第四代粉末高温合金成分选取范围研究

在综合分析第一至三代粉末高温合金发展思路并借鉴叶片合金的基础上,从高强度、高损伤容限和高工作温度要求出发,围绕固溶强化、沉淀强化和晶界强化,研究并确定了第四代粉末高温合金的成分选取范围:①γ′相形成元素Al+Ti+Ta+Nb≈9%~15%;②固溶强化元素Cr+Co+Mo+W≈30%~40%;③TCP相形成元素Cr+Mo+W≤20%;④一定量晶界强化元素(C、B、Zr、Hf)。     

含Hf和Ta新型镍基高温合金粉末中碳化物相

对含Hf和Ta新型镍基高温合金FGH98Ⅰ等离子旋转电极(PREP)雾化原始和不同温度下预热处理粉末中的碳化物相进行了研究.结果表明:原始粉末中MC′型碳化物可分为两类,一类为富Ti、Ta和Nb,另一类为含Ta、Hf和Zr.两类碳化物均含有一定量非碳化物形成元素Co和Ni及中等强碳化物形成元素Cr和Mo,并以块状、粒状分布于枝晶或胞晶间;随着预热处理温度升高,粉末中富Ti、Ta和Nb的MC′型碳化物转变为MC型碳化物,且其所含Ti、Ta和Nb的总量增大;含Ta、Hf和Zr的MC′型碳化物发生分解和转变,析出稳定的M23C6、M6C和MC型碳化物,M23C6碳化物的析出和溶解温度为950℃和1150℃,M23C6和M6C碳化物共存温度为1000～1100℃.另外,粉末中微量元素Hf和Ta主要以碳化物和γ′相参与碳化物反应.     

消除粉末高温合金中原始颗粒边界机制北大核心

采用SEM、TEM研究了高温合金粉末颗粒内部与合金中碳化物的类型和组成,采用热力学软件计算了不同Hf含量FGH4096的碳化物组成,结果表明,粉末高温合金中原始颗粒边界上的析出物为以TiC为主的MC型碳化物,MC型碳化物(TiC、ZrC、HfC、NbC、TaC)在镍基高温合金中按溶解度从大到小为TiC>NbC>TaC>ZrC>HfC。TiC在FGH4096中的溶解度较HfC的大,雾化粉末快速凝固过程中,加入Hf的FGH4096合金碳化物析出量大于不加Hf的FGH4096合金碳化物析出量,HfC均匀形核于整个粉末颗粒中。在热等静压过程中,Hf与C的结合力强而形成HfC,最终在FGH4096合金中形成(Hf,Ti,Nb)C,致使C元素无法扩散至粉末颗粒表面形成TiC,避免了原始粉末颗粒边界的形成。加入Zr消除FGH4096中原始颗粒边界的机制与添加Hf的FGH4096相同。     

粉末高温合金FGH97疲劳裂纹扩展行为

测定不同晶粒尺寸、γ'相以及不同Hf含量的粉末高温合金FGH97在650℃高温条件下的疲劳裂纹扩展速率,并将其与FGH95和FGH96两代粉末合金的疲劳裂纹扩展速率进行对比.用定量分析的方法对FGH97合金在疲劳断裂各个阶段的行为特征进行分析.较大晶粒尺寸的FGH97合金具有较低的裂纹扩展速率,合理的二次和三次γ'相匹配析出,可以获得较高的疲劳寿命;Hf元素的添加使合金的整体疲劳寿命增大;FGH97合金与FGH95和FGH96相比,具有较高的疲劳裂纹萌生抗力,更低的高温疲劳裂纹扩展速率.      

