Deformation behaviour of retained β phase inβ-eutectoid Ti-Cr alloys

Plastic deformation mode and its relation to tensile properties were investigated in retained phase of-eutectoid type Ti-Cr alloys. A plate-like single variant phase is induced during deformation of the most unstable phase having a minimum chromium content required to suppress martensitic transformation. A selected area electron diffraction pattern taken from a boundary region of the stress-induced phase plate can be explained by the idea that a single variant of phase is induced in a {3 3 2} 1 1 3 twin produced during deformation. Anisotropy in population of four phase variants decreases with increasing chromium content. On further increasing chromium content, deformation occurs by slip. Enhanced ductility is obtained in as-quenched Ti-Cr alloys accompanied by phase transformation or {332} 1 1 3 twinning during deformation, phase of as-quenched Ti-Cr alloys changes continuously from commensurate structure with sharp reflections to incommensurate structure with diffuse reflections with increasing chromium content. The obtained results in-eutectoid type Ti-Cr alloys are quite similar to those in-isomorphous type Ti-V alloys.     

Precipitate strengthening mechanisms in magnesium zinc alloy single crystals

The mechanical behaviour of Mg-5.1 wt % Zn alloy single crystals was studied in the 4.2 to 300° K temperature range. Quenched crystals have activation energies and volumes best associated with the cutting of small clusters of Zn atoms by dislocations. Fully hardened crystals contain fine ₁ and occasional ₂ precipitates with an average ₁ interparticle spacing of 330 to 660 Å. Strengthening in these crystals is mainly ascribed to the cutting of ₁ particles by dislocations. In the overaged condition ₁, ₂ and equilibrium particles are present and lead to a considerable temperature-dependence unusual for an overaged condition. Analysis of this temperature-dependence suggests that below 77° K the relatively easy cutting of ₁ particles by dislocations takes place in addition to the cutting of ₂ and particles. Above 77° K the difficult cutting of ₂ and particles alone controls the deformation, ₁ being more easily cut with the aid of thermal fluctuations.     

Some electrical characteristics of lithium and yttrium sialons

Some electrical properties of hot-pressed lithium sialons, Li _x/8Si_6–3x/4Al_5x/8O _x N_8–x havingx<5 and an yttrium sialon were measured between 291 and 775 K; the former consisted essentially of a single crystalline phase whereas the latter contained 98% glassy phase. For lithium sialons, the charging and discharging current followed al(t) t ^–nlaw withn=0.8 at room temperature. The d.c. conductivities were about 10^–13 ohm^–1 cm^–1 at 291 K and rose to 5×10^–7 ohm^–1 cm^–1 at 775 K. At high temperatures electrode polarization effects were observed in d.c. measurements. The variation of the conductivity over the frequency range 200 Hz to 9.3 GHz followed the () ⁿ law. The data also fitted the Universal dielectric law,() ^n–1 well, and approximately fitted the Kramers-Kronig relation()/()– =cot (n/2) withn decreasing from 0.95 at 291 K to 0.4 at 775 K. The temperature variations of conductivities did not fit linearly in Arrhenius plots. Very similar behaviour was observed for yttrium sialon except that no electrode polarization was observed. The results have been compared with those obtained previously for pure sialon; the most striking feature revealed being that _d.c. for lithium sialon can be at least 10³ times higher than that of pure sialon. Interpretation of the data in terms of hopping conduction suggests that very similar processes are involved in all three classes of sialon.     

Coupling of order parameter collective modes to spin density in3He-B

We derive a general expression for the dynamic spin susceptibility of³He-B which is valid for all magnetic fields. The coupling of real and imaginary modes by particle-hole asymmetry is taken into account. Then we calculate the contribution of the mode at frequency =2 – 1/4 ( is the effective Larmor frequency) to the transverse susceptibility. The spectral weight of this mode in magnetic resonance absorption is proportional to (/)^1/2 (–)², where and are particle-hole asymmetry parameters. From the experimental coupling strength of the real squashing mode to sound we estimate (–)²10^–4. The dynamic susceptibility satisfies the sum rules of Leggett. Finally we point out the difficulties in calculating the transverse NMR frequency of³He-B. These difficulties arise from theS _z =0 Cooper pairs and from the coupling ofJ _z =±1 modes forJ=1 andJ=2.     

