Session H23: Metals Structural

Cr based Alloy Cr-Y-Mo-W Oxidation Study from First Principals Molecular Dynamics Simulation

First principles molecular dynamics simulations have been performed to study the stability and oxidation progress of Cr based alloys at high temperatures. The bulk phase of cubic Cr-based alloys is investigated with density functional theory (DFT) calculations and \textit{ab initio} molecule dynamic (MD) method. Diffusion of oxygen atoms within different densities of Y, Mo, and W doping and temperatures in Cr-Y systems is studied in this research. The effects of Y, Mo, and W doping are also studied. The properties of the optimized Y, Mo, and W co-doped Cr-based alloys also studied by using \textit{ab initio} MD method. Further improvement of the oxidation resistance and surface corrosion of Cr-based alloys will be discussed.   

Enhancing mechanical toughness of aluminum surfaces by nanoboron implantation: An {\em ab initio} study

We use {\em ab initio} density functional theory to study the formation dynamics and equilibrium atomic arrangement in aluminum surfaces exposed to energetic boron aggregates. Results of our molecular dynamics simulations indicate that after using up their excess kinetic energy to locally melt the aluminum surface, boron atoms prefer to remain in subsurface sites. We perform extensive structure optimization studies to identify the optimum structural arrangement and changes in the electronic structure associated with the formation of strong Al-B bonds, which are responsible for the stability enhancement of the compound. Nano-indentation simulations based on constrained optimization suggest that presence of boron atoms enhances the mechanical hardness and wear-resistance of aluminum surfaces.   

Phase Stability and Deformation Behavior of Mo-Si-B System and effect of alloying

Molybdenum silicides are promising materials for ultra-high temperature applications above 1300 \r{ }C. One of the main drawbacks is their brittleness at low temperatures, which may be improved by additions. We employ first principles calculations with the highly precise FLAPW method to investigate the effect of alloying with 3$d$, 4$d$ and 5$d$ transition metals on phase stability, cleavage and shear characteristics of the 3-component system Mo -- Mo$_{3}$Si -- Mo$_{5}$SiB$_{2}$. We determined site preference, phase partitioning of alloying elements, and their effect on shear behavior and preferred deformation modes. We show that in Mo$_{3}$Si alloying with 3$d$ transition metals results in a significant reduction of energy barriers to shear deformation (softening effect), while 4$d$ and 5$d$ additions increase shear barriers (hardening effect). In Mo$_{5}$SiB$_{2}$, 3$d$ transition metals (except for Ti) act as weak softeners, while 4$d$ and 5$d$ show mixed behavior -- hardening for early elements and softening for late ones. The softening potency of additions increases with atomic number, but exhibits non-monotonic behavior as a result of a competition between size and electronic effects. The results are discussed in conjunction with possible pathways to ductility enhancement through alloying.   

Structural, elastic and thermal properties of cementite from Modified Embedded Atom Method (MEAM) potential

Structural, elastic and thermal properties of cementite were studied using a newly developed MEAM potential for the Fe-C alloy system. The single element potential of C correctly predicts graphite and diamond as the two stable structures. Parameters for the Fe-C alloy potential were constructed based on the structural and elastic properties of elements in the L$_{\rm{12}}$ reference structure, calculated from ab-initio simulations. Parameters were further adjusted to reproduce structural properties of cementite and the interstitial energies for Fe correctly. Pair potential parameters were optimized using a method combining Latin hypercube sampling of the N-dimensional parameter space and multi-objective optimization. Elastic constants, surface formation energies, melting temperature and specific heat of cementite were calculated using the potential. The values computed from the Fe-C alloy MEAM potential are in good agreement with DFT calculations and experiments.   

