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.

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Investigations on solidifying transparent model alloys have served frequently to gain knowledge on physical phenomena occurring during solidification of metallic alloys. However, quantitative results were only possible to obtain in thin samples where convection can successfully be suppressed. Quantitative studies on three dimensional phenomena not being affected by natural convection are thus only possible under microgravity conditions. Therefore, the European Space Agency (ESA) is planning to launch a new insert for the Material Science Glovebox (MSG) on board of the International Space Station (ISS) for studies on solidification phenomena in thick samples. Four different classes of transparent model alloys will be used to address the following scientific topics: (i) Columnar to Equiaxed Transition in Solidification Processing, (ii)Novel Peritectic Structures and In-Situ Composites; (iii) Solidification along an Eutectic Path in Binary Alloys; and (iv) Solidification along an Eutectic Path in Ternary Alloys. Here, we give details on the scientific objectives and the operational features ESA's new solidification device will offer. Introduction:

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A transparent non-facetted/non-facetted peritectic system was used for in-situ studies of peritectic growth morphologies above and below the limit of constitutional undercooling. For a hypo-peritectic alloy with near peritectic composition an oscillatory growth mode was found for velocities in the cellular/dendritic regime. It is demonstrated that the co-existence of two distinct cellular/dendritic solid/liquid interfaces which try to reach stable growth conditions causes this oscillations. Additionally, it is shown that on reducing the withdrawal speed the two distinct interfaces arrange into an isothermal coupled peritectic growth mode.