Presentation on theme: "Technion - Israel Institute of Technology"— Presentation transcript:
1Technion - Israel Institute of TechnologyFaculty of Mechanical EngineeringThe Nanomechanics Simulations LaboratoryModeling the Strength of Ni3Al Nanocubes Using Molecular Dynamics SimulationsBy: Koren Shreiber, MscGuidance: Dr. Dan Mordehai
2Smaller is StrongerAt the sub-micro meter scale, metal specimens obey different mechanical properties than their bulk counterparts.Size dependent strength.MPa compressive stresses with bulk regime, GPa with nano -particles.Bulk regimeMordehai et al. Acta Mater. 59, 2309 (2011)
3Molecular Dynamics simulation Research steps ContentsBackgroundNi3AlDislocations theoryExperimentsMolecular Dynamics simulationResearch stepsStep 1 – Validate screw dislocation properties in a perfect latticeStep 2 – Calculate the dissociation widthsStep 3 – Compression of nanoparticlesWhat’s next?
4Ni3Al LatticeNi3Al alloys are important for technological applications mainly due to their high strength at elevated temperatures ( [MPa] tensile yield stress).FCC – Face Centered Cubic (aluminum, copper, gold, lead, nickel, platinum, silver etc.)L12 ordered structureAluminum atoms in cornersNickel atoms face centeredLattice parameter a = 3.57 (Å)aAlNi
5Dislocation theory (perfect dislocations) Dislocation - crystallographic line defect within a crystal structure.Mechanisms for dislocation formation:Homogeneous nucleation in the bulk.Heterogeneous nucleation on the surface or at grain boundary.Dislocation glide - Dislocations can glide on slip planes, usually with highest density of atoms.Burgers vector (b):magnitude and directionof the lattice distortioncaused by dislocation.Screw dislocationEdge dislocation
6Partial dislocations Edislocation ̴ Gb2 (dislocation Energy) Partial dislocations – a dislocation with Burgers vector b has higher energy than a few dislocations with smaller Burgers vectors bi which satisfy b= 𝑏𝑖 .Since bi are not lattice vectors they create a planar fault between the partial dislocations.The dissociation width (the size of the “Imperfect” zone) depends on the energy of the planar faults and the elastic constants.Stacking fault
7Partial dislocations in Ni3Al (FCC) Dislocation dissociates in FCC into 2 partial dislocations with a Stacking Fault (SF)L12 structures have super-dislocations which dissociates into two super-partials dislocations with Anti-Phase Boundary (APB). The super partials dissociates into two partials dislocations with Complex SF (CSF) or Super intrinsic SF (SISF).CSFLI2
8ExperimentsUltrahigh strength of Dislocation-free Ni3Al nanocubes, Robert Maass et al.Goal: study compressive strengths of dislocation-free Ni3Al nanocubesSize-dependent ultrahigh strength (2-10 GPa)Dislocation nucleation at free surfaces as a governing plasticity mechanism in nanosized crystalsStrain “burst” (dislocation nucleation)Slip traces of dislocations on the surface
9Research goalExperiments and finite elements analysis do not give us information on the underlying dislocation mechanisms.In this research, we perform a molecular dynamics simulation, to obtain insights on the atomistic mechanisms which dominate the deformation of Ni3Al nanocubes.
10Molecular Dynamics simulation Molecular Dynamics (MD) is a computational method to determine the trajectories of atoms in phase space according to Newton’s equations of motion (F=ma).The forces acting on each atoms are derived from an interaction energy, which is calculated according to atom positions, using effective interatomic potentials.We employ an Embedded Atoms Method potential (EAM), which is reliable for FCC metals, with a set of parameters calibrated by Purja Pun, G.P. & Mishin, Y.(REF: Purja Pun, G.P. and Mishin, Y.(2009) 'Development of an interatomic potential forthe Ni-Al system', Philosophical Magazine, 89: 34, 3245 — 3267)We do not solve quantum equation every time step and .