High Strain Rate Deformation of BCC Materials: Molecular Dynamics Simulations

To improve machining processes, a good understanding of plasticity of bulk metals and at high strain rates is required. During machining processes, it is thought that strain rates of $\sim$10$^{6}$s$^{-1}$ are reached. These strain rates are currently not achievable in experimental high strain rate testing techniques. Here we investigate high strain rate deformation of bcc Fe using Molecular Dynamics simulations. Simulations include those of nano-scale single crystal pillars and of a nano-crystalline sample. We show that bcc materials may exhibit different deformation behavior at high strain rates. Under compression at strain rates of $\sim$10$^{8}$s$^{-1}$, nano-scale single crystal pillars may exhibit slip on atypical planes if the direction of maximum resolved shear stress points along that plane. The stress-strain behavior of these pillars is characterized by sudden strain bursts due to slip events. The yield stress required for these slip events can be as high as 2 GPa, much higher than the Peierls stress which is around 25 MPa for an edge dislocation in Fe. The nano-crystalline sample showed stress-strain behavior closer to that of a bulk metal. At high temperatures, pillars exhibit surface premelting. A large portion of the pillar turns amorphous when strained.   

Effect of lattice vibrations on magnetic phase transition in BCC Iron

From the first principle calculation of BCC iron, we build a classical Heisenberg model with the interaction coupling as a function of both the distance and local environments(e.g. volume). Using the Johnson potential and Finnis-Sinclair potential, we perform Monte Carlo simulations of BCC iron including the effect of lattice vibrations. The validity of classical Heisenberg model in describing the magnetic phase transition of BCC iron is explained.   

Phase-Stabilizers in Titanium Alloys

Titanium alloys exhibit three distinct crystal structures: alpha, beta and omega. For various applications alloying elements can be used to stabilize the desired phase. While alloy designers have well established rules of thumb, rigorous theory for non-equilibrium single-phase crystal stability is less well established. Here we tackle this problem using electronic structure calculations. We use two different methods based on density functional theory with pseudopotentials and plane waves, with either explicit atoms or the virtual crystal approximation (VCA). The former is highly reliable, while the latter makes a number of drastic assumptions that typically lead to poor results. Surprisingly, the agreement between the methods is good, showing that the approximations in the VCA are not important in determining the phase stability and elastic properties. This allows us to generalize, showing that the single-phase stability can be related linearly to the number of d-electrons, independent of the actual alloying elements or details of their atomistic-level arrangement. This leads to a quantitative measure of beta-stabilization for each alloying transition metal.   

Structural properties and phase stability of Ti alloys containing V and Cr by first-principles calculation

We apply the first-principles plane-wave pseudopotential method and virtual crystal approximation (VCA), to investigate structural properties and phase stability of ternary titanium alloy Ti-V-Cr. The lattice parameters of the alloy vary almost linearly with the number of d electrons in spite of the different ratio of V to Cr, which agrees with the available experimental results. At 0K, we find that an extra 0.4 electrons per atom can stabilise the beta Ti-V-Cr. Debye approximation is used to consider the temperature effect, and it is found that at 973K, with an extra 0.1 d electron per atom, the beta Ti-V-Cr can be stabilised, compared with 0.15-0.20 d electron required experimentally at 973K. It is shown that V and Cr move the Fermi energy to lower values of the density of the states of the beta phase, which accounts for the stabilisation effect.   

Dynamics of twinning dislocations in Tantalum

Twinning is one of the major deformation modes of plastic deformation in crystals, being particularly important in systems under extreme conditions of low temperature or high strain rates. Despite decades of work, the nucleation and growth mechanisms of twining are still poorly understood, especially in bcc metals. Nucleation of twinning dislocations loops on the coherent twin boundary has been considered a principal mechanism of growth of deformation twins. We have used molecular dynamics simulation to study the behavior of twinning dislocations in Tantalum, in particular the dependence of dislocation velocities on shear stress and temperature. The dynamics of edge and screw twinning dislocations is isolated and analyzed. Finally we show how kinetic parameters extracted from these simulations help inform a multiscale strength model for Tantalum that includes both twinning and slip as deformation mechanisms in the regime of high strain rates.   

Vacancy Assisted Climb in Continuum Dislocation Dynamics

Using a mesoscale continuum theory of dislocation dynamics, we study the physical effects of vacancy assisted climb. New physics emerges at high temperatures where dislocations are also able to move in the climb direction due to the absorption and emission of vacancies. We investigate this high temperature behavior using our minimal continuum dislocation dynamics model, which produces fractal cell structures in 2 and 3 dimensions. By coupling the dislocation density to a vacancy field we are able to study the effects of vacancies on diffusion-limited dislocation motion. We calibrate our model using measurements of climb velocities for straight, parallel dislocations and check the limit of no climb by freezing out vacancy motion. We use our model to explore applications of vacancy assisted climb, including dislocation creep and absorption of dislocations at grain boundaries.   

