Effect of 475 °C embrittlement on fractal behavior and tensile properties of a duplex stainless steel

The fractal dimension variations of several tension fracture surfaces of a duplex stainless steel broken at room temperature has been studied after several aging treatments performed at 475 °C for 1, 2, 6.5, 12, 24, 40, and 120 h. A dimple type of fracture mode was observed for small aging times and transgranular as well as dimple rupture for 24, 40, and 120 h of aging. The higher the time of aging is, the smaller the fractal dimension and the true fracture strain. An expected reduction of the strength with the time of aging was observed.     

Thermal oxidation of single-crystal aluminum at 550°C

Oxygen transport during oxide growth on (110), (111), and (100) Al crystals at 550°C is investigated by¹⁸O/SIMS combined with kinetic measurements and SEM and TEM observations. Starting with an electropolished surface, the experimental evidence suggests oxide growth by oxygen anion transport via local pathways through an outer amorphous Al₂O₃ layer and oxygen incorporation at the periphery of the underlying laterally growing -Al₂O₃ islands. The kinetics, island morphology and epitaxy are sensitive to substrate orientation. This oxide growth behavior is compared with oxide formation on a sputter-cleaned and annealed (111) Al surface.     

Electrochemical corrosion behavior of LDX 2101® duplex stainless steel in a fluoride-containing environment

The effect of fluoride on the electrochemical corrosion behavior of an LDX 2101® duplex stainless steel (DSS) was studied. Open-circuit potential (E_OC) and electrochemical impedance spectroscopy (EIS) measurements were carried out in artificial saliva and with the addition of fluoride (1 wt% NaF). The electrochemical corrosion behavior of the AISI 316L austenitic stainless steel (SS) was also evaluated for comparison. Both open-circuit potential and EIS results indicate that DSS and austenitic SS undergo spontaneous passivation due to spontaneously formed oxide film passivating the metallic surface, in the simulated aggressive environments. However, LDX 2101® exhibits superior corrosion resistance as compared with AISI 316L, and this improvement is ascribed to the formation of a passive film which shows a higher protective effect than the one formed on AISI 316L.     

Influence of high-intensity pulsed ion beams irradiation on oxidation behavior of 316L stainless steel at 700 ℃

316L stainless steel samples, as a widespread used material, were irradiated with HIPIB at the beam parameters of ion energy 300 keV, current density 100, 200 and 300 A/cm^2, shot number 10 and pulse duration 75 ns. The surface morphology and the phase structure in the near surface region of original and treated samples were analyzed with scanning electron microscopy （SEM） and X-ray diffractometry （XRD）. It is shown that the HIPIB irradiation can smooth the surface of the samples, and the preferred orientation is present in the surface layer of irradiated coupons. The influence of HIPIB irradiation on the oxidation behavior of 316L stainless steel at 700 ℃ for up to 100 h was investigated. Electron probe microanalysis （EPMA） was used to study the distribution of elements in the oxidation products. It is found that the oxidation behavior of the irradiated coupons depends greatly on the ion current density of HIPIB. HIPIB irradiation with ion current density of 100 A/cm^2 slightly reduces the oxidation rate with respect to the unirradiated coupon. The improvement of the oxidation resistance can be attributed to more oxide of Cr that forms on the surface of the irradiated coupons. In contrast, HIPIB irradiation with ion current density of 200 or 300 A/cm^2 is proved to be detrimental, causing a higher oxidation rate.     

Influence of silicon on the oxidation of titanium between 550 and 700°C

The oxidation behavior of Ti-Si alloys (0.25, 0.5, and 1 Wt. % Si) was investigated between 550 and 700°C; in oxygen by continuous thermogravimetry for a maximum duration of about 500 hr and, in air by daily weighing for durations from a few hundred to several thousand hours. The kinetics results revealed that the presence of silicon leads to a decrease in oxidation rate which is more evident when the temperature is raised and the silicon content is increased. Morphological and structural examinations revealed that silicon modifies the internal architecture of oxide layers when compared with unalloyed titanium; in particular, reduced porosity in the layers is observed. Analysis showed that silicon is uniformly distributed in the oxide layer. However, while part of the silicon is in solid solution in the rutile, some is also precipitated as small crystals ( <1 m at 850°C) of SiO₂, of cristobalite structure. The adherence of oxide layers to the metal substrate was measured after cooling of samples; the addition of silicon has been observed to modify, in a manner dependent on its content, the adherence of oxide layers.     

Stress corrosion cracking at constant load of 304L stainless steel in molten NaCl-CaCl2 at 570°C

The incubation and propagation times of cracks in 304L in molten NaCl-CaCl₂ at 570°C were related to the applied stress value, from creep and creep rate curves. Rest potential versus time curves were recorded simultaneously. The results showed intergranular stress corrosion cracking. When the temperature was kept at 570°C, precipitation of chromium carbide M₂₃C₆ which promoted cracking propagation, was induced. Determination of the crack rate shows that anodic dissolution at the bottom of the cracks is the main process during the stress corrosion crack propagation of 304L stainless steel in the stress range used.     