不锈钢和高熔点金属的预合金粉

供等离子喷涂用的粉末是由不锈钢和从Ti、Zr、Hf、Nb、Ta、Mo或W中选择的一种高熔点金属(占30～85at％)组成的预合金混合物。这种粉末是在含氧气氛中用等离子喷涂并生成一种耐腐蚀的粘结合金涂层。该涂层呈非晶形或微晶形结构,最好由Fe或Ni35～75at％、Cr～20at％和高熔点金属5～55at％组成。本工艺适合于喷涂不锈钢、工具钢、铜及钢基体材料,最好用喷丸清理进行预处理,并用作为粘结层的镍铬铁合金任意涂敷。     

微量元素Hf消除FGH96合金中原始粉末颗粒边界组织机制的探讨

粉末冶金高温合金中原始粉末颗粒边界组织(PPBS)由碳化物和少量碳氧化物组成。采用SEM、TEM以及AES等研究了粉末颗粒内、表面上以及合金中PPB上碳化物的结构和组成,依据热力学和扩散理论分析了Hf消除原始粉末颗粒边界组织(PPBS)的微观机制。结果表明:快速凝固粉末颗粒表面形成含有Ti、Nb、Cr、Mo、W的MC型亚稳定碳化物MC′,在HIP过程中粉末颗粒表面上的MC′相转变成稳定的MC相,以及粉末颗粒内的Ti、C元素向烧结颈处扩散,HIP后在粉末颗粒边界上形成富Ti和Nb的MC型碳化物(Ti,Nb)C。加入微量Hf后,在粉末颗粒内形成了更多更稳定的含Hf的MC型碳化物(Ti,Nb,Hf)C,C、Ti被"绑定"在碳化物(Ti,Nb,Hf)C中,抑制了C、Ti向烧结颈处扩散,从而抑制了MC型碳化物在粉末颗粒边界上的析出。     

Ta 含量对 NiCoCrAlY 涂层性能的影响

采用真空雾化的方法制备了Ta含量为0、2%及5%的NiCoCrAlYTa合金粉末,利用超音速火焰喷涂制备了三种涂层,研究了Ta对合金粉末微观组织及物相的影响,绘制了1050℃条件下涂层的高温氧化动力学曲线,研究了500h氧化试验后涂层组织和β相的分布,初步探讨了Ta元素对改善涂层抗氧化性能的作用机理。研究结果表明:Ta通过提高MCrAlY体系抗氧化元素的溶质浓度,促进了氧化膜的形成,随着Ta含量的增加,涂层的内氧化程度降低,涂层抗氧化性能提高。但含Ta涂层在长时间氧化条件下会生成CrTaO_4、AlTaO_4等尖晶石类氧化物,且Ta的氧化物PBR值较大,对涂层的抗热震性能不利。     

三代粉末高温合金的特征及发展

粉末高温合金在高温下表现出一系列优点,成为高推重比航空发动机涡轮盘等关键部件的首选材料。概述了三代粉末高温合金的发展历程和性能特点;介绍了其生产工艺流程以及热处理过程中固溶温度、冷却速度、时效对组织的影响;从析出相的析出规律(主要讨论了γ'相、碳化物、TCP相)和γ/γ'错配度方面分析了合金成分改变带来的影响,提出成分优化设计的趋势;探讨了粉末高温合金今后的发展方向,为进一步开展粉末高温合金的研究提供一定的理论依据。     

俄罗斯粉末冶金高温合金研制新进展

介绍了俄罗斯近几年用于ПС90А2、АИ222和Д27航空发动机的ЭП741НП粉末高温合金盘件的性能及使用情况,以及ЭП962НП和为第5代以及第5代半(俄罗斯划分)航空发动机设计的ВВП系列(ВВ750П、ВВ751П、ВВ752П、ВВ753П)粉末高温合金的研制最新进展。俄罗斯采用等离子旋转电极工艺(PREP)制粉+直接热等静压(As-HIP)成形制造粉末高温合金盘件。使用粒度为<100μm的粉末,为ПС90А2发动机研制的4个ЭП741НП合金盘件,其性能优于АЛ-31Ф发动机上使用的ЭП741НП合金盘件性能。为АИ222航空发动机研制的3个ЭП741НП合金盘件以及为Д27航空发动机研制的4个ЭП741НП合金盘件,其性能达到了技术文件的要求。俄罗斯装配АИ222-15С发动机的Як-130教练机通过了飞行试验,并已交付使用。ВВП系列粉末高温合金的抗拉强度、650℃下的持久强度以及低周疲劳性能均优于ЭП741НП合金,但韧性比ЭП741НП合金低。ВВ752П和ВВ753П合金盘件目前处于试制阶段,用于航空发动机涡轮和压气机的ВВ751П、ВВ750П合金盘件已开始工业试验批生产。     

