Semiconductor transformation of lead zirconate titanate (PZT)‐5H ferroelectric ceramics induced by hydrogen has been investigated. The results showed that hydrogen caused the PZT‐5H ferroelectric ceramics to transform from a yellow insulator into a black n‐type semiconductor, and the leakage current and carrier concentration increased but the resistivity decreased with increasing hydrogen concentration. During charging in H2 at a temperature higher than the Curie point, hydrogen would restrain the PZT‐5H ferroelectric ceramics to transform from cubic into tetragonal phase, resulting in the disappearance of ferroelectricity at room temperature. Charging at a temperature below the Curie point, however, did not change the crystal structure of the tetragonal ferroelectric ceramics. The properties and color of PZT‐5H ferroelectric ceramics were restored after outgassing at a high temperature.

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In this letter, hydrogen-hindered phase transition of ferroelectric ceramics from cubic to tetragonal has been studied by experiment and first principles calculation. The calculation shows that for hydrogenated tetragonal PbTiO 3 , double-lowest-energy sites of Ti along the c axis exist no longer and the only lowest energy site locates at the center of the cell. The calculation can explain the experiment that hydrogen charged above its Curie temperature can hinder phase transition of lead zirconate titanate from cubic paraelectricity to tetragonal ferroelectricity.

Hydrogen redistribution under stress-induced hydrogen diffusion and corresponding fracture behaviour of a 960 MPa grade martensitic steel were studied. Slow strain rate tensile (SSRT) tests after hydrogen pre-charging were performed and the fracture surface was observed and analysed. The strain rate ranged from 10−6 to 10−4 s−1. In the pre-charged sample with a certain hydrogen content of 0.62 ppm, hydrogen distribution was homogeneous before the SSRT test. After tensile testing, brittle fracture features appeared in the centre of the fracture surface, while ductile features appeared in the surrounding area. Brittle region size increased with the strain rate slowing down in the range from 10−4 to 5 × 10−6 s−1, while it stabilised at the strain rate slower than 5 × 10−6 s−1. Relationship between the strain rate and the brittle region size was established and discussed based on the present data of hydrogen content in the material. This paper is part of a thematic issue on Hydrogen in Metallic Alloys

Medium Mn steels are a class of the new-generation ultra-high-strength materials used in automotives. However, despite excellent ductility, they may suffer from delayed cracking and thus cause serious concerns. In this study, several medium Mn steels were tested with different prestrain and hydrogen charging conditions. The interaction and synergistic effects of prestrain and hydrogen content on hydrogen-induced delayed cracking behavior are investigated. The threshold stress of hydrogen-induced cracking (HIC) decreased during dynamic hydrogen charging under a constant load. In the process of dynamic hydrogen charging, for M7B and M10B steels, the normalized stress intensity factor σ/σb and the corresponding threshold stress rHIC decreased sharply as prestrain increased. This is because the volume fraction of retained austenite decreased with an increase in prestrain. Similarly, rHIC was reduced and the critical hydrogen content dropped drastically with increasing prestrain. For M7C, the influence of prestrain on threshold stress and hydrogen concentration was less than that of M7B. This is because the different treatment processes leads to a different stability of the retained austenite. By observing the SEM fractographs, the fracture surface of medium Mn steels showed different fracture characteristics, such as dimple fractures and intergranular and transgranular modes.

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The passivation of X65 steel in NaHCO3 solution saturated with CO2 was investigated using electrochemical and immersion tests as well as surface morphology and composition analysis. The tests were conducted using thin wire specimens at atmospheric pressure in NaHCO3 solution with different concentrations and at different temperatures. The electrochemical properties of thin wire specimens and square specimens were compared. Thin wire specimens were immersed for a long period, during which the open circuit potential (OCP) was recorded. Anodic polarization was performed after the OCP was sufficiently high and stable. Electrochemical tests revealed that the electrochemical properties of the thin wire specimens were consistent with those of the square specimens. A sudden increase in the OCP of approximately 700 mV occurred after the specimens were immersed for a long period in the tested concentration and temperature. The time at which the OCP increase occurred decreased when the NaHCO3 concentration or temperature were increased. After the OCP became stable, anodic polarization revealed favorable self‐passivation. Immersion tests on X65 steel demonstrated that the OCP increase may have been due to the filling of pores between FeCO3 grains. Small tetrahedron grains formed in the pores of large hexahedral grains to lower the porosity of the film, thus making the film more compact. Consequently, the OCP increased and anodic polarization presented self‐passivation.