Environment-sensitive Embedding Energy Research Articles

An electronic theoretical study of the high-temperature oxidation behavior of Fe-Cr-Al alloy

The formation mechanism of the oxide film on Fe-Cr-Al alloy, the effects of sulfur on the adhesion of the oxide film, and the role of RE element Y on the improvement of the oxide film adhesion were systematically studied on an electronic level, so that the physical nature of oxidation of the alloy was revealed. The results show that Al atom (compared with Cr or Fe atom ) has the lowest environment-sensitive embedding energy when O atoms are present on the alloy surface, thus leading to an outward diffusion of Al atoms from the interior of the alloy, and hence to the segregation of Al atoms on the alloy surface. Oxygen atoms are easy to combine with Al atoms to form Al2O3 oxide films on the alloy surface due to its high affinity with Al atoms. The impurity S has a lower environment-sensitive embedding energy on the interface between the Al2O3 oxide film and the alloy matrix than within the alloy matrix, suggesting that S can diffuse (segregate) to the interface. S atoms segregated on the interface weakens the cohesion of Al2O3 oxide film with the alloy matrix. Y is easy to combine with sulfur to form a stable sulfide within the interior of the alloy matrix, which inhibits the diffusion of sulfur to the Al2O3 oxide film /matrix interface. Y added into the alloy can thus markedly increase the adhesion of the oxide film with the alloy matrix, and significantly improve the high-temperature oxidation resistance of the alloy.

The ignition-proof effect of Ca and Be in Mg alloys

The atomic cluster models of α-Mg，liquid Mg and the interface between liquid/solid have been founded. The environment-sensitive embedding energy of Ca and Be in α-Mg，liquid Mg，liquid/solid interface has been calculated by recursion method. The atomic affinity energy between Mg，Ca，Be with O has been defined and calculated. The calculated results show that the solid solubility of Ca and Be is very small in α-Mg，because of their higher environment-sensitive embedding energy leads to instability in α-Mg crystal. The Ca and Be diffuse in to the liquid Mg, which has lower environment-sensitive embedding energy than the solid，and congregate on the surface of liquid Mg as the alloys solidify. Because the atomic affinity energy of Ca-O and Be-O is lower than Mg-O，The Ca and Be aggregating on the surface of liquid Mg will priorly combined with O，forming compact oxides of Ca，Be and alloys of elements，which prevent Mg alloys from burning.

The mechanism of the influence of Bi(or Sb) and rare earth on high temperature performance of AZ91 magnesium alloy

The atomic structure models of α matrix and the symmetric ［0001］ tilt boundary in AZ91 magnesium alloy were set up by using the concept of coincidence-site lattice (CSL). The total structure energy of α matrix and the grain boundary (GB)，the environment sensitive embedding energies (EESE) of alloy elements and the interaction energies between the atom clusters were calculated by using recursion method. Calculation results show that the cluster formed by Al and RE in bulk AZ1 Magnesium alloy is rather stable，but the cluster of Al，Bi(or Sb) and RE atom is unstable. When Bi(or Sb) and RE atoms coexist in AZ91 magnesium alloy，Bi or Sb will combine with RE to form dispersed particles which are mainly RE2Bi (or RE2Sb ) and RE-Sb(or RE-Bi) distributed at the grain boundary，and Al11RE3 will form in the bulk in AZ91 magnesium alloy. The consumption of Al in magnesium alloy inhibits the formation of the discontinuous precipitate phase Mg17Al12. The melting points of compounds formed by RE and Al (or Bi，Sb) are high，thus the high temperature performance of AZ91 magnesium alloy would be obviously improved.

