The effects of alloy grain size of high-Cr ferritic stainless steel were investigated using a sheet (t = 2 mm) and plate (t = 22 mm) of Cr ferritic stainless steel. The average grain size of each ...sample was measured as ∼ 25 µm and ∼ 500 µm, respectively. The large-grained plate sample exhibited a 41% oxidation rate reduction, and a slight decrease in Cr volatilization was observed in the small-grained sheet sample. A numerical examination revealed that inhomogeneous GB-related oxidation is a major cause of the different oxidation kinetics affected by metal grain size.
•A smaller grain-sized stainless steel exhibited a higher oxidation rate at 800 °C.•The large intergranular oxides, MnCr2O4, were formed at the alloy grain boundaries.•The specimen with a smaller grain size exhibited a lower Cr evaporation rate.•The Cr evaporation rate can be decreased by the oxygen vacancies in the oxide scale.
The high-temperature oxidation behavior of Fe-14Cr-9Mn-2.5Ni austenitic stainless steel in N2-21 vol％O2 environment at 900–1100 °C has been investigated. As the oxidation temperature rises, the scale ...expands from a thin bilayer to a thicker multilayer composed of an outer Fe-rich layer and an internal oxidation zone. The inner oxide consists of island Cr-rich spinel precipitates surrounded by remaining Ni-rich metallic. At 1000 °C, we observed an embedded Cr-rich dense layer that can slow down the oxidation rate. At a higher temperature (1100 °C), the outer oxide scale (Fe, Mn)2O3/(Fe, Mn)3O4/(Fe, Mn)O forms, and the oxidation rate significantly increases.
•The oxidation of Fe-Cr-Mn austenitic stainless steel with critical Cr content is studied.•As the oxidation temperature rises, the morphology of the oxide scale changes from a thin bilayer to a thicker multilayer.•An embedded Cr-rich layer at the oxidation front can slow down the oxidation rate.•The formation mechanism of the remaining Ni-Fe metallic is discussed.
•Additively manufactured alloys 718 show an excellent oxidation resistance in isothermal conditions.•TiNbO4 is identified by EBSD inside the Cr2O3 oxide scale, in agreement with thermodynamics.•AM ...samples present intergranular oxidation kinetics similar to the wrought alloy's but with deeper occurrences.•The larger the grain size, the thicker the oxide at the alloy grain boundaries.•Intergranular oxidation kinetics is controlled by both volume Al diffusion and interfacial oxygen diffusion.
The intergranular oxidation in air at 850 °C of alloy 718 produced by laser beam melting and electron beam melting was compared to that of the wrought alloy. Quantitative analyses revealed that the amount of grain boundary oxidation was similar for these alloys. However, the additively manufactured ones presented deeper and thicker oxides at grain boundaries, due to grain size heterogeneity and to a smaller number of special boundaries. Results show that intergranular oxidation kinetics follows Wagner’s theory on internal oxidation considering not only O diffusion at the intergranular oxide/metal interfaces but also Al and Ti diffusion in the bulk.
This study examined the surface oxidation properties of Cr-Mn-Fe-Co-Ni high entropy alloy (HEA) at high temperatures (900 °C, 1000 °C, 1100 °C/24 h) in the 21%O2 and 79%N2 atmosphere. An initial ...observation of microstructures confirmed that they consisted of random FCC solid solution, and the average grain size was measured at 245.5 μm. High-temperature oxidation tests of the Cr-Mn-Fe-Co-Ni HEA showed that its weight gained by 1.76, 4.45, and 9.08 mg/cm2 at 900 °C, 1000 °C, and 1100 °C, respectively, and all three temperature conditions represented typical parabolic curves. Observation of its surface found that Cr2O3 and Mn2O3 were main oxides on a specimen tested at 900 °C, while (Mn, Cr)3O4 and Mn3O4 were mostly shown to be main oxides on a specimen tested at 1000 °C and 1100 °C, despite the presence of Cr2O3. The high-temperature oxidation behavior of Cr-Mn-Fe-Co-Ni HEAs was found to have been largely affected by Cr and Mn elements. The resultant kirkendall pore generation, change in the concentration of elements, internal oxidation and Mn2O3 → Mn3O4 transformation were found to affect the oxidation resistance of Cr-Mn-Fe-Co-Ni high entropy alloy.
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•The applicability of high entropy alloy (HEA) in a severe environment is currently focused.•However, studies of high temperature oxidation of HEAs have so far been insufficient.•The surface oxidation behavior of Cr-Mn-Fe-Co-Ni HEA was investigated at high temperatures (900 °C, 1000 °C, 1100 °C/24 h).•The high temperature oxidation mechanism of Cr-Mn-Fe-Co-Ni HEA was suggested.
Three 9 wt% Cr ferrite/martensite steels (two alloyed with Si) have been exposed to oxygen-controlled LBE at 550 and 600 °C, respectively. The passivating oxide scale consists of a spinel layer plus ...internal oxidation zone (IOZ). By adding Si, the thickness of spinel layer is decreased while the IOZ is enhanced. Moreover, a Si-rich oxide layer is observed underneath the spinel layer on Si-containing samples after 2000 h exposure at 600 °C. Besides, the less visible cracks/exfoliations on Si-containing samples indicate the positive role of Si addition on scale adherence.
The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.
