Plastic Instabilities and Their Consequences in Steels and Other High Strength Alloys. The Investigation of the Fracture of Titanium Alloys by In-situ and Analytical Microstructural Techniques.

摘要

Some frequently used commercial high strength alloys have a tendency to fail catastrophically despite their ductile fracture mode. Apparently, these alloys develop instabilities during deformation which lead to local shear. The local shear areas often trigger catastrophic failure due to near adiabatic heating. An understanding of these phenomena was the object of the research reported here. Ten high strength alloys have been studied as a function of strain rate. Specifically, the investigation of fracture surfaces of high strength 4340 steel samples tested in tension at quasi-static strain rates showed indeed localized melting accompanied by radial cracking. Surprisingly, at high strain rates up to 10 3/s localized melting and radial cracking were absent; an analysis showed that this behavior was due to plastic deformation in the necked down cross-section which was heated above the ductile brittle transition temperature range, then causing the alloy to fail in a fully ductile manner. The temperature rise at the tip of a crack was modeled and calculated using defeat deformation theory. The model predicted the correct temperature range in the process zone as well as a dependence on microstructure. The latter point was confirmed experimentally on Ti-6Al-4V tensile samples with three different microstructures. The interaction of dislocation cell walls with the spatially varying stress field of a moving crack was calculated and compared with high voltage electron microscope in situ observations.