Investigation on Strengthening Mechanism of China Low-Activation Ferrite Steel upon Thermo-Mechanical Treatment

The objective of this study was to investigate the influence of strengthening mechanisms on the high-temperature mechanical properties of China low-activation ferrite(CLF-1)steel,which underwent thermodynamic design and thermo-mechanical treatment(TMT).The microstructure characterization in the normalized and tempered condition and the TMT condition was carried out using optical microscopy,X-ray diffractometer,and scanning electron microscopy with electron backscatter diffraction.High-resolution transmission electron microscopy was employed to determine the crystallographic structures of precipitated phases.The results indicated that the addition of Ti led to an increase in the allocation of C in MC phase and an enhancement in the content of MC phase.Compared to CLF-P steel in the normalized and tempered condition,a 1.5-fold increase in dislocation density and an order of magnitude improvement in MX phase density were achieved after TMT.The formation of high-density nano-scale MC phases during TMT played a significant role in precipitation strengthening due to their favorable coherent relationship with the matrix and low interfacial free energy.The excellent high-temperature mechanical properties observed in CLF-P steel after TMT can be attributed to the combined effects of precipitation strengthening,dislocation strengthening,and lath strengthening.

Effect of Silicon and Aluminum Addition on Corrosion Behavior of ODS Iron-Based Alloys in Liquid Lead–Bismuth Eutectic

The long-term corrosion behaviors of four variants of oxide dispersion strengthened(ODS)iron-based alloys in the stagnant oxygen-saturated lead–bismuth eutectic(LBE)at 550℃ were studied herein.The effects of silicon and aluminum content on the thickness,morphology and composition of the oxide scale were explored with the aid of X-ray diffraction(XRD),scanning electron microscopy(SEM),electron probe micro-analyzer(EPMA)and X-ray photoelectron spectroscopy(XPS).The addition of 1.5 wt%silicon is not able to contribute to forming a protective external silicon oxide film on the surface of aluminum-free ODS iron-based alloy,while the addition of aluminum promotes the formation of a thin and continuous alumina oxide scale.In the meantime,an appropriate amount of silicon becomes the heterogeneous nucleation site for alumina during the initial stage of oxidation,giving rise to the rapid formation of a protective alumina scale.However,excessive silicon has a negative impact on the formation of continuous alumina scale,because it may compete with aluminum to absorb more oxygen.The result of oxidation kinetics in ODS iron-based alloy shows that the parabolic rate constant of the alumina oxide scale is 3–4 orders of magnitude lower than that of the scale mainly composed of iron and chromium oxide.

Preliminary study on the fabrication of 14Cr-ODS FeCrAl alloy by powder forging

A simple powder forging process was presented herein to fabricate an Fe-14 Cr-4.5 Al-2 W-0.4 Ti-0.5 Y_(2)O_(3)ODS Fe Cr Al alloy.The forged alloy exhibits a high density that exceeds 97%of the theoretical density.The ODS alloy was investigated in terms of the residual porosity,morphology and phase structure of oxide nanoparticles,impact toughness and tensile properties.It was found that refined grains were obtained during powder forging.A residual porosity less than 1.1%has no impact on the precipitation of oxide nanoparticles.The average diameter of the oxide particles is 7.99 nm,with a number density of 2.75×10^(22)m^(-3).Almost all of the oxides are identified as orthorhombic YAl O3 particles.The refined grains and uniformly distributed oxide nanoparticles enable the alloy to show excellent mechanical strength and ductility below 700℃,and enable the ductile-to-brittle transition temperature to be close to room temperature.However,a slight decrease in strength at 1000℃and the Charpy upper shelf energy has been suggested to be due to the residual porosity.These results indicate that powder forging can be used as a promising technique for the fabrication of ODS alloys.

Rapid Hydrogen Transportation Along Grain Boundary in Nickel

The permeability and diffusivity of hydrogen in directionally solidified polycrystalline and single crystal nickel foils were measured by gas permeation method.The results showed that both hydrogen diffusivity and permeability were higher in directionally solidified nickel specimen than those in single crystal one at the temperature ranging from 300 to480 °C,and confirmed the existence of short-circuit diffusion along the grain boundaries(GBs) in the directionally solidified nickel.The results suggested that the rapid diffusion along GBs was more obviously characterized in terms of higher permeability rather than higher diffusivity.The contribution of grain boundary to hydrogen transportation was represented by the differences of diffusivity(and permeability) in single crystal nickel and directionally solidified nickel.By modifying the Fick's first diffusion law and counting the grain boundary density,the hydrogen diffusivity and permeability of rapid diffusion along GBs were calculated.The results suggested both the diffusivity and permeability fit the Arrhenius relationship well at different temperature.

Formation Mechanism of Nanoparticles in Fe-Cr-Al ODS Alloy Fabricated by Direct Oxidation Method

This study presents the fabrication of 14CrFe-Cr-Al oxide dispersion strengthened(ODS)alloy by a direct oxidation process.In order to explain how oxide nanoparticles are formed in the consolidation process,the powders after oxidation are subjected to vacuum thermal treatment at high temperatures.Differential scanning calorimeter,X-ray photoelectron spectroscopy,scanning electron microscopy and transmission electron microscopy techniques are used to detect the generation,evolution of oxides on both the surface and interior of the powder,as well as the type of oxide nanoparticles in the fabricated ODS alloy.It is found that an iron oxide layer is formed on the surface of the powder during low temperature oxidation.And the iron oxide layer would be decomposed after thermal treatment at high temperature.In the consolidation process,the oxygen required by the reaction of alumina and yttrium oxide to produce nano scale Y-Al-O particles mainly derives from the decomposition of iron oxide layer at elevated temperature and the inward diffusion of oxygen.Using the direct oxidation process,YAlO_(3) nanoparticles are dispersed in the grains and at the grain boundaries of Fe-Cr-Al ODS alloy.