Multiphase niobium aluminides fabricated via reaction synthesis

Excerpt

Niobium aluminides have been considered as a potential high temperature structural material [1‐3]. They are expected to exhibit high elastic stiffness, strength and creep resistance at elevated temperatures. One of the notable advantages of niobium aluminides are the high melting points (which can be seen on the phase diagram in Fig. 1) compared to those of other, more commonly considered aluminide systems. Titanium and nickel alumindes are generally limited to structural applications below 1000 °C [5, 6], however, niobium aluminides can have extremely high strength even above 1200 °C and the incorporation of ductile reinforcements of metallic niobium can be used to lower the ductile to brittle transition temperatures and increase room temperature fracture toughness [1]. However, melting and casting of niobium aluminides presents several difficulties due to the high temperatures required, high evaporation rate of aluminium and the wide difference in the melting points and densities of niobium and aluminium. Homogeneity and composition control has presented particular difficulties and resulted in the development of specialised melting equipment [7]. Special mould materials are used due to the high casting temperatures required and high reactivity of the Nb–Al melt. Aluminium rich phases are particularly susceptible to microcracks occurring during casting and, despite the good high temperature properties, low ductility and toughness at room temperature make it a difficult material to machine. Reaction synthesis offers a potential solution to some of these processing difficulties. Advantages include: (i) the reduction in externally applied energy, as the heat of reaction is used to heat and soften or melt the intermetallics formed, (ii) short heating and cooling times, resulting in fine scale microstructures and reduced contamination from mould materials and the atmosphere, (iii) near net shape forming, thus reducing machining requirements, and (iv) the possibility of producing composites by incorporation of a second phase in the reacting mixture [8]. However reactive processing can result in relatively high levels of porosity, due to the initial density of the reacting powder compact, volume changes during reaction and Kirkendall porosity. Furthermore, the reaction can be difficult to control and may not go to completion; also loss of shape can occur due to melting, gas evolution and shrinking during densification. Subsequent and simultaneous application of pressure by hot forging or hot extrusion can be used to remove porosity from aluminides fabricated through reaction synthesis [9, 10]. This letter presents the results from a series of reaction synthesis trials across the Nb–Al system. In light of this the suitability of simultaneous application of pressure in order to densify the intermetallics formed is considered. Interesting fine scale microstructural features have been obtained.

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