Chu Wuyang

Hydrogen-facilitated corrosion and stress corrosion cracking of austenitic stainless steel of type 310

The effects of hydrogen précharge and stress on anodic dissolution for Type 310 austenitic stainless steel (ASS) have been investigated. An experiment determining the effect of hydrogen on stress corrosion cracking (SCC) was carried out in a boiling 42 pct MgCl2 solution and in a 2.5 mo/L H2SO4 + 1 mol/L HC1 solution. The results showed that both hydrogen and stress would increase the dissolution rate, and the effects of hydrogen and stress on the dissolution rate were synergistic rather than simply additive. Hydrogen lowered the threshold stress and the shortened fracture time of SCC in a boiling MgCl2 solution by a factor of 1/5 and 10, respectively.

The mechanism of nucleation of hydrogen blister in metals

The nucleating, growing and cracking of hydrogen blister have been investigated experimentally and theoretically. The results show that atomic hydrogen induces superabundant vacancies in metals. The superabundant vacancies and hydrogen aggregate into a hydrogen-vacancy cluster (microcavity). The hydrogen atoms in the microcavity become hydrogen molecules which can stabilize the cluster. And the hydrogen blister nucleates. With the entry of vacancies and hydrogen atoms, the blister nucleus grows and the pressure in the cavity increases. When the stress induced by hydrogen pressure on the blister is up to the cohesive strength, cracks will initiate from the wall of the blister.

Investigation of stress corre- sion cracking under anodic dissolution control

This review is about stress corrosion cracking (SCC) under anodic dissolution control. The section 1 is the methods distinguishing SCC controlled by anodic dissolution from those by hydrogen. The section 2 presents hydrogen-enhanced corrosion and SCC under anodic dissolution control. The section 3 demonstrates corrosion-enhanced localized plasticity and corrosion-induced deformation localization, which are the fundaments of new mechanism of SCC. The section 4 is an overview of the proposed mechanisms of SCC under anodic dissolution control. The last section proposes a new mechanism of SCC.

Determination of tensile stress induced by oxide film during corrosion process for α-Ti

If a tensile stress generates at the metal/oxide film interface during natural corrosion, it will assist the applied stress to promote stress corrosion cracking (SCC). Such induced tensile stress has been reported during the anodic polarization of {alpha}-Ti in 0.1 mol/L H{sub 2}SO{sub 4} solution. In this study, the authors investigate the sign and magnitude of the internal stress generated during natural corrosion for {alpha}-Ti in the same solution under open circuit condition. In addition, the authors attempt to show that for all anodic dissolution controlled SCC systems, a large tensile stress will generate at the metal/passive film interface during natural corrosion. The induced stress can assist the applied stress to promote the dislocation emission and motion and make SCC nucleate in a lower applied stress range. But for the systems without SCC, the induced tensile stress is very small or compressive stress develops. If so, the mechanism of anodic dissolution controlled SCC can be clarified in the level of dislocation based on the in situ TEM observations.

Molecular dynamics simulation of the role of dislocations in microcrack healing

The molecular dynamics method is used to simulate microcrack healing during heating or/and under compressive stress. A centre microcrack in Cu crystal would be sealed under compressive stress or by heating. The role of compressive stress and heating in crack healing was additive. During microcrack healing, dislocation generation and motion occurred. When there were pre-existing dislocations around the microcrack, the critical temperature or compressive stress necessary for microcrack healing would decrease, and, the higher the number of dislocations, the lower the critical temperature or compressive stress. The critical temperature necessary for microcrack healing depended upon the orientation of the crack plane. For example, the critical temperature for the crack along the (001) plane was the lowest, i.e. 770K.

Fractal structure and fractal dimension determination at nanometer scale

Three-dimensional fractures of different fractal dimensions have been constructed with successive random addition algorithm, the applicability of various dimension determination methods at nanometer scale has been studied. As to the metallic fractures, owing to the limited number of slit islands in a slit plane or limited datum number at nanometer scale, it is difficult to use the area-perimeter method or power spectrum method to determine the fractal dimension. Simulation indicates that box-counting method can be used to determine the fractal dimension at nanometer scale. The dimensions of fractures of valve steel 5Cr21Mn9Ni4N have been determined with STM. Results confirmed that fractal dimension varies with direction at nanometer scale. Our study revealed that, as to theoretical profiles, the dependence of frsctal dimension with direction is simply owing to the limited data set number, i.e. the effect of boundaries. However, the dependence of fractal dimension with direction at nanometer scale in real metallic fractures is correlated to the intrinsic characteristics of the materials in addition to the effect of boundaries. The relationship of fractal dimensions with the mechanical properties of materials at macrometer scale also exists at nanometer scale.