消除粉末高温合金中原始颗粒边界机制

采用SEM、TEM研究了高温合金粉末颗粒内部与合金中碳化物的类型和组成,采用热力学软件计算了不同Hf含量FGH4096的碳化物组成,结果表明,粉末高温合金中原始颗粒边界上的析出物为以TiC为主的MC型碳化物,MC型碳化物(TiC、ZrC、HfC、NbC、TaC)在镍基高温合金中按溶解度从大到小为TiC＞NbC＞TaC＞...     

Microstructural study: an aid to determination of failure mechanism in nickel base superalloy blades

Cast nickel base superalloys are extensively used for high temperature gas turbine blade applications. The elevated temperature properties in these alloys are optimized through engineered microstructure, which is a combination of (a) γ-solid solution of Ni with elements such as W, Mo, Cr, Ta, Re etc, (b) γ′ precipitates, and (c) dispersed carbides. Over the years, the demand for high engine efficiency has resulted in the development of new generation superalloys with improved elevated temperature properties, especially creep resistance. These superalloys are highly alloyed with solid-solution strengthening elements and hence, impose greater technological challenges in fabrication. Due to high alloy contents, these alloys are prone to formation of detrimental phases such as the topologically close packed (TCP) phases. These phases may appear in the microstructure during the blade fabrication stages or precipitate out during exposure to high temperature and stress. In addition, the γ′ precipitates can change in morphology, shape and size during applications resulting in deterioration in high temperature mechanical properties, in general. These unfavorable microstructural changes often lead to premature failure in gas turbine engines. While analyzing these failures, the microstructural study provides important information in identifying whether the blades had faulty microstructure to start with or the abnormalities observed have resulted during exposure to service conditions. This in turn can be related to engine operating conditions. In the present paper, this has been demonstrated through analysis of two service failure cases wherein the high pressure turbine blades had failed in flight leading to aircraft accidents.     

Design of Novel Precipitate-Strengthened Al-Co-Cr-Fe-Nb-Ni High-Entropy Superalloys

A series of non-equiatomic Al-Co-Cr-Fe-Nb-Ni high-entropy alloys, with varying levels of Co, Nb and Fe, were investigated in an effort to obtain microstructures similar to conventional Ni-based superalloys. Elevated levels of Co were observed to significantly decrease the solvus temperature of the γ′ precipitates. Both Nb and Co in excessive concentrations promoted the formation of Laves and NiAl phases that formed either during solidification and remained undissolved during homogenization or upon high-temperature aging. Lowering the content of Nb, Co, or Fe prevented the formation of the eutectic type Laves. In addition, lowering the Co content resulted in a higher number density and volume fraction of the γ′ precipitates, while increasing the Fe content led to the destabilization of the γ′ precipitates. Various aging treatments were performed which led to different size distributions of the strengthening phase. Results from the microstructural characterization and hardness property assessments of these high-entropy alloys were compared to a commercial, high-strength Ni-based superalloy RR1000. Potentially, precipitation-strengthened high-entropy alloys could find applications replacing Ni-based superalloys as structural materials in power generation applications.

