Investigation on Peritectic Layered Structures by Using the Binary Organic Components TRIS-NPG as Model Substances for Metal-Like Solidification

A peritectic reaction is the transformation of a liquid and an already existing primary solid phase α to a second solid peritectic phase β(L +α→β). In the case where both solid phases occur in a dendritic manner the second phase solidifies directly from the liquid as well as by transforming from the primary solid phase. Under condition where the solidification morphology for one or both phases is planar, a peritectic alloy shows a variety of complex microstructures like isothermal peritectic coupled growth (IPCG), cellular peritectic coupled growth, discrete bands, island bands, or oscillatory tree-like structures.


Introduction
Peritectic alloys are widely used and have high economic importance. Examples of this class of material are steels, aluminum and copper alloys, rare earth magnets and composites. As the material properties are directly related to their solidification microstructures, a deeper understanding of the peritectic growth process is critical for getting an optimized quality of these alloys.
A peritectic reaction is the transformation of a liquid and an already existing primary solid phase α to a second solid peritectic phase β(L +α→β). In the case where both solid phases occur in a dendritic manner the second phase solidifies directly from the liquid as well as by transforming from the primary solid phase. Under condition where the solidification morphology for one or both phases is planar, a peritectic alloy shows a variety of complex microstructures like isothermal peritectic coupled growth (IPCG), cellular peritectic coupled growth, discrete bands, island bands, or oscillatory tree-like structures.
The theory and the conditions for possible band formations in peritectic systems without convection are described by Trivedi [1]. Since the formation of these solidification microstructures is sensitive to convection (which is nearly always present under gravity conditions) the observed solidification behavior is not well understood. To deepen the understanding of the formation of such complex peritectic structures the authors observed the dynamic of layered structure formation by using organic components [2][3][4][5][6][7] instead of metals [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27]. Experiments were carried out by affected knowingly the freedom of convection during the solidification. Additionally, comparative experiments are going on to be carried out under µg conditions aboard the International Space Station (ISS). By doing this, the influence of the natural convection on this particular peritectic solidification structures can be determined.

Research Method
Till today, only few organic systems were found that are suitable for the use as peritectic model system. It requires an appropriate concentration and temperature range for the peritectic region and both solid phases must show a non-facetted high temperature phase. The selected transparent organic model system TRIS (Trishydroxymenthyl-aminomethane)-NPG (Neopentylglycol) was analyzed by Barrio et al. [28]. It shows a peritectic region within the range of 0.47 to 0.54w% at the peritectic temperature of 410.7K.
To affect the freedom of convections, peritectic concentrations were filled in rectangular glass tube samples, either with an inner width of 100μm (near 2D) or of 600μm (more 3D). To control the process conditions, the samples are pulled through a selfconstructed micro Bridgman-furnace. This enables to specified and adjusted the temperature gradient and the solidification rate. The microscope was equipped with a CCD camera which allows taking and store time lapsed movies for a later evaluation of the experiments. The dark spots in both images and also the noticeable structure in the upper left area of the left image are contaminants on the outer glass surface. The gray lines represent the interface between the solid phases and the liquid, respectively.

Results and Discussions
An example of the result of our investigations can be seen in Figure 1. Both pictures show an isothermal peritectic coupled growth (IPCG) structure. The interface between solid and liquid can be recognized by the lateral horizontal dark gray line. The interface between the two solid phases (primary α phase and the peritectic β phase) appear in various dark grey tones depending on the depth within the sample. The growth morphologies of the IPCG are significant different in shape and growth dynamic. In 2D samples were the lamella spacing is in the same order of magnitude than the width of the sample, 3D growth features are almost suppressed. Therefore, the lamellae grow very straight in the vertical direction. In contrast, within a 3D samples, where the lamella spacing is less than the width of the sample, three-dimensional competition growth in lateral and transversal directions leads to partial bands and tulip-like oscillating lamellae.

Conclusion
For the first time, the formation of layered peritectic structures has been observed in real time by using model substances. The results show how the development of the microstructure is influenced by the competition between growth and existing convection. The future investigations under μg conditions aboard the ISS (and thus with absents of natural convection) will further deepen our understanding of the impact convection might have on the formation of peritectic layered structures.