High temperature deformation behavior of permanent casting AZ91 alloy with and without Sb addition

The elevated temperature deformation behavior of permanent cast magnesium alloy AZ91 with and without Sb addition has been investigated using slow strain rate (5.0 × 10^–4s^–1) elevated temperature tensile and constant load creep testing at 150°C and 50 MPa. The alloy with 0.4 wt% Sb showed a higher elevated temperature tensile strength and creep resistance due to the formation of thermal stable Mg₃Sb₂ precipitates and a smaller microstructure as well as the suppressing of the discontinuous precipitation. Plastic deformation of AZ91 based alloys is determined by motion of dislocation in basal plane and non-basal slip systems. The dislocation motion in a slip system is influenced by temperature, precipitates and other lattice defects. Dislocations jog, grain boundaries and/or precipitates are considered as obstacles for moving dislocations. The deformation twinning were founded in the creep process by TEM. Cross slip of dislocations was taken into account as the main softening mechanism for permanent cast AZ91 alloy during elevated temperature deformation process.     

Fermi surface and extended van Hove singularity in the non-cuprate layered perovskite superconductor Sr2RuO4

With high-resolution (22 meV) angle-resolved photoemission spectroscopy, the Fermi surface of the first copper free layered-perovskite superconductor, Sr₂RuO₄, was determined. We observed three bands to cross the Fermi energy in qualitative agreement with LDA band structure calculations; one electron-like surface encircling the point in the projected Brillouin zone, and two hole-like surfaces around the point. The most striking aspect of the measurements is the observation of an extended van Hove singularity. It is located 17 meV below the Fermi energy and extends around the point for about 0.2 Å^–1 along both the — — and the — — directions.These observations demonstrate that van Hove singularities near the Fermi surface are a more generic feature of layered oxides, and call for a clarification of their exact role in oxide superconductivity.We are grateful to D. Singh for making available his band structure calculations. D. H. Lu thanks the VW Foundation for financial support.     

Analysis of a family of unstable lattice orientations in (110) channel die compression

Summary The family of f.c.c. crystal orientations defined by loading direction (110) and any channel die constraint direction between ( ) and ( ) is kinematically unstable. We establish that the experimentally observed finite rotation of the lattice about the loading axis, for initial orientations in this range, is uniquely predicted by the constraints and critical slip-system inequalities without regard to particular hardening theory. We further establish that experimental information on the changing constraint stress would serve to distinguish among theories. Predictions of three specific hardening rules, including classical Taylor hardening and the simple theory, are illustrated for initial constraint directions ( ) and ( ). For the first of these orientations the predictions include constraint stress, lattice rotation, active and latent hardening, and overall crystal shearing to a logarithmic compressive strain of 1.0.     

The HREM study of precipitates in an Al−Zn−Mg alloy

A study of precipitate phases in an Al–Zn–Mg alloy in the T74 condition has been conducted by means of high resolution electron microscopy. It has been observed that G.P. zones, and phases exist simultaneously in the matrix. The G.P. zones are plate-like forming on (111) matrix plane. The phase has a hexagonal structure witha=0.496 nm,c=1.403 nm and the orientation relationship with the matrix is . A new orientation relationship between the phase and the matrix is found to be .     

Lattice dynamics,thermal expansion,and bulk modulus of ruthenium

The lattice dynamics, third-order elastic (TOE) constants, and the temperature variation of the volume Grüneisen function of ruthenium have been calculated using the nearest neighbor central interaction model for hexagonal metals proposed by Srinivasan and Ramji Rao. The TOE constants have been employed to calculate the low-temperature limit of the lattice thermal expansion, the Anderson-Grüneisen (AG) parameter , and the second Grüneisen constantq of ruthenium. has the value 2.79 for ruthenium. The high-temperature limit has the value 3.31, which agrees well with the experimental value 3.25 obtained from thermal expansion and specific heat data of ruthenium. Anderson's theory has been used to explain the temperature dependence of the bulk modulus of ruthenium up to 923 K and has been compared with the experimental values obtained from the elastic constant data of Fisher and Dever. The variation of the lattice parameters of ruthenium with hydrostatic pressure up to 400 kbar has been calculated from its TOE constant data using the extrapolation formula of Thurston and has been compared with the experimental results of Clendenen and Drickamer. The fit is remarkably good.     

