2111 Reaction Formed with a Ductile Metal Reinforcement Phase [19

One reactive method, often referred to as directed metal oxidation, involves reaction of molten aluminum with the oxygen in the air [45]. Normally, when aluminum metal is exposed to oxygen, a thin crust of aluminum oxide forms on the molten metal surface and acts as a barrier to further oxidation. However, if Mg and Si are added to the Al, the molten alloy wets the edges of the alumina crystals that nucleate on the surface of the Al such that the metal is wicked to the surface continually as the alumina is formed. An intertwined network of metal plus ceramic results. If the reaction temperature is high, around 1150°C, the ceramic-metal composite is mostly ceramic and has fracture toughness of only 4.7 MPa ■ m1/2. If the reaction temperature is lower, around 900°C, the percentage of metal increases and the fracture toughness also increases to about 7.8 MPa ■ m1/2. In both cases, the material has very high wear resistance and higher temperature capability than aluminum.

Other composite structures can be achieved by this directed metal oxidation process [45, 46]. If a bed or layer of particles or fibers is placed adjacent to the molten aluminum, the aluminum-alumina will grow right through to produce a particulate or fiber-reinforced ceramic-metal composite. This approach has been used with hard silicon carbide particles to achieve very wear-resistant parts that have performed well in many industrial applications.

Another interesting material that has been prepared by a reactive process combines toughening by platelet-shaped grains plus a ductile metal phase [37]. A porous perform of compacted boron carbide powder is placed in a graphite mold with zirconium metal. When heated to 1850-2000°C in an inert atmosphere, the Zr metal melts, infiltrates the preform, and reacts with the boron carbide to form a mixture of zirconium diboride platelets and zirconium carbide grains. The quantity of Zr can be varied either to be completely consumed by the reaction or to be retained as a residual metal phase to over 30% by volume. A sample with only 1% Zr had fracture toughness of 11 MPa ■ m1/2, while a sample with about 30% Zr had fracture toughness over 20 MPa ■ m1/2.

0 0

Post a comment