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.     

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.     

Quasi-static and dynamic compression behaviour of an FPTM alumina-reinforced aluminium metal matrix composite

An aluminium metal matrix composite reinforced with continuous unidirectional -alumina fibres has been compression tested at quasi-static and dynamic strain rates. In the transverse direction, the composite showed increasing flow stress (at 5% strain) and maximum stress within the studied strain rates, 10^–3–3 × 10³ s^–1. In the longitudinal direction, the maximum stress of the composite increased similarly with increasing strain rates within the range 10^–5–7 × 10² s^–1. It is shown that, if brooming of the sample ends can be suppressed, the failure stress of the composite in longitudinal compression increases significantly. Metallographic observations reveal the typical modes of damage initiation in the composite.     

Strain-rate and temperature-dependent stress-strain curves of Sn-Pb eutectic alloy

Tensile tests on cast Sn-Pb eutectic alloy have been carried out at 25, 100, 140, 170°C and several strain rates ranging from 7×10^–3–7×10^–5s^–1. The stress-strain curves obtained were strongly dependent on strain rate and temperature. Under the conditions tested, higher strainrates gave a higher stress-strain curve and a larger strain at fracture. The tensile strength may be expressed by a power function of strain rate whose exponent and coefficient are governed by a kind of Arrhenius' equation. The nominal stress-strain curves showed significant work softening due to diffuse necking; however, the true stress-strain curve exhibited slight work softening. Thus the elastic perfectly viscoplastic model can be applied to the solder.     

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.     

Superplastic deformation of Al-Li-Cu-Mg alloy sheet

Al-2.5 Li-1.2 Cu-0.6 Mg-0.12 Zr (wt%) alloy sheet was cold-rolled, solution heat-treated for 20 min at 510° C, prestrained by 3% and superplastically deformed at 450 to 540° C at strain rates between 1×10^–4 and 2.8×10^–1 sec^–1. The maximum elongation obtained was 300%. Significant cavitation occurred above about 0.5 strain at a rate (void volume/unit strain) of 4% at 540° C and 6% at 500° C. The onset of cavitation coincided with a reduction in the room-temperature tensile properties after reheat-treatment. During annealing at 500 to 540° C, grain coarsening near the sheet surface was associated with magnesium and lithium depletion. Superplastic deformation produced a fine equiaxed microstructure by dynamic recrystallization.     

Whisker reinforcement of glass-ceramics

Silicon carbide whisker reinforcement of anorthite and cordierite glass ceramics has been studied. At 25 vol% whisker loading the flexural strengths increased from 65–103 MPa to 380–410 MPa, the fracture toughnesses increased from 1.0–1.5 MPa m^1/2 to 5.2–5.5 MPa m^1/2. The strengths decline to 240–276 MPa at 1200 °C. The reasons for the decrease in strength with temperature are discussed. Whiskers from two different sources with differences in diameters and aspect ratios were evaluated and the effect of the whisker morphology on the composite properties was studied. It was found that larger diameter, higher aspect ratio whiskers result in improved composite performance. The composites were also characterized in terms of their thermal properties, i.e. thermal expansions and thermal conductivities. The thermal expansion coefficient from 25–1000 °C for anorthite-based composite was 4.6×10^–6 °C^–1 and that for the cordierite-based composite was 3.62×10^–6 °C^–1. The thermal conductivities at 1000 °C were 3.75 and 4.1 Wm^–1 K^–1 for cordierite and anorthite composites, respectively.     

Texture changes during superplastic deformation of mechanically alloyed aluminium IN90211

Superplastic deformation of mechanically alloyed aluminium IN90211 was studied by texture analysis. The textures in three deformed specimens were investigated as a function of strain using the three-dimensional crystal orientation distribution functions (CODFs). The results for the two superplastically deformed specimens (425 °C, strain rate of 1 s^–1, stress near 50 MPa, and 475 °C, initial strain rate of 77 s^–1, about 110 MPa) indicate that at strains below about 2.0, the specimen deforms by grain-boundary sliding and single (or double) slip, and at larger strains the deformation is dominated by grain-boundary sliding, multiple slip and some recrystallization. At 475°C, 330s^–1, and stress near 160 MPa, the specimen was above the superplastic regime, and the resulting texture changes with deformation were markedly different from superplastic results, and quite unusual.     

