BETA-EUTECTOID DECOMPOSITION IN RAPIDLY SOLIDIFIED TITANIUM-NICKEL ALLOYS.

The eutectoid reaction, beta yields alpha plus Ti//2Ni, has been observed in as-produced rapidly solidified ribbons and flakes of hypoeutectoid and near-eutectoid beta titanium-nickel alloys prepared by chill block melt spinning (CBMS), pendant drop melt extraction (PDME), and electron beam melting/splat quenching (EBSQ) processes. Microstructural characterization of these materials was carried out by scanning electron microscopy, transmission electron microscopy, and X-ray energy dispersive spectroscopy. The occurrence of eutectoid decomposition is attributed to the breakaway of the ribbons or flakes (while still at an elevated temperature) from the quench wheel, resulting in a lower cooling rate. Fast quenching, as obtained in the hammer and anvil process, resulted in a martensitic structure free from products of eutectoid decomposition. The eutectoid morphology was nonlamellar in hypoeutectoid alloy ribbons, while a hitherto unreported lamellar eutectoid was observed in the near-eutectoid ribbons and flakes. The formation of this lamellar eutectoid is rationalized in terms of the predominance of allotriomorphs of alpha phase and availability of sufficiently mobile alpha/beta interphase boundaries.