Isothermal Aging Time Research Articles

Effects of lattice misfit of γ/γ′ phases on hydrogen embrittlement behavior in Ni-based single crystal superalloy

The effect of γ/γ′ phase lattice misfit on hydrogen embrittlement (HE) behavior was fundamentally investigated by utilizing a Ni-based single crystal CMSX-4 superalloy with a simple microstructure that can exclude the H-trapping effects of grain boundary, c-vacancy, and misfit dislocation. Increasing isothermal aging time increased γ′ precipitate size while maintaining its volume fraction and a fully coherent interface. The magnitude of negative lattice misfit between the γ and γ′ phases increased from −0.08 to −0.22% according to γ′ precipitate coarsening, resulting in a well-developed tensile-strain field within the γ′ precipitate. The increased tensile-strain field enhanced activation energy for H-desorption in the γ′ precipitate from 31.9 to 35.1 kJ/mol. Therefore, H is preferentially distributed along the γ/γ′ interface within the γ′ precipitate even before deformation. H basically promotes the slip planarity through the H-enhanced localized plasticity (HELP) mechanism in Ni-based single crystal alloys, in which slip is the dominant deformation behavior. As the superalloy possesses a larger negative lattice misfit, H becomes trapped as diffusible-state at the tensile-strain field of the γ/γ′ interface, resulting in cuboidal brittle fracture along the {001} planes. In this process, the HELP mechanism facilitated H localization at the γ/γ′ interface. Thus, the HE behavior distinctly transitioned from HELP to an interaction between HELP and H-enhanced decohesion mechanisms (HEDE) with an increase in the magnitude of the lattice misfit. The HE behavior was investigated by correlating microstructural characterization, H-trapping behavior, and crystallographic fracture mechanism analysis.

Variety of yield strength caused by precipitations of L12-Ni3Al and B2-NiAl phases in alumina-forming austenitic steel

Alumina-forming austenitic (AFA) steel exhibits excellent high-temperature properties and has attracted significant attention in thermal power generation industry. Microstructure evolutions and yield strengths of 20Ni-AFA steel during isothermal aging are investigated. The results show that five types of phases are found in 20Ni-AFA steel during isothermal ageing: NbC, L12-Ni3Al, M23C6, B2-NiAl and Laves phases. Addition of Cu causes precipitation of L12-Ni3Al phase in 20Ni-AFA steel at the early stage of isothermal aging. L12-Ni3Al phase grows and gradually transforms into B2-NiAl phase which possesses a more stable structure with the extension in isothermal aging time. The transformation leads to yield strengths of 20Ni-AFA steel to decrease anomalously during 24-120 h isothermal aging. Yield strengths of 20Ni-AFA steel increase again as isothermal aging time continues to increase.

Optimizing temperature coefficient of Sm2Co17-type magnets through adjusting the isothermal aging process

The high-temperature magnetic performance and microstructure of Sm1–xGdx(CobalFe0.09Cu0.09Zr0.025)7.2 (x = 0.3, 0.5) magnets were investigated. With the isothermal aging time decreasing from 11 to 3 h, the temperature coefficient of intrinsic coercivity in the temperature range of 25–500 °C, β25–500 °C, was optimized from −0.167%/°C to −0.112%/°C for x = 0.3 magnets. The noticeable enhancement (∼33%) of temperature stability is correlated with the increased content of 1:5H cell boundary phase and its relatively high Curie temperature as well. However, for the x = 0.5 magnet, it is found that the presence of Sm5Co19 phases and wider nanotwin variants hinder the formation of 1:5H cell boundary phase. The insufficient 1:5H is not beneficial to the proper redistribution of Cu in cell boundary, making the x = 0.5 magnet difficult to achieve higher temperature stability. Consequently, the approach of adjusting the isothermal aging process can offer guidance for attaining superior magnetic performance in the temperature range from 25 to 500 °C for Gd-substituted Sm2Co17-type magnets.

