Local Structure Analysis of Model Metal-Metal and Metal-Metalloid Metallic Glasses
University of Wisconsin-Madison
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Materials scientists lack a general, abstract description of the atomic structure of metallic glasses (MGs). Structural descriptions developed for crystals and crystal defects are ineffective because the amorphous structure of MGs does not exhibit the necessary translation and rotational symmetries, and structure descriptions developed for oxide glasses are ineffective due to the lack of strong covalent bonds in MGs and the higher coordination number of local structures, which allows for more structural diversity. This thesis reports research on improved abstract descriptions of MG structure developed by studying both the short- and medium-range order (SRO and MRO) of simulated MGs. Hybrid reverse Monte Carlo (HRMC) simulations were used to generate MG structures with realistic MRO and molecular dynamics (MD) simulations were used to generate MG structures with low energy. In Zr-Cu-Al, a metal-metal system, subcritical fcc-like MRO exists at the nanometer scale, and better glass forming alloys can be creating by destabilizing crystal-like SRO and MRO. In the Pd82Si18 metal-metalloid system, an fcc-like region also exists, but the structure is more complicated and cannot be explained by typical topological structure metrics. A new structure metric called motif extraction based on geometry rather than topology was developed to characterize the local structure of MGs. Motif extraction applied to a canonical metal-metal glass, Zr50Cu45Al5, quenched via MD, reproduces results obtained from topological approaches and identifies a hierarchy of structures as a function of coordination number that is focused around icosahedral geometries, which are dominant in metal-metal glasses. An analogous hierarchy of structures is found in HRMC models of Zr50Cu35Al15 that focuses around fcc geometries. This suggests that one or two motifs may fundamentally dominate the structure of both glassy and crystal-like SRO in MGs. Applying motif extraction to a Pd82Si18 quenched via MD reveals a new local structure that correlates strongly with the glass transition. This motif has the same topology as another motif with opposing properties. A hierarchy of order is identified in HRMC models of Pd82Si18, analogous to the fcc-like hierarchy found in Zr50Cu35Al15. Analysis of two Al92Sm8 MGs created using different synthesis techniques reveals MRO analogous to metal-metal and metal-metalloid glasses, including the fcc-like hierarchy of order, illustrating that fcc-like close-packed order may be ubiquitous in MG systems. The structures of the two Al92Sm8 glasses are distinguished by a higher fraction of fcc order in the glass synthesized using mechanical rolling, compared to the glass to synthesized via melt-spinning. The final chapter reports experimental work identifying a new type of MRO structure in a-Si thin fims. The new MRO is consistent with larger 8-atom rings rather than the 6-atom rings that are typically found in the structure of a-Si.
metallic glass, structure, motif, short-range order
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