Superplasticity of mullite-zirconia composite

Tension tests of mullite-zirconia composite were conducted at elevated temperature. A superplastic elongation of 122% could be achieved at an initial strain rate of 2.86×10^–5s^–1 at 1550°C. Strain hardening was observed at strain rates from 1.42×10^–4 to 2.86×10^s–5s^–1 at 1550°C. The addition of zirconia grains to the mullite matrix increased the creep rate of the composite.     

High-temperature flow behaviour and concurrent microstructural evolution in an Al-24 wt% Cu alloy

Tensile specimens of an Al-24 wt% Cu alloy of grain sizes in the range 7.6–20.6 m were deformed at 400–540 °C using constant initial strain rates ranging from 5×10^–6 to 2×10^–2 s^–1. Initially the stress-strain (-) curves show work hardening which is followed by strain softening at higher strain rates and lower temperatures. At lower strain rates and higher temperatures, on the other hand, continues to increase with strain or tends to be independent of strain. Grain growth and cavitation occur to varying extents depending on strain rate and test temperature. While the grain growth can account for the work hardening at higher temperatures as well as at lower strain rates, it fails to do so at higher strain rates. The strain softening is associated with cavitation. The presence of non-steady-state flow influences the parameters of the constitutive relation to varying extents.     

Superplasticity of AlMgSi alloys

Commercial AlMgSi alloy sheets produced by thermomechanical treatment are found to be superplastic between 500 and 570°C at strain rates of 10^–5–10^{–3 –1} The strain rate sensitivity,m, is about 0.4. It was found that the highly alloyed sample contains pre-existing cavities in higher volume fraction than the alloy of lower concentration. An exponential growth of cavity volume fraction was found during superplastic deformation which is characteristic of plasticity controlled cavitation. The growth rate of the cavity volume fraction can be decreased by applying back pressure.     

Superplastic properties of a TiAl based alloy with a duplex microstructure

The superplastic properties of a engineering TiAl based alloy with a duplex microstructure were investigated with respect to the effect of testing temperatures ranging from 950°C to 1075°C and strain rates ranging from 8 × 10^–5 s^–1 to 2 × 10^–3 s^–1. A maximum elongation of 467% was achieved at 1050°C and at a strain rate of 8 × 10^–5 s^–1. The apparent activation energy was calculated to be 345 kJ/mol. Also, the dependence of the strain rate sensitivity values on strain during superplastic deformation was examined through the jump strain rate tests, and microstructural analysis was performed after superplastic deformation. It is concluded that superplasticity of the alloy at relatively low temperature and relatively high strain rate results from dynamic recrystallization, and grain boundary sliding and associated accommodation mechanism is related to superplasticity at higher temperature and lower strain rate.     

Effect of superplastic deformation on the grain size and tensile properties of Al-6.2Zn-2.5Mg-1.7Cu (7010) alloy sheet

An Al-Zn-Mg alloy (7010) was cold-rolled and annealed to produce a small recrystallized grain size, and superplastically deformed in the temperature range 475 to 520° C at strain rates to 2.8×10^–3 sec^–1. At 500° C and sec^–1 superplastic elongations up to 350% were obtained, but above about 60% elongation the residual room-temperature tensile properties after heat treatment decreased due to increasing grain-boundary cavitation. Grain growth rates were increased by superplastic strain.     

Processing maps for hot-working of powder metallurgy 1100 Al-10 vol % SiC-particulate metal-matrix composite

The hot-working characteristics of the metal-matrix composite (MMC) Al-10 vol % SiC-particulate (SiC_p) powder metallurgy compacts in as-sintered and in hot-extruded conditions were studied using hot compression testing. On the basis of the stress-strain data as a function of temperature and strain rate, processing maps depicting the variation in the efficiency of power dissipation, given by = 2m/(m+1), where m is the strain rate sensitivity of flow stress, have been established and are interpreted on the basis of the dynamic materials model. The as-sintered MMC exhibited a domain of dynamic recrystallization (DRX) with a peak efficiency of about 30% at a temperature of about 500°C and a strain rate of 0.01 s^–1. At temperatures below 350°C and in the strain rate range 0.001–0.01 s^–1 the MMC exhibited dynamic recovery. The as-sintered MMC was extruded at 500°C using a ram speed of 3 mm s^–1 and an extrusion ratio of 101. A processing map was established on the extruded product, and this map showed that the DRX domain had shifted to lower temperature (450°C) and higher strain rate (1 s^–1). The optimum temperature and strain rate combination for powder metallurgy billet conditioning are 500°C and 0.01 s^–1, and the secondary metal-working on the extruded product may be done at a higher strain rate of 1 s^–1 and a lower temperature of 425°C.     

