Development of Lightweight, Radiation- and Damage-Tolerant Micro-trusses

The main goals of this proposed work are (1)to design and fabricate 3-dimensional periodic nano- and micro-lattices that are architected to enhance stiffness and damage tolerance, (2) to conformally coat these polymer micro-trusses with either pure metallic glasses or hybrid metal-metallic glass nanolaminates and subsequently dissolve the internal polymer core to create hollow metallic micro-trusses, and (3) to investigate and model the fundamental deformation mechanisms at each relevant scale: from the atomic-level to macro-scale. The April 2012 NASA report titled “DRAFT Materials, Structures, Mechanical Systems, and Manufacturing (MSMM) Roadmap, Technology Area 12”, dictates that “Deep space human and robotic exploration and future aeronautics challenges will require a new suite of advanced technologies to ensure mission success.” This document also provides a summary of the MSMM top 10 technical challenges that are critical to NASA’s mission, among which – at the very top – are Radiation Protection (Top Challenge), Advanced Materials, and Multi-functional Structures. With the paradigm within NASA shifting to developing and demonstrating new technologies, there is a growing emphasis on developing materials with tailored properties while maintaining light weight. Such property tailoring could be achieved by demonstrating the ability to de-couple historically linked properties like mechanical strength, stiffness, toughness and density. Solving the property-decoupling challenge is instrumental in delivering the products presented by the NASA Roadmap report as it deems the lightweight composites and metallic structures to be lucrative candidates for enabling future NASA missions. 
There are two main categories of technical achievements within the scope of this proposal: 
(1) Experimental, which include progress in fabrication methodology and nanomechanical experiments and 
(2) Computational, which includes Molecular Dynamics (MD) Simulations of the small-scale metallic glass based material systems, as well as theoretical efforts.=> Investigating size-induced ductility in nano-sized metallic glasses produced by different deposition techniques. 
Progress in Experiments: 1.In this final quarter, a substantial part of our efforts has been dedicated to investigating the mechanical behavior of hollow, glassy Zr-Ni-Al nanolattices with wall thicknesses between 15nm and 500 nm deposited via sputtering.  The sputter deposition was conducted for various times to result in nanolattices with different wall thicknesses, with the nanolattice median wall thickness ranging from 10 nm in the thinnest-walled nanolattices  to  88  nm  in  the  thickest-walled  nanolattices.  Uniaxial  compression  experiments performed inside of an SEM revealed a brittle-to-deformable transition as the nanolattice wall thickness is reduced. Thick-walled nanolattices exhibited large catastrophic strain bursts involving failure of multiple nanolattice layers simultaneously. Thin-walled  nanolattices  exhibited  no  strain  bursts,  instead  undergoing  smooth  continuous deformation, with gradual layer-by-layer collapse and substantial recovery upon unloading. Structural analysis indicates all nanolattices that we made were outside the regime of Euler beam buckling or local (shell) buckling, indicating the failure mode arises from the constituent metallic glass material, not the nanolattice structure. The observed brittle-to-deformable transition as wall thickness is reduced can be understood in terms of the “smaller is more ductile or deformable” size effect that has frequently been observed in individual metallic glass nanopillars.2.We are continuing our in-situ X-ray diffraction experiments at the Advanced Photon Source (APS) to investigate the evolution of the principle peak position in S(Q) as a function of the post-sputtering annealing conditions to estimate the relative change in density as a function of thermal annealing in metallic glasses. These experiments are being performed in collaboration with Prof. W. Mao’s group at Stanford and with Qiaoshi (Charles) Zeng of Carnegie Institution. The results of these in-situ experiments reveal that annealing the samples leads to a different slope in the reduced pairwise function, as well as to a principal peak shift. This suggests that the density of the metallic glasses is increasing upon annealing and points to a reduction in the free volume. These results are consistent with our fundamental nano-tensile studies in the scope of this proposal, which revealed extreme tensile ductility reaching >10% engineering plastic strains, >150% true plastic strains, and necking down to a point during tensile straining in specimens as wide as ~150 nm at the high strengths of ~1.5 GPa (on average). Progress in Computations and Theory:1.We have embarked even more deeply into the theory of glass formation and fundamental properties of glassy metals. We characterized the atomic structures by integrating radial distribution functions (RDF) from molecular dynamics (MD) simulations for several metallic liquids and glasses: Cu46Zr54, Ni80Al20,  Ni33.3Zr66.7, and Pd82Si18. Resulting cumulative coordination numbers (CN) showed that metallic liquids have a dimension of d = 2.55 ± 0.06 from the center atom to the first coordination shell and metallic glasses have d = 2.71 ± 0.04, both less than 3. Between the first and second coordination shells, both phases crossover to a dimension of d = 3, as for a crystal. Observations from discrete atom center-of-mass position counting are corroborated by continuously counting Cu glass- and liquid-phase atoms on an artificial grid, which accounts for the occupied atomic volume. Results from Cu grid analysis show short-range d = 2.65 for Cu liquid and d = 2.76 for Cu glass. Cu grid structures crossover to d = 3 at ~3 atomic diameters. We studied the evolution of local structural dimensions during quenching and discuss its correlation with the glass transition phenomenon.