Electron Microscopy of the Primary Microstructure of Rapidly Solidified Ti–46Al–8Nb Alloy

摘要

The contradictory data on the formation of the microstructure of a refractory intermetallic Ti–46 at % Al–8 at % Nb alloy and on the phase transformations proceeding within in the Ti–Al–Nb phase diagram are analyzed and improved experimentally. To determine the primary solidified phase, a set of experiments is performed on melting of the alloy, which is synthesized in a high-purity argon atmosphere using crucibles made of an oxygen-free ceramics (99.99% AlN), and subsequent rapid volumetric isothermal solidification.Cooling from 1943 K at rates of 5, 10, and 20 K/s and subsequent quenching from 1763 K are used. Polished sections of ingots are studied by scanning electron microscopy using backscattered electron mode. Scanning electron microscopy micrographs demonstrate contrast regions of nonuniform niobium segregation, which are fixed by quenching and decorate the primary polycrystalline microstructure formed in the temperature range from 1843 (liquidus) to 1773 K (solidus). The primary crystalline phase is shown to be represented by β(Ti) dendrites, which have clearly pronounced fourfold symmetry that form during the development of sec-ondary arms.