superalloys for ultra-high-temperature uses, by mixing two types of binary alloys, Ir-20 at. pct Nb and Ni-16.8 at. pct Al, which contain fcc/L1₂ two-phase coherent structures. For alloys of various Ir-Nb/Ni-Al compositions, we analyzed the microstructure and measured the compressive strengths. Phase analysis indicated that three-phase equilibria—fcc, Ir₃Nb-L1₂, and Ni₃Al-L1₂—existed in Ir-5Nb-62.4Ni-12.6Al (at. pct) (alloy A), Ir-10Nb-41.6Ni-8.4Al (at. pct) (alloy B), and Ir-15Nb-20.8Ni-4.2Al (at. pct) (alloy C) at 1400 °C; at 1300 °C, three phase equilibria—fcc, Ir₃Nb, and Ni₃Al—existed in alloys A and C and four-phase equilibria—fcc, Ir₃Nb, Ni₃Al, and IrAl-B2—existed in alloy B. The fcc/L1₂ coherent structure was examined by using transmission electron microscopy (TEM). At a temperature of 1200 °C, the compressive strength of these quaternary alloys was between 130 and 350 MPa, which was higher than that of commercial Ni-based superalloys, such as MarM247 (50 MPa), and lower than that of Ir-based binary alloys (500 MPa). Compared to Ir-based alloys, the compressive strain of these quaternary alloys was greatly improved. The potential of the quaternary alloys for ultra-high-temperature use is also discussed.     

Ir-Nb-Si ternary refractory superalloys with a three-phase Fcc/L12/silicide structure for high-temperature applications: Phase and microstructural evolution

To find a new phase with the potential to improve the high-temperature strength of Ir-based superalloys, the novel idea of introducing silicides into the Ir-Nb binary was implemented. Hypoeutectic Ir-10Nb, eutectic Ir-16Nb, and hypereutectic Ir-25Nb alloys were used as bases, and 5 mol pct Si was added through the removal of Ir. XRD (XRD), scanning electron microscopy (SEM), and electron-probe microanalysis (EPMA) revealed the formation of a three-phase fcc/L1₂/silicide microstructure in the Ir-Nb-Si ternary after Si addition. The type of silicide formed was dependent on heat-treated temperatures and Nb content. After heat treatment at 1750 °C and 1600 °C, a tie-triangle composed of fcc/L1₂/silicide (Ir₂Si) appeared in the Ir-10Nb-5Si and Ir-16Nb-5Si alloys; in the Ir-25Nb-5Si alloy, an L1₂ and silicide (Ir,Nb)₂Si tie-line was observed. In the as-cast and 1300 °C heat-treated samples, the Ir-10Nb-5Si microstructure changed to a two-phase fcc/silicide structure, while the Ir-16Nb-5Si alloy maintained a three-phase fcc/L1₂/silicide structure. The Ir-25Nb-5Si alloy, however, had the same phases as that at 1600 °C. Silicides typically continuously or discontinuously distribute along the interdendritic regions or grain boundaries of the fcc or the L1₂ phase. With the addition of Si, it was found that both the eutectic point and solid solubility of Nb in Ir would shift toward Ir.     

Mechanical properties of Ir-Nb alloys containing Ni and Al

Two alloys made by adding 5 or 10 at. pct, respectively, of Ni-18.9 at. pct Al to an Ir-15 at. pct Nb alloy were investigated. The microstructure and compressive strength at temperatures between room temperature and 1800 °C were investigated to evaluate the potential of these alloys for ultra-high-temperature use. Their microstructural evolution indicated that the two alloys formed fcc and L1₂-Ir₃Nb two-phase structures. The fcc and L1₂ two-phase structures were examined by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The 0.2 pct flow stresses were above 1000 MPa at temperatures up to 1200 °C, about 150 MPa at 1500 °C, and over 100 MPa at 1800 °C. The strength of the quaternary Ir-base alloys at 1200 °C was even higher than that of Ir-base binary and ternary alloys. And the strength of quaternary Ir-Nb-Ni-Al was equivalent to that of the Ir-15 at. pct Nb binary alloy at 1800 °C. The compressive ductility of quaternary (around 20 pct) was improved drastically compared with that of the Ir-base binary alloy (lower than 10 pct) and the ternary Ir-base alloys (about 11 pct). An excellent balance of high-temperature strength and ductility was obtained in the alloy with 10 at. pct Ni-18.9 at. pct Al. The effect of Ni and Al on the strength of the Ir-Nb binary alloy is discussed.     

