Explanation for the linear solid/liquid interface recoil observed during directional solidification of a TRIS-NPG alloy under microgravity conditions

•Experiments with TRIS-NPG on the ISS show a linear solid/liquid interface recoil.•Initial transient simulations show that no linear interface recoil should occur.•Long-time experiments show the facetted O-phase in direct contact with the liquid.•Assuming thermal decomposition of TRIS-NPG can explain both findings.•For TRIS-NPG-X, with an increasing amount of X, measurements can be fitted. During the initial transient stage of a directional alloy solidification experiment, a solid/liquid interface asymptotically recoils from a position that is given by the liquidus temperature to a position given by the solidus temperature. Recent observations onboard the International Space Station revealed that for the organic compound TRIS-NPG, the recoil appears much larger and varies linearly with time. In addition, such conditions were found that the high-temperature non-facetted plastic phase gradually dissolves and, although it seems contradictory to the interpretation of the thermodynamics of the binary system, the low-temperature facetted phase comes into direct contact with the liquid. Both unexpected observations can be understood by assuming that the TRIS-NPG alloy gradually decomposes at the hot side of the furnace. The decomposition products are then transported to the solid/liquid interface by diffusion and the sample motion. The presence of decomposition products changes the binary alloy into a TRIS-NPG-X ternary alloy, with a liquidus temperature that decreases with an increasing amount of decomposed substances.