     

Determination of the Oxygen Content in the Complexly Alloyed Nickel Melts during Vacuum Induction Melting Using Oxygen Probes

A procedure providing an ultralow oxygen content during the melting of complexly alloyed nickel superalloys is developed. The procedure uses special oxygen probes and makes it possible to control the oxygen concentrateon in a melt rapidly. The procedure is approved on a VIM150 setup for the melting of commercial nickel superalloys. The procedure developed for the determination of the oxygen content on VIM12 and VIM150 setups can also be used for other alloys, including newly developed ones.     

The research and development of new type of ferro-base heat-resistant alloy

Nickel-base superalloys are high performance materials subject to severe operating conditions in the high temperature turbine section of gas turbine engines.Turbine blades in modern engines are fabricated from Ni-base alloy single crystals which are strengthened by ordered g’ precipitates.Turbine disks are made from polycrystal line Ni-base alloys because these components have higher strength requirements(due to higher stresses).By increasing the upper temperature limit for the next generation of disk materials,the aviation industry will see significant environmental as well as cost benefits. Researchers in the High Temperature Materials Center of the National Institute of Materials Science of Japan have recently completed their work on a new kind of disk alloys.The new disk alloys,a kind of nickel-coble-base superalloys processed by a normal cast and wrought(C & W) route,can withstand temperatures in excess of 725 degree centigrade,a 50-degree increase over C&W disks currently in operation. In this presentation,the author shows the design idea,workability and properties of these Ni-Co-base superalloys. Furthermore,the evaluation of the processing and microstructure on a full-scale processing of Ni-Co-base superalloy turbine disk are described,which demonstrated the advantages and possibility of the Ni-Co-base disc alloys at the component level.     

Nickel-base superalloys; physical metallurgy of recycling

Because of stringent performance specifications, alloys included in the category of nickel-base superalloys are some of the most metallurgically complex and expensive in the field of metallurgy. The fact that these complex alloys can be manufactured commercially to make reliable components reflects the skill which alloy producers apply to control composition to meet rigid specification of trace elements as well as alloy constituents. Nevertheless, a substantial amount of scrap is used in the manufacture of these alloys, and highly specialized operations have been developed to process this scrap. This paper provides an overview of the current use of scrap in the industry and procedures for handling, melting, and refining scrap. The effects of residual elements commonly encountered in superalloy scrap on structure and properties are reviewed. Finally, a brief assessment of trends in generation and use of scrap is presented.     

A New Oxide Morphology Map: Initial Oxidation Behavior of Ni-Base Single-Crystal Superalloys

Predictions for oxidation behavior of Ni-base superalloys become more difficult than before because of the complex alloy composition. In this study, we focus on the initial oxidation behavior of Ni-base superalloys, and we suggest a new diagram to predict the initial oxide morphology of Ni-base superalloys with 63 binary, ternary, and multicomponent Ni-base single-crystal superalloys at 1373 K (1100 °C). As a comparison of observed and calculated weight changes after one cycle at 1373 K (1100 °C) obtained by a regression analysis, 63 alloys demonstrated two distinct behaviors, which are divided heretofore into group A and group B. Microstructural observation revealed that an oxide layer in the group A alloys consists of Al₂O₃ and/or spinel or complex oxide, whereas an oxide layer in the group B alloys consists of a thick NiO layer with an Al₂O₃ internal subscale. Thermodynamic properties can reflect more effects of alloy elements in Ni-base superalloys, and Al and Cr activities, calculated by Thermo-Calc, were used as factors to predict initial oxidation morphology. Groups A and B alloys can clearly be divided according to Al and Cr activities. This was suggested as a new diagram to predict the initial oxide morphology of Ni-base superalloys, and possibly it can apply for any generation of Ni-base superalloys.     

