Phase precipitation behavior and tensile properties of as-cast Ni-based superalloy during heat treatment

The phase precipitation behavior and tensile properties of an as-cast Ni-based alloy, IN617B alloy, after solution heat treatment and long-term aging treatment were investigated. Ti(C,N), M₆C and M₂₃C₆ are the primary precipitates in as-cast microstructure. After solution heat treatment, most of carbides dissolve into the matrix except a few fine Ti(C,N) within grains. During long-term aging at 700 °C, the phase precipitation behaviors of the alloy are characterized as follows: (1) M₂₃C₆ carbides at grain boundaries (GBs) transform from film-like shape to cellular shape and gradually coarsen due to the decrease of the surface energy and element aggregation to GBs; (2) M₂₃C₆ carbides within grains have a bar-like morphology with a preferential growth direction [110] and have a cube-on-cube coherent orientation relationship with the matrix γ; (3) γ′ particles inhibit the coarsening of M₂₃C₆ within grains by constraining the diffusion of formation elements. Furthermore, the tensile strength of the alloy obviously increases, but the ductility significantly decreases after the aging for 5000 h. The alloy has a relatively stable microstructure which guarantees the excellent tensile properties during long-term aging.     

Ta对IN617B镍基高温合金凝固组织及元素偏析的影响

IN617B镍基高温合金是700℃先进超超临界燃煤电站的主要候选材料.采用OM、SEM、TEM、EDS、EPMA和热力学计算等方法研究Ta对IN617B镍基高温合金凝固行为及组织的影响.研究结果表明:IN617B合金的凝固组织呈典型的树枝晶形貌,在枝晶间和晶界处分布着网状的M6C、板条状的M23C6和颗粒状的Ti(C,...     

Grain boundary and interface chemistry of an Nd-Fe-B-based sintered magnet

The compositions of grain boundaries (GBs) and other interfaces surrounding Nd₂Fe₁₄B grains in commercial Nd-Fe-B sintered magnets have been investigated by laser-assisted three-dimensional atom probe to understand the mechanism of the coercivity enhancement by post-sinter annealing. While only a slight segregation of Nd and Pr to the GBs was confirmed in the as-sintered sample, a thin Nd-rich amorphous phase layer was observed along the GBs with Cu segregation to the interfaces in the annealed sample. The segregation of Cu to NdO_x/Nd₂Fe₁₄B interfaces was also found, suggesting that the Nd₂Fe₁₄B grains are enveloped by a Cu-enriched layer after the annealing. The concentration of Fe + Co in the thin GB layer was found to be as high as 65 at.%, and a model amorphous film processed by sputtering with the same composition as the thin GB layer was found to be ferromagnetic. Ferromagnetic behavior of the thin GB layer suggested that Nd₂Fe₁₄B grains are magnetically coupled. The coercivity mechanism of the sintered magnets is discussed based on these new findings.     

Effect of Mo/B atomic ratio on the microstructure and mechanical properties of Mo2FeB2 based cermets

Four series of Mo₂FeB₂ based cermets with the Mo/B atomic ratio in the range from 0.8 to 1.1 were prepared by reaction sintering process. The microstructure and crystalline phases were studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX) and X-ray diffractometry (XRD). The results indicate that the transverse rupture strength (TRS) of the cermets increases with an increase of Mo/B atomic ratio and shows a maximum value of 1.9 GPa at a Mo/B atomic ratio of 0.9. At a higher atomic ratio the TRS decreases. The hardness of the cermets decreases monotonically from 90.8 HRA to 88.8 HRA with an increase of Mo/B atomic ratio. In Mo-rich cermets with an atomic ratio of Mo/B above 1.0, a new M₂₃B₆ type phase (M₂₃B₆, where M represents a metal) is found. This phase has a lattice parameter a = 1.09 nm containing Mo, Fe, Cr and B with an atomic ratio close to 16:6:1:6 and is precipitated at the interface of Mo₂FeB₂ grains or at the Mo₂FeB₂ grain boundaries.     

