Self-Diffusivity Along Edge-Dislocation Singular Lines in Silver

Use was made of a recently developed surface-accumulation diffusion technique to measure the self-diffusivity of edge-type dislocation singular lines (Burgers vector along <110>) in a bent and polygonized single crystal of silver. Two factors are involved in obtaining a number for the diffusivity. One factor, the dislocation-line density, was measured by a microscopic technique recently developed, and the value of the other parameter, the effective “diameter” of a dislocation pipe, was estimated. With these parameters equal to 7.3×10⁶ dislocations per sq cm and 10^?7 cm, respectively, it was found that the self-diffusivity along edge-type dislocation lines was a factor of about two times the average grain-boundary self-diffusivity. The dislocation-pipe diffusivity is about 7×10^?7 sq cm per sec at 450°C.     

Structural changes associated with strain-induced grain boundary migration in Si-Fe

Strain-induced grain boundary migration was observed in several bicrystal sheet specimens of Si-Fe (3¼ pct Si) which were cold rolled 2 to 12 pct and then annealed at temperatures up to 1200°C. A chrome-acetic acid electroetching method was used to reveal the dislocation sites before and after grain boundary migration. Recovery effects were noted in the microstructure prior to boundary motion. Consequently, the residual strain energy in neighboring grains may determine if boundary migration with resulting increase of area occurs, and its direction of movement. Microstructural data indicate that the region initially traversed by the moving grain boundary has many structural defects in the form of low-angle boundaries and random dislocations of relatively high density. With increased distance of grain boundary migration, the density of these imperfections was found to decrease. Continued annealing at 1200°C, after boundary migration, lowered the density of random dislocations in the swept region to a limiting value of about 2×10⁶ lines per sq cm.     

High temperature deformation behaviors of a high Nb containing TiAl alloy

In the present paper, high temperature tensile and creep behaviors of Ti–45Al–9(Nb,W,B,Y) alloy with duplex (DP) microstructure were investigated. In addition to tensile tests at 815 °C and a strain rate range of 1 × 10⁻⁴ s⁻¹−1 × 10⁻³ s⁻¹ and tensile, creep tests at 760 °C and 815 °C under the stress of 180 MPa, the microstructure evolutions during tensile and creep tests were studied. The results show that high temperature high Nb containing TiAl alloy with DP microstructure has a good balance between ductility and strength and intermediate creep resistance. The tensile properties have the strain rate dependence, and ultimate tensile strength (UTS) and yield strength (YS) vs. strain rate obey a single-logarithm linear relationship. Minimum creep rate is affected by the test temperature and stress. Using loading change experiment a stress exponent of 4.3 is determined. DP microstructure is unstable after long-term exposure at high temperatures, and the spheroidization of lamella and recrystallization along grain boundaries occur during the high temperature deformation. It is assumed that the diffusion-assisted climb of dislocations might be the controlling mechanism at the minimum creep rate stage.     

Thermal-cycling creep of γ-TiAl-based alloys

In two-phase TiAl-based alloys, the coexisting α₂ and γ phases exhibit a thermal expansion mismatch, so that increased creep rates during thermal cycling may be expected. Creep deformation of two γ-TiAl-based alloys was investigated during thermal cycles between 900 and 300 or 350°C with applied tensile stresses of 32.5 or 37.0 MPa. Measured thermal cycling creep rates were compared with isothermal creep rates calculated at the effective average temperature. No creep enhancement was measured upon thermal cycling within uncertainties of 1.6×10⁻³% strain per cycle.     

