Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session J25: Focus Session: Cooperative Phenomena in Plasticity I |
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Sponsoring Units: DMP Chair: Peter Ispanovity, Eotvos University Room: 203B |
Tuesday, March 3, 2015 2:30PM - 2:42PM |
J25.00001: Robust Scaling of Strength and Elastic Constants and Universal Cooperativity in Disordered Colloidal Micropillars Daniel Gianola, Daniel Strickland, Yun-Ru Huang, Peter Derlet, Daeyeon Lee We study the uniaxial compressive behavior of disordered colloidal free-standing micropillars composed of a mixture of 3 and 6 $\mu $m polystyrene particles. Mechanical annealing enables variation of the packing fraction across the phase space of colloidal glasses. The measured strengths and elastic moduli of the micropillars span almost three orders-of-magnitude despite similar plastic morphology governed by shear banding. We measure a robust correlation between strengths and elastic constants that is invariant to humidity, implying a critical strain of $\sim $0.01 that is strikingly similar to that observed in metallic glasses and suggestive of a universal mode of cooperative plastic deformation. We estimate the characteristic strain of the underlying cooperative plastic event by considering the energy necessary to create an Eshelby-like ellipsoidal inclusion in an elastic matrix. We find that the characteristic strain is similar to that found in experiments and simulations of other disordered solids with distinct bonding and particle sizes, suggesting a universal criterion for the elastic to plastic transition in glassy materials with the capacity for finite plastic flow. In addition, we measure the statistics of load-drops for specimens at three packing fractions. At higher packing fractions, the load-drops scale as a power-law with an exponent close to mean field theory (MFT) predictions. However, the scaling at the lowest packing fraction deviates from MFT. [Preview Abstract] |
Tuesday, March 3, 2015 2:42PM - 2:54PM |
J25.00002: ABSTRACT WITHDRAWN |
Tuesday, March 3, 2015 2:54PM - 3:06PM |
J25.00003: Scale-free avalanche dynamics in crystal plasticity Pater Dusan Ispanovity, Lasse Laurson, Michael Zaiser, Stefano Zapperi, Istvan Groma, Mikko Alava We investigate the properties of strain bursts (dislocation avalanches) occurring during plastic deformation of crystalline matter using two dimensional discrete dislocation dynamics (DDD). We perform quasistatic stress-controlled simulations with three DDD models differing in the spatiotemporal discretization and the mobility law assumed for individual dislocations. We find that each model exhibits identical avalanche dynamics with the following properties: (i) strain burst sizes follow a power law distribution characterized by an exponent $\tau\approx 1.0$ and (ii) the distribution in truncated at a cutoff that diverges with increasing system size at any applied stress level. It has been proposed earlier that plastic yielding can be described in terms of a continuous phase transition of depinning type and its critical point is at the yield stress. We will demonstrate, however, that our results are inconsistent with cutoff scaling in depinning systems (like magnetic domain walls or earthquakes) and that the system behaves as critical at every stress level. We, therefore, conclude that in the models studied plastic yielding cannot be associated with a continuous phase transition. [Preview Abstract] |
Tuesday, March 3, 2015 3:06PM - 3:42PM |
J25.00004: Large Scale 3-D Dislocation Dynamics and Atomistic Simulations of Flow and Strain-Hardening Behavior of Metallic Micropillars Invited Speaker: Satish Rao Experimental studies show strong strengthening effects for micrometer-scale FCC as well as two-phase superalloy crystals, even at high initial dislocation densities. This talk shows results from large-scale 3-D discrete dislocation simulations (DDS) used to explicitly model the deformation behavior of FCC Ni (flow stress and strain-hardening) as well as superalloy microcrystals for diameters ranging from 1 - 20 microns. The work shows that two size-sensitive athermal hardening processes, beyond forest and precipitation hardening, are sufficient to develop the dimensional scaling of the flow stress, stochastic stress variation, flow intermittency and, high initial strain-hardening rates, similar to experimental observations for various materials. In addition, 3D dislocation dynamics simulations are used to investigate strain-hardening characteristics and dislocation microstructure evolution with strain in large 20 micron size Ni microcrystals (bulk-like) under three different loading axes: 111, 001 and 110. Three different multi-slip loading axes, $<111>$, $<001>$ and $<110>$, are explored for shear strains of $\sim$0.03 and final dislocation densities of $\sim$1013/m2. The orientation dependence of initial strain hardening rates and dislocation microstructure evolution with strain are discussed. The simulated strain hardening results are compared with experimental data under similar loading conditions from bulk single-crystal Ni. Finally, atomistic simulation results on the operation of single arm sources in Ni bipillars with a large angle grain boundary is discussed. The atomistic simulation results are compared with experimental mechanical behavior data on Cu bipillars with a similar large angle grain boundary. [Preview Abstract] |
Tuesday, March 3, 2015 3:42PM - 3:54PM |
