Mechanical Behavior at the Nanoscale V: Modeling
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Mechanical Behavior of Materials Committee, TMS: Nanomechanical Materials Behavior Committee
Program Organizers: Christopher Weinberger, Colorado State University; Megan Cordill, Erich Schmid Institute of Materials Science; Garritt Tucker, Colorado School of Mines; Wendy Gu, Stanford University; Scott Mao; Yu Zou, University of Toronto
Tuesday 8:30 AM
February 25, 2020
Room: Santa Rosa
Location: Marriott Marquis Hotel
Session Chair: Christopher Weinberger, Colorado State University; Shawn Coleman, Army Research Laboratory
8:30 AM
Modeling Deformation Twinning in BCC Transition Metals: Anik Faisal1; Christopher Weinberger1; 1Colorado State University
Deformation twinning is an important mode of deformation mechanism in BCC transition metals under high rate and low temperature conditions. Atomistic simulations of twin structures in BCC transition metals reveal two types of twin structure: conventional reflection twin structure and isosceles twin structure. In some cases, the conventional reflection twin structure is energetically favorable while in other cases, the isosceles structure is favorable. In this work, energetics of these twin structures is examined with the inclusion of interaction between dissociating partial dislocations and generalized stacking fault energy using both MD and DFT calculations. The energetics of nucleation of both twin structures coupled with the rate and temperature dependence of twin formation provide basis for developing multiscale models of deformation twinning in BCC metals. This model will offer useful insights into understanding multiscale deformation phenomena that initiate at nanoscale and mechanical properties of bulk BCC transition metals.
8:50 AM
Uncovering the Nanoscale Mechanisms Governing Thermomechanical Properties in Solute-stabilized Nanocrystalline Alloys: Ankit Gupta1; Gregory Thompson2; Garritt Tucker1; 1Colorado School of Mines; 2University of Alabama
Nanocrystalline (NC) materials attract significant interest in terms of potential engineering applications and building behavioral understanding. The addition of specific solutes has been predicted and experimentally shown to provide stability again grain growth at elevated temperatures; dictated by enthalpies of mixing and segregation of NC alloys. However, the vast majority of existing studies assume only thermal annealing conditions; however, under thermomechanical (TM) loading, stresses and strains will alter the thermodynamic criteria for stability as well as the underlying mechanisms of deformation accommodation. In this study, stability of NC alloys under thermo-mechanical loads is investigated using experiments and atomistic simulations. A strongly segregating Ni (P) alloy system is chosen. The underlying mechanisms dictating the thermo-mechanical stability, as a function of alloy grain size and solute content, are studied. The dependence of GB structure on solute segregation and its role in deformation accommodation is also discussed.
9:10 AM
Strong Strain Hardening in Ultrafast Melt-quenched Nanocrystalline Cu: the Role of Fivefold Twins: Amir Hassan Zahiri1; Pranay Chakraborty1; Yan Wang1; Lei Cao1; 1University of Nevada Reno
Low ductility due to the absence of strain hardening effect in the nanocrystalline and nanotwinned metals is one of the recent challenges. In this work, we studied melt-quenched nanocrystalline Cu under compression, which contains high-density of fivefold twins (ffts), twin boundaries, and stacking faults and we observed sustained strain hardening effect. The molecular dynamics simulations shows that the observed strain hardening is due to contribution of numerous dislocation reactions, constant nucleation and dislocations impedance, and restricted twin boundary migration in fft networks. We find that dislocations can nucleate and impede by the ffts and migration of the fft boundary is restricted by its own core. Moreover, due to the gliding of two different Shockley partial dislocations in the opposite directions fft boundary migrates by two atomic planes directly. Finally, dislocation transmission observed among the fft boundaries. This work presents the advantage of ffts over nanotwins to overcome the strength-ductility trade-off.
9:30 AM
Investigating the Mechanical Behavior of Nano-architected Materials via Multiscale Discrete Defect Element Method: Phu Cuong Nguyen1; Ill Ryu1; 1University of Texas at Dallas
The multiscale defect element method (DDEM) has developed to couple conventional continuum crystal plasticity finite element (CPFEM) with a discrete defect (DD) modeling. The multiscale model could account for complex multi-physical phenomena, which would play an important role in obtaining a fundamental understanding of deformation mechanism at small scale. Using the model, we investigate the mechanical behavior of nano-architected materials. The mechanical response of nanoscale octet-truss lattice structures under compression obtained from this model is compared with experimental results and a scaling model for the strength of the nanolattice structures is developed. The multiscale model will expand our knowledge on governing mechanism of plasticity controlled by defects in crystalline materials.