The controlling creep processes in TiAl alloys at low and high stresses

The creep response of a nearly-lamellar Ti–47Al–4(W, Nb, B) alloy is studied at 760 °C in a wide stress range 100–500 MPa. The alloy exhibits excellent creep resistance with a minimum creep rate of 1.2×10⁻¹⁰/s at 100 MPa and the time to 0.5% creep strain of 1132 h at 140 MPa. The controlling creep process is probed by analysis of the post-creep dislocation structure and by observation of incubation period during stress reduction test. The results indicate that creep is controlled by dislocation climb at low stresses (Class II type) and by jog-dragged dislocation glide at high stresses (Class I type). The transition from Class II to Class I type creep occurs at about 180 MPa. The excellent creep resistance of the studied alloy compared to other W containing TiAl alloys is attributed to its highly stable lamellar microstructure consisting eventually of coarse gamma laths.     

Einfluß der Aufkohlung auf das Kriechverhalten einer FeNiCr-Hochtemperaturlegierung

Effects of Carburization of the Creep Behaviour of a FeNiCr-High Temperature Alloy Incoloy 800 and Incoloy 800 doped with 1% Nb were carburized at 1000 °C in CH₄–H₂ to 0.83% C (mass content). The undoped alloy shows relatively coarse large M₂₃C₆ carbides at the grain boundaries, the alloy with 1% Nb has mainly fine carbides in the grains. Creep experiments were performed with the carburized and uncarburized specimens at 1000 °C, in which creep rates were attained in the range 10^?9… 10^?7 s^?1 of secondary creep. The stress dependence of the creep rate indicates two creep mechanisms: diffusion creep at low stresses and dislocation creep at high stresses. The diffusion creep is faster for both alloys after carburization. The dislocation creep is retarded by carburization for the undoped alloy. At about equal creep rate ε = 10^?7 s^?1 the carburized specimens have a longer lifetime. The fracture is brittle for Incoloy 800 in the uncarburized and carburized state, characterized by void and crack formation and poor reduction in area. The fracture of the carburized Incoloy 800 with 1% Nb is rather ductile with less void formation. The results indicate that carburization does not deteriorate the creep behaviour of the FeNiCr alloy if the reached carbon content is not too high. An addition of Nb is very favorable for the creep properties after carburization.     

Creep microstructures and creep behaviors of pure molybdenum sheet at 0.7 Tm

The creep behavior of high purity molybdenum (99.995 wt.%) is investigated at temperatures of 1600 to 2000 °C (0.61–0.77 T_m) by direct ohmic heating. The stress and temperature dependency of creep strain and rupture time are described through optical microstructure observations. Under low load and low temperature conditions, coarse secondary recrystallized grains caused by dynamic recrystallization are observed far from the crack tip. In contrast, under high load and high temperature conditions, coarse secondary recrystallized grains are only fully formed near the crack tip, while coarse secondary recrystallized grains and small primary recrystallized grains coexisted further away from the tip. The recrystallized grain size of the Mo-B sheet is smaller than that of the Mo-A sheet, and small primary and large secondary recrystallized grains are mixed throughout whole specimens of the Mo-B sheet. Mo-A sheet shows elongated ductile fracture, but Mo-B sheet shows irregular brittle fracture under the same conditions. The steady-state creep strain rate at 1800 °C is found to be 7.34 × 10^− 6, 2.83 × 10^− 5 and 1.53 × 10^− 4 s^− 1 under a constant stress of 5, 10 and 20 MPa, respectively. The stress exponent is estimated to be 3.85–3.98 and the strain activation energy for steady state creep is 362–413 kJ/mol.     

Effect of Ca addition on the as-cast microstructure and creep properties of Mg-5Zn-5Sn magnesium alloy

The effect of Ca addition on the as-cast microstructure and creep properties of Mg-5Zn-5Sn magnesium alloy was investigated. The results indicate that adding 1.0 wt.% Ca to Mg-5Zn-5Sn alloy can effectively refine the as-cast microstructure of the alloy, and the CaMgSn phase with high thermal stability is formed in the alloy. In addition, adding 1.0 wt.% Ca to Mg-5Zn-5Sn alloy can also improve the creep properties of the alloy. After adding 1.0 wt.% Ca to Mg-5Zn-5Sn alloy, the second creep rate of the alloy at 150°C and 50 MPa for 100 h decreases from 4.67 × 10⁻⁸ to 1.43 × 10⁻⁸ s⁻¹. The strengthening mechanism is mainly attributed to the microstructural refinement and the formation of CaMgSn phase.     

