Oxidation behavior of a Zr–Cu–Al–Ni amorphous alloy in air at 300–425 °C

The oxidation behavior of Zr–30Cu–10Al–5Ni bulk metallic glass and its crystalline counterpart was studied over the temperature range of 300–425 °C in dry air. In general, the oxidation kinetics of both amorphous and crystalline alloys followed a two- or three-stage parabolic rate law at T⩾350 °C, while at 300 °C the amorphous alloy oxidized following a linear behavior. The oxidation rate constants for the amorphous alloy are slightly higher than those for the crystalline alloy at 350–400 °C. The scale formed on the amorphous alloy consists of mainly tetragonal-ZrO₂ at 300 °C, while a mixture of monoclinic-ZrO₂ (m-ZrO₂) and tetragonal-ZrO₂ (t-ZrO₂) and some CuO were detected at higher temperatures. The scale formed on the crystalline alloy, on the other hand, consists of mainly Al₂O₃, some tetragonal-ZrO₂, and a slight amount of monoclinic-ZrO₂ at 300 °C. At higher temperatures, the crystalline alloy consists of mainly monoclinic-ZrO₂, some CuO and Cu₂O, and limited tetragonal-ZrO₂. It is suggested that the formation of Al₂O₃ (at 300 °C) and CuO/Cu₂O (at 350-400 °C) on the crystalline alloy is responsible for the reduced oxidation rates as compared with those of amorphous alloy.     

Oxidation resistance of SHS Fe–Al–Si alloys at 800 °C in air

Alloys formed by Fe–Al intermediary phases have lower density than common metallic high-temperature alloys and good high-temperature oxidation resistance. Previously it was proved that silicon additions to these alloys enable to produce them efficiently by reactive sintering and improves the wear resistance. In this work, the oxidation resistance of the novel Fe–Al–Si alloys containing 10–30 wt. % of aluminium and 5–30 wt. % of silicon produced by the reactive sintering technology was tested. Cyclic and isothermal oxidation tests were carried out at 800 °C in air. Tested alloys exhibit excellent oxidation resistance, which increases with silicon content up to 20 wt. %. The reasons are discussed in terms of the phase composition of the oxide layer and the changes in chemical composition under the oxide layer during oxidation.     

Oxidation of chromium at 890°–1200°C

Cr was oxidized in 1 atm of oxygen at 980, 1090 and 1200°C for periods up to 100 hr. Surface preparation has a large effect on scaling; electropolished Cr oxidizes rapidly. Non-uniform oxide layers form exhibiting nodule growth, blistering, wrinkling and multilayered ballooning. These and other observations indicate that compressive stresses develop during film thickening. This suggests that anion as well as cation diffusion takes part in the growth process and that new oxide forms within the oxide layer. The resulting continuous plastic deformation is considered in interpreting the oxidation kinetics. Best values of the rate constants are derived from measurement of layer thickness at selected areas on the metallographic cross-section.. Moisture did not affect the rate. Cr is at least as oxidation resistant as Fe-25 Cr alloy.     

Oxidation behavior of ZrC-based composites in static laboratory air up to 1300 °C

Two ZrC-based composites, viz., ZrC + 20 vol.%SiC and ZrC + 20 vol.%SiC + 20 vol.%ZrB₂, were prepared by hot pressing. The oxidation behavior of the composites was studied in the temperature range of 800 °C–1300 °C in air. From the mass gain tests, the surface and cross-sectional SEM observation and EDS analysis of the oxidized samples, it was found that the ZrC + 20 vol.%SiC composite seemed to have a high oxidation resistance below 1000 °C, but the ZrC + 20 vol.%SiC + 20 vol.%ZrB₂ composite appeared a more excellent oxidation resistance up to 1200 °C. It is main reason that the oxidation rate of ZrC is higher than that of SiC below 1300 °C, meaning that SiO₂ from SiC is insufficient to form a dense tight protection film; however, the addition of ZrB₂ particles promotes the formation of more borosilicate above 1000 °C, which exerts more protective effect by self-healing mechanism.     

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.     

The oxidation behavior of high Cr and Al containing Nb-Si-Ti-Hf-Al-Cr alloys at 1200 and 1250 °C

The oxidation behavior of two alloys containing different content of Al and Cr from the Nb-Si-Ti-Hf-Al-Cr system has been evaluated at 1200 and 1250 °C. The alloy compositions in atomic percent are Nb-24Ti-16Si-2Hf-2Al-10Cr (B1), and Nb-24Ti-16Si-2Hf-6Al-17Cr (B2). The oxidation kinetic of B1 alloy at 1200 and 1250 °C followed a mixed parabolic-linear law, while the oxidation kinetic of B2 alloy at 1200 and 1250 °C followed a parabolic law. The weight gain of B2 alloy was 18.9 mg/cm² after oxidation at 1200 °C for 100 h, which was a seventh of the value of that of B1 alloy. Besides, oxidation became more severe as temperature increased to 1250 °C. The oxide scales of B2 alloy consisted of CrNbO₄, TiNb₂O₇ and SiO₂, which were relatively compact and protective. In addition, the oxidation mechanism of Nb-Si based alloys were also discussed.     

