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Sheet material of the Al–Mg–Si–Cu alloy 6013-T6 was additionally aged at 191 °C for up to 100 h and thermally exposed at temperatures ranging from 85 to 150 °C for time periods up to 3000 h. Microstructure, tensile properties and corrosion behaviour were investigated in the different heat treatment conditions using differential scanning calorimetry and transmission electron microscopy as well as performing tensile tests and various corrosion tests. Hardening precipitates present in the peak-aged microstructure were the metastable β″ and Q′ phases. Alloy 6013-T6 exhibited sensitivity to pitting and intergranular corrosion when immersed in chloride solutions, but was immune to intergranular stress corrosion cracking. Increasing aging at 191 °C resulted in progress in the precipitation sequence and coarsening of the precipitates. However, the degradation of tensile properties due to overaging was slight, and no change in corrosion behaviour was observed. Prolonged thermal exposure at temperatures between 85 and 120 °C did not cause significant alterations in microstructure, tensile properties, and corrosion performance of peak-aged 6013 sheet, indicating thermal stability of this alloy when used in fuselage applications.

Sheet materials of the alloys 6056 and 2024A were joined using the friction stir welding technique. Similar and dissimilar butt welds were produced. Strength and corrosion behaviour of the joints were investigated in different heat treatment conditions. Hardness profiles across the weld revealed minima in the heat affected zones, being very pronounced at the advancing side of the joints. The hardness of the nugget zone increased after post-weld artificial aging. Strengths of postweld heat treated similar 6056 and dissimilar 2024A/6056 joints approached 90 and 85 %, respectively, of the ultimate tensile strength of 6056-T7X. As found by electrochemical measurements, alloy 2024A-T3 exhibited the most noble corrosion potential and artificially aged alloy 6056 the most active potential. Besides microstructural changes caused by the welding process, galvanic coupling affected the corrosion behaviour of friction stir welds, in particular with joints of dissimilar aluminium alloys, as indicated by accelerated corrosion of alloy 6056 in the nugget region. Intensified corrosion attack was also observed in the heat affected zones.

Al-based intermetallic coatings are widely used as bond coats and for oxidation protection of turbine blades and engine components. Here we present and discuss the structural and thermal properties of novel unbalanced-magnetron sputtered Al-based coatings (single-phase intermetallic Al2Au, binary Al-Zr, and nano-structured Al-Zr-Y) developed to protect gamma-TiAl from environmental attack at elevated temperatures. Al-Zr films exhibit a coarse-grained dual-phase Al3Zr2-Al2Zr structure and are nano-structured by alloying with ~5, 10, and 14 at% Y.Combined dynamic differential-scanning calorimetry and thermogravimetric analyses up to a temperature of 1150 °C reveal that the Al2Au film is very stable with only marginal mass gain from oxidation found between 800 and 1000 °C. High temperature X-ray diffraction shows that this coating retains its (311) texture up to 900 °C where Al2O3 formation leads to the depletion of Al in Al2Au and subsequently the precipitation of intermetallic AlAu. When gamma-TiAl is coated with Al2Au and exposed to cyclic oxidation tests at 750 and 850 °C good oxidation resistance is obtained as a protective oxide layer is formed.Dual-phase Al3Zr2-Al2Zr coatings form ZrO2 and Al2O3 in oxidizing atmosphere. However, the phase transition from monoclinic (m-)ZrO2 to tetragonal (t-)ZrO2 with the accompanying volume change causes flaking of the oxide. Yttrium addition to the Al-Zr films stabilizes the cubic (c-) and t-ZrO2 and hence avoids the fatal tetragonal-monocline transformation. The thermally grown c-ZrO2 based oxides allow good adhesion to thermal barrier coatings which are themselves based on c-ZrO2.

The oxidation behaviour of an orthorhombic Ti–22Al–25Nb alloy, bare and with protective coatings, was investigated at 750°C in air under quasi-isothermal and thermal cycling conditions. As found by post-oxidation analysis, the uncoated substrate material was severely degraded by formation of spalling oxide scales and ingress of oxygen and nitrogen causing nitride precipitation, internal oxidation and interstitial embrittlement. Metallic Ti–51Al–12Cr coatings as well as nitride coatings based on Ti–Al–Cr–Y–N, either monolithically grown or with superlattice structure, provided an effective diffusion barrier against oxygen. The excellent oxidation resistance of the TiAlCr coatings was associated with the ternary Ti(Al,Cr)2 Laves phase promoting the formation of a protective alumina scale. The different intermetallic phases formed in the interdiffusion zone caused neither cracking nor spallation of the protective coating. Both, monolithically grown TiAlCrYN and superlattice TiAlYN/CrN coatings, exhib...