Crystallography and structural evolution during reverse transformation in an Fe–17Cr–0.5C tempered martensite

The mechanism and the crystallography of austenite and δ-ferrite formation from tempered martensite at temperatures of 900–1200°C have been studied by means of transmission electron microscopy in an Fe–17Cr–0.55C alloy. It was found that austenite nucleates within ferrite at low angle, high angle and twin-related lath boundaries as well as at high angle equiaxed grain boundaries in contact with M₂₃C₆ grain/lath boundary carbides. The austenite grains are in a cube–cube relationship with the M₂₃C₆ carbide particles and bear the Kurdjumov–Sachs orientation relationship with at least one of the adjacent ferrite grains. They are often in the Kurdjumov–Sachs relationship with both ferrite laths separated by a high angle boundary as far as the laths had formed from the same austenite. The {111}_A close packed plane of γ precipitate is parallel to the {110}_F plane most parallel to the grain boundary. The close packed planes of some austenite grains nucleating at the high angle lath boundaries are parallel to the close packed planes of both ferrite laths. These crystallographic features often result in a single variant of austenite orientation at a grain boundary. After nucleation, the austenite grains grow by the migration of both semicoherent and incoherent interfaces. These results demonstrate that a specific orientation relationship is preferred for the austenite nucleation, but is not necessary for the subsequent growth. The kinetics of austenite growth are controlled by chromium diffusion. The δ-ferrite particles precipitate at high temperatures as a non-equilibrium phase. No rational orientation relationship between δ-ferrite and retained austenite was found. The experimental results are discussed qualitatively in terms of the thermodynamic predictions using the software ThermoCalc, assuming local equilibrium at the moving interfaces.     

Carburization of W- and Re-rich Ni-based alloys in impure helium at 1000 °C

The surface and microstructure stability of experimental W- and Re-rich Ni-based alloys in an impure-helium environment containing only CO and CO₂ as impurities (ppm level) have been investigated at 1000 °C. All the alloys carburized during 50 h of exposure, and, depending on the alloy composition, different carbides of the type M₆C, M₇C₃ and M₂₃C₆ formed on the alloy surface, in grain interiors and at grain boundaries. Microprobe analysis and Calphad-based calculations indicated that the chromium carbides (particularly Cr₂₃C₆) were enriched by rhenium. Extended exposure (225 h) led to the disappearance of surface transient carbides and the growth of surface oxide Cr₂O₃ occurred.     

Co-Cr-W-Ni合金中碳化物析出机制的研究

Co-Cr-W-Ni合金中碳化物的类型和析出机制对合金的力学性能影响显著,因此,本文用XRD、SEM-EBSD和TEM技术研究了Co-Cr-W-Ni合金时效后碳化物的析出类型、分布特征和析出机制。实验结果表明,Co-Cr-W-Ni合金中碳化物的主要类型为M₇C₃、M₆C和M₂₃C₆;M₂₃C₆型碳化物主要分布于孪晶与g相晶粒的三叉交界处,与基体g相是立方-立方的取向关系;M₆C型碳化物退化分解并原位析出M₇C₃型碳化物是Co-Cr-W-Ni合金中一种重要的碳化物析出机制。     

Oxidation properties of Fe-5Cr-4Al (wt.%) alloys in oxygen at temperatures 1000°C–1320°C

Oxidation kinetics of a parent Fe-5Cr-4Al alloy subjected to two types of anneals were investigated at temperatures ranging from 1000°C to 1320°C. The alloy annealed at 850°C exhibited a rapid transient oxidation stage associated with growth of nodules containing iron oxides and internal precipitation of -Al₂O₃ in the alloy beneath these nodules. The nodules nucleated and grew from sites located in the regions of the alloy grain boundaries during the period of rapid alloy grain growth. Nodular growth virtually ceased when a continuous -Al₂O₃ film formed at the nodule-alloy interface. The alloy subjected to anneal at 1000°C and at the reaction temperature to stabilize the alloy grain size tended upon oxidation to form a protective -Al₂O₃, layer by parabolic kinetics at temperatures to 1250°C. If this alloy was oxidized in stages at 1000°C, a protective -Al₂O₃ scale was formed up to 1320°C. The temperature coefficient of the parabolic oxidation kinetics was consistent with diffusion processes at boundaries of the -Al₂O₃ grains playing an essential role during growth of this protective oxide layer.     

Formation and morphology of M<Subscript>7</Subscript>C<Subscript>3</Subscript> in low Cr white iron alloyed with Mn and Cu

The presence of M₇C₃ carbide in white iron enhances its wear resistance because of high hardness. Scanning electron microscopy (SEM) revealed its morphology as a pencil-like hexagonal structure. On the basis of the SEM observations, elemental distribution studies, and differential thermal analysis (DTA) of some heat-treated hypoeutectic white irons alloyed with Cr, Mn, and Cu, it is concluded that M₇C₃ carbides form as a result of attainment of a favorable condition in the liquid phase present at the austenite grain boundaries. Segregation of phosphorus in the intercellular regions and formation of a copper-rich intermetallic is responsible for the formation of this liquid phase. Austenite was found to nucleate first, followed by the nucleation and growth of M₇C₃ carbide in its vicinity, because of rejection of C and Cr during formation of austenite. The rosette structure generally observed is formed from the joining of M₇C₃ carbides by precipitation of secondary carbides.     

