Solidification behaviour and microstructure evolution of refractory metals-based alloys under rapid solidification

• Main Author: Feitosa, Leandro Moraes
• Other Authors: Dong, Hong ; Tsamis, Alkis
• Published: University of Leicester 2018
• Subjects: 620
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.755395

In this thesis, the solidification behaviour and microstructure evolution of Ta-Al-Fe, Nb-Al-Co, Nb-Al-Fe, Ta-Al-Ti and Nb-Si-Ti alloys rapidly solidified via arc melting, suspended droplet alloying and additive manufacturing techniques were examined. Loss of interfacial equilibrium resulted in an extended solute solubility with significant undercooling due to nucleation constraints, leading to unexpected phases. For Ta-Al-Fe alloys, when Al < 10 at.% and Fe < 4 at.%, the peritectic reaction, L+A2 → σ, is suppressed and the eutectic, L → A2 + μ, occurs with formation of a halo of μ on primary σ phase. For Co-rich Nb-Al-Co alloys, when Nb > 20 at.%, the quasi-peritectic reaction, L+Co2AlNb → C36+CoAl does not occur, C36 and CoAl phases form through solid-state precipitation. A halo of C14 forms on primary CoAl but limited vice-versa. In Nb-Al-Fe alloys, failure to initiate coupled growth of NbAl3+C14 leads to a two-phase halo of C14+Nb2Al. The quasi-peritectic reaction, L+Nb2Al → (Nb)+μ is suppressed, forming the eutectic Nb2Al+μ instead. The ternary eutectic, L+C14+Nb2Al → μ, is limited with μ forming primarily. For Ti-Al-Ta alloys, the quasi-peritectic reactions, L+β → α+σ and L+σ → α+κ, occur at a very narrow window; good agreement for γ and ε phases is found with non-equilibrium formation of ε. For additive manufactured Nb-Si-Ti alloys, an increase in scanning speed led to microstructure refinement. Due to elemental additions, Nb3Si is suppressed with formation of Nbss+Nb5Si3. C15 Laves phase formed when Cr > 5 at.%, with HfO2 when Hf is added. For Ti > 22 at.% and SI > 18 at.%, a Ti-rich Nb5Si3 forms due to rejection of Ti solute. Hot isostatic pressing and heat treatment led to crystal structure changes, densification and phase coarsening. Diffusion and local super-saturation led to the split of Nbss into an energetically stable and homogeneous Tiss phase.