The Ni3Al-Ni3Cr-Ni3Ru section of the Ni-Cr-Al-Ru system

An investigation is reported of the 75 at% nickel section of the Ni-Cr-Al-Ru system at 1523 and 1273 K. Constitutional data obtained by electron probe microanalysis, X-ray diffraction and microscopical examination are presented as partial isothermal sections. At 1523 K, the major part of the section consists of phase, while the aluminium-rich region contains a and+ region; the extent of the solid solution of chromium and ruthenium in totals ~ 4 at%. The ruthenium-rich corner of the section shows a two-phase region consisting of + ruthenium-rich solid solution. At 1273 K the,+ and + ruthenium regions increase in extent. The/ mismatch values in the equilibrated alloys studied lie in the range ~ –0.08 to –0.39%. Constitutional features of as-cast alloys are also reported.     

Steady-state creep in a 25 wt % Cr-20 wt % Ni austenitic stainless steel

Steady-state creep behaviour of a 25 wt % Cr-20 wt % Ni stainless steel without precipitates was studied in the stress range 9.8 to 39.2 MPa at temperatures between 1133 and 1193 K. The results of stress-drop tests indicate that, in the steady-state creep region, diffusion-controlled recovery creep is dominant. Such recovery creep can be accounted for in terms of the composition of the internal stress, _i=_s+_c, except in the case of fine-grained specimens where d<80 m, whered is the mean grain diameter, _s is possible to reduce easily and is comparable to the driving stress for creep, and _c is the persistent stress field due to metastable substructure. In the fine-grained specimens, it is suggested that the steady-state creep is dominantly controlled by grain boundaries.     

Stability of metastable phases and microstructures in the ageing process of Al–Mg–Si ternary alloys

Precipitation behaviour of Al–Mg–Si alloys, with balanced (Mg/Si=2), excess silicon (Mg/Si<2) and excess magnesium (Mg/Si>2) compositions, were studied by differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and Vickers hardness tests. Four significant exothermic peaks were observed in DSC curves which were attributed to metastable clusters, , and stable phases. The peaks corresponding to and were formed closely in the DSC curves but showed different behaviour in isothermal annealing. The additional peak verifying the precipitation of phases, which has recently been proposed by some workers, was not detected. Transmission electron microscope observations and Vickers microhardness tests showed that precipitates played a major role in improving the hardness, but not precipitates. © 1998 Chapman & Hall     

Interplay of Nuclear Magnetism and Superconductivity

The ratio of nuclear saturation magnetization and superconducting critical field, ₀ M _sat / B _S0*, classifies the strength of mutual influence of nuclear magnetism and superconductivity. In order to investigate the interplay of both phenomena for the three distinct cases 1, 1, and 1 we have measured the ac susceptibility of Al, of the intermetallic compound AuIn ₂ , and of the metal hydride TiH _2.07 at ultralow temperatures, 17 K T 1 K, as function of static field 0 B 15 mT. For Al, the interplay enables an absolute measurement of the nuclear magnetization. For AuIn ₂ , we get a steep decrease of B _S (T) and a broadening of the superconducting transition in its nuclear ferromagnetic phase. Surprisingly, the nuclear ferromagnetic state coexists with type-I superconductivity in AuIn ₂. The metal hydride TiH _2.07 , which is under present investigation, is a good candidate to show reentrant superconductivity.     

