Atomistic study of diffusion-mediated plasticity and creep using phase field crystal methods

Abstract

The nonequilibrium dynamics of diffusion-mediated plasticity and creep in materials subjected to constant load at high homologous temperatures is studied atomistically using Phase Field Crystal (PFC) methods. Creep stress and grain size exponents obtained for nanopolycrystalline systems, m 1.02 and p 1.98, respectively, closely match those expected for idealized diffusional NabarroHerring creep. These exponents are observed in the presence of significant stress-assisted diffusive grain boundary migration, indicating that Nabarro-Herring creep and stress-assisted boundary migration contribute in the same manner to the macroscopic constitutive relation. When plastic response is dislocation-mediated, power law stress exponents inferred from dislocation climb rates are found to increase monotonically from m 3, as expected for generic climb-mediated natural creep, to m 5.8 as the dislocation density ρ d is increased beyond typical experimental values. Stress exponents m 3 directly measured from simulations that include dislocation nucleation, climb, glide, and annihilation are attributed primarily to these large ρ d effects. Extrapolation to lower ρ d suggests that m 4 − 4.5 should be obtained from our PFC description at typical experimental ρ d values, which is consistent with expectations for power law creep via mixed climb and glide. The anomalously large stress exponents observed in our atomistic simulations at large ρ d may nonetheless be relevant to systems in which comparable densities are obtained locally within heterogeneous defect domains such as dislocation cell walls or tangles. The tendency of a solid material to gradually and irreversibly deform or even flow under low loads at high temperatures is termed creep deformation 1-6 . Low loads and high temperatures in this context are σ σ y and T T m /2, respectively, where σ y is the yield stress and T m is the equilibrium melting temperature of the material. This type of slow plasticity not only alters material microstructure, shape, and properties over extended time periods, but is also a primary cause of mechanical failure in materials such as gas turbine blades that operate in high temperature load bearing environments 1-3 .The plastic flow that occurs during creep can involve conservative defect evolution mechanisms, such as dislocation glide and grain boundary sliding, but is generally facilitated by thermally activated defect motion along local stress and/or chemical potential gradients, and is therefore inherently diffusive in nature. Vacancy diffusion in particular tends to be a central facilitator of deformation, either directly during diffusional creep or indirectly during dislocation or power law creep, as discussed further in the following. Moreover, it is the collective evolution of different defect populations over multiple length and time scales that leads to the observed diverse macroscopic phenomenology of creep. If the characteristic time scales associated with the diffusion of individual defects are almost entirely inaccessible to most at...

Structural evaluation and deformation features of interface of joint between nano-crystalline Fe–Ni–Cr alloy and nano-crystalline Ni during creep process

“…Studies on Coble creep for bcc [27,29], fcc [30,31] and hcp [32] NC metals using molecular dynamics simulation have been reported in literature. The implication of stress-assisted grain boundary migration on diffusional creep deformation process has been investigated to some extent using atomistic simulations [33][34][35][36]. Creep behavior of nano joint using atomistic simulation is not studied till date although creep behavior study for brazing joints using continuum damage mechanics simulation [37,38] and combined creep-plastic behavior investigation of the interface between thermal barrier coatings and bond-coat using constitutive model are reported in literature recently [39].…”

Self-consistent modeling of anisotropic interfaces and missing orientations: Derivation from phase-field crystal

“…It has be shown that the method is derivable from the fundamental classical density functional theory (CDFT) [30,31], with certain approximations based on the Ramakrishnan and Yussouff [32] free energy formalism. Since its inception, the method has evolved to model structural phase transformations [33][34][35][36], alloy systems [30,37,38], multiferroic composite materials [39], order-disorder systems [40], nucleation and polymorphism [41,42], amorphous or glass transitions [43,44], quasicrystals [45] and crystal plasticity [46]. Through coarse graining procedures, the methodology may be used to generate models that operate on mesoscopic length scales.…”

A Unified Constitutive Model for Creep and Cyclic Viscoplasticity Behavior Simulation of Steels Based on the Absolute Reaction Rate Theory

“…Consequently, the operational cycles cause serious thermomechanical fatigues and creep-fatigue failures. An important source of inelasticity in the material structure is creep and viscoplasticity [1,2], which might be produced by microstructural changes [3,4]. Especially within such practical engineering materials, more than one damage mechanism might exist, such as the creepviscoplasticity interaction.…”