Effect of ion implantation on the strength of sapphire at 300–600°C

Ion implantation with ¹¹B⁺ or ²⁸Si⁺ at 1000°C doubled the ring-on-ring flexure strength of c-plane sapphire disks tested at 300°C but had little effect on strength at 500 or 600°C. Disks were implanted on the tensile surface with 2 × 10¹⁷ B/cm² (half at 40 keV and half at 160 keV) or 1 × 10¹⁷ Si/cm² (80 keV). Sapphire implanted with 1 × 10¹⁸ B/cm² had only half as much flexure strength at 300° or 500°C as sapphire implanted with 2 × 10¹⁷ B/cm². Implantation with B, Si, N, Fe or Cr had no effect on the c-axis compressive strength of sapphire at 600°C. Boron ion implantation (2 × 10¹⁷ B/cm², half at 40 keV and half at 160 keV) induced a compressive surface force per unit length of 1.9 × 10² N/m at 20° and 1.4 × 10² N/m at 600°C. The infrared emittance at 550–800° of B-implanted sapphire at a wavelength of 5 m increased by 10–15% over that of unimplanted sapphire. Infrared transmittance of sapphire implanted with B, Si or N at either 1000°C or 25°C is within 1–3% of that of unimplanted material at 3.3 m. Implantation with Fe or Cr at 25°C decreases the transmittance by 4–8% at 3.3 m, but implantation at 1000°C decreased transmittance by only 2–4% compared to unimplanted material.     

Possibility of grain refinement for superplasticity of a Mg-AI-Zn alloy by pre-deformation

For a commercial Mg-Al-Zn alloy sheet, tensile tests are carried out under various strains, strain rates and temperatures to investigate the possibility of grain refinement by dynamic recrystallization during pre-deformation. It is found from the microstructural observations that relatively fine grains of 10 m or so, in diameter are attained under the conditions of 250°C and 8.3 × 10^–4 s^–1. The specimen pre-strained under this condition also exhibits a fairly good superplasticity of total elongation beyond 300%.     

High thermal expansion KAlSiO4 ceramic

Orthorhombic kalsilite (KAlSiO₄) was prepared by solid-state reaction from K₂CO₃, Al₂O₃, and SiO₂. The axial thermal expansion coefficients of the orthorhombic kalsilite were 1.6×10^–5°C^–1 for the a-axis, 1.6×10^–5°C^–1 for the b-axis, 2.8×10^–5°C^–1 for the c-axis, and 2.0×10^–5°C^–1 for the average from room temperature to 1000°C. A high thermal expansion ceramic consisting of the orthorhombic kalsilite was prepared by sintering. The densification was promoted by adding Li₂CO₃. The KAlSiO₄ ceramic sintered at 1200°C for 2 h with 5 wt% Li₂CO₃ had a bending strength of 65 MPa and linear thermal expansion coefficient of 2.2×10^–5 °C^–1 from room temperature to 600°C.     

Cooling rate measurements on pure iron rapidly solidified by piston quenching

Cooling curves have been monitored during rapid solidification of pure iron, using a rapid response thermocouple embedded in one of the quenching pistons. Cooling rates are found to be typically 10⁶ to 10⁷ K sec^–1 in the vicinity of the solidification point at 1500° C, falling to 2 × 10⁴ to 3 × 10⁵ K sec^–1 at 500° C. Heat-flow analysis shows that cooling conditions during rapid solidification are clearly non-Newtonian, with heat transfer coefficients of 3 × 10⁵ to 6 × 10⁵Wm^–2 K^–1 and Nusselt numbers of 0.5 to 1.0. Cooling rates, heat transfer coefficients and Nusselt numbers are higher for piston quenching than for other rapid solidification processes such as melt spinning. Piston-quenched iron microstructures can be ferritic or martensitic depending on the cooling rate during rapid solidification.     

Superplastic behaviour of YBa2Cu3O7−x /Ag superconductors

High-temperature deformation characteristics of YBa₂Cu₃O_7–x oxide (YBCO) and YBa₂Cu₃O_7–x /Ag composite (YBCO/Ag) in uniaxial compression have been investigated. A compression test was carried out at temperatures from 780–930°C at initial strain rates between 10^–6 and 10^–4 s^–1. YBCO/Ag composites with fine, dense and equiaxed grains were compressed over 120% with no indication of failure at higher temperatures, and the strain-rate sensitivity exponent, m, was found to be about 0.42–0.46 between 890 and 930°C. They are considered to be one indication of superplasticity. The activation energy for deformation was 500–580 KJ mol^–1. The specimens suffered grain growth slightly during the deformation at 930°C and the majority of growth might be a function of exposure time, temperature and silver content, but each grain maintained the equiaxed shape after extensive superplastic deformation. This is consistent with a grain-boundary sliding mechanism. The silver at grain boundaries acts to decrease the activation energy for deformation and promote the grain-boundary sliding.     

