Structural mechanism of scale for 12Cr–W–Mo–Co steels with enhanced oxidation resistance

The phase constituent, morphologies, layer structures of the scale of 12Cr–W–Mo–Co heat resistant steel (HRS) formed in dry air and air with 10% vapour were systematically investigated. The interface between the scale and ferritic/martensitic matrix of this HRS was also studied. For the scale formed in air, single particle- and sheet-shaped oxide layer, which are composed of (Fe, Co, Cr)₂O₃, were formed. The scale combines with steel matrix via coherent or semi-coherent structure. For the scale formed in air with 10% vapour, the oxides take the shape of particulate. Layering phenomenon has been observed, i.e. the external layer is composed of (Fe, Co)₂O₃/(Fe, Co)₃O₄, the internal layer with spinel (Fe, Co, Cr)₃O₄ and the transition layer with Cr rich and Cr poor regions. The interface between the transition layer and the matrix is tight and steady, but the region linking the internal scale and transition layer is shaky and brittle. Both the oxidation processes in air and air with 10% vapour are considered to be controlled by diffusion mechanism.     

The Oxidation Behaviour of Alloys Based on the Ni–Co–Al–Ti–Cr System

The isothermal oxidation behaviour of a series of quinary Ni–Co–Al–Ti–Cr alloys were studied at 800 °C. Alloys with higher Cr concentrations exhibited lower mass gain after 100-h exposure, as did the alloys richest in Ni and Al for a given Cr concentration. Extensive internal oxidation and nitridation was also observed in all alloys, except those containing the highest concentrations of Ni and Al. All alloys studied generated continuous chromium oxide layers, beneath which alumina particles were observed. Compositional analysis of the subscales identified shallower Cr concentration gradients in alloys containing equiatomic levels of Ni and Co, suggesting increased availability of Cr in the alloy. Thermodynamic calculations confirmed that these alloys contained higher concentrations of Cr in their γ matrices as a result of a combination of both the elemental partitioning behaviour and the increased mole fraction of γ′ precipitates forming in the alloy.     

Solidification structures of Ti–Al–Cr alloys

Phase transformations in the ternary Ti–Al–Cr alloy system have been studied by combining preliminary phase equilibria calculations and microstructural studies of a number of model alloys. The results have contributed to a better understanding of phase equilibria in the Ti–Al–Cr alloy system above 1273 K. A liquid surface projection has been tentatively proposed. Micro-twins have been observed in the monolithic γ phase within a B2 matrix. This supports a previously proposed mechanism for the formation of such a structure in a B2 matrix. The results also suggest that there is no representative orientation relationship between γ and the Ti(Cr,Al)₂ Laves phase. The L1₂ τ phase can be in direct equilibrium with the liquid phase. The ω phase stability has also been studied. The stability of the ω phase is attributed to the electron density of the prior B2 phase. This leads to changes in the effective heat of formation of the ω structure, as concluded from total energy LMTO calculations.     

Interfacial reaction between Co–Cr–Mo alloy and liquid Al

The interfacial reaction between Co–Cr–Mo alloy and liquid Al was investigated using immersion tests. Microstructure characterization indicated that the Co–Cr–Mo alloy was corroded by liquid Al homogeneously, with the formation of a (Co,Cr,Mo)₂Al₉ layer close to alloy matrix and “(Cr,Mo)₇Al₄₅ + Al” layer close to Al. Kinetics analysis showed that the corrosion of the Co–Cr–Mo alloy followed a linear relationship with the immersion duration. Compared with pure Co–liquid Al reaction system, the alloying of Cr and Mo changed the solid–liquid interface structure, but the corrosion of the solid metal was still dominated by the dissolution of an intermetallic layer.     

