Copper-zirconium tungstate composites exhibiting low and negative thermal expansion influenced by reinforcement phase transformations

A fully-dense Cu-75 vol pct ZrW₂O₈ metal matrix composite was fabricated by hot isostatic pressing of Cu-coated ZrW₂O₈ particles. A small amount of the high-pressure γ-ZrW₂O₈ phase was created during the cooldown and depressurization following densification; near complete transformation to γ-ZrW₂O₈ was achieved by subsequent cold isostatic pressing. The thermal expansion behavior of the composite between 25 °C and 325 °C was altered by the cold isostatic pressing treatment, and also depended on the length of time that had passed between thermal cycles. The measured thermal expansion coefficients within specific temperature ranges varied from −6 · 10⁻⁶ K⁻¹ to far above the thermal expansion coefficient of the copper matrix. The complex temperature-dependent expansion/contraction behavior could be justified by considering the evolution of phase transformations taking place in the ZrW₂O₈ phase, which were observed by in-situ synchrotron X-ray diffraction measurements.     

Effect of heat and thermomechanical treatments on the linear thermal expansion coefficient of an N30K10T3 invar

The N30K10T3 invar that has a temperature of the onset of martensite transformation of austenite M _s ≈ −80°C and a Curie point θ_C ≈ 200°C after water-quenching from 1150°C is studied. The decomposition of a supersaturated solid solution is shown to substantially influence the linear thermal expansion coefficient. The alloy is studied in the following three initial states: after quenching, after phase transformation-induced hardening (γ → α_m → γ_p.h), and after cold (20°C) plastic deformation by 30%.     

Microstructural evolution of a nanocrystalline Ti−47Al−3Cr alloy during annealing in the α+γ-phase field

Prealloyed, gas-atomized (GA) Ti−47Al−3Cr alloy powder, containing about 70 pct of the α₂ (Ti₃Al) phase and 30 pct of the γ (TiAl) phase, was fully amorphized by mechanical alloying. The amorphous phase was stable during heating to 600°C, but decomposed at higher temperatures, with an exothermic reaction peak at 624°C as the material transformed to a mixture of α₂ and γ and then to a fully γ structure at 722°C. A nanocrystalline compact with a mean grain size of 42 nm was obtained by hot isostatic pressing (HIP'ing) of the amorphous powder at 725°C. Isothermal annealing experiments were conducted in the two-phase α+γ field, at 1200°C, using holding times of 5, 10, 25, and 35 hours, followed by air cooling. The X-ray diffractometry and analytical transmission electron microscopy investigations carried out on annealed and air-cooled specimens revealed only the presence of the γ grains, which coarsened on annealing. Initially, the grains grew, followed by a saturation stage after annealing for 25 hours, with a saturation grain size of about 1 μm. This grain growth and saturation behavior can be described with a normal grain growth mechanism in which a permanent pinning force is taken into account. Twins formed in the γ grains as a result of annealing and air cooling and exhibited a common twinning plane of (111) with the matrix phase. The minimum γ grain size in which twinning occurred in the annealed specimens was determined to be 0.25 μm, which suggests that twinning is energetically unfavorable in the nanometer-sized grains. N. SRISUKHUMBOWORNCHAI, formerly Master's Student, IMAP, University of Idaho     

