Multistage sintering approach to prepare Ni3Al/α-Al2O3 composites

The nickel aluminide intermetallic matrix composites (IMC), Ni76Al24B0.1 with either 5 or 10 vol pct α-Al2O3, were synthesized through a multistage sintering approach from the elemental powders of Ni, Al, and oxide of α-Al2O3. An electroless nickel-boron (Ni-B) plating process was adopted to improve the contacted interface between the reinforced oxide ceramics and the metal matrix, as well as to supply the atomic scale boron in the metallic matrix of the IMCs. The entire process comprises steps involving preparing a powdery starting material, sealing it within a metal sheath or can, compacting or cold deforming it, preliminarily heating the compacted material at a relatively low temperature, executing a pore-eliminating (mechanical deforming) process to eliminate the pores resulting from the preceding heating step, and sintering the material at a relatively high temperature to develop a transient liquid phase to heal or to eliminate any microcracks, crazes, or collapsed pores from the previous steps. Most of all, it is important that contact with a heat absorbent material, such as a metal sheath, produces the Ni2Al3 phase during preliminary heating. This new phase is a brittle and crispy material with a low melting point (1135 °C). It has been found to play an important role in preventing any significant cracks during the pore-eliminating process and in developing a transient liquid phase in the following 1200 °C sintering step. This multistage sintering with a heat absorbent process is beneficial for producing a product that has large dimensions, a desirable shape, good density, and excellent mechanical 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, respectively.