Effects of deviations from stoichiometry on the deformation behaviour of FeAl at intermediate temperatures

The stress–strain behaviour and creep of polycrystalline and monocrystalline FeAl phases with various deviations from stoichiometry was studied at temperatures up to 1000°C with emphasis on intermediate temperatures between 600°C and 1000°C. The results revealed strong effects of impurity precipitates besides the strong effects of the point defects. The transition of FeAl from low-temperature behaviour to high-temperature behaviour occurs at about 1000°C for 10⁻³ s⁻¹ compression rate and at lower temperatures for lower rates. Creep was observed with subgrain formation and stress exponents primarily in the range 4–5. The apparent activation energy for creep is of the order of the activation energy for bulk diffusion only above 950°C, whereas much higher activation energies were observed for creep at lower temperatures. Stress–strain behaviour with yield points was observed in the intermediate temperature range 600–1000°C.     

High-temperature compressive creep of liquid phase sintered silicon carbide

Creep of liquid phase sintered SiC has been studied at temperatures between 1575 and 1700°C in argon under nominal stresses from 90 to 500 MPa. Creep rates ranged from 3×10⁻⁸ to 10⁻⁶/s, with an activation energy of 840±100 kJ/mol (corresponding to carbon and silicon self-diffusion), and a stress exponent of 1.6±0.2. The crept samples showed the presence of dislocation activity, generally forming glide bands and tangles. Degradation of the mechanical properties due to cavitation or reaction of the additives was not detected. SEM and TEM microstructural characterization and analysis of the creep parameters leads to the conclusion that the creep mechanisms operating are grain boundary sliding accommodated by lattice diffusion and climb-controlled dislocation glide operating in parallel. Other possible operating mechanisms are discussed and the data are compared with published data.     

Creep mechanism of as-cast Mg-6Al-6Nd alloy

The creep mechanism of as-cast Mg-6Al-6Nd alloy was studied. The stress exponent for creep is 5.8 under the applied stresses of 50–70 MPa at 175°C. The activation energy for creep is 189 kJ·mol⁻¹ under the applied stress of 70 MPa in the range of 150–200°C. The true stress exponent and threshold stress for creep are calculated as 4.96 and 10.2 MPa, respectively. The true stress exponent indicates that its creep mechanism belongs to the dislocation climb-controlled creep, which is in agreement with the microstructure changes before and after creep. The high value for stress exponent is attributed to the interaction of Al₁₁Nd₃ phase with dislocations. The activation energy is more than the self-diffusion activation energy of Mg, which is attributed to the load transfer taking place from the matrix to Al₁₁Nd₃ phase during creep.     

The resistance to sliding along Coulombic shear faults in ice

Sliding experiments were performed along Coulombic shear faults in laboratory-grown freshwater ice over a range of sliding velocities (4 × 10⁻³–8 × 10⁻⁷ m s⁻¹) and temperatures (−3, −10 and −40 °C). The Coulombic failure criterion was used to describe the observed linear relationship between the shear stress along and the normal stress across the fault. From this relationship the coefficient of friction was determined. At each temperature the coefficient of friction peaks at a transitional velocity (∼8 × 10⁻⁶ m s⁻¹). For a given velocity the coefficient of friction increases with decreasing temperature. We propose that the peaked shape of the coefficient of friction vs. sliding velocity graphs are a consequence of a change in sliding behavior from “ductile-like” at low velocities to “brittle-like” at higher velocities. The velocity-strengthening and velocity-weakening friction regimes are attributed to creep and to frictional melting, respectively.     

