DEVELOPMENT OF MoS_2-CONTAINING Ni-Cr BASED ALLOYS AND THEIR HIGH-TEMPERATURE TRIBOLOGICAL PROPERTIES

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Tribological property and high temperature oxidation behavior of Ni-based alloy

Ni-Cr based alloys were prepared by hot-pressing the mixture of strengthening phases Mo, Al, Ti, and lubricant phase MoS2. The hardness, tribological properties as well as the high temperature oxidation properties were evaluated, The results show that the strengthening phases can improve the mechanical properties of Ni-Cr based alloy obviously, and the wear and friction properties of Ni-based alloy with strengthening phase can be improved. Its friction coefficient and wear rate rubbed with Al2O3 ceramic disk are about 0.4 and 10 -14m3/（N·m）, respectively, and the oxidation process is mainly affected by Cr2O3.

Effect of high magnetic field on phase transformation temperature and microstructure of Fe-based Alloys

The effect of a high magnetic field up to 30T on phase transformation temperature and microstructure of Fe-based alloys has been reviewed. A high magnetic field accelerates ferrite transformation, changes the morphology of the transformed microstructures and increases the A3 and A1 temperature. In a magnetic field of 30T, the A1 temperature increases by about 37.1℃ for Fe-0.8C, the A3 temperature for pure Fe increases by about 33.1℃. The measured transformation temperature data are not consistent with calculation results using Weiss molecular field theory. Ferrite grains are elongated and aligned along the direction of magnetic field in Fe-0.4C and Fe-0.6C alloys by ferrite transformation, but elongated and aligned structure was not found in pure Fe, Fe-0.05C alloy and Fe-1.5Mn-0.11C-0.1V alloy.

DIRECTIONALLY SOLIDIFIED MICROSTRUCTURE OF AN ULTRA-HIGH TEMPERATURE Nb-Si-Ti-Hf-Cr-Al ALLOY

The directionally solidified samples of an ultra-high temperature Nb-Si-Ti-Hf-Cr-Al alloy have been prepared with the use of an electron beam floating zone melting (EBFZM) furnace, and their microstructural characteristics have been analyzed. All the primary dendrites of Nb solid solution (Nbss), eutectic colonies of Nba, plus (Nb, Ti)3 Si/(Nb, Ti)5 Si3 and chains of (Nb, Ti)3 Si/(Nb, Ti)5 Si3 plates align along the growth direction of the samples. With increasing of the withdrawing rate, the microstructure is refined, and the amounts of Nbss+ (Nb, Ti)3 Si/(Nb, Ti)5 Si3 eutectic colonies and (Nb, Ti)3 Si/(Nb, Ti)5 Si3 plates increase. There appear nodes in the (Nb, Ti)3 Si/(Nb, Ti)5 Si3 plates.

High-temperature oxidation behavior of Nb_(67-x)W_(15)Si_(18)Hf_x(x=0, 5 and 10) alloys

The oxidation behaviors of Nb67-xW15Si18Hfx (x=0, 5, 10) alloys were studied at 1 250℃in air. It is found that the Nb67W15Si18 alloy has the best oxidation resistance among the three alloys; and Hf addition is harmful to the oxidation resistance of the Nb67W15Si18 alloy. The oxides formed on the Nb67W15Si18 alloy are mainly Nb12WO33 and NbO2, and that on the Nb62W15Si18Hf5 and Nb57W15Si18Hf10 alloys is Nb2O5. Effect of Hf on the oxidation behavior of the Nb67-xW15Si18Hfx alloys has been discussed based on microstructures and kinetics of oxidation.