Combined atomistic–crystal plasticity analysis of the effect of beta phase precipitates on deformation and fracture of lamellar γ + α2 titanium aluminide

Atomistic simulations based on the use of interatomic potentials and a finite element method based on the crystal plasticity theory are combined to investigate the deformation and fracture behaviour of polycrystalline lamellar -TiAl + ₂-Ti₃Al material containing 10 vol % of body centred cubic beta phase precipitates. The effects of both stable beta phase precipitates, which deform by slip, and metastable beta phase precipitates, which deform by a combination of stress-induced martensitic transformation and slip, are studied. To model the cracking along the grain boundaries and the matrix–precipitate interfaces, the grain boundaries and interfaces are modelled using a cohesive zone approach. The grain boundary–interface potentials are determined by carrying out atomistic simulations of the grain boundary–interface normal separation (decohesion) and sliding.The results obtained suggest that incompatibilities in the plastic flow between the adjacent grains in the single-phase material give rise to a large build-up in tensile hydrostatic stress in the region surrounding certain three-grain junctions, which, in turn, leads to nucleation of the grain boundary cracks and ultimate failure. The stable beta phase precipitates located at the three-grain junctions in the two-phase material help accommodate the incompatibilities in plastic flow, doubling the strain to failure. The lattice expansion, which accompanies martensitic transformation in the metastable beta phase precipitates, further delays nucleation of the grain boundary–interface cracks giving rise to an additional increase in the fracture strain.     

Structural features of Σ = 19, [110] GaAs tilt grain boundaries

= 19, [110] tilt grain boundaries have been observed to facet parallel to particular planes; the facets lie along _A/ _B, ( )_A/ )_B and ( )_A/( )_B. The structural unit of the = 19 ( )_A/( )_B [110] boundaries consists of 5- and 7-member rings, which are similar to the core structure of a/2[110] edge dislocations. The polarities in each grain on either side of the boundaries has been confirmed by CBED methods; a lower number of anti-site type cross-boundary bonds occur along the boundaries compared to when the polarity of one grain is reversed. The presence of 7-member rings and anti-site cross-boundary bonds results in a more open atomic structure at the boundary, shortening the distance between the first and the second {331} atomic planes from the boundary plane by 40%.     

Rheological behaviour and microstructure of stir-casting zinc-aluminium alloys

The influence of processing variables on the rheological and microstructural behaviour of stir-cast (rheocast) ZA-27 alloy (Zn-27 wt % Al-2wt % Cu) has been investigated experimentally. A concentric cylinder viscometer with shear rate range up to 650 sec^–1 was used to measure the apparent viscosity of the slurries. During continuous cooling and at high shear rates (300–640 sec^–1), non-dendritical materials obey the power-law fluid model, i.e. where _a is the apparent viscosity and the shear rate. At lower shear rates (125 sec^–1), the slurries display dendritical-liquid mixture with viscosity up to 50 poises. Microstructural studies of continuously cooled materials reveal a clear tendency of primary particles to cluster. This phenomenon could be explained by the reduction of the amount of entrapped liquid in the particles.     

Anisotropic lattice coherency of GaAs nanocrystals deposited on Si(100) surface by molecular beam epitaxy

Lattice coherency and morphology of GaAs nanocrystals grown on Si(100) substrate have been studied by transmission electron microscope in order to see growth mechanism of the nanocrystals. GaAs nanocrystals consisting of four {111} facet planes and a rectangular basal plane with four sides along [01 ] and [011] directions have grown on the Si surface. Either (011) or (01) lattice planes along the minor axis on the rectangular basal plane in the GaAs nanocrystal are completely-coherent with {011} lattice planes in the Si substrate. On the other hand, another (01) or (011) lattice planes along the major axis are partially-coherent with those in the Si substrate. When the lattice planes of the either (011) or (01), which is randomly determined by local atomic structures, become partially-coherent with those in the Si substrate to relax accumulated lattice strain, the growth rate of nanocrystal is remarkably increased along the direction parallel to unstrained (011) or (01) planes which prevents from each area of the strained {011} planes in the nanocrystals increasing. The anisotropic lattice coherency between the GaAs nanocrystals and the Si substrate causes the anisotropic morphology of the GaAs nanocrystals which is elongated the directions parallel to the strained {011} planes.     

Optimization of multilayer thermal insulation

An iteration method is developed for determination of the thicknesses of layers of a multilayer thermal insulation with minimum mass, with consideration of temperature limitations. The penalty function method is employed.Notation M(h) target function - _i thickness of the i-th layer - p_i density of material in i-th layer - n number of layers of thermal insulation - y spatial coordinate - t time - Y_i, i = 0, 1, 2, ..., n coordinates of layer boundaries - Cⁱ(T) volume heat capacity of material in i-th layer - ⁱ(T) thermal conductivity coefficient of material in i-th layer - (y) initial temperature distribution - q thermal flux - t_c right-hand value of time interval - T _max ⁱ , i = 1, 2, ..., n maximum admissible temperatures on i-th boundary - penalty function - penalty parameters - g_i function considering temperature limitations - transformed function - k number of successive unconditional minimization problem - l number of iteration in search for local minimum - ,, s parameters of conjugate gradient method Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 2, pp. 286–291, August, 1980.     

