Mechanical Behavior of Nanostructured Materials: Metallic Glass and High Entropy Alloys
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS Structural Materials Division, TMS: Mechanical Behavior of Materials Committee, TMS: Nanomechanical Materials Behavior Committee
Program Organizers: Xinghang Zhang, Purdue University; Yuntian Zhu, North Carolina State University; Joseph Poon, University of Virginia; Suryanarayana Challapalli, University of Central Florida; Enrique Lavernia, University of California, Irvine; Haiyan Wang, Texas A&M University

Tuesday 2:00 PM
February 28, 2017
Room: 30D
Location: San Diego Convention Ctr

Funding support provided by: AJA International; Hysitron Inc.

Session Chair: Joseph Poon, University of Virginia; Jürgen Eckert, Erich Schmid Institute of Materials Science; Peter Liaw, University of Tennessee

2:00 PM  Cancelled
Interfaces in Colloidal Crystals: Frans Spaepen1; 1Harvard School of Engrg & Appl Sciences
    Colloidal systems are solid particles, possibly charged, suspended in a liquid. At sufficiently high density, they form liquid and crystalline (fcc, bcc) phases. If the particles have a diameter on the order of a micrometer, they can be tracked in time and space by confocal microscopy. This makes it possible to model complex materials problems at the particle (i.e., atomic) level. The crystal-liquid interface can be studied by quantitatively observing the formation of crystal nuclei in the liquid. Its interfacial stiffness can be measured from the thermal fluctuations in its position at equilibrium. Its mobility can be measured directly from the attachment and detachment rates during melting and crystallization. Colloidal polycrystals contain a variety of high-angle boundaries, whose structure, stiffness and mobility can be mapped out by the same methods.

2:25 PM  Invited
Comparing Amorphous Alloy Synthesis Employing Melt Spinning & Mechanical Alloying: Andrew Cheung1; Gary Shiflet1; 1University of Virginia
    There is interest in using powdered metallic glasses with compaction methods to synthesize bulk amorphous alloys. The compositional space of metallic glasses has predominately been explored through the process of rapid quenching, however, considerable effort has also been employed to produce glass alloys with mechanical alloying. To different degrees, both rapid quenching and ball milling are dependent on thermodynamic and kinetic processes and to employ powdered metallic glasses created by ball milling interchangeably with those from rapid quenching, the amorphous nature of the glasses resulting from each method must be the same. Their chemistries and subsequent crystallization paths should match. This presentation will report on binary and ternary system compositions that form metallic glasses through ball milling which are different from chemistries required to form glasses through rapid quenching. Observations on alloys near stable compounds for ball milling and eutectics for rapid cooling will be discussed.

2:50 PM  Invited
Tailoring the Mechanical Behavior of Metallic Glasses: Juergen Eckert1; 1Montanuniversität Leoben
    Tailoring intrinsic heterogeneity and length-scale modulation is a promising approach to overcome the limited plasticity of bulk metallic glasses. One way is to introduce microstructural heterogeneities on different length-scales. This ranges from µm-sized ductile second phase precipitates embedded in the glassy matrix down to nanometer-scale heterogeneities, which promote local shear events. Additionally, deformation-induced martensitic transformation or twinning can be exploited for plasticity enhancement. Moreover, the short and medium-range order of metallic glasses can also be tuned through mechanical pre-treatment (e.g., cold-rolling, channel-die compression, severe plastic deformation) at different temperatures. This changes the deformation behavior of inherently brittle metallic glasses remarkably, inducing a considerable increase of plastic strain via modulating internal stresses on different length-scales. The effectiveness of such treatments for ductilization will be analyzed in the light of possible structural changes and inhomogeneous stress / strain distribution.

3:15 PM  Invited
Deviations from High-Entropy Configurations in the AlxCoCrCuFeNi Alloys: Louis Santodonato1; Yang Zhang2; Mikhail Feygenson3; Chad Parish1; Michael Gao4; Richard Weber5; Joerg Neuefeind1; Zhi Tang6; James Morris1; Peter Liaw7; 1Oak Ridge National Laboratory; 2The University of Illinois at Urbana-Champaign; 3Juelich Centre for Neutron Science; 4National Energy Technology Laboratory; 5Materials Development, Inc.; 6Alcoa Technical Center; 7The University of Tennessee
    The goal of the present research is to gain better understanding of the elemental distributions and the evolution of the configurational entropy during the solidification of high-entropy alloys. Ultimately, we wish to understand how the atomic-level configurational properties affect the macroscopic engineering properties. Model alloys in the AlxCoCrCuFeNi family will be discussed, with an emphasis on integrated theoretical and experimental studies, such as ab initio molecular dynamics simulations, neutron scattering, synchrotron X-ray diffraction, high-resolution electron microscopy, and atom-probe tomography. It will be shown that even when these materials undergo cooling transformations involving chemical ordering and elemental segregation, a significant amount of disorder remains, due to the distributions of multiple elements in the major phases. The results suggest that the high-entropy-alloy-design strategy may be used to develop a wide range of complex materials, which are not limited to single-phase solid solutions.