Structural stability of the Laves phase Cr2Ta in a two-phase Cr–Cr2Ta alloy

Detailed microstructural analysis of a two-phase alloy of composition Cr–9.8 at.% Ta and lying in the Cr–Cr₂Ta region of the Cr–Ta binary system confirmed that the existing phase diagram is inaccurate; in the cast and annealed condition (1273 K/24 h), blocky primary Cr₂Ta precipitates were observed although the phase diagram indicates the eutectic composition to be ∼13 at.% Ta. The eutectic structure is composed of Cr solid solution and the Laves phase Cr₂Ta; the morphology is primarily lamellar although the rod morphology was occasionally observed. The Laves phase eutectic microconstituent exhibits the C14 (2H) hexagonal structure with a low stacking fault (basal faults) density and an average composition corresponding to 28.5 at.% Ta. After a prolonged high-temperature anneal (1573 K/168 h), the morphology breaks down to form discrete particles of Cr₂Ta; the C14, C36 and C15 structures were all recognized in this annealed condition, often more than one form being present in a single precipitate. The C15 structure was not twinned but contained some stacking faults on the {111} planes. Composition measurements confirmed that these structural transformations were accompanied by composition changes, the precipitates becoming more Ta-rich as they transitioned from the C14 via the C36 to the C15 phase. These observations are coupled with the results from earlier studies to present a discussion on factors that influence the stability and C14/C36/C15 transformation kinetics.     

Distribution of Elements Within Carbides of Multicomponent Alloys of the Ni – Cr – Co – Al – W – Re – Ta – Mo – Nb – C System

Metal Science and Heat Treatment - Specific features of alloying element distribution within carbides of multicomponent alloys with the basic composition Ni – 5% Cr – 9% Co – 6%...     

Corrosion Behavior of Ni–20Cr–18W–1Mo Superalloy in Supercritical Water

The corrosion behavior of Ni–20Cr–18 W–1Mo superalloy in supercritical water 500 °C/25 MPa for 200 h is investigated using gravimetry, SEM/EDS, XPS, and TEM. The oxide films show a layered structure with Ni rich in the outer layer, and Cr rich in the inner layer, consisting of an outer Ni(OH)2and NiO layer, including some Cr(OH)3, and an inner Cr2O3, Ni Cr2O4, and WO3 layer. Mo elements are not oxidized. The oxide films grow via a mixed mechanism,namely metal dissolution/oxide precipitation mechanism and solid-state growth mechanism. The effects of secondary and primary carbides on the weight-gain trend and oxide formation are discussed.     

单晶镍基高温合金超高温蠕变期间的变形机制

通过蠕变性能测试及组织形貌观察,研究了6％Re-5％Ru(质量分数)单晶镍基高温合金的超高温蠕变行为和变形机制.结果 表明,该合金在1160℃/120 MPa条件下的蠕变寿命为206 h.稳态蠕变期间,位错在基体中滑移和攀移越过筏状γ'相是合金的变形特征,基体中溶解的高浓度难熔元素可增加位错运动阻力.蠕变后期,切入筏状...     

Re及温度对单晶镍基合金晶格常数及错配度的影响

通过对不同状态及不同Re含量单晶镍基合金进行高、低温X射线衍射谱线测定及组织形貌观察,研究Re含量及温度对单晶镍基合金中γ、γ′两相晶格常数及错配度的影响。结果表明:铸态合金中γ、γ′两相有较大的晶格常数及错配度;经完全热处理后,立方γ′相以共格方式嵌镶在γ基体中,合金中两相的晶格常数及错配度略有减小;长期时效使γ′相粗化后,两相之间出现界面位错,使合金中两相的晶格常数及错配度绝对值增加。随Re含量增加,合金中γ、γ′两相在室温的晶格常数增大,错配度的绝对值减小。与γ′相相比,γ基体相有较大的膨胀系数,因此,随温度提高,合金中两相晶格错配度的绝对值增大。     

