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

Abstract

Ti27.5Al13Nb is an intermetallic alloy that incorporates the orthorhombic Ti 2 AlNb phase (O phase) and the ¡ 2 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 Ti27.5Al13Nb alloy was replaced with an equivalent amount of Fe and/or Mo, yielding three new derivative alloys: Mo-replaced Ti27.5Al8.7Nb1Mo, Fe-replaced Ti27.5Al5.5Nb1Fe and (Mo and Fe)replaced Ti27.5Al4.9Nb1Mo0.5Fe. 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 600³1000 µm for the B-free alloy to 100³250 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 ¡ 2 -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.

High Cycle Fatigue and Very High Cycle Fatigue of Orthorhombic (O + α₂)-Type Ti–27.5Nb–13Al Alloy with and without B and Fatigue Striation Analysis in Comparison with (α + β)-Type Titanium Alloys

“…With the addition of 0.1 pct B, the prior B2 grain size in the as-cast condition was reduced drastically, from 600³ 1200 µm for the B-free alloy to 100³250 µm. 18,20) Figure 1 shows the lamellar and duplex microstructures of B-free and 0.1 pct B-modified Ti27.5Al13Nb alloys taken A d v a n c e V i e w on planes parallel to the rolling direction. When the B-free alloy was cooled from the single B2 phase region, a typical lamellar microstructure with similarly aligned ¡ 2 phase lamellae (seen as a white contrast phase) within the prior B2 grains and a massive ¡ 2 phase at the grain boundaries 17,18) were observed (Fig.…”

“…2022) The width of each TiB was 1³2 µm, and the aspect ratio of the width to the length was ³10. 18) Due to the presence of these streaks of TiB, the growth of B2 grains was blocked when the alloy was held in the single B2 phase region, and thus the B2 grain size was greatly reduced. In conjunction with this B2 grain refinement, a reduction of colony size was observed, which went from ³500 µm to ³70 µm by the addition of 0.1 pct B.…”

“…6) To refine the grain, the authors tried to add a minor amount of boron (B) in this Ti27.5Al13Nb alloy, 18) since it is reported that TiB formed during the solidification process acts as obstacle against grain growth, 19) and we found that the prior ¢-grain sizes of this alloy in the as-cast condition was reduced by about an order of magnitude, from 1200 to 200 µm, by the addition of 0.1 pct B. 18) As another means to improve ductility, the A d v a n c e V i e w microstructure of this alloy was modified to a duplex by applying a thermomechanical treatment, in which hot deformation of the alloy is performed in the high temperature (B2 + ¡ 2 ) two-phase region, followed by annealing in the same (B2 + ¡ 2 ) two-phase region. By subsequent stabilization annealing in the (O + ¡ 2 ) two-phase region existing on the low temperature side, the B2 phase transforms into the (O + ¡ 2 ) lamellar microstructure.…”

“…By subsequent stabilization annealing in the (O + ¡ 2 ) two-phase region existing on the low temperature side, the B2 phase transforms into the (O + ¡ 2 ) lamellar microstructure. 17,18) Consequently, a duplex microstructure consisting of a globular ¡ 2 phase and an (O + ¡ 2 ) lamellar microstructure was obtained. By applying these two grain-refining methods separately or concurrently, the RT ductility was markedly increased.…”

“…Therefore, in a previous study, we evaluated the creep properties of this Ti27.5Al13Nb alloy at 1023 K under 250 MPa stress and discussed the creep mechanism operating in this alloy under different microstructural conditions. 18) Since titanium material is often used in automobile and aircraft parts such as turbine blades, turbine discs, and spring coils, where very low alternating or cyclic stresses operate constantly and the service life extends well into the very high cycle (VHCF) regime, both fatigue and creep are regarded as the most important characteristics for such applications. The £-TiAl alloy can be considered a promising candidate alloy for use in such parts.…”