Intermetallic compound evolution and mechanical properties of Cu/Sn58Bi/Cu 3D structures blended with B4C nanoparticles during isothermal aging

The Cu/Sn58Bi/Cu and Cu/Sn58Bi-0.6B4C/Cu joints were prepared by transient liquid phase (TLP) bonding technology to verify the reliability of low-temperature connection joints. Intermetallic compound (IMC) evolution at the interface under different isothermal aging times was analyzed, and the mechanical properties of the joints were tested. The results show that two kinds of IMC, lamellar Cu3Sn and fan-shaped Cu6Sn5, are formed on both sides of the joint. With increasing soldering time, the thickness for both kinds of IMC become thicker, and it is significantly thicker for the IMC near the cold end. Furthermore, the IMC growth rate of the Cu/Sn58Bi-0.6B4C/Cu solder joint is significantly lower. When the isothermal aging time reaches 36 h, the Cu/Sn58Bi/Cu joint has been almost composed of IMC, while it needs a longer time for the Cu/Sn58Bi-0.6B4C/Cu joint. The growth of Cu3Sn IMC is in line with parabolic law, being mainly controlled by volume diffusion near the hot end, while the grain boundary diffusion significantly affects it at the cold end. The joints shear strength decreases gradually with extending bonding time, but it is always higher for the Cu/Sn58Bi-0.6B4C/Cu joint. When the heating time is not long, the fracture mainly appears at the Bi-rich phase in the solder matrix. With increasing bonding time, the location of fracture gradually transformed from the Cu6Sn5 grains to the junction of the two IMC and Cu3Sn grains, along with the intercrystalline fracture and transcrystalline fracture. In summary, B4C nanoparticles can significantly improve the joint reliability and enhance its mechanical properties.

Influence of exchange coupling interaction on coercivity in 2:17-type SmCo-based high temperature permanent magnets

The intergranular exchange coupling interaction in Sm(CobalFe0.10Cu0.10Zr0.03)7.2 high temperature permanent magnets with different isothermal aging time was studied. The magnet isothermally aging for 24 h with higher coercivity at room temperature exhibits a weaker exchange coupling, while that aging for 4 h with a relatively small temperature coefficient of coercivity shows a stronger exchange coupling. Microstructure and microchemistry analysis indicated that Cu gradually enriches in the cell boundaries with the prolongation of isothermal aging time, decreasing the exchange coupling strength. Theoretical analysis results revealed that with the increase of temperature, the cells gradually get exchange decoupled and demagnetized independently, and the exchange coupling strength between cells decreases, which slows down the decline of coercivity. It is proposed that an optimized temperature coefficient of coercivity requires a stronger exchange coupling at room temperature, conducing to a more significant compensation effect with the increase of temperature.

Coarsening Evolution of γ′ Phase and Failure Mechanism of Co-Ni-Al-Ti-Based Superalloys During Isothermal Aging

Novel Co-based superalloys, as potentially ideal aero-engine hot section materials, have a higher temperature bearing capacity and better oxidation resistance than Ni-based superalloys. Coarsening evolution of γ′ phase and failure mechanism of Co-Ni-Al-Ti-based superalloys during the isothermal aging process at 1073 K were investigated using multiple characterizations and testing methods. The results show that γ′ phase is uniformly distributed on the γ phase matrix, and coarsening with the increase in isothermal aging time, which results in a decrease in maximum tensile strength. Furthermore, Mo element is preferred to distribute in γ′ phase and provides stronger solution strengthening effect than Cr element, which determines more excellent mechanical properties of 2Mo superalloy than that of 2Cr superalloy. The coarsening rate of γ′ phase in the 2Cr superalloy is significantly higher than that in the 2Mo superalloy. Grain boundary failure is dominant in isothermal aging, and the cracks nucleate and expand along the vertical direction of loading stress on the grain boundary. The current work suggests that the coarsening of the γ′ phase, reduction in the volume fraction of γ′ phase, and formation of ′-precipitate depleted zone (PDZ) near the grain boundary during aging controls the deterioration of mechanical properties in Co-Ni-Al-Ti-based superalloys.

Insight on nano-sized precipitate-hardened Ti-Mo 11Cr-11Ni martensitic stainless steel: Experimental Evidence and Related Patents Investigation

