Tsong P. Perng

Comparison of hydrogen gas embrittlement of austenitic and ferritic stainless steels

Hydrogen-induced slow crack growth (SCG) was compared in austenitic and ferritic stainless steels at 0 to 125 °Cand 11 to 216 kPa of hydrogen gas. No SCG was observed for AISI 310, while AISI 301 was more susceptible to hydrogen embrittlement and had higher cracking velocity than AL 29-4-2 under the same test conditions. The kinetics of crack propagation was modeled in terms of the hydrogen transport in these alloys. This is a function of temperature, microstructure, and stress state in the embrittlement region. The relatively high cracking velocity of AISI 301 was shown to be controlled by the fast transport of hydrogen through the stress-induced α′ martensite at the crack tip and low escape rate of hydrogen through the γ phase in the surrounding region. Faster accumulation rates of hydrogen in the embrittlement region were expected for AISI 301, which led to higher cracking velocities. The mechanism of hydrogen-induced SCG was discussed based upon the concept of hydrogen-enhanced plasticity.

Comparison of organic and inorganic germanium compounds in cellular radiosensitivity and preparation of germanium nanoparticles as a radiosensitizer

Purpose: The aim of this work is to compare the radiosensitizing effect between organic and inorganic germanium compounds and to investigate whether nanometer-sized germanium particles can act as radiosensitizers. Materials and methods: Bis (2-carboxyethylgermanium) sesquioxide (Ge-132), germanium oxide (GeO2) and germanium nanoparticles were used in this study. Cell viability was determined by clonogenic survival assay. Cellular DNA damage was evaluated by alkaline comet assay, confocal microscopy and the cellular level of phospho-histone H2AX (γ-H2AX). Results: Nanometer-sized germanium particles were fabricated. They have a similar radiosensitizing effect as that of GeO2. Conversely, Ge-132 did not enhance the radiosensitivity of cells. Comet assay was employed to evaluate the level of DNA damage and confirmed that inorganic germanium compounds enhanced cellular radiosensitivity. Notably, the comet assay indicated that the nanoparticle itself caused a higher level of DNA damage. The possibility that germanium nanoparticles per se caused DNA damage was ruled out when the cellular level of γ-H2AX was examined. Conclusions: We demonstrated that inorganic but not organic germanium compounds exerted radiosensitizing effect in cells. Nanometer-sized germanium particles were fabricated and were able to enhance the radiosensitivity of cells. Confounding effect may occur when comet assay is used to estimate the level of DNA damage in the presence of germanium nanoparticles.

Room temperature vibrational photoluminescence and field emission of nanoscaled tris-(8-hydroxyquinoline) aluminum crystalline film

The nanoscaled tris-(8-hydroxyquinoline) aluminum (AlQ3) crystalline film was synthesized by vapor condensation. It was stacked with nanometer-sized rods, approximately 100 nm wide and 1 μm long, and had a surface roughness of about 100 nm. The vibronic progression with several separated peaks was observed in the photoluminescence spectrum at room temperature. It is attributed to the crystallinity of AlQ3 and the coupling of vibrations of the individual ligands to the fluorescence transition. The emission current was also observed with a turn-on field of 12.0 V/μm, and a current density of about 0.8 mA/cm2 at 22 V/μm. Therefore, the AlQ3 crystalline film provides a choice for field emission.

Embrittlement of amorphous Fe40Ni38Mo4B18 alloy by electrolytic hydrogen

The effects of hydrogen on the tensile properties and fracture processes at room temperature were investigated. Specimens were tested at various strain rates in air or under different cathodic charging-current densities. The slopes of the stress-strain curves were essentially identical for all the specimens, except that the fracture points varied under different test conditions. Macroscopically, hydrogen only affected the elastic deformation behavior, but microscopically, the embrittlement was caused by the heterogeneous nucleation of localized plastic deformation. The degree of hydrogen embrittlement increased as the charging current increased or as the strain rate decreased. With the same charging current and time, longer dynamic charging resulted in more severe embrittlement. Before fracture took place, the strength of the alloy could be completely restored if hydrogen had been removed. Hydrogen diffusivity and solubility were used to draw the time-dependent hydrogen concentration profiles for the specimens under different charging conditions. The difference in the mechanical properties was correlated with the hydrogen concentration within the specimen.

