The role of deformation twins in brittle crack propagation in iron–silicon steel

Crack initiation and propagation in polycrystalline metals and alloys can be characterized by the crack driving force and the resistance to fracture. Interfaces such as grain, sub-grain and interphase boundaries are microstructural features that can resist crack propagation. For iron–silicon polycrystalline steels, brittle fracture occurs predominately by transgranular cleavage but intergranular fracture is enhanced by embrittling heat-treatments. In this paper, we consider the role of deformation twin boundaries on the brittle crack propagation and fracture resistance of poly and single crystals of Fe–3 wt.% Si steel. Three-point bend, impact and miniaturized disc tests have been undertaken at temperatures in the range of 77–273 K. The fractographic features have been characterized with attention being given to (i) the role of the {1 1 2} deformation twins on the propagation of the {0 0 1} cleavage cracks and (ii) the process-zone of the propagating cleavage cracks. The results are discussed with reference to three-dimensional model predictions.     

Orientation dependence of plastic deformation in nickel-based single crystal superalloys: Discrete–continuous model simulations

The anisotropic mechanical response of single-crystal nickel-based superalloys is simulated. At 1123 K, two uniaxial tensile loading cases are simulated: one along [0 0 1] and another along [1 1 1]. Resulting stress–strain curves, stress distributions, interfacial dislocation structures are analysed. In accordance with experiments, the simulations show an anisotropic yield strength. The applied strain is accommodated by dislocations propagating through matrix channels on octahedral slip systems. The net result appears as slip bands along the cubic directions, even though no cubic slip systems are activated. In the [0 0 1] case, the plastic flow is distributed more or less evenly among the three matrix channels, whereas in the [1 1 1] case it is mainly concentrated in one single channel. Typical zig–zag configurations are observed. The elementary mechanisms controlling their formation are explained. Cross-slip does not play any role there. The hardening anisotropy between both loading cases is related to strong differences between the interfacial dislocation microstructures.     

Liquid Ni–Fe penetration and recrystallisation in tungsten

Melt penetration in grain boundaries of solid tungsten has been investigated.Solid tungsten rods have been exposed to a nickel–iron melt saturated with tungsten and the penetration depth and the shape of the liquid channels have been examined. The solid tungsten samples have been treated in different ways like cold working, annealing and recrystallisation, before melt exposure. Important parameters for the penetration process are stresses, surface tensions, solution and kinetic effects. A new theoretical model for the penetration mechanism in cold worked samples is proposed.Rapid recovery of the grains in the penetrated areas of the cold worked samples was observed. This is discussed, as well.     

The role of strontium in modifying aluminium–silicon alloys

Small amounts of strontium can transform the morphology of the eutectic silicon phase present in Al–Si casting alloys from coarse plate-like to fine fibrous networks. In order to understand this industrially important but hitherto insufficiently understood effect, the strontium distribution was studied in atomic resolution by atom probe tomography and in nanometre resolution by transmission electron microscopy. The combined investigations indicate that Sr co-segregates with Al and Si within the eutectic Si phase. Two types of segregations were found: (i) nanometre-thin rod-like co-segregations of type I are responsible for the formation of multiple twins in a Si crystal and enable its growth in different crystallographic directions; (ii) type II segregations come as more extended structures, restrict growth of a Si crystal and control its branching. We show how Sr enables both kinds of mechanisms previously postulated in the literature, namely “impurity-induced twinning” (via type I) and growth restriction of eutectic Si phase (via type II).     

Cutting performance of multilayer diamond coated silicon nitride inserts in machining aluminum–silicon alloy

Aluminum–silicon (Al–Si) alloy is very difficult to machine and diamond tools are considered by far the best choice for the machining of these materials. Experimental results in the machining of the Al–Si alloy with diamond coated inserts are presented. Considering the fact that high adhesive strength and fine surface morphology play an importance role in the applications of chemical vapor deposition (CVD) diamond films, multilayer technique combining the hot filament CVD (HFCVD) method is proposed, by which multilayer diamond-coating on silicon nitride inserts is obtained, microcrystalline diamond (MCD)/ nanocrystalline diamond (NCD) film. Also, the conventional monolayer NCD and MCD coated inserts are produced for comparison. The as-deposited diamond films are characterized by field emission scanning electron microscopy (FE-SEM) and Raman spectrum. All the CVD diamond coated inserts and uncoated insert endure the aluminum-silicon alloy turning to estimate their cutting performances. Among all the tested inserts, the MCD/NCD coated insert exhibits the perfect behavior as tool wear due to its very low flank wear and no diamond peeling.     