Electronic theoretical study of the mechanism of the oxide scale formation of Fe-Cr-Al alloy

The atomic structure models of the surface，the interface of oxide scale and bulk of Fe-Cr-Al alloy were set up by using corresponding computer programming, and the electronic structure parameters （environment-sensitive embedding energy，affinity energy，total energy and total density of states） of alloying elements on the surface，the interface of oxide scale and bulk of Fe-Cr-Al alloy, were calculated by the recursion method. The formation mechanism of the oxide film，the influences of sulfur and rare earth metals on the adhesion of oxide film of Fe-Cr-Al alloy and on the formation course of the oxide film were studied systematically on electronic level. The results show that the driving force of Al atom to segregate on to the surface was larger than that of Cr and Y atom. This leads to the inward diffusion of O atoms and the outward diffusion of Al atoms at the early stage of oxidation，resulting in a surface layer rich in Al and O atoms. The affinity force between oxygen and Al was larger （the affinity energy being lower），oxygen is easy to combine with Al atoms to form Al2O3 oxide scale. Yttrium is segregated to the surface，which inhibits the diffusion of Al to the surface of Fe-Cr-Al alloy. The lateral growth of oxide film is effectively controlled，the occurrence of the convoluted morphology of the scale is avoided，the adhesion of the oxide film to the Fe-Cr-Al matrix is increased by the addition of Y. The diffusion of impurity S to the interface of Fe-Cr-Al matrix and Al2O3 oxide scale increases the total energy of the interface，decreases the total density of states，which reduces the stability of the interface，weakens the cohesion of Fe-Cr-Al matrix and Al2O3 oxide scale.

Study of corrosion effect of Pt on Ti alloys by recursion method

The electronic structure parameters of pure Ti and TiPt alloy, such as density of states, Fermi energy level, environment-sensitive embedding energy and order energy have been calculated by recursion method. The environment-sensitive embedding energy induced by Pt in Ti crystal is greater than that on Ti surface, so Pt is prone to gather on Ti surface. The order energy between Pt atoms is positive, which leads Pt atoms to form intermetallics with bulk Ti atoms. The electric potential of intermetallics is lower than that of Ti atom cluster at Ti alloy surface, so cells will form between intermetallics and Ti atom clusters, then the intermetallic will decompose in corrosive medium. With the deposition of Pt, a rough electrocatalytic Pt layer would form on the surface of Ti alloy. This electrocatalytic Pt layer enhances the passivation of Ti alloy and improves the corrosion resistance of Ti alloy.

Theoretical study of ignition-proof mechanism of RE element in Mg alloys

The atomic cluster models of α-Mg, liquid Mg and the interface between liquid/solid have been founded by computer program. The environment-sensitive embedding energy of RE elements in α-Mg, liquid Mg, liquid/solid interface has been calculated by recursion method. The atomic affinity energy between Mg, La, Y with O has been defined and calculated. The calculated results show that the solid solubility of La and Y is very small in α-Mg, because their higher environment-sensitive embedding energy leads to instability in α-Mg crystal. The RE elements diffuse to the liquid Mg, which has lower environment-sensitive embedding energy than the solid, and congregate on the surface of liquid Mg as the alloy solidifies. Because the atomic affinity energy of RE-O is lower than Mg-O(The atomic affinity energy between Mg, La, Y with O are Mg-O：-14.9338eV，La-O：-19.0608 eV，Y-O：-19.5050 eV, respectively), the RE elements congregating on the surface of liquid Mg will priorily combined with O, forming compact RE oxides, which prevent Mg alloys from burning.

The electronic theory study of the influence of Pd on the passivation of Ti alloys

The electronic structure parameters of Ti alloys, such as density of states, Fermi energy level, environment-sensitive embedding energy etc, have been calculated by recursion method. The larger environment-sensitive embedding energy of Pd in the bulk of Ti alloy than that on the surface of Ti alloy indicates that Pd is apt to segregate on the surface. The negative formation energy of Pd clusters suggests that the distribution of Pd atoms on the surface of Ti alloy is in the form of atom clusters. The range of the local density of states of Pd is narrow, while that of Ti is wide. So a peak appears in the total density of states of Ti alloys between -20 and -15 eV. This peak has a significant effect on the Fermi level. It makes the Fermi energy of the surface Ti alloys with more Pd atoms low. So the microcells form between the area with more Pd atoms and the area with fewer or without Pd atoms on the surface of Ti alloys. Ti dissolves preferentially in the corrosive media. This leads to form a layer of Pd atomic clusters on Ti alloy surface. The Pd atomic layer serves as the electrocatalytic surface, facilitating the passivation of Ti alloys and improving the corrosion resistance.

