<i>In-situ</i> Observation of Melting Behavior of Al-6.4 mass%Si Alloy with Spherical Primary Crystal by SIMA Process

The strain-induced melt activation (SIMA) process is used to obtain fine spherical primary crystals by introducing strain into dendritic primary crystals and then heating them to a semi-solid state. In-situ observations using a laser scanning microscope elucidated the refinement and spheroidization processes during heating to the semi-solid state, which had previously only been inferred. The inverse pole figure map obtained by electron backscatter diffraction shows that, after strain introduction, the dendritic primary crystals were divided into multiple regions with different crystal orientations. During heating to the semi-solid state, linear gaps due to thermal grooving were observed in the primary crystal from about 500°C onward. Linear gaps appear at various locations in the primary crystals. When the eutectic temperature was reached, the liquid phase flowed into the gaps. The results indicate the following: In the SIMA process, a difference in crystal orientation occurs within the primary crystal when strain is induced, which is thought to function as a grain boundary. This is considered the main factor contributing to the refinement of the primary crystals in the SIMA process. Furthermore, it was confirmed that refinement occurred as the liquid phase flowed into the grain boundaries within the primary crystals.