Sybrand van der Zwaag

Charting the complete elastic properties of inorganic crystalline compounds

The elastic constant tensor of an inorganic compound provides a complete description of the response of the material to external stresses in the elastic limit. It thus provides fundamental insight into the nature of the bonding in the material, and it is known to correlate with many mechanical properties. Despite the importance of the elastic constant tensor, it has been measured for a very small fraction of all known inorganic compounds, a situation that limits the ability of materials scientists to develop new materials with targeted mechanical responses. To address this deficiency, we present here the largest database of calculated elastic properties for inorganic compounds to date. The database currently contains full elastic information for 1,181 inorganic compounds, and this number is growing steadily. The methods used to develop the database are described, as are results of tests that establish the accuracy of the data. In addition, we document the database format and describe the different ways it can be accessed and analyzed in efforts related to materials discovery and design.

Increasing the reliability of solid state lighting systems via self-healing approaches: A review

Reliability issues in solid state lighting (SSL) devices based on light emitting diodes (LED) is of major concern as it is a limiting factor to promote these optoelectronic devices for general lighting purposes. This postulate is even truer for high power devices in which high current and thus high thermal load are involved. In order to increase reliability and lighting efficacy, LED designs related to thermal management are evolving parallel to LED research and development. However there are still some issues mainly related to the degradation of LED’s constituents with time involving a faster decay of the lightning efficacy. In order to increase reliability of SSL devices, components presenting self-repairing properties could be implemented. In this review we will first briefly expose the state of the art on inorganic semiconductor based LED research and development, trends and challenges that lead to an increase of lighting efficiency. In a second part the different failure mode occurring for SSL devices have been compiled highlighting what are the main mechanism influencing and limiting LED reliability. Strong from this knowledge, in the last part, self-healing concepts will be proposed to further improve LED’s reliability.

Determination of Martensite Start Temperature in Engineering Steels Part I. Empirical Relations Describing the Effect of Steel Chemistry

The dependency of the martensite start (ms) temperature upon composition of engineering steels has been examined by analyzing the results predicted by an artificial neural network (ANN) model and thermodynamic data. Two new formulas, the simple linear and binary interaction ones, have been statistically derived and applied to predict the is temperature in an Fe-C-Si-Mn-Cr-Mo system. It is shown that the separation of the influence of interactions from that of individual alloying elements is successful since most of the statistical results are reasonable and thus have been physically interpreted. The thermodynamic calculations show that the alloying elements have similar influence upon the is and A 3 temperatures. The apparent effect of carbon depends largely on C-X interactions. C-Mn and C-Mo interactions weaken the effect of carbon while that of C-Si interaction intensifies the role of C. This is supported by phenomenological results and has been physically interpreted. The interactions between substitutional alloying elements have also significant influence upon the is temperature. The Si-Mn interaction strongly increases the Ms while Si-Mo interaction significantly decreases the is. So far, there is no proper physical explanation for this though supportive evidence has been obtained from phenomenological results. in and Mo have the weakest apparent interaction, that is, their influence can be simply added up. Moreover, a semi-physical model has been built to predict the is temperature from a critical temperature, which can be calculated thermodynamically. It shows that the semi-physical method gives a satisfactory prediction of is with a standard error of 15.3°C. Evaluation of nine common empirical methods indicates that the Kung and Rayment (KR) formula gives the best predicting results amongst them.

A mathematical model for the dissolution kinetics of Mg2Si-phases in Al–Mg–Si alloys during homogenisation under industrial conditions

A numerical analysis of the homogenisation treatment of aluminium alloys under industrial circumstances is presented. The basis of this study is a mathematical model which is applicable to the dissolution of stoichiometric multicomponent phases in both finite and infinite ternary media. It handles both complete and incomplete particle dissolution as well as the subsequent homogenisation of the matrix. The precipitate volume fraction and matrix homogeneity are followed during the entire homogenisation treatment. First, the influence of the metallurgical parameters, such as particle size distribution, initial matrix concentration profile and particle geometry on the dissolution- and matrix homogeneity kinetics is analysed. Then, the impact of the heating-rate and local temperature on the homogenisation kinetics is investigated. Conclusions for an optimal homogenisation treatment of aluminium alloys may be drawn. The model presented is general but the calculations were performed for the system Al‐Mg‐Si with an Al-rich matrix and Mg2Si-precipitates.

