TYC Soiree @ Imperial: Atoms and the continuum: from micromechanics to solidification



Thursday March 26th, 2015
Time: 5.00pm
Venue: Room G01, Royal School of Mines, Imperial College London, South Kensington Campus
Contact: Ms Hafiza Bibi

Use of 3-D Dislocation Dynamics and Large Scale Atomistic Simulations in Understanding the Mechanical Behavior Of Metallic Systems.

Dr Satish Rao
Institute of Mechanical Engineering, EPFL, Lausanne


Two examples of the use of large scale atomistic and 3-D dislocation dynamics simulations in understanding the mechanical behavior of metallic systems: a) Size affected mechanical behavior of metallic micropillars and b) Unusual high strengths of concentrated solid solution high entropy alloys, are presented and discussed.

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 – 20mm. 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 20mm 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 ~0.03 and final dislocation densities of ~ 1013/m2. The orientation dependence of initial strain hardening rates and dislocation microstructure evolution with strain are discussed. Also, atomistic simulation results on the operation of single arm sources in Ni bipillars with a large angle grain boundary and a low angle twin boundary is discussed. The atomistic simulation results are compared with experimental mechanical behavior data on Cu bipillars with a similar large angle grain boundary or a twin boundary.

Finally, molecular statics and molecular dynamics simulations of a/2<110> dislocation behavior for a model FCC Ni30Co30Fe20Ti20 alloy, as well as simulations for a/2<111> dislocations in a model BCC Ni16Co16Fe36Ti30 alloy are discussed. Stacking fault energies for the FCC alloy as well as unstable stacking fault energies on the (110) plane for the BCC alloy are determined as a function of average composition. The core structure of a/2<110> dislocations in the FCC alloy are shown to have Shockley partial splitting variations along the dislocation line. Similar core structure variations along the dislocation line are found for a/2<111> dislocations in a model BCC alloy. The correlation lengths for dislocation line fluctuations in these alloys are determined and discussed. Molecular dynamics simulation results on the critical stress to move a/2<110> dislocations in the FCC alloy and a/2<111> dislocations in the BCC alloy as a function of temperature are compared with those of pure elements and experiment.

Why Solidification? Why Phase-Field?

Professor Ingo Steinbach
Ruhr University-Bochum, Interdisciplinary Centre for Advanced Materials Simulation.


‘‘Solidification’’ is a branch of pattern formation in theoretical physics. ‘‘Phasefield’’ is an applied tool in engineering. This strange combination of basic and applied research is reviewed against its historical background: a story of failure and success. The main achievements in both fields are highlighted, and future perspectives are briefly discussed.







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