Embrittlement of Metal by Solute Segregation-Induced Amorphization

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, $48\times {10}^6$-atom reactive-force-field molecular dynamics simulations provide the missing link. Namely, an order-of-magnitude reduction of grain-boundary shear strength due to amorphization, combined with tensile-strength reduction, allows the crack tip to always find an easy propagation path.

Figure 1

S-induced amorphization of Ni crystal. (a) Radial distribution function $g(r)$ of Ni fcc crystal with 0% and 24% S doping. (b) Full width at half maximum (FWHM) of the first peak of $g(r)$ as a function of S concentration. (c) Bond angle distribution of Ni fcc crystal with 24% S doping and that of amorphous Ni.

Figure 2

Crossover of fracture modes caused by amorphous sulfide intergranular phases in nanocrystalline Ni. (a) A snapshot of MD simulation, where prenotched nanocrystalline Ni is fractured by applying uniaxial strain. (b) and (c) are close-ups of the fracture simulations for NC-Ni and NC-$\mathrm{Ni}+20\%\mathrm{S}$, respectively, where gray, blue and yellow colors represent Ni atoms inside grains ($>0.5\mathrm{nm}$ from GBs), Ni atoms within 0.5 nm from GBs, and S atoms, respectively. White arrows in (b) point to transgranular fracture surfaces.

Figure 3

Grain-boundary shear-strength reduction by S-induced amorphization. Generalized stacking fault energy (GSFE) of Ni $\Sigma 5(012)$ GB without S (a), with a monolayer of segregated S (b), and with amorphous sulfide GB phase (c). Also shown are shear stress calculated from the derivative of the GSFE with respect to displacement along the [100] direction without S (d), with a monolayer of segregated S (e), and with amorphous sulfide GB phase (f). (g) Schematic showing the sliding of one grain with respect to the other to calculate GSFE. (h) Schematic of opening (mode-I) and sliding (mode-II) fractures in front of the main crack.

Figure 4

Ductility of nanocrystalline Ni with and without S segregation. (a) Close-up of fractured NC-Ni, where the atoms are color-coded according to the common neighborhood parameter (blue represents atoms with perfect fcc structure). Green stripes represents twinning boundaries. (b) The inset magnifies a deformation twinning in one of the grains, where yellow and blue colors denote the twinning boundary and perfect atoms, respectively. (c) Number of intragrain defect atoms for NC-Ni (red) and NC-$\mathrm{Ni}+20\%\mathrm{S}$ (blue). (d) Stress-strain relation ahead of the notch for NC-Ni (red) and NC-$\mathrm{Ni}+20\%\mathrm{S}$ (blue).