Microstructural development in cast and aged Ni-Al-Cr-based alloys derivedfrom the B2 type beta-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 perfo rmance. Similar microstructures can also form the basis of high-temperature shape memory alloys, due to martensitic transformation of the beta-phase t o an L1(0)-type product. One route by which beta-gamma/gamma' microstructur es can be produced involves the use of chromium as a gamma-stabilizer. Howe ver, microstructural development in such a case is complicated by the forma tion 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 a lloys, 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 transmi ssion electron microscopy. The paper discusses the formation of L1(0) marte nsite, intradendritic gamma', interdendritic gamma/gamma' and alpha-Cr prec ipitation. (C) 1999 Kluwer Academic Publishers.