Indirect evidence for the existence of the Mn partitioning spike during the austenite to ferrite transformation

The growth kinetics during a final cooling down following cyclic partial phase transformations in the intercritical austenite + ferrite region has been investigated for a Fe–0.17 Mn–0.023C alloy (wt.%). For the first time, a specific growth retardation stage was observed. The growth retardation is attributed to the residual Mn spikes in the austenite, as a result of the prior partial phase transformations. The experimental observations are in good agreement with theoretical calculations based on the local equilibrium concept.

Experimental observations on thecorrelation between microstructure andfracture of multiphase steels

Abstract The aim of this study is to clarify the role of the microstructure in damage evolution. The influence of the transformation induced plasticity effect on the crack initiation and the impact of the different phases in development of cracks are important factors that control the fracture mechanisms. The fracture mechanisms of multiphase steels have been investigated on the basis of extensive light optical microscopy and scanning electron microscopy investigations. On the micro-scale, two failure modes appear: cleavage and ductile fracture, depending on the stress–strain conditions, the internal cleanness, the volume fraction of the retained austenite and the position of the austenite and the martensite grains. If a martensite or an austenite grain fails inside a bainitic island, the crack develops rapidly leading to cleavage fracture. If the failure starts inside the ferritic matrix due to void initiation at hard phases, the emerged void causes ductile damage to the surrounding ferrite.

Repeated crack healing in MAX-phase ceramics revealed by 4D in situ synchrotron X-ray tomographic microscopy.

MAX phase materials are emerging as attractive engineering materials in applications where the material is exposed to severe thermal and mechanical conditions in an oxidative environment. The Ti2AlC MAX phase possesses attractive thermomechanical properties even beyond a temperature of 1000 K. An attractive feature of this material is its capacity for the autonomous healing of cracks when operating at high temperatures. Coupling a specialized thermomechanical setup to a synchrotron X-ray tomographic microscopy endstation at the TOMCAT beamline, we captured the temporal evolution of local crack opening and healing during multiple cracking and autonomous repair cycles at a temperature of 1500 K. For the first time, the rate and position dependence of crack repair in pristine Ti2AlC material and in previously healed cracks has been quantified. Our results demonstrate that healed cracks can have sufficient mechanical integrity to make subsequent cracks form elsewhere upon reloading after healing.