Electronic theoretical study on the corrosion and passivation mechanism of Ti metal

The atomic cluster models of the crack in αTi formed by dislocation accumulation were set up. The electronic structure of Ti was calculated by using recursion method, and the influence of O, Cl, and Pd on the electronic structure of Ti was studied. The calculated results show that, the total density of states near Fermi level is lowerd due to the existence of O, which leads to the decrease of the chemical activity of Ti. The binding energy of Ti is reduced by oxygen. The affinity between O and Ti is large, so O is easy to react with Ti to form the oxide film. The stability of Cl in Ti and the affinity with Ti are not as good as O. Cl is difficult to substitute the O atom on the surface of Ti, so the oxide film of Ti is very stable, the phenomenon of over passivation can not occur. The environment-sensitive embedding energy of Pd in the crack is smaller than in αTi grain, so Pd is easy to diffuse to the crack and makes the environment sensitive embedding energy of H rise obviously, which leads to the weakening of H diffusion to the crack and the improvement of the stress-corrosion resistance of Ti.

The grain boundary segregation of microalloying elements and the ultra-refinement of steel

A model of Σ5〈001〉/(210) high angle grain boundary (GB) in austenite phase in steel was set up with computer programming. The environment-sensitive embedding energies (ESE) of C, N and microalloying elements in grains or in GB core are c alculated by recursion method, respectively. The segregation and interaction of C, N and microalloying elements in the GB core were discussed. Calculation resul ts show that: discrete light impurities C and N tend to segregate in the GB area , forming atomic clusters; microalloying elements are liable to distribute in pe rfect austenite grains. Ti, V, Nb occupy the top sites of the trigonal prism for med by iron in the GB core, and Ti, V, Nb can also form atomic clusters in GB co re like C or N. When the temperature is decreased and the concentration of C, N and microalloying elements reach to the limit of solubility, the C, N compounds of microalloying elements precipitate from the matrix in GB core of austenite in steel. These compounds can act as heterogeneous nuclei of austenite phase in th e course of recrystallization and retard the growth of austenite grains, leading to the refinement of austenite grains. Nb is the most efficient refiner of micr oalloying elements Nb, Ti, V.

Electronic theoretical study of stress corrosion mechanism of Ti metal

In order to reveal the nature of the stress corrosion of Ti theoretically, the a tomic cluster model of α-Ti and the crack formed by dislocation accumulation wa s set up. The electronic structure parameters (Fermi energy level, structure ene rgy, surface energy, cluster energy and environment-sensitive embedding energy) of α-Ti and the crack were calculated by using the recursion method. The calcul ated results show that the environment-sensitive embedding energy of H atoms in the region of crack is smaller than that in the perfect area of α-Ti, so H atom s are apt to segregate at the crack. The positive cluster energy of H atoms in t he region of the crack in Ti metal shows that H atoms can not form cluster and h ave the tendency of forming ordered phases (hydrides). When H atoms accumulate o n the surface of the crack, they reduce the surface energy of the crack, making the crack propagation easier. The Fermi energy of the tip of crack is higher th an that in other areas, so electrons move to the perfect region of α-Ti from th e tip of crack, leading to the formation of the potential difference between the crack tip and the perfect region of α-Ti, then the dissolution-corrosion proce ss of crack tip as anode occurs under the action of the electrolyte.