Structure and mechanical properties of boron-doped cubic zirconium trialuminides

Cubic (L1₂) ternary zirconium trialuminides macroalloyed with Cu(Al₅CuZr₂), Mn(Al₆₆Mn₉Zr₂₅), and Cr(Al₆₇Cr₈Zr₂₅) (atomic percent) and doped with 50 and 100 ppm boron were fabricated by induction melting. Their as-cast microstructures are characterized by a small amount of porosity (1 to 2 pct) and second phase (2 to 3 pct). Boron seems to slightly enhance porosity (up to 3.3 pct) in Al₅CuZr₂ +100 ppm B alloy, and it also promotes some compositional inhomo-geneity in Al₆₆Mn₉Zr₂₅ alloy. Vickers microhardness and compressive properties at room temperature (RT), peak strength temperature (500 °C to 600 °C) and 900 °C were investigated. Microcracking development was also investigated in Al₅CuZr₂ +100 ppm boron alloy exhibiting a stepped load-deflection curve. Vickers microhardness strongly depends on load, similarly to boron-free cubic ternary zirconium and titanium trialuminides, and increases in a systematic way with increasing boron content which seems to indicate a solid solution strengthening effect. At RT, 0.2 pct offset yield strength is not increased by the boron doping in most of the alloys studied except for Al₆₆Mn₉Zr₂₅ + 50 ppm B alloy. Permanent deformation (apparent ductility) at ultimate compressive strength is not enhanced by boron doping. In Al₅CuZr₂ +100 ppm B alloy microcracks start nucleating and proliferating in the elastic region of load-deflection curve in characteristic “bursts” accompanied by a “click” sound and the appearance of a discernible step on the load-deflection curve. Pre-existing pores are observed to be active centers of microcracking.     

Microhardness and compressive mechanical behavior of L12 titanium trialuminides

Vickers microhardness and compressive mechanical properties of Ll₂ Al₃Ti + X intermetallics, where X = Cr, Mn, Fe, and Cu, were studied. Compressive tests were carried out at the temperature range from room temperature to 900 °C. At room temperature, both Vickers microhardness and yield strength (0.2 pct offset) decrease with increasing the long-range order parameterS. Essentially the same behavior of yield strength is observed at all compressive test temperatures up to 900 °C. In general, apparent compressive ductility (permanent deformation) increases with increasing test temperature, although with different rates, for most of the alloys, except Ll₂ Al₃Ti + Mn (high Ti), which is characterized by the lowest long-range order parameterS. Compressive ductility increases with increasing long-range order at all temperatures, but the highest rate of increase occurs at high temperatures above approximately 600 °C and the rate of increase at room temperature is minimal. At low temperatures, deformation-induced microcracks were formed in large numbers in all of the alloys studied, even in the matrix free of preexisting flaws. These microcracks can propagate catastrophically under tensile deformation conditions at a stress level barely approaching 0.2 pct offset prior to the onset of gross plastic flow, leading to a premature fracture. At high temperatures, the initiation of deformation-induced microcracks is inhibited. The mechanical behavior of L1₂ titanium trialuminides is discussed within a general framework of ductile-to-brittle transition.     

