On the Primary Phase Microstructure of Solidifying Ti-46Al-8Nb Refractory Intermetallic Alloy

The reported contradictory data on microstructure formation of the refractory intermetallic Ti–46Al–8Nb (at.%) alloy and on the high-temperature phase transformations proceeding within the Ti–Al–Nb phase diagram are analysed and clarified experimentally. To determine the primary solidifying phase, a set of experiments is performed on melting of the alloy specimens with low oxygen contamination in a high purity argon atmosphere using crucibles made of oxygen-free ceramics (99.99% AlN), and subsequent rapid solidification. Volumetrically-isothermal cooling from 1943 K at rates of 5, 10, and 20 K/s and following quench of mini-ingots from 1763 K are used. Specimens were studied by scanning electron microscopy (SEM) in backscattered electron (BSE) mode. SEM-BSE micrographs demonstrate contrasting shadow regions of non-uniform niobium segregation, which are fixed by quench and decorate the primary polycrystalline microstructure formed within the temperature range between 1843 (liquidus) and 1773 K (solidus). The primary crystallizing phase is proven to be represented by β(Ti) dendrites, which have clearly pronounced fourfold (bcc-lattice) symmetry being formed with secondary dendrite arms development. The solidification path is shown to be described with single- phase scheme L→L+β(Ti)→β(Ti); no peritectic β(Ti)→α(Ti) bcc-hcp phase transformation revealed within the mushy state of alloy.