Correlation between interfacial segregation and surface-energy-induced selective grain growth in 3% silicon–iron alloy

Effects of final reduction and interfacial segregation of sulfur on surface-energy-induced selective grain growth have been investigated in 3% silicon–iron alloy strips with various bulk content of sulfur. Interfacial segregation kinetics of sulfur varies with annealing atmosphere: a convex profile under vacuum or hydrogen and a gradual increase under argon. This is because the segregated sulfur evaporates or gasifies to hydrogen sulfide during final vacuum or hydrogen annealing, resulting in a sulfur-depleted zone just below the strip surface. The surface-energy-induced selective growth of a grain at time t is determined by the concentration of segregated sulfur. The selective growth rate depends on the combined effect of the segregated sulfur and the final reduction that determines the average grain size. For obtaining (110)[001] Goss texture, the final reduction should, therefore, be controlled, depending on the bulk content of sulfur which influences directly the segregation kinetics of sulfur and thus the texture development.     

Effects of laser irradiation on iron loss reduction for Fe–3%Si grain-oriented silicon steel

The effects of laser irradiation on iron loss reduction for Fe–3%Si grain-oriented silicon steel sheet were investigated. The local tensile residual stress states near the laser irradiated cavity lines were observed by using the new X-ray stress measurement method for a single crystal. Although the higher laser power induced the larger tensile residual stresses, the minimum iron loss was obtained at the medium tensile residual stress conditions of about 100–200 MPa. The increase of Vickers hardness was observed with increasing laser power, which was the mark of the plastic deformations induced by the laser irradiation. The tensile residual stress reduces eddy current loss and the plastic deformation increases hysteresis loss of the material. The total iron loss is determined by the balance of these two effects of laser irradiation.     

Simulation of the three-dimensional morphology of solidification porosity in an aluminium–silicon alloy

A novel extension of the cellular automata technique for microstructural modelling is presented, allowing simulation of the evolution of the complex three-dimensional morphology of porosity during the solidification of an aluminium–silicon alloy. The complex morphology arises due to the restriction of the growth of the pores by the developing solid phase. The model predicts the average properties of the porosity formed, together with the distribution in size and morphology.The model is used to determine the influence of a variety of applied conditions (e.g. thermal history, pressure, hydrogen content) and material properties (nucleation behaviour, alloy composition) upon the pore morphology, as characterized by the average and extreme dimensions. The relative magnitude of the effect of each parameter and the interactions between parameters upon the porosity are statistically analysed. The simulated pore size shows the largest sensitivity to applied pressure, hydrogen content and solidification time, together with interactions between solidification time and pressure. These results are in good agreement with previously reported experimental behaviour.     

Columnar to equiaxed transition of eutectic in hypoeutectic aluminium–silicon alloys

Directional solidification of unmodified and strontium modified binary, high-purity, aluminium–7 wt% silicon and commercial A356 alloys has been carried out to investigate the mechanism of eutectic solidification. The microstructure of the eutectic growth interface was investigated with optical microscopy and Electron Backscattering Diffraction (EBSD). In the commercial alloys, the eutectic solidification interface extends in the growth direction and creates a eutectic mushy zone. A planar eutectic growth front is observed in the high-purity alloys. The eutectic aluminium has mainly the same crystallographic orientation as the dendrites in the unmodified alloys and the strontium modified high-purity alloy. A more complex eutectic grain structure is found in the strontium modified commercial alloy. A mechanism involving constitutional undercooling and a columnar to equiaxed transition explains the differences between pure and commercial alloys. It is probably caused by the segregation of iron and magnesium and the activation of nucleants in the commercial alloy.     

Evolution of solute clustering in Al–Cu–Mg alloys during secondary ageing

The evolution of an atomistic-level nanostructure during the early stages of secondary ageing of a rapid hardening Al–1.1Cu–1.7Mg (at.%) alloy has been characterized by a combination of atom probe tomography (APT) and transmission electron microscopy. Quantitative APT analysis reveals key changes in the evolution of solute clusters during secondary ageing (T6I4 condition) that correlate with secondary hardness increments. The microstructures that cause peak hardness differ between the T6 and T6I4 tempers – the former is a result of solute clustering as well as the precipitation of GPB zones and S phase, whereas in the latter, secondary ageing promotes only the formation of solute clusters. Cu–Mg clusters with high Mg:Cu ratio have the most strengthening potency during secondary ageing in T6I4 heat treatment.     

