Microstructural development in cast and aged Ni-Al-Cr-based alloys derived from the B2 type β-NiAl structure

The formation of intermetallic compounds consisting of nickel-rich B2-type NiAl (beta-phase) ductilized by two-phase A1 (gamma)/L1(2) (gamma') regions provides the possibility of combining ductility and high-temperature performance. Similar microstructures can also form the basis of high-temperature shape memory alloys, due to martensitic transformation of the beta-phase to an L1(0)-type product. One route by which beta-gamma/gamma' microstructures can be produced involves the use of chromium as a gamma-stabilizer. However, microstructural development in such a case is complicated by the formation of alpha-Cr precipitates. This paper examines microstructural development and stability in cast Ni-25 at % Al-14 at % Cr, Ni-29 at % Al-22 at % Cr and Ni-27 at % Al-8 at % Cr alloys, together with a more complex material, namely, Ni-20 at % Al-13 at % Co-9 at % Cr-4 at % Ti-1 at % Mo-1 at % V. Both the as-cast condition and samples aged at 850 and 1100 degrees C for 140 h are examined using transmission electron microscopy. The paper discusses the formation of L1(0) martensite, intradendritic gamma', interdendritic gamma/gamma' and alpha-Cr precipitation. (C) 1999 Kluwer Academic Publishers.