Studies on hot deformation of sintered cobalt

The hot deformation behavior of sintered cobalt powder was studied. The Co powder prepared by thermal decomposition of cobalt oxalate was subsequently compacted by cold isostatic pressing (CIP) and sintered at 1300 °C under H₂ atmosphere. Cobalt rods of 95 pct theoretical density were obtained. Strain rate change tests in compression were conducted in the temperature range of 900 °C to 1300 °C by changing strain rates from 0.001 to 3.2 s⁻¹. Uninterrupted hot compression tests at constant strain rates and selective temperatures were also conducted. Microcracks as well as surface cracks were observed in the samples tested below 1200 °C. It was observed that the strain rate sensitivity (SRS) increased with increasing temperature and decreasing strain rate, with the maximum SRS of 0.3 being obtained at 1285 °C and strain rate of 10⁻³ s⁻¹. Despite the higher SRS at low strain rates, the hot workability of sintered cobalt was found to be poor. Extensive grain boundary microcracking was observed, with the density being lowered after deformation. However, the samples tested at higher strain rates showed less microcracking and an increase in density. On the basis of the results, it was concluded that ease of grain boundary sliding at lower strain rates and higher temperatures was responsible for the poor workability at these conditions.     

The effect of processing on the hot workability of Ti-48Al-2Nb-2Cr alloys

The hot compression behavior and microstructure evolution of ingot metallurgy (I/M) and powder metallurgy (P/M) processed samples of the near-γ Ti-aluminide alloy, Ti-48Al-2Nb-2Cr (at. pct), were determined. Three I/M conditions and two P/M conditions were examined in this study. Hot compression tests were performed in the temperature range of 1100 °C to 1300 °C at strain rates ranging from 1.67×10⁻¹/s to 1.67×10⁻⁴/s. The P/M materials consolidated by either hot isostatic pressing (“hipping”) or extrusion exhibited the best hot workability in most cases. The P/M materials possessed finer, more homogeneous microstructures than the I/M materials. It was also noted that improved workability was observed in materials with equiaxed microstructures without any lamellar constituents.     

Phase equilibria in the system Fe-Zn-O at intermediate conditions between metallic-iron saturation and air

The phase equilibria in the FeO-Fe₂O₃-ZnO system have been experimentally investigated at oxygen partial pressures between metallic iron saturation and air using a specially developed quenching technique, followed by electron probe X-ray microanalysis (EPMA) and then wet chemistry for determination of ferrous and ferric iron concentrations. Gas mixtures of H₂, N₂, and CO₂ or CO and CO₂ controlled the atmosphere in the furnace. The determined metal cation ratios in phases at equilibrium were used for the construction of the 1200 °C isothermal section of the Fe-Zn-O system. The univariant equilibria between the gas phase, spinel, wustite, and zincite was found to be close to pO₂=1 · 10⁻⁸ atm at 1200 °C. The ferric and ferrous iron concentrations in zincite and spinel at equilibrium were also determined at temperatures from 1200 °C to 1400 °C at pO₂ = 1·10⁻⁶ atm and at 1200 °C at pO₂ values ranging from 1 · 10⁻⁴ to 1 · 10⁻⁸ atm. Implications of the phase equilibria in the Fe-Zn-O system for the formation of the platelike zincite, especially important for the Imperial Smelting Process (ISP), are discussed.     

Characterization of Ti4AlN3

Bulk samples of Ti₄AIN₃ were fabricated by reactive hot isostatic pressing (hipping) of TiH₂, AlN, and TiN powders at 1275 °C for 24 hours under 70 MPa. Further annealing at 1325 °C for 168 hours under Ar resulted in dense, predominantly single-phase samples, with <1 vol pct of TiN as a secondary phase. This ternary nitride, with a grain size of ≈20 μm on average, is relatively soft (Vickers hardness 2.5 GPa), lightweight (4.6 g/cm³), and machinable. Its Young’s and shear moduli are 310 and 127 GPa, respectively. The compressive and flexural strengths at room temperature are 475 and 350 MPa, respectively. At 1000 °C, the deformation is plastic, with a maximum compressive stress of ≈450 MPa. Ti₄AlN₃ thermal shocks gradually, whereby the largest strength loss (50 pct) is seen at a ΔT of 1000 °C. Further increases in quench temperature, however, increase the retained strength before it ultimately decreases once again. This material is also damage tolerant; a 100 N-load diamond indentation, which produced an ≈0.4 mm defect, reduces the flexural strength by only ≈12 pct. The thermal-expansion coefficient in the 25 °C to 1100 °C temperature range is 9.7±0.2 × 10⁻⁶ °C⁻¹. The room-temperature electrical conductivity is 0.5 × 10⁶ (Θ · m)⁻¹. The resistivity increases linearly with increasing temperature. Ti₄AlN₃ is stable up to 1500 °C in Ar, but decomposes in air to form TiN at ≈1400 °C. graduated from the Department in June of 1999 with an MS thesis.     