On the oxidation of the third-generation single-crystal superalloy CMSX-10

The oxidation behavior of the third-generation nickel-base single-crystal superalloy CMSX-10 is examined. Since the in-service performance of the alloy is of the greatest practical significance, a detailed study is made of the microstructural degradation of a turbine blade that had been removed prematurely from a commercial gas turbine engine. The results are augmented with isothermal oxidation tests conducted in the laboratory for 100 hours, at temperatures of 800 °C, 900 °C, and 1000 °C. Scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis, microhardness testing, and X-ray diffraction (XRD) were employed. It was found that the oxidation of CMSX-10 at temperatures below 1000 °C does not produce either Al₂O₃ or the spinel Ni(Cr,Al)₂O₄, both of which are found in the internal oxidation zone of the earlier generations of superalloys. Surprisingly, it is demonstrated conclusively that the oxidation of CMSX-10 generates the β phase (NiAl). This reaction, which to the authors’ knowledge has not yet been reported, is termed self-aluminization. The XRD studies demonstrate that the internal oxidation of CMSX-10 produces (Ni,Co)Ta₂O₆, (Ni,Co)WO₄, CrTaO₄, and Cr(W,Mo)O₄. There is indication that the formation of the δ phase (Ni₂Al₃) slows the oxidation rate at 1000 °C.     

Laser-Surface Alloying of Nimonic 80 with Silicon and Aluminum and its Oxidation Behavior

In the current study, laser-surface alloying (LSA) of Nimonic 80 (a Ni-based superalloy) was conducted using a high-power continuous wave (CW) CO₂ laser by simultaneous feeding of predetermined proportion of elemental Si and Al powders with an Ar shroud. After LSA, the microstructure of the alloyed zone was carefully analyzed and found to consist of several intermetallic phases of Ni and Si. The microhardness of the alloyed zone was significantly increased to 500?VHN compared with 250?VHN of the as-received substrate. The high-temperature oxidation resistance of the laser-surface-alloyed specimens (under isothermal conditions) was improved (at temperature ranges between 1223?K and 1423?K [950?°C and 1150?°C]) compared with as-received Nimonic. Even though LSA enhanced resistance to oxidation up to a limited period, continued exposure to extended hours (at a given temperature) led to spallation of scale. It seems that a SiO₂-rich adherent scale is responsible for enhanced protection against oxidation in the laser-surface-alloyed specimens. However, the presence of Al₂O₃ in the oxide film enhances the resistance to spallation by increasing the scale adherence at a higher temperature. The results are supported by a suitable thermodynamic calculation.     

Hot Deformation Behavior of As-Cast 2101 Grade Lean Duplex Stainless Steel and the Associated Changes in Microstructure and Crystallographic Texture

The hot deformation behavior of 2101 grade lean duplex stainless steel (DSS, containing ~5 wt pct Mn, ~0.2 wt pct N, and ~1.4 wt pct Ni) and associated microstructural changes within δ-ferrite and austenite (γ) phases were investigated by hot-compression testing in a GLEEBLE 3500 simulator over a range of deformation temperatures, T _def [1073 K to 1373 K (800 °C to 1100 °C)], and applied strains, ε (0.25 to 0.80), at a constant true strain rate of 1/s. The microstructural softening inside γ was dictated by discontinuous dynamic recrystallization (DDRX) at a higher T _def [1273 K to 1373 K (1000 °C to 1100 °C)], while the same was dictated by continuous dynamic recrystallization (CDRX) at a lower T _def (1173 K (900 °C)]. Dynamic recovery (DRV) and CDRX dominated the softening inside δ-ferrite at T _def ≥ 1173 K (900 °C). The dynamic recrystallization (DRX) inside δ and γ could not take place upon deformation at 1073 K (800 °C). The average flow stress level increased 2 to 3 times as the T _def dropped from 1273 to 1173 K (1000 °C to 900 °C) and finally to 1073 K (800 °C). The average microhardness values taken from δ-ferrite and γ regions of the deformed samples showed a different trend. At T _def of 1373 K (1100 °C), microhardness decreased with the increase in strain, while at T _def of 1173 K (900 °C), microhardness increased with the increase in strain. The microstructural changes and hardness variation within individual phases of hot-deformed samples are explained in view of the chemical composition of the steel and deformation parameters (T _def and ε).

