A. Martín-Meizoso

Simulation of the microstructural evolution during liquid phase sintering using a geometrical Monte Carlo model

The present paper proposes a Monte Carlo model to simulate the evolution of the microstructure during liquid phase sintering. The model mainly assumes that the system temperature is constant and that the composition of the liquid phase is completely homogeneous, but it is also capable of simulating the final cooling stage of the sintering process. It works with solidification, or melting, probabilities of volume elements (voxels) that are obtained accounting for the local geometry via the closest surrounding neighbours and avoiding the use of thermodynamic probabilities. This method reproduces an isotropic behaviour during solidification and melting and gives growth and melt rates proportional to surface curvatures. The proposed model provides realistic simulations of the microstructural evolution that takes place in real materials during liquid phase sintering.

Interfacial cracking in thin film structures

Abstract Interfacial fracture is a critical failure mode identified in reliability tests of multilayer thin film structures used in microelectronics. This paper reviews the main techniques developed so far to measure interfacial toughness in thin film sandwich structures, together with the models used to extract the fracture parameters of the interface. This background is used to present Cross Sectional Nanoindentation (CSN), a new mechanical test specifically designed for measuring the fracture toughness of thin film interfaces. The main advantages of this new technique are the high spatial resolution, which makes it suitable for studying patterned structures, and the direct observation of the interfacial crack front, not possible with other test configurations. A numerical model based on the elastic theory of plates has been used to calculate the interfacial toughness for ceramic-ceramic systems from CSN test results. Closed form analytical solutions, developed for two limiting cases, are consistent with the numerical approach. The CSN technique has been successfully applied to silicon nitride-silicon oxide thin films, commonly used as electrical isolators in microelectronic devices.

Original article: Diffusional Monte Carlo model of liquid-phase sintering

Abstract: Liquid-phase sintering (LPS) is a consolidation process for metallic and ceramic powders. At given temperature conditions, the process occurs with constant amount of liquid phase. However, the evolution of solid-particle shape is observed, namely, the rounding of particles and the growth of big particles at the expense of the small ones, which is known as Ostwald ripening. In this work, we propose a Monte Carlo (MC) model to simulate the microstructural evolution during LPS. The model considers the change of state of the discretising elements, namely voxels, of the system. The microstructural evolution proceeds accounting for both the geometrical characteristics of the particles, such as the number of solid neighbours, and the amount of solute contained in or surrounding a randomly chosen voxel. This has been implemented in terms of two probability distribution functions (PDFs). The diffusion of solute has also been considered by means of the implementation of a three-dimensional finite-difference algorithm. The diffusional MC model that we present is able to reproduce the Ostwald ripening behaviour and, in particular, results match the case in which the process is limited by the diffusion of the solute in the liquid phase.

Standard surface defects produced by water vapour oxidation in steels used in fossil fuel fired power plants

Abstract The present work describes the population of surface defects produced by steam in steels typically used in superheater, reheater and water wall tubes of fossil fuel fired power stations at working temperatures. An accurate statistical description of material defects (in the as-manufactured condition and of those defects induced in service), and the service conditions (temperature, stress, time, ambient…) is necessary to assess correctly the life of a component on a sound, scientific bases and not only in an empirical way. Oxidation-induced defects were generated in the laboratory using Ar, saturated with water vapour for different times and at various temperatures, representative of those expected in service. The population of surface defects has been measured by conventional metallographic procedures. An evolution is observed, with time and temperature, of the mean size of surface defects in carbon steels but not in SA-213-T22 ferritic steel. The surface density of defects for carbon steel SA-210 decreases with time by growth and coalescence of neighbouring pits. For carbon steel SA-178-C, the oxidation takes place by lateral growth of the pits that meet, eventually spalling the metal. The defect densities in samples that operated under real conditions (true service 1, TS.1 and TS.2) are smaller than in the as-received material and laboratory-oxidised samples. A coalescence mechanism among neighbouring pits is also observed for these samples.

Localized stress redistribution after fibre fracture in brittle matrix composites

Abstract A model is detailed that describes the redistribution of stress, after individual fibre failure, amongst the intact reinforcing fibres of a ceramic matrix composite. The load drop carried by a single fibre when it breaks is balanced by extra loads in the intact fibres which are calculated using elastic solutions and variational mechanics. The magnitude of stress redistribution is shown to follow a d − m relationship, where d is the distance from the broken fibre and m depends on the fibre volume fraction and a non-dimensional parameter that normalizes the influence of the stress and the elastic properties of the composite. The model has been applied to two composites, SiC/SiC and CAS/SiC. It is shown that the stress redistribution is localized in both composites, the stress increments in the closest intact fibres being more than one order of magnitude greater than predicted by the assumption of global stress redistribution.