Microstructural Stability and Hot Deformation of γ–γ′–δ Ni-Base Superalloys

Nickel-base superalloys exhibit excellent high-temperature mechanical and physical properties and remain the first choice for structural components in advanced gas turbine engines for the aerospace propulsion and power generation applications. In response to the increasing demand for more efficient solutions and tighter requirements linked to gas turbine technologies, the properties of nickel-base superalloys can be improved by modification of their thermo-mechanical and/or compositional attributes. Recent investigations have revealed the potential use of ternary eutectic γ–γ′–δ Ni-base superalloys in advanced gas turbines due to high temperature mechanical properties that are comparable to state-of-the-art polycrystalline Ni-base superalloys. With properties largely dependent on microstructural strengthening mechanisms, both the composition and thermo-mechanical processing parameters of this novel class of alloys need to be optimized concurrently. The hot deformation characteristics of four γ–γ′–δ Ni-base superalloys with varying levels of Nb were evaluated at temperatures and strain rates between 1353 K and 1433 K (1080 °C and 1160 °C) and 0.01 to 0.001/s, respectively. Evidence of dislocation-based plasticity was observed following deformation at low temperatures and high strain rates, while high temperatures and low strain rates promoted superplasticity in these alloys. The extent of the microstructural changes and the magnitude of the cavitation damage which occurred during deformation was found to vary as a function of the alloy composition.     

Metallurgy,Microstructure and Properties of a New Nitride Strengthened Nickel‐Chromium Superalloy

The nitrogen content of Ni‐base superalloys for high temperature service is generally kept below about 0.05 wt.‐% to avoid detrimental precipitation of nitrides. These nitrides are said to have a harmful influence on mechanical properties and workability of these alloys. However, some recent studies and research conducted with nitrogen strengthening of Ni‐Cr‐alloys have resulted in an alloy with excellent physical and mechanical properties. The applied PESR (Pressurized Electro‐Slag Remelting)‐Technology provided up to 1.0 wt‐% nitrogen in a NiCr7030‐alloy.The homogeneously distributed nitrides prevent the alloy from excessive grain growth thus providing stable mechanical properties, i.e. impact toughness even after long term exposure. The new alloy easily exceeds R_m= 850 MPa at room temperature and 600 MPa at 600 °C as relevant design values. This paper introduces this new alloy with its very special metallurgy, microstructure, and its physical and mechanical properties.     

Bifilm Defects in Ni-Based Alloy Castings

The Ni-base superalloys, which are normally melted and cast in a vacuum, entrain their surface-oxide film during turbulent pouring of the melt; unfortunately at this time, this process is universally practiced for investment castings of these materials. The entrained film becomes a bifilm crack automatically, so that cast alloys have a large population of cracks that controls their failure behavior. The problems of the growth of single crystals and the welding of polycrystalline alloys are reviewed to illustrate the central role of bifilms in the cracking of turbine blades, the heat-affected zones of welds, and the reliability of properties. It has been demonstrated that improved gravity pouring systems can reduce these problems significantly, but only countergravity filling of molds is expected to result in defect-free castings. Recent cases in which turbine blades failed in service are examined, and the central role of bifilm defects in these failures is discussed.     

Hydrogen permeation and diffusion in iron-base superalloys

In this work, the hydrogen permeation and diffusion in three iron-base superalloys, JBK-75, Incoloy903 and GH35A, were investigated. A gaseous permeation technique was employed to measure the hydrogen permeabilities and diffusivities of specimens with different, heat treatments for a given alloy over the temperature range of 210–430°C. The effects of strengthening phase γ′ on the hydrogen permeation and diffusion in the alloys were examined. The results showed that the dependences of the hydrogen permeabilities and diffusivities in various alloys on the temperature obeyed Arrhenius relationship over the test temperature range and that the hydrogen permeation behavior in these alloys was essentially independent of their heat-treated conditions, i.e. which was not significantly affected by γt́ precipitates in the alloys. The hydrogen permeation parameters of the iron-base superalloys were approximately consistent with those of the austenitic stainless steels. The hydrogen transport in these alloys with f.c.c. structure was generally controlled only by the lattice diffusion of hydrogen.