     

Phase formation in electrodeposited and thermally annealed Al-Mn alloys

Aluminum-manganese alloys with compositions ranging from 0 to 50 wt pct Mn were electrodeposited onto copper substrates from a chloroaluminate molten salt electrolyte containing MnCl₂ at temperatures of 150 °C to 325 °C. The structures of these electrodeposits were then compared to those observed when metastable electrodeposits were thermally annealed at 200 °C to 610 °C. The alloys were characterized by scanning electron microscopy, transmission electron microscopy (TEM), energy dispersive spectroscopy, and X-ray diffraction. At deposition temperatures of 150 °C to 250 °C, no stable structure other than the strongly supersaturated and highly dislocated Al-face-centered cubic (fcc) solid solution is observed. An amorphous phase and body-centered cubic (bcc) Al₈Mn₅ are observed at higher manganese compositions. In the temperature range of 250 °C to 325 °C, some of the phases predicted by the equilibrium phase diagram, such as Al₆Mn and Al₁₁Mn₄, are electrodeposited. The direct deposition of the icosahedral and decagonal phases has been demonstrated at 325 °C. Thermal annealing of the amorphous phase at temperatures higher than 225 °C results in its transformation to the icosahedral phase with a grain size much smaller than that obtained in the electrodeposited icosahedral phase. Additional annealing at higher temperatures does not result in any detectable coarsening of the icosahedral phase; instead, crystals of Al₆Mn or Al₁₁Mn₄ grow into the regions once occupied by the icosahedral phase. The crystalline Al₆Mn phase which forms as the result of thermal annealing shows a structural deviation from the equilibrium phase. As-deposited alloys comprised of 2-to 3-nm-thick amorphous regions separated by fcc-Al grains failed to crystallize after 30 minutes annealing at 500 °C.     

Tensile properties and microstructure of haynes 25 alloy after aging at elevated temperatures for extended times

The tensile properties of Haynes 25 alloy have been measured after various aging treatments, time, and temperature: as received; and aged at 600 °C for three months; 800 °C for 6 and 12 months; and 1000 °C for 3 and 6 months. Contour plots in temperature-ln (time) space were constructed based on the literature and our own data, detailing changes in yield strength, ultimate tensile strength, and tensile elongation. Scanning electron microscopy and transmission electron microscopy observations of the Haynes 25 alloy microstructure provided an explanation of why the properties changed with aging. Intense lattice distortions after aging at 600 °C, the presence of an α-Co₃W, a L1₂-ordered, fcc phase, a=0.357 nm, after aging at 800 °C, and the nucleation and growth of W₃Co₃C carbides from aging at 800 °C and 1000 °C produced the changes in tensile properties. We did not observe either the Co₂W Laves phase or Co₇W₆ γ phase in any of the material conditions we examined, using TEM of thin foils: as received and aged at 600 °C, 800 °C, and 1000 °C. Other researchers believe these phases cause a loss of ductility in the Haynes 25 alloy with prolonged high-temperature exposure.     

Microstructure and properties evolution of plasma sprayed Al_2O_3-Y_2O_3 composite coatings during high temperature and thermal shock treatment

Al_2O_3-Y_2O_3 composite powder with TiO_2 additive was plasma sprayed to prepare Al_2O_3-Y_2O_3 composite coatings.The micro structure and properties evolution of the Al_2O_3-Y_2O_3 coatings during high temperature and thermal shock resistance were investigated.The results show that the micro structure of the Al_2O_3-Y_2O_3-TiO_2 coating is more uniform than that of the Al_2O_3-Y_2O_3 coating.Meanwhile,amorphous phase is formed in the two coatings.The Al_2O_3-Y_2O_3(-TiO_2) coatings were heat treated for 2 h at temperatures of 800,1000 and 1200℃,respectively.It is found that the microstructure and properties of the two coatings have no obvious change at 800℃.Some of the amorphous phase is crystallized at1000℃,and meanwhile Y_2O_3 and Al_2O_3 react to form YAG phase and YAM phase.At 1200℃,all of the amorphous phases are crystallized.After heat treatment,the micro hardness of the two coatings is increased.The thermal shock resistance of the Al_2O_3-Y_2O_3 system coatings can be improved by using TC4 titanium alloy as substrate and with NiCrAlY bonding layer.Moreover,the Al_2O_3-Y_2O_3-TiO_2 coating exhibits better thermal shock resistance due to the addition of TiO_2.     

