Mechanistic studies in combustion synthesis of Ni₃Al and Ni₃Al-matrix composites

Abstract

Nickel aluminides exhibit limited ductility and toughness at room temperature. One way to improve these characteristics is by adding ceramic reinforcements to the matrix. In this paper, we have studied the combustion synthesis of Ni₃Al and Ni₃Al-matrix composites, using the self-propagating high-temperature synthesis (SHS) mode. First, studies of the Ni₃Al synthesis were carried out by quenching the reaction during its progress, which revealed the mechanism of the synthesis. The influence of Al₂O₃ and SiC whiskers or particulates, and B₄C particulates added to the reaction mixture prior to combustion synthesis, was investigated next. It was found that, in general, reinforcements are heat sinks and limit the propagation of the reaction. Also, whiskers impede the flow of formed liquid to a larger extent than do particulates. Al₂O₃ is inert and matrices reinforced with up to 2 wt. % Al₂O₃ are composed essentially of Ni₃Al grains. However, both B₄C and SiC react with the Ni-Al matrix and lead to complex phases. In particular, B₄C readily forms a Ni-Al-B liquid phase and disrupts dramatically the progress of the Ni₃Al matrix synthesis.