Micro- and Nano-Mechanical Behavior of Materials: Micro/Nano-Mechanics II
Program Organizers: Sundeep Mukherjee, University Of North Texas; Mahmoud Baniasadi, Georgia Southern University; Meysam Haghshenas, University of Toledo

Monday 2:00 PM
November 2, 2020
Room: Virtual Meeting Room 36
Location: MS&T Virtual

Session Chair: Corinne Packard, Colorado School of Mines; Meysam Haghshenas, The University of Toledo


2:00 PM  Invited
Persistent, Inelastic Cycling Behavior in Rare Earth Orthophosphate Ceramics: Corinne Packard1; 1Colorado School of Mines
    Rare earth orthophosphate ceramics are relatively low-strength, refractory materials that have shown promise as toughness-enhancing fiber coatings in oxide-oxide ceramic matrix composites. Previous nanoindentation studies of these materials have shown energy-recovering behavior upon unloading, both discretely as pop-outs and continuously as a slope change (‘elbows’) in compositions in both the monazite and xenotime structures. This work reports the behavior of several rare earth orthophosphate materials under cyclic nanoindentation loading with blunt and sharp tips on both polycrystalline and single crystal materials. We find that these materials exhibit highly reversible cyclic deformation, dissipate more energy than shape memory ceramics, and are not limited to a narrow temperature range.

2:20 PM  
Exploring Small-scale Quasicrystal Plasticity in Unknown Temperature Regimes and Compositions: Changjun Cheng1; Yu Zou1; 1University of Toronto
    Quasicrystals, sometimes, can be regarded as an intermediate state between regular metals (crystal) and metallic glasses (amorphous), in terms of structural and chemical complexity. A long-standing problem, however, significantly impedes practical usage of quasicrystals: steady-state plastic deformation has only been found at high temperatures or under confining hydrostatic pressures; at low and intermediate temperatures, they are very brittle, suffer from low ductility and formability and, consequently, their deformation mechanisms are still not clear. Here, this presentation systematically studies the deformation behavior of icosahedral Al-Pd-Mn and decagonal Al-Ni-Co quasicrystals using a micro-thermomechanical technique over a range of temperatures (25-500 °C), strain rates, and sample sizes accompanying microstructural analysis. Here we show interesting phenomena relevant to small-length scales, including: cracking, serrated flows, diffusion and phase transformation. QC thin films made by co-sputtering is also studied.

2:40 PM  Invited
In Situ Nanomechanics of Ni-based Superalloys and Bond Coating: Effect of Temperature: Sanjit Bhowmick1; Eric Hintsala1; Douglas Stauffer1; 1Bruker
    High-strength structural materials such as Ni-based superalloys and diffusion bond coats are widely used in challenging environments and with exposure to mechanical fatigue, particle impact, and erosion at elevated temperatures. Diffusion platinum-aluminide bond coats are an example of compositionally and microstructurally graded coatings with significant variation in engineered mechanical properties across the cross-section. In this study, an SEM nanomechanical instrument with an integrated high-temperature stage and an active tip heating was used to conduct pillar compression of aluminide bond coating and substrate at room temperature to well above 800degC. This is the first study of an in situ nanomechanical testing of any sample at such higher temperature with capturing deformation events in detail at that temperature. With combined analysis of chemistry and microstructural changes, the results were used to understand local mechanical properties variation as a function of temperature.

3:00 PM  Invited
Temperature-dependent Intermittent Plasticity of Nb Microcrystals: Quentin Rizzardi1; Douglas Stauffer2; Jaime Marian3; Robert Maass1; 1University of Illinois at Urbana-Champaign; 2Bruker Nano Surfaces; 3University of California, Los Angeles
    Intermittent microplasticity via dislocation avalanches indicates scale-invariance, which is a paradigm shift away from traditional bulk deformation. Recently, we have developed an experimental method to trace the spatiotemporal dynamics of correlated dislocation activity (dislocation avalanches) in microcrystals (Phys. Rev. Mat. 2 (2018) 120601; Phys. Rev. Mat. 3 (2019) 080601). Here we exploit the temperature sensitive deformation of bcc metals. A marked change of the slip-size distribution is observed in the studied microcrystals, with increasingly small event-sizes dominating with decreasing temperature. This shows how a reduction in thermal energy increasingly suppresses the length-scale of dislocation avalanches, indicating how long-range correlations become gradually limited to the scale of the lattice. Our results further show that the stress-strain response is composed of strain-increments that are either thermally activated or essentially athermal. Temperature-dependent small-scale testing in combination with state-of-the-art discrete dislocation dynamics (DDD) simulations of Nb microcrystals are used to reveal these insights.

3:20 PM  
Characterization of Dislocation Avalanches by In Situ Micropillar Compression Tests with Simultaneously Detected Acoustic Emission: Dávid Ugi1; Péter Ispánovity1; Michal Knapek2; Kristián Máthis2; Zoltán Dankházi1; István Groma1; 1Loránd Eötvös University; 2Charles University
    In the size regime around few µm the plastic deformation of single crystals becomes inhomogeneous and unpredictable. Therefore, it is necessary to provide an in-depth experimental investigation of these elementary deformation processes. In my talk, I will show experimental results achieved with the combination of two methods: (i) in situ nanoindentation testing used to determine the stress-strain characteristics of micrometer size Zn pillars, and (ii) acoustic emission measurement typically used to monitor plastic activities in bulk materials. The nanoindentation experiment was carried out in a SEM while a piezoelectric detector was attached to the sample. Our simultaneous measurements show real correlation between the dislocation avalanches associated with the measured stress drops and the detected acoustic signals. The statistical analysis of the sequence of acoustic signals reveals long-range temporal correlations and a complex internal structure of a single event and, thus, opens new perspectives in the interpretation of the acoustic signals.