Partial phase relationships in Ir-Nb-Ni-Al and Ir-Nb-Pt-Al quaternary systems and mechanical properties of their alloys

Two quanternary systems, Ir-Nb-Ni-Al and Ir-Nb-Pt-Al, were successively investigated to assess their possible use in ultra-high-temperature applications. The phase relationships concentrated on the fcc/L1₂ two-phase region were primarily established, and the mechanical properties were studied. Ir-Nb-Ni-Al quaternary alloys around the Ir-rich or Ni-rich corners of the Ir-Nb-Ni-Al tetrahedron showed a coherent fcc/L1₂ two-phase structure, analogous to that of Ni-base superalloys; however, most of the alloys presented three or four phases with two types of L1₂ phases. Although these alloys showed a high compressive strength at high temperature, they exhibited a higher creep rate than Ir-base binary and ternary alloys. Another quanternary system, Ir-Nb-Pt-Al, showed promising results. Only an fcc/L1₂ two-phase structure was found in all the alloys investigated with compositions ranging from the Ir-rich side to the Pt-rich side, and the lattice misfit between the fcc and L1₂ phases was small. The high-temperature strength at 1200 °C of Ir-Nb-Pt-Al alloys was higher than that of Ir-Nb-Ni-Al alloys with the same Ir content (at. pct). Moreover, Ir-Nb-Pt-Al alloys exhibited excellent creep resistance at 1400 °C and 100 MPa. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.     

Metallurgical reactions in two industrially strip-cast aluminum-manganese alloys

Precipitation, phase transformation, subgrain growth, and recrystallization that occur during heat treatment of two strip-cast, cold-rolled, high manganese aluminum alloys have been studied mainly by transmission electron microscopy (TEM). The alloys differ in silicon content. The isothermal heat treatments have been performed in a salt bath at temperatures between 330 °C and 530 °C for times up to 1000 hours. Size distributions for each type of secondary particle have been determined. After short annealing times, small quasicrystals precipitated and subsequently transformed to α phase. The densities of these precipitates controlled dislocation movement and regulated subgrain sizes. Prolonged heating resulted in peritectoid reactions to Al₆Mn or Al₁₂Mn. Recrystallization, which is associated with the formation of Al₁₂Mn, is advanced by increasing the silicon content; the nucleation and growth of Al₁₂Mn occurs only at the expense of other phases that stabilize the subgrain network. Formerly with SINTEF, Oslo, Norway     

Structural study of electrodeposited aluminum-manganese alloys

Aluminum-manganese alloys with compositions ranging between 0 and 27 wt pct Mn were electrodeposited at 150°C onto copper substrates from a chloroaluminate molten salt electrolyte with a controlled addition of MnCl₂. The specimens were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and X-ray diffraction. The addition of small amounts of Mn results in the formation of a supersaturated fcc solid solution of Mn in Al. At the higher Mn content, an amorphous phase is established. The highly faceted crystalline surface of pure Al and Al−Mn solid solution becomes smooth and nearly specular when the amorphous phase is present. The amorphous phase appears in the form of rounded grains and has a lower limit of Mn concentration close to the Al₆Mn composition. There is a concentration discontinuity between the above limit and the higher Mn concentration limit of the fcc phase (about 9 wt pct). Appearance of the amorphous phase in the alloy results in a decrease in the Mn concentration in solid solution to about 2 wt pct. Crystallization of the amorphous phase starts at the fcc-amorphous phase interface at 230°C. As a result of treatment at 230 °C to 340 °C, the amorphous phase completely transforms into Al₆Mn, while the fcc phase is unaffected. Prior to crystallization, the amorphous phase shows a modification that could be interpreted as the formation of a fine-grained icosahedral phase. The formation and distribution of phases by electrodeposition and rapid solidification are discussed.     

