| Popis: |
To determine the nature of low temperature fracture and its effects on macroscopic mechanical characteristics, a quantitative fractographic investigation of the fracture surfaces of niobium, Nb-Ti (45 and 50 wt % Nb) alloys and composites is carried out using scanning electron microscopy (SEM). Statistical analysis of the SEM data indicates that fracture in the 20–4.2 K range is mixed in nature, with the ductile mechanism of nucleation, growth and coalescence of microdiscontinuities playing the dominant role. The theoretical shear strength is realized in ductile micronecks of the Nb-Ti alloys. The temperature dependence of plastic deformation localized in microvolumes under fracture is studied. The effects of different structures and processing factors on the low temperature behaviour of various mechanical properties of the composites with bcc and fcc components are considered. It is concluded that the low temperature localization of plastic deformation in a bcc reinforcing component plays a dominant role in the degradation of the mechanical characteristics of composites with a soft fcc matrix. |