Tensile creep behaviour of a fibre-reinforced SiC-Si3N4 composite

The tensile creep behaviour of a SiC-fibre-Si₃N₄-matrix composite was investigated in air at 1350 C. The unidirectional composite, containing 30 vol % SCS-6 SiC fibres, was prepared by hot pressing at 1700 C. Creep testing was conducted at stress levels of 70, 110, 150 and 190 MPa. An apparent steady-state creep rate was observed at stress levels between 70 and 150 MPa; at 190 MPa, only tertiary creep was observed. For an applied stress of 70 MPa, the steady-state creep rate was approximately 2.5×10^–10 s^–1 with failure times in excess of 790 h. At 150 MPa, the steady-state creep rate increased to an average of 5.6×10^–8 s^–1 with failure times under 40 h. The creep rate of the composite is compared with published data for the steady-state creep rate of monolithic Si₃N₄.     

Cyclic surface deformation behaviour of ZrH2-purified iron: Effects of solute carbon,strain amplitude,strain rate and environment

The surface deformation behaviour in ZrH₂-purified interstitial-free iron was studied during fatigue in the push-pull plastic strain control mode under various combinations of plastic strain amplitude (5×10^–4, 5×10^–3), strain rate (5×10^–4, 3× 10^–2s^–1) and environment (ultrahigh vacuum, oxygen). Comparative tests of vacuum-melted commercially pure iron (CPI) containing 170 p.p.m. C were also conducted to investigate the effect of interstitials. At a plastic strain amplitude of 5×10^–4, the environmental effect is clearly exhibited regardless of the strain rate and the presence of interstitials. Fatigue in ultrahigh vacuum produces very fine slip lines not only in interstitial-free iron but also in vacuum-melted CPI. In the presence of oxygen, fatigue produces prominent slip lines, but those developed in CPI are more intense and coarser than those developed in interstitial-free iron. At the higher plastic strain amplitude of 5×10^–3, the gaseous environmental effect on the cyclic surface deformation is insignificant. The surface deformation behaviour is discussed in terms of the environmental effect and the basic mechanisms that govern the cyclic plasticity of iron.     

The evaluation of hot-working characteristics of cobalt-based implant alloys

The hot-working characteristics of wrought Co-Ni-Cr-Mo implant alloy during ingot-to-billet conversion were evaluated using a Gleeble-2000A simulator. The hot tensile test at 700–1 320 °C was used to determine the optimum hot-working parameters at a strain rate equivalent to that of conventional press forging to ensure acceptable hot workability. Hot ductility and deformation resistance as a function of temperature can be clearly established. The fracture surfaces of the tensile specimens were examined to correlate them with the hot tensile ductility values at various temperatures. The poor ductility at temperatures above 1300 °C was attributed to the incipient melting of grain boundaries. The effect of temperature and strain rate on the flow-stress behaviour and microstructures were investigated by uniaxial compression testing in the temperature range 900–1200 °C and strain rate, , range of 0.01–10s^–1. The strain-hardening and steady-state behaviour were described from the measured true stress-true strain curves.     

Processing maps for hot working of Cu-Ni-Zn alloys

The constitutive behaviour of — nickel silver in the temperature range 700–950 °C and strain rate range 0.001–100 s^–1 was characterized with the help of a processing map generated on the basis of the principles of the dynamic materials model of Prasadet al Using the flow stress data, processing maps showing the variation of the efficiency of power dissipation (given by 2m/(m+1) wherem is the strain-rate sensitivity) with temperature and strain rate were obtained, -nickel silver exhibits a single domain at temperatures greater than 750 °C and at strain rates lower than 1s^–1, with a maximum efficiency of 38% occurring at about 950 °C and at a strain rate of 0.1 s^–1. In the domain the material undergoes dynamic recrystallization (DRX). On the basis of a model, it is shown that the DRX is controlled by the rate of interface formation (nucleation) which depends on the diffusion-controlled process of thermal recovery by climb. At high strain rates (10 and 100s^–1) the material undergoes microstructural instabilities, the manifestations of which are in the form of adiabatic shear bands and strain markings.     

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.     

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.     

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.     