The stress corrosion cracking behaviour of plate material of the aluminium alloys 2024-T351, 8090-T8171, 7475-T651, and 7075-T7351 was investigated performing constant load tests. Short transverse tensile specimens were permanently immersed in aerated aqueous 0.6 M Na2Cl solutions with additions of Na2SO4, NaNO3, NaHCO3, NH4HCO3, Na2HPO4, Na2SO3 or Na2CO3. The concentration of the added salts was 0.06 M. The applied stress was 100 MPa, except with 7075-T7351 specimens, which were loaded at 300 MPa. Environment induced failure was not observed in neutral 0.6 M NaCl solution. The various salts added promoted intergranular stress corrosion cracking with the alloys 2024-T351, 8090-T8171, and 7475-T651. Threshold stresses were generally below 100 MPa. For 8090-T8171 exposed to chloride containing electrolytes with additions of sulfate, hydrogen phosphate, or sulfite, threshold stresses were approximately 100 MPa or higher. Similar results were obtained for 7475-T651 plate when immersed in chloride-hydrogen phosphate and chloride-carbonate solutions. Alloy 7075-T7351 was resistant against intergranular stress corrosion cracking. Specimens suffered pitting corrosion during immersion in the corrosive environments. Failure observed with 7075-T7351, in particular when exposed to the chloride-nitrate solution, was associated with reduction of cross-sectional area due to pitting and transgranular stress corrosion cracking. Anioneneinflusse auf das Spannungsrisskorrosionsverhalten von Aluminiumlegierungen Das Spannungsrisskorrosionsverhalten von Plattenmaterial der Aluminiumlegierungen 2024-T351, 8090-T8171, 7475-T651 und 7075-T7351 wurde in kurzer Querrichtung untersucht. Dauertauchversuche wurden unter konstanter Belastung in wassrigen 0,6 M NaCl Losungen mit 0,06 M Zusatzen von Na2SO4, NaNO3, NaHCO3, NH4HCO3, Na2HPO4, Na2SO3 oder Na2CO3 durchgefuhrt. Die aufgebrachte Spannung betrug 100 MPa, auser bei Zugproben der Legierung 7075-T7351, die mit 300 MPa belastet wurden. Wahrend bei Auslagerung in neutraler Natriumchlorid-Losung kein Versagen beobachtet wurde, losten die zugegebenen Salze bei den Legierungen 2024-T351, 8090-T8171 und 7475-T651 interkristalline Spannungsrisskorrosion aus. Die kritischen Grenzspannungen lagen uberwiegend unter 100 MPa. Bei Auslagerung in chloridhaltigen Elektrolyten mit beigegebenen Sulfat-, Hydrogenphosphat- oder Sulfitionen waren die kritischen Grenzspannungen fur die Legierung 8090-T8171 etwas hoher; sie hatten einen um 100 MPa oder daruber liegenden Wert. Gleiches galt fur die Legierung 7475-T651 in chloridhaltigen Losungen mit Hydrogenphosphat- oder Carbonatzusatzen. Plattenmaterial der Legierung 7075-T7351 war in den untersuchten Elektrolyten bestandig gegen interkristalline Spannungsrisskorrosion. Die Proben wurden jedoch durch Lochkorrosion geschadigt. Querschnittsverminderung infolge Lochkorrosion und transkristalliner Spannungsrisskorrosion fuhrten zum Versagen von Zugproben der Legierung 7075-T7351 innerhalb der maximalen Prufdauer von 30 Tagen, insbesondere bei Auslagerung in eine chlorid-nitrathaltige Losung.

