Solidification Process and Mechanical Behavior of the Nb/Nb₅Si₃ Two Phase Alloys in the Nb-Ti-Si System

Abstract

Design of two-phase alloys consisting of niobium solid solution (A2) and -Nb 5 Si 3 (D8 l ) phases in the Nb-Ti-Si ternary system is pursued for ultra-high temperature structural applications. Compositional and annealing conditions are determined for the formation of A2/D8 l lamellar microstructure via eutectoid decomposition of Nb 3 Si (tP32) phase in the Nb-Si binary and Nb-Ti-Si ternary system. Addition of titanium is found to result in increased room temperature toughness, but decreased high temperature strength. Enhancement of mechanical properties is achieved by applying the directional solidification technique. Mechanical properties of the alloys are discussed from microstructural point of view.

Dislocation character and operative slip systems in α-Nb5Si3 tested at 1673K

“…Previous studies have also demonstrated that the decomposition of the high temperature phase Nb 3 Si into the equilibrium (Nb)/a-Nb 5 Si 3 i two phases through an eutectoid reaction is so sluggish that the Nb 3 Si phase is retained in the as-solidified condition [17,18]. Thus the directionally solidified alloys were annealed at 1673 K for 500 h, by which retained high temperature phase Nb 3 Si completely transforms into (Nb) and a-Nb 5 Si 3 [19,20]. High temperature compression tests were conducted under vacuum at temperatures up to 1673 K with a nominal strain rate of 1.0 Â 10 À4 s À1 .…”

Fracture toughness and high temperature strength of unidirectionally solidified Nb–Si binary and Nb–Ti–Si ternary alloys

“…(1), a larger sized ductile particle can provide an alloy with higher toughness increment, wherein the crack bridging mechanism is operative. In addition, it has been shown that the addition of Ti enhances the ductility of bcc solid solutions in the Nb-Ti-Cr [11,[40][41][42], Nb-Ti-Al [43], Nb-Ti [20], and Nb-Ti-Si [17,24] systems. Namely, the number of (Nb) particles that contribute to the crack bridging toughening would be comparably smaller in the ternary alloys than in the binary alloys, but once the (Nb) particles bridged a crack, larger amount of energy absorption can be expected from the plastic deformation of the (Nb) particles.…”

“…5.0 and 14 m, respectively [36,37]. The yield strength of the (Nb) phase is estimated from the Vickers hardness of the Nb solid solution phase [24] using the following equation [38]:…”

“…The development of a (Nb)/Nb 5 Si 3 lamellar microstructure through Nb 3 Si decomposition has been found to be effective in improving the toughness [24,25,27]. However, alloys with fully lamellar microstructure are still brittle at room temperature, and thus, the alloy design to develop a "duplex" microstructure, where coarse (Nb) particles co-exist with the (Nb)/Nb 5 Si 3 lamellae, is necessary to provide some compressive ductility at room temperature for the alloys [24]. This fact suggests that relatively coarse (Nb) particles formed from liquid plays a significant role in the toughening of the alloys.…”

“…At the same time, the toughness improvement through microstructure control has been attempted in eutectic or hypereutectic Nb-Si alloys that are expected to show lower damage tolerance but better oxidation resistance and high temperature strength than Nb-rich Nb-Si alloys [23][24][25][26][27]. The development of a (Nb)/Nb 5 Si 3 lamellar microstructure through Nb 3 Si decomposition has been found to be effective in improving the toughness [24,25,27].…”