Mechanistic studies in combustion synthesis of Ni 3 Al and Ni 3 Al-matrix composites

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Paul J. McGinn Center for Materials Science and Engineering, Department of Electrical Engineering, University of Notre Dame, Notre Dame, Indiana 46556 (Received 14 December 1992; accepted 11 January 1994)

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 A12O3 and SiC whiskers or particulates, and B 4 C 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. A12O3 is inert and matrices reinforced with up to 2 wt. % A12O3 are composed essentially of Ni3Al grains. However, both B 4 C and SiC react with the Ni-Al matrix and lead to complex phases. In particular, B 4 C readily forms a N i - A l - B liquid phase and disrupts dramatically the progress of the Ni3Al matrix synthesis.

I. INTRODUCTION Intermetallic compounds such as nickel aluminides are considered suitable materials for structural applications owing to their low density and good corrosion resistance at high temperature.1 However, their low ductility and lack of creep resistance at high temperature place limitations on their use.2 Significant improvement in the ductility and toughness has been achieved through the addition of ternary elements.3 For example, microalloying of Ni3Al by 0.1 wt. % boron leads to enhanced tensile ductility.4 An alternative approach designed to overcome the detrimental characteristics of intermetallics is the addition of continuous or discontinuous ceramic reinforcements such as A12O3 fibers or particulates. Reinforced intermetallic matrix composites can be produced by a variety of powder processing techniques, e.g., rapid solidification processing, mechanical alloying, etc.5 Among these, solid-state combustion synthesis, which includes Self-Propagating High-Temperature Synthesis (SHS) and thermal explosion, has recently attracted significant interest. In this method, the exothermic reaction between powder constituents is initiated and becomes self-sustaining to yield the final product progressively without requiring additional heat. Because no complex equipment is required and processing

correspondence should be addressed to this author. 1184

J. Mater. Res., Vol. 9, No. 5, May 1994

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times are on the order of seconds/minutes, this method presents a simple and attractive alternative to time- and energy-consuming conventional methods. Reviews of the combustion synthesis technique and its prospects have rece

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