A dual phase structure mainly consisting of martensite and reversed austenite has been achieved in Ti-Mo martensitic stainless steels, which were austenitized, cryogenically frozen, and further aged at various temperatures to investigate the effects of microalloying elements and aging treatment on the microstructural evolution and mechanical properties. Interestingly, the results indicated that the Vickers hardness increased with isothermal aging time after aging at a relatively low temperature, i.e. 480 °C, but it decreased with aging time after aging at a temperature higher than 520 °C. For example, the Vickers hardness (HV0.1) values of specimens aged at 480 °C were 645 ± 37 (1 h), 657 ± 31 (4 h), and 673 ± 29 (8 h). In contrast, specimens aged at 640 °C had Vickers hardness (HV0.1) values of 389 ± 15 (1 h), 383 ± 16 (4 h), and 375 ± 16 (8 h). The greatest ultimate tensile strength (UTS), 1730 MPa, was that of the Ti-Mo martensitic stainless steel aged at 480 °C for 4 h. The strengthening of Ti-Mo martensitic stainless steel can be primarily ascribed to the numerous nano-sized intermetallic particles, i.e., η-Ni3Ti, that precipitated within the martensite matrix upon aging treatment. Furthermore, more reversed austenite with various shapes, namely, film-like, granular, and blocky, formed when the aging temperature was the relatively low temperature of 520 °C and effectively enhanced the ductility of the martensitic stainless steel.

Interaction between η-Ni3Ti and reversed austenite within Custom 465 stainless steel: experimental evidence and related patents investigation

After various aging treatment conditions, precipitated nano-sized intermetallic compound particles, i.e., η-Ni3Ti, with a rod-like shape and reversed austenite with various shapes, i.e. lath, blocky, and granular morphologies, were distributed within martensite laths of Custom 465 stainless steel in the present study. Identified via STEM mapping and corresponding diffraction patterns, the precipitates of η-Ni3Ti had a hexagonal closed-packed (HCP) crystal structure and adopted a Burgers orientation relationship, i.e., {011}α′ // {0001}η ; <111‾>α′ // <112‾0>η, with the martensite matrix. Furthermore, the size of the tiny η-Ni3Ti particles increased with the isothermal aging temperature and time, indicating that the precipitation behavior obeyed the thermal activation mechanism. Despite forming simultaneously upon aging treatment, the formations of reversed austenite and η-Ni3Ti particles were mutually independent due to the excess content of Ni in the precipitation hardened (PH) stainless steel. In addition, the existence of such reversed austenite also led to the inhomogeneous distribution of dislocations due to the discrepancy in the thermal expansion coefficients of reversed austenite and martensite, which further resulted in the inhomogeneous distribution of η-Ni3Ti particles and a wider Vickers hardness distribution, especially after aging at low temperature.

Comparison of intermetallic compound growth and tensile behavior of Sn-3.0Ag-0.5Cu/Cu solder joints by conventional and microwave hybrid heating

Microwave hybrid heating (MHH) is a possible approach for soldering; yet, the long-term reliability of solder joints is still unclear. In this work, a comparative study was carried out on the influence of isothermal aging on SAC305/Cu solder joints by conventional reflow and MHH. The conventional reflow oven was set to 260 °C at 60 s holding time, and MHH was carried out at high operating power at 40 s. The reflowed sample was isothermally aged at 150 °C for various aging times (0, 240, 480 and 960 h) in a laboratory oven. The conventional reflow and MHH transformed the IMC structure from scallop-like to a planar-like Cu6Sn5 with a linear increase in thickness and a thin layer of Cu3Sn was observed at 240 h of aging. While Ag3Sn particles grew larger at longer aging time. The MHH shows a thinner Cu6Sn5 layer, and a smaller grain size compared to conventional reflow. The ultimate tensile strength (UTS) for conventional reflow (32.7 MPa) is 26.8% lower compared to MHH (44.7 MPa). The UTS for MHH is greater than conventional reflow as the isothermal aging time increases.

Dynamic Observation of Interfacial IMC Evolution and Fracture Mechanism of Sn2.5Ag0.7Cu0.1RE/Cu Lead-Free Solder Joints during Isothermal Aging.

Dynamic observation of the microstructure evolution of Sn2.5Ag0.7Cu0.1RE/Cu solder joints and the relationship between the interfacial intermetallic compound (IMC) and the mechanical properties of the solder joints were investigated during isothermal aging. The results showed that the original single scallop-type Cu6Sn5 IMC gradually evolved into a planar double-layer IMC consisting of Cu6Sn5 and Cu3Sn IMCs with isothermal aging. In particular, the Cu3Sn IMC grew towards the Cu substrate and the solder seam sides; growth toward the Cu substrate side was dominant during the isothermal aging process. The growth of Cu3Sn IMC depended on the accumulated time at a certain temperature, where the growth rate of Cu3Sn was higher than that of Cu6Sn5. Additionally, the growth of the interfacial IMC was mainly controlled by bulk diffusion mechanism, where the activation energies of Cu6Sn5 and Cu3Sn were 74.7 and 86.6 kJ/mol, respectively. The growth rate of Cu3Sn was slightly faster than that of Cu6Sn5 during isothermal aging. With increasing isothermal aging time, the shear strength of the solder joints decreased and showed a linear relationship with the thickness of Cu3Sn. The fracture mechanism of the solder joints changed from ductile fracture to brittle fracture, and the fracture pathway transferred from the solder seam to the interfacial IMC layer.

