IP: 207.241.225.236 On: Thu, 25 Nov 2021 14:54:31 Copyright: American Scientific Publishers Delivered by Ingenta Materials Express Article Copyright © 2021 by American Scientific Publishers All rights reserved. Printed in the United States of America 2158-5849/2021/11/1841/015 doi:10.1166/mex.2021.2091 www.aspbs.com/mex Nanograin size effects on deformation mechanisms and mechanical properties of nickel: A molecular dynamics study Alexandre Melhorance Barboza * , Ivan Napoleão Bastos, and Luis César Rodríguez Aliaga Universidade do Estado do Rio de Janeiro, UERJ, Instituto Politécnico, Rua Bonfim, 25. 28625-570, Nova Friburgo, Rio de Janeiro, Brazil ABSTRACT The grain size refinement of metallic materials to the nanometer scale produces interesting properties compared to the coarse-grained counterparts. Their mechanical behavior, however, cannot be explained by the classical deformation mechanisms. Using molecular dynamics simulations, the present work examines the influence of grain size on the deformation mechanisms and mechanical properties of nanocrystalline nickel. Samples with grain sizes from 3.2 to 24.1 nm were created using the Voronoi tessellation method and simulated in tensile and relaxation tests. The yield and ultimate tensile stresses follow an inverse Hall-Petch relationship for grain sizes below ca. 20 nm. For samples within the conventional Hall-Petch regime, no perfect dislocations were observed. Nonetheless, a few extended dislocations were nucleated from triple junctions, suggesting that the suppression of conventional slip mechanism is not uniquely responsible for the inverse Hall-Petch behavior. For samples respecting the inverse Hall-Petch regime, the high number of triple junctions and grain boundaries allowed grain rotation, grain boundary sliding, and diffusion-like behavior that act as competitive deformation mechanisms. For all samples, the atomic configuration analysis showed that Shockley partial dislocations are nucleated at grain boundaries, crossing the grain before being absorbed in opposite grain boundaries, leaving behind stacking faults. Interestingly, the stress relaxation tests showed that the strain rate sensitivity decreases with grain size for a specific grain size range, whereas for grains below approximately 10 nm, the strain rate sensitivity increases as observed experimentally. Repeated stress relaxation tests were also performed to obtain the effective activation volume parameter. However, the expected linear trend in pertinent plots required to obtain this parameter was not found. Keywords: Nanocrystalline Nickel, Molecular Dynamics, Deformation Mechanisms, Hall-Petch, Strain Rate Sensitivity. 1. INTRODUCTION The relationship between yield stress and grain size for conventional polycrystalline metals and alloys can be described by the classic Hall-Petch (HP) relationship [1, 2] in which a reduction in grain size causes an increase in material’s strength due to decreasing intra-granular sources ∗ Author to whom correspondence should be addressed. and movement of dislocations [3]. Nanocrystalline (NC) materials, in its turn, are a particular case of polycrys- talline materials wherein the average grain size, d, is smaller than 100 nm. These class of materials exhibit altogether different or superior properties than those of large grain size materials [4]. Regarding mechanical prop- erties, one striking feature of NC materials is the deviation from the HP relationship [5]. For smaller grains, usually below 20 nm [6], the results are controversial; some works Mater. Express, Vol. 11, No. 11, 2021 1841