Experimental isothermal section at 1200 °C of V–Cr–Si system

In the present work, the isothermal section at 1200 °C of the V–Cr–Si phase diagram was experimentally studied. The samples were prepared by arc melting and characterized using scanning electron microscopy/energy-dispersive spectroscopy and electron probe microanalyzer. The continuous solution phases (Cr,V)₃Si, (Cr,V)₅Si₃ and (Cr,V)Si₂ were confirmed. The solubilities of Cr in V₆Si₅ and of V in CrSi have been measured. A ternary phase (Cr,V)₁₁Si₈ was observed and its homogeneity range, which is at constant Si ratio, was determined at 1200 °C.     

Thermal expansion of silicon at temperatures up to 1100 °C

Thermal expansion of CVD single crystal silicon was measured with a push-rod dilatometer up to 1100 °C for different crystallographic orientations of the specimen. Thermal analysis, Laue analysis and X-ray diffraction were used to verify silicon crystal orientation and absence of possible phase transformations. Coefficients of technical thermal expansion have been calculated in this temperature range and their variations with temperature have been demonstrated. These differences might cause anisotropy in thermal stresses, which has been calculated and compared with experimental values of dry-oxidised silicon wafers.     

Friction and wear behavior of laser-sintered iron–silicon carbide composites

Laser sintering is currently one of the most popular techniques to develop innovative materials for many of the high tech industrial applications owing to its ability to build complex parts in a short time. As such, material researchers are focusing on developing advanced metal matrix composites through selective laser sintering method to develop an intricate component eliminating delay in production time. In the light of the above, the present work focuses on developing iron–silicon carbide (nickel coated) composites using direct metal laser sintering technology. A laser speed of 50, 75, 100 and 125 mm/s were adopted. Metallographic studies, friction and wear test using pin-on-disc have been carried out on both the matrix metal and its composites. Load was varied from 10 to 80 N while sliding velocity was varied from 0.42 to 3.36 m/s for a duration of 30 min. A maximum of 7 wt.% of silicon carbide has been successfully dispersed in iron matrix by laser sintering. Increased content of SiC in iron matrix has resulted in significant improvement of both hardness and wear resistance. Lower the sintering speed, higher is the hardness and wear resistance of both the matrix metal and its composites. However, coefficient of friction of composites increased with increased SiC under identical test conditions. SEM observations of the worn surfaces have revealed extensive damage to the iron pins, when compared with that of the composites.     

Isothermal oxidation behavior of Ti3Al-based alloy at 700–1000 °C in air

The isothermal oxidation behavior of a Ti₃Al-based alloy (Ti-24Al-14Nb-3V-0.5Mo-0.3Si, molar fraction, %) at 700– 1 000 °C in air was investigated. The oxidation kinetics of tested alloy approximately obeys the parabolic law, which shows that the oxidation process is dominated by the diffusion of ions. The oxidation diffusion activity energy is 241.32 kJ/mol. The tested alloy exhibits good oxidation resistance at 700 °C. However, when the temperature is higher than 900 °C, the oxidation resistance becomes poor. The XRD results reveal that the oxide product consists of a mixture of TiO₂ and Al₂O₃. Serious crack and spallation of oxide scale occur during cooling procedure after being exposed at 1 000 °C in air for 16 h. According to the analysis of SEM/EDS and XRD, it is concluded that the Al₂O₃ oxide forms at the initially transient oxidation stage and most of it keeps in the outer oxide layer during the subsequent oxidation procedure.     

Scale composition and oxidation mechanism of the Ti–46Al–8Nb alloy in air at 700 and 800 °C

It is known that the oxide scale formed on TiAl alloys is generally composed of a mixture of alumina (Al₂O₃) and titania (TiO₂). The presence of niobium changes the activities of Ti and Al and influences the kinetics of oxidation and oxide layer composition. In this work, the Ti–46Al–8Nb alloy was subjected to cyclic oxidation in air at 700 °C (for 2 and 24 h) and 800 °C (for 300 h). Scale composition was analyzed by means of different techniques including X-ray photoelectron spectroscopy, X-ray diffraction and secondary ion mass spectroscopy. The scale consisted of several layers. The outer layer was built of alumina (amorphous or with very fine grains), whereas the inner layer – mainly of titania. After a longer exposure at a higher temperature (800 °C), niobium-rich precipitates and aluminum oxide grains were detected near to the alloy/scale interface and titanium nitride was found in the inner parts of the scale. Oxidation mechanism was studied by two-stage oxidation method using oxygen-18 and oxygen-16 isotopes combined with SIMS analyses. The distribution of oxygen isotopes over the alloy/scale interface indicated mixed inward/outward diffusion at the of reacting species. The experiments using Au markers showed that after longer oxidation time the inward diffusion was a predominant transport process.     

