Use of a dislocation-based boundary element model to extract crack growth rates from depth distributions of intergranular stress corrosion cracks

A dislocation-based boundary element model was used to simulate intergranular stress corrosion crack propagation in virtual microstructures. A Monte Carlo approach was used in which the propagation of approximately 100 cracks was calculated for different Voronoi generated microstructures. At every simulation step the model gave the position of the crack tip together with stress intensity factors K_I and K_II. Using a simple power-law-type crack growth rate $\mathit{da}/\mathit{dt}=D_p{K}^{m_p}$, the depth of each particular crack can be calculated knowing the time the samples were exposed to the stress and corrosive environment. Existing experimental data giving crack depth distributions for Alloy 600, and XM-19 and 304 stainless steel are investigated and the best-fit crack growth law established. Alloy 600 in a light water reactor environment and XM-19 in high-temperature water both lead to m_p = 3. While for 304 stainless steel in the more aggressive K₂S₄O₆/H₂SO₄ (pH 2) an exponent m_p = 0.8 was found.     

Magnetic and spin-dependent transport properties of reactive sputtered epitaxial Ti1−xCrxN films

Reactive-sputtered epitaxial Ti_1?xCr_xN films are ferromagnetic in the range of 0.17 ? x ? 0.51 due to the Cr–N–Cr double-exchange interaction below the Curie temperature (T_C). The T_C first increases, then decreases as x increases, and a maximum of 120 K appears at x = 0.47. All of the films are metallic with a transition near T_C. A resistivity minimum ρ_min is observed below 60 K in the films with 0.10 ? x ? 0.51 due to the effects of the weak localization and electron–electron interaction. The negative magnetoresistance (MR) is caused by the double-exchange interaction below T_C and the weak localization can also contribute to MR below T_min where ρ_min appears. The x-dependent electron carrier densities reveal that the ferromagnetism is not from the carrier-mediated mechanism. The anomalous Hall resistivity follows the relation of ${\rho }_{\mathit{xy}}^{\text{A}}\propto {\rho }_{\mathit{xx}}^2$, which is from the side-jump mechanism.     

Electrochemical short crack effect in environmentally assisted cracking of a steam turbine blade steel

Measurement has been made of the corrosion fatigue short crack growth rate in a 12Cr steam turbine blade steel subjected to low frequency trapezoidal loading in aerated and deaerated 300 ppb¹ Cl⁻ and 300 ppb ${\text{SO}}_4^{2-}$, simulating early condensate chemistry. No difference in growth rate compared to that for long cracks was observed in deaerated solution but significantly enhanced growth rate was obtained in aerated solution for a short crack of length less than 250 μm. Complementary stress corrosion cracking tests were conducted but to ensure crack development at modest applied stresses the environment adopted was aerated 35 ppm Cl⁻, representing a severe system upset. In this case, the growth rate of the short crack was up to 20 times higher than that for a long crack (>6 mm), even though the crack length had reached 1.6 mm. An explanation for both sets of data based on the difference in potential drop between a short and long crack is expounded.     

Grain boundary diffusion and segregation of Ni in Cu

Grain boundary (GB) diffusion of ⁶³Ni in polycrystalline Cu was investigated by the radiotracer technique in an extended temperature interval from 476 to 1156 K. The independent measurements in Harrison’s C and B kinetic regimes resulted in direct data of the GB diffusivity D_gb and of the so-called triple product P = s · δ · D_gb (s and δ are the segregation factor and the diffusional GB width, respectively). Arrhenius-type temperature dependencies for both the D_gb and P values were measured, resulting in the pre-exponential factors $D_{\mathrm{gb}}^0=6.93\times {10}^{-7}$ m² s⁻¹ and P₀ = 1.89 × 10⁻¹⁶ m³ s⁻¹ and the activation enthalpies of 90.4 and 73.8 kJ mol⁻¹, respectively. Although Ni is completely soluble in Cu, it reveals a distinct but still moderate ability to segregate copper GBs with a segregation enthalpy of about −17 kJ mol⁻¹.