Prediction of Mechanical Behaviour in Ni-Base Superalloys Using the Phase Field Model of Dislocations

The "Phase-Field Model of Dislocations" (PFMD) was used to simulate shearing of gamma-prime precipitate arrays in single crystal turbine blade superalloys. The focus of the work has been on the cutting of the L12 ordered precipitates by a{111} dislocation ribbons during Primary Creep. The Phase Field Model presented incorporates specially developed Generalised Stacking Fault Energy (γ-surface) data obtained from atomistic simulations. The topography of this surface determines the shearing mechanisms observed in the model. The merit of the new γ-surface, is that it accounts for the formation of extrinsic stacking faults, making the model more relevant to creep deformation of superalloys at elevated temperatures

Effective gamma-surfaces in {111} plane in FCC Ni and L1(2) Ni3Al intermetallic compound

The well-known concept of γ surfaces originally suggested by Prof. Vitek is extended to the case of a shift of one part of a crystal with respect to the other part in two adjacent {111} planes of a stacking fault. The proposed approach is used to construct the effective γ surfaces in the {111} plane in fcc Ni and L12 Ni3Al of intermetallic compound. It is shown that Ni3Al can have five metastable stacking faults in this plane, namely, an intrinsic superlattice stacking fault, an extrinsic superlattice stacking fault, a complex stacking fault, an antiphase boundary, and a complex extrinsic stacking fault. The existence of the latter in the {111} plane in the L12 Ni3Al intermetallic compound has not been reported previously. The shift of the crystal in two close-packed planes in fcc Ni makes it possible to avoid the formation of a high-energy stacking fault which arises when constructing an ordinary γ-surface, as well as to self-consistently include the extrinsic stacking fault into consideration. © 2013, Pleiades Publishing, Ltd

MD simulations of collision cascades in the vicinity of a screw dislocation in aluminium

Molecular dynamics has been applied to study the interaction of collision cascades with 1=2h110i screw dislocations in aluminium. Primary knock-on atoms (PKA) with energy EPKA ¼ 5 keV were initiated in Al crystals at temperatures ranging from 100 K to 600 K at different distances from the dislocation line. Capture of the displaced atoms from the collision cascade region by the dislocation core has been observed at all simulation temperatures and led to the formation of a helical dislocation segment. Apart from that, in the material subjected to external loading a displacement cascade in the vicinity of a screw dislocation can trigger dislocation cross-slip. Under applied stress the dislocation unpins from the helical segment with the formation of a mobile dislocation loop with 1=2h110i Burgers vector. The number of residual vacancies and self-interstitial atoms produced in collision cascades near 1=2h110i screw dislocations is evaluated and compared with the number of Frenkel pairs formed in collision cascades in the pristine material under the same irradiation conditions. © 2013 Elsevier B.V

Molecular dynamics simulations of radiation damage in D0(19) Ti3Al intermetallic compound

Molecular dynamics (MD) has been applied to simulate the radiation damage created in displacement cascades in D019 Ti3Al structural intermetallics. Collision cascades formed by the recoil of either Al or Ti primary knock-on atoms (PKA) with energy EPKA = 5, 10, 15 or 20 keV were considered in Ti3Al single crystals at T = 100, 300, 600 and 900 K. At least 24 different cascades for each (EPKA, T, PKA type) set were simulated. A comprehensive treatment of the modelling results has been carried out. We have evaluated the number of Frenkel pairs, fraction of Al and Ti vacancies, self-interstitial atoms and anti-sites as a function of (EPKA; T, PKA type). Preferred formation of both Al vacancies and self-interstitial atoms in D019 Ti3Al exposed to irradiation has been detected.© 2013, Elsevier B.V

Interaction of collision cascades with an isolated edge dislocation in aluminium

The velocity-Verlet molecular dynamics has been applied to study the radiation damage created in collision cascades in an aluminium crystal harbouring an isolated edge dislocation with 1/2〈110〉 Burgers vector. The total of more than 150 displacement cascades formed by the recoil of primary knock-on atoms with energy EPKA=5keV in crystals at temperatures 100K⩽T⩽600K were simulated. Three different mechanisms of the interaction of collision cascades with edge dislocations in aluminium have been observed. At low and room temperatures the dislocations climb by absorption of displaced atoms from the collision cascade region. Capture of vacancies and displaced atoms by dislocation core at high temperatures leads to dislocation climb up and down. At room and especially at high temperatures dislocation climb by absorbing displaced atoms is accompanied by the formation of stacking fault tetrahedra in the vicinity of the dislocation line. In order to quantify the redistribution of radiation defects, the number of residual vacancies and self-interstitials created in collision cascades near edge dislocations was evaluated and compared with the number of Frenkel pairs formed in collision cascades in the pristine material under the same irradiation conditions. © 2012, Elsevier B.V

Continuum and discrete models of dislocation pile-ups. II. Pile-up of screw dislocations at the interface in a bimetallic solid

The methodology developed in the precursor to this paper is used to find the positions of n screw dislocations in a pile-up against an interface bonding two crystalline solids. The pile-up is caused by a constant applied stress and, as n , the dislocations are located with sufficient accuracy to predict the large but finite stress distribution at the interface. Such a prediction is impossible using a conventional continuum dislocation density

Interconnection of continuum and discrete models of dislocation pile-ups

A new asymptotic approach for analysing pile-ups of large numbers of dislocations is described. As an example, the pile-up of n identical screw or edge dislocations on a single slip plane under the action of an external loading in the direction of a locked dislocation in that plane is considered. As n -> infinity the continuum number density of the dislocations can be easily obtained whereas direct evaluation of the discrete dislocation positions from the set of force balance equations is not straightforward. However, in the framework of our method these positions can be revealed using the corresponding dislocation density

Preferential formation of al self-interstitial defects in gamma-TiAl under irradiation

Empirical dynamic calculations were used to observe a distinct increase in aluminium interstitial defects compared to titanium interstitial species remaining after a displacement cascade (even though Frenkel formation energies for both species were found to be similar). Thermodynamic data from static ab-initio models support this interesting result. Calculations were then used to determine whether these interstitials are free to move and therefore have the possibility of migrating to a surface where a passive Al2O3 layer can be formed. © 2012, Elsevier Ltd

Asymptotics of Edge Dislocation Pile-Up against a Bimetallic Interface

The approach developed in preceding papers is extended to derive the equilibrium positions of n edge dislocations in a linear pile-up stressed by a constant applied loading against an interface in a bimetallic solid. As n → ∞, the dislocations in the inner region are located with sufficient accuracy that the stress distribution at the interface can be evaluated by a simple computational procedure. Such a prediction is impossible using a conventional continuum dislocation theory. © 2009 SAGE Publications