Determination of the isothermal sections of the Al-Ni-Si ternary system at 750 °C and 850 °C

The phase equilibria of the Al-Ni-Si ternary system at 850 °C and 750 °C have been investigated using scanning electron microscopy (SEM) and electron-probe microanalysis (EPMA). Isothermal sections at 850 °C and 750 °C were constructed based on experimental data from 53 alloys heat treated at 850 °C for 1200 hours and at 750 °C for 1440 hours, respectively. The phase equilibria among the following intermetallics and solid-solution phases are described: Ll₂-Ni₃(Al,Si), B2-NiAl, Ni₅Si₂, δ-Ni₂Si, ϑ-Ni₂Si(τ ₄), Ni₃Si₂, NiSi, NiSi₂, Ni₂Al₃, NiAl₃, Ni₂AlSi(τ ₂), Ni₃Al₆Si(τ ₃), Ni₁₆AlSi₉(τ ₅), the fcc solid solution, and the diamond (Si) phase. In addition, a phase, temporarily designated as Ni₅(Al,Si)₃(τ ₆), was observed for the first time at both 750 °C and 850 °C. This phase is probably the stabilization of Ni₅Al₃ by Si to higher temperatures than the binary Ni₅Al₃, which is only stable at <∼700 °C.     

Evolution of microstructures in the nickel modified titanium trialuminides near the L12 phase field

This study focuses upon the evolution of microstructures during solidification processing of several intermetallic alloys around the Ll₂ phase in the Al-rich corner of the Al-Ti-Ni ternary system. The alloys were produced by double induction melting and subsequent homogenization followed by furnace cooling. The microstructure was characterized by means of optical and scanning electron microscopy with energy-dispersive spectroscopy (EDS) analysis and X-ray diffraction. The microstructural evolution in homogenized alloys was dependent on both nickel and titanium content. Very fine precipitates of Al₂Ti were observed within the Ll₂ phase in alloys containing 62 to 65 at. pct Al and at least 25 at. pct Ti. The Al₂Ti precipitates are stable at least up to 1000 °C and undergo complete dissolution at 1200 °C. In alloys containing around 66 at. pct Al and 25 to 31 at. pct Ti, phases such as Al₃Ti, Al₅Ti₂, and Al₁₁Ti₅ were observed. A modified room temperature isotherm in the Al-Ti-Ni ternary system is proposed, taking into account the existence of Al₂Ti, Al₁₁Ti₅, Al₅Ti₂, and Al₃Ti in equilibrium with the Ll₂ phase. It seems that at room temperature, the Ll₂ phase field for homogenized alloys is extremely small. It will be practically impossible to obtain a single-phase microstructure at room temperature in the Al-Ti-Ni ternary alloys after homogenization at 1000 °C followed by furnace cooling. S. BISWAS, formerly Graduate Student, Department of Mechanical Engineering, University of Waterloo     

Features of the structure of porous titanium nickelide during reaction sintering with an aluminum additive

Porous cylindrical samples of a titanium nickelide-based alloy containing 0.5, 1.5, and 2.0 at % Al of 2.5 × 30 mm in size were obtained by two-stage sintering. The first stage was solid-phase sintering at t ≥ 900°C, and the second stage was liquid-phase sintering at t > 1000°C. Starting from existing notions of the reaction diffusion in the Ti-Ni-Al system, the structure of the obtained alloy is analyzed. It is established by scanning electron microscopy and electron probe microanalysis that a multiphase alloy is formed during the first sintering. This alloy contains isolated regions of the TiNi phase, the isolation of which prevents the propagation of the front of the martensite phase transformation in the ready sample. The heterophase structure of the Ti₅₀Ni_{50 − x} Al _x porous alloy, which is formed after the first sintering, becomes more uniform after carrying out the second one at a higher temperature.     

