Yan-Bin Wang

In-situ TEM observation of dissolution-enhanced dislocation emission, motion and the nucleation of SCC for Ti24Al11Nb alloy in methanol

Many experiments have showed that anodic polarization can facilitate ambient creep for various metals and alloys. Anodic polarization can decrease the yield strength, and enlarge the plastic zone on the surface ahead of a loaded crack tip. The density of dislocations in a region very close to the fracture surface of SCC was much higher than that far away from the fracture surface. All of these experiments showed that anodic dissolution facilitated the localized plastic deformation. Up to now, however, direct proof of anodic dissolution or corrosion-enhanced dislocation emission, multiplication and motion is lacking. Using a special constant deflection device, the dislocation configuration change ahead of a loaded crack tip before and after anodic dissolution together with the initiation of SCC for Ti-24Al-11Nb alloy in methanol can be in-situ observed in TEM. The results showed that the localized anodic dissolution could facilitate dislocation emission, multiplication and motion and SCC with size of nanometers would initiate in the dislocation-free zone (DFZ) or at the original crack tip after the dissolution-enhanced dislocation emission and motion reached a critical condition.

Threshold stress intensity for hydrogen—Induced cracking of tubular steel

The susceptibility of tubular steels to hydrogen-induced cracking (HIC) depends on metallurgical as well as environmental factors. Hydrogen atoms, produced as a result of corrosion of the inside wall, diffuse through the pipewall and are trapped at heterogeneous sites in the steel. When the hydrogen reaches a critical concentration at some site, which depends on the composition and microstructure of steel, blistering and/or hydrogen-induced cracking (HIC) will occur. The critical concentration of diffusible hydrogen for blistering in the tubular steel was C{sub th} = 8.38 ppm. Hydrogen-induced fracture under constant load could occur even though the concentration of diffusible hydrogen, C{sub 0}, was less than the C{sub th}. The threshold stress intensity for HIC in the tubular steel decreased with the increase in diffusible hydrogen concentrations, C{sub 0}, i.e., K{sub IH}(MPam{sup 1/2}) = 46 {minus} 12.5lnC{sub 0}(ppm).

Investigation of fractal dimensions of hydrogen-induced brittle fracture of titanium aluminide

Abstract A relation between the stress intensity factor K I ∗ required for brittle crack initiation and propagation and the fractal dimension DF of the fracture surface was derived, i.e. ln K I ∗ =( 1 2 ) ln 2γE′ + ( 1 2 ) ln (d f /L 0 )(1−D F ) where df is the fracture unit, L0 is the length of the straight projection line of the fracture profile, γ is the real surface energy, and E′ = E (plane stress) or E/(1 − v2) (plane strain).The real surface energy can be calculated on the basis of the measured linear relation on ln K I ∗ vs.Df. The equation is not only suitable for overload fracture but also for delayed fracture, e.g. hydrogen-induced cracking and stress corrosion cracking. The experiment results showed that the hydrogen-induced delayed cracking occured in Ti-24Al-11Nb alloy during dynamic charging, and the threshold stress intensity factor was very low, i.e.KIH/KIC = 0.43. The experimental relation between the stress intensity factor K I ∗ and DF was consistent with the theoretical equation.

Interphase stress corrosion crack

A kind of interphase SCC has been found for the Ti-24Al-11Nb alloy tested in the methanol solution. Only the furnace cooled specimens with lower KISCC initiated and propagated preferentially along the α2β interphase boundary or nucleated simultaneously along the interphase boundary and in the individual phase rather than propagation through the α2β interface.

Intergranular stress study of TC11 titanium alloy after laser shock peening by synchrotron-based high-energy X-ray diffraction

The distribution of residual lattice strain as a function of depth were carefully investigated by synchrotron-based high energy X-ray diffraction (HEXRD) in TC11 titanium alloy after laser shock peening (LSP). The results presented big compressive residual lattice strains at surface and subsurface, then tensile residual lattice strains in deeper region, and finally close to zero lattice strains in further deep interior with no plastic deformation thereafter. These evolutions in residual lattice strains were attributed to the balance of direct load effect from laser shock wave and the derivative restriction force effect from surrounding material. Significant intergranular stress was evidenced in the processed sample. The intergranular stress exhibited the largest value at surface, and rapidly decreased with depth increase. The magnitude of intergranular stress was proportional to the severity of the plastic deformation caused by LSP. Two shocks generated larger intergranular stress than one shock.The distribution of residual lattice strain as a function of depth were carefully investigated by synchrotron-based high energy X-ray diffraction (HEXRD) in TC11 titanium alloy after laser shock peening (LSP). The results presented big compressive residual lattice strains at surface and subsurface, then tensile residual lattice strains in deeper region, and finally close to zero lattice strains in further deep interior with no plastic deformation thereafter. These evolutions in residual lattice strains were attributed to the balance of direct load effect from laser shock wave and the derivative restriction force effect from surrounding material. Significant intergranular stress was evidenced in the processed sample. The intergranular stress exhibited the largest value at surface, and rapidly decreased with depth increase. The magnitude of intergranular stress was proportional to the severity of the plastic deformation caused by LSP. Two shocks generated larger intergranular stress than one shock.