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.     

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.     

Effect of DC electric field on the tensile deformation of ultrafine-grained 3Y-TZP at 1450–1600 °C

Two test types were employed to determine the effect of DC electric field E = 5–8.3 V cm⁻¹ applied in the loading direction on the tensile flow stress of 3Y-TZP (d_o = 0.44 μm) at 1450–1600 °C. In type a, the field was applied continuously throughout the tensile test; in type b, the field was alternately applied and removed during the course of the tensile test. The field reduced the flow stress in both test types, but with an appreciably larger effect in the type a tests. The field, however, had no discernable effect on the stress exponent $n=\mathrm{\partial }\log \dot{\epsilon }/\mathrm{\partial }\log \sigma$ determined by strain rate change tests. The reduction in flow stress in the type b tests reversed upon removal of the field. It was interpreted to result from both Joule heating and a decrease in the energy to form the rate-controlling Zr⁴⁺ vacancies in the space charge zone adjacent to the grain boundaries. The calculated width of the space charge zone λ (=1.5–7.8 nm) and the ratio of the applied electric potential to the reduction in stress Δϕ_a/Δσ (=18–19 μV/MPa) were in accord with predictions. The larger reduction in the flow stress in the type a tests is attributed to a greater retardation of grain growth by the field.