The effect of Nd and Pr on the oxidation behavior of a Fe–13Cr alloy

The influence of neodymium (Nd) and praseodymium (Pr) on the oxidation behavior of a Fe–13Cr alloy has been studied in oxygen at 800 °C for 24 h. Additions (⩽0.03 wt.%) of either Nd or Pr reduce the oxidation rate of Fe–13Cr and change the morphology of the oxides formed on Fe–13Cr.     

Effect of water vapor on oxidation and creep behavior of incoloy 800 at temperatures between 850 and 980°C

The oxidation kinetics and creep behavior of Incoloy 800 were studied in air and in water vapor charged with oxygen. Oxidation kinetics were determined by thermogravimetry. The creep behavior of samples tested in air and in H₂OO₂=11 atmospheres was investigated by electron microscopic techniques. The oxidation rate of samples tested in air was governed by a p-conducting chromia interlayer. Depending upon water vapor pressure, chromia became n-conductive. As a consequence, a continuous rate-governing silica interlayer formed. The oxygen activity in the alloy was established by the dissociative pressure of the respective diffusion-rate-governing oxide scale. The stability of the dispersion-hardening Ti(C, N) particulates within the alloy was affected by the respective oxygen activity. In the samples tested in air, partial oxidation of the particulates due to enhanced oxygen activity caused a loss of coherency with the matrix. As a result, the deformation behavior during creep was changed to planar-slip mode, bringing about loss of creep resistance and ductility.     

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.     

Low strain rate deformation behavior of a Cr–Mn austenitic steel at −80 °C

The deformation behavior of a Cr–Mn austenitic steel during interrupted low strain rate uniaxial tensile testing at ?80 °C has been studied using X-ray diffraction (XRD), electron backscatter diffraction and transmission electron microscopy. Continuous γ → ε → α′ martensite transformation was observed until failure. High dislocation densities were estimated in the austenite phase (～10¹⁵ m^?2), and for the α′-martensite they were even an order of magnitude higher. Dislocation character analysis indicated that increasing deformation gradually changed the dislocation character in the austenite phase to edge type, whereas the dislocations in α′-martensite were predominantly screw type. XRD analyses also revealed significant densities of stacking faults and twins in austenite, which were also seen by transmission electron microscopy. At low strains, the deformation mode in austenite was found to be dislocation glide, with an increasing contribution from twinning, as evidenced by an increasing incidence of ∑3 boundaries at high strains. The deformation mode in α′-martensite was dominated by dislocation slip.     

Effect of dwell condition on fatigue behavior of a high-Nb TiAl alloy at 750 °C

The strain-controlled low cycle fatigue (LCF) and creep-fatigue interaction (CFI) tests of a newly developed Ti-45Al-8Nb-0.2W-0.2B-0.02Y (at.%) alloy were carried out at 750 °C in air. The hysteresis loop, cyclic stress response and life modeling as well as failure mechanism of the alloy were investigated in detail. It was revealed that the tensile and compressive mean stresses would generate when the dwell condition was introduced at minimum and maximum strain, respectively. In addition, the dwell condition, especially for the compressive dwell condition, would significantly decrease the fatigue life. The typical continuum damage accumulation(CDA) and modified CDA life models proposed in the present study were employed to predict both LCF and CFI life of the alloy, which showed that the modified CDA life model had a higher accuracy than the typical CDA life one. Moreover, only single crack initiation source was observed at 92% (i.e. 11/12) of LCF fracture while multiple crack initiation sources at 84% (i.e. 31/37) of CFI fracture. Apparently different from LCF specimen showing more transgranular appearance, CFI specimen shows more intergranular appearance.     

The influence of microstructure on the isothermal and cyclic-oxidation behavior of Ti-48Al-2Cr at 800°C

The isothermal and cyclic-oxidation behavior of the intermetallic Ti48Al-2Cr (at. %) alloy were studied at 800°C in air. Emphasis was placed on the effect of microstructures, in a range relevant for practical applications; i.e., duplex, near gamma, nearly lamellar, and fully lamellar; obtained by various heat treatments. The oxidation kinetics of the intermetallic alloy showed initially the formation of a relatively protective oxide scale. After an exposure time of about 10 hr the oxidation rate increased significantly, due to a Loss of protectivity of the oxide scale. The growth rate of the oxide scale, as well as its composition, structure and morphology showed no major relation to the microstructure of the base material. The oxidation of Ti-48Al-2Cr in air, initially resulted in the formation of -Al₂O₃, TiO₂ (rutile), Ti₂AlN, and TiN. After Longer exposure times, the mixed-oxide scale with an alumina-rich Layer at the outside was overgrown by the fast-growing TiO₂, responsible for the rapid kinetics. Using¹⁸O/¹⁵N experiments some mechanistic aspects were discussed in relation to the existence of a nitrogen-rich Layer near the scale/alloy interface. Thermal-cyclic-oxidation experiments up to 3000 1-hr cycles showed that spallation of the oxide scale initiated after about 175 1-hr cycles. Also in this case the growth rate of the oxide scale as well as its composition, structure and morphology showed no major relation to the microstructure of the base material.     