Initiating, growing and cracking of hydrogen blisters

The growing process of a hydrogen blister in a wheel steel was observed in situ with an optical microscope, and the fracture surfaces formed from broken blisters on a wheel steel and bulk metallic glass were investigated. The initiating, growing, cracking and breaking of hydrogen blisters are as follows. Supersaturated vacancies can increase greatly during charging and gather together into a vacancy cluster (small cavity). Hydrogen atoms become hydrogen molecules in the vacancy cluster and hydrogen molecules can stabilize the vacancy cluster. The small cavity becomes the nucleus of hydrogen blister. The blister will grow with entering of vacancies and hydrogen atoms. With increasing hydrogen pressure, plastic deformation occurs first, the hydrogen blister near the surface extrudes, and then cracks initiate along the wall of the blister with further increasing hydrogen pressure. A cracked blister can grow further through propagating of cracks until it breaks.

DOMAIN SWITCHING AND THE CHANGE IN INDENTA- TION CRACKS FOR BaTiO3 SINGLE CRYSTAL UNDER THE ELECTRIC FIELD PERPENDICULAR TO THE POLARIZATION

As a ferroelectric material, BaTiO3 single crystal has the domain structure which can be changed by the application of mechanical stress and electric field. Therefore, the fracture behavior of the crystal is closely related with the domain switching. To understand the relationship between the fracture behavior and the domain switching clearly, the change of the indentation cracks and domain switching around the indentation under electric field perpendicular to the polarization of the samples were investigated through in situ observations by a differential interference contrast microscopy. The results show that for in-plane polarized sample, after completion of domain switching under the in-plane electric field perpendicular to the polarization of the crystal, the indentation cracks and the domains around the indentation are the same as the new indentation on the sample with new polarization state. In the case of applying in-plane electric field on the anti-plane polarized sample, the speed of the domain switching increases in the initial stage and decreases in the end stage, it has the maximum value as half of the domain switching completed. And the speed fluctuates in the first stage.

Study on delayed cracking of conductive notch under electric field in PZT-5H ferroelectric ceramics

Electric-field-induced delay cracking of conducting notch in PZT-5H ferroelectric ceramics has been studied using a compact specimen with a notch filled in conductive silver paste. The critical electric field that induces instant failure of the PZT-5H specimen is shown to be EF = 14.7±3.2 kV/cm. When an electric field lower than EF, but higher than EDF = 9.9 kV/cm was applied, a micro-crack formed at the conductive notch tip instantly, propagating slowly until the specimen failure. When the electric field was lower than EDF, the micro-crack propagated a short distance, and then stopped. When the electric field was lower than EK = 4.9 kV/cm, no cracks formed at the conductive notch tip instantly, however, a delay micro-crack would form and propagate. When the electric field was lower than EDK=2.4 kV/cm, no cracks formed and delay propagation occurred. A model for electric charge emission and concentration at a conductive notch is proposed to explain the delay cracking of conducting notch.

SUB–CRITICAL CRACK GROWTH IN AIR AND DOMAIN SWITCHING FOR BaTiO3 SINGLE CRYSTAL UNDER CONSTANT DEFLECTION

BaTiO_3 single crystal with superior dielectric,pyroelectric,piezoelectric and electrooptic properties has triggered much attention due to numerous potential applications in microelectronic devices,sensors and micro/nanoelectromechanical systems.However,one of the main disadvantages for BaTiO_3 single crystal is that after long-term working under mechanical field or electric field,cracks may be nucleated and propagated.The crack nucleation and propagation in ferroelectric materials are closely related to the domain configurations,and their relationship with crack propagation,nucleation and domain switching,is not known clearly,so it is imperative to take more effort to research this relationship.Here,the relationship of domain switching with crack propagating quickly as well as sub-critical crack growth in ambient air for constant deflection sample of BaTiO_3 single crystal has been in situ studied using a polarized light microscopy.The results indicate that domain switching occurs first and the volume fraction of switched bands increases during loading.When the load reaches to a critical value,crack will be initiated and propagated quickly.Keeping the constant deflection, the domain switch could still occur but very slowly,i.e.,adsorption of water in air is able to promote domain switching.As a result of adsorption enhanced domain switching,cracks are propagated slowly in air,that is say,sub-critical crack growth or stress corrosion cracking in air could occur for BaTiO_3 single crystal under constant deflection.