EVALUATION OF Nb SOLID SOLUTIONS AS MATRIX OF COMPOSITES FOR VERY HIGH TEMPERATURE STRUCTURAL APPLICATIONS

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High temperature deformation behavior was investigated by using Nb solid solution single crystals in order to obtain fundamental data for development of Nb base in-situ composites for very high temperature structural applications. Nb-Ta and Nb-Mo alloys were se1ected to meet this objective, because Ta has a very small size misfit and shear modulus difference to Nb, while Mo has a significant size misfit and shear modulus difference to Nb. hi addition, both Ta and Mo have no scavenging effect in alloying with Nb, which means that the addition of these elements reveals the intrinsic nature of solid solution strengthening.Solid solution strengthening of Nb solid solutions, Nb-Ta and Nb-Mo, at low and intermediate temperatures can be interpreted in terms of size misfit and i-s (interstitial-substitutional) interaction. TEM observations indicate that deformation microstructures consist of long, straight screw dislocations, suggesting that screw dislocations interact strongly with Ta-O and Mo-O.This interaction is confirmed by the fact that marked anisotropy appears in the orientation dependence of yield stress and slip plane depending on oxygen content.At high temperatures yield stress of Nb-Ta-O decreases rapidly with increasing temperature, while that of Nb-Mo-O keep a high stress level up to about 1500K.Deformation microstructures of Nb-Mo-O at high temperatures consist dominantly of edge dislocations, suggesting the interaction of edge dislocations with Mo-O.Internal friction measurements reveal that at low and intermediate temperatures screw dislocations interact with Snoek atmosphere generated by Ta-O and Mo-O which receive the i-s effect, whereas at high temperatures edge dislocations interact with Cottrell atmosphere generated by Mo-O.