In-Situ Synchrotron Diffraction Studies on Transformation Strain Development in a High Strength Quenched and Tempered Structural Steel—Part I. Bainitic Transformation

In-situ phase transformation behavior of a high strength (830 MPa yield stress) quenched and tempered S690QL1 (Fe-0.16C-0.2Si-0.87Mn-0.33Cr-0.21Mo (wt pct)) structural steel during continuous cooling under different mechanical loading conditions has been studied. Time-temperature-load resolved 2D synchrotron diffraction patterns were recorded and used to calculate the phase fractions and lattice parameters of the phases during heating and cooling cycles under different loading conditions. In addition to the thermal expansion behavior, the effects of the applied stress on the elastic strains during the formation of bainite from austenite and the effect of carbon on the lattice parameter of bainitic ferrite were calculated. The results show that small tensile stresses applied at the transformation temperature do not change the kinetics of the phase transformation. The start temperature for the bainitic transformation decreases upon increasing the applied tensile stress. The elastic strains increase with increase in the applied tensile stress.     

Effect of the Removal of a Surface Layer on the Mechanical Properties and the Static Stress–Strain Curves of an Austenitic–Martensitic Sheet TRIP Steel

The influence of the removal of a surface layer with a high strain-induced martensite content on the mechanical properties and the shape of stress–strain curves of austenitic–martensitic VNS9-Sh TRIP steel is studied by room-temperature tests. The removal of a surface layer 5–20 μm thick by electropolishing is shown not to decrease the mechanical properties of this steel and not to change the shape of its stress–strain curves, which have a developed yield plateau. This effect can be related to the presence of a long (up to 1%) stage of microyield in this steel. The existence of a yield plateau in the stress–strain curves of VNS9-Sh steel in the initial state and after the removal of a surface layer can also be explained by the simultaneous operation of three plastic deformation mechanisms, namely, slip, twinning, and martensitic transformation, during deformation.     

<Emphasis Type="Italic">In-Situ</Emphasis> Synchrotron X-ray Diffraction Studies on Effects of Plastic and Elastic Loading on bcc Phase Transformations of a 3rd Generation 1 GPa Advanced High Strength Steel

In this paper, we describe the effects of mechanical loading on bcc-to-bcc phase transformations of an Advanced High Strength Steel during cooling. In-situ synchrotron diffraction was employed to measure time–temperature–load diffraction patterns. Calculations were made of the volume fractions of the phases, the transformation kinetics, and the austenite lattice parameter during cooling and simultaneous loading. In addition, volume fractions and lattice parameters of retained austenite at room temperature under different loading conditions were obtained. The results show that applying a load during cooling of the fcc phase significantly increases the volume fraction of a bcc phase before the start of the martensitic transformation. The kinetics of phase transformations were affected by the applied loads. The volume fraction and lattice parameter of retained austenite at room temperature vary in different samples and the highest retained austenite and the largest lattice parameter were obtained in the sample subjected to the highest load.     

Riction properties of composite materials of the system TiN - Ain. II. Effect of oxide films on friction and wear of a ceramic material — Steel system

Friction and wear are studied for materials of the system TiN — AlN preliminary oxidized at 800–1100°C. It was established that thin oxide films containing Al₂TiO₅ and α-Al₂O₃, that promote a decrease in frictional wear, form on the surface of composite materials of the system TiN — AlN. Our assumptions are confirmed that the improvement in tribological properties of TiN — AlN composites is caused by forming oxide screening layers that prevent direct contact between the ceramics and steel counter-body. At high rates (V=16 m/sec) and pressure (P=2.0 MPa) the oxide films form more rapidly. Translated from Poroshkovaya Metallurgiya, Nos. 1–2(411), pp. 121–124, January–February, 2000.     

Effect of Thermal Buoyancy on Fluid Flow and Inclusion Motion in Tundish without Flow Control Devices——Part II :Inclusion Motion

1Introduction and Mathematical Formulations Understanding the behavior of nonmetallic in-clusions in the continuous casting process is i mpor-tant to cope with quality problems such as blistersinultra-lowcarbon coils andinclusion defects in heavyplates.In…     

Modelling of the temperature and the velocity of ceramic powder particles in a plasma flame—II. Zirconia

The model developed in Part I of this study, has been extended to a ZrO₂ particle here. It was found that the temperature and the velocity of a ZrO₂ particle, computed by assuming a constant average plasma temperature, deviate significantly from those calculated by considering the variation of the plasma temperature with the distance from the nozzle, as in the actual case. Again, it was found that the particle size reduction due to vaporisation cannot be neglected for particles moving close to the axis of the flame. The injection distance and the injection velocity were identified as two important spray parameters. It was noted that ZrO₂ particles within a certain size range should be used for developing coatings. The computations also revealed that the particles travelling in the peripheral region of the flame, do not undergo complete melting. It was experimentally verified that the particle size reduces significantly due to vaporisation.     

Surface stress and the chemical equilibrium of small crystals—II. Solid particles embedded in a solid matrix

The equilibria of small solid particles embedded in a solid matrix are considered. Three interface quantities are of significance; an interfacial free energy representing the work of creating the interface and two interfacial stresses. One represents the work of stretching the interfaces while the other represents the work of stretching one crystal holding the other fixed and thereby altering the structure of the interface. The isotropic case is developed in detail. Several new effects result from the partial accommodation in the matrix of the stress field arising from interfacial stress. An elastic accommodation factor modifies the capillary contribution to the pressure in the particle, the chemical potentials and the Gibbs-Thomson effect. Diffusion potentials but not chemical potentials are constant throughout the system. Coherent and incoherent nucleation is reexamined.     