J25.00005: Slip Dynamics in Small Scale Crystals Robert Maass, Peter Derlet, Julia Greer, Cynthia Volkert Classical work showed that dislocation velocities are strongly dependent on applied stress. Numerous experiments have validated this for individual or groups of dislocations in macroscopic crystals by using imaging techniques combined with either mechanical data or time resolved topological data. Developments in small scale mechanical testing allow to correlate the intermittency of collective dislocation motion with the mechanical response. Discrete forward surges in displacement can be related to dislocation avalanches, which are triggered by the evolving dislocation sub-structure. We study the spatiotemporal characteristics of intermittent plastic flow in quasi-statically sheared single crystalline Au crystals with diameters between 300 nm and 10000 nm, whose displacement bursts were recorded at several kHz (Scripta Mater. 2013, 69, 586; Small, available online). Both the crystallographic slip magnitude, as well as the velocity of the slip events are exhibiting power-law scaling as. The obtained slip velocity distribution has a cubic decay at high values, and a saturated flat shoulder at lower velocities. No correlation between the slip velocity and the applied stress or plastic strain is found. Further, we present DD-simulations that are supportive of our experimental findings. The simulations suggest that the dynamics of the internal stress fields dominate the evolving dislocation structure leading to velocities that are insensitive to the applied stress -- a regime indicative of microplasticity. [Preview Abstract] |
Tuesday, March 3, 2015 3:54PM - 4:06PM |
J25.00006: ABSTRACT WITHDRAWN |
Tuesday, March 3, 2015 4:06PM - 4:18PM |
J25.00007: ABSTRACT WITHDRAWN |
Tuesday, March 3, 2015 4:18PM - 4:54PM |
J25.00008: Strength and Dislocation Structure Evolution of Small Metals under Vibrations Invited Speaker: Alfonso Ngan It is well-known that ultrasonic vibration can soften metals, and this phenomenon has been widely exploited in industrial applications concerning metal forming and bonding. In this work, we explore the effects of a superimposed small oscillatory load on metal plasticity, from the nano- to macro-size range, and from audible to ultrasonic frequency ranges. Macroscopic and nano-indentation were performed on aluminum, copper and molybdenum, and the results show that the simultaneous application of oscillatory stresses can lower the hardness of these samples. More interestingly, EBSD and TEM observations show that subgrain formation and reduction in dislocation density generally occurred when stress oscillations were applied. These findings point to an important knowledge gap in metal plasticity -- the existing understanding of ultrasound softening in terms of the vibrations either imposing additional stress waves to augment the quasi-static applied load, or heating up the metal, whereas the metal's intrinsic deformation resistance or dislocation interactive processes are assumed unaltered by the ultrasound, is proven wrong by the present results. Furthermore, in the case of nanoindentation, the Continuous Stiffness Measurement technique for contact stiffness measurement assumes that the imposed signal-carrier oscillations do not intrinsically alter the material properties of the specimen, and again, the present results prove that this can be wrong. To understand the enhanced subgrain formation and dislocation annihilation, Discrete Dislocation Dynamics (DDD) simulations were carried out and these show that when an oscillatory stress is superimposed on a quasi-static applied stress, reversals of motion of dislocations may occur, and these allow the dislocations to revisit repeatedly suitable configurations for annihilation. DDD, however, was unable to predict the observed subgrain formation presumably because the number of dislocations that can be handled is not large enough. Subgrain formation was directly predicted by a new simulation method of dislocation plasticity based on the dynamics of dislocation density functions. [Preview Abstract] |
Tuesday, March 3, 2015 4:54PM - 5:06PM |
J25.00009: To Slip or Snap: Finite Length Chains and Yield Mechanisms in Polyethylene Fibers Thomas O'Connor, Mark Robbins Understanding the microscopic mechanisms of yield in oriented polymer fibers is a long -standing problem. Advances in polymer processing have produced highly ordered polyethylene (PE) fibers with tensile strengths between 4-7 GPa, but these values are far less than the theoretical limiting strength of 25 Gpa due to C-C bond scission. This reduction in strength is caused by the presence of defects within the fiber. The simplest type of defect is chain ends which reflect the finite length of polymer chains. The presence of chain ends allows a polymer fiber to yield by chain slip without scission of covalent bonds. Here we present extensive united atom (UA) and all atom (AA) molecular dynamics simulations of crystalline PE fibers subjected to uniaxial tension. The fibers are fully aligned crystals constructed from chains of finite length N, with N spanning 3 orders of magnitude ($10^{1}- 10^{4}$ monomers). We explore the yield behavior of these systems and relate it to the dynamics of the underlying chain end defects. UA tensile strengths are systematically smaller than AA by a factor of about 3. Both show a saturation in tensile strength as N rises above 500 monomers. This reflects a saturation in the stress for chain ends to slip and implies a maximum tensile strength of ~6 Gpa. [Preview Abstract] |
Tuesday, March 3, 2015 5:06PM - 5:18PM |
J25.00010: X-Ray Speckle Measurements of Hysteresis in a Shape Memory Alloy Michael Rogers, Mark Sutton Shape memory alloys (SMAs), such as the ternary alloy CuAlNi, are metals that have the fascinating ability to ``remember'' their original shape: Once deformed, the simple act of heating can thermomechanically return them to their original configuration. At the heart of this process is a martensitic phase transition, a solid-solid transition that can be induced by either a temperature change or by external stress. Cycling between phases of SMAs reveals hysteresis in their stress-strain relationships and transformation temperatures. Moreover, these characteristics evolve over many transformation cycles. We report on in-situ X-ray Photon Correlation Spectroscopy (XPCS) measurements of the microstructural evolution of a CuAlNi alloy that underlies these hysteresis effects. By simultaneously monitoring changes in X-ray speckle patterns from the two solid phases as the alloy was thermally cycled through both partial and full transformations, we have seen reversible microstructural hysteresis loops and irreversible loops that reveal mesoscopic plastic deformation in the material. [Preview Abstract] |
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