高温时效时间对定向凝固DZ951镍基高温合金组织和持久性能的影响

采用光学显微镜和扫描电子显微镜研究了高温时效时间对DZ951合金组织及其持久性能的影响。结果表明,合金在1130℃高温时效时,碳化物由铸态时的骨架状变成块状,呈不连续状分布在晶界。随着保温时间的延长,γ′立方度降低,尺寸增加。保温6h时,γ′粗化形筏。合金在1130℃高温时效时,由于γ′部分固溶,并在随后的冷却过程中析出细小球形γ′相,随着保温时间的增加,细小γ′相的体积分数增多。DZ951合金在1100℃,60MPa的持久寿命随保温时间增加而降低,延伸率增加。γ′粗化形筏对合金持久寿命是不利的。     

Quantifying the Influences of Carbides and Porosities on the Fatigue Crack Evolution of a Ni-Based Single-Crystal Superalloy using X-ray Tomography

The detrimental effects of carbides and porosity on the fatigue crack initiation and propagation of nickel-based single-crystal superalloys have been reported by many previous studies. However, few studies have quantitatively compared the fatigue damaging effects of carbides and pores on the fatigue crack evolution. In this study, a high-resolution X-ray computed tomography (XCT) characterization of a DD5 nickel-based single-crystal superalloy during fatigue test was performed. The evolution of carbides, pores and cracks at all stages was observed and tracked. In order to quantify the 3D microstructures, a new damage factor that correlates the morphology of fracture surface with crack evolution behaviors was proposed. It was found that porosity was more detrimental than carbides in crack initiation and propagation during fatigue tests. Furthermore, pore spacing has been found to be the most significant factor among all controlling pore characteristics in the crack initiation stage and sphericity is the most critical pore characteristic in the crack propagation stage. Therefore, by statistically analyzing the evolution of carbides and pores during fatigue tests in this study, the underlying fatigue cracking mechanism of nickel-based superalloys is revealed.     

层错能和温度对不同Co含量的镍基高温合金蠕变性能的影响

研究了2种不同Co含量的镍基高温合金分别在650 °C/630 MPa,725 °C/630 MPa和760 °C/630 MPa条件下的蠕变变形组织。通过透射电镜分析了温度和层错能对蠕变变形机制的影响。结果表明,对于所选取的高温合金来说,温度的提升可以有效促使蠕变变形机制由层错转变为孪晶。这表明孪晶的形成更大程度上取决于温度。此外,合金Co含量的提升以及层错能的下降都会使层错和孪晶延伸并穿过γ基体和γ′析出相,该方式提升了材料的蠕变抗力以及蠕变寿命。     

The cyclic-oxidation behavior of several nickel-base single-crystal superalloys without and with coatings

Several commerical single-crystal superalloys (CMSX-2, CMSX-3, CMSX-4, CMSX-6, SRR 99) and some laboratory versions of one of them (CMSX-4) with various Y additions were investigated concerning their cyclic-oxidation resistance in air at 1000 and 1150°C. The investigations also included two materials (CMSX-6, SRR 99) with an RT-22 coating. Weight changes and acoustic emission were recorded up to 1000 cycles and scales, coatings, and substrates were characterized by metallography, SEM, and microprobe in postexperimental investigations. The best cyclic-oxidation behavior and excellent resistance to spalling even at 1150°C were shown by a laboratory version of CMSX-4 containing between 10 and 60 ppm Y. While at 1000°C interdiffusion can be taken as tolerable for the coated alloys, there is rapid degradation of the coating by interdiffusion at 1150°C.     

