Showing papers by "Andrej Atrens published in 2013"

An Hydrogen Evolution Method for the Estimation of the Corrosion Rate of Magnesium Alloys

Abstract: Due to the negative difference effect, a special electrochemical phenomenon for magnesium and its alloys, significant errors can be introduced into corrosion rate measurements when using traditional electrochemical techniques. Also, the classical weight-loss method only provides final corrosion information (or an integrated corrosion rate), and moreover errors may easily be introduced into the final result during the removal of corrosion products and in the calculation of the corrosion rate.

A critical review of the influence of hydrogen on the mechanical properties of medium-strength steels

Abstract: As medium-strength steels are promising candidates for the hydrogen economy, it is important to understand their interaction with hydrogen. However, there are only a limited number of investigations on the behavior of medium-strength steels in hydrogen. The existing literature indicates that the influences of hydrogen on the tensile properties of medium-strength steels are mainly the following: (i) the steel can be hardened by hydrogen, as demonstrated by an increase in the yield stress or ultimate tensile stress; (ii) some steels can be embrittled by hydrogen, as revealed by lower yield stress or ultimate tensile stress; (iii) in most cases, these steels may experience hydrogen embrittlement (HE), as indicated by a reduction in ductility. The degree of HE mainly depends on the test conditions and the steel. The embrittlement can lead to catastrophic brittle fracture in service. The influence of hydrogen on the fatigue properties of medium-strength steels is dependent on many factors such as the stress ratio, temperature, yield stress of the steel, and test frequency. Generally, the hydrogen influence on fatigue limit is small, whereas hydrogen can accelerate the fatigue crack growth rate, leading to a shorter fatigue life. Inclusions are an important factor influencing the properties of medium-strength steels in the presence of hydrogen. However, it is not possible to predict the influence of hydrogen for any particular steel that has not been experimentally evaluated or to predict service performance. It is not known why similar steels can have different behavior, ranging from good resistance to significant embrittlement. A better understanding of the microstructural characteristics is needed.

Stress corrosion cracking of high-strength steels

Abstract: The mechanisms of stress corrosion cracking (SCC) and hydrogen embrittlement were recently reviewed by Lynch in this journal. The present review, in contrast, focuses on the rate-limiting step of the SCC of low-alloy high-strength steels in water and particularly focuses on the influence of the applied stress rate on the SCC of lowalloy high-strength steels. Linearly increasing stress tests of low-alloy high-strength steels in distilled water indicated that the stress corrosion crack velocity increased with increasing applied stress rate until the maximum crack velocity, corresponding to v in fracture mechanics tests in distilled water. Moreover, the crack velocity was dependent only on the applied stress rate and was not influenced by the steel composition. The rate-limiting step could be the rupture of a surface film, which would control the rate of metal dissolution and/or the production and transport of hydrogen to the crack tip or to the regions ahead of the crack tip.

Thermal desorption spectroscopy study of the interaction of hydrogen with TiC precipitates

Abstract: Thermal desorption spectroscopy (TDS) was used to study hydrogen-trap interactions for an experimental steel (0.025 wt%C-0.09%Ti). After lab processing, the microstructure consisted of small (∼20 μm) ferrite grains containing nanometer TiC precipitates. After hot and cold rolling, the material contained some hydrogen (originated from the hot rolling) in irreversible traps, the TiC precipitates. After annealing in hydrogen, the TDS spectra consisted of a high temperature peak, attributed to irreversible trapping by TiC precipitates. Annealing slightly increased the TiC precipitate size. Both the peak temperature and peak area increased with increasing annealing temperature. The increase in peak area occurred together with the increase in TiC precipitate size. The TDS spectra for samples annealed at 800 °C, and electrochemically charged, contained (i) a low temperature peak which decreased in height with increasing desorption time, and (ii) a high temperature peak that did not change significantly with desorption time, and was similar to those after gaseous charging. The low temperature peak was attributed to reversible traps such as grain boundaries, whereas the high temperature peak was attributed to irreversible trapping by TiC precipitates. The high temperature TDS peak was composed of constituent peaks with essentially the same activation energy of 145 kJ/mol.

Influence of a high magnetic field on the microstructure and properties of a Cu–Fe–Ag in situ composite

Abstract: The influence of a magnetic field during heat treatments was studied for a deformation-processed Cu−14Fe−0.1Ag in situ composite produced by thermo-mechanical processing. A high magnetic field during initial heat treatment promoted the spheroidization and refinement of Fe dendrites, and decreased the diffusion activation energy of Fe atoms in the Cu matrix, which promoted Fe atom precipitation. The resultant in situ composite had thinner Fe fibers, higher tensile strength and better conductivity. A high magnetic field during intermediate heat treatment increased the conductivity and tensile strength. The strength, conductivity and elongation to fracture of a Cu−14Fe−0.1Ag in situ composite could be improved simultaneously using a high magnetic field during the initial, intermediate and final heat treatment. The following combination of properties could be produced by the Cu−14Fe−0.1Ag in situ composite at η =7.8 after isochronic aging for 1 h using 10 T magnetic induction intensity: (i) 1149 MPa tensile strength, 60.3% IACS conductivity and 3.3% elongation; or (ii) 1093 MPa, 61.9% IACS and 3.5%; or (iii) 1006 MPa, 63.7% IACS and 3.7%.