A deformation processed β-Ti + Y metal–metal composite

A Ti-20V-20Y deformation processed metal–metal composite was deformed axisymmetrically by extrusion and swaging to a true strain of 5.9. Tensile strength, ductility, Y phase thickness and spacing, and preferred crystallographic orientation were examined at several levels of true strain as the deformation progressed. The Ti-V metastable BCC solid solution matrix developed a (110) fiber texture. The Y second phase developed a fiber texture that constrained the Y phase to deform in plane strain. Relatively high tensile ductility was observed at all levels of deformation processing strain.     

X-ray Diffraction Study of Mixed-Layer Compounds in the Pseudobinary System SnTe–Bi2Te3

X-ray diffraction studies of cleaved single-crystal specimens and powders demonstrate that the SnTe–Bi₂Te₃ system contains a homologous series of nSnTe · mBi₂Te₃ layered compounds (n = 1, 2; m = 1–3). In addition to the known compound SnBi₂Te₄, the compounds SnBi₄Te₇ (n = 1, m = 2) and SnBi₆Te₁₀ (n = 1, m= 3), structural analogues of GeBi₄Te₇ and GeBi₆Te₁₀, are identified. SnBi₄Te₇ has a 12-layer structure with lattice parameters a = 0.4392(1) nm andc = 2.399(1) nm (sp. gr. P m1). SnBi₆Te₁₀ has a 51-layer structure with a= 0.4391(1) nm and c= 10.264(5) nm (sp. gr. R m). The new structural data for nSnTe · mBi₂Te₃ compounds and earlier results are used to reassess the SnTe–Bi₂Te₃ phase diagram.     

Effects of grain-boundary configuration on the high-temperature creep strength of cobalt-base L-605 alloys

The effects of grain-boundary configuration on the high-temperature creep strength are investigated using commercial cobalt-base L-605 alloys with low carbon content in the temperature range 816 to 1038° C (1500 to 1900° F). Serrated grain boundaries are formed principally by the precipitation of tungsten-rich b c c phase (the same as ₂ phase found in Ni-20Cr-20W alloys) on grain boundaries by a relatively simple heat treatment in these alloys. The creep rupture properties are improved by strengthening of grain boundaries by the precipitation of tungsten-rich bcc (₂) phase. The specimens with serrated grain boundaries have longer rupture lives and higher ductility than those with normal straight grain boundaries under low stress and high-temperature creep conditions, while the rupture lives and the creep ductility of both specimens are almost the same under high stresses below 927° C. The matrix of the alloys is strengthened by the precipitation of carbides at temperatures below 927° C and by the precipitation of tungsten-rich ₂ phase at 1038° C during creep. It is found that there is an orientation relationship between tungsten-rich a2 phase particles and-Co matrix, such that (0 1 1)₂ ¶ (1 1 1) _-Co and [1 1]₂ ¶ [1 0] _-Co. The fracture surface of specimens with serrated grain boundaries is a ductile grain-boundary fracture surface, while typical grain-boundary facets prevailed in specimens with straight grain boundaries.     

Testing of ungrounded voltage transformers

Conclusions In the investigated ungrounded voltage transformers the error _c is negligible in comparison with the error due to the class of accuracy. Nevertheless, in acceptance tests of newly designed ungrounded transformers it is desirable to determine the error and specify it in technical documentation. This should be done for the reason that as the accuracy of ungrounded transformers increases the error become comparable with the error corresponding to the class of accuracy of the transformer. Moreover, the error may prove to be dominant in metrological certification by methods [3, 4].Translated from Izmeritel'naya Tekhnika, No. 1, pp. 55–57, January, 1980.     

Isothermal decomposition of the ß′ phase in a Cu-Zn-Al alloy

The isothermal decomposition of theβ′ phase in a Cu-25 wt % Zn-6 wt % Al alloy was investigated using transmission electron microscopy and electron probe microanalysis. The ordering of theβ phase was too rapid to be suppressed during quenching from the solution annealing temperature. On ageing the alloy in the temperature range 603 to 703 K, theβ′ phase was found to decompose into a mixture of α+γ phases by the precipitation of fine and equiaxed γ-phase particles distributed uniformly throughout the matrix of α. The orientation relationship between α and γ was identified as $$\begin{gathered} (001)_\alpha ||(001)_\gamma \hfill \\ [010]_\alpha ||[010]_\gamma \hfill \\ \end{gathered} $$ The growth rate of the γ phase precipitates exhibited a maximum at 653 K. Electron probe microanalysis showed that the γ phase precipitates were enriched in aluminimum and depleted in zinc, compared to the α matrix. In addition to the uniform distribution of intragranular γ phase precipitates, heterogeneous precipitation of the α phase was observed along the grain boundaries indicative of a direct transformation ofβ′ to α in these regions: this reaction was found to be pronounced as the ageing temperature was increased up to 773 K.     

