Thermal and mechanical stability of nanograined FCC metals

The mechanisms governing and factors controlling the thermal and mechanical stability of nanograined free-standing face-centered cubic thin films were investigated through in situ transmission electron microscopy annealing and straining experiments. A variety of sample preparation techniques were developed to investigate the active mechanisms. The results obtained from the select face-centered cubic metals studied were used to develop a general understanding of face-centered cubic metals with microstructure limited to the nanometer scale. The films were analyzed, both prior to and following the in situ transmission electron microscopy experiments, via a range of analytical techniques in order to characterize chemical and microstructural details. The mechanisms observed were compared to the pertinent theories and models.;In situ transmission electron microscopy heating and annealing experiments were performed on free-standing pulsed-laser deposited Au, Cu, and Ni thin films. The grain growth of pulsed-laser deposited Ni films was studied and the growth rate was found to be a function of time, temperature, film thickness, and surface abnormalities. The grain growth was classified as abnormal in nature resulting in a bimodal grain size distribution. Abnormal grain growth was found to increase with an increase in film thickness. This increase was attributed to the presence of a higher density of preferred nanograins for abnormal grain growth in thicker films, although the mechanisms that induce the rapid growth were not determined. A higher percentage of abnormal large grains were found along ridges templated from the substrate, and in regions with extensive electron beam exposure. Post-annealing analysis of pulsed-laser deposited Ni films revealed an unexpected myriad of microstructural defects including dislocations, twins, stacking faults, dislocation loops, and stacking-fault tetrahedra, as well as a metastable hexagonal closed-packed phase. The production of these defects reflects the excess free volume of the grain boundaries produced by the deposition.;Pulsed-laser deposited Au and Cu films were deposited at room temperature with similar deposition conditions to nominally the same film thicknesses and annealed for similar times and temperatures as the pulsed-laser deposited Ni films. In contrast to the Ni, the Au and Cu films exhibited normal-like grain growth in which no set of grains were dominated the microstructural rearrangement and the grain size distribution was similar to a log normal distribution. In contrast to the Ni, the microstructure of the Au and Cu films contained twins and dislocations only. The understanding gained from the studies of the thermal stability of Au, Cu, and Ni films was used to develop processing routes to improve the mechanical properties of free-standing pulsed-laser deposited Ni films through control of the grain size distribution.;In situ transmission electron microscopy straining experiments were performed on Al and Ni free-standing films using a variety of microfabricated and custom-built devices. Failure in ultra-fine columnar grained evaporated 99.99% pure Al films with a wide grain size distribution was found to transition from intergranular brittle fracture to shear fracture with an increase in film thickness. Film thickness was the dominating length scale influence on the active mechanisms in comparison to other possible factors such as grain size, gauge length, and gauge width. It is proposed that the dependence in failure mode on film thickness is a result of grain boundary grooving, which has greater impact in thinner films. The large grain size distribution in the as-deposited Al film serendipitously demonstrated a toughening mechanism for brittle-like nanograined metal films. It was found that large grains within a nanograined matrix can accommodate significant plasticity and can act to prevent crack propagation, thus toughening the films. This phenomenon was verified via in situ transmission electron microscopy straining of pulsed-laser deposited Ni films annealed under a condition known to produce a bimodal grain size distribution. The toughening in the pulsed-laser deposited Ni films was attributed to a combination of mechanisms including stress-driven grain growth, plasticity in the large grains, crack blunting, and finally ligament rupture mechanisms. The results observed in the deposited thin films were compared to the mechanisms active in ultra-fine grained Al fabricated by equal channel angular pressing. These samples do not suffer from the surface roughness effects found in the deposited films and the texture is not <111> dominant. In contrast to the deposited films, the equal channel angular pressed Al showed extensive dislocation activity during deformation similar to bulk face-centered cubic metals.;The results of these in situ transmission electron microscopy annealing and straining experiments provide clarification of some of the mechanisms operating during the microstructural evolution of nanograined face-centered cubic free-standing thin films under thermal and mechanical loading, while also raising many previously unaddressed questions on the active mechanisms under these loads.