The interaction between oxygen and bismuth in dilute solution in copper at 1300 °C

The activities of bismuth (in the range 0.0021 < X_Bi< 0.0053) and oxygen (in the range 3 X 10⁻⁴ < X_o < 0.044) in liquid copper at 1300 ° have been measured by equilibrating pure copper specimens with gaseous atmospheres of known oxygen potential and partial pressure of bismuth. The value of γ_Bi^o in liquid copper at 1300 ° was obtained as 3.09 and the variation of γ_Bi with mole fraction of oxygen was obtained as log γ_Bi = −0.91X_o + 0.49, which yields ɛ_O^Bi = −2.1. The observed variation of log γ_o with mole fraction of bismuth is widely scattered but is in fair agreement with the thermodynamically-consistent expression, log γ_o = −0.9LY_Bi − 0.51.     

High-temperature mechanical behavior of Ti-6Al-4V alloy and TiC p /Ti-6Al-4V composite

Mechanical behaviors at 538 °C, including tensile and creep properties, were investigated for both the Ti-6Al-4V alloy and the Ti-6Al-4V composite reinforced with 10 wt pct TiC particulates fabricated by cold and hot isostatic pressing (CHIP). It was shown that the yield strength (YS) and ultimate tensile strength (UTS) of the composite were greater than those of the matrix alloy at the strain rates ranging from approximately 10⁻⁵ to 10⁻³ s⁻¹. However, the elongation of the composite material was substantially lower than that of the matrix alloy. The creep resistance of the composite was superior to that of the matrix alloy. The data of minimum creep strain rate vs applied stress for the composite can be fit to a power-law equation, and the stress exponent values of 5 and 8 were obtained for applied stress ranges of 103 to 232 MPa and 232 to 379 MPa, respectively. The damage mechanisms were different for the matrix alloy and the composite, as demonstrated by the scanning electron microscopy (SEM) observation of fracture surfaces and the optical microscopy examination of the regions adjacent to the fracture surface. The tensile-tested matrix alloy showed dimpled fracture, while the creep-tested matrix alloy exhibited preferentially interlath and intercolony cracking. The failure of the tensile-tested and creep-tested composite material was controlled by the cleavage failure of the particulates, which was followed by the ductile fracture of the matrix.     

Refractory and ceramic materials

The paper examines the consolidation of 95 mole% ZrO₂-2 mole% CeO₂-3 mole% Y₂O₃ nanocrystalline powder in cold uniaxial double-action pressing, cold isostatic pressing (60 and 120 MPa), and sintering. Five starting powders are produced by processing a suspension after hydrothermal decomposition in different conditions. It is established that a homogeneous microstructure forms only in a material from the powder subjected to two homogenizing grindings. After cold uniaxial pressing and cold isostatic pressing, the sintered samples reach a relative density of 0.96 to 0.94. The bending strength is 600 to 660 MPa. The efficient consolidation of ceramics requires comprehensive processing of starting nanocrystalline powders to modify their morphology. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 7–8 (456), pp. 45–58, 2007.     