9:50 AM
Dislocation-twinning Competitions in Body-centered Cubic Metallic Nanowires: Chaoming Yang1; Liang Qi1; 1University of Michigan
Without preexisting deformation defects, a nanowire (NW) can reach near-ideal tensile strength and deform by nucleating defects, such as dislocations and deformation twinning, from its free surfaces. In particular for the body-centered cubic (BCC) NWs of refractory metals growing along <100>, deformation twinning is commonly observed and is responsible for the pseudoplasticity under <100> tension. Recently we build a modified embedded-atom method (MEAM) potential of Niobium (Nb) that can reproduce its ideal tensile strength behavior. Based on MEAM potentials of Nb and Molybdenum (Mo), molecular dynamics simulations show different deformation behaviors for NWs under <001> tension. Mo NWs always have the typical deformation twinning, but there are interesting competitive phenomena between dislocations and deformation twinning in Nb NWs. These competitions depend on temperatures and NW geometry. Analyses of the effects of interatomic bonding characteristics to these dislocation-twinning competitions deepen our understanding of deformation mechanisms at the nanoscale.
10:10 AM Break
10:30 AM Cancelled
Atomistic Study of Ceramic Grain Boundary Deformation: Shawn Coleman1; Qi An2; Matthew Guziewski1; 1CCDC Army Research Laboratory; 2University of Nevada, Reno
Grain boundaries and interfaces can be engineered to promote improved mechanical properties and to influence the fracture response in brittle ceramics. For decades experiential process engineers have found that adding select dopants to ceramics promoted isolated disorder at the interface, which in turn increased the macroscopic fracture toughness with minimal cost to strength. To speed up the design of new ceramics and ceramic composites, atomic models are being built to study how grain boundaries and interfaces can be engineered to selectively tune their properties. Specifically, this work investigates the deformation and failure of boron carbide and silicon carbide grain boundaries using density functional theory and molecular dynamics. Here we showcase how chemistry and disorder influence grain boundaries mechanical properties.
11:10 AM
Superelasticity and Superplasticity in Shape Memory Ceramic Nanoparticles: Ning Zhang1; Mohsen Asle Zaeem2; 1University of Alabama; 2Colorado School of Mines
Martensitic phase transformation in zirconia-based ceramics has attracted considerable attention due to its associated shape memory effect. However, degradation of superelastic and shape memory properties is observed experimentally only after a few loading cycles. By means of molecular dynamics technique, this work aims to reveal the nanoscale responsible plastic deformation mechanisms in yttria stabilized tetragonal zirconia (YSTZ) nanoparticles under compression. Simulation results show that up to 7.8% superelastic strain and 13.8% superplastic strain are achieved in single crystal particles. By heating to ~600 K, the deformed particle is observed to regain its original shape. However, with the increase of loading-unloading-heating cycles the required temperature for completely shape recovery increases, up to ~1300 K. Even more, 1%-3% residual strains are left after several cycles. Although self-healing is observed, amorphous phase accumulation in the phase transformation region is observed to be responsible for the degradation of shape memory property in YSTZ nanoparticles.
11:30 AM
Failure Mechanisms of Core-shell Nanostructures: Raghuram Santhapuram1; Arun Nair1; 1University of Arkansas
Engineering a nanostructure to enhance the mechanical and physical properties can be achieved by either changing the material composition or by varying the geometry of the nanostructure. Core shell nanostructures (CSN) is an example where the geometry and material composition play an important role in improving the mechanical properties. In this study, we use molecular dynamics method to understand the mechanical response of the CSN’s with different metallic core materials and an amorphous shell under nanoindentation. The use of different core materials with different stacking fault energies and an amorphous shell will help in selecting appropriate materials for the CSNs for specific applications. Preliminary results with an aluminum core and an amorphous silicon shell have shown full recovery from plastic deformation under nanoindenation. We plan to discuss the ratio between core radii to shell thickness that will generate a deformation resistant CSN.