Ar ion irradiation hardening of high-Cr ferritic/martensitic steels at 700 °C

High-Cr ferritic/martensitic (FM) steels are being considered for applications as fuel cladding or core structures for Generation-IV reactors. Because high temperatures approaching 923-973 K (650-700 °C) are envisioned in the designs of Generation IV reactors, irradiation response of high-Cr FM steels at the high temperatures requires investigations. Response of two high-Cr FM steels P92 and 11Cr to irradiation at 973 K (700 °C) was investigated through Ar ion irradiation in combination with damage simulations, nanoindentation measurements and microstructure analyses. Irradiation hardening occurred in both steels after Ar ion irradiation at 973 K (700 °C) to 10 dpa, providing the first evidence that irradiation hardening can occur at a high irradiation temperature of 973 K (700 °C) in high-Cr FM steels. Argon bubbles with a very high number density and an average diameter of about 2.6-3 nm formed in the two steels after the irradiation. The irradiation hardening occurred in the two steels is attributed to the formation of these high-number-density fine argon bubbles produced by the irradiation homogeneously distributed in the matrix. Difference in the magnitude of irradiation hardening between the two steels was also discussed.     

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.     

Plastic flow of the mechanically alloyed Fe-0.6%O at temperatures of 550–700°C

Creep of steel Fe-0.6%O produced by the method of powder metallurgy has been studied in a temperature range of 550–700°C at flow stresses from 100 to 400 MPa. It has been shown that the creep of the material is characterized by high values of the apparent activation energy for deformation, which considerably exceeds the value of the activation energy for self-diffusion in α iron, and by high values of the stress exponent in the power law of creep. An analysis of the deformation behavior of the alloy showed that there are observed high threshold stresses as a result of retardation of moving dislocations by small incoherent particles of oxides. Taking into account the threshold stresses and the temperature dependence of the shear modulus, it has been established that the deformation behavior of the powder material is described by a power law of creep. The true values of the stress exponent were found to be approximately 8, and the values of the true activation energy for deformation, to be close to the activation energy for bulk (at T = 700°C) and pipe (at T = 550–650°C) self-diffusion.     

The effect of metallurgical pretreatment on the kinetics of oxidation of iron at 700°C in pure gaseous oxygen

Continuous weight gain/time measurements have been made using cold rolled sheet samples of Hellefors “Remko” iron and Metals Research “Ferrovac E” iron. The effects of cold work on the oxidation characteristics of these irons in oxygen at 700°C were obtained by comparison with the oxidation kinetics of the same cold rolled specimens after various metallurgical treatments. It is suggested that the observed higher oxidation rate of untreated cold worked specimens is due to the greater vacancy sink concentration in the substrate metal.Since the impurity content of the two irons differed, an attempt was made to assess the effect of impurity on iron oxidation. In agreement with the observations of other workers the less pure “Remko” iron oxidized at the slower rate in both the cold worked and annealed state even though initially the oxidation rates were similar for both irons. It is suggested that after an initial period of oxidation the impurities accumulate at the substrate metal surface and act as barriers restricting further oxidation.     

Phase Equilibria of the Al-Fe-Si-Ti Quaternary System at 700 °C

The phase equilibria in two isothermal sections of the Al-Fe-Si-Ti quaternary system with Al fixed at 90 and 75 at.% respectively, and the related ternary subsystems at 700 °C, have been investigated by scanning electron microcopy coupled with energy dispersive spectroscopy and x-ray diffraction. Only one three-phase region, L-Al + TiAl₃ + FeAl₃ is found in the Al-rich portion of Al-Fe-Ti system, and four three-phase regions, including the region of L-Al + τ₆ + τ₄, are confirmed in the Al-rich corner of the Al-Fe-Si system. Two four-phase regions and five four-phase regions have been identified in the 90 at.% Al and 75 at.% Al sections, respectively. No new ternary and quaternary compounds are found in the present work.     

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.     

Layer growth kinetics and wear resistance of martensitic precipitation hardening stainless steel plasma nitrocarburized at 460°C with rare earth addition

To study the effect of rare earth (RE) addition on low temperature plasma nitrocarburizing of martensitic precipitation hardening stainless steel, 17-4PH stainless steel was plasma nitrocarburized at 460 °C for different times with RE addition. The modified layers were tested by optical microscope, scanning electron microscope, X-ray diffraction, microhardness tester and pin-on-disc tribometer. The experimental results show that the layer depth of plasma RE nitrocarburized layer can be increased up to 56% compared with plasma nitrocarburizing without RE addition. Incorporation of RE element is beneficial to the formation of nitrogen and carbon expanded martensite (α′_N). The surface microhardness of plasma RE nitrocarburized layer can be increased to 1286 HV and higher up to 80 HV than that obtained from the conventional treated one. The friction coefficient of martensitic stainless steel can be dramatically decreased by low temperature plasma nitrocarburizing with RE addition, and the friction coefficient of the modified specimens decrease gradually with increasing process time in the present test condition.     

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.     

The Isothermal Section of the Zr-Sn-Cu Ternary System at 700 °C

The isothermal section of the Zr-Sn-Cu ternary system at 700 °C was investigated by using x-ray diffraction, scanning electron microscope and energy dispersive spectroscopy. A new ternary compound τ (Zr_25.3Cu_66.1Sn_8.6) was observed in the Cu-rich corner of this system. The previous known ZrCuSn and ZrCuSn₂ ternary compound were confirmed.     

