Papers by Author: Ragnvald H. Mathiesen

Abstract: In the frame of ESA-MAP (Microgravity Application Promotion) project entitled XRMON (In situ X-Ray MONitoring of advanced metallurgical processes under microgravity and terrestrial conditions), a microgravity (μg) experiment in the XRMON-GF (Gradient Furnace) setup was successfully launched in 2012 on board MASER 12 sounding rocket. During this experiment, in situ and real time observations of the formation of the solidification microstructures in diffusive conditions were carried out for the first time by using X-ray radiography. In addition, two reference experiments with the same control parameters but in ground-based conditions were performed to enable us a direct comparison with the μg experiment and therefore to enlighten the effects of gravity upon microstructure formation. This communication reports on fragmentation phenomenon observed during those experiments. For 1g upward solidification, fragmentations mainly take place in the upper part of the mushy zone. After their detachments, dendrite fragments are carried away by buoyancy force in the bulk liquid where they are gradually remelted. For μg experiment and horizontal solidification, this type of fragmentation is not observed. However, a great number of fragmentations are surprisingly revealed by in situ observation in the deep part of the mushy zone, when the liquid fraction is very small. Moreover, as soon as they are detached, the dendrite fragments move toward the cold part of the mushy zone, even in the case of μg experiment. The observations suggest that sample shrinkage may be at the origin of this fragment motion.

Abstract: The Lipton Glicksman Kurz (LGK) growth model is commonly used to predict growth rates for equiaxed dendrites in solidifying mushy zones. However, the original LGK method treats an isolated dendrite growing in an infinite volume of liquid. In an equiaxed mushy zone, with multiple nucleation events, thermal and solutal interactions take place between the equiaxed dendrites. A modified version of the LGK model was developed that allows for measurement of the solute build-up ahead of the dendrites. To investigate the validity of the model, comparisons are made with results obtained from in-situ synchrotron X-ray videomicroscopy of solidification in a Bridgman furnace of an Al-12wt.%Ge alloy inoculated with Al-Ti-B grain refiner. Comparisons between the original LGK and modified LGK models are presented for discussion. The modified LGK model shows realistic tip temperature trends.

Abstract: Hypermonotectic alloys are distinguished by a temperature region for which the homogeneous melt decomposes into two liquid phases. In Al-based hypermonotectics, the minority phase is much higher in density than the matrix melt phase and consequently macro-segregation due to sedimentation is an inherent problem when casting these alloys. However, under the correct solidification conditions, it may be feasible to counteract sedimentation by thermocapillary forces that arise due to the thermosolutal dependence of the surface tension between the two liquid phases. The current investigation involved in-situ X-ray video microscopy studies during directional solidification of Al-Bi samples of various compositions employing a Bridgman furnace. It was found that large undercoolings were required to initiate the L  L1 + L2 reaction and L2 nucleation occurred heterogeneously on the monotectic front. L2 droplets were then set in collective size-dependent motion by hydrodynamic forces coupled to external fields (i.e. temperature gradient and gravity) and internal fluctuations. The resulting flow fields are superimposed onto the short range coagulation mechanisms, such as diffusion coupling, and are all crucial in determining the final size distribution and dispersion of the Bi-rich phase in the cast material.

Abstract: In-situ synchrotron X-ray diffraction has been applied in order to study grain growth in an ultra-fine grained (D~400 nm) 6060 aluminium alloy at 270°C. The submicron grain structure was produced by Equal Channel Angular Pressing (ECAP) to an effective strain of ~6 without rotation of the billet. As the material was textured after ECAP, the initial stages of grain growth were seldom detected, but in the grain size interval available for studies a grain growth exponent of 3.6±0.3 was obtained. By interpolation of the grain growth curves to D=D0 (determined by EBSD) the effect of growth on the softening of the alloy was estimated. The interpolated average curve indicates that the initial stages of softening are not due to uniform grain growth, but rather reconfiguration and annihilation of dislocations as well as overaging of hardening precipitates.

Abstract: In-situ synchrotron X-ray radiography has been used to study columnar and equiaxed dendritic growth in directional solidification of Al-Cu alloys employing a Bridgman furnace. Nominal spatial and temporal resolutions of 1.5 µm and 150 ms, respectively, were obtained with a 1.3 %1.3 mm2 field of view, and a signal-to-noise above 99.5 %. Dedicated processing software has been developed to allow for quantitative extraction of data such as solid-liquid interface morphology, local propagation velocities and constitutional gradients from the images. The data collected also contain unprecedented in-situ observations on dendrite fragmentation. The limited field of view together with a slight sample position dependency in the heat transfer coefficients made it necessary to impose thermal gradients, G > 10 K/mm, to have reproducible solidification processes. Non-vanishing horizontal G-components contributed to convection that at some occasions resulted in observable effects on growing crystals or on liquid segregates.