UC Santa Barbara

A better understanding of failure mechanisms in metals allows engineers to build more reliable structural components.One such failure mechanism is ductile failure, defined by the nucleation, growth, and eventual coalescence of voids in a material.

This thesis focuses on how crystallographic orientation, as well as triaxiality, grain boundary misorientation, grain or phase boundary inclination, phase orientation relationship, and material differences between two phases affect the growth of voids in polycrystalline metals.

Void growth in face-centered cubic (FCC) and body-centered cubic (BCC) metals is studied using two crystal-plasticity (CP) fast-Fourier transform (FFT)-based models: a small-strain dilatational viscoplastic (DVP) FFT algorithm and a large-strain (LS) elasto-viscoplastic (EVP) FFT algorithm. The macroscopic response, including the growth of the void, is in good agreement between the DVP-FFT and LS-EVP-FFT algorithms.

The study begins with an investigation of the growth of intragranular voids in single-crystal and polycrystal FCC microstructures. This study indicates that the loading type plays a significant role in the relationship between the crystal orientation and void growth. In strain-rate controlled simulations, voids in the hardest [111] crystals grow the fastest in time, whereas in stress-controlled simulations, voids in the softest [100] crystal grow the fastest in time. Void growth rate increases with triaxiality, which has been widely observed in the literature. Then, intergranular voids in FCC bicrystalline and tricrystalline simulations are studied. This study indicates that the grain boundary misorientation and inclination have very little effect on the overall rate of void growth. Crystal orientation remains a strong indicator of the rate of void growth, although there are some voids at grain junctions that appear to grow faster than within any constituent single crystal. Grain boundary inclination affected how quickly the voids grew within one part of the bicrystal or tricrystal simulations because of the stress and strain rate states in which the inclination angle placed each constituent grain. Finally, the study concludes with a focus on voids at FCC/BCC phase interfaces. This study confirms that the behaviors seen in single-phase bicrystals apply in most cases to biphase interfaces, but the hardening rate difference between Cu and Ta caused the void growth to exceed the average growth rate of a void in each crystal alone. Finally, the void grew slightly faster in the BCC phase when more slip systems were available.