Air Oxidation of Two Nanophase Ni–Ag Alloys at 600–700°C

Two nanophase Ni-base alloys containing 50 and 25 at.% Ag prepared by mechanical alloying, denoted Ni–50Ag and Ni–25Ag were oxidized in air at 600 and 700°C for 24 hr. Ni–50Ag underwent internal oxidation of nickel, associated with the formation of a continuous outermost layer of silver metal with scaling rates larger than those for pure nickel. On the contrary, Ni–25Ag formed a continuous NiO layer surmounted by a discontinuous silver layer and internal oxidation was suppressed. The oxidation rate of Ni–25Ag decreased with time much more rapidly than predicted by the parabolic rate law during the initial stage and eventually became parabolic, with rate constants much lower than those for the oxidation of pure nickel. These results are attributed to the two-phase nature and, particularly, to the very small grain size of the two alloys.     

Phase equilibria in the Mg-rich corner of the Mg-Zn-La system at 350°C

1. Introduction Mg-based alloys are attractive for applications because of the light weight and a high specific strength. Among the common alloying elements, the addition of zinc could make the magnesium alloys strengthen by precipitation [1-2]. The further addi- tion of the rare-earth elements to the Mg-Zn alloys is more effective on the precipitation-strengthening [3-4]. But phase diagrams of the Mg-Zn-RE system, as the basis of the alloying, have hardly been built up [5-6]. The phase equ…     

Enhancement Mechanism of Cerium in 316LN Austenitic Stainless Steel During Creep at 700 °C

Effect of cerium (Ce) on creep strength and microstructure of 316LN austenitic stainless steel (316LN steel) at 700 °C/150 MPa was investigated by scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM) and thermodynamic calculations. Addition of 0.032 wt% Ce to 316LN steel results in a prominent increase in creep life from 313 to 556 h. Ce enriches in titanium nitride nanoparticles, increases slightly the activity and diffusion coefficient of Mo, and facilitates the formation of fine and dense intragranular Laves phase precipitates. Thus the creep strength is remarkably enhanced by Ce addition in 316LN steel through the intragranular Laves phase precipitation strengthening. It reveals a new insight into the improvement effect of rare earth (RE) elements such as Ce on creep strength of austenitic stainless steels, which inspired the design of RE-microalloying heat-resistant steels.     

Phase relations in the Fe-rich region of the Fe-Gd-Mo ternary system at 800 °C

The phase relation in the Fe-rich region of the Fe-Gd-Mo ternary system at 800 °C has been investigated using x-ray powder diffraction followed by Rietveld refinement and electron probe microanalysis. A partial isothermal section contains one ternary ThMn₁₂-type phase, four solution phases (Fe,Mo)₁₇Gd₂, (Fe,Mo)₂₃Gd₆, (Fe,Mo)₃Gd, and (Fe,Mo)₂Gd, one binary phase μ(Fe,Mo), and a liquid phase. The homogeneity range of the ternary ThMn₁₂-type phase is compared to that observed at 1200 °C.     

Phase relations in the Fe-rich region of the Fe-Gd-Mo ternary system at 800 °C

The phase relation in the Fe-rich region of the Fe-Gd-Mo ternary system at 800 °C has been investigated using x-ray powder diffraction followed by Rietveld refinement and electron probe microanalysis. A partial isothermal section contains one ternary ThMn₁₂-type phase, four solution phases (Fe,Mo)₁₇Gd₂, (Fe,Mo)₂₃Gd₆, (Fe,Mo)₃Gd, and (Fe,Mo)₂Gd, one binary phase μ(Fe,Mo), and a liquid phase. The homogeneity range of the ternary ThMn₁₂-type phase is compared to that observed at 1200 °C.     

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.     

Investigation of the nitrogen-nickel-titanium system: The isothermal section at 900 °C

Phase relations in the ternary system N-Ni-Ti have been established experimentally for the 900 °C isothermal section assisted by thermodynamic modeling. Phase equilibria are characterized by an appreciable amount of up to ≅11 at. % N dissolved in the octahedral interstices of the crystal structure of Ti₂Ni. Two-phase equilibria are formed at 900 °C among the pairs Ti₂N + Ti₂NiN_x, TiN_1-x + Ti₂NiN_x, TiN_1-x + TiNi_1-x, and TiN_1-x + Ni(Ti)_x. The investigation is based on X-ray powder diffraction, metallography, scanning electron microscopy, and electron probe microanalysis techniques on about 50 alloys, which were prepared by arc melting or high-frequency levitation melting of appropriate blends of Ti,Ni-powders with TiN used to introduce nitrogen. Key experiments related to the N solubility in the Ti₂Ni phase have been triggered by an interactively performed thermodynamic modeling. The experimental results are in fine agreement with the thermodynamic calculation.     

The oxidation of Iron-Chromium-Manganese alloys at 900°C

The oxidation of nine ternary iron-chromium-manganese alloys was studied at 900°C in an oxygen partial pressure of 26.7 kPa. The manganese concentration was set at 2, 6, and 10 wt. %, and chromium at 5, 12, and 20 wt. %. The scales formed on the low-chromium alloys consisted of (Mn,Fe)₂O₃, -Fe₂O₃, and Fe₃O₄. These alloys all exhibited internal oxidation and scale detachment upon cooling. The scales formed on the higher-chromium alloys were complicated by nodule formation. Initially, these scales had an outer layer of MnCr₂O₄ with Cr₂O₃ underneath, adjacent to the alloy. With the passage of time, however, nodules formed, and the overall reaction rate increased. This tendency was more marked at higher manganese contents. Although these alloys contained a high chromium content, the product chromia scale usually contained manganese. It was concluded that the presence of manganese in iron-chromium alloys had an adverse effect on the oxidation resistance over a wide range of chromium levels.