Rupture locations of friction stir welded joints of AA2017-T351 and AA6061-T6 aluminum alloys

The tensile rupture locations of friction stir welded joints of AA2017-T351 and AA6061-T6 aluminum alloys were examined. The experiments show that the rupture locations of the joints are different for the two aluminum alloys, which are influenced by the welding parameters. When the joints are free of welding defects, the AA2017-T351 joints are ruptured in the weld nugget adjacent to the thermo-mechanically affected zone on the advancing side and the rupture surfaces appear as oval contours of the weld nugget, while the AA6061-T6 joints are ruptured in the heat affected zone on the retreating side and the rupture surfaces are inclined at a certain degree to the bottom surfaces of the joints. When welding defects are present in the joints, the AA2017-T351 joints are ruptured in the weld center, while the AA6061-T6 joints are ruptured on the retreating side near the weld center. The rupture locations of the joints are dependent on the internal structures of the joints and can be explained through them.     

Pack Aluminide Coatings Formed at 650 °C for Enhancing Oxidation Resistance of Low Alloy Steels

THE COMMERCIAL ALLOY STEELS containingtypically9-12wt.%Cr and1wt.%Mo are low costmaterials developed for critical strength engineeringapplications at temperatures up to700°C[1-2].Thecomposition and microstructure of these materials arenormally optimised to provide the required mechanicalstrength and creep/fatigue resistance.However,thehigh temperature oxidation resistance is often notsufficient to prevent premature failure caused byoxidation degradation,particularly in high temperat…     

The Oxidation of a Directionally Solidified Ni-Al-Cr3C2 Alloy at 1100 and 1200°C in Oxygen

The oxidation behavior of a directionallysolidified Ni-Al-Cr₃C₂ alloy ofover all composition Ni-12.3Cr-6.9Al-1.8C (wt.%) hasbeen investigated at 1100 and 1200°C under 1 atmoxygen. Experiments have also been carried out on specimens having the samecomposition but with a nonaligned structure. At1100°C, in both cases and unlike conventionalnickel-base superalloys with the same chromium andaluminum contents, aluminium was found to oxidize internallybeneath an external Cr₂O₃ scale.Although the volume fraction of the internalprecipitates was significant, they showed no tendency tocoalesce into a compact subsurface layer, but formed randomly distributed clustersin the alloy matrix. The kinetics of oxidation andmorphologies of the oxide scales were not substantiallyaffected either by thermal cycling or by the alloy microstructure. At the higher temperature,continuous Al₂O₃ scales formedbeneath thick layers of transient nickel andnickel-chromium-rich oxides; no internal precipitationof aluminum-rich oxides was observed. However, internal degradation of thedirectionally solidified specimens at 1200°C wasquite significant, due to in situ oxidation of primarycarbides. The multilayered scales formed at 1200°C spalled extensively on cooling as a consequenceof loss of contact, starting preferentially at theintersections of the Cr₂C₃ fiberswith the alloy-scale interface. The observed behaviorcan be attributed to a reduction in the availability of chromiumbecause of the multiphase structure of the alloy; this,in turn, resulted in an increase in the flux of oxygeninward, leading to internal oxidation of aluminum at 1100°C. The almost exclusive externaloxidation of aluminum becomes possible at 1200°C,probably because of an increase in the diffusivity ofaluminum in the alloy matrix.     

Investigation of the Fluorine Microalloying Effect in the Oxidation of TiAl at 900°C in Air

High-temperature oxidation resistance of gamma titanium aluminides can be achieved by the formation of a continuous scale of slowly growing Al₂O₃. The formation of such a scale was favored by the addition of small amounts of fluorine. It is shown that fluorine can have a beneficial effect on oxidation resistance in a certain F-range which is quantified by thermodynamic calculations and by experimental investigations. It is assumed that the F-effect offers a significant potential for improvement of the oxidation resistance of technological TiAl alloys.     