High temperature flexural strength and oxidation behavior of hot-pressed B4C–ZrB2 ceramics with various ZrB2 contents at 1000–1600 °C in air

High temperature flexural and oxidation behavior of hot-pressed B₄C–ZrB₂ ceramics with various ZrB₂ addition amounts was measured and analyzed in the temperature range of 1000 to 1600 °C in ambient air atmosphere. The high temperature flexural strength is a strong function of temperature. With the temperature increasing, the high temperature flexural strength decreases. The oxidation behavior was investigated in detail, and the oxidation mechanism was summarized. The purpose of this study is to give fundamental understanding of the high temperature behavior of B₄C based ceramics.     

Oxidation of Fe–10Cr in O2 and in O2 + H2O environment at 600 °C: A microstructural investigation

Oxidation of Fe–10Cr in dry and wet O₂ was studied at 600 °C for up to 168 h. Oxide microstructure was investigated by STEM/EDX, FIB/SEM and TEM. Oxidation in dry O₂ gives a Cr-rich protective (Fe_1−xCr_x)₂O₃ scale. The same protective oxide initially forms in O₂ + H₂O environment, but after an incubation period scale breakdown is triggered by CrO₂(OH)₂ evaporation that depletes the substrate in Cr and converts (Fe_1−xCr_x)₂O₃ to FeCr spinel oxide. Internal oxidation occurs after breakaway. Alternating external and internal oxidation result in the inward-growing scale showing a characteristic banded morphology.     

Creep- and coarsening properties of Al–0.06 at.% Sc–0.06 at.% Ti at 300–450 °C

Upon aging at 300–450 °C, nanosize, coherent Al₃(Sc_1−xTi_x) precipitates are formed in pure aluminum micro-alloyed with 0.06 at.% Sc and 0.06 at.% Ti. The outstanding coarsening resistance of these precipitates at these elevated temperatures (61–77% of the melting temperature of aluminum) is explained by the significantly smaller diffusivity of Ti in Al when compared to that of Sc in Al. Furthermore, this coarse-grained alloy exhibits good compressive creep resistance for a castable, heat-treatable aluminum alloy: the creep threshold stress varies from 17 MPa at 300 °C to 7 MPa at 425 °C, as expected if the climb bypass by dislocations of the mismatching precipitates is hindered by their elastic stress fields.     

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.     

Comparison between the oxidation of iron in oxygen and in steam at 650–750 °C

Oxidation of iron was investigated in oxygen and in steam at 650–750 °C by TGA, OM, SEM, and XRD. In oxygen, parabolic kinetics and multilayer oxide scales composed of typical three iron oxides were observed. Noticeably, the scale adhesion was very poor. In steam, linear-parabolic shape kinetics and multilayer oxide scales with fewer pores were found. As indicated by Pt marking, inward-growing process in steam was considered to result in the improvement of scale contact. Based on the experimental results, scaling mechanism of iron in steam was discussed.     

Oxidation of P92 steel weldment between 600 and 800 °C in air

P92 alloy with a composition of Fe-9.1Cr-0.5Mo-1.7W (wt.%) was welded, and its oxidation behavior was studied at 600, 700 and 800 °C for up to 6 months in air. The oxidation resistance decreased in the order of the base metal, weld metal, and the heat affected zone. The morphology and the composition of the scales formed on these samples were similar. The scales were either uniform in thickness or nodular. The scales consisted mainly of Fe₂O₃. As oxidation progressed, thick, nodular oxide scales formed. The alloying elements such as Cr, W, and Mn tended to incorporate in the lower part of the oxide scale.     

The mechanism of oxidation of Ni-Cr-Al alloys at 1000°–1200°C

The isothermal oxidation behaviour of Ni-Cr-Al alloys, containing 15 or 29%Cr and 1 or 4%Al, has been studied in 1 atm O₂ at 1000° or 1200°C, using thermogravimetric methods, metallography, electron probe microanalysis and scanning electron microscopy. The addition of 1%Al to Ni-15 or 29%Cr improves the oxidation resistance progressively with time, by developing a region continuously increasing in α-Al₂O₃ content at the base of the initial Cr₂O₃ scale. Addition of 4%Al to the same alloys permits the much more rapid formation of a steady-state, surface α-Al₂O₃-rich scale. The competitive and collaborative roles of Al and Cr in promoting protective scales are discussed.     

Oxidation of Fe–Si,Fe–Al and Fe–Si–Al alloys in CO2–H2O gas at 800 °C

Binary Fe–(1, 2, 3)Si and Fe–(2, 4, 6)Al, and ternary Fe–(2, 3)Si–(4, 6)Al alloys (all in wt%) were oxidised in Ar–20% CO₂, with and without H₂O, at 800 °C. All binary alloys except Fe–6Al, in all gases, formed a thin outer layer of Fe₃O₄, an intermediate Fe₃O₄ + FeO layer, an inner FeO + Fe₂SiO₄ (or FeAl₂O₄) layer and internally precipitated SiO₂ (or FeAl₂O₄). Ternary alloys and Fe–6Al developed a protective Al₂O₃ layer beneath Fe₂O₃ in Ar–20% CO₂. Water vapour affected ternary alloy oxidation only slightly, but Fe–6Al oxidized internally in high H₂O-content gas, and its scale was non-protective.