ADVANCED NICKEL-BASED AND NICKEL-IRON-BASED SUPERALLOYS FOR CIVIL ENGINEERING APPLICATIONS

The use of high-temperature materials is especially important in power station construction, heating systems engineering, furnace industry, chemical and petrochemical industry, waste incineration plants, coal gasification plants and for flying gas turbines in civil and military aircrafts and helicopters. Particularly in recent years, the development of new processes and the drive to improve the economics of existing processes have increased the requirements significantly so that it is necessary to change from well-proven materials to new alloys. Hitherto, heat resistant ferritic steels sufficed in conventional power station constructions for temperatures up to 550℃ newly developed ferritic/martensitic steels provide sufficient strength up to about 600 - 620℃. In new processes, e.g. fluidized-bed combustion of coal, process temperatures up to 900℃ occur. However, this is not the upper limit, since in combustion engines, e.g. gas turbines. Material temperatures up to 1100℃ are reached locally. Similar development trends can also be identified in the petrochemical industry and in the heat treatment and furnace engineering. The advance to ever higher material temperatures now not only has the consequence of having to use materials with enhanced high-strength properties, considerable attention now also has to be given to their chemical stability in corrosive media. Therefore not only examples of the use of high-temperature alloys for practical applications will be given but also be contributed to some general rules for material selection with regard to their high-temperature strength and corrosion resistance.

Tribological properties of nickel-graphite composite against different counterfaces at elevated temperatures

Nickel-based graphite-containing composites were prepared by powder metallurgy method. The effect of graphite addition on mechanical properties of nickel-based alloy was investigated and the tribological properties from room temperature to 600 ℃ were tested by a pin-on-disk tribometer with alumina,silicon nitride and nickel-based alloy as counterfaces. The microstructure was analyzed by X-ray diffraction(XRD) and scanning electron microscope(SEM) attached with energy dispersive spectroscopy(EDS). The worn surfaces at high temperature were observed by optical microscope and SEM. The results show that the tensile strength and hardness of composites decrease after adding graphite,while the friction and wear properties are all improved by adding 12%(mass fraction) graphite. Compared with the counterface of alumina and silicon nitride,the friction coefficients and wear rates are lower when the composite rubs against nickel-based alloy containing molybdenum disulfide.

Material Characterization of Ni Base Alloy for Very High Temperature Reactor

The generation of highly efficient electricity and the production of massive hydrogen are possible using a very high temperature reactor （VHTR） among generation IV nuclear power plants. The structural material for an intermediate heat exchanger （IHX） among numerous components should be endurable at high temperature of up to 950 °C during long-term operation. Impurities inevitably introduced in helium as a coolant facilitate the material degradation by corrosion at high temperature. In the present work, the surface reactions available under controlled impure helium at 950 °C were investigated based on the thermodynamics and the corrosion tests were performed in a temperature range of 850-950 °C during 10-250 h for commercial Alloy 617 as a candidate material for an IHX. Moreover, the mechanical property and microstructure for nickel-based alloys fabricated in laboratory were evaluated as a function of the processing parameters such as hot rolling and heat treatment conditions. From the reaction rate constant obtained from an impure helium control system for a material evaluation, it was predicted that the outer oxide layer thickness, internal oxide depth, and carbide- depleted zone depth reach about 116, 600 and 1000 μm, respectively when Alloy 617 is exposed to an impure helium environment at 950 ~C for 20 years. For Ni-Cr-Co-Mo alloy, subsequent annealing and a combination of cold working and subsequent annealing following solution annealing caused increases in the grain boundary carbide coverage and size. The angular distribution of the grain boundary as well as the carbide distribution was also changed leading to a consequent improvement of the mechanical property at 950 °C in air.

Room-temperature and high-temperature magnetic permeability of Co-doped nanocrystalline alloys

Influence of composition and annealing temperature on structure and magnetic properties of amorphous and nanocrystalline Fe78.4-xCoxSi9B9Nb2.6Cu1 （x=27.4, 40.0, 51.0, 78.4） alloys was investigated by X-ray diffraction （XRD） and the temperature dependence of permeability. According to the initial crystallization temperature （Tx1） from differential scanning calorimetry （DSC） curves of as-quenched amorphous alloys, 490-700 ℃ isothermal annealing was carded out to obtain the characteristic nanocrystalline structure. Furthermore, the soft magnetic properties were measured by temperature evolution of magnetic permeability to obtain the correlation between Co content, annealing temperature and magnetic permeability. The results show that, on the one hand, the annealing temperature exerts a significant effect on phase structure and initial permeability （μi）. The higher-temperature （from 550 to 610 ℃） annealed Co content nanocrystalline samples can remain higher μi at elevated temperature. On the other hand, partial substitution Fe by Co can improve the high-temperature magnetic stability;however, the room-temperature permeability of higher Co content alloys decreases obviously at the same time. This phenomenon was analyzed from the viewpoint of the saturation magnetic induction （Bs）, magnetic anisotropy （〈K〉） and magnetostriction （λs）.

