Corrosion fatigue behavior of extruded AZ80, AZ61, and AM60 magnesium alloys in distilled water

Rotary bending fatigue tests were conducted in laboratory air and distilled water using three extruded magnesium (Mg) alloys AZ80, AZ61, and AM60 with different chemical compositions. In laboratory air, the fatigue strengths at high stress levels were similar in all alloys because cracks initiated at Al-Mg intermetallic compounds, whereas AZ80 with the largest Al content exhibited the highest fatigue strength at low stress levels, which was attributed to the crack initiation due to cyclic slip deformation in the matrix microstructure. In distilled water, fatigue strengths were considerably decreased due to the formation of corrosion pits in all alloys, and the difference of fatigue strength at low stress levels among the alloys disappeared, indicating that the addition of Al that improved the fatigue strength in laboratory air was detrimental to corrosion fatigue. __________ Translated from Problemy Prochnosti, No. 1, pp. 141–145, January–February, 2008.     

In vitro degradation performance and biological response of a Mg-Zn-Zr alloy

The feasibility of a Mg-Zn-Zr alloy for biomedical applications was studied through microstructure characterization, corrosion tests in different biological media, and cell proliferation, differentiation and adhesion tests. Corrosion tests showed that the ZK60 alloy in the as-extruded state with finer grain sizes exhibited slower corrosion rates than the same alloy in the as-cast state. The tests in different biological fluids showed that the corrosion rates of the as-cast and as-extruded ZK60 alloy in DMEM + FBS were the highest, while those in Hank's solution were the lowest. The corrosion rate of the as-extruded ZK60 alloy was similar to the corrosion rates of other commercial magnesium alloys, namely the die-cast AZ91D, die-cast AM50, extruded AZ31 and extruded WE43 alloys. The results obtained from the indirect cytotoxicity evaluation showed that the 100% concentrated cast and extruded ZK60 alloy extracts resulted in significantly reduced cell numbers and total protein amounts, as compared to the negative control. The cell number and total protein amount increased with the gradual dilution of the extracts, but the protein normalized ALP activity showed an opposite trend. For the direct assay, L-929 and MG63 cells exhibited good adhesion with spread pseudopod on the surface of extruded ZK60 alloy samples after 24 h culture. In short, the as-extruded ZK60 alloy could be a good candidate material for biodegradable implants.     

Fatigue strength of welded cover plates on 6261 aluminium alloy I-beams

S−N data were acquired in bending from some 50 extruded 6261 aluminium alloy I-beams (100 mm deep, 75 mm wide and 4.2 mm thick) with cover plates welded to them. Cover plate end details were rectangular, oval or rhomboid in shape with either welded or unwelded transverse ends. Fatigue strengths of the various end details varied by about 25% at lives of either 10⁵ cycles or 2×10⁶ cycles. The S−N data were compared with design curves from British Standard CP 118:1969 and BS 8118 Part 1:1991 BS 8118 provided a conservative life estimate over the range tested (10⁴−2×10⁷ cycles), while the predictions of CP 118 were non-conservative at lives > 2±10⁶ cycles. A simple two-dimensional finite-element analysis of a model cover plate/I-beam geometry was linked with growth rate data obtained from SENB specimens, to provide a life prediction S−N curve from a Paris law integration. There was good agreement between the results from the model and the experimental data.     

Superplasticity and grain boundary sliding in rolled AZ91 magnesium alloy at high strain rates

The superplastic deformation characteristics and microstructure evolution of the rolled AZ91 magnesium alloys at temperatures ranging from 623 to 698 K (0.67–0.76 T_m) and at the high strain rates ranging from 10⁻³ to 1 s⁻¹ were investigated with the methods of OM, SEM and TEM. An excellent superplasticity with the maximum elongation to failure of 455% was obtained at 623 K and the strain rate of 10⁻³ s⁻¹ in the rolled AZ91 magnesium alloys and its strain rate sensitivity m is high, up to 0.64. The dominant deformation mechanism in high strain rate superplasticity is still grain boundary sliding (GBS), which was studied systematically in this study. The dislocation creep controlled by grain boundary diffusion was considered the main accommodation mechanism, which was observed in this study.     