The datasets obtained during the current study are available from the corresponding author on reasonable request.
•Si addition reduces microcracks and exfoliations of the oxide layer.•The growth rate of the spinel layer is decreased at 550 and 600 °C by adding Si.•An inner Si-rich oxide layer is observed underneath the spinel layer at 600 °C.
The oxidation behavior of 17Cr2Ni2MoVNb steel in air was studied at different temperatures using advanced characterization techniques. The variation in oxidation products with temperature was ...revealed, and the selective oxidation mechanism at the front of the inner oxide layer was determined. Factors affecting the morphology, composition, and structure of the oxide near the interface were determined. A novel internal oxidation model with a multilayer core/shell structure induced by defects and interfaces was proposed, which was mainly controlled by the diffusion pathway of oxygen, thermodynamic properties, and intragranular defects.
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•The oxidation behavior of 17Cr2Ni2MoVNb steel were studied using various techniques.•The morphologies, compositions, and structures of the oxides were determined.•The mechanism of internal oxidation for 17Cr2Ni2MoVNb steel was described in detail.•A novel internal oxidation model for 17Cr2Ni2MoVNb steel was established.
Copper is equipped with excellent electrical and thermal properties. However, its poor mechanical properties and poor high-temperature stability prevent its extensive used in structural materials. ...Therefore, establishing an effective strengthening method that enables copper to increase its strength and high-temperature tolerance can promote wide-ranging copper applications. An effective method is the addition of ceramic particles as a reinforcement phase to improve the mechanical properties of copper. The preparation processes of several ceramic particles reinforced copper-based composites are discussed in this paper, including three powder metallurgy processes as mechanical alloying, internal oxidation, and wet chemistry, as well as friction stir processing, reactive spray deposition, and liquidus in situ reaction. The effects of each process on the mechanical properties of copper-matrix composites are introduced.
•Preparation of copper-matrix composites reinforced by the second phase is reviewed.•Mechanical alloying has the characteristics of simple process and low production cost.•Ti, La, and so on in CMCs is of great significance for improving the stability and strength of copper.•Friction stir process is mainly used to modify copper surface.
The oxidation of ZM6 alloy in 20 %O2 + Ar at 500 ℃ was investigated by advanced characterization. The surface oxide scale had a duplex structure: a MgO outer layer and a MgO/Nd2O3 composite inner ...layer. Although the outward diffusion of Mg2+ was effectively inhibited by the duplex oxide scale, the inward diffusion of oxygen was not. Particularly, Mg12Nd precipitates and the alloy grain boundaries were operative as fast diffusion paths for the internal oxidation. Once the duplex oxide scale was cracked/exfoliated by the growth/thermal stresses, violent oxidation would then occur, resulting in the formation of white MgO powders.
•A duplex surface oxide scale with a MgO outer layer and a MgO/Nd2O3 composite inner layer was formed.•The internal oxide precipitates had a core-shell structure, Nd2O3 core wrapped by MgO shell.•The oxidation of ZM6 alloy was dominated by internal oxidation.•Violent oxidation at 100–150 h is attributed to the exhaustion of Nd in the near-surface matrix during the prior oxidation.
•The oxidation kinetics of CoCrFeMnNi and CoCrFeMnNb0.25Ni alloys followed parabolic rate law.•At 700 and 800 °C, CoCrFeMnNb0.25Ni alloy has better antioxidant activity than CoCrFeMnNi alloy, but ...opposite at 900 °C.•In the oxidation process, Nb plays a dual role.•The new Laves phase at the oxidation scale/alloy interface hinders the outward diffusion of active metal cations.•The Laves phase inside the alloy becomes the channel for oxygen ions to diffuse inward and forms internal oxidation.
The high temperature oxidation behavior of CoCrFeMnNbxNi high-entropy alloys in air was studied. As-cast CoCrFeMnNb0.25Ni alloy is a duplex-phase structure of FCC plus Laves phase. The oxidation kinetics of alloys followed parabolic rate law and the corresponding oxidation rate constants were calculated. In the oxidation process, Nb plays a dual role: On the one hand, the new Laves phase at the oxidation scale/alloy interface hinders the outward diffusion of active metal cations and improves the oxidation resistance; On the other hand, the Laves phase inside the alloy becomes the channel for oxygen ions to diffuse inward and forms internal oxidation.
We investigate the oxidation of TiZrNbHfTa high entropy alloy between 550 °C and 650 °C as a surface hardening method. Coarse-grained specimens with grain boundaries perpendicular to the surface ...exhibit catastrophic oxidation, while ultrafine-grained, cold-rolled specimens show a continuous mass gain and the formation of an adherent, µm-sized, vitreous oxide layer. Underneath, TiZrNbHfTa decomposes into a bcc- and an hcp-phase, while selective internal oxidation of hafnium and zirconium occurs upon oxygen inward diffusion. Oxygen concentration- and microhardness-depth profiles confirm an increase in oxygen concentration and hardness at the surface, raising the initial hardness by four times to 1522 ± 64 HV0.5.
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•Microstructure has a significant impact on the oxidation behavior of TiZrNbHfTa.•A coarse-grained microstructure causes catastrophic oxidation.•An adherent oxide layer only forms on ultafine-grained, cold-rolled specimens.•Oxidation of cold-rolled TiZrNbHfTa increased the surface hardness by four times.
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