Reactive diffusion in the system vanadium–silicon

The diffusion-controlled growth of vanadium silicides (V₃Si, V₅Si₃, V₆Si₅, VSi₂) was studied on bulk V–Si diffusion couples annealed for 2–36 h at 1150–1390°C. The layer growth kinetics was parabolic for all of the silicides. Only at 1150 and 1200°C was an induction period observed before the formation of a continuous V₆Si₅ layer. The parabolic growth constants of the II kind for the exclusive growth of each silicide from the adjacent phases were calculated from the parabolic constants of the I kind measured on the V–Si diffusion couples. The rate constants of the II kind were in turn related to the diffusion properties of the silicides. As a result, the interdiffusion coefficient, taking into account the diffusion of both elements, was obtained for each phase. The resulting activation energies were 240 kJ mol⁻¹ for V₃Si, 250 kJ mol⁻¹ for V₅Si₃ and 190 kJ mol⁻¹ for VSi₂. The activation energies scale well with the melting point of the compounds. For V₆Si₅, the activation energy is strongly dependent on the set of thermodynamic data used in the calculation owing to the uncertainty in the decomposition temperature of this phase.     

Experimental investigation and Ginzburg–Landau modeling of the microstructure dependence of superconductivity in Cu–Ag–Nb wires

The type-II superconducting properties of heavily deformed Cu–Ag–Nb wires, containing only 4 wt% (4.18 vol.%) of elongated Nb filaments as a separate superconducting phase, were investigated as a function of microstructure, temperature, total wire strain, and external magnetic fields. The microstructure of the wires was examined using optical and electron microscopy. The experimental observation of the proximity effect, i.e. of the penetration of the superconducting state into the normal resistive Cu–Ag matrix leading to bulk-superconductivity, is explained in terms of the experimentally determined topology of the microstructure in conjunction with Ginzburg–Landau theory. The pronounced drop of the critical temperature and of the critical magnetic field with increasing wire strain is explained in terms of the reduced thickness of the ductile Nb filaments which is at large strains of the order of the Ginzburg–Landau correlation length in Nb.     

Role of scandium additions in primary silicon refinement of hypereutectic Al–20Si alloys

This study investigates the effect of Sc on the formation of primary silicon during the solidification of hypereutectic Al–20Si alloys. The evolution of the microstructure was studied using thermal analysis. The results show that the addition of 0·2 and 0·4 wt-% Sc suppresses the nucleation of primary silicon due to the formation of ScP particles instead of AlP particles. For large Sc additions, at the centre of the samples, the primary silicon has a star-shaped morphology with thin branches. The addition of Sc decreases the P refinement ef?ciency in hypereutectic Al–20Si alloys, which may be a result of the formation of a ScP phase. The results suggest that the addition of Sc poisons the nucleant particles of the primary and eutectic silicon.     

Orientation dependence of the γ-α’ martensitic transformation in single crystals of austenitic stainless steels with low stacking-fault energy

The development of the γ-α’ martensitic transformation (MT) upon tensile deformation of single crystals of austenitic stainless steels of compositions (wt %) Fe-17% Cr-12% Ni-2% Mn-0.75% Si (I) and Fe-18% Cr-12% Ni-2% Mo-0.015% C (II) has been studied as a function of the crystal-axis orientation and test temperature by X-ray diffraction and electron microscopy. It has been established that the orientation dependence of the slip deformation preceding the γ-α’ MT is determined by two factors, namely, the orientation dependence of slip deformation preceding the γ-? MT and the orientation dependence of the work U necessary for the formation of the α’-martensite crystals. The orientation dependence of slip deformation preceding the γ-? MT leads to the γ-α’ MT in the [\(\bar 1\)11], [\(\bar 1\)23], [011], and [012] crystals with different defect densities and, correspondingly, at different stress levels. In the [001] crystals, no γ-α’ MT is observed macroscopically since the γ-? MT in these crystals is suppressed. It has been established that the γ-α’ MT in the [\(\bar 1\)11], [011], [\(\bar 1\)23], and [012] crystals can be developed at T = 300 K after preliminary deformation at T = 77 K. The development of the γ-α’ MT at T = 300 K is physically related to the growth of the α’-martensite nuclei formed upon plastic deformation at T = 77 K.     