Intergranular Critical Current Density in Polycrystalline (Bi-Pb)-2223 High Temperature Superconductor by ac Susceptibility Measurements

The field dependence of real () and imaginary () component of ac susceptibility of superconductors within the critical state model can conveniently be used for evaluating the critical current when field amplitude is larger than the penetration field. A method to analyze the real () and imaginary () component of fundamental ac susceptibility with the objective of extracting the temperature dependence of critical current density J _c(T) is reported. The procedure makes use of the ac susceptibility data of two polycrystalline (Bi-Pb)-2223 samples measured with different excitation amplitudes below and close to the critical temperature. The temperature dependence of J _c is extracted using the isothermal scan over and data. Results obtained from this procedure are found to be in fair agreement with J _c(T) calculated from traditional loss-maximum data.     

Structure and Magnetism of the Composite Crystal Ca0.824CuO2

We have studied the magnetic state from a viewpoint of crystallographic features of the 1-D chain compound Ca_0.824CuO₂. A possible spin-hole arrangement in the magnetically coexisting state was determined by analyzing the local structural distortion in the CuO₂ chain by means of a modulated-crystal-structure analysis. The essential periodic sequence expected is (: up- and down-spin, : hole), which can be regarded as a kind of spin-1/2 ferromagnetic-antiferromagnetic alternating Heisenberg chain.     

Underpinning of a simple blunt flaw fracture initiation relation

With the cohesive process zone representation of the micro-mechanistic processes that are associated with fracture as a basis, the author is involved in a wide-ranging research programme, the objective being to extend the fracture mechanics methodology for sharp cracks to blunt flaws, so as to take credit for the blunt flaw geometry. In earlier work, a Mode I fracture initiation relation has been derived, subject to the restriction that the process zone size s is small compared with the flaw depth (length) and any characteristic dimension other than the flaw root radius . The relation gives the critical elastic flaw-tip peak stress _pcr, and has been derived using a two-extremes procedure, whereby the separate, and indeed exact, solutions for small and large s/ values are blended together to give an all-embracing relation that is valid for all s/ values. _pcr is expressed in terms of the process zone material parameters and geometrical parameters but, for a wide range of flaw geometry parameters, _pcr essentially depends on only one geometrical parameter . This paper provides underpinning for the general thrust of the two-extremes procedure by appealing to exact results for the complete spectrum of s/ values from analyses of appropriate Mode III models. Results obtained by applying the two-extremes procedure are shown to be in very good agreement with the exact results.     

Polymorphism in Sr3Fe2O7−x

The compound Sr₃Fe₂O_7–x , with variable iron valence, was investigated by X-ray powder techniques, both at room and at high temperatures. If the material is examined in massive form, a single phase called -Sr₃Fe₂O_7–x appears as previously reported in the literature. This -phase is tetragonal and exhibits the lattice parameters: a=3.874 and c=40.314 Å. Two other phases, called and -Sr₃Fe₂O_7–x , respectively, can be obtained on heating the finely powdered material when laid on a flat platinum support. The form is stable up to 1275° C, while the form is revealed only above 1275° C and changes always into -Sr₃Fe₂O_7–x when quenched. Both and phases are tetragonal, with a=4.001 and c= 58.251 for the form and a=4.013, c=57.092 Å for the form. The transition involves a true phase equilibrium, while the transformation is possible only by means of a suitable mechanical treatment of the material.     