The thermal capacity and enthalpy of gadolinium sesquiselenide over a wide temperature range

Measurements have been made on the thermal capacity of γ-Gd₂Se₃ at 58.88–298.34 K. Values have been obtained for the thermal capacity, entropy, reduced Gibbs energy, and enthalpy under standard conditions: C°_p = 125.87 ± 0.5 J· mole⁻¹ · K⁻¹; S°(298.15 K) = 196.5 · 1.6 J · mole⁻¹ · K⁻¹; Φ°(298.15 K) = 103.6 ± 1.6 J · mole⁻¹ · K⁻¹; H°(298.15 K)-H°(0) = 27681 ± 138 J · mole⁻¹. The enthalpy of Gd₂Se₃ has been measured and the major thermodynamic functions have been calculated for the solid and liquid states over the temperature range 450–2300 K. The temperature dependence of the enthalpy in the ranges 300–1800 K and 2000–2300 K are represented: H°(T)-H°(298.15 K) = = 1.1949 · 10⁻² · T² + 122.38 · T + 347402 · T⁻¹ − 38716 and H°(T)-H°(298.15 K) = 262.81 · T-− 196047, respectively. The calculated temperature, enthalpy, and entropy of melting for Gd₂Se₃ are: T_m = 1925 ± 40 K, Δ_mH° (Gd₂Se₃) = 68.5 kJ · mole^-1, Δ_mS°(Gd₂Se₃) = 35.6 J · mole⁻¹ · K⁻¹. __________ Translated from Poroshkovaya Metallurgiya, Nos. 3–4(448), pp. 56–61, March–April, 2006.     

The effect of homogenization treatment and hot isostatic pressing on microporosity in cast steel

The effect of high temperature homogenization treatment and hot isostatic pressing on microporosity in unidirectionally solidified AISI 4330 low alloy steel ingots was investigated. The volume fraction and size distribution of micropores in the as-cast, homogenized, and hot isostatically pressed conditions were determined using X-ray microradiography and optical metallography. The vol pct of spherical, nonspherical and total microporosity increased with distance from the chill in a linear fashion in both the as-cast and the homogenized conditions. The vol pct of microporosity decreased with increasing homogenization time. Nonspherical micropores gradually spheroidized during the treatment. Elimination of microporosity was substantially accelerated by carrying out homogenization under high isostatic pressure. Measured vol pct microporosity at 3 cm from the chill before and after vacuum homogenization at 1315°C for 30 h were 2 × 10⁻² and 1 × 10⁻², respectively. All observable microporosity was eliminated by hot isostatic pressing for 1 h at 1260 and 1O38°C with corresponding pressures of 29,000 and 27,000 psi, respectively. It is proposed that vacancy diffusion to grain boundaries is the rate controlling mechanism.     

Effects of sulfur on the fatigue and fracture resistance of interfaces between γ-Ni(Cr) and α-Al2O3

Dramatic effects of S on the adhesion and fatigue resistance of interfaces between γ-Ni(Cr) and α-Al₂O₃, have been explicitly demonstrated and quantified. This has been achieved by using two bonding conditions: one involving solid-state diffusion (SSDB) and another through a liquid phase (LPB) that forms at temperatures above a eutectic that releases a S-rich liquid. Upon SSDB, there is no significant S excess at the interface, whereas LPB forms a thin interphase with a high local concentration of S. The SSDB materials have interfaces with such high toughness (above 300 J m⁻²) and fatigue resistance that mode I cracks divert into the Al₂O₃, rather than propagate at the interface. Conversely, the LPB materials delaminate at the interface, solely as a result of the residual stresses from thermal expansion misfit, with an interface toughness in the range 2 to 7 J m⁻².     