Thermal expansion of the cubic (3C) polytype of SiC

Thermal expansion of the cubic beta or (3C) polytype of SiC was measured from 20 to 1000° C by the X-ray diffraction technique. Over that temperature range, the coefficient of thermal expansion can be expressed as the second order polynominal: ₁₁=3.19×10^–6+ 3.60×10^–9 T–1.68×10^–12 T ² (1/° C). It increases continuously from about 3.2×10^–6/° C at room temperature to 5.1×10^–6/° C at 1000° C, with an average value of 4.45 × 10^–6/° C between room temperature and 1000° C. This trend is compared with other published results and is discussed in terms of structural contributions to the thermal expansion.     

Volume change and light scattering during mechanical damage in polymethylmethacrylate toughened with core-shell rubber particles

Mechanical damage was investigated in polymethylmethacrylate toughened with core-shell (hard core) rubber particles. During a tensile experiment, volume changes, light absorption, light scattering and a small strain elastic modulus were recorded. Light scattering was quantitatively related to the number of damaged particles and a fast partial unloading technique allowed determination of the non-elastic part of these changes in material properties. Experiments performed between 10^–5 and 10^–1s^{–1 and between 20 and 70 °C} showed time-temperature transitions. These appeared to be different for each property, and measurement of the activation energy for each parameter enabled microscopic damage mechanisms to be inferred. Three types of microstructural damage were observed: pure matrix plasticity at very low strain rates or high temperatures, rubber cavitation at correlated locations at medium strain rates and temperatures, and disordered cavitation, rubber tearing and matrix plasticity at high strain rates or low temperatures. The experimental mean stress triggering rubber cavitation was compared with the predicted value.     

Thermal properties of MoSi2 and SiC whisker-reinforced MoSi2

The heat capacity, thermal conductivity and coefficient of thermal expansion of MoSi₂ and 18 vol % SiC whisker-reinforced MoSi₂ were investigated as a function of temperature. The materials were prepared by hot isostatic pressing between 1650 and 1700 °C, the hold time at temperature being 4 h. The heat capacity of MoSi₂ showed an increase from about 0.44 Wsg^–11K^–1 at room temperature to 0.53 at 700 °C. Whisker reinforcement increased heat capacity by about 10%. Thermal conductivity exhibited a decreasing trend from 0.63 Wcm^–1 K^–1 at room temperature to 0.28 Wem^–1 K^–1 at 1400°C. Whiskers reduced conductivity by about 10%. The thermal expansion coefficient increased from 7.42 °C^–1 between room temperature and 200 °C to 9.13 °C^–1 between room temperature and 1200 °C. There was a 10% decrease resulting from the whiskers. The measured data are compared with literature values. The trends in the data and their potential implications for high-temperature aerospace applications of MoSi₂ are discussed.     

Diffusion of tin (IV) in silicate glazes and glasses

The diffusion coefficients of Sn(IV) in an aluminosilicate glass and a commercial glaze have been measured from 809 to 1505° C. Two experimental techniques have been used. In one method, single crystals of SnO₂ were embedded in either the powdered glass or sealed into a bar of the glass. After the diffusion anneal, the Sn(IV) concentration profile was determined by EPMA. In the other method, radioactive ¹¹³Sn was used as a tracer and the profile determined by measuring the X-ray emission. The results gave a good agreement between the two methods. The diffusion coefficients in the glaze ranged from 7×10^–20 m² sec^–1 at 809° C to 1.9×10^–14 m² sec^–1 at 1250° C and in the glass, from 5.6×10^–15 m² sec^–1 at 1307° C to 1.6×10^–11 m² sec^–1 at 1505° C.     

Thermal expansion of TiAl + TiB2 alloys and model calculations of stresses and expansion of continuous fiber composites

Thermal expansion values for three TiAl alloys with different additions of TiB₂ can be represented using a third-order equation at temperatures between 20 and 800°C. Expansion values were obtained on both heating and cooling temperature cycles. The total expansion at 800°C is between 0.917 and 0.931% for three different samples. The expansivity increases from about 10×10^–6°C^–1 at 80°C to 14×10^–6°C^–1 at 750°C. A five-coaxial cylinder elastic model for multizone-coated continuous fiber composites is developed for predicting stresses and thermal expansion of composites. Either isotropic or transversely isotropic material properties can be assigned to the various cylinder zones.Paper presented at the Tenth International Thermal Expansion Symposium, June 6–7, 1989, Boulder, Colorado, U.S.A.     