Investigation of the interfacial reactions and growth behavior of interfacial intermetallic compound between Sn37Pb solder and Au/Ni/Kovar substrate

The interfacial reactions and microstructural evolution of Sn37Pb/Kovar with Au/Ni platings solder joints were investigated during isothermal aging at 120 °C, 150 °C and 170 °C for different duration, respectively. The experimental results indicated that a thin interfacial intermetallic compounds (IMCs) layer formed at the Sn37Pb/Kovar interface after reflowing process. The accurate compositions of thin IMCs were double-layers of (Ni, Au)3Sn4 and AuSn4, which were confirmed by utilizing TEM-EDS analysis. However, during the aging process, the compositions of interfacial IMCs of solder joint obviously changed, which transformed from AuSn4/(Ni, Au)3Sn4 to (Au, Ni)Sn4/Ni3Sn4 and the AuSn4 IMC formed in the bulk solder gradually changed to the (Au, Ni)Sn4 IMC. Furthermore, the total IMC thickness increased dramatically with increasing isothermal aging time, as well as aging temperature. The growth process of IMC layer is mainly dominated by the volume diffusion mechanism during the solid-state aging. The square of growth rate constant (K) equals to 2.559 × 10−17, 1.307 × 10−16 and 7.781 × 10−16 m2s−1 for the aging temperature of 120 °C, 150 °C and 170 °C, respectively. The activation energy (Q) value of the Sn37Pb/Kovar couple was calculated to be approximate 95.96 kJ·mol−1.

Effect of Isothermal Aging Time on Magnetic Properties and Microstructure of Sm1-xDyx(Co0.695Fe0.2Cu0.08Zr0.025)7.2 magnets

The ordering process of 2:17R disordered phase with microtwin structure is a very important process during aging in 2:17-type Sm–Co magnets. In this paper, Sm1– x Dy x (Co0.695Fe0.2Cu0.08Zr0.025)7.2 x = 0, 0.5) magnets with different degrees of ordering were prepared. The effect of different isothermal aging times on the magnetic properties of magnets with microtwin structure has been investigated. After isothermal aging for 0, 0.5, 2, 6, and 12 h, the coercivity and remanence of magnets without Dy doped increased gradually, while almost no change in the coercivity and remanence is observed in magnets with x = 0.5. After the slow cooling process, the coercivity about 20 kOe is obtained in all Dy-free magnets with different aging times. On the contrary, with an increase in the ordering degree of 2:17R disordered phase, the coercivity of Dy-doped magnets at room temperature enhances from 5.82 to 7.88 kOe. The temperature coefficient of abnormal coercivity between 20 °C and 100 °C of Sm0.5Dy0.5(Co0.695Fe0.2Cu0.08Zr0.025)7.2 magnets with isothermal aging for 12, 20, and 40 h reduces from +0.176%/°C to +0.065%/°C, which could be due to more content of Dy diffuse from cell boundary to cell phase.

Phase transition and hardness evolution of a Ti-5Al-5Mo-1Fe-1Cr alloy subjected to isothermal aging

In this paper, a Ti-5Al-5Mo-1Fe-1Cr alloy was isothermally aged at 500 °C for different periods. Phase transformation took place during the aging treatment. The phase transformation was studied by X-ray diffraction spectroscopy and transmission electron microscopy. The as-quenched sample was composed of matrix β (body-centered cubic structure) phase, ωath (hexagonal structure) phase and minor α (hexagonal close-packed structure) phase. The results showed that aging treatment promoted the β → ω → α phase transformations in the as-quenched alloys. Different isothermal aging time affected the contents and morphologies of the three phases inside the alloy and thus the hardness. It was found that the content of hard ω phase plays an important role in the hardness of the Ti-5Al-5Mo-1Fe-1Cr alloy. The hardness of alloy showed an overall decreasing trend as the aging time increased to 2 h, except for the temporary slight increase of the hardness at an aging time of 1 h.

