Materials Science Forum Vols. 790-791

Abstract: Columnar to equiaxed transition (CET) was studied in a peritectic TiAl-based alloy with chemical composition Ti-45.1Al-4.9Nb-0.25C-0.2B (at.%). Solidification experiments were conducted in a Bridgman-type apparatus using cylindrical moulds made of high-purity Y₂O₃. The methodology containing appropriate etching and observations under flat light in stereo-microscope was used to identify the morphology of primary β phase grains and position of CET in the samples˰ All samples prepared by power down-technique showed sharp CET. The position of the CET measured from the beginning of the sample depends on the applied cooling rate and increases from approximately 65 to 115 mm by decreasing cooling rate from 50 to 15 K/min. Based on terrestrial experiments, the future work focused on microgravity and hypergravity CET experiments and numerical modeling is proposed. A Bridgman furnace front tracking method will be applied in future work to complement the experimental results here as part of the European Space Agency GRADECET programme. This modeling will input directly into planned microgravity and hypergravity CET experiments.

Abstract: The Ni₂FeGa Heusler alloy is synthesized by arc melting in argon atmosphere. It shows two phase microstructure, γ-phase ( disordered fcc ) and Austenite ( ordered bcc, L₂₁ ). Phase identification and microstructural characterization were carried out using XRD, SEM and TEM. Solidification at various undercoolings upto 215 °C was performed using flux undercooling technique. B₂O₃ was used as the flux that provides an inert atmosphere and isolates the molten pool from the quartz tube. The solidified microstructure of the undercooled samples were analyzed and the result indicates γ-phase to be the primary phase to form. The samples are also textured. XRD patterns indicate different texture at different undercoolings. Possible mechanisms for such changes will be discussed. The competitive nucleation mechanism can not also be ruled out as the SEM micrographs show the globular morphology of γ-phase likely due to defragmentation of primary dendrites. Thermal analysis by DSC shows incongruent melting of Ni₂FeGa Heusler alloy which strengthen the argument of poor nucleation ability of L₂₁ ordered intermetallic austenite phase as compared to primary γ-phase. Up to achieved undercooling limits, γ-phase forms as the primary phase competitively with the L₂₁ ordered phase. Studies indicate that competitive nucleation mechanism is a likely mechanism to explain the phase selection.

Abstract: Dynamic coarsening of austenite dendrite in lamellar cast iron has been studied for a hypoeutectic alloy. The common morphological parameter to characterize dynamic coarsening, secondary dendrite arm space has been replaced by the Modulus of primary dendrite ( M_PD ) and the Hydraulic diameter of the interdendritic space ( D^Hyd_IP ) to interpret the dynamic coarsening with respect to the local solidification time. The obtained results demonstrate the coarsening process of both the solid and liquid phase. The interdendritic space is increasing as the contact time between the solid and liquid phase increases. The ratio between the D^Hyd_IP/M_PD is strongly dependent on the precipitated fraction primary austenite indicating clearly the morphology variation during coarsening. The interrupted solidification method demonstrate that the observed coarsening process is not only a combination of the increasing fraction precipitated solid phase and the rearrangement of the solid liquid interphase curvature but the volume change due to density variation is also contribute to the coarsening process. Keywords: dendrite morphology, hydraulic diameter, interdendritic space, gray iron.

Abstract: Investigation of dynamic coarsening in lamellar cast iron is extended over a wide interval ranging from hypoeutectic to eutectic composition. The dendrite morphology is defined on as-cast samples produced under various cooling rates. The as-cast morphology is considered being close to the one at the end of solidification. The obtained relations describing the coarsening process as a function of local solidification time and fraction austenite are compared to results obtained from interrupted solidification experiments. By using the Modulus of primary dendrite (M_PD) and the Hydraulic diameter of the interdendritic space (D^Hyd_IP) become possible to characterize the coarseness of a wide range of lamellar cast irons solidified under various cooling rates.

Abstract: An experimental approach employing temperature and concentration gradients is presented that is suitable for determining impurity diffusion coefficients in a single experimental cycle. The Al-Cu system is used to illustrate the feasibility of the method. In a single phase α-Al solid solution, concentration gradients are generated by exposing a cylindrical sample to steep temperature gradients and by annealing until the initially formed mushy zone is re-solidified. The annealing is performed such that a symmetric, ramp shaped profile in the form of a roof is generated. The sample is then again exposed to a temperature gradient at somewhat lower temperatures for an extended time period. The symmetric profile then becomes asymmetric due to the varying diffusion coefficient along the sample. Information on the pre-exponential factor D₀ and the activation energy for diffusion Q_D is retrieved from the asymmetry of the resulting concentration profile. The asymmetry becomes increasingly pronounced with longer diffusion times, yielding an increasing accuracy of the diffusion coefficients. The experimental approach is generally applicable to alloy systems with finite solubility.