Processing and characterization of Ti2AlC, Ti2AlN, and Ti2AlC0.5N0.5

In this article, we report on the fabrication and characterization of Ti₂AlC, Ti₂AlN, and Ti₂AlC_0.5N_0.5. Reactive hot isostatic pressing (hipping) at ≈40 MPa of the appropriate mixtures of Ti, Al₄C₃ graphite, and/or AlN powders for 15 hours at 1300 °C yields predominantly single-phase samples of Ti₂AlC_0.5N_0.5, 30 hours at 1300 °C yields predominantly single-phase samples of Ti₂AlC. Despite our best efforts, samples of Ti₂AlN (hot isostatic pressed (hipped) at 1400 °C for 48 hours) contain anywhere between 10 and 15 vol pct of ancillary phases. At ≈25 μM, the average grain sizes of Ti₂AlC_0.5N_0.5 and Ti₂AlC are comparable and are significantly smaller than those of Ti₂AlN, at ≈100 μm. All samples are fully dense and readily machinable. The room-temperature deformation under compression of the end-members is noncatastrophic or graceful. At room temperature, solid-solution strengthening is observed; Ti₂AlC_0.5N_0.5 is stronger in compression, harder, and more brittle than the end-members. Conversely, at temperatures greater than 1200 °C, a solid-solution softening effect is occurring. The thermal-expansion coefficients (CTEs) of Ti₂AlC, Ti₂AlN, and Ti₂AlC_0.5N_0.5 are, respectively, 8.2 × 10⁻⁶, 8.8 × 10⁻⁶, and 10.5 × 10⁻⁶ °C⁻¹, in the temperature range from 25 °C to 1300 °C. The former two values are in good agreement with the CTEs determined from high-temperature X-ray diffraction (XRD). The electrical conductivity of the solid solution (3.1 × 10⁶ (Θ m)⁻¹) is in between those of Ti₂AlC and Ti₂AlN, which are 2.7 × 10⁶ and 4.0 × 10⁶ Θ ⁻¹ m⁻¹, respectively.     

Thermodynamic properties of liquid copper-lead alloys

Activities of lead in liquid copper-lead alloys were measured in the temperature range 1000 to 1200 °C at intervals of 50 °C by the dew-point technique. Various partial and integral molar properties of the liquid alloys were evaluated from the data, and the boundaries of liquid immiscibility in the Cu-Pb phase diagram were calculated. The activity coefficients of lead and copper in dilute solutions are represented by: In γ^o _Pb = (346/T@#@) + 0.181, and In γ^o _cu = (3852/T) − 0.945. The temperature dependence of Gibbs energy self-interaction coefficients for lead and copper are given by: ε^Pb _Pb = − (7828/T) − 3.506, and ε^Cu _Cu = − (8804/T) + 3.140. Various coefficients have the following values at 1200 °C: γ^o _Pb = 12.58,ε^Pb _Pb = − 8.85.η^Pb _Pb = − 52,197 J/gfw, σ^Pb _Pb = 38.15 J/gfw-K, γ^o _Cu = 5.31, ε^Cu _Cu = −2.92,η^Cu _Cu = − 63,970 J/gfw, and σ^Cu _Cu = 19.19 J/gfw-K.     

The diffusivity of sulfur in wüstite and its solubility in wüstite and γ iron in equilibrium with each other and a liquid oxysulfide

The solubility of sulfur in wüstite in equilibrium with γ iron and liquid oxysulfide was found to be 0.011 wt pct at 1050°C. The sulfur solubility in γ iron in equilibrium with wüstite and liquid oxysulfide was also determined at 1050°, 1150°, and 1250°C and found to be 135, 165, and 160 ppm respectively. These values are considerably lower than the sulfur-solubility in y iron in the binary Fe-S system saturated with pyrrhotite. The diffusivity of sulfur in wüstite was determined by oxidizing Fe-S alloys in mixtures of CO and CO₂, and analyzing the entire sample for sulfur afterwards. From the amount of sulfur diffused through the growing wüstite layer into the gas phase, the diffusivity of sulfur in wüstite was evaluated, and found to be 4.1 × 10⁻⁸ and 9.6 × 10⁻⁷ cm²/s at 1050° and 1250°C respectively. These values are of the same order as the self-diffusivity of iron in wüstite in equilibrium with iron at the same temperatures.     