A kinetic model for the degradation of benzothiazole by Fe3+-photo-assisted Fenton process in a completely mixed batch reactor

The degradation of benzothiazole in aqueous solution by a photo-assisted Fenton reaction has been studied in a batch reactor in the pH range 2.0–3.2 and for H₂O₂ and Fe(III) concentrations respectively between 1.0×10⁻³–1.5×10⁻¹ and 1.0×10⁻⁶–4.0×10⁻⁶ M.

A kinetic model has been developed to predict the decay of benzothiazole at varying reaction conditions. The use of kinetic constants from the literature in the model allows to simulate the system behavior by taking into account the influence of pH, hydrogen peroxide, Fe(III) and sulfate concentrations and the ionic strength.     

Evaluation of the stress corrosion cracking behaviour of damage-tolerant Al---Li sheet using the slow strain rate testing technique

Slow strain rate tests were performed on longitudinal tensile specimens of 8090-T81 sheet under permanent immersion conditions in various synthetic environments. Strain rates were in the range 10⁻⁷−10⁻⁴ s⁻¹. Environmentally assisted cracking is observed in aqueous chloride-carbonate-hydrogencarbonate solutions. Near neutral 3.5% NaCl solution and also 3% NaCl solution with hydrogen peroxide added do not promote stress corrosion cracking with 8090-T81 alloy sheet. The degradation of ductility found with tensile specimens immersed in the latter corrosive environments is caused by localized corrosion independent of stress. Fracture energy data obtained from slow strain rate tests in substitute ocean water reveal a large scatter. Again, the deterioration observed is not related to stress corrosion cracking. Slow strain rate tests were also carried out with longitudinal tensile specimens of 2091-T8X and 2091 CPHK-T8X alloy sheet using an aqueous solution of 3% NaCl + 0.3% H₂O₂. For the alloy 2091 CPHK-T8X, similar results were obtained to those with 8090-T81, whereas 2091-T8X sheet is prone to environment-induced cracking in the aqueous chloride-peroxide solution.     

Highly conducting indium tin oxide (ITO) thin films deposited by pulsed laser ablation

Highly conducting and transparent indium tin oxide (ITO) thin films were prepared on SiO₂ glass and silicon substrates by pulsed laser ablation (PLA) from a 90 wt.% In₂O₃-10 wt.% SnO₂ sintered ceramic target. The growths of ITO films under different oxygen pressures (P_O2) ranging from 1×10⁻⁴–5×10⁻² Torr at low substrate temperatures (T_s) between room temperature (RT) and 200°C were investigated. The opto-electrical properties of the films were found to be strongly dependent on the P_O2 during the film deposition. Under a P_O2 of 1×10⁻² Torr, ITO films with low resistivity of 5.35×10⁻⁴ and 1.75×10⁻⁴ Ω cm were obtained at RT (25°C) and 200°C, respectively. The films exhibited high carrier density and reasonably high Hall mobility at the optimal P_O2 region of 1×10⁻² to 1.5×10⁻² Torr. Optical transmittance in excess of 87% in the visible region of the solar spectrum was displayed by the films deposited at Po₂≥1×10⁻² Torr and it was significantly reduced as the P_O2 decreases.     

Characterization of indium zinc oxide thin films prepared by pulsed laser deposition using a Zn3In2O6 target

Using a Zn₃In₂O₆ target, indium-zinc oxide films were prepared by pulsed laser deposition. The influence of the substrate deposition temperature and the oxygen pressure on the structure, optical and electrical properties were studied. Crystalline films are obtained for substrate temperatures above 200°C. At the optimum substrate deposition temperature of 500°C and the optimum oxygen pressure of 10⁻³ mbar, both conditions that indeed lead to the highest conductivity, Zn₃In₂O₆ films exhibit a transparency of 85% in the visible region and a conductivity of 1000 S/cm. Depositions carried out in oxygen and reducing gas, 93% Ar/7% H₂, result in large discrepancies between the target stoichiometry and the film composition. The Zn/In (at.%) ratio of 1.5 is only preserved for oxygen pressures of 10⁻²–10⁻³ mbar and a 93% Ar/7% H₂ pressure of 10⁻² mbar. The optical properties are basically not affected by the type of atmosphere used during the film deposition, unlike the conductivity which significantly increases from 80 to 1400 S/cm for a film deposited in 10⁻² mbar of O₂ and in 93% Ar/7% H₂, respectively.     