Electronic theory for the refinement mechanism of ultrafine steel

The model of 1/2［110］ edge dislocation in austenite phase was set up with comp uter programming. The environmentsensitive embedding energy (ESE) of C, N and alloy elements in grains or in the area of edge dislocation was calculated by us ing recursion method. The enrichment and interaction of C, N and alloy elements in the area of dislocation were discussed. Calculation results show that: discre te light impurities C and N tend to aggregate in the dislocation area, forming C aldwell air mass above the edge dislocation; the alloy element is liable to dist ribute in perfect austenite grains. Ti, V, Nb and Cr, which are strong or middli ng strong elements for carbide formation, are easy to aggregate in the edge disl ocation area. They can also form the Caldwell air mass above the edge dislocatio n like C or N. But Ni, which is non_carbide formation element, aggregates below the dislocation line or distributed in the perfect grains. The light impurity ca n enforce the aggregation of strong carbide forming elements in the edge disloca tion area. When the temperature is decreased, and when the concentration of C, N and alloy elements reach the high limit of concentration, the C, N compound of alloying elements will precipitate from the matrix in the edge dislocation area of austenite in the steel. These compounds can act as heterogeneous nuclei of au stenite phase in the course of recrystallization, leading to the refinement of a ustenite grains.

The study on the ultrafine mechanism of steels: strain-induced phase-transform ation form austenite to ferrite

The models of interphase boundaries of austenite/ferrite(γ/α) in the undeformde austenite and deformed austenite, and the model of 12[1 1 0] edge dislocation in austenite matrix were set up with computer programming. The interfacial energ y of γ/α interphase boundaries in the undeformde austenite and deformed austen ite, the environment sensitive embedding energies (EESEESE) of C, N a nd microalloying elements Nb, Ti or V (in the form of air mass) in the area of e dge dislocation in austenite were calculated by using recursion method. The refineme nt mechanism of steels in the deformation process was discussed. Interfacial ene rgy calculation results show that the most preferred ferrite nucleation sites ar e the area of high density dislocations such as deformation bands, the austenite grain boundaries and subbounaries, which leads to a considerable increase in th e nucleation rate. This is why ferrite grains formed in the deformed austenite a re far more smaller than those in undeformed austenite. The EESEESE calcu lation results show that C,N microalloying elements (Nb, Ti or V) in form of the air mass, C,N together with microalloying elements (Nb or Ti) in the form air m ass are all easy to aggregate to the area of edge dialocations, which results in the carbonitride precipitation in there (deformation band, grain boundary or gr ain subboundary). The growth of ferrite grains is considerably suppressed by def ormation due to impingement with carbonitride particles and other ferrite grains , which leads to the further refinement of ferrite grains.

A study on the mechanism of the influence of Bi, Sb alloying on microstructure and properties of AZ91 magnesium alloy

The atomic structure model of symmetric ［0001］tilt boundary of α phase in AZ91 magnesium alloy was set up by using the concept of coincidence-site lattice (CSL). The total structure energies of α matrix and grain boundary (GB), the environment sensitive embedding energies (EESE) and the interaction energies of Al, Bi and Sb were calculated by using recursion method. The alloying behaviour of Al, Bi, Sb in AZ91 magnesium alloy was discussed. Calculation results show that the total structure energies of α matrix and grain boundary (GB) are decreased due to the presence of Al, Bi or Sb, which suggests AZ91 alloy is strengthened due to solid solution strengthening. Alloying elements tend to be distributed uniformly in α matrix, and are apt to occupy the top sites of the trigonal prism formed by Mg in the GB core. The aggregation of Bi or Sb to GBs,being easier than Al, suppresses the aggregation of Al, which accelerates the formation of continuous precipitates, improves the high temperature properties. The major alloying element Al and the additional elements Bi or Sb in small amounts can form the ordered phases in AZ91 alloys: Mg１７Al１２, Mg3Bi2 and Mg３Sb２, respectively, and the quantity of the ordered phase is greater in GBs than in α(Mg) matrix. Bi or Sb additions to AZ91 retards the aggregation of Al, and Mg3Bi2 or Mg３Sb２ precipitates along boundaries greatly improve the high temperature properties.

