Ductile-phase toughening in V-V3Si in situ composites

Abstract

This article describes the room-temperature fracture behavior of ductile-phase-toughened V-V 3 Si in situ composites that were produced by arc melting (AM), cold-crucible induction melting (IM), and cold-crucible directional solidification (DS). Composites were produced containing a wide range of microstructures, interstitial impurity contents, and volume fractions of the ductile V-Si solid solution phase, denoted (V). The fracture toughness of these composites generally increases as the volume fraction of (V) increases, but is strongly influenced by the microstructure, the mechanical properties of the component phases, and the crystallographic orientation of the (V) phase with respect to the maximum principal stress direction. For eutectic composites that have a (V) volume fraction of about 50 pct, the fracture toughness increases with decreasing ''effective'' interstitial impurity concentration, [I] ϭ [N] ϩ 1.33 [O] ϩ 9 [H]. As [I] decreases from 1400 ppm (AM) to 400 ppm (IM), the fracture toughness of the eutectic composites increases from 10 to 20 MPa. Further, the fracture ͌m toughness of the DS eutectic composites is greater when the crack propagation direction is perpendicular, rather than parallel, to the composite growth direction. These results are discussed in light of conventional ductile-phase bridging theories, which alone cannot fully explain the fracture toughness of V-Si in situ composites.

Niobium Silicide High Temperature In Situ Composites

“…However, they do not completely describe the trend in the toughening behavior that has been reported (Bewlay et al, 1995;Henshall et al, 1997). In particular, the effect of volume fraction of the Nb solid-solution phase on composite toughness is only partially described by models for ductile phase toughening.…”

“…A liquidus surface projection of the metal-rich region (up to 37.5%Si) of the NbTi-Si phase diagram is shown in Figure 6 (Bewlay et al, 1997. Figure 6 also shows the eutectic between Nb 3 Si and (Nb).…”

“…Intermetallic-based compound materials, such as Nb or Mo silicides, have been combined with metallic second phases in order to generate composites with a combination of attractive hightemperature properties and acceptable low-temperature properties. Nb-silicide based in-situ composites with Nb 3 Si and/or Nb 5 Si 3 silicides have been shown to have great potential because of their attractive balance of high-and low-temperature mechanical properties Bewlay et al, 1996Bewlay et al, , 1997. These materials have the potential to surpass the performance of Ni-based superalloys.…”

Deformation Mechanisms in Niobium Silicide-Based Composites

“…(General treatments of the fracture toughness characteristics of particulate, fibrous, and layered MMCs are given respectively in Chapters 3.07, 3.08, and 3.11, this volume.) There have been extensive studies of the fracture mechanisms in composites from binary Nb-Si alloys Mendiratta et al, 1991;Mendiratta and Dimiduk, 1993;Bewlay etal., 1995aBewlay etal., , 1997cRavichandran, 1992;Henshall et al, 1997;Nekkanti and Dimiduk, 1990). Typically fracture toughness measurements have been performed using single edge notched bending specimens (SENB), and the procedures provided by ASTM standard E-399 (1983).…”

“…However, anisotropy of creep properties has not been examined and requires further consideration. In addition, although the effect of volume fraction of (Nb) on toughness has been well defined (Bewlay et al, 1995a;Henshall et al, 1997), the effect of volume fraction (Nb) on creep performance is less well understood. Although solid components are attractive for early engine testing of the RMICs, due to the simplicity of machining from stock to final part, applications in hollow parts will take greatest advantage of the RMIC high temperature capability.…”