Understanding the Corrosion of Mg and Mg Alloys

Abstract: This article summarizes current, fundamental knowledge of the corrosion of magnesium, and highlights some latest developments. Mg is a most reactive metal. The high available reaction energy, and because the simultaneous exchange of two electrons is forbidden by quantum mechanics, means that there is a highly reactive intermediate that can chemically split water. Thus, part of the corrosion rate cannot be measured with electrochemical techniques. Furthermore, the evolving hydrogen can insulate part of the corroding surface from electrochemical measurement. It is best practice to simultaneously use several measurement techniques, and to compare the measured corrosion rates in the same units.

Thermal desorption spectroscopy study of experimental Ti/S containing steels

Abstract: This research studied hydrogen trapping by Ti/S containing precipitates in two experimental steels, C076 and C077. Ingots were hot and cold rolled to sheets of 1·2 mm thickness. Precipitates were formed during a well designed heat treatment. Thermal desorption spectra (TDS) were measured after electrochemical hydrogen charging. C077 and C076 produced low temperature TDS peaks at ∼90 and ∼140°C, with activation energies of ∼34 kJ mol−1, consistent with hydrogen trapping at grain boundaries. C076 had a higher hydrogen content attributable to the larger grain boundary area. C076 also had higher temperature TDS peak at ∼270°C, with an activation energy of ∼58 kJ mol−1, attributed to hydrogen trapping associated with Fe1·2Ti0·8S2 precipitates.

Microstructure and Properties of a Deformation-Processed Cu-Cr-Ag In Situ Composite by Directional Solidification

Abstract: Cu-7Cr-0.07Ag alloys were prepared by casting and directional solidification, from which deformation-processed in situ composites were prepared by thermo-mechanical processing. The microstructure, mechanical properties, and electrical properties were investigated using optical microscopy, scanning electronic microscopy, tensile testing, and a micro-ohmmeter. The second-phase Cr grains of the directional solidification Cu-7Cr-0.07Ag in situ composite were parallel to the drawing direction and were finer, which led to a higher tensile strength and a better combination of properties.

Neutron residual stress measurements in rails

Abstract: Rails were among the first objects of study by neutron diffraction strain measurement and the first experiments were done as early as the late 1980s. This interest is easy to explain: the problem of rail fracturing is critical from the public safety point of view and the penetrating ability of neutrons suggested the possibility of breakthrough experiments and fast progress in this field. It was well-established that residual stresses, both near-surface and interior, played a significant role in the development of defects which led to rail failure. This suggested three distinct approaches of neutron diffraction strain measurement that could contribute to various problems of the rail industry...In the present paper, we describe recent results in the application of neutron diffraction to the long-standing problems described above...

Overview of the Mg Corrosion Mechanism

Abstract: Our recently developed plug-in specimens allow direct comparison between corrosion rates measured by weight loss, hydrogen evolution and from Tafel extrapolation of polarization curves. These have allowed identification of some of the reasons why Tafel extrapolation has previously not provided a good measure of steady state corrosion for Mg, because: (i) of crevice corrosion in the specimen mount, (ii) of decoupling of the Mg corrosion from the electrochemical measurements; and (iii) Tafel extrapolation has often been used to study Mg corrosion soon after specimen insertion into the solution, despite the fact that Mg alloy corrosion is often non-linear, the corrosion rate accelerates to a steady state rate after an initial period of low corrosion rate.

A Novel Heat Treatment for Excavator Dipper Teeth Manufactured from Low‐Carbon Low‐Alloy Steel

Abstract: This research examined the metallurgy of production of forged excavator teeth manufactured from low-carbon low alloy steel. A novel partial forging-remnant-heat hardening technology was proposed. The critical temperature of isothermal annealing for 40 Cr steel was determined experimentally to be 650°C. Production dipper teeth were produced using this novel technology. Their properties were compared with dipper teeth produced by the alternative routes. The novel technology provided optimum microstructure, good mechanical properties, and a lower economical cost, congruent with the low carbon economy. Copyright

The tensile properties of NiCrMo1 steel under conditions of hydrogen charging studied using the linearly increasing stress test

Abstract: The tensile properties of NiCrMo1 steel were investigated using the linearly increasing stress test (LIST) in air and with hydrogen charging. Hydrogen charging was carried out by applying an increasingly negative applied potential to -1550 mVAg/AgCl in acidified 0.1 M Na2SO4, pH 2 solution. The LIST results showed that the yield stress of the steel was similar in air or with hydrogen charging. SEM examination showed that the specimens tested in air had no surface cracks, and the fracture surfaces comprised dimples. The failure in air was due to ductile overload. The specimens tested with hydrogen charging had obvious surface cracks in the necked region, and their length increased to about 550 mm with a more negative potential. However, the fracture surfaces were nevertheless dominated by ductile feature. These results imply that the influence of hydrogen was only associated with the final ductile fracture after the onset of necking.

