The effects of gravity on the combustion characteristics and microstructure of metal-ceramic composites (HfB 2 /Al and Ni 3 Ti/TiB 2 systems) were studied under both normal and low gravity conditions. Under normal gravity conditions, pellets were ignited in three orientations relative to the gravity vector. Low gravity combustion synthesis (SHS) was carried out on a DC-9 aircraft at the NASALewis Research Center. It was found that under normal gravity conditions, both the combustion temperature and wave velocity were highest when the pellet was ignited from the bottom orientation; i.e., the wave propagation direction was directly opposed to the gravitational force. The SHS of 70 vol pct Al (in the Al-HfB 2 system) was changed from unstable, slow, and incomplete when ignited from the top to unstable, faster, and complete combustion when ignited from the bottom. The hydrostatic force (height ϫ density ϫ gravity) in the liquid aluminum was thought to be the cause of formation of aluminum nodules at the surface of the pellet. The aluminum nodules that were observed on the surface of the pellet when reacted under normal gravity were totally absent for reactions conducted under low gravity. Buoyancy of the TiB 2 particles and sedimentation of the Ni 3 Ti phase were observed for the Ni 3 Ti/TiB 2 system. The possibility of liquid convective flow at the combustion front was also discussed. Under low gravity conditions, both the combustion temperature and wave velocity were lower than those under normal gravity. The distribution of the ceramic phase, i.e., TiB 2 or HfB 2 , in the intermetallic (Ni 3 Ti) or reactive (Al) matrix was more uniform.

Combustion synthesis (SHS) of HfB 2 -Al composite materials with a wide range of HfB 2 -to-Al ratios corresponding to either metal (Al) or ceramic (HfB 2 ) matrix was carried out with the emphasis on 60 and 70 vol pct Al. The effects of composition and green density of pellets on the combustion characteristics were studied. Combustion temperature, wave velocity, and reaction mode all changed drastically with composition and green density. The combustion mechanisms were also studied using temperature profile analysis. The combustion zone can be divided into preflame and main reaction zones, and the width of the latter was much larger than that of the former. It was also found that the combustion reaction was initiated at the melting of the aluminum and consisted of a two-step reaction sequence corresponding to the initial formation of Al 3 Hf and, subsequently, HfB 2 compounds. The formation of Al 3 Hf triggered the HfB 2 formation according to the following reaction mechanism:Al Hf ϩ 2B → HfB ϩ 3Al 3 2

Combustion synthesis (SHS) of Ni 3 Ti-TiB 2 metal matrix composites (MMCs) was selected to investigate the effect of gravity in a reaction system that produced a light, solid ceramic particle (TiB 2 ) synthesized in situ in a large volume (Ͼ50 pct) of the liquid metallic matrix (Ni 3 Ti). The effects of composition, green density of pellets, and nickel particle size on the combustion characteristics are presented. Combustion reaction temperature, wave velocity, and combustion behavior changed drastically with change in reaction parameters. Two types of density effects were observed when different nickel particle sizes were used. The structures of the combustion zones were characterized using temperature profile analysis. The combustion zone can be divided into preflame, reaction, and afterburning zones. The combustion mechanism was studied by quenching the combustion front. It was found that the combustion reactions proceeded in the following sequence: formation of liquid Ni-Ti eutectic at 940 ЊC → Ni 3 Ti ϩ NiTi phases → reduction of NiTi with B → TiB 2 ϩ Ni 3 Ti.

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