DSC Study on Mg-Si Phases in As Cast AA6xxx

As cast billets of AA6xxx alloys require a homogenisation treatment to make the material suitable for hot extrusion. During this homogenisation treatment several processes take place such as the transformation of interconnected plate-like β-A15FeSi intermetallics into more rounded discrete α-A112(FeMn)3Si particles and the dissolution of β-Mg2Si particles. Precipitation and dissolution of Mg-Si phases in as cast material were investigated for various Fe contents (0.02 wt%, 0.19 wt% and 0.65 wt%) by means of DSC measurements and optical microscopy. Eutectic melting was also studied. In as cast material, the amount of Fe-containing intermetallics increases with Fe content. It appears that the amount of precipitation of β and β Mg-Si phases during the DSC scan is lower for higher Fe content and that the corresponding activation energies are similar and independent of the Fe content. The amount of eutectic melting is dependent on the time involved with the DSC scan. Introduction As cast billets of AA6xxx require a homogenisation treatment to make the material suitable for hot extrusion. During this homogenisation treatment several processes take place such as the transformation of interconnected plate-like β-A15FeSi intermetallics into more rounded discrete αc-A112(FeMn)3Si particles and the dissolution of β-Mg2Si particles [1]. Transformation of βAl5FeSi to αc-A112(FeMn)3Si intermetallics is important because it improves the ductility of the material [2]. Dissolution of β-Mg2Si is also important since it will give maximum age hardening potential for the extruded product and will suppress the generation of surface defects due to local melting [3,4]. Although the precipitation and dissolution of Mg-Si phases in already homogenised and aged material have been studied in detail [5-7], such studies are not found for as cast materials. It is known that the concentration of Fe influences the β-Al5FeSi to αc-Al12(FeMn)3Si transformation kinetics and the amount and morphology of these phases [8]. Also some influence of Fe on the precipitation and dissolution of the Mg-Si phases might be expected. A strong influence of the Fe content on the diffusion of Mg and Si or their solubility is not anticipated since for a very low Fe content the maximum solubility of Fe in Al has already been reached. Therefore, the activation energies of dissolution and precipitation of Mg-Si phases are expected to be independent of Fe content. However, an indirect influence of the Fe content might be present since Fe will affect the morphology and spatial distribution of the Fe containing intermetallics in the eutectic and therefore that of the Mg2Si particles in the eutectic. Furthermore, the Si concentration in the matrix may depend on the Fe content because of the incorporation of Si in the Fe containing intermetallics. The aim of this study is to investigate precipitation, dissolution and eutectic melting of Mg-Si particles in the as cast structure of AA6xxx for various Fe contents. To this purpose both optical microscopy and Differential Scanning Calorimetry (DSC) measurements were performed. Experimental Materials and microstructural characterisation. Table 1 shows the compositions of three as cast Al-Mg-Si-Fe alloys. The Mg and Si contents in each alloy are approximately equal whereas the Fe content varies between 0.02 and 0.65 wt%. Since these alloys do not contain Mn, the Fe containing intermetallics consist of β-Al5FeSi and/or αc-Al12Fe3Si. The alloys were cast in steel moulds of 200 mm x 150 mm x 400 mm and subsequently air cooled. All samples for optical characterisation and DSC measurements were taken at least 20 mm from the edge of the ingot to get a similar microstructure and composition. Table 1. Alloy composition [wt%] for alloys A, B and C. Alloy Mg Si Fe Other Al A 0.75 0.52 0.02 <0.01 Balanced B 0.76 0.45 0.19 <0.01 Balanced C 0.82 0.49 0.65 <0.01 Balanced In the optical micrographs, the Al matrix appears as light grey areas, the β-Al5FeSi and αc-Al12Fe3Si intermetallics as medium grey particles and the β-Mg2Si as dark grey particles. Also differences in morphology between the different types of particles are visible. The volume fraction of the intermetallics in each casting was determined. By automatic SEM and EDX measurements the volume fraction of αc-Al12Fe3Si and β-Al5FeSi was determined, which method is described in detail in Ref. [9]. DSC measurements. DSC measurements were performed using a Perkin and Elmer DSC7 with a sample weight of approximately 60 mg. Pure aluminium was used as a reference. Temperature scans were made from 20°C to 605°C with constant heating rates of 5, 10, 20 or 40 °C/min. A baseline was obtained by fitting a polynomial at points in the curve where no reactions occurred. The activation energies of the precipitation of β and β Mg-Si and the precipitation and dissolution of β-Mg2Si were obtained by the Kissinger method [4,10,11]. The activation energy Q is determined from the slope of the straight line obtained by plotting ln(Tp/φ) versus 1/RTp, where Tp is the peak temperature of the reaction, φ is the heating rate and R is the gas constant. Results Optical microscopy. Fig. 1 shows micrographs of two as cast microstructures (alloys B and C). The intermetallics and the Mg-Si particles are clearly distinguishable. The particles are plate-like and appear as needles in the micrographs [12]. The Mg-Si particles are precipitated as β-Mg2Si on the boundaries of the Fe containing particles, as isolated β-Mg2Si particles on the boundaries of the Al-dendrites, or as much smaller β-Mg2Si or β Mg-Si precipitates in the matrix. The larger βMg2Si particles, present in the interdendritic liquid accompanying the eutectic reaction during casting, are more spherical with a diameter of approximately 2 μm or larger. The β-Mg2Si or β Mg-Si particles in the Al matrix have a needle shape and their width is smaller than 0.5 μm. They are precipitated from the supersaturated solid matrix during cooling of the casting. Close to the Fe containing particles there is a solute depleted zone where no Mg-Si particles are precipitated. (a) (b) Fig. 1 Optical micrographs of alloys B (a) and C (b). Table 2 gives an overview of the measured volume concentration of β-Al5FeSi and αc-Al12Fe3Si intermetallics. Also the calculated volume concentration of intermetallics is given assuming that all Fe in the alloy is bound in the intermetallic phases. The volume fraction of intermetallics increases as the Fe content increases. The calculated intermetallic volume fraction is slightly higher than the measured fraction, since some Fe is in solid solution or precipitated in small particles which were not detected by the imaging system. Measurement of the amount of Mg-Si and β-Mg2Si particles present in the matrix was not possible because most particles were too small to analyse quantitatively by optical or SEM measurement. Table 2. Volume concentrations [vol.%] of intermetallic phases present. Alloy αc-Al12Fe3Si (measured) β-Al5FeSi (measured) αc-Al12Fe3Si + β-Al5FeSi (calculated) A <0.1 <0.1 <0.1 B 0.3 ± 0.1 0.1 ± 0.1 0.5 C 0.6 ± 0.2 0.6 ± 0.2 1.9 DSC measurements. DSC scans of alloys A, B and C are shown in Fig. 2 for a heating rate of 40°C/min. Six peaks are visible in the plots, indicated by (1) to (6). The exothermic peak (1), with a maximum at approximately 320°C, is due to the formation of β Mg-Si particles [5]. The exothermic peak (2), with a maximum at approximately 350°C, is caused by the formation of β Mg-Si particles [5]. There is a large overlap of the peaks (1) and (2). The exothermic peak (3), with a maximum at approximately 450°C, is caused by the precipitation of β-Mg2Si particles [5]. The endothermic peak (4), with maximum at approximately 540°C, is due to the dissolution of the Mg2Si particles. The small endothermic peak (5), with an onset of approximately 578°C, is caused by eutectic melting of Al+Mg2Si+β-Al5FeSiaL+α -Al8Fe2Si [3]. It appears only in the alloy with a high Fe content (alloy C). The endothermic peak (6), with an onset temperature of approximately 587°C, is caused by the eutectic melting of Al+Mg2SiaL [13]. 200 300 400 500 600 -0.10 -0.05 0.00 0.05 0.10 0.15 (5) Alloy: A B C (6)

