K.W. Gao

In situ SEM study of formation and growth of shear bands and microcracks in bulk metallic glasses

Abstract Formation and growth of shear bands and microcracks in bulk metallic glasses of Zr 57 Cu 15.4 Ni 12.6 Al 10 Nb 5 and Zr 41.2 Cu 12.5 Ni 10 Ti 13.8 Be 22.5 have been investigated through in situ tensile tests in scanning electron microscope using a single-edge notched specimen. Atom force microscope was used to study the three-dimension pattern of fine shear bands. The results show that besides shear stress, normal stress plays also an important role in forming and growing of shear bands, which appear first during loading. Mode II shear microcrack will initiate and propagate first in the shear planes along shear bands. The mode II cracks will open and become a shallow I+II complex crack because there is a component of normal stress. As soon as the complex crack (or cracks) becomes a mode I crack penetrating the thickness through propagating from the surface (or surfaces) toward the centre, the specimen will fracture immediately.

Correlation between passive film-induced stress and stress corrosion cracking of α-Ti in a methanol solution at various potentials

Abstract The flow stress of a specimen of α-Ti before unloading is different with the yield stress of the same specimen after unloading and forming a passive film through immersing in a methanol solution at various constant potentials. The difference is the passive film-induced stress. The film-induced stress and susceptibility to stress corrosion cracking (SCC) in the methanol solution at various potentials were measured. At the stable open-circuit potential and under anodic polarization, both film-induced tensile stress σ p and susceptibility to SCC had a maximum value. The film-induced stress and SCC susceptibility, however, decreased steeply with a decrease in potential under cathodic polarization. When the potential V ≤−280 mV SCE , the film-induced stress became compressive; correspondingly, susceptibility to SCC was zero. Therefore, the variation of film-induced stress with potential was consistent with that of susceptibility to SCC. A large film-induced tensile stress is the necessary condition for SCC of α-Ti in the methanol solution. The symbol and amount of the film-induced stress were related to the compositions of the passive film, which have been analyzed using the X-ray photoelectron spectrum (XPS).

Correspondence between hydrogen enhancing dezincification layer-induced stress and susceptibility to SCC of brass

Dezincification layer formed during corrosion or stress corrosion cracking (SCC) of brass in the ammonia solution could induce an additive stress. The effect of hydrogen on the dezincification layer-induced stress and the susceptibility to SCC were studied. The dezincification layer-induced stress was measured using the deflection method and the flow stress differential method. The susceptibility to SCC was measured using a slow strain rate test. Results showed that both the dezincification layer-induced stress and the susceptibility to SCC increased with increasing hydrogen concentration, and the hydrogen concentration dependence of the susceptibility to SCC was in a good agreement with that of the dezincification layer-induced stress. Hydrogen in brass facilitates the selective dissolution of Zn in the ammonia solution, and then enhances the dezincification layer-induced stress.

Delayed fracture of lead zirconate titanate ferroelectric ceramics under sustained electric field

In this letter, we report on delayed fracture of lead zirconate titanate ceramics with a Zr/Ti ratio of 52/48(PZT-5) ferroelectric ceramics in silicon oil under sustained electric field, and that in silicon oil or moist atmosphere under sustained mechanical load. The experimental results show that sustained electric fields may cause delayed fracture of PZT-5 ceramics and there is a threshold field for the delayed fracture. The threshold electric field is less than one third of the critical electric field to cause instant fracture and is also half of the coercive field of the PZT-5 ceramics.

The effect of hydride and martensite on the fracture toughness of TiNi shape memory alloy

The effect of hydride and hydrogen-induced martensite on the fracture toughness of a TiNi shape memory alloy has been investigated using a notched tensile specimen. The results show that the fracture toughness of the hydrogenated samples decreases linearly with increasing hydrogen concentration, and the relative fracture toughness loss is as high as 96%. The relative fracture toughness loss induced by the martensite, however, is about 1.8%, and is independent of hydrogen concentration. The decrease in the fracture toughness of hydrogenated samples is almost completely due to the hydride. The relative fracture toughness loss induced by the hydride, ΔKICTiNiH/KIC, increases with increasing hydride content, WTiNiH, i.e., ΔKICTiNiH/KIC(%) = 93[1−exp(−WTiNiH/9.5)]. Microcracks can generate along the hydrides during charging at , but do not further increase the relative fracture toughness loss induced by the hydride.

