Multistage sintering approach to prepare Ni3Al/alpha-Al2O3 composites

The nickel aluminide intermetallic matrix composites (IMC), Ni76Al24B0.1 wi th either 5 or 10 vol pct alpha -Al2O3, were synthesized through a multista ge sintering approach from the elemental powders of Ni, Al, and oxide of al pha -Al2O3 An electroless nickel-boron (Ni-B) plating process was adopted t o improve the contacted interface between the reinforced oxide ceramics and the metal matrix, as well as to supply the atomic scale boron in the metal lic matrix of the IMCs. The entire process comprises steps involving prepar ing a powdery starting material, sealing it within a metal sheath or can, c ompacting or cold deforming it, preliminarily heating the compacted materia l at a relatively low temperature, executing a pore-eliminating (mechanical deforming) process to eliminate the pores resulting from the preceding hea ting step, and sintering the material at a relatively high temperature to d evelop a transient liquid phase to heal or to eliminate any microcracks, cr azes, or collapsed pores from the previous steps. Most of all, it is import ant that contact with a heat absorbent material, such as a metal sheath, pr oduces the Ni2Al3 phase during preliminary heating. This new phase is a bri ttle and crispy material with a low melting point(1135 degreesC). It has be en found to play an important role in preventing any significant cracks dur ing the pore-eliminating process and in developing a transient liquid phase in the following 1200 degreesC sintering step. This multistage sintering w ith a heat absorbent process is beneficial for producing a product that has large dimensions, a desirable shape, good density, and excellent mechanica l properties. The resulting elongation of tensile tests in air reaches 14.6 and 8.9 pct for the present 5 and 10 vol pct powder metallurgy IMCs, respe ctively.