The possibility of forming the topologically close-packed Laves phase has been investigated in various Ni-base superalloy systems that contain a significant amount of Nb. Simplified alloy compositions consisting of various combinations of major alloying elements, including Cr, Fe, and Co, were prepared; a high level of Nb was added in each alloys to simulate the dendritic segregation in the real casting process. In addition to the fee dendrites, the as-cast microstructure through a slow solidification rate developed the regions of eutectic decomposition at the end of solidification. The eutectic regions consisted of only two phases in every alloy studied. The intermetallic phases that formed the eutectic with the fee Ni matrix were identified by SEM-EDAX, X-ray diffraction, and DTA analysis. The results suggested that Laves phase was not expected to exist in Ni-Cr-Co base alloys, and that other Nb-hardened superalloys, especially those Ni-Cr-Fe base alloys, would likely develop Laves phase in the Nbsegregated regions during casting. Alloy chemistry theory was proposed to discuss the alloying effect on the existence of Laves phase in Ni base alloy systems. The combination of Cr and Fe alloying additions would be the essential 'criteria to allow Laves phase appeared in a slow solidification process. The DTA analysis indicated that the alloy with the Laves phase had a low eutectic melting point. These important results can provide a comprehensive understanding of what observed in those Nb-strengthened superalloys with complex chemical compositions.

In this study, the effects of adding TiB2 particles to eutectic Al + Mg2Si phases in aluminum alloys were analyzed. The eutectic Al + Mg2Si phases were modified effectively when a large amount of TiB2 was added, and changes in the shape, size, and distribution of the eutectic Al + Mg2Si phases were confirmed using a polarizing microscope and FE-SEM. The crystal structure of the TiB2 particles and Mg2Si phases were analyzed using HR-TEM, and the analysis confirmed that the TiB2 particles can act as heterogeneous nucleation sites. This paper intends to clarify the principle of phase modification of the eutectic Al + Mg2Si phases by TiB2 particles and proposes a new mechanism to improve Mg2Si phase modification when TiB2 particles are added.