Continuously cast Cu-Al-Ni shape memory wires with a unidirectional morphology

The possibility of applying the Ohno continuous casting (OCC) process to the fabrication of Cu-Al-Ni shape memory alloy wires has been investigated. It was established that Cu-Al-Ni shape memory alloy wires of ∼2-mm diameter can be continuously cast by the OCC process. Cast wires were found to be of unidirectional structure with a matrix phase of β ₁ in which γ ₂ particles were precipitated in the form of dendrites along the subgrain boundaries. The γ ₂ precipitates became fewer and the size became finer with an increase in casting speed. The M _s transformation temperature was in the range −19 °C to −3 °C depending on casting speed, giving rise to stress-induced martensitic transformations at room temperature. The wires cast at slower speeds (50 to 90 mm min⁻¹) displayed shape memory attributes on heating to 90 °C, whereas the wires produced at higher casting speeds (110 to 150 mm min⁻¹) exhibited superelasticity at room temperature. In addition, the fatigue failure of the wires cast at 150 mm min⁻¹) occurred after 6500 to 7500 bend cycles, which is comparable to that of a single crystal of similar composition.     

Kinetic study of the chlorination of gallium oxide

The kinetics of the chlorination of gallium oxide in chlorine atmosphere was studied between 650 °C and 800 °C. The calculations of the Gibbs standard free energy variation with temperature for the reaction Ga₂O_3(S)+3Cl_{2 (g)}→2GaCl_3(g)+1.5O_{2 (g)} show that direct chlorination is favorable above 850 °C. Thermogravimetric experiments were performed under isothermal and nonisothermal conditions. The effect of temperature, gas flow rate, and Cl₂ partial pressure were studied. The solids were characterized by X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The nonisothermal results showed that chlorination of Ga₂O₃ starts at approximately 650 °C, with a mass loss of 50 pct at 850 °C. The isothermal results between 650 °C and 800 °C indicated that the reaction rate increased with temperature. The correlation of the experimental data with different solid-gas reaction models showed that the results are adequately represented by the model proposed by Shieh and Lee: X=1−{1−b ₂₂[b ₂₁ t+e ^−b ₂₁ ^t−1]}^1/(1−γ). From this model, it was found that the rate of reaction for the chlorination of Ga₂O₃ is of the order 0.68 with respect to Cl₂ and the activation energy is 113.23 kJ/mol. On the other hand, the order of the activation rate of the interface surface is 0.111 with respect to Cl₂ and its activation energy is 23.81 kJ/mol.     

Microstructural evolution of a nanocrystalline Ti-47Al-3Cr alloy during annealing in the α + γ-phase field

Prealloyed, gas-atomized (GA) Ti-47Al-3Cr alloy powder, containing about 70 pct of the α ₂ (Ti₃Al) phase and 30 pct of the γ (TiAl) phase, was fully amorphized by mechanical alloying. The amorphous phase was stable during heating to 600 °C, but decomposed at higher temperatures, with an exothermic reaction peak at 624 °C as the material transformed to a mixture of α ₂ and γ and then to a fully γ structure at 722 °C. A nanocrystalline compact with a mean grain size of 42 nm was obtained by hot isostatic pressing (HIP’ing) of the amorphous powder at 725 °C. Isothermal annealing experiments were conducted in the two-phase α+γ field, at 1200 °C, using holding times of 5, 10, 25, and 35 hours, followed by air cooling. The X-ray diffractometry and analytical transmission electron microscopy investigations carried out on annealed and air-cooled specimens revealed only the presence of the γ grains, which coarsened on annealing. Initially, the grains grew, followed by a saturation stage after annealing for 25 hours, with a saturation grain size of about 1 μm. This grain growth and saturation behavior can be described with a normal grain growth mechanism in which a permanent pinning force is taken into account. Twins formed in the γ grains as a result of annealing and air cooling and exhibited a common twinning plane of (111) with the matrix phase. The minimum γ grain size in which twinning occurred in the annealed specimens was determined to be 0.25 μm, which suggests that twinning is energetically unfavorable in the nanometer-sized grains.