Oxidation behavior of three commercial ODS alloys at 1200°C

The isothermal-oxidation behavior of three oxide-dispersion-strengthened (ODS) alloys, viz., MA 956, ODM 751, and PM 2000, has been examined in air at 1200°C for exposure times up to 4800 hr. During exposure all the alloys formed an external scale of alpha alumina (-Al₂O₃). The growth rate of alumina on MA 956 was significantly faster than that formed on ODM 751 resulting in an oxide layer which was about twice as thick after 4800 hr. The oxide-grain morphology on MA 956 was essentially equiaxed containing irregularly shaped, titanium-rich particles, whereas the oxide formed on ODM 751 was slightly finer, distinctly columnar and contained elongated yttrium-rich particles. Spalling of the oxide layer occurred after approximately 2400 hr on MA 956, whereas only slight spalling occurred on ODM 751 even after the longest exposure time. Experiments revealed that the initial surface roughness of PM 2000 can contribute significantly to spalling by enabling the growth of highly convoluted scale layers which are mechanically unstable under compressive stresses (buckling). Internal porosity is also observed in all three alloys after exposure. The pores were generally spherical with small Ti-, Al-, Y-rich particles distributed over their internal surfaces. The amount of porosity increases to a maximum and then slowly decreases.     

Sub-Scale Depletion and Enrichment Processes During High Temperature Oxidation of the Nickel Base Alloy 625 in the Temperature Range 900�C1000?��C

Numerous chromia-forming austenitic steels and nickel-base alloys contain chromium-rich strengthening precipitates, e.g. chromium-base carbides. During high temperature exposure the formation of the chromia base oxide scale results in chromium depletion in the alloy matrix and consequently in dissolution of the strengthening phase in the sub-surface zone. The present study describes the oxidation induced phase changes in the chromium depletion layer in case of alloy 625, a nickel base alloy in which the strengthening precipitates contain hardly any or only minor amounts of chromium. Specimens of alloy 625 were subjected to oxidation up to 1000 h at 900 and 1000 °C and analyzed in respect to oxide formation and microstructural changes using light optical microscopy, scanning electron microscopy, energy and wavelength dispersive analysis, glow discharge optical emission spectroscopy, and X-ray diffraction. In spite of the fact that the alloy precipitates ??-Ni₃Nb and/or (Ni, Mo)₆C contain only minor amounts of chromium, the oxidation induced chromium depletion results in formation of a wide sub-surface zone in which the precipitate phases are depleted. However, in parallel, substantial niobium diffusion occurs towards the alloy surface resulting in formation of a thin layer of ??-Ni₃Nb phase adjacent to the alloy/oxide interface. By modeling phase equilibria and diffusion processes using Thermo-Calc and DICTRA it could be shown that the phase changes in the sub-scale zone are governed by the influence of alloy matrix chromium concentration on the thermodynamic activities of the other alloying elements, mainly niobium and carbon. The ??-phase depletion/enrichment process is caused by a decreasing niobium activity with decreasing chromium concentration whereas the (Ni,Mo)₆C dissolution finds its cause in the increasing carbon activity with decreasing chromium content.     

Air Oxidation of FeCoNi-Base Equi-Molar Alloys at 800–1000°C

The oxidation behavior of FeCoNi, FeCoNiCr, and FeCoNiCrCu equi-molar alloys was studied over the temperature range 800–1000 °C in dry air. The ternary and quaternary alloys were single-phase, while the quinary alloy was two-phase. In general, the oxidation kinetics of the ternary and quinary alloys followed the two-stage parabolic rate law, with rate constants generally increasing with temperature. Conversely, three-stage parabolic kinetics were observed for the quaternary alloy at T 900°C. The additions of Cr and Cu enhanced the oxidation resistance to a certain extent. The scales formed on all the alloys were triplex and strongly dependent on the alloy composition. In particular, on the ternary alloy, they consist of an outer-layer of CoO, an intermediate layer of Fe₃O₄, and an inner-layer of CoNiO₂ and Fe₃O₄. Internal oxidation with formation of FeO precipitates was also observed for this alloy, which had a thickness increasing with temperature. The scales formed on the quaternary alloy consisted of an outer layer of Fe₃O₄ and CoCr₂O₄, an intermediate layer of FeCr₂O₄ and NiCr₂O₄, and an inner layer of Cr₂O₃. Finally, the scales formed on the quinary alloy are all heterophasic, consisting of an outer layer of CuO, an intermediate-layer of CuO and Fe₃O₄, and an inner-layer of Fe₃O₄, FeCr₂O₄, and CuCrO₂. The formation of Cr₂O₃ on the quaternary alloy and possibly that of CuCrO₂ on the quinary alloy was responsible for the reduction of the oxidation rates as compared to the ternary alloy.     