Fabrication of nano-crystalline W-Ni-Fe pre-alloyed powders by mechanical alloying technique

Nano-crystalline pre-alloyed powders of W-Ni-Fe were fabricated by high energy ball milling mechanical alloying (MA) technique. The change of appearances and the crystallite sizes of powders before and after high energy ball milling were investigated by XRD, TOPAS P software, SEM and EDS. The results show that the nano-crystalline pre-alloyed powders can be fabricated by 5 h high energy ball milling. During the MA process, the diffusion of W, Ni and Fe happens in the process of repeated welding and fracturing. As a result, nano-crystalline supersaturated solid solutions are formed. The crystallite sizes won't be refined after 10 h ball milling. The crystallite sizes of different compositions are almost the same under the same MA condition. Due to the toughening mechanism of rare earth element, the powders of 90W-4Ni-2Fe-3.8Mo-0.2RE alloy are seriously agglomerated after ball milling compared with the other alloys. It can be concluded that the optimal sintering temperature of 90W-4Ni-2Fe-3.8Mo-0.2RE pre-alloyed powders after 15 h mechanical alloying is 1 300-1 350 ℃.

32Cr3Mo1V表面激光熔覆TiC/钴基高温合金组织和性能研究

针对铝热轧辊套表面因高温摩擦磨损失效的问题,采用同轴送粉式激光熔覆在32Cr3Mo1V表面制备不同质量分数(0%、5%、10%、15%)TiC/Co-08高温耐磨熔覆涂层。分析了该熔覆层的显微组织形貌、相组成及微区成分;测试了显微硬度、高温摩擦磨损性能。结果表明,Co-08涂层主要由γ-Co及晶间碳化物组成,其磨损形式为氧化磨损和磨粒磨损。TiC添加量为5%时,晶间组织碳化物含量减少,涂层中Fe元素含量升高,涂层显微硬度降低,摩擦系数有所升高,磨损形式为粘着磨损和磨粒磨损。TiC含量≥10%时,晶间碳化物含量升高,涂层中析出细小的TiC颗粒,有效提高了涂层的显微硬度,其摩擦系数也明显降低,磨损机制主要为氧化磨损及轻微的磨粒磨损。当TiC添加量为5%时,涂层显微组织变得粗大且显微硬度及高温耐磨性均降低。随着TiC含量的继续增加,涂层中重新析出小尺寸TiC增强体,且晶间碳化物含量也有所上升,其显微硬度及高温耐磨性也随TiC含量的增加而升高。在TiC含量≥10%时,TiC/Co-08复合涂层具有良好的高温耐磨性。

孔挤压强化对GH4169孔结构高温疲劳裂纹扩展行为影响研究

为探究孔挤压强化对镍基高温合金GH4169孔结构疲劳裂纹扩展行为的影响规律,针对孔挤压强化后螺栓孔特征模拟件开展残余应力松弛及疲劳裂纹扩展试验。结果表明,经热松弛稳定后,孔边依旧可以保持至少400 MPa的残余压应力水平,且孔壁表面残余应力松弛幅度高于内部;孔挤压强化后疲劳裂纹扩展速率降低一个数量级,随着相对挤压量增加,疲劳裂纹扩展速率降低幅度增大,疲劳裂纹扩展寿命增幅为4~22倍。结合有限元仿真及断口分析,孔挤压强化对疲劳裂纹扩展的影响机制可归结为松弛至稳定后的残余压应力抵消部分外载应力,导致有效应力强度因子范围减小,使得疲劳裂纹扩展速率降低。基于该机制,利用残余应力修正的Walker模型对疲劳裂纹扩展寿命进行预测,误差在1.8倍分散带以内。