Orientation dependence of the γ-ɛ martensitic transformation in single crystals of austenitic stainless steels with low stacking fault energy

X-ray diffraction and electron microscopy were used to study the development of the γ-? martensitic transformation (MT) upon tensile deformation of single crystals of (I) the Fe-17% Cr-12% Ni-2% Mn-0.75% Si and (II) Fe-18% Cr-12% Ni-2% Mo-0.015% C (wt %) austenitic stainless steels as a function of the crystal-axis orientation and the test temperature T. It has been shown that a decrease in the test temperature to T<173 K in single crystals of steels I and II with a low stacking fault energy (γ₀=0.01–0.015 J/m²) leads to a γ-?-α’ MTs upon plastic deformation. It has been established that the degree of deformation preceding the γ-? MT depends on the crystal-axis orientation and the γ₀ magnitude. In the [011] and $[\bar 1 11]$ crystals, the γ-? MT upon tension is developed already at early stages of plastic flow, at ?≤3%, whereas in the $[\bar 1 23]$ and [012] crystals it occurs after a substantial deformation by slip, at ?=16–70%. In the [001] crystals, no γ-? MT is revealed by X-ray diffraction, but 1–2% ? phase is observed by electron microscopy. The physical cause for the observed orientation dependence in the γ-? MT is related to the effect of external stresses σ on the degree of splitting of perfect dislocations a/2〈110〉 to Shockley partial dislocations a/6〈211〉, which form nuclei of the ? phase.     

Bioleaching of marmatite in high concentration of iron

Bioleaching of marmatite with a culture of Thiobacillus ferrooxidans and Thiobacillus thiooxidans in high concentration of iron was studied,the results show that the zine leaching rate of the mixed culture is faster than that of the sole Thiobacillus ferrooxidans,the increasing iron concentration in leaching solution enhances the zine leaching rate.The SEM analysis indicates that the chemical leaching residues is covered with porous solid layer of elemental sulfur,while elemental sulfur is not found in the bacterial leaching residues.The primary role of bacteria in bioleaching of sphalerite is to oxidize the chemical leaching products of ferrous ion and elemental sulfur,thus the indirect mechanism prevails in the bioleaching of marmatite.     

Carbon content dependence of grain growth mode in VC-doped WC–Co hardmetals

Carbon content dependency of grain growth mechanism and grain growth inhibition mechanism in VC-doped WC–Co hardmetals is investigated. VC-doped WC–Co hardmetals with three different carbon contents were sintered with liquid phase and then rapidly quenched to freeze up the structure at the sintering temperature. In these samples, spatial distributions and atomic scale structures of V-rich phases are investigated using transmission electron microscopy (TEM) and related techniques. In these measurements, doped V is found in liquid phase as solute, in large (W,V)C_x precipitates and in interface segregations. Further detailed observations and discussions are carried out for the (W,V)C_x segregated at the WC grain/Co phase interfaces. These (W,V)C_x phases change their form from planar films to small islands depending on the carbon content. The WC grain/Co phase interfaces are fully covered by planar (W,V)C_x in the sample of low carbon content. On the other hand, the WC grain/Co phase interfaces are partially covered by (W,V)C_x islands in the material of high carbon content. During sintering, the WC grains in this sample grew much faster than those in the sample of low carbon content. These structural differences are discussed in terms of WC grain/(W,V)C_x interface energy.     

A role of fractions of mobile grain boundaries in secondary recrystallization of Fe–Si steels

Grain growth during annealing of polycrystalline materials is influenced by the type of grain boundaries a grain encounters as it grows. Simple computer experiments have been performed to ascertain which type of boundaries are responsible for the abnormal grain growth (AGG) in Fe–3% Si steels. In modelling abnormal grain growth, two different assumptions are used, the first one is that the coincidence site lattice (CSL) boundaries have high mobility and the second is that high energy boundaries are more mobile than other boundaries. The results of the computer experiments support the latter model for abnormal grain growth in Fe–3% Si steels. Finally, the importance of fractions of mobile grain boundaries on the development of Goss texture is discussed.