Microstructure, Tensile and Creep Properties of Minor B-Modified Orthorhombic-Type Ti–27.5Al–13Nb Alloy and Its Nb-Replaced Mo- and Fe-Containing Derivatives

Ti–27.5Al–13Nb is an intermetallic alloy that incorporates the orthorhombic Ti2AlNb phase (O phase) and the α2 phase, and was previously developed by the authors for high-temperature applications. The aim of the present study was to develop less expensive orthorhombic alloys. For this purpose, a portion of the cost-prohibitive Nb in this baseline Ti–27.5Al–13Nb alloy was replaced with an equivalent amount of Fe and/or Mo, yielding three new derivative alloys: Mo-replaced Ti–27.5Al–8.7Nb–1Mo, Fe-replaced Ti–27.5Al–5.5Nb–1Fe and (Mo and Fe)-replaced Ti–27.5Al–4.9Nb–1Mo–0.5Fe. Further, a minor amount of B (boron), i.e., 0.1 pct B, was added to these derivative alloys and baseline alloy. The microstructures and corresponding tensile and creep properties were examined in four cases: B-free and B-modified alloys with fully lamellar microstructures, and B-free and B-modified alloys with duplex microstructures. With the addition of 0.1 pct B, the prior B2 grain size of each ingot was drastically reduced by about one order of magnitude, from 600∼1000 µm for the B-free alloy to 100∼250 µm, and thereby a refined fully lamellar microstructure was obtained. However, this high degree of grain refinement did not markedly improve ductility at room temperature. A duplex microstructure consisting of a globular α2-phase and a lamellar microstructure led to the improved ductility and tensile strength. The addition of B exerts either a positive or negative effect on the creep properties depending on the compositions and microstructures, creep test temperature and applied stress. The Mo-replaced alloys had better creep properties than the baseline alloy, whereas creep properties of the Fe-replaced and (Mo and Fe)-replaced alloys were considerably inferior to those of the baseline alloy. Among the four alloys, the 0.1 pct B-modified Mo-replaced alloy with a duplex microstructure exhibited the highest ductility of 4.7 pct at room temperature and higher tensile strength up to 1073 K and better creep properties than the other two derivative alloys and the baseline alloy.