TEM analysis of 2205 duplex stainless steel to determine orientation relationship between M23C6 carbide and austenite matrix at 950 °C

Microstructures of 2205 duplex stainless steel were examined using transmission electron microscopy. During isothermal heating at 950 °C, M₂₃C₆ carbide was formed on the austenite grain boundaries with two types of morphologies: “larva” and “triangle”. The orientation relationship between the M₂₃C₆ carbide and the austenite matrix is cubic-to-cubic. In addition, these two types of precipitates have a twin relationship with each other. Based on the STEM-EDS data, the silicon content of triangle M₂₃C₆ carbide is higher than that of the larva M₂₃C₆ carbide, revealing that the silicon content in the M₂₃C₆ carbide plays an important role in determining the orientation relationship between the M₂₃C₆ carbide and the austenite matrix.     

Hot corrosion and oxidation behavior of a novel Pt + Hf-modified γ′-Ni3Al + γ-Ni-based coating

A new type of Pt + Hf-modified γ′-Ni₃Al + γ-Ni-based coating has been developed in which deposition involves Pt electroplating followed by combined aluminizing and hafnizing using a pack cementation process. Cyclic oxidation testing of both Pt + Hf-modified γ′ + γ and Pt-modified β-NiAl coatings at 1150 °C (2102 °F), in air, resulted in the formation of a continuous and adherent α-Al₂O₃ scale; however, the latter developed unwanted surface undulations after thermal cycling. Type I (i.e. 900 °C/1652 °F) and Type II (i.e. 705 °C/1300 °F) hot corrosion behavior of the Pt + Hf-modified γ′ + γ coating were studied and compared to Pt-modified β and γ + β-CoCrAlY coatings. Both types of hot corrosion conditions were simulated by depositing Na₂SO₄ salt on the coated samples and then exposing the samples to a laboratory-based furnace rig. It was found that the Pt + Hf-modified γ′ + γ and Pt-modified β coatings exhibited superior Type II hot corrosion resistance compared to the γ + β-CoCrAlY coating; while the Pt + Hf-modified γ′ + γ and γ + β-CoCrAlY coatings showed improved Type I hot corrosion performance than the Pt-modified β.     

Influence of titanium addition on the microstructure of the novel ferrous-based stainless steel

The microstructural characteristics of the Fe-9Al-30Mn-1C-5Ti (wt.%) alloy were determined by scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray spectrometry. The microstructure of the alloy was essentially a mixture of (γ + TiC_x + (α + B2 + DO₃)) phases during solution treatment between 950 °C and 1150 °C. The TiC_x carbide had a face-center-cubic structure with a lattice parameter a of 0.432 nm. When the as-quenched alloy was subjected to aging treatment at temperatures of 450-850 °C, the following microstructural transformation occurred: (γ + TiC_x + κ + (α + DO₃)) → (γ + TiC_x + κ + (α + B2 + DO₃ + TiC_x)) → (γ + TiC_x + κ + κ′ + (α + B2 + DO₃)) → (γ + TiC_x + (α + B2 + DO₃)). Addition of Ti promotes the formation of the α phase at high temperatures.     

Effects of equal channel angular extrusion and aging treatment on R phase transformation behaviors and Ti3Ni4 precipitates of Ni-rich TiNi alloys

Ni-rich TiNi alloys were subjected to the effect of multiple equal channel angular extrusion (ECAE) treatments by B_C path at 500 °C. The characteristics of R phase transformation in aging treatment were dissimilar in the appearance and the temperature range to those counterparts induced by ECAE treatments. The fine lens-like shape Ti₃Ni₄ particles precipitated mainly in the regions of near grain boundaries and on the tangled grain boundaries after ECAE treatments. The effects and mechanisms of aging treatments and ECAE treatments on R phase transformation behaviors and Ti₃Ni₄ precipitates were investigated and discussed.     

