Notch tolerance of high-strength Cu-based nanocomposites

Understanding flaw tolerance in composites is critical for both the design and the application of reliable structural materials. Through uniaxial tension experiments, we explored notch-related plastic deformation mechanisms in both Cu-alumina and Cu-Ag-Zr composites via real-time strain mapping. Experiments demonstrated that, in samples under tensile loading with notch pairs from both materials, highly strained regions first emerged from notch tips and then developed into strain-concentration bands. Under further loading, each of these bands changed in shape from straight to elliptical as it formed a bridge across to its opposite notch tip. When any sample came under tension, its notch tip radius increased, and its strain-concentration bands gradually moved toward its notch segment center. High plastic strain became localized at the notch tips, of course, but also within the strain-concentration bands. Notch-strengthening, on the other hand, appeared mainly within the strain-concentration bands. The two composites showed different ductility within these bands. Alumina-particle-strengthened Cu had low elongation that eventually resulted in an abrupt fracture somewhere within these bands. By contrast, Ag-fiber-strengthened Cu-Ag-Zr nanocomposites had high elongation in strain-concentration bands, accompanied by clear notch blunting. In other words, Ag-fiber-strengthened Cu had higher flaw tolerance than alumina-particle-strengthened Cu, a difference we attributed to the differences between malleable fibers and hard particles.