Formation and dynamics of grain boundaries in directional solidification

摘要

The solidification of non-faceted alloys has been studied for about twenty years from a fundamental point of view as a pattern-forming model system. Directional solidification of transparent organic alloys in thin samples -the sample is pulled at fixed speed V in an externally imposed temperature gradient G- is a choice method to observe stationary growth regimes. The solidification rate is then equal to the pulling velocity, and the propagating solid-liquid interface, which appears immobile in the laboratory reference frame, can be observed in situ by optical microscopy. It is well known that the stationary regime of the planar front is stable at low velocity, i.e. for V less than a threshold value V_c, above which it undergoes a cellular, or Mullins-Sekerka instability. In fact, experimentally, a closer examination of the planar solidification front generally reveals the existence of small cusped grooves at the solid-liquid interface. These grooves are attached to grain boundaries. Their depth and apex angle are determined by Young's law and are a function of the surface tension of the grain boundary (Fig. 1). Some of the observed grooves are attached to pre-existing large-angle grain boundaries, i.e., grain boundaries with a large disorientation angle (typically, more than 5°), between the two neighboring crystals or grains. Here we report the appearance of new grooves in the course of solidification, for velocities well below V_c. These grooves are shallow, and reveal the formation of low-angle grain boundaries, or subboundaries (disorientation angles typically less than 1°). In other words, a solidification induced polygonization process is evidenced. We show that the governing mechanism of this process is a coupling between the diffusion-controled dynamics at the interface and the dislocation forest in the solid.