Equilibrium and metastable solidification in Ti-Al-Nb and Al-Ni systems

The presented work reports on the solidification studies in two alloy systems: the niobium bearing γ-TiAl, relevant for the automotive and aero-engine applications, and aluminium rich Raney-Ni, precursor alloys for catalyses used in the chemical industry. The time-resolved observations of equilibrium liquid-solid phase transformations, as well as non-equilibrium solidification from the undercooled melt, are performed by combination of in situ structural studies using high-energy X-rays at a synchrotron source and the electromagnetic levitation technique. Containerless processing assured the contamination-free environment leading to high undercooling levels even at moderate cooling rates. For the critical part of the Ti-Al-Nb phase diagram an equilibrium involving the liquid phase is deduced from the phase transformations gathered on heating periods of levitation experiment. New experimental data on the partial liquidus and solidus surfaces are delivered as well as the information on the nature of the reactions along the univariant lines. These data provide a valuable contribution to the reassessment of the thermodynamic description. The primary phase selection as function of undercooling is studied in ternary Ti-Al-Nb alloys. The metastable formation of the cubic β phase within the primary solidification region of the hexagonal α phase is observed with increasing melt undercooling. Furthermore, the microstructure evolution of the β solidifying Ti-46Al-8Nb alloy discloses the transition to the thermal growth mode for ∆T>200−250 K, accompanied by complete solute trapping. Supplemented with the data on the solidification velocity determined as function of melt undercooling, this results are discussed within the local non-equilibrium model of the free dendrite growth. The in situ observations of the non-equilibrium solidification of the binary Al-Ni system give insight into multiple phase transformation sequence. The achieved undercooling levels up to 320 K for the aluminium alloys containing 18–31.5 at.% Ni did not alter the primary phase selection. However, during further cooling of L+Al3Ni2 semisolid samples the peritectic formation of a metastable decagonal quasicrystalline phase is observed providing a critical undercooling below the peritectic temperature of Al3Ni phase is reached. On further cooling the metastable phase subsequently transforms into the equilibrium Al3Ni. A similar solidification pathways are expected for the Raney-Ni alloys produced by gas atomisation, where the associated high cooling rates allowed to retain the metastable phase at room temperature