The Aqueous and Surface Chemistry of Activation in the Flotation of Sulphide Minerals—A Review. Part II: A Surface Precipitation Model

The activation of sulphide minerals by heavy metal ions at medium to alkaline pH solutions is reviewed. The pH-dependent characteristics of adsorption are identically indicated by adsorption isotherms, surface potential and flotation responses. The mechanisms involved are explored. It is found that the initial adsorption process is controlled essentially by surface precipitation of the activator hydroxide. The real activation of flotation is obtained during the second stage—the surface conversion step. The industrial practice of activation and flotation with a highly alkaline solution highlights the proposed model and mechanisms. The mechanism underlying the intermediate pH depression of flotation of the activated mineral is discussed.     

Application of minimum energy formalism in a multiple slip band model for fatigue—II. Crack nucleation and derivation of a generalised Coffin-Manson law

A situation of multiple parallel slip bands is considered. Each slip band is modelled as an accumulation of edge dipoles. The dislocation dipole density within the slip bands continually increases with fatigue cycling. The strain energy density within the slip bands consequently builds up with cycling until a critical cycle number when it becomes energetically favorable to nucleate a microcrack within one of the slip bands. This is proposed to be the crack nucleation cycle number. The above criterion based on the strain energy density is used to derive an explicit expression for the number of cycles for crack nucleation. It is shown that there is a parallel between this criterion for crack nucleation and other criteria that are based on the accumulation of a certain critical amount of damage. A generalised Coffin-Manson law for crack initiation is also derived. The size and most likely site of the just-nucleated crack is discussed.     

Overview no. 76: Mechanism of deformation and development of rolling textures in polycrystalline f.c.c. metals—II. Simulation and interpretation of experiments on the basis of Taylor-type theories

Simulations of f.c.c. rolling textures based on homogeneous slip under conditions of full and relaxed constraints (Taylor-type theories) are presented. The characteristic peak and fibre components found in the resultant ODFs for the various relaxed shear possibilities are analysed in great detail and compared with the experimentally observed textures in Al and the CuZn alloys given in Part I. Special attention has been paid to the questions of stability and metastability of different orientations, the position of the fibres in orientation space, the influence of initial textures, the pathways that individual orientations take together with their rates of rotation along and their arrival rates at the fibres. Finally the topological distribution of grains in connection with the compatibility problem of the different stable orientations caused by relaxed shears is discussed.     

Structural Features and Properties of Alloy 84% WC ― 16% Co,Obtained by Hot Pressing in the Solid and Liquid Phases. Part 1. Effect of the Temperature at which the Specimens are Prepared on Their Density and Structure

Studies are made of changes in the porosity and structure of a hard alloy obtained by hot pressing and sintering within a broad range of temperatures (950-1450°C). The effect of solid- and liquid-phase annealing on their porosity and structure is also examined. It is shown that the hot-pressed specimens are denser than the sintered specimens within the solid-phase region, other conditions being equal. Solid-phase annealing of hot-pressed and sintered specimens increases the density of both types of specimens, but the annealing operation is more effective in the former case. It is established that the open pore channels are 2-3 times smaller in the hot-pressed specimens than in the sintered specimens and are no larger than the average size of the tungsten carbide particles. Active growth of the carbide particles and redistribution of the ductile phase are observed even in the solid phase. The carbide particles grow considerably faster (by a factor of four) at sintering temperatures above 1200°C.     

Structural Features and Properties of Alloy 84% WC ― 16% Co,Obtained by Hot Pressing in the Solid and Liquid Phases. Part 2. Influence of the Temperature at which the Specimens are Made on Their Physicomechanical Properties

We have studied the changes that occur in the resistivity, the transverse bending strength, and the fracture toughness (cracking resistance) of a hard alloy obtained by hot pressing at about 500 MPa and sintering over a wide range of temperatures (950-1450°C) as well as the how those parameters are affected by solid-phase and liquid-phase annealing. The porosity dependence of the resistivity is shown not to be single-valued. Other factors affect the resistivity, e.g., the degree of particle interaction and the state of the structural components, which vary with the porosity. The resistivity curve for hot-pressed specimens has an inflection in the region of 1200°C. The resistivity increases at a faster rate at lower temperatures. In the temperature range studied the porosity dependence of the transverse bending strength and the fracture toughness is linear: _b = ⁰ _b(1 – 3.53) and K _1c = K ⁰ _1c (1 – 3.44). Prolonged solid-phase annealing of hot-pressed specimens improves their mechanical properties owing to a decrease in porosity.     

Effect of interatomic potential on the calculated energy and structure of high-angle coincident site grain boundaries—II. (100) Twist boundaries in Cu,Ag and Au

Earlier calculations on (100) high-angle twist boundaries in Al are extended to Cu, Ag and Au. Recent pseudopotentials of Dagens together with three empirical potentials for Cu are applied. Boundaries with values of Σ, the inverse density of coincidence sites, ranging between 5 and 73 are considered. In contrast to our earlier results for the Al pseudopotential of Dagens et al., the relaxed grain-boundary energy, E_Σ, is found to increase smoothly as function of the twist angle, θ, for all six potentials considered here; i.e. “cusps” in the E_Σ(θ) curve are not observed. Rather similar boundary structures are obtained for the noble-metals; these differ drastically from those derived by means of the pseudopotential for Al. From a comparison with (100) twist boundaries in ionic crystals with NaCl structure it is concluded that also in the f.c.c. metals a volume expansion at the interface which increases smoothly with θ should be present. Interfacial structures determined without consideration of this expansion are hence concluded to be systematically in error. The article concludes with a summary of the properties of (100) twist boundaries in the f.c.c. and NaCl structures which are thought to be valid independent of the detailed form of the interatomic potential chosen.