Los Alamos National Laboratory,
Keywords: metallic Composites, nanomechanics, electron microscopy, extreme environments, toughness, strength, ductility
Summary:This presentation reports on recent findings within the Center for Integrated Nanotechnologies that determine the effects of interfacial structures and damage gradients on mechanical response at length scales varying from nanometers to millimeters. Three focus areas are presented: 1.) Enhanced structural performance of BULK metallic layered nanocomposites processed via Severe Plastic Deformation as a function of decreasing layer thickness. 2.) In-situ TEM and SEM straining of metallic and metal-ceramic nanocomposites with enhanced toughness and 3.) High-throughput mechanical probing of nanoscale radiation damage gradients. 1.) Enhanced structural performance of BULK metallic layered nanocomposites: In bulk multi-phase composite metals containing an unusually high density of heterophase interfaces, the bi-metal interface controls all defect-related processes. Quite unconventionally, the constituent phases play only a secondary role. With the ‘right’ characteristics, these bi-material interfaces can possess significantly enhanced abilities to absorb and eliminate defects. Through their unparalleled ability to mitigate damage accumulation induced under severe loading and/or severe environments (such as elevated stress, high strain rate, high temperature, and radiation environments), they will provide their parent composite with a highly effective healing mechanism and consequently a robustness not possible in existing advanced structural materials. Bulk (> cm3) laminar composites with controllable layer thicknesses down to the submicron or nano-scale range can be fabricated via accumulative roll bonding (ARB), a severe plastic deformation (SPD) processing technique. Imposing over thousands of percent strain, ARB refines the microstructure of ordinary coarse-grained composite metals down to submicron and nanoscales. ARB is an ideal material processing technique because: it produces a 2-D layered microstructure, it imposes monotonic deformation in a familiar manner (rolling), and it allows for controllable accumulated strain and layer thickness (from 1 mm to 10 nm). 2.) In-situ TEM/SEM straining of metallic and metal-ceramic nanocomposites: When layer thicknesses in nanocomposites drop below 10nm, materials with limited room temperature ductility and toughness such as ceramics and Magnesium can acquire new interface-dominated properties including enhanced deformability. In Al-TiN nanolaminates with layer thickness below 5 nm, cracking in ceramic TiN was suppressed with codeformation evident in both layers. In-situ TEM straining demonstrates a profound size effect in enhancing plastic co-deformability in nanoscale metal-ceramic multilayers, as well as direct validation of ex-situ and 3-D elastic-plastic deformation models. 3.) High-throughput mechanical probing of nanoscale radiation damage gradients: We discuss applications of spherical nanoindentation stress-strain curves in characterizing the local mechanical behavior of materials with modified surfaces. Using ion-irradiation on tungsten as a specific example, we show that a simple variation of the indenter size (radius) can identify the depth of the radiation-induced-damage zone, as well as quantify the behavior of the damaged zone itself. Using corresponding local structure information from electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) we look at (a) the elastic response, elasto-plastic transition, and onset of plasticity in ion-irradiated tungsten under indentation, (b) correlate these changes to the different grain orientations in tungsten as a function of (c) irradiation from different sources (such as He, W, and He+W).