Experimental characterization of deformation mechanisms in nanocrystalline thin films using in situ techniques
Johns Hopkins University
The properties of a material depend immensely on its microstructure. The ability to characterize a material’s microstructure and develop predictive models for how the material will respond when, for example, load is applied is a key component of designing with engineering materials. Metals that are nanocrystalline (i.e. grain size < 100 nm) have unique properties owing to the ubiquity of grain boundaries within the microstructure. This dissertation presents new methods of testing and characterizing materials at the nanoscale. In situ experiments were performed to measure the velocity of mobile grain boundaries utilizing conventional transmission electron microscope (TEM) imaging. The average velocity of migrating grain boundaries was calculated to be on the order of 0.1 nm s)*, significantly higher than the velocity predicted from diffusion-based processes. Additional in situ experiments were conducted that utilize straining in combination with orientation imaging microscopy are utilized to determine the character of boundaries migrating in response to high stresses. From these experiments, no correlation between grain boundary character and mobility was found. Also, by utilizing through these experiments, deformation twinning was observed in nanocrystalline copper thin films. It was observed that twin nucleation and growth proceeds from grain boundaries. Additionally, many physical phenomena are affected by local stress state within a crystal. A new technique is presented that provides the capability to map elasticiii strains in polycrystalline materials with nanoscale resolution. This technique, initially developed for analysis of semiconductor devices, was applied successfully to engineering metals and ceramics for the first time. The strain resolution for this technique was calculated for polycrystalline copper (0.15%) and hot-pressed boron carbide (0.078%) specimens. The elastic strain near grain boundary facets was measured and compared to a simple model, finding that the measured strain values generally agree with the residual strains expected from thermal anisotropy. Strain values were also measured in polycrystalline magnesium near deformation twins and a low-angle tilt boundary.
Mechanical testing, nanocrystalline, grain boundary character, in situ straining