Materials Science Forum Vol. 508

Abstract: The final morphology of liquid metallic emulsions, produced in a temperature gradient, depends on the interfacial gradient force acting on small metallic droplets. This force is proportional to the temperature coefficient of the interfacial energy between the two immiscible liquid phases. In the present paper first a widely used equation of Young, Goldstein and Block for the steady-state velocity of liquid droplets under the influence of the temperature gradient is discussed. Then a new equation is proposed for the temperature dependence of the interfacial energy.

Abstract: Metal foams are quite a challenge to materials scientists due to their difficult manufacturing. In all processes the foam develops in the liquid or semiliquid state. Liquid-metal foams are complex fluids which contain liquid metals, solid particles and gas bubbles at the same time. An X-ray transparent furnace was developed to monitor liquid metal foam evolution. Aluminium foams - similar to the commercial Metcomb foams - were produced by feeding argon or air gas bubbles into an aluminium composite melt. The foam evolution was observed in-situ by X-ray radioscopy under normal gravity. Drainage and rupture were evaluated during the 5 min foam decay and 2 min solidification. Argon blown foams showed significant drainage and cell wall rupture during the first 20 s of foam decay. Air blown foams were stable and neither drainage nor rupture occurred. We demonstrated the feasibility of experiments during parabolic flight or drop tower campaigns. However, the development of a foam generator for low gravity is needed.

Abstract: The microstructure of cast Zn-25wt%Al alloy inoculated by addition of a Zn-4wt%Ti master alloy (ZnTi4) has been studied using scanning electron microscopy and electron back-scatter diffraction (EBSD). It is found that Ti(Al,Zn)₃ particles act as nucleation centres for grains of the primary solid solution of Zn in Al (α' phase). The Ti(Al,Zn)₃ particles evolve in melt from the TiZn₃ particles in the ZnTi4 master alloy. EBSD shows that α' phase dendrites are in the same crystallographic orientation as the Ti(Al,Zn)₃ particles on which they nucleate. It is also found that some of the Ti(Al,Zn)₃ particles do not have any well-defined crystallographic orientation relationship with the α' phase. These particles were probably pushed and then engulfed by growing α' grains which had already nucleated on other particles.

Abstract: The eutectoid transformation of the spheroidal graphite cast iron (S.G.I.) has been investigated with “in situ” dilatometer, which was made for the investigation of the cast iron alloys. The investigation of the eutectoid transformation has been taking place by evaluation of the “insitu” dilatation curves in connection with metallographic examinations, chemical analyses and thermodynamic calculations of the phase equilibriums. By dilatometric curves it is possible to follow the exact eutectoid transformation of austenite. On a basis of numerous quantitative relations, as the relation between the ferrite and pearlite fractions in the as-cast SGI, which was determined by the analysis of the dilatometric curves and the composition, the ratio between ferrite and pearlite in the microstructure could be determined in a very short time. From the kinetics of austenite transformation and temperature dependence of the ferrite or pearlite growth the following characteristic temperatures of the eutectoid transformation have been established: the ferrite nucleation o Tα , the beginning of the ferrite growth Tα , and pearlite growth Tp , respectively. Kinetic curves, which show the fraction of the single microstructure constituents in the microstructure in dependence of the transformation time for mainly ferrite SGI, are good represented by the physical sigmoidal Boltzmann model.

Abstract: The present work aims to analyze the dispersion process (path) of the solid particles by moving a spherical particle from the feeding-nozzle outlet till the penetration of the melted surface (by the laser beam) and then its path in the metal bath. In laser surface treatment technologies with injection of hard particles in the melting bath it is necessary for the particles to work against the surface stress to penetrate through the surface of the bath into the melt. The theory shown was used in precalculations for the experiments carried out dispersing carbide particles (WC, TaC, NbC). Powder of particles of all three types were dispersed by means of the Ar carrying gas into an C15 steel melt by CO₂ laser. Evaluating the results of this process taking into consideration different specific technological parameters (speed of the beam, powder feeding speed) one can conclude that all three carbide types can penetrate into the melt.