Piezoelectric and mechanical properties of novel composites of PZT and a liquid crystalline thermosetting resin

A novel series of lead zirconate titanate (PZT) ceramic–polymer composites has been developed and characterized. The matrix polymer is a liquid crystalline thermosetting resin (LCR) based on a HBA-HNA backbone and phenylethynyl end-groups. The composites show excellent high temperature processability. The dielectric properties were studied as a function of PZT volume fraction and processing conditions. Piezoelectric behaviour was compared to Yamada et al. model for 0–3 composites. For a moderate PZT volume fraction a high value for the piezoelectric stress constant of g 33 = 48 mV m/N was measured, which, in combination with a good chemical and thermal resistance of the polymer matrix, makes the material a good candidate for sensor applications at elevated temperatures. The liquid crystalline thermosetting character of the polymer imparts interesting high temperature post-formability.

Characterization of self-healing polymers : From macroscopic healing tests to the molecular mechanism

Over the last few years, several testing methods have been introduced for the detection and quantification of autonomous and thermally stimulated healing in polymers. This review summarizes some of the most prominent state-of-the-art techniques for the characterization of polymer healing occurring at the microscopic and macroscopic levels during the repair of damage such as scratches, cracks, or ballistic perforations. In addition to phenomenological investigation of the self-healing process, a range of physical characterization techniques have been explored for elucidation of the underlying healing mechanism at the molecular or polymer network level. The present state of visual methods, spectroscopic techniques, scattering techniques, and dynamic methods is described. A short outlook is provided, discussing the future challenges and expected new trends in the characterization of self-healing polymers.