Steady-state creep of a particulate SiC/6061 Al composite

The creep behavior of a 10 vol.% silicon carbide particulate reinforced 6061 Al composite produced by powder metallurgy (PM) has been examined by creep tests in both tension and compression at 400°C. The tensile creep data covering minimum creep rates of the orders 10⁻⁹ to 10⁻⁴/s show an apparent stress exponent n_app≈13, but a comparison with compressive creep data reveals that some high strain-rate data in tension are due to the transition to the tertiary stage. Analysis of the data is made only for the steady-state creep rate, together with that for an unreinforced PM 6061 Al alloy, by incorporating a threshold stress. This gives a stress exponent n=3 for the matrix alloy, whereas the composite data show such a trend that the n value gradually changes from 3 to 1 as the effective stress increases. A new method of steady-state creep data analysis is formulated by taking account of the interface-confined diffusional flow and thereby the finding above is reasonably assessed.     

Some Aspects of Slip in Germanium

Germanium single crystals strained in tension at 600°C slip on the {111} plane and, macroscopically at least, in the <110> direction. Deformation is inhomogeneous: various localized rotations are observed, as is a banding consistent with secondary slip banding. The structure after deformation is polygonized with a domain size of about 2×10^?3 cm. In relaxation tests, an incubation period prior to flow is observed, of duration inversely related to temperature and applied stress. Under continuous loading, there is a sharp first yield point. A critical resolved shear stress therefore must be cited with respect both to temperature and to rate of loading. At 600°C, when loading proceeds at 2900 psi per min, it is 1310 psi. The yield point phenomenon is suggestive of Cottrell’s solute atom atmosphere theory and five points of qualitative agreement with this theory are found.     

Transgranular Stress-Corrosion Cracking

Abstract

The rates of dissolution of atoms from steps of different widths on a metal surface are calculated. It is shown that there is a critical spacing, below which the rate of dissolution possible from a step falls rapidly. This critical spacing is related to the solution viscosity and hence the solution composition and temperature by the relationship:

l _crit = 8 παb³η10¹³ √2V₂(ε_s ? ε_∞)/ε_s²KT³ For a stainless steel immersed in 42% MgCl₂ solution at 150°, the calculated critical value of the slip line spacing is 6 × 10^?5cm. This agrees well With the experimental observations, as a steel which has a spacing of 4 × 10^?5 cm. takes fifty times longer to fail in a stress-corrosion test under these conditions than does a steel with a spacing of 7 × 10^?5 cm. The reactivities of steps of different widths on copper alloys are also calculated and it is shown that slip line spacings found in cold worked copper-zinc alloys containing between 10–30% Zn lie in the critical range where the reactivity of the step changes rapidly with step width.     

Tensile superplastic deformation of YBa2Cu3O7−x high-TC superconductors

The superplastic behavior of polycrystalline YBa₂Cu₃O_7−x was investigated. Tensile elongations above 85% were achieved for fine-grained (<1 μm) microstructures tested in the temperature range of 800–875°C and strain rates varying from 6×10⁻⁶ to 10⁻³/s. It is suggested that the dominant superplastic deformation mechanism is grain boundary sliding accommodated and controlled by interface reaction, characterized by a stress exponent of n=2, a grain size exponent of p=1.5, and an activation energy of Q_sp=515±104 kJ/mol. A Langdon–Mohamed deformation mechanism map was constructed. Overlaying the available experimental data onto the map, including the results from creep studies, gives an insight into the deformation mechanisms involved in high-temperature deformation of YBa₂Cu₃O_7−x.     