Thermal expansion and bulk modulus of cobalt

The ten third-order elastic (TOE) constants of hexagonal cobalt, which has nearly the idealc/a value, have been evaluated using the nearest-neighbor central interaction model for hexagonal materials proposed by Ramji Rao and Srinivasan. These TOE constants have been employed to evaluate the low-temperature limit of the thermal expansion, the Anderson-Grüneisen (AG) parameter , and the second Grüneisen constantq of cobalt. has the value 2.08 for cobalt. The temperature variation of the volume Grüneisen gamma of cobalt has been calculated on this model. The high-temperature limit has the value 2.13, which agrees well with the experimental value obtained from thermal expansion and specific heat data of cobalt. Anderson's theory has been used to explain the temperature dependence of the bulk modulus of cobalt and compared with the experimental values obtained from the elastic constant data of Fisher and Dever. The calculatedB _s values agree with the experimental values to within 1%.     

Distribution of extension rates of growth fronts along Rosiwal's line in the growing two-dimensional cell model

A growing two-dimensional cell model is defined as follows. In an area there are Poisson-distributed nuclei. Arising from these nuclei, grains start to grow simultaneously. All grains grow circularly with the same constant radial growth rate . During the process of growth no new nuclei are formed. If two grains touch each other, growth is stopped there by formation of a straight grain boundary. We arbitrarily put a straight line, called Rosiwal's line, into the area. While grains are growing many straight grain boundaries and circular growth fronts cross Rosiwal's line. At a fixed fraction transformed, F(=crystallized area/total area), we consider the different extension rates of growth fronts (growing borders) along Rosiwal's line, v( v<), in the left (or right) direction. The number of grains that have a growth front along Rosiwal's line into the left (or right) direction depends on F. Although the number changes with variation of F, we obtained theoretically the surprising result that the distribution density of reduced extension rates V = v/ , w(V), does not depend on F, and is always V ^–2(V ²–1)^–1/2. In order to verify this result we found an experimental possibility to realize the growing two-dimensional cell model.     

The crystallography and morphology of Mo2C in ferrite

The orientation relationships between hexagonal Mo₂C precipitates (H) in ferrite (B) have been determined by electron diffraction to an accuracy of ±2°. With one exception, the 19 results are consistent with the previously reported Pitsch and Schrader (P/S) orientation relationship. However, these more accurate determinations show clearly that there is a systematic deviation of up to 5.5° from the exact P/S relationship and that this deviation consists of a small rotation about the parallel close packed directions-[100]_B// . The long direction of the Mo₂C needles has been determined unequivocally in terms of the orientation relationship to be [100]_B// . Moiré fringes between precipitate and matrix have been used to improve the accuracy of the orientation relationship results and to determine the lattice parameters of the carbide precipitates investigated. The Moiré fringe analysis has shown small systematic departures from the exact parallelism between [100]_B and along the length of Mo₂C needles and a lowering of the carbide lattice parameter that is consistent with the replacement of Mo by Fe in the carbide. The orientation relationship results, including the observed systematic deviation from the exact P/S relationship, are shown to be consistent with the edge-to-edge model.     

Creep of (Mg,Fe)O single crystals

The creep behaviour of (Mg, Fe)O single crystals compressed along 1 0 0 has been investigated over the temperature range 1300 to 1500° C, at stresses between 20 and 70 MPa, for oxygen partial pressures between 10^–4 and 10² Pa, and with iron concentrations between 70 and 11 900 p.p.m. Under these conditions, the dependence of the steady-state strain rate on stress, temperature, oxygen partial pressure, and iron concentration can be summarized by the flow law, exp (–445 kJ mol^–1/RT. These results suggest that the steadystate strain rate is controlled by dislocation climb with a jog velocity which is limited by lattice diffusion of oxygen by a vacancy pair mechanism. The activation energy for creep, 445 kJ mol^–1 is larger than that reported for self-diffusion of oxygen, 330 kJ mol^–1, because the formation energy for jogs is relatively large, 115 kJ mol^–1.     

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.