Mechanoelectrochemical Behavior of Pure Magnesium and Magnesium Alloys Stressed in Aqueous Solutions

A new synergistic effect of corrosion and stress on the viscoelasticity of pure magnesium and magnesium alloys has been shown. This phenomenon named corrosion creep has been studied in 99.9653% Mg, die-cast AZ91D (Mg–9% Al–1% Zn), AM50 (Mg–5% Al–0.4% Mn), and AS21 (Mg–2.3% Al, 0.23% Mn, 1.10% Si) alloys. Creep tests were carried out at 25°C in air and in the borate buffer aqueous solution (pH 9.3). It is found that the highest sensitivity to creep in the corrosive environment is observed in the alloy with the highest Al content. This agrees with the data obtained earlier in the study of mechanoelectrochemical behavior and corrosion fatigue of Mg alloys. However, in air, the creep behavior of all alloys at room temperature is approximately the same. Creep life of pure magnesium and its alloys significantly decreases in a corrosive environment. Corrosion-creep cracks filled with magnesium hydroxide particles were observed by SEM–EDS analysis at the surface of AS21 alloy. The thickness of the oxide layer defined by AES in samples of AZ91D, AM50, and AS21 alloys placed for 2 h into the borate solution amounts to 540, 1320, and 1440 nm, respectively. This can be explained with the account for surface phenomena.     

Thermal analysis and solidification pathways of Mg–Al–Ca system alloys

Mg–Al–Ca alloys are creep resistant magnesium alloys with high application potentials. The solidification pathways and microstructure formation in this alloy system are still under discussion. In this paper, the solidification behavior of AZ91 and AM50 with Ca addition (AZC91x and AMC50x alloys) was investigated by a computer-aided cooling curve analysis (CA-CCA) system. Microstructure and phase identification were carried out by SEM and EDX analysis. The results show that the Ca-containing phase formation mainly depends on Ca content and Ca/Al ratio. With increasing the Ca/Al ratio these phases transform from Al₂Ca to (Mg, Al)₂Ca and Mg₂Ca. Moreover, Ca addition decreases the liquidus temperature of Mg–Al alloys, but influences the solidus temperature in a more complex way. Increasing the Ca content also decreases the solid fraction at which dendrite coherency occurs. The relationship between solidification interval, dendrite coherency point, formation of Ca-containing phases and hot tearing is also discussed.     

Application of ultrasound for fatigue testing of lightweight alloys

The use of aluminium and magnesium alloys offers a great potential for weight reduction in automotive applications. Load-bearing car components are subjected to 10⁸ cycles and more during service, and the high-cycle fatigue properties of construction materials are therefore of great interest.
The time-saving ultrasound fatigue testing method has been used to study the fatigue properties of a high-pressure, die-cast magnesium alloy AZ91  hp and a post-forged, cast-aluminium alloy AlSi7Mg0.3 in ambient air and saltwater (5wt% sodium chloride) spray. In ambient air, fatigue cracks in AZ91  hp emanate from voids, and it is possible to correlate void areas with the numbers of cycles-to-failure. Post-forging of AlSi7Mg0.3 reduces the numbers and size of voids. The remaining small voids (void areas smaller than 9000  μm² ) do not significantly reduce lifetimes. Saltwater deteriorates the fatigue properties of both the lightweight alloys. With increasing numbers of cycles, the influence of the corrosive liquid on fatigue strength becomes more pronounced.     

Effect of substituting cerium-rich mischmetal with lanthanum on high temperature properties of die-cast Mg–Zn–Al–Ca–RE alloys

Mg–Zn–Al–Ca–RE alloys have been found to be promising materials for substituting aluminum alloys used for automatic transmission case applications in the automobile industry. Particularly, Mg–0.5%Zn–6%Al–1%Ca–3%RE (ZAXE05613) alloy exhibits comparable creep resistance as ADC12 die-casting aluminum alloy that is currently used for automatic transmission case applications. Changing the rare earth (RE) content of the alloy from mischmetal to lanthanum gives a further improvement in the creep properties of the alloy. Lanthanum addition results in the crystallization of a large amount of acicular Al₁₁RE₃ (Al₁₁La₃) compound along the grain boundaries as well as across the grain boundaries and this effectively controls grain boundary sliding and dislocation motion in the vicinity of the grain boundaries. As a result, die-cast ZAXLa05613 alloy exhibits a higher creep resistance than that of ZAXE05613 alloy.     

