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

利用Nb固溶体单晶研究了铌基原位复合材料的高温变形行为，为开发用于高温结构的铌基原位复合材料提供基础数据。选择Nb-Ta和Nb-Mo合金是为了满足这一目标，因为Ta对Nb的尺寸失配和剪切模量差异很小，而Mo对Nb的尺寸失配和剪切模量差异很大。在与Nb的合金化过程中，Ta和Mo的加入均没有清除作用，表明Ta和Mo的加入具有固溶强化的本质。Nb固溶体Nb- ta和Nb- mo在低温和中温下的固溶体强化可以用尺寸失配和i-s（间隙-取代）相互作用来解释。TEM观察表明，变形组织由长而直的螺旋位错组成，表明螺旋位错与Ta-O和Mo-O相互作用强烈。屈服应力和滑移面的取向随氧含量的变化呈现出明显的各向异性，证实了这种相互作用。在高温下，Nb-Ta-O的屈服应力随着温度的升高而迅速降低，而Nb-Mo-O的屈服应力在达到1500K左右时保持较高的应力水平。Nb-Mo-O在高温下的变形组织以边缘位错为主，表明边缘位错与Mo-O相互作用。内摩擦测量结果表明，在低温和中温下，螺旋位错与Ta-O和Mo-O产生的Snoek气氛相互作用，并产生i-s效应，而在高温下，边缘位错与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.