Reaction sintering of NiAl and TiB2-NiAl composites under pressure

Intermetallic matrix composites are a new class of engineering materials fo r high temperature structural applications in oxidizing and aggressive envi ronments. Attempts have been made in the present investigation to synthesiz e 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 fa bricated from the mixtures of elemental Ni and Al and TiB2 powders by high pressure reaction sintering (HPRS) and reactive hot pressing (RHP). The HPR S and RHP were carried out at 3 GPa and 900 degrees C, and 20 MPa and 1650 degrees 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 (Ni2 Al3, Ni3Al). It was necessary to anneal these compacts at 1100 degrees C to obtain TiB2-NiAl composites with single phase NiAl matrix. The densities o f the HPRS compacts were similar to 99%. The hardness and fracture toughnes s values were in the range IO to 20 GPa and 3.9 to 5.7 MPa root m, respecti vely. The RHP compacts contained AlB2, Ni2B and NiTi2 phases in addition to those present in the HPRS compacts. The RHP composites were fully dense. T hese had superior hardness (15-22 GPa) but inferior fracture toughness (2.9 -3.8 MPa root m) compared to those obtained by HPRS. (C) 1998 Elsevier Scie nce S.A. All rights reserved.