3:40 PM Break

4:00 PM  Invited
Universal Parameter to Quantitatively Predict Metallic Glass Properties: Evan Ma1; 1Johns Hopkins University
    Quantitatively correlating the amorphous structure in metallic glasses with their physical properties has been a long-sought goal. Here we introduce “flexibility volume” as a universal indicator of the structural state, to correlate with properties on both atomic and macroscopic levels. The flexibility volume is assessed via atomic vibrations that probe local configurational space and interaction between neighboring atoms, and is defined in a simple form to be measurable both computationally and experimentally. We show that this indicator deterministically predicts the shear modulus, which is at the heart of key properties of metallic glasses. The flexibility volume correlates strongly on the one hand with atomic packing topology, and on the other hand with the activation energy for thermally activated relaxation and the propensity for stress-driven shear transformations, explaining the mechanical heterogeneities. The concrete structure-property correlations discovered are robust for all metallic glass compositions, processing conditions and length scales.

4:25 PM  
Brittle-to-ductile Transition in Metallic Glass Nanowires: Daniel Sopu1; Mihai Stoica1; Jürgen Eckert2; 1IFW Dresden; 2Erick Schmid Institute of Materials Science
    Experimental studies of the tensile behavior of metallic glass nanowires show a wide range of failure modes, ranging from ductile-to-brittle shear failure. Simulations on the deformation behavior of nanoscaled metallic glasses report an unusual extended strain softening and are not able to reproduce and explain the brittle-like fracture deformation as found in experiments. Using large-scale molecular dynamics simulations we provide an atomistic understanding of the deformation mechanisms of metallic glass nanowires and differentiate the extrinsic size effects and aspect ratio contribution to plasticity. The ductile-to-brittle transition is explained in terms of critical nanowire aspect ratio and structural rejuvenation process. These results may help to better understand the deformation mechanisms of metallic glass nanostructures and demarcate ductile and catastrophic failure.

4:45 PM  
Strain Delocalization and “Ductile” Fracture Behaviors of Metallic Glass: Zhe Fan1; Jin Li1; Yingchao Yang2; Qiang Li1; Sichuang Xue1; Haiyan Wang3; Jun Lou2; Jian Wang4; Xinghang Zhang3; 1Texas A&M University; 2Rice University; 3Purdue University; 4University of Nebraska-Lincoln
    It is well adopted that most metallic glasses are brittle as deformation induces low density of shear bands and severe shear localization proceeding catastrophic failure. Here we show that by introduction of crystalline phase with the appropriate dimension, shear delocalization for metallic glasses can be achieved as a manifestation of ductile dimples in amorphous phase. Dimple sizes can be tailored by the dimension of amorphous phase. Besides, unlike instantaneous crack propagations for most metallic glasses, in this study, we demonstrate that cracks in amorphous phase can travel in a highly periodic fashion. While reducing the layer thickness of amorphous phase (same as the crystalline phase) down to certain length scale, the peak stress increases following a “Smaller is Stronger” trend. However under certain film thickness, inherent brittleness takes over, resulting in brittle fracture mode. We acknowledge the financial support by NSF-CMMI under grant No. 1161978.

5:05 PM  Cancelled
Structural Evolution and Deformation Characteristics of Nanocrystalline Equiatomic AlCrCuCoFeNi High-entropy Alloy: Ramya Sree Ganji1; Koteswararao Rajulapati1; 1University of Hyderabad
    Mechanical alloying has been used to synthesize nanocrystalline equiatomic AlCrCuCoFeNi high-entropy alloy. It resulted in a single phase fcc based solid solution after 60 h of milling with a very fine crystallite size of 8 nm. Bulk samples are made by spark plasma sintering of the milled powders at 750 oC. The structural evolution was tracked using x-ray diffraction, scanning electron microscopy and transmission electron microscopy. It appears that the complex solid solution formed during milling is unstable on exposure to elevated temperature and a phase separation has occurred. Vickers microindentation, depth sensing nanoindentation and compression testing have been employed to probe the mechanical behavior of the sintered alloy. The observed deformation characteristics are correlated with the microstructural features.