The effects on fracture toughness of ductile-phase composition and morphology in Nb-Cr-Ti and Nb-Si in situ composites
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INTRODUCTION
SHAH and Antontl] and Anton and Shah[2] evaluated several refractory intermetallic alloys for use at temperatures exceeding superalloy use temperatures. Test results from Cr2Nb indicated a very low creep rate and an inherently good oxidation resistance; however, this material is also known to be extremely brittle, as are most refractory intermetallics. Anton and Shaht31 subsequently showed that the concept of ductile-phase toughening was a possible method for overcoming this extreme brittleness. They determined, by microhardness indentation, that additions of Nb deflected cracks and caused them to blunt. Those results provided the basis of research designed to further explore the effects of ductile-phase toughening in the Cr2Nb/Nb system. Other similar systems in which research has been reported are Nb/NbsSi3,[4.5] Nb/Nb3AI,I6I Ti/TisSi3,[7] and V/V3Si,]8~where in each case the ductile phase is a transition metal in solid solution with Si or A1. A summary of the mechanisms that have been found for ductile-phase toughening systems was recently published by Chan.t91 Maximum toughness in these systems occurs when the matrix transition metal alloy is tough and ductile. Investigation of the toughening mechanisms in the Nb/Cr2Nb system continued the work begun by Anton and Shah[3] with fracture toughness evaluation of several twophase Nb-Cr binary alloys, or "in situ composites." All the compositions evaluated were found to have low values of toughness (4 to 7 MPa~mm)relative to structural engineering materials (>20 MPa~/m). Also, the slope of the fatigue crack growth curve was very steep. These results indicated that a detailed evaluation of the fracture characteristics of
D.L. DAVIDSON, Institute Scientist, and K.S CHAN, Staff Scientist, are with the Southwest Research Institute, San Antonio, TX 78238. D.L. ANTON, Senior Research Scientist, is with United Technologies Research Center, East Hartford, CT 06108. Manuscript submitted January 9, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
alloys in this system would not be very useful. It was determined, however, that the Nb solid-solution phase exhibited low toughness, and that thermomechanical treatments would not improve the fracture characteristics to an acceptable level. In other words, the "ductile-phase toughening" mechanism was not functioning as envisioned because the toughening phase was not ductile enough. Alloying was sought as the method for enhancing the toughness of the Nb solid solution, thereby enhancing the toughness of the composite. This article reports on the toughness evaluations made of the Nb-Cr alloys and how alloying with Ti increased toughness of the solid-solution matrix alloys and in situ Cr2Nb composites. The low toughness values found for the binary alloys were attributed to the embrittling effects of Cr additions to Nb. Toughness improvements to the solid-solution alloy and composite matrices with Ti additions are then presented in the context of electronic bonding. Finally, the toughness of Nb-Cr-Ti composites are co
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