D. C. electrical properties of hot-pressed nitrogen ceramics

The d.c. electrical properties of some hot-pressed polycrystalline nitrogen ceramics have been measured between 18 and 500° C in applied electric fields up to 1.1×10⁴ Vcm^–1. The materials examined were Si₃N₄, 5wt%, MgO/Si₃N₄ and two sialons havingz=3.2 andz=4.0. The conduction in all the materials showed similar general features. The time dependent charging (I _c) and discharging currents (I _D) were observed which followed a I(t)t_–n law at room temperature withn=0.7 to 0.8. The exponentn forI _c decreased with increasing temperature. The current density-field (J _s-E) characteristics were ohmic in applied fields of less than 3×10³ Vcm^–1; conductivity increased with electric field above that range. Above about 280° C, a was independent ofE, its temperature dependence following log T ^–1. Below about 230° C conductivity fitted a exp (–B/T _1/4) law in both low and high fields. There is a good correlation between the temperature and field variations of time dependent current and the steady current. The conductivities were in the range of 10^–15 to 10^–16–1 cm^–1 at 18° C and rose to 4×10^–10 to 2×10^{–12 –1} cm^–1 at 500° C. The activation energies were in the range of 1.45 to 1.80 eV and 0.05 to 0.15 eV at above 300° C and near room temperature respectively. Various models to explain the data are considered.     

Rate-dependent fracture toughness of pure polycrystalline ice

A series of three-point bend tests using single edge notched testpieces of pure polycrystalline ice have been performed at three different temperatures (–20°C, –30°C and –40°C). The displacement rate was varied from 1 mm/min to 100 mm/min, producing the crack tip strain rates from about 10^–3 to 10^–1 s^–1. The results show that (a) the fracture toughness of pure polycrystalline ice given by the critical stress intensity factor (K _IC) is much lower than that measured from the J—integral under identical conditions; (b) from the determination of K _IC, the fracture toughness of pure polycrystalline ice decreases with increasing strain rate and there is good power law relationship between them; (c) from the measurement of the J—integral, a different tendency was appeared: when the crack tip strain rate exceeds a critical value of 6 × 10^–3 s^–1, the fracture toughness is almost constant but when the crack tip strain rate is less than this value, the fracture toughness increases with decreasing crack tip strain rate. Re-examination of the mechanisms of rate-dependent fracture toughness of pure polycrystalline ice shows that the effect of strain rate is related not only to the blunting of crack tips due to plasticity, creep and stress relaxation but also to the nucleation and growth of microcracks in the specimen.     

Carbonization of coal-tar pitch into lump needle coke in a tube bomb

The carbonization of coal-tar pitches and their QI (quinoline insoluble)-free fractions was studied by evaluating their lump cokes produced in a tube bomb at various temperatures (470 to 550° C) and pressures (0 to 75 kg cm^–2 gauge). The lump coke from QI-free fractions had a comparable anisotropic development and coefficient of thermal expansion (CTE). The carbonization temperature and pressure were found to influence strongly the properties of the cokes. At the highest temperature of 550° C, the most appropriate pressure for the best needle coke was 15 kg cm^–2 G (gauge). Either higher or lower pressure increased the CTE value of coke. In contrast, at a moderate temperature of 500° C, the higher pressure produced the better coke. At the lowest temperature of 470° C, it took 10 h to complete the carbonization, and the lowest pressure allowed the best extent of uniaxial arrangement. Among the cokes prepared under the present conditions, the Carbonization at 500° C under 40 kg cm^–2 G produced the best needle coke with a CTE value as low as 0.1 × 10^–6° C^–1. The carbonization scheme leading into the needle coke is discussed for a better understanding of how the carbonization temperature and pressure cooperatively influence the quality of the resultant coke, in relation to the carbonization reactivity of coal-tar pitch.     

Effect of sulfide ions on the stress corrosion behaviour of Al-brass and Cu10Ni alloys in salt water

The stress corrosion cracking (SCC) behavior of Al-brass and Cu10Ni alloys was investigated in 3.5% NaCl solution in absence and in presence of different concentrations of Na₂S under open-circuit potentials using the constant slow strain rate technique. The results indicated that the Cu10Ni alloy is more susceptible to stress corrosion cracking than as-received Al-brass at strain rate of 3.5 × 10^–6 s^–1 in 3.5% NaCl in presence of high concentration of sulfide ions (1000 ppm). The sulfide ions (up to 500 ppm) has no effect on the stress corrosion cracking of the annealed Al-brass in 3.5% NaCl at two strain rates of 7.4 × 10^–6 and 3.5 × 10^–6 s^–1. The results support film rupture for Al-brass and sulfide stress corrosion cracking assisted with pitting corrosion for Cu10Ni at slip steps as the operating mechanisms.