Hot deformation behavior of TiAl alloys prepared by blended elemental powders

A nearly full dense Ti-45Al-7Nb-0.4W (at.%) alloy billet with dimension of 120 mm in diameter and 50 mm in height was fabricated by reactive sintering of blended elemental powders. The high temperature deformation behavior was investigated by isothermal compressive tests, performed at temperature in 1000–1200 °C with strain rates from 1 × 10^?3 s^?1 to 1 × 10^?1 s^?1. Results indicate that the dependence of flow stress on temperature and strain rate is well fit for a hyperbolic-sine relationship using the Zener–Hollomon parameter. The measured apparent activation energy Q and stress exponent are determined as 420 kJ mol^?1 and 3.7, respectively. High oxygen content, high Nb content and fine grain size are main reasons for the high activation energy and high strength of PM TiAl alloy. An appropriate set of deformation processing parameters of 1200 °C and 1 × 10^?3 s^?1 are recommended for the present TiAl alloy.     

Electrical and Mechanical Properties of 3D-Printed Graphene-Reinforced Epoxy

Recent developments in additive manufacturing have demonstrated the potential for thermoset polymer feedstock materials to achieve high strength, stiffness, and functionality through incorporation of structural and functional filler materials. In this work, graphene was investigated as a potential filler material to provide rheological properties necessary for direct-write three-dimensional (3D) printing and electrostatic discharge properties to the printed component. The rheological properties of epoxy/graphene mixtures were characterized, and printable epoxy/graphene inks formulated. Sheet resistance values for printed epoxy/graphene composites ranged from 0.67 × 10² Ω/sq to 8.2 × 10³ Ω/sq. The flexural strength of printed epoxy/graphene composites was comparable to that of cast neat epoxy (~ 80 MPa), suggesting great potential for these new materials in multifunctional 3D-printed devices.     

Crystal Structure of Delta-Prime Plutonium And the Thermal Expansion Characteristics Of Delta,Delta-Prime,and Epsilon Plutonium

The σ′ plutonium is found to be body-centered-tetragonal with two atoms per unit cell at 000 and 1/2, 1/2, 1/2. The unit cell dimensions at 477°C are α = 3.339 ± 0.003Å and c = 4.446 ± 0.007Å, which lead to a calculated density of 16.01 g per eu cm. Each plutonium atom is surrounded by 12 others at a mean distance of 3.275?. The linear coefficient of thermal expansion perpendicular to the c-axis is 10⁶ ?_a = 305 ±35 per °C, and parallel to the c-axis is 10⁶ ?_c=?659 ± 67 per °C. The average linear coefficient, (2?_a + ?_c)/3, is equal to 10⁶?_L = ?18 ± 28 per °C. The linear coefficient of themal expansion of face-centered-cubic σ plutonium is found to be 10⁶ ?_L=?8.6 ± 0.3 per °C, and of body-centered-cubic ∈ plutonium 10⁶ ?_L = 36.5 ±1.1 per °C.     

Superplastic deformation behaviour of friction stir processed 7075Al alloy

Commercial 7075Al rolled plates were subjected to friction stir processing (FSP) with different processing parameters, resulting in two fine-grained 7075Al alloys with a grain size of 3.8 and 7.5 μm. Heat treatment at 490 °C for 1 h showed that the fine grain microstructures were stable at high temperatures. Superplastic investigations in the temperature range of 420–530 °C and strain rate range of 1×10⁻³–1×10⁻¹ s⁻¹ demonstrated that a decrease in grain size resulted in significantly enhanced superplasticity and a shift to higher optimum strain rate and lower optimum deformation temperature. For the 3.8 μm 7075Al alloy, superplastic elongations of >1250% were obtained at 480 °C in the strain rate range of 3×10⁻³–3×10⁻² s⁻¹, whereas the 7.5 μm 7075Al alloy exhibited a maximum ductility of 1042% at 500 °C and 3×10⁻³ s⁻¹. The analyses of the superplastic data for the two alloys revealed a stress exponent of 2, an inverse grain size dependence of 2, and an activation energy close to that for grain boundary self-diffusion. This indicates that grain boundary sliding is the main deformation mechanism for the FSP 7075Al. This was verified by SEM examinations on the surfaces of deformed specimens.