Thermal and dielectric properties of zirconyl phosphate compact

Experimental results are presented on the measurements of thermal expansion (up to 1500°C), thermal conductivity (up to 1000°C), dielectric constant (up to 450 °C) and tan (up to 800 °C) of zirconyl phosphate compacts obtained by sintering at 1600°C. The thermal expansion coefficient of the samples at the temperature below 1100°C was less than 1.7 × 10^–6°C^–1. The samples showed a definite shrinkage at temperatures of 1110 and 1470°C due to the phase transformations. The expansion at 1500°C was less than that at 1100°C probably because of the phase transformation. The thermal conductivity at room temperature was a very small value (0.0046 to 0.0065 cal s^–1 cm°C^–1 cm^–2). The dielectric constant was close to 9. The value of tan° (–0.0001) measured is one of the lowest values for ceramic materials.     

Modeling of Constitutive Behavior for Epon 828/T-403 at High Strain Rates

Experimental results on the uniaxial compressive stress-strainbehavior of Epon 828/T-403 over a strain rate range from 1.1 × 10^–4 to 5.2 × 10³ s^–1were simulated using a viscoelastic viscoplastic constitutive modeldeveloped by Hasan and Boyce (1995). An optimal combination of materialparameters for the constitutive model was determined by curve-fittingthe experimental results. A comparison between the modeling andexperimental results shows that this model, with proper parameters, iscapable of capturing the strain-rate effects, as well as the typicalpolymeric compressive stress-strain behavior, of Epon 828/T-403, whichincludes the stages of linearly elastic, nonlinearly elastic, yield-like(peak) behavior, strain softening, and nearly perfect flow. For all thestress-strain behavior modeled over seven decades of strain rate span,the maximum error between modeling and experimental results is less than25%. The temperature increase induced by the dissipated plasticwork is also computed and found to be between 3 and8°C at these strain rates.     

Crack formation and oxidation in superplastically deformed Si3N4

Crack formation and oxidation during superplastic deformation of Si₃N₄ were studied and the superplastic forming ability of Si₃N₄ was discussed. Tensile deformation tests were conducted under a 1 atm nitrogen atmosphere at 1600 °C, and under a constant crosshead speed with an initial strain rate of 2×10^–5s^–1. The microstructures of superplastically deformed specimens were observed by SEM. The relation between chemical composition and microstructure was determined by EPMA. After 280% deformation (at fracture), the formation of regions rich in glassy phase was observed. These regions, supposed to be formed due to oxidation of cracks, appeared just before fracture. The present material is capable of being deformed up to strains of 210%.     

Plastic deformation behavior in dual-phase Ni–31Al intermetallics at elevated temperature

Plastic deformation behavior of dual-phase Ni–31Al intermetallics at elevated temperature was examined. It was found that the alloy exhibited good plasticity under an initial strain rate of 1.25 × 10⁻⁴ s⁻¹ to 8 × 10⁻³ s⁻¹ in a temperature range of 950–1075 °C. A maximum elongation of 281.3% was obtained under an initial strain rate of 5 × 10⁻⁴ s⁻¹ at 1000 °C. The strain rate sensitivity, m value was correlated with temperature and initial strain rate, being in the range of 0.241–0.346. During plastic deformation, both the two phases Ni₃Al and NiAl in dual-phase Ni–31Al could co-deform without any void formation or debonding, the initial coarse microstructure became much finer after plastic deformation. Dislocation played an important role during the plastic deformation in dual-phase Ni–31Al alloy, the deformation mechanism in dual-phase Ni–31Al could be explained by continuous dynamic recovery and recrystallization.     

A new grain-refinement process for superplasticity of high-strength 7075 aluminium alloy

The superplasticity of high strength 7075 aluminium alloy has been improved to a great extent by the new thermomechanical treatment proposed. This treatment (TMPA) includes solution treatment, overageing, warm-rolling deformation, recrystallization and an artificial ageing process. The maximum elongation may be up to 2100% under deformation at an initial strain rate of 8.33×10^–4s^–1 and a temperature of 510 °C, which is much higher than reported before. Observation of the microstructure changes revealed that the excellent superplastic elongation of the alloy seems mainly to be due to a decrease in the grain growth rate of the alloy and a reduction in the number of cavities nucleated during superplastic deformation.