Stress corrosion cracking and hydrogen-induced cracking of amorphous Fe74.5Ni10Si3.5B9C2

Abstract Stress corrosion cracking (SCC) in NaCl solution and hydrogen-induced cracking (HIC) during dynamic charging of Fe 74.5 Ni 10 Si 3.5 B 9 C 2 amorphous alloy were investigated through sustained load tests. The normalized threshold stress of SCC was σ SCC / σ F =0.04, where σ F is fracture strength in air. Anodic polarization and addition of As 2 O 3 into the solution did not change σ SCC / σ F , but cathodic polarization increased σ SCC / σ F from 0.04 to 0.31. Cathodic polarization increased but anodic polarization decreased the time to failure during SCC at the constant load of σ =0.27 σ F . The normalized threshold stress of HIC, σ HIC / σ F , was linearly decreased with the increase in logarithm of hydrogen concentration ( C 0 ) and kept a minimum constant when C 0 was larger than a critical value, i.e., σ HIC / σ F =1.58−0.36ln C 0 ( C 0 ⩽74.4 wppm) and 0.1 ( C 0 ⩾74.4 wppm). The threshold stress of HIC during dynamic charging with the maximum current was larger than that of SCC at open-circuit potential. Fracture surfaces of HIC were also different with that of SCC. Experiments indicated that SCC of the amorphous alloy in the NaCl solution is controlled by anodic dissolution process instead of hydrogen.

Martensite caused by passive film-induced stress during stress corrosion cracking in type 304 stainless steel

Abstract After a pre-crept specimen of type 304 stainless steel underwent stress corrosion cracking (SCC) at constant load in a 42% MgCl2 solution for 2 h, α′ martensite on the surface and over the entire specimen increased in volume fraction by 14 and 1.5%, respectively. If the specimen pre-crept in the silicon oil was immersed in the 42% MgCl2 solution at a cathodic potential of −600 mVSCE, in which no SCC occurs, the α′ martensite volume fraction increased by only 0.6%. The critical hydrogen concentration for the formation of hydrogen-induced α′ martensite was 50.9 wppm, which was larger than that which entered into the sample during SCC. The difference between the flow stress of the 304 steel before unloading and the yield stress of the same specimen extended in air after unloading and immersed in the 42% MgCl2 solution to form a passive film is defined as a passive film-induced stress. The passive film forming at the open-circuit potential generated a large tensile stress, and that at a cathodic potential of −600 mVSCE generated a small compressive stress. Therefore, the observed martensite transformation during SCC can be attributed to a passive film-induced tensile stress.

Molecular dynamics simulation of cross-slip and the intersection of dislocations in copper

The molecular dynamics method is used to simulate cross-slip by thermal activation at 30 K and the intersection of dislocations in copper containing 1.6 × 106 atoms using the embedded atom method potential. The results show that an extended screw dislocation can recombine through thermal activation at 30 K into a constriction on the surface because of stress imbalance and the constriction will split again in the other slip plane. Removing the constriction along the extended dislocation results in a cross-slip of the screw dislocation at low temperature. After the intersection between a moving right-hand screw dislocation DC and a perpendicular left-hand dislocation BA, whose ends are fixed on the surfaces, an extended jog corresponding to a row of one-third vacancies forms in BA and a trail of vacancies behind DC. If the intersected dislocation is a right-hand screw dislocation AB, the jog formed in AB corresponds to a row of one-third interstitials and the point defects behind DC are interstitials. After the intersection between screw and edge dislocations, the jog formed in the edge dislocation corresponds to a row of one-third vacancies and there are no point defects behind the screw dislocation.

Stability of nano-scale ferroelectric domains in a LiNbO3 single crystal: The role of surface energy and polar molecule adsorption

The stability of nano-scale ferroelectric domains in a LiNbO3 single crystal under varied atmospheric humidity levels was studied using piezoelectric force microscopy. Experimental results showed that the nano-scale domains fabricated by the tip field of the atomic force microscope changed as the environmental humidity changed; the c− domains expanded or shrank with increases or decreases in the environmental humidity (that is, with the amount of adsorbed H2O molecules on the domain surface), while the c+ domains transformed in the opposite sense. The surface energy of the domains is responsible for these transformations.

The effect of humidity on nano-scaled domain switching in LiNbO3 single crystal

This paper deals with the influence of relative humidity on nano-scaled domain switching caused by the electric field of atomic force microscopy (AFM) in a LiNbO3 single crystal. Experimental results show that the switched domain size under AFM tip does not increase monotonously with relative humidity. The domain radius increases slightly in lower relative humidity conditions (< 40%), and decreases dramatically in higher relative humidity conditions (40–99%). The domain is not created under a relative humidity of 99%. Experiments on the AFM force-distance curve prove that there is a water bridge between the tip and the sample surface, whose size increases with the rise of humidity. The influence of water bridges on the effective electric field acting on the domain switching in the sample is discussed.