Microstructures and Properties of Ds in-Situ Composites of Nb-Ti-Si Alloys

Abstract

In-situ composites based on binary Nb-Si alloys and consisting of a Nb solid solution with Nb3Si or Nb5Si3 have shown a promising combination of low temperature and high temperature mechanical properties. The environmental resistance and room temperature fracture toughness of these composites can be further enhanced by additions such as Ti, Hf, Cr, and Al. In the present study, ternary Nb-Ti-Si alloys were prepared by directional solidification to generate aligned two and three phase composites containing a Nb solid solution with Nb3Si and/or Nb5Si3. The present paper will describe microstructures, phase equilibria and fracture toughness of these composites. The improvement in the room temperature fracture toughness over binary Nb-Nb5Si3 composites is discussed.

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

“…This would be explained by the halo effect 46) as follows: during solidification of the primary D8 m phase, niobium atoms are rejected into the liquid phase and enriched enough to form A2 phase in front of the solid/liquid interface, and thereby, the peritectic reaction is skipped over the subsequent eutectic reaction. Although the solidification pathway in the ternary alloys with 10 at%Ti is similar to that in the binary alloys, 28,45) the formation of A2/tP32 two-phase from liquid phase is not the invariant reaction in the ternary alloys. Therefore such fine A2/tP32 eutectic microstructures as seen in the binary alloys are not formed in the ternary alloys.…”

“…28) In the present study the alloys with (Nb+Ti) rich composition in the Nb-Ti-Si ternary system are annealed at various temperatures for the evaluation of the phase regions regarding A2, tP32 and D8 l phases. Based upon the XRD and EPMA conducted on annealed alloys, the isotherm at 1673 K for the Nb-corner in the Nb-Ti-Si phase diagram has been constructed as shown in Fig.…”

“…47) Thereby many eutectoid nodules would nucleate at the A2/ Fig. 3 a) A proposed three-dimensional illustration, 28) and b) the vertical section at 25%Si of the Nb-Ti-Si ternary phase diagram.…”

“…Much effort has been made to clarify the effect of alloying elements with the Nb-Si binary alloy system upon the mechanical properties and oxidation resistance. For example, addition of Al or Cr leads to improved oxidation resistance but also to increased DBTT of ductile phases, [27][28][29][30] addition of Mo or W enhances high temperature compressive strength but lowers room temperature toughness, 31,32) and Ti addition improves both room temperature toughness and oxidation resistance. 2,28,29) The recent development in the Nb-silicide based composites is reviewed in the several papers.…”

“…For example, addition of Al or Cr leads to improved oxidation resistance but also to increased DBTT of ductile phases, [27][28][29][30] addition of Mo or W enhances high temperature compressive strength but lowers room temperature toughness, 31,32) and Ti addition improves both room temperature toughness and oxidation resistance. 2,28,29) The recent development in the Nb-silicide based composites is reviewed in the several papers. [33][34][35][36][37][38] Extensive studies have been made on the Nb-silicide based Fig.…”