Abstract: Rapid solidification represents a very attractive approach to develop new Al alloys in an economically convenient way. The lower segregation content, refined grains, higher ultimate tensile and yield strengths combined to a good ductile properties confer to these materials an interesting position also in the so critical automotive and/or aeronautical applications. The current paper presents results of an analysis concerning Zn-Al alloys with a new metastable microcrystalline structure, where Copper has been used as alloying element. With addition of elements as Ti and B modification of the microstructure has been reached. In order to study the influence of the cooling rate on the microstructure and structural transformations castings has been realized with melt spinning technique, in both steel and sand moulds. For morphological investigations optical and scanning electron microscopy has been employed. By dilatometric analysis and X-Ray diffraction technique the thermodynamic factors, the kinetic effects, phase transformations and the volume changes related to the transformations produced at the eutectoid temperature have been monitored. For the aforementioned field of applications the most favourable composition has been chosen: based on the up to date outcomes, by modifying the original alloy with some elements a quite homogeneous structure combined with good mechanical behaviour has been obtained.

Abstract: High Cr and Ni content steels are widely used in many manufacturing processes in the chemical and petrochemical industry. The automotive industry has also recognized the necessity of heat resistant alloys for a long time, for example, to apply them to exhaust systems to endure thermal loading and oxidation during the operation of engines. Various heat resistant alloys such as cast irons, stainless steels, and Ni-base super alloys have been considered as candidate materials of automotive exhaust systems. Among those candidates, ferritic stainless steels attracted a lot of attention due to their favorable low thermal expansion, sufficient mechanical strength at elevated temperature and excellent corrosion resistant properties [1]. Currently they are the leading engineering materials in several fields of applications that require resistance to wear, corrosion [2,3], creep or thermal fatigue [4]. The high corrosion resistance of these steels is due to alloying elements such as Cr, Ni and Mo. If the ferritic stainless steels are alloyed with strong carbide-forming elements, such as Mo, Ti, V and Nb, hard phases, MC carbides can be obtained in the soft ferrite phase [5,6]. The improvement of the properties of FeCrNi cast steels is directly related to the development of the microstructure, which mainly consists of a ferritic matrix and carbides and/or dispersed intermetallics [7,8]. The improvement is not always the hardening. The hardness is usually limited by the casting and the subsequent machining, so an annealing process is also inserted.

Abstract: Design and characterization of modified Cobalt base alloy for biological applications have been studied and compared. In particular, modification has been realized by addition of Mo, Ti and Zr to better fit the requirements for dental applications. On the samples morphological and surface analysis including residual stress determination have been considered. As a result of this study, a positive effect of Ti addition has been demonstrated. Contrarily, a simultaneous addition of Ti and Zr does not promote any enhancement as microstructure and properties concern.

Abstract: In this paper we present two types of currently used vacuum systems on the HPDC and the preparation phase of an experimental tool. To investigate the properties of HPD castings we have designed a die geometry that complies with requirements of vacuum and non-vacuum systems. We used computer simulation to shape up the runner-and overflow-system. In the course of our work we examined specimens with differing thickness and alloys with different compositions to check the changes of mechanical properties and the effect of vacuum system.

Abstract: The convection pattern and the evolution of mushy zone, temperature and solidification structure are measured during solidification of NH₄Cl-70%H₂O solution in a water-cooled copper mold with transparent sidewalls. The natural convection and crystal sedimentation are measured via Particle Image Velocimetry (PIV) technique. This experiment is simulated using a 5-phase mixed columnar-equiaxed solidification model proposed by current authors [Comp. Mater. Sci. 50 (2010) 32-4]. The 5 phases comprise the extradendritic melt, the solid dendrite and interdendritic melt inside the equiaxed grains, the solid dendrite and interdendritic melt inside the columnar grains. Melt convection and crystal sedimentation are considered. It is demonstrated that the experimentally observed flow patterns and the solidification structure can be qualitatively reproduced. Reasons for the quantitative deviation between the simulation and experiment are discussed. Analysis of the modeling results in details and improvement of the calculation accuracy will be in a subsequent step.