Strength and ductile-phase toughening in the two-phase Nb/Nb 5 Si 3 alloys
- PDF / 3,395,516 Bytes
- 11 Pages / 613 x 788.28 pts Page_size
- 84 Downloads / 217 Views
INTRODUCTION
ADVANCES in aerospace technology are paced by the continuing search for light weight durable materials which retain their strength and stifflaess at very high temperatures. There are a large number of intermetallic compounds, some of which have the potential for being developed into structural materials for service in turbine engines and hypersonic vehicles at temperatures from 1000 ~ to 1600 ~ [1-41 Most of the intermetallics with high melting temperatures have complex, low-symmetry crystal structures which possess strong directional atomic bonds. It is generally believed that the strong bonding is responsible for retention of mechanical properties, such as high strength, stiffness, and creep resistance (due to low diffusivity) at high temperatures, and the compiled data on these materials have shown that these properties directly scale with melting temperature. I~1However, this same attribute often contributes to a lack of ductility and low fracture resistance at low temperatures. These compounds may be exploited for high-temperature structural use provided their low-temperature fracture resistance can be enhanced without compromising the attractive hightemperature properties. The microstructural aspects of toughening and fracture have been treated in numerous studies of conventional multiphase metallic alloys, and the results have been summarized in a recent review article, t51 However, only limited research has been carried out on toughening mechanisms in brittle intermetallic compounds. Conventional approaches to intermetallics have employed appropriate alloying and thermomechanical processing to enhance toughness; an example of this is the toughening of titanium aluminide alloys. [6.7.8] The addition of brittle reinforcements which provide crack-deflection, crackMADAN G. MENDIRATTA, Group Manager, Metals and Ceramics, is with the Materials Research Division, Universal Energy Systems, Inc., Dayton, OH 45432-1894. JOHN J. LEWANDOWSKI, Associate Professor, is with the Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, OH 44106. DENNIS M. DIMIDUK, Group Leader, is with WRDC/MLLM, Materials Laboratory, Wright-Patterson AFB, OH 45433-6533. Manuscript submitted March 28, 1990. METALLURGICAL TRANSACTIONS A
bridging, and pull-out toughening is an approach which is being explored in ceramic-matrix composites and may be exploited in the intermetallic systems. An example[gj of the latter material is the a2-based titanium aluminide (Ti-24 at. pct A1-11 at. pct Nb) reinforced by continuous silicon carbide fibers (SCS-6). An additional possibility is to provide damage tolerance by the addition of a welldispersed ductile phase in a brittle matrix. In general, this latter case may be achieved by "phase blending, "[l~ provided that thermochemical stability is unimportant in the final product. However, for high-temperature structural use, thermochemical stability is an important consideration, and, therefore, composite systems whose components exhibit a high degree of inh
Data Loading...