深冷处理过程中Inconel 617合金的微观组织演变（英文）

研究了Inconel 617合金在不同深冷处理时间和次数条件下微观组织的演变规律。结果表明,深冷处理对Inconel 617合金的微观组织结构有很大的影响,随着深冷处理时间的延长,晶粒尺寸减小,但是随着深冷处理次数的增加,晶粒尺寸又增大,并且细化晶粒的晶界处具有高的应力。晶格常数与晶粒尺寸的变化相反。深冷处理试样析出了MC、M₆C、M₂₃C₆的简单和复杂碳化物,导致了位错的塞积。深冷处理24 h后试样的几何必须位错(GND)密度增大,当24 h循环处理2次后,GND密度显著降低。此外,深冷处理导致Inconel 617合金的旋转立方织构和旋转铜织构转变为黄铜织构、P织构、高斯织构。     

Oxidation behavior of a Ni3Al PM alloy

The oxidation behavior of a Ni₃Al powder-metallurgical (PM) alloy doped with boron was investigated by means of discontinuous isothermal tests in the temperature range of 535° to 1020°C for exposures of up to 150 hr. The oxidation kinetics were characterized by a sharp decrease in the oxidation rate at about 730°C which is associated with a change in the oxidation mechanism. Below 730°C, the scale exhibited an outer NiO layer and an internal-oxidation zone consisting of a fine dispersion of alumina in a diluted Ni-Al solid solution. Between these two layers a very thin layer of nickel could be observed. Above 730°C, a three-layered scale was observed consisting of an outer NiO layer, an intermediate layer that depending on temperature consisted of a mixture of nickel and aluminum oxides or NiAl₂O₄, and an inner layer of Al₂O₃, which accounts for the higher oxidation resistance. Oxidation at the higher temperatures resulted in extensive void formation at the scale/metal interface which led to poorly adherent scales. It is worth noting that at the early oxidation stage the scale was characterized by planar interfaces. Roughening of the air/scale and, specially, the scale/metal interfaces after long exposures at the low-temperature range or after short times at higher temperatures could be related to the formation of the inner Al₂O₃ layer at the grain boundaries which favor oxygen penetration through the grain interior.     

Effects of hot compression on carbide precipitation behavior of Ni—20Cr—18W—1Mo superalloy

The effect of hot compression on the grain boundary segregation and precipitation behavior of M₆C carbide in the Ni–20Cr–18W–1Mo superalloy was investigated by thermomechanical simulator, scanning electronic microscope (SEM) and X-ray diffraction (XRD). Results indicate that the amount of M₆C carbides obviously increases in the experimental alloy after hot compression. Composition analyses reveal that secondary M₆C carbides at grain boundaries are highly enriched in tungsten. Meanwhile, the secondary carbide size of compressive samples is 3–5 μm in 10% deformation degree, while the carbide size of undeformed specimens is less than 1 μm under aging treatment at 900 and 1000 °C. According to the thermodynamic calculation results, the Gibbs free energy of γ-matrix and carbides decreases with increase of the compression temperature, and the W-rich M₆C carbide is more stable than Cr-rich M₂₃C₆. Compared with the experimental results, it is found that compressive stress accelerates the W segregation rate in grain boundary region, and further rises the rapid growth of W-rich M₆C as compared with the undeformed one.     

RETRACTED ARTICLE: Dislocation-particle interaction in precipitation strengthened Ll<Subscript>2</Subscript>-ordered Ni<Subscript>3</Subscript>Al

Transmission electron microscope investigation has been performed on the particle-dislocation interactions in Ni₃Al-based intermetallics containing various types of fine precipitates. In an Ll₂-ordered Ni₃Al alloy with 4 mol.% of chromium and 0.2–0.5 mol.% of carbon, fine octahedral precipitates of M₂₃C₆ type carbide, which has a cube-cube orientation relationship with the matrix, appear during aging. Typical Orowan loops are formed in Ni₃Al containing fine dispersions of M₂₃C₆ particles. In the alloys with appropriate titanium content, fine precipitates of coherent disordered γ are formed during aging. The γ precipitates are initially spherical or rounded cubic in shape and grow into platelets as aging proceeds. Loss of coherency is initiated by the introduction of dislocations at the γ/γ′ interface and results in step formation at the dislocations. The γ precipitates become globular after the loss of coherency. In the γ′ phase hardened by the precipitation of the disordered γ phase, dislocations are attracted into the disordered γ phase and cut through the particles during deformation at any stage of aging. In Ni₃Al containing a fine dispersion of disordered γ, superdislocations are strongly attracted to the disordered particles and dissociate on the (111) plane in the γ particles, while they dissociate on the (010) plane in the matrix. It is shown by comparison that the strengthening due to attractive interaction is more effective than that due to repulsive interaction. The roles of the variation of the interaction modes and of the dissociation of superdislocations in the matrix and particles are discussed in connection with the optimum microstructures of Ll₂-ordered intermetallics as high temperature structural materials.     