Influence of heating rate and aging temperature on omega and alpha phase precipitation in Ti[sbnd]35Nb alloy

Metastable β Ti alloys based on Nb exhibit features that make them suitable for load-bearing biomedical applications, so it is crucial to evaluate their phase transformation and properties. This paper discusses the effects of aging heat treatments on the microstructure, phase transformation and mechanical behavior of Ti35Nb alloy. Samples were solution heat treated in the β field, quenched and aged under different conditions of heating rates, isothermal aging temperature and isothermal aging time. To enhance the alloy's mechanical behavior, thermal analysis and high temperature X-ray diffraction experiments coupled with Vickers hardness tests were employed to determine the temperature range of isothermal ω phase precipitation, as well as α phase precipitation temperature, thereby providing information to optimize aging heat treatment conditions. Results indicate that Ti35Nb alloy subjected to a heating rate of 30 °C/min and an isothermal aging temperature of 500 °C applied for 4 h would ensure high mechanical strength and low elastic modulus. The application of these aging conditions led to a microstructure consisting of very refined α phase precipitates homogeneously dispersed in the β phase matrix, presenting an ultimate tensile strength of 972 MPa, elongation of 7.5% and elastic modulus of 78 GPa, making this aged alloy a promising candidate for biomedical applications. These results shed light on the correlations between phase transformations, microstructure and mechanical behavior in high solute content TiNb alloys for biomedical applications.

Crystalline and magnetic microstructures of iron-rich Sm(Co0.65Fe0.26Cu0.07Zr0.02)7.8 sintered magnets: Isothermal aging effect

Crystalline and magnetic microstructures of the iron-rich Sm(Co0.65Fe0.26Cu0.07Zr0.02)7.8 anisotropic sintered magnets after isothermal aging process were investigated systematically by using transmission electron microscope (TEM) and magnetic force microscopy (MFM). As the isothermal aging time (t) increases from 0.5 to 20 h, the remanences of the specimens almost keep constant (11.4 kGs), while the intrinsic coercivity (Hcj) shows an increase from 1.13 to 7.93 kOe, which is much higher than the well-known value of the order of 1 kOe. TEM investigation indicates that the cellular microstructures are incomplete together with partially phase transformation of 1:7H to 2:7R and 5:19H when t = 0.5 h. When t reaches 5 h, the phase transformation is almost completed. With t increasing from 2.5 to 20 h the average cellular size of the specimens increases from ∼90 to ∼150 nm and the density of the lamellae phase increases from ∼0.014 to ∼0.033 1/nm. MFM investigations show that the average surface width (Ws) decreases from 0.86 to 0.67 μm initially and then increases to 0.98 μm and the main domain width (Wm) decreases from 2.17 to 1.21 μm initially and then increases to 1.51 μm. And the Wm is about two times of Ws for the specimens because of the smaller magnetostatic energy of the surface layer of the specimens with c-axis parallel to the imaging plane. It is impressive that the domain walls form in straight line through cellular structures as t is less than 5 h, and in a zig-zag shape when t is over 5 h. This is the first time to reveal systematically the domain structures for the iron-rich 2:17-type Sm-Co magnets during isothermal aging process. The evolution of the magnetic microstructures is ascribed to the variation of the effective intrinsic magnetic properties of cell and cell boundary phases originating from both the phase transformation and development of cellular structures.

Study on the High Temperature Interfacial Stability of Ti/Mo/Yb0.3Co4Sb12 Thermoelectric Joints