Effect of stress ratio on the cyclic tension corrosion fatigue life of notched steel BS970:976M33 in sea water with cathodic protection

Steel conforming to BS970:976M33 has been fatigue tested in notched form in air and in synthetic sea water, both freely corroding and cathodically protected at −1050 mV with respect to the saturated Ag/AgCl reference electrode. The cyclic and mean stresses were varied to study the effects of changes in the stress ratio, R(σ_min/σ_max), from 0.05 to 0.75, on the fatigue life response. Compared with the fatigue performance obtained in air at R=0.05 free corrosion lowered the fatigue strength at 10⁶ cycles to failure from 430 MPa to 135 MPa and cathodic protection changed it to 270 MPa. In each condition changes in R from 0.05 to 0.5 lowered the fatigue strength in the short life range by approximately 50% but had a much smaller effect, of approximately 10%, at a fatigue life of 10⁶ cycles.     

Temperature dependence of the growth of gallium oxide on CoGa(100)

The oxidation of a CoGa(100) surface at high temperatures has been studied by scanning tunnelling microscopy (STM) and auger electron spectroscopy (AES). When CoGa(100) is oxidised at a sufficiently high temperature (>600 K), an ordered Ga₂O₃ film is formed. The stability of the film depends on the sample temperature and partial oxygen pressure of the ambient gas. At negligible oxygen pressure (<10⁻¹¹ mbar) the oxide is stable up to 850 K. At an oxygen pressure of 10⁻⁶ mbar the oxide is stable up to 930 K and some of the oxide remains present up to 970 K. The oxide film is found to be very uniform. The thickness of the film is constant and independent of the oxidation temperature (600 K<T<930 K), oxygen pressure (<10⁻⁶ mbar), and exposure (10⁻⁴–10⁻² mbar.s≈10²–10⁴ L). We find a clear improvement of the order of the oxide film surface with increasing oxidation temperature. In STM images, a domain structure of the oxide film is observed. The size of the domains increases by a factor of 5–10 when the oxidation temperature is increased from 700 to 900 K.     

Surface and electrical studies of CuO:V2O5 thin films

Results from the studies of multicomponent CuO:V₂O₅ bulk material and thermally evaporated thin films of highly conducting bulk composition prepared at different substrate temperatures are thus compared and discussed. The electronic conductivity is enhanced on increase in the substrate temperature T_s and reaches a maximum value of 12.3 × 10⁻⁶Ω⁻¹ cm⁻¹ for T_s = 423 K. X-ray photoelectron spectroscopy studies indicate an increase in the reduced states of vanadium and copper ions in going from the bulk glass to the thin film. Dynamic secondary-ion mass spectroscopy studies on thin films over a depth of 3000 Å show a strong dependence of T_s on the Cu-to-V intensity ratio. Even though stoichiometric values for thin films are achievable by varying the T_s, the oxidation states of Cu in these films are predominantly monovalent. The electrical behaviors of these materials and their thin film counterparts are finally being discussed in relation to the surface analysis data.     

Mechanical properties of Friction Stir Processed 2618/Al2O3/20p metal matrix composite

The effect of Friction Stir Processing (FSP) on the mechanical properties of 2618 aluminium alloy reinforced with 20% of alumina particles aluminium alloy has been studied in the present paper. The material was processed into the form of sheets of 7 mm thickness after T6 treatment and was tested in tension and fatigue at room temperature.

Tensile tests were also performed at higher temperatures and different strain rates in the nugget zone, in order to analyse the superplastic properties of the recrystallized material and to observe the differences with the parent materials as a function of the strong grain refinement due to the Friction Stir Process. The high temperature behaviour of the material was studied, in longitudinal direction, by means of tensile tests in the temperature and strain rate ranges of 400–500 °C and 10⁻³–10⁻¹ s⁻¹, respectively.

Fracture surfaces of the deformed fatigue test specimens were comprehensively examined in a scanning electron microscope equipped with field emission gun to determine the macroscopic fracture mode and characterize the fine-scale topography and microscopic mechanisms governing fatigue fracture.

The mechanisms governing fatigue life, cyclic deformation and fracture characteristics are analysed in function of magnitude of applied stress, intrinsic micro structural evolution and material deformation behaviour.     

