The influence of stress on the pitting susceptibility of a 12%CrMoV martensitic stainless steel

The influence of applied macrostress on the pitting susceptibility of a 12%CrMoV martensitic stainless steel in a de-aerated caustic solution containing chloride at 373K has been investigated and discussed.     

Effect of boron on phosphorus-induced temper embrittlement

Combined equilibrium and non-equilibrium grain boundary segregation of solute atoms in dilute ternary alloys is modelled through consideration of site competition between two solutes. Model predictions are made for a low-alloy steel containing boron. The predicted results indicate that the kinetics of phosphorus segregation are dramatically facilitated by quenched-in vacancies, and the magnitude of the segregation, however, is substantially suppressed by the competition of boron with phosphorus for segregation sites, and in turn the phosphorus-induced embrittlement may be alleviated.     

Effect of substrate curvature on residual stresses and failure modes of an air plasma sprayed thermal barrier coating system

A set of aerofoil shaped air plasma sprayed thermal barrier coated (APS-TBC) specimens were adopted in this paper to investigate the stress distributions in the ceramic top coat (TC) and the thermally grown oxide (TGO), the mechanism of local crack generation and propagation at the TC/BC (bond coat) interface. The failure mode of the TBC system, the distribution of asperities at TC/BC interface, thickness of the TC and BC, and the TC microstructure were found to be influenced by substrate curvature. Residual stress was therefore measured across the thickness of the TC, along the undulating TGO and mapped at locations of asperities where failure tended to occur to interpret the initiation of local failure. The role of the TGO was investigated via its chemical bonding with the TC and the decohesion occurring at the TGO/BC interface. The crack propagation at the interface has been discussed with respect to the macro-failure of the TBC system.     

A model describing neutron irradiation-induced segregation to grain boundaries in dilute alloys

A model describing neutron irradiation-induced grain boundary segregation at a given temperature is established for dilute alloys based on a complex diffusion mechanism and combined with Mc-Lean’s equilibrium segregation model. In the model, irradiation-enhanced solute diffusion is taken into consideration. The diffusion equations are more rigorously solved than in earlier models, so that an accurate definition of the grain boundary solute concentration is given as a function of time. The effect of the temperature dependence of dislocation density is accommodated and the estimation method for complex diffusion is reappraised. Theoretical predictions are made for segregation of phosphorus in neutron-irradiatedα-Fe. There exists a transition temperature below which combined irradiation-induced nonequilibrium and irradiation-enhanced equilibrium segregation is dominant and above which thermal equilibrium segregation is dominant. The peaks in the temperature dependence of segregation shift to lower temperatures with decreasing neutron dose rate and/or increasing neutron dose. The combined radiation-induced nonequilibrium and radiation-enhanced equilibrium peak segregation temperature is about 150 °C forP grain boundary segregation in neutron-irradiatedα-Fe at dose rate=10⁻⁶ dpa/s and dose=1 dpa. The thermal equilibrium segregation peak is around 550 °C for the same conditions. Comparison of some experimental and predicted results shows that the predictions are generally consistent with the observations.     

Microstrucure of explosively welded 0.1 Pct carbon mild steel tubes to 0.2 Pct carbon mild steel plate

The microstructures developed within trial explosive welds between 16 mm diam, 1.2 mm wall thickness tubes of 0.1 pct C 0.4 pct Mn mild steel and a 0.2 pct C 0.8 pct Mn mild steel base plate have been examined. A weld zone typically ∼20 μm wide is formed in which solid-phase bonding is interrupted by pockets of localized melting. The complex microstructure develops as a result of both severe plastic deformation and rapid cooling. Plastic deformation, limited to regions close to the weld interface and the internal surface of the tube, was confirmed by the high density of dislocations in the α phase. Twinning on {112} planes occurs within discrete regions of the tube plate and is discussed in terms of the geometrical arrangement of the tubes in the plate. The morphology of the product in the fusion pockets has been compared with the massive and acicular martensite which is typical of quenched low-carbon steels. The high-pressure shock waves that develop during the collision of the tube and tube plate result in pronounced local deformation adjacent to the weld junction. The closely interwoven microstructure produced has been interpreted as the result of a pressure-induced transformation.     

An analysis of stress corrosion crack growth by anodic dissolution

An analysis of the distribution of electrode potential within a stress corrosion crack which is growing by an anodic dissolution process has been used to define the electrode potential at the tip of the crack. This potential is used to predict the kinetics of crack growth. The influence of the applied stress intensity and the electrochemical properties of the crack tip and surface on the growth rate have been considered for low alloy steels in concentrated hydroxide solution and aluminum alloys in acidic chloride solution. Crack growth rates obtained in high concentration solutions are extrapolated to lower concentration solutions which may be expected in service environments. Predicted crack growth rates are in good agreement with published data.     

The sintering of creep-induced cavities in a low alloy ferritic steel (1Cr1Mo0.75V)

Sintering heat treatments at temperatures in the range 1173 to 1273 K have been undertaken in an atmospheric pressure of argon gas to remove grain boundary cavities within a lCrlMo0.75V ferritic steel (Durehete 1055) following creep rupture after service operation at 838 K. High sensitivity density measurements and optical metallography have been used to monitor the progressive sintering of cavities. The results are discussed in relation to existing models which describe the sintering of cavities by grain boundary diffusion.     

Creating Three-Dimensional Models to Investigate Brittle Fracture in Polycrystalline Metals

Three-dimensional models with irregular grain geometries and appropriate physical properties are needed to investigate fracture in polycrystalline metals and alloys. Creating such models is challenging but achievable using a two-stage process, suitable for any polycrystal. The processes described in this paper are illustrated by examples of brittle fracture in ferritic steel, zinc and nickel. The predicted crack path in a model is compared with the grain boundary fracture seen in three point bend specimens of nickel embrittled by sulphur.     

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.