S. van der Zwaag

The Lattice-Parameters of Austenite and Ferrite in Fe-C Alloys as Functions of Carbon Concentration and Temperature

Despite its relevance for various calculations involving phase transformations in Fe-C alloys, little information is available on the lattice parameter of austenite at elevated temperatures and its dependence on the carbon concentration. Furthermore, severe scatter exists in the literature for the lattice parameter at high temperature. Most literature data were acquired using X-ray diffraction, although neutron diffraction seems to be a more suitable technique penetrate and probe a large volume of material, the advantages of neutron diffraction over X-ray diffraction are its reduced sensitivity to surface decarbonization and improved crystal statistics (both decarbonization and grain growth can occur during diffraction experiments at high temperatures requiring long exposure times). In this work in-situ neutron diffraction experiments on Fe-C alloys were performed to determine the lattice parameter of the austenitic and ferritic phases in a temperature region from just below to just above the bi-phasic austenite/ferrite region.

Magnetic and X-ray diffraction measurements for the determination of retained austenite in TRIP steels

The accurate determination of the volume fraction of retained austenite is of great importance for the optimization of transformation induced plasticity (TRIP) steels. In this work, two aluminium-containing TRIP steels are studied by means of magnetization and X-ray diffraction (XRD) measurements. By fitting the field dependence of the approach to saturation in the magnetization curves, the saturation magnetization is determined, which is linearly related to the volume fraction of retained austenite. Moreover, information with respect to the microstructure can be obtained from the fitting parameters and the demagnetizing factor for the magnetization curve. The volume fractions obtained from the magnetization measurements are compared with data from XRD measurements. A discussion of the data suggests that magnetization measurements lead to more reliable results and a more sensitive detection of the retained austenite than XRD measurements.

The Concept of Filament Strength and the Weibull Modulus

The strength of high-performance filaments is a complex parameter which can not be fully described with a single value. The Weibull model is used to describe the intrinsic statistical nature of the fracture strength. Possibilities and limitations of the Weibull model are illustrated. The relationship between material properties and the parameters in the Weibull model is discussed.

Influence of Cross-linkers on the Cohesive and Adhesive Self-Healing Ability of Polysulfide-Based Thermosets

Synthetic systems with intrinsic self-repairing or self-healing abilities have emerged during the past decade. In this work, the influence of the cross-linker and chain rigidity on the healing ability of thermoset rubbers containing disulfide bonds have been investigated. The produced materials exhibit adhesive and cohesive self-healing properties. The recovery of these two functionalities upon the thermally triggered healing events has shown to be highly dependent on the network cross-link density and chain rigidity. As a result, depending on the rubber thermoset intrinsic physical properties, the thermal mending leading to full cohesive recovery can be achieved in 20-300 min at a modest healing temperature of 65 °C. The adhesive strength ranges from 0.2 to 0.5 MPa and is fully recovered even after multiple failure events.

Ferrite formation in Fe-C alloys during austenite decomposition under non-equilibrium interface conditions

The growth of ferrite during the decomposition of supersaturated austenite in binary Fe-C alloys is studied using a numerical model that allows non-equilibrium conditions at the moving α-γ-interface. A numerical method is presented to account for non-equilibrium interface conditions during diffusional growth. In this model, the driving force for interface migration is given by the difference of the chemical potentials of the austenitic and the ferritic Fe-lattice at the interface. Carbon rearrangement, resulting from the movement of the interface, affects the driving force for interface migration during the transformation, thus establishing a mixed mode of growth control. The mode of control is predicted to vary from essentially a diffusion controlled mode at very low undercooling towards essentially an interface controlled mode at deep undercoolings. The results from this model are in good agreement with several sets of experimental growth rates and the carbon concentration profiles of partially transformed Fe-C alloys.

Dilatometric analysis of phase transformations in hypo-eutectoid steels

Dilatometry is a useful technique to obtain experimental data concerning transformation kinetics in ferrous alloys. This technique is commonly used in cooling experiments to study the austenite decomposition of hypo-eutectoid steel grades. In the standard analysis of the dilatation signal there are two factors that are normally neglected. During the pro-eutectoid ferrite formation the austenite enriches in carbon, resulting in a non-linear temperature dependence of the specific austenitic volume. Furthermore, the specific volume of the formed ferrite is considerably different from that of the formed pearlite. In total not taking into account these two effects can lead to an error in the determined fraction ferrite of up to 25%. A method is presented that takes into account the two above-mentioned factors. In order to determine both the fraction ferrite and the fraction pearlite, in the analysis the temperature range of the transformation is divided into a ferrite-formation range and a pearlite-formation range. Two possible criteria for this division are discussed, and it is shown that the choice does not have an essential influence on the results.

Assessment of different techniques for quantification of α-Al(FeMn)Si and β-AlFeSi intermetallics in AA 6xxx alloys

Abstract During homogenisation of AA 6xxx aluminium alloys, the platelike β-AlFeSi intermetallic phase will transform to a less Si-rich and more spheroidised α-Al(FeMn)Si phase which is more favourable for extrusion. In this study, several quantitative methods, which determine the relative volume fraction of α-Al(FeMn)Si and β-AlFeSi, are compared and an assessment of each method is made. The methods used are optical microscopy, scanning electron microscopy (SEM) in combination with electron dispersive X-ray (EDX) using polished samples, and X-ray diffraction (XRD) on intermetallics, extracted through selective dissolution of the Al matrix. The highest accuracy is obtained by using SEM/EDX analysis and applying two criteria.

A semi-quantitative description of the kinetics of structural relaxation in amorphous Fe40Ni40B20

Abstract Measurements are presented of the change of length and Youngs modulus during structural relaxation of amorphous Fe 40 Ni 40 B 20 . The results have been analysed in terms of a model which ascribes the effects to a combination of chemical and topological structural relaxation (CSRO reap. TSRO). It was possible to separate both contributions. TSRO is the slower process and starts when CSRO is nearly completed. CSRO is a process containing a spectrum of activation energies between 150 and 250 kJ/mol. The TSRO contribution can be well described by the free volume model with a single activation energy of 250 kJ/mol, which corresponds to the value found from viscosity measurements.

Self-healing behaviour in man-made engineering materials: bioinspired but taking into account their intrinsic character

Man-made engineering materials generally demonstrate excellent mechanical properties, which often far exceed those of natural materials. However, all such engineering materials lack the ability of self-healing, i.e. the ability to remove or neutralize microcracks without (much) intentional human interaction. This inability is the unintentional consequence of the damage prevention paradigm underlying all current engineering material optimization strategies. The damage management paradigm observed in nature can be reproduced successfully in man-made engineering materials, provided the intrinsic character of the various types of engineering materials is taken into account.

The effect of thin hard coatings on the Hertzian stress field

Abstract In this paper the effect of thin rigid coatings on the stress field generated by a Spherical indentation on a flat halfspace is investigated using a finite-element program. It is shown that thin hard coatings can have a significant effect on the radial and circumferential stresses. Of particular interest is the reduction in the maximum (radial) tensile stresses, since this indicates that hard coatings can be used to protect brittle materials against elastic contact damage. The stress reduction increases with the Youngs modulus and thickness of the coating. The reduction in the substrate stresses is, however, accompanied by an increase in the maximum tensile stresses in the coating itself.