Critical Strengths for Slip Events in Nanocrystalline Metals: Predictions of Quantized Crystal Plasticity Simulations

Abstract

This article studies how the monotonic and cyclic stress-strain response of nanocrystalline (NC) metals is affected by the grain-to-grain distribution of critical strengths (s c ) for slip events, as well as plastic predeformation (e pre p ). This is accomplished via finite element simulations that capture large jumps in plastic strain from dislocation slip events-a process referred to as quantized crystal plasticity (QCP). [1] The QCP simulations show that s c and e pre p significantly alter the monotonic and cyclic response at small strain, but only s c affects the response at large strain. These features are exploited to systematically infer the s c and e pre p characteristics that best fit experimental data for electrodeposited (ED) NC Ni. Key outcomes are the following:(1) the s c distribution is truncated, with an abrupt onset of slip events at a critical stress;(2) e pre p = À0.4 pct, signifying precompression; (3) there is reverse slip bias, meaning that reverse slip events are easier than forward events; and (4) highly inhomogeneous residual stress states can be enhanced or reduced by tensile deformation, depending on e pre p .

Probing the Relation Between Dislocation Substructure and Indentation Characteristics Using Quantized Crystal Plasticity

“…The hardness H and indenter load P take the form [24] TavgAcontact -I '^c ~ P ^ I ^contact (10) The first equality defines H and the oc relation acknowledges a Tabor relation [25] between H and flow strength T^vg. The second equality employs the definition of T;,vg from Eqs.…”

“…The FE model geometry is i; = 20 .5, W = T = H= 10 s. Local (a) shear stress and (b) shear strain on an active slip system a =[1][2][3][4][5][6][7][8][9][10][11][011] in a subsurface cell (see shaded cells,Fig. 5(a)), for the quantized Q(800 MPa, 4%) and continuum C(800 MPa) cases, with R= 20 s and W=T=H=^Qs T' and y" proceed in a smooth manner for the Continuous Crystal Plasticity (C) case; however, for the Quantized Crystal Plasticity (Q) case with (800 MPa, 4%), y" jumps by A7p( = 4%) when IT" reaches 800 MPa.…”

Crystalline–Amorphous Nanostructures: Microstructure, Property and Modelling

“…Microscale models with the presumed deformation mechanisms and corresponding physical parameters and functions are able to capture the influence of microstructure characteristics on mechanical responses of NCs [ 138 , 139 , 140 , 141 ]. Figure 16 illustrates a quantized crystal plasticity model that was developed to study the distinctive mechanical behaviors of nanocrystalline metals resulting from grain size reduction [ 142 , 143 ]. This model was motivated by molecular dynamics simulations of dislocation loop propagation across nanograins, which showed that the grain-averaged plastic strain jumps by discrete amounts.…”

X-Ray Diffraction Studies of Forward and Reverse Plastic Flow in Nanoscale Layers During Thermal Cycling

“…During cooling, σ Ni increases ∼linearly by ∼500 MPa as plastic strain increases by ε p,Ni ≈ 0.1%. Gaps in the unloading vs. loading traces have been observed in conventional [39,40] and nanocrystalline metals [41] following complete unloading and/or stress reversal under isothermal conditions. Here, the Ni layers are unloaded to only 60% of the initial stress.…”