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

研究了Nb固溶体单晶的高温变形行为，为超高温结构用Nb基自生复合材料的研制提供了基础数据。Nb-Ta和Nb-Mo合金是为了满足这一目标，因为Ta与Nb的尺寸失配和剪切模数差很小，而Mo与Nb的尺寸失配和剪切模数差很大。此外，Ta和Mo在与Nb的合金化过程中都没有清除作用，这意味着这些元素的加入揭示了固溶强化的本质。Nb-Ta和Nb-Mo固溶体在中低温度下的固溶强化可以从尺寸失配和I-S(间隙-置换)相互作用两个方面来解释。透射电子显微镜观察表明，Nb-Ta-O的变形组织由长直的螺旋位错组成，表明螺旋位错与Ta-O和Mo-O有很强的相互作用。Nb-Ta-O的屈服应力随温度的升高而迅速下降，而Nb-Mo-O的屈服应力随温度的升高而迅速下降，而Nb-Mo-O的高温变形组织主要由刃型位错组成。内耗测量表明，在中低温度下，螺型位错与Ta-O和Mo-O产生的Snoek气氛相互作用，而在高温下，边缘位错与由Mo-O产生的Cottrell气氛相互作用。

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.