To improve the interfacial stability at high temperatures, n-type skutterudite (SKD) thermoelectric joints with sandwich structures of Ti/Mo/Yb0.3Co4Sb12 were successfully designed and fabricated. In this structure, Mo and Ti were introduced as the barrier layer with the goal of suppressing the interfacial diffusion and the buffer layer with the goal of enhancing the bonding strength, respectively. To evaluate the high temperature interfacial behavior of the Ti/Mo/Yb0.3Co4Sb12 joints, thermal shocking between 0 °C and 600 °C and isothermal aging at a temperature range of 550 °C to 650 °C were carried out in vacuum. During the isothermal aging process, Ti penetrates across the Mo layer, and finally diffuses into the Yb0.3Co4Sb12 matrix. By increasing the isothermal aging time, Ti continuously diffuses and reacts with the elements of Sb and Co in the matrix, consequently forming the multilayer-structured intermetallic compounds of Ti3Sb/Ti2Sb/TiCoSb. Diffusion kinetics was investigated and it was found that the interfacial evolution of the Ti/Mo/Yb0.3Co4Sb12 joints was a diffusion-controlling process. During the diffusion process, the formed Mo-Ti buffer layer acts as a damper, which greatly decelerates the diffusion of Ti towards the Yb0.3Co4Sb12 matrix at high temperatures. Meanwhile, it was found that the increase in the contact resistivity of the joints mainly derives from the inter-diffusion between Ti and Yb0.3Co4Sb12. As a result, the Ti/Mo/Yb0.3Co4Sb12 joint demonstrates the excellent stability of the interfacial contact resistivity. Service life prediction was made based on the stability of the contact resistivity, and it was found that the Ti/Mo/Yb0.3Co4Sb12 joint is qualified for practical applications at 550 °C.

Microstructural evolution of a titanium- and cobalt-modified nickel-based repair alloy during exposure to high temperatures

Spray forming (SF) is a near net shape manufacturing process in a wide variety of alloys. A titanium- and cobalt-modified repair alloy has been prepared by the SF process and was presented excellent properties of friction and wear in previous published article. In this paper, the microstructural evolution of the alloy exposed at 750 °C for different isothermal aging time was analyzed. Microstructure observations reveal that equiaxed grains formed in the ingots whether after aging process or not. The size and the volume fraction of γ′ precipitates increase with the ongoing of aging process, which could result in the change of the lattice misfit of γ′/γ and the precipitation of new σ, η phases. Intergranular fracture and transgranular fracture are main fracture mechanisms in aged state. Its tensile properties were attributed to the size and the volume fraction of γ′ precipitates, γ′/γ misfit, grain boundary carbides and the number of harmful phases (σ and η phases) during aging process.

Evolution of Grain Boundary Precipitates in an Al-Cu-Li Alloy During Aging

The grain boundary microstructure of Al-Cu-Li alloy AA2050 was investigated for different isothermal aging times to rationalize intergranular corrosion (IGC) characteristics. In the underaged condition, the dominant grain boundary precipitates are fine T1 (Al2CuLi). Extended aging revealed that grain boundaries were decorated by large T1 precipitates and S′ phase (Al2CuMg), with S′ growth not dimensionally constrained. Such a transition in the precipitate type at grain boundaries is a unique feature of the Al-Cu-Li system.

Experimental investigation of the ordering pathway in a Ni-33 at.%Cr alloy

The present study involves a detailed experimental investigation of the concurrent compositional clustering and long-range ordering tendencies in a Ni-33 at.%Cr alloy, carried out by coupling synchrotron-based X-ray diffraction (XRD), transmission electron microscopy (TEM), and atom probe tomography (APT). Synchrotron-based XRD results clearly exhibited progressively increasing lattice contraction in the matrix with increasing isothermal aging time, at 475 °C, eventually leading to the development of long-range ordering (LRO) of the Pt2Mo-type. Detailed TEM and APT investigations revealed that this LRO in the matrix is manifested in the form of nanometer-scale ordered domains, and the spatial distribution, size, morphology and compositional evolution of these domains have been carefully investigated. APT results also revealed the early stages of compositional clustering prior to the onset of long-range ordering in this alloy and such compositional clustering can potentially be correlated to the lattice contraction and previously proposed short-range ordering tendencies.

Microstructure and oxidation properties of 9Cr–1.7W–0.4Mo–Co ferritic steel after isothermal aging

Microstructural evolutions of 9Cr–1.7W–0.4Mo–Co ferritic heat-resistant steel after isothermal aging were investigated by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The kinetic models are employed to describe the mechanisms of aging hardening and oxidation resistance. The results show that the recovery of martensite lath occurs, and the precipitates with large particle size forms during aging. The alloying elements in the interior of δ-ferrite contribute to the formation of the precipitates located at grain boundaries. The increase of W element gathered on the grain boundaries indicates the forthcoming formation of Laves phase. Relative percentage of hardening firstly increases, and then decreases with the increase of tempering parameter. Both the power exponent, n, and the oxidation rate, kp, increase with the increase of aging time. In addition, the values of n are less than 2, and the value of kp is from 0.012 mg/cm2/h up to 0.049 mg/cm2/h with the increase of isothermal aging time.