Solute Segregation and Grain Boundary Cohesion of Magnesium Binary Alloys: A First-Principles Study

The occurrence of segregation in dilute alloys under irradiation is a highly unusual phenomenon that has recently attracted attention, stimulated by the interest in the fundamental properties of alloys as well as by their applications. The fact that solute atoms segregate in alloys that, according to equilibrium thermodynamics, should exhibit full solubility, has significant practical implications, as the formation of precipitates strongly affects physical and mechanical properties of alloys. A lattice Hamiltonian, generalizing the so-called ‘ABV’ Ising model and including collective many-body inter-atomic interactions, has been developed to treat rhenium solute atoms and vacancies in tungsten as components of a ternary alloy. The phase stability of W–Re-vacancy alloys is assessed using a combination of density functional theory (DFT) calculations and cluster expansion (CE) simulations. The accuracy of CE parametrization is evaluated against the DFT data, and the cross-validation error is found to be less than 4.2 meV/atom. The free energy of W–Re-vacancy ternary alloys is computed as a function of temperature using quasi-canonical Monte Carlo simulations, using effective two, three and four-body interactions. In the low rhenium concentration range (<5 at.Re), solute segregation is found to occur in the form of voids decorated by Re atoms. These vacancy-rhenium clusters remain stable over a broad temperature range from 800 K to 1600 K. At lower temperatures, simulations predict the formation of Re-rich rhenium–vacancy clusters taking the form of sponge-like configurations that contain from 30 to 50 at.Re. The anomalous vacancy-mediated segregation of Re atoms in W can be rationalized by analyzing binding energy dependence as a function of Re to vacancy ratio as well as chemical Re–W and Re-vacancy interactions and short-range order parameters. DFT calculations show that rhenium–vacancy binding energies can be as high as 1.5 eV if the rhenium/vacancy ratio is in the range from 2.4 to 6.6. The predicted Re clustering agrees with experimental observations of precipitation in self-ion irradiated W-2 Re alloys and neutron-irradiated alloys containing 1.4 at.Re.

Computer models for the prediction of solute segregation in alloys under irradiation require expressions for the diffusion coefficients which appear in the generalized Fick equations used in these models. These expressions are obtained by analyses of atomic transport caused by the movement of vacancies and interstitials generated by the radiation. The present paper provides expressions for the required diffusion coefficients (ten in number) in terms of the properties of atomic models which are appropriate for many bcc transition metals including α-Fe.

The state of knowledge of radiation-induced solute segregation (RIS) in alloys is reviewed, with special emphasis on solute segregation in steels. Background information is summarized and some theoretical perspective is provided. Examples illustrating the factors affecting RIS are presented and some important outstanding issues that impact the current understanding of RIS are discussed.

Void formation and solute segregation in ion-irradiated niobium-base alloys

Effect of solute segregation on the intrinsic stacking fault energy of Co-based binary alloys: A first-principles study

Solute segregation and interface stability in ODS ferritic alloys: A first-principles study with experimental validation

Solute segregation and precipitation in dilute alloys during irradiation have been studied by means of the Johnson-Lam kinetic model. The model is based on a combination of chemical reaction rates and diffusion equations for free defects, solutes, and bound defect-solute complexes. The enrichment of solute at sink surfaces and solute depletion in the matrix have been calculated as functions of temperature, damage rate, defect-solute bind-ing energy, and initial solute concentration. Using parameters appropriate for Be in Ni, significant solute segregation is found in the temperature range from 0.2 to 0.7 Υm. The temperature for maximum segregation is higher for the high displacement rates typically used in charged-particle bombardment experiments than for the low displacement rates used in fast-reactor irradiations. The solute concentration at the sink surface builds up at high temperatures, without surpassing the solubility limit, until a steady state is at-tained. However, at lower temperatures solute enrichment at sinks becomes larger and the solubility, in general, becomes lower. Precipitation will occur when the local solute concentration reaches that of the phase boundary. The solute concentration at the precipi-tate-matrix interface is determined by the solubility limit, and precipitation continues until the matrix is sufficiently solute-depleted to achieve a steep concentration gradient that will balance the defect-induced solute flow by back-diffusion. Hence, the steady-state matrix composition is determined by radiation conditions and is independent of the initial alloy composition when precipitation occurs. The solute depletion at steady state is more severe at low displacement rates than at high rates. The calculations are quali-tatively compared with recent experimental observations of the temperature and com-positional dependence of solute precipitation in the Ni-Be system.