Dielectric relaxation behaviors of pure and Pr6O11-doped CaCu3Ti4O12 ceramics in high temperature range

Pure and Pr₆O₁₁-doped CaCu₃Ti₄O₁₂ (CCTO) ceramics were prepared by conventional solid-state reaction method. The compositions and structures were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The influences of Pr-ion concentration on dielectric properties of CCTO were measured in the ranges of 60 Hz-3 MHz and 290-490 K. The third phase of Ca₂CuO₃ was observed from the XRD of CCTO ceramics. From SEM, the grain size was decreased obviously with high valence Pr-ion (mixing valence of Pr³⁺ and Pr⁴⁺) substituting Ca²⁺. The room temperature dielectric constant of Pr-doped CCTO ceramics, sintered at 1323 K, was an order of magnitude lower than the pure CCTO ceramics due to the grain size decreasing and Schottky potential increasing. The dielectric spectra of Pr-doped CCTO were flatter than that of pure CCTO. The loss tangent of Pr-doped CCTO ceramics was less than 0.20 in 2 × 10²-10⁵ Hz region below 440 K. The complex impedance spectra of pure and Pr-doped CCTOs were fitted by ZView. From low to high frequency, three semicircles were observed corresponding to three different conducting regions: electrode interface, grain boundary and grain. By fitting the resistors R and capacitors C, the activation energies of grain boundary and electrode contact were calculated. All doped CCTOs showed higher activation energies of grain boundary and electrode than those of pure CCTO ceramics, which were concordant with the decreasing of dielectric constant after Pr₆O₁₁ doping.     

The influence of grain-boundary segregation of Y in Cr2O3 on the oxidation of Cr metal. II. Effects of temperature and dopant concentration

The oxidation behavior of pure Cr and Cr implanted with Y was studied as a function of temperature (900 and 1025°C) and ion-implantation dose (1×10¹⁵ and 2×10¹⁶ Y ions/cm²). The microstructures of the Cr₂O₃ scales were affected by both of the variables studied. Yttrium ions segregated at the grain boundaries in the Cr₂O₃ scales formed on the implanted alloys and the concentration of Y at the grain boundaries decreased with a decrease in the dose of implanted Y. The mechanism of growth of the Cr₂O₃ scales was altered by the presence of the Y ions at the Cr₂O₃ grain boundaries only when a critical concentration of Y at the grain boundaries was exceeded.     

Decomposition of martensite by discontinuous-like precipitation reaction in an Fe–17Cr–0.5C alloy

The mechanism of martensite decomposition and the kinetics of carbide precipitation have been studied in an Fe–17 wt% Cr–0.55 wt% C alloy. The morphology of carbide precipitates formed within the decomposed regions and the crystallography of their formation were examined by means of transmission electron microscopy after tempering at 735°C for various times. The martensite decomposition starts within less than 10 s, but it is not completed even after 10 min. The reaction initiates with the nucleation of fine cementite particles preferentially at the prior austenite grain boundaries and occasionally at the martensite lath boundaries. Cementite particles are related to the ferritic matrix with the Bagaryatsky orientation relationship. The decomposition of martensite proceeds heterogeneously by the migration of a reaction front. Various carbide morphologies were observed in the region close to the reaction front: rod-like, spherical or lamellae. The kinetics of martensite decomposition changes from carbon diffusion controlled to chromium diffusion controlled. After long time tempering, the alloy carbides, M₂₃C₆ and M₇C₃, precipitate at the reaction front. The M₂₃C₆ carbides are related with respect to the ferrite by the Kurdjumov–Sachs orientation relationship. Two specific orientation relationships were found between the M₇C₃ carbide and the ferrite, which are related to each other by a rotation of 30° about their common axis of [0001]_h//[110]_α. One of them has previously been reported. The specific features of discontinuous-like precipitation in martensite are discussed and are attributed to the presence of carbon and chromium atoms, which have different mobilities. The driving forces for diffusion of carbon and chromium were qualitatively determined with the software and database ThermoCalc by assuming local equilibrium at the moving interfaces.