Influence of columnar microstructure of a sputtered NiAl coating on its oxidation behavior at 1000 °C

The influence of columnar microstructure of a sputtered nanocrystalline NiAl coating on its oxidation behavior at 1000 °C in air was investigated. Results showed that the existence of columnar boundaries significantly affected its initial oxidation kinetics. However, no such obvious effect on the scale thickness was observed after long term oxidation.     

Effect of Pressures on the Corrosion Behaviours of Materials at 625°C

The corrosion behaviors of austenitic stainless steels (SS) 310, 304 and Ni- and Fe-based A-286 exposed to 0.1 MPa, 8 MPa and 29 MPa at 625°C for 1000 h were investigated. These represent exposure to superheated steam, subcritical and supercritical water (SCW) at 625°C, respectively. As SS 310 showed the smallest weight change, the oxide cross-sections made from 310 samples were examined by transmission electron microscopy. The results revealed a single-layer oxide at 0.1 MPa and dual-layer oxides at 8 MPa and 29 MPa, followed by a Cr-depleted region into the austenite substrate. The compositions of the inner oxides at 8 MPa and 29 MPa are Cr-rich and largely similar to those of the single-layer oxides at 0.1 MPa exposure. These results suggest that corrosion testing in superheated steam may be a suitable surrogate for scoping tests of materials under SCW conditions at >650°C.     

Effect of Co on oxidation and hot corrosion behavior of two nickel-based superalloys under Na2SO4–NaCl at 900 °C

Nickel-based superalloys with and without Co by partial replacement of W were prepared using double vacuum melting. A comparison of the oxidation in air and hot corrosion behaviors under molten 75wt.%Na₂SO₄+ 25wt.%NaCl at 900 °C were systematically investigated. The results showed that partial replacement of W with Co promoted the formation of chromia scale and consequently decreased the oxidation rate. Besides, the addition of Co also retarded the internal oxidation/nitridation of Al and consequently promoted the growth of Al₂O₃ scale, which further decreased the scaling rate and improved the adhesion of scale. Moreover, the addition of Co also further improved the hot corrosion resistance under molten Na₂SO₄–NaCl salts.     

Effect of be addition on the oxidation behavior of Mg−Ca alloys at elevated temperature

Beryllium was added to Mg−Ca alloys to study their ignition-proof properties. The ignition temperatures of Mg−2Ca alloys were increased dramatically with increasing Be addition. Thermogravimetric measurement revealed that the oxidation of Mg−2Ca alloys was slowed down by Be addition. After elevated temperature exposure to air, the Mg−2Ca alloy was partially ignited, while the surface of Be-containing alloys was smooth without any partial ignition. SEM, low-angle XRD, and AES observations indicated that the surface of Becontaining alloys became compact and dense, and the oxide film formed at elevated temperature mainly consisted of CaO together with MgO and BeO. It was found that the CaO enriched oxide layer acted as an impermeable barrier to the inward diffusion of oxygen and thus further oxidation was prevented.     

Degradation behavior of stainless steel in PbO–CaO–SiO2 slag in the presence of sulphur

Five stainless steel grades are subjected to PbO–CaO–SiO₂–S slag at 1200 °C. The degradation phenomena are identified as liquid slag and liquid metal corrosion, oxidation and sulphidation. The relation between sulphidation and steel and slag composition is discussed. For the slag with the lowest PbO/SiO₂ ratio, sulphidation is mainly recognized through (Fe, Cr)_xS_1?x at the surface and in the subsurface of the steel, especially for the steel grades with the lowest Cr content. For the slag with the highest PbO/SiO₂ ratio, sulphidation is mostly pronounced in the steel grades with the highest Ni content through the formation of a liquid (Ni, Pb, S) phase.     

Long-time oxidation of iron at 1100°C in air as a function of time

Long-time oxidation experiments, 180 hr, 360 hr, and 660 hr, were carried out on AREMA iron in ambient air at 1100°C. Fe samples, 3 mm in diameter and 39 mm in length, were completely oxidized. Weight-gain measurements, electron microanalysis, and X-ray diffraction (XRD) analysis were used as experimental techniques. The completely oxidized samples remained cylindrical, compact, and solid. The content of hematite in the scale mass increased exponentially from 88.1% (180 hr) to 88.4% (660 hr), with the limit value 100% at t . No wustite and no pure Fe traces have been found in completely oxidized samples.