Theoretical and practical aspects of testing potential biomaterialsin vitro

In vitro methods provide a necessary and useful adjunct toin vivo studies in testing potential biomaterials. One of the most important functions is the screening for toxic effects of the biomaterials. The spectrum of changes elicited ranges from cell death to alterations of cell adhesion, proliferation and biosynthetic activity. Such test systems may involve the direct contact of cells with the biomaterials or the use of soluble extracts of the latter. The rapid, cost-effective and highly sensitivein vitro methods have to be weighed against the problem of validity of extrapolation to thein vivo situation. The first step in testing potential biomaterials may be termed general biocompatibility testing and usually involves the use of various cell lines (i.e. transformed cells) which can be easily cultivated and passaged in the laboratory for long periods. Although the latter is convenient and highly sensitive for recognizing and excluding toxic materials at an early stage in the process of toxicity testing, this method cannot be regarded as exhaustive. It is proposed that such screening methods be followed by a secondin vitro phase, in which primary and early passage cells of a type relevant to the proposed application of the medical device are used. This specific biocompatibility testing is an attempt to simulate thein vivo situation as closely as possible. A further component of such a testing scheme involves the use of relevant biological parameters, such as cell adhesion or the production of specific biosynthetic products by the relevant cell type. It is thus possible to construct a spectrum ofin vitro changes, ranging from marked inhibition of growth with frank cell death (not biocompatible) to marked stimulation of relevant growth and other cell biological parameters (biocompatible and bioactive). An expansion ofin vitro testing methods can offer a method to tailor biomaterials for specificin vivo applications. In conclusion, it must be stressed that allin vitro experimentation, whether general of specific, cannot replace the subsequentin vivo testing. Both components are necessary in an adequate scheme for testing potential biomaterials.     

The formation of single-phase Si-Al-O-N ceramics

Si-Al-O-N ceramics have been prepared by hot-pressing mixtures of Si₃N₄, AIN and SiO₂ (with an addition of 1% MgO) having varying ratios of AIN/SiO₂. Microstructural analysis by transmission electron microscopy and Auger electron spectroscopy has demonstrated the progressive increase in grain-boundary silicate glass in pressings prepared from compositions with excess SiO₂ to compositions given by the formula Si_6–zAl_zO_zN_8–z. This formula represents the simple substitution of Al for Si atoms and O for N atoms in the hexagonalSi₃N₄ crystal. Microstructures for this balanced composition are essentially single phase, consiting of non-faceted, sub-micron, grains with a grain-boundary segregate layer of glass-forming silicate composition, containing impurity and additive metal ions, which may be detected only by Auger spectroscopy. This microstructure is in contrast with unbalanced compositions which contain faceted grains joined by a glassy silicate phase which is easily detected by electron microscopy. Final microstructural analysis combined with observations of density and phase content with progress of hot-pressing has confirmed the important role of liquid silicate formation and a solution-reprecipitation mechanism for densification. The presence of a 1% MgO additive is shown to accelerate this process, forming a low melting point silicate by reaction with SiO₂, assisting the early solution of AIN and the reprecipitation of substituted crystals.     

Transverse Spin Diffusion in a Dilute Spin-Polarized Degenerate Fermi Gas

We re-examine the calculation of the transverse spin-diffusion coefficient in a dilute degenerate spin-polarized Fermi gas, for the case of s-wave scattering. The special feature of this limit is that the dependence of the spin diffusion coefficient on temperature and field can be calculated explicitly with no further approximations. This exact solution uncovers a novel intermediate behaviour between the high field spin-rotation dominated regime in which D H^–2 , D T^–2 , and the low-field isotropic, collision dominated regime with D = D T^–2 . In this intermediate regime, D_, T^–2 but D D. We emphasize that the low-field crossover cannot be described within the relaxation time approximation. We also present an analytical calculation of the self-energy in the s-wave approximation for a dilute spin-polarized Fermi gas, at zero temperature. This emphasizes the failure of the conventional Fermi-liquid phase space arguments for processes involving spin flips. We close by reviewing the evidence for the existence of the intermediate regime in experiments on weakly spin-polarized ³ He and ³ He– ⁴ He mixtures.     

AC electrical conductivity of electron beam evaporated Cu-GeO2 thin cermet films

AC electrical properties of 410 nm think 30 at.wt% Cu-70 at.wt% GeO₂ thin films are reported for the frequency range 10⁴ to 10⁶ Hz and temperature range 150 to 425 K. The loss tangent (tan ) and the dielectric loss (/₀) are found to show striking minima around a cut-off frequency 10⁵ Hz. In the lower frequency range (10⁵ Hz), ₁() ^s T ⁿ is obeyed with s (0 to 0.51) increasing as a function of temperature and n (0.10 to 0.14) showing a very weak temperature dependence. In the higher frequency region (10⁵ Hz), ₁() and /₀ increase sharply leading to the quadratic behavior of ₁() with s equal to 2. These processes are discussed by analyzing an equivalent circuit which shows that at lower frequencies, the effects of series resistance in leads and contacts can be neglected, while at higher frequencies such effect give rise to spurious ² dependance for the conductance. A weakly activated AC conductivity and a frequency exponent s that increases with increasing temperature suggest that the low frequency behavior originates from carrier migration by tunneling process.     

