Mechanical Properties and Fracture Behavior of Directionally Solidified NiAl-V Eutectic Composites
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INTRODUCTION
THERE has been considerable interest in NiAl as a candidate material for high-temperature applications in aerospace engines, largely due to its attractive combination of properties.[1,2] Beside the high melting temperature [1947 (1674 C)], it offers exceptional oxidation resistance [up to 1573 K (1300 C)], moderate density (5.90 g/cm3), high thermal conductivity, and low brittleto-ductile-transformation-temperature (BDTT). However, like most other intermetallic compounds, NiAl has its share of inherent problems. At room temperature, NiAl suffers from low ductility and poor fracture toughness, while at elevated temperatures exhibits low yield strength and creep resistance. In order to achieve the desirable balance of properties, two approaches have been considered. First one is improving the high-temperature strength by dispersion strengthening and the other, compositing NiAl with continuous-fiber reinforcements as a means of improving both the toughness and the strength.[2] Although a large volume of work on dispersion strengthening of the NiAl has resulted in improved high-temperature strength and creep resistance, no significant improvements have been made in increasing the low-temperature fracture toughness. Another method of imparting damage tolerance into intrinsically brittle intermetallics is by ductile-fiber[3] and ductile phasetoughening.[4,5] A special emphasis has been devoted to SRDJAN MILENKOVIC, Head, is with the Solidification Processing and Engineering Group, IMDEA Materials Institute, C/Eric Kandel 2, 28906 Getafe, Madrid, Spain. Contact e-mail: srdjan. [email protected] RUBENS CARAM, Professor, is with the Department of Materials Engineering, State University of Campinas, P.O. 6122, 13083-970 Campinas, SP, Brazil, and also with the FEM/ UNICAMP, Rua Mendeleyev, 200, 13083-860 Campinas, SP, Brazil. Manuscript submitted February 26, 2014. Article published online July 9, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
the incorporation of refractory metal or intermetallic (Heusler or Laves) phases through directional solidification (DS) of eutectic alloys. DS eutectics can be considered as natural composites since their structure consists of two or more separate solid phases under normal solidification conditions. The advantages DS eutectics have over single phase intermetallics include an improvement in both strength and toughness by various intrinsic and extrinsic mechanisms, and a natural chemical and mechanical compatibility between the reinforcement and the matrix, which is extremely important feature for high-temperature applications in hostile environments for extended periods of time. The NiAl forms a number of pseudobinary eutectic systems with either a nominally ductile refractory phase (Cr, Mo, Re, V, W) or with intermetallic phase of the Heusler or Laves type (Hf, Nb, Ti, Ta). Significant progress has been made through this in situ composite system approach.[6–15] From these studies, effects of the type of reinforcement phase on the key properties (fracture toughness and creep
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