The stress corrosion cracking behaviour of 7475 plate material in the tempers T651 and T7351 was investigated performing constant load tests. Short transverse specimens were permanently immersed in aerated aqueous 0.6 M sodium chloride solutions with additions of 0.03 M sodium sulphate, 0.03 M sodium nitrate and 0.1 M sodium bicarbonate. Chloride solutions containing sulphate, nitrate or/and bicarbonate promoted environment-induced cracking. A short transverse threshold stress below 50 MPa was found for 7475-T651 plate material. Neutral 0.6 M NaCl solution was less conducive to stress corrosion cracking. Specimens of alloy 7475 in the overaged temper T7351 failed at applied stresses above 300 MPa. This failure resulted from overload fracture caused by a reduction of cross-sectional area due to pitting corrosion, as confirmed by fractography showing severe pitting attack. Fractographic examinations of 7475-T7351 specimens failed during immersion in chloride-nitrate-bicarbonate containing solutions with or without addition of sulphate after long exposure time periods revealed slight transgranular stress corrosion cracking, too. Pronounced transgranular environment-induced cracking was observed on the fracture surfaces of specimens which were exposed to an aqueous solution containing chloride, sulphate, nitrate, and bicarbonate for 30 days at applied stress and were subsequently tensile tested in an inert environment.

There is a major drive to introduce γ-TiAl into gas turbine engines in order to reduce weight. However, this will require the development of coatings that protect against oxidation at high temperature, but do not adversely affect the mechanical properties. This work reports the high temperature degradation mechanisms of a nanoscale CrAlYN/CrN multilayer coating deposited on γ-TiAl(8Nb) by a combined high power impulse magnetron sputtering / unbalanced magnetron sputtering. Detailed TEM/STEM of FIB prepared specimens from isothermal static oxidation tests at 850°C for up to 1030 hours is presented. The evolution of the complex oxide structure and the implications for future coating development is discussed.

Sheet material of the Al-Mg-Si alloy 6061 in the tempers T4 and T6 was thermally exposed at temperatures ranging from 85 to 120°C for 1000 h. The microstructure, tensile properties and the corrosion behaviour in the different heat treatment conditions were investigated using differential scanning calorimetry and transmission electron microscopy as well as performing tensile tests and various corrosion tests. The additional heat treatments, which should simulate aging during long-term service usage, caused an increase in strength of 6061-T4 sheet, associated with changes in the naturally aged microstructure. Thermal exposure at 120°C for 1000 h resulted in tensile and corrosion properties being similar to those obtained for peak-aged sheet. Alloy 6061 in the T6 temper exhibited microstructural stability when additionally heat treated at 85 and 120 °C for 1000 h. No significant alterations in the microstructure, tensile properties, and corrosion performance were observed after exposure to slightly elevated temperatures.

La meilleure approche pour decrire l'influence magnetique sur la diffusion est d'ajuster l'enthalpie d'exces magnetique a l'energie magnetique du fer determinee experimentalement

The oxidation behaviour of two γ-TiAl-based alloys coated with thermal barrier coatings (TBCs) was investigated in air at 900 °C for up to 1000 h under quasi-isothermal conditions. Samples bare or with a 4 μm thick Ti–51Al–12Cr (at.%) protective layer were coated with yttria-stabilized zirconia using electron-beam physical vapour deposition. Prior to TBC deposition, the specimens were pre-oxidised in air and low-pressure oxygen. Thermal barrier coatings on samples of the alloy Ti–45Al–8Nb–0.2B–0.15C (at.%) did not spall off during the maximum exposure length, whereas failure occurred with pre-oxidised specimens of the alloy Ti–45Al–6Nb–1Cr–0.4W–0.2B–0.5C–0.2Si (at.%). Scanning electron micrographs revealed an outer oxide scale with a columnar structure grown by outward diffusion of Ti and Al cations. Below this columnar oxide layer, a broad porous zone formed consisting predominantly of titania. A discontinuous nitride layer was observed in the transition region between oxide scale and substrate. Spallation of the TBC was associated with cracking in the porous titania-rich layer. The zirconia top coat exhibited an excellent adherence to the thermally grown oxide scales. Pre-exposure in low-pressure oxygen did not improve the oxidation resistance. Post-oxidation microstructural examinations of specimens coated with Ti–Al–Cr revealed that the protective layer was severely deteriorated. On the degraded Ti–Al–Cr coating, an outer scale with columnar structure formed beneath the TBC.