HETEROGENEOUS NUCLEATION MECHANISMS OF TiB_2 PARTICLES IN Al ALLOYS

摘要

The benefits of grain refining wrought and foundry Al alloys are widely acknowledged. To induce grain refinement, Al-Ti-B master alloys are commonly added to the melt prior to casting. Understanding the nucleation mechanisms of these refiner particles is important with respect to optimizing current commercial casting practices and improving future refining potency. However, using conventional solidification casting techniques, heterogeneous nucleation is difficult to investigate as a-Al grain growth obscures the nucleation sites. A metallic glass technique has been successfully applied allowing the nucleation mechanisms to be examined using Transmission Electron Microscopy (TEM). The technique freezes the crystalline growth of a-Al at an early stage in a glass and allows the study of the orientation relationship between nucleant and nuclei using diffraction and chemical analysis. The effects of Si and Zr which at higher concentrations can have a detrimental effect on the ultimate grain size (UGS) achieved by the grain refiner addition were examined. It has been found that the chemical composition of the refiner particles and their interfaces could play an important role in the nucleation mechanism, which has implications for current casting practices.