The Influence of Yttrium Implantation on the Oxidation of Impure Iron Containing C and Mn at 700°C

A study of impurity-yttrium interactions hasbeen performed during iron oxidation [p(O₂)= 0.04 Pa, T = 700°C]. Yttrium-implanted specimensalways exhibit better oxidation behavior compared withblank specimens. On pure iron or the Fe 0.054 wt.%C alloy thebeneficial effect is attributed toFe₂YO₄ formation. With themanganese-containing alloys (Fe 0.2 wt.%Mn), theprotective effect of yttrium is attributed to YMnO₃ formation. The best oxidationbehavior is obtained with implanted Fe0.18 wt.%Mn-0.041wt.%C alloys due to the formation of an YMnO₃oxide subscale at the scale-alloy interface. Yttriumimplantation also hinders carbon segregation at theoxide-alloy interface. This effect ensures better scaleadherence. With the most-impure alloy, yttriumimplantation also changes the growth process fromexternal cation diffusion to predominant inward-oxygendiffusion.     

Oxidation Behavior of Tribaloy T-800 Alloy at 800 and 1,000 °C

The oxidation behavior of Co-based Tribaloy T-800 alloy has been studied isothermally in air at 800 and 1,000 °C, respectively. The results showed that the oxidation mechanism was dependent on the exposure temperature. The oxidation of the alloy followed subparabolic oxidation kinetics at 800 °C. The oxide scale at this temperature exhibited a multi-layered structure including an outer layer of Co oxide, a layer composed of complex oxide and spinel, a nonuniform Mo-rich oxide layer, an intermediate mixed oxides layer and an internal attacked layer with different protrusions into Laves phase. During 1,000 °C exposure, it followed linear kinetics. The oxidation rendered a relatively uniform external Cr-rich oxide layer coupled with a thin layer of spinel on the top surface and voids at local scale/alloy interface and intergranular region together with internal Si oxide at 1,000 °C.     

Effect of CeO2 on the Isothermal Oxidation of Electrode- posited Ni-Al Composite Coatings at 950 °C

采用微米Al和纳米Ce O2颗粒与Ni复合的方法在Ni基上制备了3种不同Al含量的Ni-Al-Ce O2复合镀层,作为对比采用相同的工艺制备了不含纳米CeO 2颗粒的Al含量相当的Ni-Al复合镀层。微米Al颗粒尺寸范围为1~5m,纳米Ce O2颗粒平均尺寸为10 nm。SEM/EDAX和TEM分析结果表明,纳米CeO 2颗粒的添加细化了基体Ni的晶粒。950℃下恒温氧化实验结果表明:在相同的Al颗粒含量条件下,不管Ni-Al复合镀层是形成NiO还是Al2O3氧化膜,纳米CeO 2颗粒的加入都提高了Ni-Al镀层的抗氧化性能。同时对Ce O2影响Ni-Al复合镀层的氧化性能进行了详细的分析。     

Oxidation protection of 20Cr-25Ni-Nb stainless steel at 1300°C

Enhanced environmental protection of chromia-forming advanced metallic alloys at normal operating temperature, typically 900°C, may be provided by two well-established approaches—incorporation of reactive elements into the protective oxide scale or an amorphous ceramic coating acting as a diffusion barrier. The continued effectiveness of such approaches, namely by cerium and yttrium ion implantation and with a vapor deposited amorphous silica coating, in reducing oxidation of 20Cr-25Ni-Nb stainless steel in a carbon-dioxide-based environment has been examined during 0.5 and 1 hr transients to 1300°C. The influence of pre-oxidation of the ion-implanted, silica-coated, and uncoated steel for extended periods in the same environment at 825–900°C has also been established.     