Production of High Purity Mg-X Rare Earth Binary Alloys using Zr

Abstract: An Fe content lower than the tolerance limit is critical in controlling corrosion rates of Mg alloys. The possibility of reducing Fe below the tolerance in Mg melts was studied using Zr as a precipitating agent. The experiments were carried out on Mg-X binary alloys with rare-earths, X = Y, Ce, Gd, Nd, and La. The laboratory scale results show that Zr is effective in reducing the Fe content from the Mg melt for Mg-X binary alloys. Purification occurs by the precipitation from the melt of Fe rich precipitates, and the settling of the precipitates to the bottom of the melt. Any desired Fe content down to one wt ppm can in principle be achieved by appropriate melt treatment. The experimental results are discussed with respect to calculated phase diagrams.

Steels for the hydrogen economy

Abstract: Our research program seeks to understand the influence of hydrogen on steels for the hydrogen economy. The present paucity of affordable hydrogen-proof materials poses strong technological and economic barriers to the hydrogen economy. Our research seeks to understand the behaviour of medium-strength-steels. Our research includes : ( i ) understanding the hydrogen influence on steels using the linearly increasing stress test ( LIST) and fatigue tests for electrochemically charged specimens; ( 2 ) evaluating the hydrogen fugacity equivalent to the electrochemical hydrogen charging , using ( i ) electrolytic permeability experiments and ( ii ) thermal desorption spectroscopy ( TDS ) . The approach for each is described. The theoretical underpinnings are explained. The first results are presented and the future research directions explained.

Effect of alloying elements on corrosion behaviour of binary magnesium alloys

Abstract: The corrosion behaviour of as-cast high-purity magnesium (HP Mg), as-cast and solution-heat-treated Mg-X alloys (X = 1% Mn, 5% Sn, 0.3% Ca, 5% Zn, 6% Al, 0.1% Zr, 0.3% Si, 0.1% Sr) was evaluated by PW (from weight loss), PAH (from evolved hydrogen volume) and Pi (from polarization curves) from immersion test in 3.5% NaCl solution saturated with Mg(OH)2. The corrosion rates of all the alloys were faster than that of HP Mg. There was a clear correlation between corrosion rates with type of alloying rather than the quantity of alloying. Although the solution heat treatment dissolved the second phase particles of the Mg-X alloys, there were some particles remaining in the microstructure. Therefore, the acceleration effect of the alloying elements can be interpreted as the mechanism of micro-galvanic corrosion caused by the particles.

The tensile properties of NiCrMo1 steel under conditions of hydrogen charging studied using the linearly increasing stress test

Abstract: The tensile properties of NiCrMo1 steel were investigated using the linearly increasing stress test (LIST) in air and with hydrogen charging. Hydrogen charging was carried out by applying an increasingly negative applied potential to -1550 mVAg/AgCl in acidified 0.1 M Na2SO4, pH 2 solution. The LIST results showed that the yield stress of the steel was similar in air or with hydrogen charging. SEM examination showed that the specimens tested in air had no surface cracks, and the fracture surfaces comprised dimples. The failure in air was due to ductile overload. The specimens tested with hydrogen charging had obvious surface cracks in the necked region, and their length increased to about 550 mm with a more negative potential. However, the fracture surfaces were nevertheless dominated by ductile feature. These results imply that the influence of hydrogen was only associated with the final ductile fracture after the onset of necking.

Evaluation from first principles the structural stability of Mg containing different amounts of Al atoms under high pressure

Abstract: The structural stability and phase transition of magnesium (Mg) containing different amounts of Al under high pressure was studied by means of first-principles total energy calculations. The cohesive energy calculations showed that the hcp and bcc structures of Mg-4.17 at%Al and Mg-8.33 at%Al were of the strong structural stability. The enthalpy for hcp and bcc structures of Mg was dependent upon the Al content. With increasing Al content from 0 to 8.33 at%, the enthalpy for hcp and bcc structures increased monotonously. Based on the enthalpy differences of the hcp and bcc structures under different pressures, the phase transition pressure under which the hcpbcc structural phase transition may take place for pure Mg, Mg-4.17 at%Al and Mg-8.33 at%Al was 60 GPa, 70 GPa and 85 GPa, respectively, indicating that with the increasing Al content, the phase transition pressure became higher and the hcpbcc transition was more difficult.