Cyclic deformation behavior of double-slip-oriented copper single crystals I: coplanar double slip orientation on 011-1&#x0304;11 side of the stereographic triangle

The cyclic deformation behaviors of [2̄33] coplanar double-slip-oriented and [4̄ 18 41] single-slip-oriented copper single crystals were investigated at constant plastic shear strain amplitude γ_pl in the range of about 10⁻⁴–10⁻² at ambient temperature in air. It was revealed that the cyclic deformation behavior of copper single crystal oriented on the 011-1̄11 side is distinctly dissimilar from that on the 001-1̄11 and 001-011 sides in the stereographic triangle. The plot of initial hardening rate θ_0.2 against γ_pl of [2̄33] crystal exhibits two regions as presented for single-slip-oriented crystals. The critical strain amplitude (≈3.5×10⁻³), corresponding to the occurrence of the secondary hardening stage in the cyclic hardening curve of the [2̄33] crystal, was found to be an intermediate value between that for single-slip-oriented single crystals and polycrystals. The result shows that the cyclic hardening behavior of the [2̄33] crystal, as compared with that of single-slip-oriented crystals, is more close to that of polycrystals. Instead of a clear plateau, the cyclic stress–strain (CSS) curves of the [2̄33] crystals shows a quasi-plateau over the range of about 3.0×10⁻⁴–2.0×10⁻³, which would be greatly attributed to the mode of dislocation interactions between slip systems operating in the crystal. The habit plane of two types of deformation bands DBI and DBII, formed in the cyclically deformed [2̄33] crystal, are perpendicular to each other strictly, and they develop with increasing applied strain amplitude.     

Ratcheting and low-cycle fatigue characterizations of extruded AZ31B Mg alloy with and without corrosive environment

The ratcheting and low-cycle fatigue behaviors of extruded AZ31B in air and in PBS are investigated. To realize the corrosive environment and monitor the axial strain of AZ31B tested in PBS, a miniaturized chamber attachment is self-designed and an eddy current sensor is employed. Stress strain loop and strain evolution under fully symmetric and asymmetric stress cycling with and without corrosive environment are presented. The influences of twinning and detwinning, ratcheting fatigue interaction and fatigue corrosion are all discussed. Considering the effect of ratcheting and corrosion environment on damage evolution of extruded AZ31B, a modified Ellyn’s model is presented and the results agree with the experimental observations very well.     

Development of Bi-oxide superconducting tapes and coils

We have developed Bi-2212 and 2223 tapes. For Bi-2212, two double stacked pancake type coils were fabricated using Bi-2212/Ag tapes prepared by a combination of the continuous dip-coating process and melt-solidification. A small coil (13 mm inner bore, 46.5 mm outer diameter) was inserted in a conventional superconducting magnet system. In a bias field of 20.9 T, the generated field of the coil was 0.9 T, at an I_c of 310 A (criterion 10⁻¹³ Ωm) at 1.8 K. Thus, the superconducting magnet system achieved the generation of a field of 21.8 T in the full superconducting state. A large coil (20 mm inner bore, 94 mm outer diameter) generated a field of 2.6 T (I_c = 385 A (10⁻¹³ Ωm)) at 4.2 K and 1.53 T (I_c = 225 A (10⁻¹³Ωm)) at 20 K in self-field. For Bi-2223, tapes were prepared by the powder-in-tube technique using Ag-10% Cu-x%M (x = 0–1.0, M = Ti, Zr, Hf or Au) alloy sheaths. The high J_c values of 5–7 × 10⁴ A cm⁻² at 4.2 K and 14 T were obtained for the tapes doped with x = 0.03–0.1 at.% Ti, 0.1 at.% Zr, 0.1 at.% Hf or 0.3% Au. These tapes have a modified Bi-2223 grain structure at the sheath/core interface and also a dense and more aligned microstructure, resulting in higher J_c values.     

Relaxation of residual stress with fatigue loading

Investigations have been carried out to study the relaxation of the surface residual stress in 0.23% C steel due to the application of fatigue loading. The residual stress was induced in the specimen by pre-straining and was measured by X-ray back reflection method using Cr-K radiation. The surface residual stress induced, depends on the plastic strain and appears to bear a relation of the type σ_R = σ_o(e_p)^0.78. The decay of the residual stress appears to depend on log N, given by the relation σ_R1 = σ_RO−K log N, where N is the number of fatigue cycles. The constant K depends on the initial value of the residual stress.