Abstract: The laser surface-treatment methods have been quickly developed by appearing of lasers with high power beam and can increase the hardness, of the surface. A very hard, wear-resisting layer can be produced by the dispersing of ceramic grains. The essence of the technology is, that such a material (compound-phase, e.g.: metal-oxide, carbide, nitride, etc.) is added to the surface layer melted by laser, which does not solve or solves only partly in the metal-melt. This work studies the effect of the different technological parameter (such as, power of the laserbeam, motion speed, amount of the ceramic particles etc.) on the different microstructure accrued during the laser surface alloying. The desired microstructure has homogeneous carbide distribution in the matrix. But it is embarrassed by several conditions. The aim of this present work is to find out the reasons for the inhomogeneous ceramic particle distribution inside the matrix and to discover these embarrassing conditions.

Abstract: Metal Matrix Composites (MMCs) have great technical potential as they combine the ductility of metals with the hardness of ceramics: the reinforced material has improved mechanical properties. MMCs are interesting for the use in automotive or aerospace applications, e.g. in rolling bearings or turbine components. Recent manufacture is done by expensive methods like powder metallurgy to disperse the ceramic particles homogeneously in the metal matrix. In this contribution, we report on an approach to understand the basic mechanisms governing direct casting of MMCs. The basic problem is the particle-solid/liquid-interface interaction during dendritic solidification of metallic melts containing ceramic particles. The experimental idea is to deeply undercool the melt below its melting point. This results in a fast propagating solidification front when the solidification occurs. Due to the rapid solidification, the microstructure is frozen instantaneously and can be investigated post-mortem. It is expected that particles are incorporated in the material in the case of rapid solidification. Experimental techniques are electromagnetic levitation under terrestrial conditions and low gravity conditions during parabolic flight. With the electromagnetic levitation technique, samples were undercooled up to 150 K under its melting points despite the presence of particles. The experiments under low gravity show the importance of the reduction of melt convection on particle distribution within the rapidly solidified sample.

Abstract: The evolution of our understanding of dendritic and eutectic growth is presented. The control of both phenomena is essential in solidification processing. In this article, the development of the most important concepts which evolved over essentially one century will be described. Some of the original contributions which came from solid state transformation theory will also be included. This paper traces the history of the experimental and theoretical (analytical) work but leaves the numerical approach of the last 25 years for future papers.

Abstract: The as-cast properties of components with a columnar grain structure are very different from those with an equiaxed one. Under certain solidification conditions, zones of both structures can occur in an alloy casting; the boundary between the zones is the columnar-to-equiaxed transition (CET). A front-tracking model of dendritic solidification has been developed, which can predict the nucleation and growth of solid in undercooled liquid during a casting process. The growth process is described by dendrite tip kinetics, and is fully coupled to a fixed-grid control volume model of heat transfer during solidification. Using the front-tracking model, two methods for predicting the likelihood of an equiaxed zone forming ahead of a columnar front have been formulated, namely, an indirect method and a direct method. The indirect method is based on modelling the growth of the columnar front in the absence of equiaxed nucleation. The bulk liquid undercooling is monitored and an equiaxed indicator is calculated at each time step based on the extent of such undercooling at that time. The equiaxed indicator is a measure of the relative likelihood of an equiaxed zone forming. In the direct method nucleation and growth of individual equiaxed grains is treated ahead of the advancing columnar front. In this case, if impingement of neighbouring fronts is treated, the simulation to complete solidification will yield the macrostructure and the CET. In this paper, details of both methods of equiaxed prediction are presented. Results from the indirect method are compared to experimental results found in literature and agreement is found.

Abstract: The coarsening of secondary dendrite arms is discussed in terms of two different driving forces, which are the curvature effect and the temperature gradient zone melting (TGZM) effect. It is shown that the driving force due the TGZM effect can be higher than that due to the curvature effect at high temperature gradients and equal to each other at medium temperature gradients. For such high and medium temperature gradients, simple analytical models are proposed to predict the coarsening kinetics of secondary arms during solidification in a binary alloy. The prediction of the present analytical model agrees with experimental data obtained from the solidification of Fe – 1.6 wt. % Mn – 0.75 wt. % C steel.