J.X. Li

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.

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.

Hydrogen Embrittlement of Rail Steels

Abstract The resistance to hydrogen embrittlement (HE) of rail steels was studied. Hydrogen blistering, hydrogen-induced plasticity loss (HIPL) Iδ under slow strain rate test (SSRT), and hydrogen-i...

Role of Hydrogen in Stress Corrosion Cracking of X-60 Pipeline Steel in Soil Containing Water

The role of hydrogen and anodic dissolution in stress corrosion cracking (SCC) of API X-60 pipeline steel in soil containing water has been studied. The results show that the relative elongation reduction induced by hydrogen, IHE, increases linearly with increases in the logarithm of hydrogen concentration, CH, i.e., IHE(%) = 27.9 + 11.5 lnCH(wppm). Hydrogen can enter the sample during SCC in soil containing water at open circuit and cathodic polarization. Therefore, the relative elongation reduction during SCC, ISCC, is the sum of ISCC(H), ISCC(AD), and ISCC(HAD), i.e., ISCC = ISCC(H) + ISCC(AD) + ISCC(HAD), where ISCC(H) is the relative elongation reduction induced by the hydrogen that entered the sample during SCC, ISCC(AD) is that induced by the anodic process during SCC, and ISCC(HAD) is that caused by the coupling of hydrogen and the anodic process in a hydrogen-enhanced anodic process. For SCC at open circuit, there is no hydrogen-enhanced anodic dissolution, and ISCC = ISCC(H) + ISCC(AD). Experime...

Fretting Wear Behavior and Mechanism of Inconel 690 Alloy Related to the Displacement Amplitude

ABSTRACT Fretting wear tests on Inconel 690 alloy were carried out at different displacement amplitudes. The results indicated that with an increase in displacement amplitude, the ratio of tangential force to normal load and wear volume increased. The fretting mode gradually transformed from mostly stick, mixed stick–slip, to full sliding, showing the competition of fretting-induced fatigue cracking and fretting-induced wear. There was a gradient plastic strain created by fretting, which resulted in the formation of a nanocrystalline tribologically transformed structure (TTS) and plastic deformation layers. The plastic strain in the plastic deformation layer gradually increased with an increase in displacement amplitude.

Effect of humidity on indentation crack growth in lead-free ferroelectric ceramics

The effect of humidity on growth of unloaded indentation crack in KNN free-lead ferroelectric ceramics has been investigated. The results showed that crack growth of unloaded indentation in lead-free ferroelectric ceramics could occur in humid air of 70% and 90%RH without electric field and mechanical stresses, but did not in air with RH≤30%. The growth of indentation crack could occur in dry air when the field was larger than the threshold field Eth(y)=0.01EC (normal to the poling direction) or Eth(z)=0.05EC (parallel to the poling direction) and the larger the field, the shorter the incubation time. The increment of crack growth in humid air under sustained field, Δc, was composed of three parts, i.e., Δc=Δc1 + Δc2 + Δc12, where Δc1 was the increment in humid air without field, Δc2 that in dry air under sustained field and Δc12 that induced by combined effect of electric field and humidity because of humidity promoting domain switching. Crack growth of unloaded indentation could occur during hydrogen charging and the threshold stress intensity factor of hydrogen-induced delayed cracking, KIH, as well as the fracture toughness of hydrogenated ceramics, KIC(H), decreased with increasing hydrogen concentration.