Reaction sintering of NiAl and TiB2–NiAl composites under pressure

Intermetallic matrix composites are a new class of engineering materials for high temperature structural applications in oxidizing and aggressive environments. Attempts have been made in the present investigation to synthesize TiB2–NiAl composites, in which the matrix phase NiAl was produced in situ by reaction synthesis. The composites with 10, 15 and 30 vol.%NiAl were fabricated from the mixtures of elemental Ni and Al and TiB2 powders by high pressure reaction sintering (HPRS) and reactive hot pressing (RHP). The HPRS and RHP were carried out at 3 GPa and 900°C, and 20 MPa and 1650°C, respectively. The different phases in the sintered compacts were identified by X-ray diffraction and microstructural studies. In HPRS, the reaction was incomplete which gave rise to various intermediate phases (Ni2Al3, Ni3Al). It was necessary to anneal these compacts at 1100°C to obtain TiB2–NiAl composites with single phase NiAl matrix. The densities of the HPRS compacts were ∼99%. The hardness and fracture toughness values were in the range 10 to 20 GPa and 3.9 to 5.7 MPa√m, respectively. The RHP compacts contained AlB2, Ni2B and NiTi2 phases in addition to those present in the HPRS compacts. The RHP composites were fully dense. These had superior hardness (15–22 GPa) but inferior fracture toughness (2.9–3.8 MPa√m) compared to those obtained by HPRS.