Segregation and interaction of rare earth and iron elements on grain boundaries in ZA27 alloys

The model of liquidphase of ZA27 cast alloy was set up in terms of the molecul ar dynamics simulation. The atomic structural models of the α phaseliquid int erf ace and the grain boundary of α phase with the coincidencesite lattice were c on structed by computer programming. The environmentsensitive embedding energy (E S E) of rare earth (or iron) atom in grain, on grain boundary and on α phaseliq uid interface was calculated by Recursion method. The bond order integrals (BOI) be tween Fe, RE atom and Al were also computed. The results show that RE and iron e lements are more stable on the α phaseliquid interface than in grain, which e xpl ains the fact of very small solid solubility of RE and iron elements in α phase , and the enrichment of RE and iron elements in the solid-liquid growth front whe n solidifying. This leads to the segregation of RE and iron atoms on the grain b oundaries and the formation of the complicated REcompounds.

Electronic Structure and Doping Effect of the Σ11(11-3)/[110] Grain Boundary in Ni

The first-principles discrete variational method is employed to study the effect of boron and phosphorus impurities on the electronic structure of the NiΣ11(113)/[110] grain boundary. The calculated results show that boron slightly decreases the bonding between the host Ni atoms but, on the other hand, it forms a strong bonding state with its Ni neighbours. Phosphorus strongly decreases the bonding between the host Ni atoms. The calculated environment-sensitive embedding energies show that boron has a strong site-competition ability and can successfully drive out phosphorus from the grain boundary region.

Effect of boron and sulphur on the electronic structure of grain boundaries in Ni

The first-principles discrete variational method is employed to study the effect of boron and sulphur on the electronic structure of the Ni Σ11(1 1 3 ̄ )/[1 1 0] grain boundary (GB). The calculated results show that boron does not strongly influence (only slightly decreases) the bonding between the atoms of the metal. In addition, B forms the strong bonding state with its neighbouring metal atoms. Our study also indicates that S strongly decreases the bonding between the atoms of the metal, and that the bonding tendency between S and the atoms of the metal across the GB plane is very weak. The calculations of environment-sensitive embedding energies show B has the strong site-competition ability and can successfully drive out S from the GB region. We conclude that the influence of impurities segregating on the GBs is closely associated with their effects on: (i) the decrease of the bonding between the atoms of the metal due to the presence of impurities; (ii) the bonding between the impurity atom and the atoms of the metal; and (iii) the site-competition ability of impurity atoms.

First-principles calculations of energies of impurities and doping effects at grain boundaries in nickel

The atomic structure of the ∑ 11(11—)/[110] grain boundaries (GB) in Ni is set up by molecular dynamics (MD) simulation using a first-principles potential. The binding energies and the site-competition ability (environment-sensitive embedding energies) of impurities on the GB environment are calculated using the first-principles discrete variational method (DVM). It turns out that B and C have the highest binding energies to the Ni GB and can successfully drive out the harmful impurities from the GB. Our calculations show that P and S embrittle the Ni GB and B, C and N enhance it. We conclude that the influence of impurities on the GB strength is closely associated with their effects on: (i) The decrease of the cohesion between host atoms due to the presence of impurities; (ii) the cohesion between impurity atoms and host atoms; and (iii) the site-competition ability of impurities.