The Air Oxidation of Two-Phase Fe-Cu Alloys at 600-800°C

The air oxidation of three Fe-Cu alloyscontaining 25, 50, and 75 wt.% Cu has been studied at600-800°C. The oxidation followed the parabolic lawonly approximately with rates lower than those of thepure constituent metals. The scales were alwayscomposed of an inner layer containing a mixture ofcopper metal and iron oxide and of an outer oxide layerwhose composition depended on the copper content of the alloy. For the two alloys richer in ironthe external layer was composed mostly of iron oxideswith some copper-rich particles which oxidized only inthe external-scale zone. For the alloy richest in copper the external layer contained a complexmixture of iron oxides, copper particles and doubleFe-Cu oxides surmounted by an outermost copper-oxidelayer. No significant iron depletion was observed in the alloys beneath the region of internaloxidation. The peculiar scale microstructure observedfor these alloys is considered mainly as a consequenceof their two-phase microstructure and of the limited solubilities of the two components in oneanother.     

Oxidation of zirconium between 400° and 800°C

The vacuum microbalance method is used to study the oxidation reaction for two surface preparations over the temperature range of 400° to 800°C. The results fit in well with the authors previous work at temperatures of 200° to 425°C and with the work of other groups at higher temperatures. An analysis of the rate data shows that the cubic rate law fits the experimental data best for the abraded specimens. However, the parabolic rate law can be fitted to the data if an initial deviation is disregarded. With chemically polished specimens, a good fit is obtained with the parabolic rate law. The parabolic rate law constant A gives two straight lines when plotted as log A vs 1/T. For the temperature range of 200° to 525°C an energy of activation of 18,200 cal per mol is calculated while a value of 28,600 cal per mol is calculated for the temperature range of 525° to 750°C. The results of this work bring together the previously determined high-temperature oxidation studies of Cubicciotti with the early low-temperature studies of Gulbransen and Andrew.     

Air Oxidation of Ni–Ti Alloys at 650–850°C

The oxidation of pure Ni and three Ni–Ti alloys containing 5, 10, and 15 wt.% Ti over the temperature range 650–850°C in air was studied to examine the effect of titanium on the oxidation resistance of pure nickel. Ni–5Ti is a single-phase solid solution, while the other two alloys consisted of nickel solid solution (-Ni) and TiNi₃. The oxidation of Ni–Ti alloys at 650°C follows an approximately parabolic rate law and produces a decrease in the oxidation rate of pure Ni by forming an almost pure TiO₂ scale. At higher temperatures, Ni–Ti alloys also follow an approximately parabolic oxidation, and their oxidation rates are close to or faster than those of pure Ni. Duplex scales containing NiO, NiTiO₃ and TiO₂ formed. Some internal oxides of titanium formed, especially at 850°C. In addition, the two-phase structure of Ni–10Ti and Ni–15Ti was transformed into a single-phase structure beneath the scales.     

超声波冲击技术对AA6061-T6空蚀行为的影响

AA6061-T6铝合金因较高的比强度、良好的可成型性广泛应用于汽车工业中,但较差的空蚀性能极大的降低了其使用寿命。利用超声波冲击技术在AA6061-T6表面制备了塑性形变强化层,研究塑性形变强化层的微观组织、残余应力及显微硬度分布和抗空蚀性能。结果表明,超声波冲击强化后铝合金表面择优取向由原始的(200)转变为(111),并形成约140μm塑性变形层。与未强化试样相比,强化试样表面显微硬度提高了80.7%,并形成140μm硬化层;表面植入残余压应力达到-259 MPa,残余压应力层深达到700μm。超声波冲击强化处理后的空蚀性能提高2.36倍;空蚀表面形貌分析表明强化后试样空蚀机制由韧性断裂转变为脆性断裂和疲劳破坏。研究表明超声波冲击强化后试样表面晶粒细化、择优取向转变、硬度提高以及较高残余压应力等增强了材料的抗空蚀性能。     

Coordination of Pre-oxidation Time and Temperature for a Better Corrosion Resistance to CO2 at 550 °C

Oxidation of Metals - Influence of pre-oxidation treatment on the oxidation resistance of F91 to CO2 at 550 °C was investigated. The formation of the thin scale of (Fe, Cr)2O3...     