W1813N无磁不锈钢表面激光熔覆Ni60与WC-12Co/Ni25涂层的组织结构和磨损行为

利用激光熔覆技术分别在W1813N无磁性不锈钢表面制备高硬度镍基自熔性合金Ni60（60HRC）涂层和低硬度Ni25基WC-12Co复合涂层。利用扫描电子显微镜（SEM）、能谱（EDS）、X射线衍射仪（XRD）和台阶仪,分析激光熔覆制备Ni60涂层和WC-12Co/Ni25复合涂层的显微组织、相组成和磨损行为。利用显微硬度、摩擦系数、磨痕轮廓对比两种涂层的耐磨性和磨损机制。结果表明,Ni60涂层显微组织主要为树枝晶和等轴晶,且Cr23C6, Cr2B等强化相弥散分布在?-Ni和FeNi固溶体晶界;而WC-12Co/Ni25复合涂层中WC-12Co颗粒弥散镶嵌于低硬度Ni25基质,复合涂层中WC-12Co颗粒体积比达到32.5%。复合涂层中最大和最小显微硬度差异达到648 HV。尽管两种涂层的摩擦系数相近,但复合涂层的磨损体积仅为Ni60涂层的10%,Ni60涂层表面的磨痕特征为犁沟状和塑性粘附,复合涂层磨痕表面为WC碎屑和塑性粘附,因此Ni60涂层的磨损失效机制为磨粒磨损和粘着磨损,而复合涂层磨损机制为粘着磨损,以上结果表明WC-12Co/Ni25复合涂层具有更好的耐磨性。     

MA956高温合金活动模板电解加工小孔技术研究

对采用活动模板电解加工技术在MA956铁基高温合金薄板上加工小孔的工艺方法进行了讨论。研究了电解液温度、加工电压、电源占空比和脉冲频率等参数对小孔加工结果的影响。研究表明:采用10%w.t.NaNO_3电解液,在电解液温度为30℃、加工电压为40 V、电源占空比为30%、电源脉冲频率为400 Hz时,能在MA956高温合金薄板上加工获得锥度小的小孔。     

Oxidation Behavior of Nickel-Base Single-Crystal Superalloy with Rhenium-Base Diffusion Barrier Coating System at 1,423 K in Air

The oxidation behavior of the nickel-base single-crystal superalloy TMS-82+ coated with a duplex Re(W)–Cr–Ni/Ni(Cr)–Al layer was investigated in air at 1,150 °C for up to 100 h. The coating layer was formed by electroplating Re(Ni) and Ni(W) films on the alloy, followed by Cr-pack cementation at 1,300 °C, and as a result, forming a continuous Re(W)–Cr–Ni diffusion-barrier layer. A Ni film containing fine Zr particles was then electroplated on the duplex layer, followed by Al pack cementation at 1,000 °C for 1 and 5 h to form an Al reservoir layer with a duplex Ni₂Al₃/γ-Ni layer, which changed quickly to γ-Ni phase containing (10∼13)at.% Al for the 1 h Al-pack coat and a mixture of γ′-Ni₃Al and β-NiAl phases for the 5 h Al-pack coat during high-temperature oxidation. A protective α-Al₂O₃ scale formed during oxidation at 1,150 °C in air, and parabolic rate constants of 7.4 × 10⁻¹¹ and 6.6 × 10⁻¹⁰ kg² m⁻⁴ s⁻¹ were obtained for the 1 h- and 5 h-Al pack-coatings, respectively. There was little change in the structures of the superalloy substrate after oxidation at 1,150 °C in air for up to 100 h. It was found that the Re(W)–Cr–Ni layer remained stable, acting as a diffusion barrier between the alloy substrate and Al reservoir layers.     

镍代钴WC－9（Ni－Co）硬质合金的研究

研究了配碳量与Ｎｉ／Ｃｏ比对ＷＣ－９（Ｎｉ－Ｃｏ）硬质合金抗弯强度的影响。实验结果表明：ＷＣ－３Ｎｉ－６Ｃｏ的配碳量是６．０４ｗｔ％时，其抗弯强度与ＷＣ－９Ｃｏ硬质合金相当；Ｎｉ的分布均匀性对抗弯强度的影响极其重要，粗粒Ｎｉ粉或（和）较多η相的存在都会使硬质合金形成脆性断裂，直接影响合金的抗弯强度。