Johann Mogeritsch

Experimental Observation of Convection during Equiaxed Solidification of Transparent Alloys

3D samples of NH4Cl-H2O solutions were solidified under defined experimental conditions. The occurring melt convection was investigated by Particle Image Velocimetry (PIV). The occurrence of NH4Cl crystals was observed optically and first attempts were made to quantitatively measure its number density, size distribution and sedimentation rate by PIV and Particle Tracking (PT). In order to prove the reproducibility of the results several experimental runs with equal and slightly modified conditions were analyzed.

In-situ observation of coupled growth morphologies in organic peritectics

Systematic directional solidification experiments were done with TRIS-NPG organic peritectic alloys to investigate peritectic solidification close to the limit of constitution undercooling. The experiments were carried out as in-situ observation in a horizontal micro Bridgman furnace. Under specific conditions isothermal peritectic coupled growth (PCG) were obtained for alloys within the hypo-peritectic region. Isothermal PCG was obtained either by (i) reducing the growth velocity from above the critical value for morphological stability of both solid phases to a value below, or by (ii) long-time growing with a constant velocity below the critical value for morphological stability of both solid phases. The later happens via island banding, where nucleation and lateral growth of the peritectic phase compete with growth of the primary phase. Interphase spacings and the widths of α and β lamellae were measured as function of growth velocity for a fixed composition and as function of composition for a fix growth velocity.

In Situ Observation of Solidification in an Organic Peritectic Alloy System

Up to date very few organic substances have been reported that show a non-faceted/non-faceted (nf/nf) peritectic phase diagram in a temperature range suitable for direct observation in a micro Bridgman furnace setup. Sturz et al. [1] and Barrio et al. [2] studied the peritectic phase diagram for the organic model alloy TRIS (Tris(hydroxymenthyl)aminomethane) - NPG (Neopentylglycol). The phase diagram is based on thermal analysis by means of DSC measurements [1, 2] and evaluation of lattice parameters measured with x-ray diffractometry [2]. In the current work we present investigations on the system TRIS – NPG that have been obtained by optical investigations of directional solidification in a micro Bridgman-furnace with various initial alloy concentrations and pulling rates in a fixed temperature gradient. The phase diagram [1, 2] was confirmed by direct comparison of DSC measurements and optical investigations. Furthermore we present in situ observations of solidification in the peritectic region. They show a solidification behavior that was clearly distinguishable from the solidification in hyper- and hypoperitectic regions of the phase diagram.

In-situ observation of the dynamic of peritectic coupled growth using the binary organic system TRIS-NPG

This paper shows the evolution from banded structure to peritectic coupled growth and describes the mechanism which leads to such microstructure formation. Experimental investigations on binary organic TRIS-NPG (Trishydroxymethyl-aminomethane, Neopen- tylglycol) alloys at concentrations in the peritectic region were carried out in order to study the formation of peritectic microstructures. The experiments were done vertically with relatively large rectangle glass tubes so that slow natural convection occurred. Each time a sample was hold unmoved in the vertical micro Bridgman-furnace for 2 hour to establish a constant temperature gradient. Afterwards, the samples were moved at several pulling rates through the furnace and a camera recorded the dynamic of the solid/liquid interface. The in-situ observations show layered structures for concentrations within the hyper-peritectic region. Carefully evaluations of the results exhibit, that (i) the solid/liquid interface is curved in observation direction toward the glass walls, (ii) the initial existing peritectic phase grows at micrometer-sized liquid channels inside the primary phase toward to the solid/liquid/glass wall junction, (iii) afterwards the peritectic phase spreads along the liquid-primary phase boundary and forms a so-called band, then (iv) the primary phase gets largely overgrown and unstable peritectic coupled growth (PCG) forms; (v) this PCG is inherent unstable and disappears and occurs in several cycles; until (vi) finally stable growth of the primary phase prevails.

Recurring Instability of Cellular Growth in a Near Peritectic Transparent NPG-TRIS Alloy System

In this study, a near peritectic transparent metal-like solidifying NPG-TRIS alloy was directionally solidified in a thin sample under process conditions which favor cellular growth. Dif-ferent to an identical horizontally processed sample, the vertically processed sample revealed a cel-lular array which was curved. This curving was attributed to a convection-induced higher alloy con-centration at the sample edges compared to the sample center. Surprisingly, it was found that the curved cellular array is inherently unstable as three times dendrite-like structures shot forwards. As origin of these rapid advancements two possibilities were discussed. Either a convection-induced variation of the solute boundary layer might be responsible. Or it can be explained by a more gener-ic approach which due to the curved form of the cellular array considers extended freedom for cel-lular branching in combination with a relative large deviation of the preferential crystal growth di-rection from the heat flow direction. For the discussed phenomena the presence of the peritectic phase within the intercellular spacing was found to be not of importance.