11Step 1 – construct a Ni3Al lattice In order to examine the ability of the potential to describe dislocation properties we constructed an “infinite” (fully periodic) Ni3Al lattice with 360K atoms.1311 Å309 Å10 ÅScrew dislocation dipoles – two dislocations in opposite directions.Finally, we relaxed the systemA screw dislocation dipole was introduce into the computational cell on (111) slip planes, according to isotropic elastic displacements.Burgers vector
12Step 1 – Visualizing dislocations Atoms in dislocations are identified according to BOP (bond order parameters)BOP – A set of symmetry parameters that defines the local symmetry in the lattice. For instance, the BOP of an atom in the perfect bulk is different from the one in the CSF.We used Atomeye for visualization.bCSFAPBdipole
13Dissociation width calculation for one typical dipole Step 2 - Calculate the dissociation widthsDissociation width calculation for one typical dipoleSimulation results (MD and excel)APB converge to 7b (35 Å)CSF converge to 2b (10 Å)
14Analytical results (maple) Step 2 – Calculate the dissociation widthsWidth of planar faults follows from the energy balance between the faults energy and the elastic interaction forces between partial dislocationsAnalytical results (maple)The results are in good agreement with our MD simulation
15Step 3 - compression of nanoparticles We constructed a Ni3Al nanocube (size 17.9x17.9 nm, 250k atoms) and compressed it with a virtual planar indenter (a repulsion force field which propagates at a constant rate towards the nanocube).178.5 Å
16Step 3 - compression of nanoparticles The forces on the indenter are extracted from the simulation.Stress & strain are calculated:Traces of deformation twinning made by a glide of twinning dislocations can be observed on the surface.Nucleation at 5 GPaElastic zone
20Ni3Al - MotivationNi-base super alloys are important for technological applications due to their:High strength following precipitation hardening at elevated temperature ( [MPa] tensile yield stress).Low density (~7 g/cm3) resulting lightweight.High resistance to creep deformation.High oxidation & corrosion resistance.precipitatebulk
21Undeformed particles after extraction ExperimentsDeformation behavior of free standing single-crystalline Ni3Al-based nanoparticles , J. Schloesser et al.Undeformed particles after extractionCompression testing on free standing and single crystalline Ni3Al nanocube (̴ 300 nm)Dislocation nucleation of Undeformed (defect free) particlesStrength of 2-3 GPastrain “burst”Tungsten needlePicking up particlecompression
22Partial dislocations in Ni3Al (FCC) Dislocation dissociation/separation in FCC – 2 partial dislocations with Stacking Fault (SF)L12 structures have super-dislocations which dissociates into two super-partials dislocations with Anti-Phase Boundary. The super partials dissociates into two partials dislocations with Complex SF or Super intrinsic SFAnomaly of Ni3Al - With the help of thermal activation, dislocation configuration is not planar and cannot glide (Kear-Wilsdorf lock)FCCCSFLI2Lock
23Molecular Dynamics simulation Molecular Dynamics (MD) is a computational method to determine the trajectories of atoms in phase space according to Newton’s equations of motion (F=ma).The forces acting on each atoms are derived from an interaction energy, which is calculated according to atom positions, using effective interatomic potentials.We employ an Embedded Atoms Method potential (EAM), which is reliable for FCC metals, with a set of parameters calibrated by Purja Pun, G.P. & Mishin, Y.(REF: Purja Pun, G.P. and Mishin, Y.(2009) 'Development of an interatomic potential for the Ni-Al system',Philosophical Magazine, 89: 34, 3245 — 3267)We do not solve quantum equation every time step and .
24About UsThe aim of the Nano Mechanics Simulations Laboratory is to develop and employ atomic and nanoscale simulation techniques to study the mechanical properties of nanometer-size specimens and surfaces.Understanding mechanical properties on very small scales:Fundamental understanding of plasticityProvide design guidelines for reliable nano and micro devices.Traditional approaches developed for bulk materials can no longer be used.