Cracking of amorphous Fe40Ni38Mo4B18 induced by static charging with hydrogen

Cracking in amorphous ribbon of Fe40Ni38Mo4B18 without external loading could be induced by cathodic charging with hydrogen in 0.1 N H2SO4 + 5 mg/L NaAsO2. However, before cracking was initiated, the effect of hydrogen on the mechanical properties could be eliminated if hydrogen had been removed. A series of static charging experiments was carried out to study the cracking characteristics in this alloy. The diffusivity and concentration of hydrogen were obtained from both permeation and cathodic charging /thermal evolution experiments. The cause of cracking by static charging could be attributed to the build up of an internal hydrogen pressure around heterogeneous sites. The critical pressures for crack initiation were calculated based on the diffusivity and concentration data.

Hydrogen Permeation through Coated and Uncoated WASPALOY

Hydrogen permeability, diffusivity, and solubility have been measured for a Ni-base superalloy, WASPALOY,* over the temperature range of 200 to 560 °C. Measurements were made with various surface conditions. The hydrogen diffusivity and permeability values for Pd-coated WASPALOY were between those for pure nickel and for austenitic stainless steel. Hydrogen in uncoated WASPALOY had consistently lower effective diffusivity and permeability than in the Pd-coated condition. Gold-plating on WASPALOY or adding H2O to H2 gas substantially reduced both transport parameters, presumably due to slower surface or interface kinetics and lower permeability of hydrogen in the gold layer. Independently measured hydrogen solubility determined by equilibration of bulk specimens with H2 gas was roughly 60 pct of the solubility obtained by dividing the effective diffusivity into the permeation constant. This is discussed on the basis of internal trapping, which reduced the effective diffusivity and resulted in a higher apparent solubility.

Hydrogen compatibility of Femnal alloys

Five Femnal alloys with various ferrite/ austenite were prepared by mainly adjusting the carbon content. Hydrogen compatibility of these alloys was investigated. A slow extension rate tensile test in one atmosphere of hydrogen gas showed that hydrogen embrittlement (HE) increased with the content of ferrite in the alloys. The alloy with a full austenite phase was strong, ductile, and essentially immune to HE, whereas the one with full ferrite was weaker, very brittle, and suffered serious damage in hydrogen. A sustained load test was further conducted in hydrogen gas at 25°C to 75 °C for the alloys with 10, 35, and 65 pct ferrite. No subcritical crack growth occurred in the alloy with 10% ferrite. Typical three-stage environment-assisted crack growth was observed for the alloys with 35 and 65 pct ferrite. A kinetics study of the subcritical crack growth indicated that the cracking rate was basically controlled by hydrogen transport through the ferrite grains. Austenite phase could be embrittled provided sufficient hydrogen was dissolved. To reduce HE in Femnal alloys, the content of ferrite should be reduced; this can be achieved mainly by controlling the carbon content in the alloys.

Amorphization of Ti1−x Mn x

Three amorphous Ti1−xMnx alloy powders, withx = 0.4, 0.5, and 0.6, were prepared by mechanical alloying (MA) of the elemental powders in a high-energy ball mill. The amorphous powders were characterized by X-ray diffraction (XRD) and high-resolution transmission elec- tron microscopy (HRTEM). The crystallization temperatures for these alloys detected by dif- ferential scanning calorimetry (DSC) varied from 769 to 830 K. The calculated enthalpies of mixing in these amorphous phases are relatively small compared with those for other Ti-base binary alloys. The criteria for solid-state amorphization reaction are examined. It is suggested that the kinetics of nucleation and growth favors the formation of the amorphous phases and the supply of atoms for nucleation and growth is predominantly through the defective regions induced by MA.

Hydrogen Gas-Rechargeable Metal Hydride Electrode for Ni-MH Battery

Some nonbreakable pellets made of an AB 5 alloy and metallic powder Ag or Ni were used as the anode in a Ni-MH battery such that the anode could be charged in hydrogen gas and discharged in electrolyte. The performance isinfluenced by the alloy properties, electrode porosity, hydrogen pressure, discharge current, electrolyte polarization, and contamination in the system. For the pellet Ag/AB 5 = 10, continuous discharge can proceed but only for a limited time. The lower porosity of the pellet may be the main problem. For the pellet Ni/AB 5 = 25, it can be discharged unlimitedly at a constant hydrogen pressure of 20 atm. When discharged with a fixed amount of hydrogen gas, the adsorbed hydrogen is immediately discharged on the surface rather than being absorbed into the bulk, and the discharge capacity comes mostly from the gaseous hydrogen rather than from the hydrogen already dissolved in the bulk of alloy. Therefore, the pellet acts more like a catalyst for hydrogen dissociation than as an absorbent. Furthermore, a minimum of ∼7 atm of hydrogen pressure is required for the system to operate properly. It is suggested that, in order for such a system to operate better under the wet condition, a higher hydrogen gas pressure be maintained to accelerate the hydrogenation rate and more porous pellets be used for faster reaction kinetics.