Effect of composition on the transformation behaviour of duplex 316 weld metal

Several 316 manual metal arc weld metals produced with commercial and experimental consumables were aged between 600 and 850 °C. This resulted in the dissolution of the -ferrite with the appearance of new austenite, M₂₃C₆, intermetallics , and phase. During transformation, the -ferrite becomes progressively enriched in Cr, Mo and Si and, depending on the local composition of the -ferrite, eventually transforms to intermetallic or phases. A transformation model has been developed which indicates that solute diffusion via the / interface is the controlling mechanism. This enrichment process has been found to be important in controlling the formation of intermetallics. In the weld metals containing controlled residual additions of Ti and Ti + B, ageing resulted in the a fine dispersion of MX phase in the austenitic matrix.     

Fatigue crack growth behaviour of tungsten carbide-cobalt hardmetals

Fatigue crack propagation studies have been carried out on a range of WC-Co hardmetals of varying cobalt content and grain size using a constant-stress intensity factor double torsion test specimen geometry. Results have confirmed the marked influence of mean stress (throughK _max), which is interpreted in terms of static modes of fracture occurring in conjunction with a true fatigue process, the existence of which can be rationalized through the absence of any frequency effect. Dramatic increases in fatigue crack growth rate are found asK _max approaches that value of stress intensity factor ( 0.9K_IC) for which static crack growth under monotonic load (or static fatigue) occurs in these materials. Lower crack growth rates, however, produce fractographic features indistinguishable from those resulting from fast fracture. These observations, and the important effect of increasing mean free path of the cobalt binder in reducing fatigue crack growth rate, can reasonably be explained through a consideration of the mechanism of fatigue crack advance through ligament rupture of the cobalt binder at the tip of a propagating crack.     

The effective surface energy of brittle materials

The effective surface energy of four brittle materials, alumina, poly(methylmethacrylate), glass, and graphite, is calculated from load/deflection curves of notched bars deformed in three-point bending. Two of the methods, which are commonly used in fracture mechanics studies,viz the modified Griffith treatment and the compliance analysis method, are concerned with the effective surface energy at the initiation of fracture, _I . The third method, the work of fracture test, is concerned with the mean effective surface energy over the whole fracture process, _F . The two estimates of _I give consistent values, and there is no systematic variation of _I with notch depth. Values of _F decrease with increasing notch depth as the fracture process becomes more controlled. For alumina _{I F} . For PMMA and glass _I > _F because of a multiplicity of crack sources during fracture initiation. For graphite _I < _F because of subsidiary cracking as fracture proceeds.     

Kinetic interpretation ofα- andβ-Si3N4 formation from oxide-free high-purity silicon powder

A kinetic analysis of the isothermal nitridation of high-purity oxide-free silicon powder is described. The kinetic analysis suggests that the and polymorphs of Si₃N₄ are formed by separate and parallel reaction paths. This analysis provides for the decoupling and quantitative kinetic interpretation of- and-Si₃N₄ formation reactions. Consistent with existing microstructural and thermodynamic evidence, the-forming reaction is shown to obey a first-order rate law, whereas a phase-boundary controlled rate law describes the-forming reaction. A kinetic model employing these rate laws is developed and is used to predict the/ phase ratio as a function of isothermal reaction temperature and extent of reaction. The/ phase ratios so obtained are shown to be in good agreement with experimental observations made under a variety of reaction conditions.