Cyclic Oxidation Behavior of the Ti–6Al–4V Alloy

The cyclic oxidation behavior of the Ti–6Al–4V alloy has been studied under heating and cooling conditions within a temperature range from 550 to 850 °C in air for up to 12 cycles. The mass changes, phase, surface morphologies, cross-sectional morphologies and element distribution of the oxide scales after cyclic oxidation were investigated using electronic microbalance, X-ray diffractometry, scanning electron microscopy and energy dispersive spectroscopy. The results show that the rate of oxidation was close to zero at 550 °C, obeyed parabolic and linear law at 650 and 850 °C, respectively, while at 750 °C, parabolic—linear law dominated. The double oxide scales formed on surface of the Ti–6Al–4V alloy consisted of an inner layer of TiO₂ and an outer layer of Al₂O₃, and the thickness of oxide scales increased with an increasing oxidation temperature. At 750 and 850 °C, the cyclic oxidation resistance deteriorated owing to the formation of voids, cracks and the spallation of the oxide scales.     

Oxidation Mechanism of Cu2O to CuO at 600–1050°C

To clarify the oxidation mechanism of Cu₂O to CuO, Cu₂O oxidation was studied at 600–1050 °C under 1atm O ₂. The Cu₂O specimens were prepared through completely oxidizing 99.99999 and 99.5 pure copper at 1000°C in an Ar + 1 O ₂ atmosphere. The oxidation kinetics of Cu₂O specimens prepared from both purity levels followed the logarithmic law, not the parabolic law or the cubic law as reported in the literature. The activation energy for Cu₂O oxidation is relatively high in the lower-temperature range, but becomes very small or even negative at higher temperatures. The logarithmic oxidation rate law can be explained by Davies et al.s model related to grain-boundary diffusion in the oxide layers. The very small or negative activation energies in the higher-temperature range can be attributed to the very small thermodynamic driving force and the fast lateral growth of CuO grains related to a sintering effect. The influence of small amount of impurities is also discussed.     

Oxidation behavior of oxidation protective coatings for C/C-SiC composites at 1 500 °C

Porous carbon/carbon preforms were infiltrated with melted silicon to form C/C-SiC composites. Three-layer Si-Mo coating prepared by slurry painting and SiC/Si-Mo multilayer coating prepared by chemical vapor deposition(CVD) alternated with slurry painting were applied on C/C-SiC composites, respectively. The oxidation of three samples at 1 500 °C was compared. The results show that the C/C-SiC substrate is distorted quickly. Three-layer Si-Mo coating is out of service soon due to the formation of many bubbles on surface. The mass loss of coated sample is 0.76% after 1 h oxidation. The sample with SiC/Si-Mo multilayer coating gains mass even after 105 h oxidation. SiC/Si-Mo multilayer coating can provide longtime protection for C/C-SiC composites and has excellent thermal shock resistance. This is attributed to the combination of dense SiC layer and porous Si-Mo layer. Dense SiC layer plays the dual role of physical and chemical barrier, and resists the oxidation of porous Si-Mo layer. Porous Si-Mo layer improves the thermal shock resistance of the coating.     

X-ray Diffraction Study of a Quasicrystalline Alloy Stable at High Temperature

Crystalline A1-Cu-Fe-Mg alloy powders were produced by means of rapid solidification.During annealingprocess,the quasicrystalline icosahedral phase formed at about 625°C and was stable until 900°C.Thisquasicrystalline phase is considered to be a stable one.The forming process of this phase was studied in detail.