The effect of Cr alloying on defect migration at Ni grain boundaries

Mass transport along grain boundaries in alloys depends not only on the atomic structure of the boundary, but also its chemical make-up. In this work, we use molecular dynamics to examine the effect of Cr alloying on interstitial and vacancy-mediated transport at a variety of grain boundaries in Ni. We find that, in general, Cr tends to reduce the rate of mass transport, an effect which is greatest for interstitials at pure tilt boundaries. However, there are special scenarios in which it can greatly enhance atomic mobility. Cr tends to migrate faster than Ni, though again this depends on the structure of the grain boundary. Further, grain boundary mobility, which is sometimes pronounced for pure Ni grain boundaries, is eliminated on the time scales of our simulations when Cr is present. We conclude that the enhanced transport and grain boundary mobility often seen in this system in experimental studies is the result of non-equilibrium effects and is not intrinsic to the alloyed grain boundary. These results provide new insight into the role of grain boundary alloying on transport that can help in the interpretation of experimental results and the development of predictive models of materials evolution.

Molecular dynamics study of the influence of uniaxial deformation on the migration velocity of tilt boundaries in nickel

The molecular dynamics method was used to study the influence of elastic uniaxial deformation on the migration velocity of tilt boundaries with misorientation axes [100] and [111] in nickel. The dependences of the migration velocity at a temperature of 1600 K on the misorientation angle were obtained. It is shown that the high-angle [100] and [111] tilt boundaries migrate at approximately the same velocity, while the low-angle [111] boundaries migrate approximately twice as fast as the [100] boundaries. The obtained dependences of the migration velocity of the boundaries on the value of uniaxial deformation in almost all cases turned out to be nonmonotonic and had a maximum at a tension value of about 1%. With a further increase in tension, migration slowed down, which is most likely explained by a decrease in the surface tension of the boundaries and, accordingly, in the driving force due to the finite sorption capacity of grain boundaries with respect to the free volume. Under elastic compression, in most cases, a monotonic decrease in the migration velocity was observed, which is due to a decrease in free space during compression and a decrease in the mobility of atoms at the boundary.

Direct insight into the structure-property relation of interfaces from constrained crystal structure prediction

A major issue that prevents a full understanding of heterogeneous materials is the lack of systematic first-principles methods to consistently predict energetics and electronic properties of reconstructed interfaces. In this work we address this problem with an efficient and accurate computational scheme. We extend the minima-hopping method implementing constraints crafted for two-dimensional atomic relaxation and enabling variations of the atomic density close to the interface. A combination of density-functional and accurate density-functional tight-binding calculations supply energy and forces to structure prediction. We demonstrate the power of this method by applying it to extract structure-property relations for a large and varied family of symmetric and asymmetric tilt boundaries in polycrystalline silicon. We find a rich polymorphism in the interface reconstructions, with recurring bonding patterns that we classify in increasing energetic order. Finally, a clear relation between bonding patterns and electrically active grain boundary states is unveiled and discussed.

Молекулярно-динамическое исследование влияния концентрации вакансий на скорость миграции границ наклона в никеле

The influence of vacancy concentration on the migration rate of high-angle tilt boundaries with misorientation axes <111> and <100> in nickel was studied by the method of molecular dynamics. It is shown that the dependence of the migration rate on the concentration of vacancies introduced at the initial stage of modeling has a maximum near 1%. The decrease in the migration rate with a further increase in the free volume is mainly due to the deceleration of the boundary by low-mobile vacancy clusters, which at high vacancy concentrations the boundary is no longer capable of sorbing. The second reason for the decrease in the migration rate with an increase in the concentration of vacancies above 1% is a decrease in the surface tension of grain boundaries and, accordingly, the driving force of their migration due to the finite sorption capacity of the boundaries with respect to the excess free volume.

Влияние концентрации вакансий на скорость миграции границ наклона в никеле: молекулярно-динамическое моделирование

The effect of vacancy concentration on the migration rate of high-angle tilt boundaries with misorientation axes <111> and <100> in nickel was studied by the method of molecular dynamics. It is shown that the dependence of the migration rate of grain boundaries on the vacancy concentration is nonmonotonic and has a maximum at a concentration of vacancies introduced at the initial stage of about 1%. With a further increase in concentration, especially above 4%, the migration rate of the considered boundaries decreased as a result of deceleration of boundaries by low-mobility vacancy clusters attached to it, which the boundary could no longer sorb, as in the case of relatively small clusters.