Interfacial dislocation network in precipitation strengthened alloys during creep: a discrete dislocation dynamics (DDD) study in three dimensions

Ni-base superalloys show an intricate network of dislocations around γ′ precipitates during high-temperature low-to-intermediate stress creep. With an aim to understand the formation of this interfacial dislocation network on the surfaces of unsheared, cuboidal γ′ precipitates, we perform three-dimensional discrete dislocation dynamics simulations at constant stress in a model system containing superellipsoidal inclusions. The exponents of the superellipsoid are adjusted to fit the cuboidal shape of γ′. We use a fault-energy-based back-force model to describe interactions between dislocations and structurally inhomogeneous inclusions. The model incorporates climb of edge dislocation segments on non-glissile planes through a modified dislocation mobility law for face-centred cubic crystals. Athermal repulsive intersection cross-slip is considered for the screw segments. We systematically show the evolution of dislocation network as a function of applied stress, inter-particle spacing, and ratio of glide-to-climb mobility. We scale the simulation box and the inclusions by the same factor in order to keep the volume fraction of inclusions constant in all cases. Although the dislocation density increases with the increase in applied stress as well as inter-particle spacing, the onset of steady-state in all cases is marked by a constant mobile-to-immobile dislocation density (ρ m/ρ im) ratio. For the range of stresses and inter-particle spacings considered in this study, the steady-state ρ m/ρ im remains nearly the same. Our simulations indicate a power-law behaviour where the stress exponent n ≈ 4 suggests dislocation climb to be the rate-controlling mechanism. The simulated morphological features of the dislocation network formed on the surfaces of the inclusions at steady-state (e.g., hexagonal nets due to dislocation reactions) are similar to those observed experimentally in single-crystalline superalloys crept at high temperatures and low stresses. Moreover, we obtain a relationship between length scale associated with dislocation density and applied stress. © 2021 IOP Publishing Ltd

A phase-field study of domain dynamics in ferroelectric BCT-BZT system

We present a thermodynamically consistent phase-field model describing the free energy of perovskite-based BCT-BZT solid solution containing an intermediate morphotropic phase boundaries. The Landau coefficients are derived as functions of composition of zirconium. The electrostrictive and elastic constants are appropriately chosen from experimental findings. The resulting Landau free energy is constructed to describe the stable polarization states as a function of composition. The evolution of the polarization order parameters at a particular composition is described by a set of time-dependent Ginzburg-Landau (TDGL) equations. Additionally, we solve Poisson's equation and mechanical equilibrium equation to account for the ferroelectric/ferroelastic interactions. We have performed two dimensional and three-dimensional simulations with appropriate electrical boundary conditions to study the effect of external electric field on domain dynamics in BCT-BZT system at the equimolar composition

Realization of rhombohedral, mixed, and tetragonal like phases of BiFeO3 and ferroelectric domain engineering using a strain tuning layer on LaAlO3(001) substrate

BiFeO3 (BFO), a room temperature multiferroic, undergoes a series of structural transformations under varying strain conditions by utilizing appropriate substrates for a specific strain condition. In this study, epitaxial thin films of BFO were grown on La0.7Sr0.3MnO3 +/-delta (LSMO), a strain tuning layer on LaAlO3[LAO (001)] substrates, using pulsed laser ablation. LSMO layers of varying thicknesses from 2 nm to 20 nm were grown followed by a BFO layer of a fixed thickness (20 nm). A strained layer of similar to 2 nm thick LSMO stabilizes the tetragonal like phase of BFO. Increasing the thickness of the LSMO layer to 10 nm results in a mixed phase with rhombohedral (R) and tetragonal (T) domains, and a further increment of the LSMO layer thickness to 20 nm stabilizes the rhombohedral phase of BFO. The tetragonal phase with weak monoclinic distortion possessed 180 degrees domains with dominant out-of-plane polarization components. However, the mixed phase (R + T) possessed various plausible polarization components in both out-of-plane and in-plane directions. Further, a thermodynamically consistent model based on the phase field approach was implemented to investigate the role of strain on the formation of domain patterns with various polarization components and piezoelectric coefficients. The simulated domain structure exhibited a similar transformation on the dominant polarization components as observed in experiments across different phases of BFO. Our simulations show that the elastic constraint along the z-direction enhances the tetragonality of BFO. The piezoelectric (d(33)) coefficient was found to be similar to 46 pm/V for the 20 nm mixed phase BFO, which was nearly a 200% increment compared to the single phase BFO thin films on LAO. Published under license by AIP Publishing

Abstracts of National Conference on Research and Developments in Material Processing, Modelling and Characterization 2020

This book presents the abstracts of the papers presented to the Online National Conference on Research and Developments in Material Processing, Modelling and Characterization 2020 (RDMPMC-2020) held on 26th and 27th August 2020 organized by the Department of Metallurgical and Materials Science in Association with the Department of Production and Industrial Engineering, National Institute of Technology Jamshedpur, Jharkhand, India. Conference Title: National Conference on Research and Developments in Material Processing, Modelling and Characterization 2020Conference Acronym: RDMPMC-2020Conference Date: 26–27 August 2020Conference Location: Online (Virtual Mode)Conference Organizer: Department of Metallurgical and Materials Engineering, National Institute of Technology JamshedpurCo-organizer: Department of Production and Industrial Engineering, National Institute of Technology Jamshedpur, Jharkhand, IndiaConference Sponsor: TEQIP-