EMBRITTLEMENT AND CRACK-GROWTH IN HIGH-TEMPERATURE INTERMETALLICS

Ordered intermetallic compounds based on transition metal aluminides have been proposed as structural materials for advanced aerospace applications. Development of such materials, offering benefits in density and/or operating temperature range, has concentrated on the aluminides of nickel, titanium, and iron, either as monolithic materials or as matrixes for composites. However, despite attractive elevated temperature properties, the utilisation of these materials has been hindered by their poor low temperature ductility and susceptibility to environmental embrittlement. This analysis of published work and current experimental results reveals a variety of environmental embrittlement mechanisms that afflict the three major aluminide types to varying degrees in either oxygen or hydrogen containing atmospheres at ambient or elevated temperatures. Embrittlement by hydrogen occurs in all the aluminide types at ambient temperature by loss of cohesive strength at interfaces, by interaction with dislocations (iron and nickel based), or by hydride formation (titanium based). The last mechanism is also operative at elevated temperature. In the presence of dissolved oxygen, titanium aluminides show little ductility, associated with pinning of 1/2[110] dislocations through a deepening of the Peierls valley. This effect is operative up to about 600-800-degrees-C. Although unaffected at room temperature, Ni3Al based alloys exhibit ductility loss at 600-800-degrees-C in the presence of oxygen, owing to grain boundary oxygen diffusion and localised embrittlement at crack tips. Mechanisms are discussed that address the observed environmental embrittlement in aluminides. In all cases, suitable alloying attenuates the effect or offers the best prospect of so doing.