Precipitation of (Ti,V)2Al(C,N) in multiphase Al-Ti-V alloys

Precipitation of the complex carbonitride (Ti,V)2AlC(N) in multiphase Al-Ti-V alloys with compositions based on Al3Ti has been studied using analytical transmission electron microscopy. Plate-like precipitate particles were observed in association with a range of phases, including δ-Al3(Ti,V), ξ′-Ti2Al5, ξ-Ti5Al11, ζ-Al8V5 and the metallic β-(Ti,V) following chill-casting and subsequent isothermal heat treatments. They were formed preferentially along the interphase boundaries between the δ (or ξ′, ξ) and β phases in heat treated duplex microstructures. Electron diffraction confirmed the crystal structure of the precipitates (designated h phase) to be hexagonal (a = 0.297 nm, c = 1.354 nm), consistent with carbides or nitrides of the form M2 AlC or M2AlN (M = Cr, V, Ti, Nb). The orientation relationship most common between the h and δ phases was such that (0001)h∥ (111)δ and [11–0]h∥[10]δ while a relationship of the form (0001)h∥(110)β,ζ and [11–0] h∥[11]β,ζ was observed between the h and b.c.c. β and ζ phases. Energy dispersive X-ray analysis indicated that the h phase has a stoichiometry of the form (Ti,V)2Al, while electron energy loss spectra revealed the presence of both C and N, implying that the precipitate is the product of a solid solution between isostructural Ti2AlC(N) and V2AlC(N).