The influence of Sc on thermal stability of a nanocrystalline Al-Mg alloy processed by cryogenic ball milling

The microstructural evolution during annealing of a cryogenically ball-milled Al-7.5Mg-0.3Sc (in wt pct) was examined using differential scanning calorimetry and transmission electron microscopy (TEM). The as-milled alloy was a supersaturated fcc solid solution with an average grain size of ∼25 nm and heterogeneous grain morphologies and size distributions. Calorimetric measurements at a constant heating rate of 32 K/min indicated two exothermic events in association with recovery from 100 °C to 240 °C and recrystallization from 300 °C to 450 °C. Prior to recrystallization, the precipitation of Al₃Sc may occur at low annealing temperatures producing a nonuniform dispersion of approximately spherical particles with diameters of 4 to 5 nm. Recrystallization gave rise to heterogeneous microstructures with bimodal grain size distributions, which may result from the heterogeneity of microstructure in the as-milled state. The heterogeneous microstructures of the recrystallized Al-Mg-Sc alloy were similar to those observed in the recrystallized Sc-free Al-Mg alloy.     

Effect of heat treatments on <Emphasis Type="Italic">In-Situ</Emphasis> Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/TiAl<Subscript>3</Subscript> composites produced from squeeze casting of TiO<Subscript>2</Subscript>/A356 composites

In-situ Al₂O₃/TiAl₃ intermetallic matrix composites were fabricated via squeeze casting of TiO₂/A356 composites heated in the temperature range from 700 °C to 780 °C for 2 hours. The phase transformation in TiO₂/A356 composites employing various heat-treatment temperatures (700 °C to 780 °C) was studied by means of differential thermal analysis (DTA), microhardness, scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and X-ray diffraction (XRD). From DTA, two exothermic peaks from 600 °C to 750 °C were found in the TiO₂/A356 composites. The XRD showed that Al₂O₃ and TiAl₃ were the primary products after heat treatment of the TiO₂/A356 composite. The fabrication of in-situ Al₂O₃/TiAl₃ composites required 33 vol pct TiO₂ in Al and heat treatment in the range from 750 °C to 780 °C. The hardness (HV) of the in-situ Al₂O₃/TiAl₃ composites (1000 HV) was superior to that of nonreacted TiO₂/A356 composites (200 HV). However, the bending strength decreased from 685 MPa for TiO₂/A356 composites to 250 MPa for Al₂O₃/TiAl₃ composites. It decreased rapidly because pores occurred during the formation of Al₂O₃ and TiAl₃. The activation energy of the formation of Al₂O₃ and TiAl₃ from TiO₂ and A356 was determined to be about 286 kJ/mole.     

Investigation of (Cr,Co)7C3-fcc-graphite equilibrium in the temperature interval 1373 to 1473 K

In the current study, the phase equilibria between fcc, graphite and M₇C₃ (M = Cr,Co) have been studied at 1373 °K, 1423 °K, and 1473 °K. The solubility of Co in the M₇C₃ phase and the solubility of Cr and C in the fcc phase have been determined by the high-temperature equilibration and quenching technique. Appropriate mixtures of Cr₇C₃ + Co or M₇C₃ + Co + C were equilibrated and subsequently quenched in liquid nitrogen. The quenched samples were characterized by X-ray diffraction and by metallographic examination. The studies were carried out on the samples to determine the homogeneity of the sample as well as the phases and their composition. From the results, the compositional regions of the three-phase triangle M₇C₃ + fcc + graphite could be accurately determined. The results show that the Co solubility in the Cr₇C₃ in the experimental temperature interval is higher than previous investigations performed at higher temperatures.