Disorder-induced melting in nickel: implication to intergranular sulfur embrittlement

Heuer, J. K.; Okamoto, P.R.; Lam, Nghi Q.; Stubbins, James F.

doi:10.1016/s0022-3115(02)00707-9

Cited by 42 publications

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“…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.…”

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“…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].…”

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Embrittlement of Metal by Solute Segregation-Induced Amorphization

et al. 2010

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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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Embrittlement of Metal by Solute Segregation-Induced Amorphization

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“…This is in agreement with experimental results; the tensile strength of bulk fcc Ni finally become about one-tenth of its original strength by introducing sulfur. 12) (5) For more than one atomic layer's segregation (7.2 atom/nm 2 ), the sulfur atoms begin to neighbor each other. Over this concentration, the decreasing rate in tensile strength and cohesive energy becomes slightly larger; this trend is more apparent for Ni than Fe.…”

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“…In other words, the enegy gain by stabilization of S atom on the metal surface compensates the energy loss of S-S repulsion in the grain boundary, and therefore keeps the average segregation energy large. Figure 9 shows the comparison between calculated tensile strength and experimental ultimate tensile strength 12) of fcc Ni. Intergranular concentration is defined as concentration within 0.5 nm region from the grain boundary plane in both sides.…”

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Grain Boundary Decohesion by Sulfur Segregation in Ferromagnetic Iron and Nickel —A First-Principles Study—

2006

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Using first-principles calculations, we simulate grain boundary decohesion (embrittlement) in ferromagnetic bcc FeAE3(111)[1 "1 10] and fcc NiAE5(012)[100] symmetrical tilt grain boundaries by progressively adding sulfur atoms to the boundaries. We calculate the segregation energy of sulfur atom, tensile strength, and cohesive energy of the grain boundaries. We show that a certain amount of sulfur segregation (two atomic layers, 14.4 atom/nm 2 ) is energetically possible to realize considering the calculated segregation energies. At this concentration, the tensile strength and the cohesive energy of the grain boundaries reduce by one order of magnitude comparing with no segregation case.

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“…This conclusion is only valid if the segregation process starts and terminates instantly, and we know that segregation can take hours or days (2). As shown in (1), when GB0 (GB2) sites are occupied by S, the binding energy of GB2 (GB0) sites reduces greatly as a result of the repelling interaction between S atoms.…”

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Comment on "Grain Boundary Decohesion by Impurity Segregation in a Nickel-Sulfur System"

Wang

Olson

2005

Science

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Yamaguchi et al.(1) examined the embrittlement of nickel (Ni) by progressively adding sulfur (S) atoms to a grain boundary (GB). From first-principles calculations, they concluded that S atoms tend to aggregate at the GB and that the repulsive S-S interactions induce boundary expansion, thus weakening Ni-Ni binding across the boundary. The agreement between their calculated critical S concentration and the measured data (2) suggests that the GB embrittlement of Ni is due to the aggregation of S segregants. Although we believe that the first-order calculations of Yamaguchi et al.(1) are reliable, we question the interpretations of the calculated binding energies and argue that the distribution of S near the GB remains uncertain.According to the Yamaguchi et al. calculations, the average binding energy for a S atom on site GB0/GB2 ( Fig. 1) with a 100% occupation is -4.75/-4.66 eV, and the binding energy drops to -4.23 eV when both GB0 and GB2 sites are fully occupied. With the calculated segregation energy DE seg , which is defined as the energy lowering when an impurity moves from inner bulk (binding energy 0 -2.96 eV/S) to the GB region, Yamaguchi et al. estimated the occupation number using McLean_s equation of equilibrium segregation (3), C GB 0 EC bulk exp(-DE seg /RT )^/ E1 þ C bulk exp(-DE seg /RT )^, with the impurity concentration in the bulk and the temperature as parameters. They noted that a binding energy of -4.23 eV is large enough for S atoms to segregate fully to the GB0 and GB2 sites, and went on to discuss the volume expansion effect of such GB0-GB2 S combinations.This conclusion is only valid if the segregation process starts and terminates instantly, and we know that segregation can take hours or days (2). As shown in (1), when GB0 (GB2) sites are occupied by S, the binding energy of GB2 (GB0) sites reduces greatly as a result of the repelling interaction between S atoms. For instance, if an S monolayer is formed at GB0 sites first, then the binding energy for a 1/4 monolayer of S at GB2 sites decreases from -4.67 to -3.45 eV. The occupation probability is only on the order of 1% under the experimental conditions in (2) (T 0 918 K and C bulk 0 25 atomic parts per million), according to McLean_s equation. This means that a high concentration of GB0-GB2 pairs is unlikely to appear at the Ni GB.To conduct a more comprehensive search for S-S pairs at the NiS5 (012) tilt GB, we calculated the binding energy of S in the form of both GB0-GBn (n 0 1, 3, 4, 5, 6) and GB2-GBm (m 0 1, 3, 4, 5, 6, -2, -3, -4, -5, -6) pairs at one monolayer concentration using the same code EVienna Ab initio Simulation Package (4)^and parameters reported in (1). Our calculations demonstrate that although the GBn (n 0 3, 4, 5, 6) site is still less stable than the GB0 site, the binding energy difference is greatly reduced when GB0 is occupied by S. For example, a GB0-GB3 pair is 0.05 eV less stable than a GB0-GB2 pair; in an isolated 1/4 monolayer, S at the GB3 site is 1.16 eV less stable than at the GB2 site (1). Interesti...

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Disorder-induced melting in nickel: implication to intergranular sulfur embrittlement

Cited by 42 publications

References 21 publications

Embrittlement of Metal by Solute Segregation-Induced Amorphization

Embrittlement of Metal by Solute Segregation-Induced Amorphization

Grain Boundary Decohesion by Sulfur Segregation in Ferromagnetic Iron and Nickel —A First-Principles Study—

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Disorder-induced melting in nickel: implication to intergranular sulfur embrittlement

Cited by 42 publications

References 21 publications

Embrittlement of Metal by Solute Segregation-Induced Amorphization

Embrittlement of Metal by Solute Segregation-Induced Amorphization

Grain Boundary Decohesion by Sulfur Segregation in Ferromagnetic Iron and Nickel &mdash;A First-Principles Study&mdash;

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Grain Boundary Decohesion by Sulfur Segregation in Ferromagnetic Iron and Nickel —A First-Principles Study—