Energetics of Ideal Grain Boundary Fracture in Iron and the Thermodynamic Criterion of Impurity Embrittlement

ABSTRACTModeling of grain boundary (GB) relaxation during ideal fracture, and the fracture energetics of a Σ3 (111) GB in Fe was performed using the modified Finnis-Sinclair semi-empirical method, and utilizing the so-called “environment-sensitive embedding energies” of impurity atoms, introduced earlier by the author. The calculations were done for both the clean GB, and GB with the following impurity atoms: H, B, C, N, O, P and S. The ideal fracture was modeled by separating the two halves of crystal normal to the GB, step-wise, minimizing the total crystal energy at every step. The interplanar distances were varied, while the Fe interatomic spacing within the hexagonal planes was held fixed. When the distance between the two crystal halves: one with the impurity and another without, exceeded the interatomic interaction cut-off radius (3.6 Å), two different free surfaces - with and without the impurity - emerged. The GB and surface energies were found both for the pure Fe, and that with impurity atoms at the GB or free surface. Both the (111) GB energy and the (111) surface energy of pure Fe agree well with experimental data and results of previous semi-empirical modeling. In general, the correlation between the embrittling/ cohesion enhancing effect of impurities in GB and the difference between the GB and free surface energies agrees with the thermodynamic criterion of embrittlement.

Environment Sensitive Embedding Energies of Impurities, and Grain Boundary Stability in Tantalum

AbstractMetalloid impurities have a very low solubility in Tantalum, and therefore prefer to segregate at the grain boundaries (GBs). In order to analyze the energetics of the impurities on the Tantalum GB, the LMTO calculations were performed on a simple 8-atom supercell emulating a typical (capped trigonal prism) GB environment. The so-called “environment-sensitive embedding energies” were calculated for Hydrogen, Boron, Carbon, Nitrogen, Oxygen, Phosphorus, and Sulphur, as a function of the electron charge density due to the host atoms at the impurity site. The calculations showed that, at the electron density typical of a GB, Carbon has the lowest energy (followed by Nitrogen and Boron) and thus would compete with the other impurities for the site on the GB, tending to displace them from the GB. The above energies were then used in a modified Finnis-Sinclair embedded atom approach for calculating the cohesive energies and the equilibrium interplanar distances in the vicinity of a (111) Σ3tilt GB plane, both for the clean GB and that with an impurity. These distances were found to oscillate, returning to the value corresponding to the equilibrium spacing between (111) planes in bulk BCC Tantalum by the 10th-12th plane off the GB. Carbon, Nitrogen and Boron somewhat dampen the deformation wave (making the oscillations less than in the clean GB), while Oxygen, Phosphorus and Sulphur result in an increase of the oscillations. The cohesive energies follow the same trend, the GB with Carbon being the most stable. Thus, Carbon, Nitrogen and Boron may be thought of as being cohesion enhancers, while Oxygen, Phosphorus and Sulphur result in decohesion effects.

Environment Sensitive Embedding Energies of Impurities, and Grain Boundary Relaxation in Iron

ABSTRACTImpurities, such as H, P, S, B, etc, have a very low solubility in iron, and therefore prefer to segregate at the grain boundaries (GBs). In order to analyze the energetics of the impurities on the iron GB, the LMTO calculations were performed on a simple 8-atom supercel 1 emulating a typical (capped trigonal prism) GB environment. The so-called “environment-sensitive embedding energies” were calculated for H, B, C, N, O, Al, Si, P, and S, as a function of the electron charge density due to the host atoms at the impurity site. It was shown that, at the electron charge density typical of a GB, B and C have the lowest energy among the analyzed impurities, and thus would compete with them for the site on the GB, tending to push the other impurities off the GB. The above energies were then used in a modified Finnis-Sinclair embedded atom approach for calculating the equilibrium interplanar distances in the vicinity of a (111) σ3 tilt GB plane, both for the clean GB and that with an impurity. These distances were found to be oscillating, returning to the equilibrium spacing between (111) planes in bulk BCC iron by the 10th-12th plane off the GB plane. H, B, C, N and O actually dampen the deformation wave (making the oscillation amplitudes less than in the clean GB), while, Al, Si, P and S result in an increase of the oscillations. The effect of B, C, N and O may be interpreted as cohesion enhancement; this conclusion supports our earlier first-principles results [1] on B and C.