Computational Thermodynamics and Kinetics: Role of Defects in Phase Stability and Transformations
Sponsored by: TMS Functional Materials Division, TMS Materials Processing and Manufacturing Division, TMS Structural Materials Division, TMS: Chemistry and Physics of Materials Committee
Program Organizers: Vahid Attari, Texas A&M University; Sara Kadkhodaei, University Of Illinois Chicago; Eva Zarkadoula, Oak Ridge National Laboratory; Damien Tourret, IMDEA Materials Institute; James Morris, Ames Laboratory
Thursday 2:00 PM
March 3, 2022
Room: 255C
Location: Anaheim Convention Center
Session Chair: Soumendu Bagchi, Los Alamos National Laboratory; Vahid Attari, Texas A&M University
2:00 PM Invited
NOW ON-DEMAND ONLY - CALPHAD-guided Grain Boundary Phase Diagrams and Opening Possibilities for Grain Boundary Engineering: Reza Darvishi Kamachali1; Lei Wang1; Anna Manzoni1; Birgit Skrotzki1; Gregory Thompson2; 1Federal Institute for Materials Research and Testing (BAM); 2Department of Metallurgical Materials Engineering, The University of Alabama
Engineering grain boundaries requires a quantitative description of both their segregation and specific solubility behavior. This is only possible when a full thermodynamic description of the grain boundaries is given. Recently we have proposed a density-based model for grain boundary thermodynamics that enables CALPHAD-guided derivation of grain boundary phase diagrams and related full-field simulations for multi-component alloys. Combining this model with experimental investigations, in this talk, we present new aspects of interfacial segregation and phase transformation revealed in polycrystalline alloys. Solute segregation and consequent precipitation in Al-Cu-Mg and Al-Cu-Li alloys will be discussed. We investigate the effects of elastic interactions on the segregation and phase stability at grain boundaries and demonstrate the application of the model to nanocrystalline alloys, with numerous grain boundaries of various characters. A general strategy for grain boundary engineering will be presented.
2:30 PM
A First-principles Analysis of the Temperature Dependence of Stacking Fault Energies and Cross-slip Barrier in Mg and Its Alloys: Julian Brodie1; Maryam Ghazisaeidi1; 1Ohio State University
Magnesium is interesting due to its high strength, low density, recyclability, and applications in automotive and aerospace industries. However, Mg has poor room temperature ductility due to its hexagonal-closed-packed structure which results in an insufficient number of available slip systems at low temperatures. However, recent theoretical studies show improving the room temperature ductility of Mg may be accomplished by decreasing the cross-slip energy barrier between the pyramidal I and II planes in Mg via dilute solute additions. As the calculations were performed at 0K, calculations at higher temperatures are needed. We use Density Functional Theory to compute the stacking fault energy on relevant slip planes in Mg and its alloys. We then use phonon information extracted using Density Functional Perturbation Theory and the Quasiharmonic Approximation to determine the temperature dependence of the stacking faults. Finally, we determine the cross-slip energy barrier as a function of temperature for the Mg alloys.
2:50 PM
Effect of Electric Fields on Bulk and Surface Driven Dislocation Behavior in fcc Metals: Soumendu Bagchi1; Danny Perez1; 1Los Alamos National Laboratory
High-gradient electric-fields are inevitably encountered in technologies ranging from low-cost compact linear accelerators to miniaturized electronic devices. Material functionality under extreme field conditions can heavily depend on the coupling between electro-thermal loading and microstructural deformation. While difficult to explicitly access through state-of-the art experiments, relevant mechanisms of field-induced material evolution can be directly probed by atomistic simulations. Using a charge-equilibration-molecular dynamics framework, we model the effect of surface electric fields and thermo-mechanical stresses on the plastic deformation of fcc metals. Bulk dislocation multiplication mechanisms e.g. Frank-Read source activation is shown to enhance local field-induced stresses through the creation of surface slipped-steps, which can facilitate further activation. We also investigate the possibility to nucleate dislocation loops from slipped surface-steps by quantifying the related activation energy barriers. By performing fixed-end nudged elastic band calculations, we show that nucleation barriers can be significantly reduced with increasing surface electric field and slip-step heights.
3:10 PM
Does Vibrational Motion Explain the Latent Heat of Melting in Materials?: Camille Bernal-Choban1; Claire Saunders1; Stefan Lohaus1; Doug Abernathy2; Brent Fultz1; 1California Institute of Technology; 2Oak Ridge National Laboratory
It is well established that atomic vibrations dominate the entropy of solids and liquids. The challenge persists in determining if the difference in vibrational entropy upon melting is larger than the change in entropy from other sources. Inelastic neutron scattering (INS) measurements were performed on powder samples of Ge, Sn, Pb, and Bi from room temperature to 200 K above the melting temperature of each material. Experimental vibrational spectra were processed using customized multiphonon and multiple scattering corrections. Subsequent analysis, informed by vibrational-transit theory, was used to calculate the vibrational density of states (DOS) for each phase. Preliminary results for Ge showed that the difference between the solid and liquid DOS accounts for over 50% of the latent heat. The separation of diffusional versus collective motion through melting will also be discussed. Similar methods and analysis will be presented for Sn, Pb, and Bi.
3:30 PM Break
3:50 PM
Thermodynamic Explanation of the Invar Effect by Computation and Experiments: Stefan Lohaus1; Matthew Heine2; Pedro Guzman1; Camille Bernal1; Olle Hellman3; David Broido2; Brent Fultz1; 1California Institute of Technology; 2Boston College; 3Linkoping University
Phonons, electrons, spins and their interactions give the entropy and free energy of magnetic materials. A Maxwell relation implies that the pressure dependence of these entropy contributions must somehow sum to zero, to give Invar materials their known zero thermal expansion. With experiments and computations, we evaluated these thermodynamic contributions from lattice vibrations and spin disorder versus pressure for Fe-36%Ni Invar. From nuclear resonant inelastic X-ray scattering and Mössbauer spectroscopy at varying temperatures and pressures, we measured the phonon density of states and the magnetization, and determined their individual contributions to the entropy of Invar. Calculations of the phonon modes performed with an ab initio effective potential method that includes the magnetic disorder are able to capture the anomalies in the thermal expansion, and are in good agreement with experiments. The low thermal expansion of Invar is explained by the cancellation of entropy effects from lattice vibrations and spin disordering.
4:10 PM
Entropic Effects on Thermally Activated Dislocation Cross-slip: Yifan Wang1; Wei Cai1; 1Stanford University
Dislocation cross-slip is one of the main microscale mechanisms involved in the temperature dependence of stage-III strain-hardening of face-cubic centered (fcc) metals. So far, atomistic investigations of cross-slip rate focused on determining the activation enthalpy of cross-slip under stress and the entropic effects were usually neglected. We show that the rate predicted based only on activation enthalpy underestimates the cross-slip rate by 6-orders of magnitude compared to direct molecular dynamics (MD) simulation. Based on harmonic transition state theory (HTST) with corrections of anharmonic soft modes, we develop a fully atomistic model that successfully predicts the cross-slip rate of fcc nickel under a much wider range of stress and temperature conditions. We find that thermal expansion and thermal softening effects contribute significantly to the activation entropy and the rate due to a non-negligible coupling between the activation enthalpy to the hydrostatic component of the stress.