Embrittlement of Metal by Solute Segregation-Induced Amorphization

Abstract

Impurities segregated to grain boundaries of a material essentially alter its fracture behavior. A prime example is sulfur segregation-induced embrittlement of nickel, where an observed relation between sulfur-induced amorphization of grain boundaries and embrittlement remains unexplained. Here, 48 Â 10 6 -atom reactive-force-field molecular dynamics simulations provide the missing link. Namely, an orderof-magnitude reduction of grain-boundary shear strength due to amorphization, combined with tensilestrength reduction, allows the crack tip to always find an easy propagation path. DOI: 10.1103/PhysRevLett.104.155502 PACS numbers: 62.20.mm, 81.05.Bx, 81.07.Bc The modification of chemical bonds due to a small amount of impurities segregated to grain boundaries (GBs) controls the mechanical properties of materials [1]. Despite decades of intense experimental [2,3] and theoretical [4][5][6] efforts, however, mechanisms of such GB mechanochemistry [7] are not well understood. In the case of sulfur (S) segregation-induced embrittlement of nickel (Ni), which is important for the development of next-generation nuclear reactors [8], Heuer et al. performed tensile tests of notched specimens with varying amount of S segregation to GBs [3]. (The maximum range of S segregation was determined to be 0.5 nm on either side of a GB.) They observed a transition from transgranular ductile fracture to intergranular brittle fracture at a critical S concentration of 15:5 AE 3:4% at GBs. In addition, they measured another critical S concentration for amorphization of Ni face-centered cubic (fcc) crystal during S þ -ion implantation [3]. The critical S concentration of 14:2 AE 3:3% for amorphization coincides with the critical S concentration for GB embrittlement within experimental error. These experiments clearly demonstrate an essential relation between amorphization and embrittlement. The central question is: What are the atomistic mechanisms that relate amorphization to embrittlement?Answering this question poses a so called multiscale simulation challenge [9], i.e., coupling quantummechanical accuracy to describe solute chemistry with large length scales to incorporate long-range stress fields and microstructures such as grains and amorphous GB phases. Recent developments in chemically reactive atomistic simulation methods and parallel computing technologies that are scalable over 10 5 processors (see [10], Fig. S1) [11], combined with those in nanomechanical experiments [12], have set the stage to address this challenge. Namely, simulations and experiments can now study the mechanical properties of nanoscale materials at the same length scale. Here, we perform large molecular dynamics (MD) simulations (see [10], Supplementary Methods) based on reactive force fields (REAXFF) [13,14], which are trained and validated (Supplementary Tables S1 and S2,[10]) by quantum-mechanical calculations based on the density functional theory [15], in order to study the effect of S segregation in nanocrystalline Ni.We first investigate the effe...

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“…The reduction of the interfacial mobility of Ni with the addition of S is surprising given the well-known embrittlement of Ni materials 77 due to the presence of S impurities in the grain boundaries of polycrystalline Ni, an effect suggesting that in this situation, S might be greatly increasing the GB interfacial mobility, thus compromising the structural integrity of Ni polycrystalline materials. To better understand the trend in Figs.…”

The ReaxFF reactive force-field: development, applications and future directions

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