Load transfer in ceramic matrix composites

The authors attempt to determine the load transfer behaviour in cracking of ceramic matrix composites reinforced with continuous fibres. It is concluded that large underestimations of the fibre loads result from the global load transfer assumption.

Resilience and ductility of Oxy-fuel HAZ cut

Cutting processes affect the material to a deeper or shallower attached-to-the-cut zone. Its microstructure, its hardness and mechanical properties are changed. Also the cutting process introduces surface roughness and residual stresses. In most cases it is recommended to remove this region by grinding, in order to keep a smoother surface, free from the above mentioned effects. This work presents the characterization results of the Heat Affected Zone (HAZ) of a steel plate of grade S460M, with a thickness of 25 mm, cut by flame oxyfuel gas cutting. The HAZ microstructure is observed (and the depth of the HAZ measured), the hardness profile and the stress vs. strain curves until fracture are measured by testing micro-tensile samples, instrumented with strain gauges. Micro-Tensile specimens are 200 microns in thickness and were obtained from layers of the HAZ at different distances from the oxy-fuel cut. The obtained stress-strain curves are compared with the hardness measurements and the observed metallography.

Geometrical Monte Carlo model of liquid-phase sintering

Liquid-phase sintering (LPS) is an industrial process used to consolidate materials composed of two different kinds of metallic and/or ceramic powders. At constant temperature, the amount of the present liquid-phase is constant. However, the shape of particles of solid phase changes over time. In general, the rounding of particles and the growth of big particles at the expense of the small ones are observed. This process is known as Ostwald ripening. In this work, we propose a Monte Carlo (MC) model to simulate the microstructural evolution during LPS. The discretizing elements of the system, namely the voxels, change state between solid and liquid, according to previously defined melting and solidification probability distribution functions (PDFs). The generated PDFs take into account the geometrical characteristics of the system particles in terms of number of solid neighbours that surround a randomly chosen voxel. The geometrical MC model that we present is able to reproduce the Ostwald ripening behaviour and, in particular, matches the case in which the process occurs limited by the attachment/detachment of the solid phase to/from the surface of the particle.

A mechanistic approach to the life prediction of 316L stainless steel under combined creep/fatigue cycling

Abstract In this work, the relaxation stress behaviour of a 316L stainless steel cycled at high temperature has been analysed and a critical strain rate, which marks the transition from a transgranular to an intergranular dominated damage regime, was obtained. This strain rate has been used to partition the relaxation strain into its transgranular and intergranular components. Coffin-Manson type relationships for both damage regimes have been derived and the corresponding intrinsic parameters, ductility and irreversibility damage coefficient, determined. A cumulative damage rule, resulting from equalling the sum of the transgranular and intergranular damages to unity, allows the calculation of the material life under creep/fatigue conditions. These predicted values have been tested against previously reported data obtained from the same material. It is shown that, irrespective of the imposed strain range, the duration of the hold period and the ratio between the transgranular and intergranular components of the damage, predicted and experimental lives are in close agreement. The points determined by the calculated transgranular and intergranular components of the overall damage fall closely along the failure locus as defined by the cumulative damage rule.

HIPERC: a novel, high performance, economic steel concept for linepipe and general structural use

The HIPERC project has examined the effects of alloying elements and processing conditions in low carbon, < 0.09 wt %., niobium containing, 0.05 - 0.12 wt.%, steels. Laboratory-scale heats and pilot rolling trials simulating air and water-cooled plate production as well as hot-rolled strip production have been made. The effects of C, Mn, Ni, Cu, Cr, Mo, Nb, Ti and B, on transformation characteristics and temperatures of recrystallisation have been determined along with regression equations for characterisation of microstructure, tensile and impact properties, and for the weldability of these steels. The properties of products processed commercially to plate and coil-plate and made into pipe and to plate for structural use were determined and these compared well with the values predicted from the regression equations. The project has shown that excellent combinations of strength, toughness and weldability can be obtained using this steel type. Additional experiences have been gained in the processing of these steels through three commercial rolling mills and benefits were seen with this steel type due to higher production rates and lower amounts of surface dressing compared with steels currently being used to satisfy equivalent property specifications. Recommendations on the limits for niobium in Euronorms have been proposed; concerns relating to weldability have been addressed by proposing varying limits based on the carbon and manganese contents of the steel. This report makes the output of this project available to CEN working groups to support the revision of Euronorms based on the gathered data.