Stacking-fault energy and anti-Invar effect in FeMn alloys at high temperature

High-Mn steels (20-30at\%Mn, 2-4wt\%Si and Al) are of interest for the automotive industry due to their outstanding mechanical properties. Their deformation behavior has been empirically correlated to the stacking-fault energy (SFE), an important quantity in steel design that can be calculated \emph{ab-initio}. Using state-of-the-art methods within density-functional theory together with Monte Carlo simulations, we calculate the free energy of the Fe-22.5at\%Mn binary alloy between 300-800~K. Experimentally, the alloy is completely random and in the paramagnetic state, which we model via the coherent potential approximation and the disordered local moment approach, respectively. We treat magnetic excitations by including longitudinal spin-fluctuations and find that the FeMn alloy is an itinerant paramagnet. Our calculations confirm the experimentally observed strong magneto-volume coupling, realized in the anti-Invar behavior. We then obtain the structural stability and the SFE from free energy differences and find very good agreement with measurements. Our results demonstrate that the interplay between magnetic excitations and the thermal lattice expansion is the main factor determining the anti-Invar effect, the \emph{fcc-hcp} martensitic transformation temperature and the SFE.   

Microstructural and mechanical characterization of 0.2mass\% Carbon content steel

The The microstructures of low carbon content steel are comprised of bainite, martensite, tempered martensite and retained autenite. These structures are obtained by different heat treatments. The effect of heat treatment on microstructure and mechanical properties were investigated using X-ray diffraction, focused ion beam - scanning electron microscope (FIB-SEM), electron backscatter diffraction (EBSD), and nanoindentation. The experimental misorientation distribution revealed most grain boundaries had misorientation range between 50$^{\circ}$ and 60$^{\circ}$. The lattice relation between bainite and parent austenite is Kurdjomov-Sachs ($<111> || <110>$). FIB-SEM images and nanoindentation were revealed the grain size can influence the hardness.   

Ab initio-based interatomic potentials for body-centered cubic refractory metals

A fundamental understanding of transformation and deformation processes in the bcc refractory metals (V, Nb, Ta, Mo, and W) is vital for designing new bcc-based commercial alloys with desired properties. Such an understanding is aided by computational methods capable of reaching length and time scales needed for meaningful simulations of phase transformations and extended defects responsible for plastic deformation. Classical interatomic potentials are indispensable for simulating such phenomena inaccessible to first-principles methods. We develop accurate and robust embedded-atom method (EAM) [1] and modified-EAM (MEAM) [2] potentials for the bcc metals by fitting the model parameters to accurate first-principles data. The potentials are applicable for studying mechanical and thermodynamic properties, yielding excellent agreement with both experiments and first-principles calculations. {\newline} {\newline} [1] M. S. Daw and M. I. Baskes, Phys. Rev. Lett. 50, 1285 (1983). {\newline} [2] M. I. Baskes, Phys. Rev. Lett. 59, 2666 (1987).   

Influence of Nano-Alumina and Micro-Size Copper on Microstructure and Mechanical Properties of Magnesium Alloys AZ31

In this paper, magnesium composites are synthesized through the addition of nano-alumina and micron size copper particulates in AZ31 magnesium alloy using the technique of disintegrated melt deposition. The simultaneous addition of Cu and nano-alumina particulates led to an overall improvement in physical, microstructural characteristics and mechanical response of AZ31. Small size and reasonably distributed second phases were formed. The 0.2{\%} yield strength increased from 180 to 300 MPa (67{\%}), while the ductility increased by almost 24{\%}. The overall tensile properties assessed in terms of work of fracture improved by 66{\%}. An attempt is made to correlate the tensile response of composites with their microstructural characteristics. The results suggest that these alloy composites have significant potential in diverse and wider engineering applications.