Ni solute segregation and associated plastic deformation mechanisms into random FCC Ag, BCC Nb and HCP Zr polycrystals

Increased interest has been paid to grain boundary segregation in alloy K-500 due to severe intergranular cracking recently observed in forged bars. However, little systematic study of this segregation has been performed so far. A detailed auger electron spectroscopy (AES) study of grain boundary segregation in alloy K-500 has been carried out as a function of alloy chemistry. To determine C segregation, the C and O contamination rates in a vacuum chamber were measured and the necessary condition for C grain boundary segregation determination was established. It has been found that severe C, Al, and Cu segregation to grain boundaries occurred and depended on alloy chemistry. High bulk Ni and low bulk Al promoted C and Al grain boundary segregation, and low bulk Ni and high bulk Al significantly enhanced Cu segregation to grain boundaries. The depth profiles of intergranularly segregated elements also showed different features for high and low Ni content alloys. In high Ni alloys, C and Al levels dropped continuously as a function of distance from the grain boundaries but the Cu level dropped only slightly. In low Ni alloys, the Al and C levels rose from relatively low grain boundary levels to a peak at a certain distance from the grain boundary where the high grain boundary Cu level dramatically dropped. Transmission electron microscope (TEM) observation revealed a grain boundaryγ′-depleted zone followed by a region with coarser and denserγ′ particles in low Ni and high Al alloys but quite uniformly distributedγ′ particles with no depleted zone in high Ni and low Al alloys. These can be explained by the observed segregation behavior. The occurrence of Cu segregation is explained according to available theories about surface segregation in binary Ni-Cu alloys, and the segregation of C and Al to grain boundaries is suggested to be probably due to their interaction with Ni and Cu.

Defect buildup and solute segregation in alloys under pulsed irradiation

On the thermal stability and grain boundary segregation in nanocrystalline PtAu alloys

ABSTRACTHomogenisation has been used to eliminate solute segregation of CrMnFeCoNi high-entropy alloys (HEAs) before thermomechanical processing. To overcome the deficiencies caused by high-temperature homogenisation, we cancelled the homogenisation and studied the solute segregation feature and its effect on the grain growth and the mechanical property. The HEA forms equiaxed grains with the solute segregation in the range of tens of micrometres after cold rolling and recrystallisation. The grain growth in recrystallisation still abides by the classical grain growth kinetics, but with a higher power index of 3.33 and activation energy of 392.4 kJ mol−1 than those of the homogenised HEA, indicating a solute-drag effect. The relationship between the yield stress and grain size follows the Hall–Petch dependence with a higher intrinsic strength.

Ti-Al alloys have excellent high-temperature performance and are often used in the manufacture of high-pressure compressors and low-pressure turbine blades for military aircraft engines. However, solute segregation is easy to occur in the solidification process of Ti-Al alloys, which will affect their properties. In this study, we used the quantitative phase-field model developed by Karma to study the equiaxed dendrite growth of Ti-4.5% Al alloy. The effects of supersaturation, undercooling and thermal disturbance on the dendrite morphology and solute segregation were studied. The results showed that the increase of supersaturation and undercooling will promote the growth of secondary dendrite arms and aggravate the solute segregation. When the undercooling is large, the solute in the root of the primary dendrite arms is seriously enriched, and when the supersaturation is large, the time for the dendrite tips to reach a steady-state will be shortened. The thermal disturbance mainly affects the morphology and distribution of the secondary dendrite arms but has almost no effect on the steady-state of the primary dendrite tips. This is helpful to understand the cause of solute segregation in Ti-Al alloy theoretically.

Solute segregation and precipitate stability in irradiated alloys

Radiation-induced solute segregation in metallic alloys