Computational Thermodynamics and Kinetics: Phase Stability II
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Chemistry and Physics of Materials Committee, TMS: Computational Materials Science and Engineering Committee
Program Organizers: Nana Ofori-Opoku, Canadian Nuclear Laboratories; Eva Zarkadoula, Oak Ridge National Laboratory; Enrique Martinez Saez, Clemson University; Vahid Attari, Texas A&M University; Jorge Munoz, University of Texas at El Paso
Wednesday 2:00 PM
March 17, 2021
Room: RM 54
Location: TMS2021 Virtual
Session Chair: Peter Galenko, Friedrich Schiller University Jena
2:00 PM
Thermodynamic Stability of the Light Elements Doping in Sm(Fe,Co)12 Compounds: Arkapol Saengdeejing1; Ying Chen1; 1Tohoku University
SmFe12-based compounds have been considered one of the most promising candidates for the next generation of high performance permanent magnet materials. SmFe12-based compounds exhibit excellent intrinsic hard magnetic properties with lesser amount of rare earth elements compare to other hard magnetic materials. However, it is difficult to synthesis the pure SmFe12 in bulk due to its thermodynamic instability. Additional alloying elements helps stabilizing the structures but also degrades the magnetic properties. Finding suitable alloying elements which effectively stabilize the SmFe12 structure and have minimum effect on the magnetic properties is experimentally expensive and time consuming. In this work, we performed systematic first-principles calculations to explore the finite temperature stability of various light alloying elements combing with several transition metal elements in SmFe12-based compounds. Several promising systems are predicted such as Sm(Fe,Co)12B, Sm(Fe,Co,B)12, and Sm(Fe,Co,V,Ti)12 to be thermodynamically stable while still maintaining decent magnetic properties.
2:20 PM
First-principles Investigation of the Phase Structures and Stabilities in Mg-Zn Alloys: Du Cheng1; Kang Wang1; Bi-Cheng Zhou1; 1University of Virginia
Mg-Zn alloys exhibit strong age-hardening effect and have become promising bases for lightweight structural materials with improved strength. However, the stoichiometries, atomic structures, phase stabilities and formation mechanisms of the various nanoscale structures and intermetallic compounds formed during the heat treatment in this system still remain unclear. Here we use a combined approach of first-principles calculations, cluster expansion and Monte Carlo simulation to investigate the atomic structures and thermodynamic stabilities of the experimentally reported compounds as well as orderings on the HCP lattice. The morphologies and formation mechanisms are further inferred from the strain analysis. In addition to the structure and stability of commonly observed compounds, the potential Guinier-Preston zones are identified from preferred HCP orderings and the relations among MgZn2 Laves phase, Mg4Zn7 and β_1^' precipitates are discussed in details.
2:40 PM
Stability and Phase Transition of Cristobalite in SiO2: Ying Chen1; Nguyen-Dung Tran1; Hao Wang2; Masanori Kohyama3; Satoshi Kitaoka4; Tetsuo Mohri1; 1Tohoku University; 2Shanghai University; 3AIST; 4Japan Fine Ceramics Center (JFCC)
SiO2 has many polymorphs with plentiful phase transformations. The present work focus on the Cristobalite phases of SiO2 which is taken as a promising oxidation protective film in environmental barrier coating. β-Cristobalite is stable at high temperature, while the phase transition to α-Cristobalite happens accompanying a large volume shrinkage at 270°C, which leads to the SiO2 film broken or peeled off. First-principles calculation has been conducted to get a guideline to suppress such α-β Cristobalite phase transition. Electronic structure, phonon vibration calculation are conducted on α-Cristobalite, β-Cristobalite and α-Quartz to investigate the stability and the tendency of phase transformation. Nitrogen-doping is then attempted and a structure with Si-N-N-Si-O ring formed from the pair N2 substituting two oxygen atoms at the vertexes of neighboring O-O-O-Si tetrahedron gives obviously lower energy than other configurations, this mechanism can be explained by analysis of local energy density and local stress density.
3:00 PM
Phase stability and Atomic Diffusion in fcc Fe-Ni Alloys: Interplay between Magnetic and Chemical Degrees of Freedom: Kangming Li1; Chu-Chun Fu1; Maylise Nastar1; 1DEN-Service de Recherches de Métallurgie Physique, CEA, Université Paris-Saclay
Various modelling efforts have been devoted to understand the phase stability and atomic diffusion in Fe-Ni systems, a well-known prototype of austenitic steels. However, the effects of magnetic excitations and transitions on these properties, which are known to be important in Fe-alloys, are still unclear and often neglected. A close interplay with chemical order is also expected as Fe-Ni systems undergo chemical and magnetic transitions within a narrow temperature interval. In this study, we construct ab initio-parametrized effective interaction models, including both chemical and magnetic variables and the vibrational entropy, for fcc Fe-Ni alloys. We perform Monte Carlo simulations to obtain the thermodynamic and diffusion properties over the whole concentration range and across the chemical/magnetic transition temperatures. The correlated effects of chemical and magnetic degrees of freedom on the phase diagram and vacancy- and self-interstitial-mediated diffusion are discussed.
3:20 PM
Dislocation Formation Mechanism in Polycrystalline HCP Zr and Zr-2.5wt.%Nb Alloy: Cong Dai1; Nana Ofori-Opoku1; 1Canadian Nuclear Laboratories
The deformation-induced dislocations in polycrystalline HCP metals have a great influence on the materials mechanical properties. Many factors include grain orientations, loading conditions and grain boundary types would affect the nucleation and kinetics of these dislocations, and they are investigated by atomistic simulations based on empirical potentials. The stress strain curves, dislocation densities and stacking fault energies are calculated to study the dislocation formation mechanism and its response on the materials mechanical properties. Different types of dislocations are identified during the deformation, and they are likely formed from grain boundaries. The alloying samples are found more deformation-resistance compared to pure ones, since the alloying atoms segregated at grain boundaries would pin dislocations and make them more difficult to emit. Our work shows an approach to investigate the materials deformation behaviors by atomistic structural analysis.