微量元素Zr对高温合金铸件凝固缺陷及持久性能的影响

高温合金中微量元素含量虽然极低,但是其对合金的高温性能产生的重要作用不可忽略。以Zr为例,其存在对高温合金铸造特性和高温力学性能的影响仍需要进一步探索。研究了微量合金元素Zr对K417合金凝固过程中凝固缺陷的形成、组织演变以及持久性能的影响。研究结果表明,Zr的加入会影响合金铸态组织中共晶组织的含量,同时也影响凝固过程中的固液相线。随着Zr含量的增加,凝固温度区间先下降后增加;从而影响合金铸造流动性以及组织凝固缺陷的形成。900℃、314 MPa下的持久性能测试结果表明,随着Zr含量的增加,寿命先延长后降低,Zr含量为0.07%时达到最大值。为铸造高温合金母合金中微量元素含量的控制,提高铸件冶金质量提供实验支撑。

增材制造镍基高温合金成形过程数值模拟研究进展

增材制造技术为镍基高温合金复杂零部件的制造带来了前所未有的机遇,然而在实验研究和实际生产中仍然面临着较大的竞争压力,制约了增材制造镍基高温合金的快速发展。近年来,不同尺度的模拟方法逐步应用于指导镍基高温合金的增材制造和开发。宏观尺度模拟关注成形过程中的热历史、成形控制、残余应力分布和力学行为;介观尺度模拟主要用于解决成形过程中的激光吸收、熔池内熔体流动、熔化凝固、缺陷形成以及裂纹防治等问题;微观尺度模拟则聚焦于制造过程中构建材料的微观组织演化;而多尺度模拟通过耦合不同类型的模型,实现了材料成形过程中的跨尺度研究。本文通过综述宏观、介观和微观以及多尺度条件下镍基高温合金增材制造过程数值模拟研究进展,分析了不同模拟方法对于解决增材制造镍基高温合金成形和控性相关问题的策略和思路。最后,针对如何推动数值模拟在增材制造镍基高温合金开发中的应用进行了展望,并指出其发展方向。

航天用镍基高温合金粉末床熔融技术研究进展

采用激光粉末床熔融技术(LPBF)制造镍基高温合金具有较大的生产潜力,在航空航天领域有着广泛的应用,因此这一技术受到了高度关注。近年来,为了更好地了解LPBF-镍基高温合金的性能和使用极限,相关学者对其开展了大量的研究工作。本文基于近年来多项LPBF-镍基高温合金的研究,阐述了该领域的研究目标和重要发现,并列举了LPBF镍基高温合金在航天工业中的具体应用。总体分析了工艺参数和热处理对镍基高温合金及其复合材料增材构件组织及力学性能的影响,以及LPBF过程中常用的建模方法及研究进展。最后,对LPBF-镍基高温合金的未来发展方向提出了展望。

多元氯化物熔盐中表面改性Inconel 625的腐蚀行为

氯化物熔盐作为传热蓄热介质,对太阳能热发电储能系统中金属部件产生严重腐蚀,而针对镍基合金在氯化物熔盐中的高温腐蚀行为研究较少。以镍基合金Inconel 625和经过表面改性后的Inconel 625(ST In625)为研究对象,利用组织表征、微拉伸试验及高温熔盐浸泡试验,研究不同样品的力学性能变化以及在二元和三元氯化物熔盐中的腐蚀行为。结果表明:ST In625合金强度明显提升,拉伸塑性有所下降。腐蚀初期,两种样品在氯化物熔盐中形成的氧化膜以Cr_(2)O_(3)和NiCr_(2)O_(4)为主,ST In625表面产生的腐蚀坑较少;腐蚀后期,在两种样品表面均检测到NiO,二元氯化物熔盐中两种样品表面未生成明显氧化层,出现明显腐蚀坑,而三元氯化物熔盐表面会生成新的Cr_(2)O_(3)和NiCr_(2)O_(4)絮状氧化层,无腐蚀坑出现。因此,在氯化物熔盐中ST In625的腐蚀失重较小,耐蚀性明显提高,且两种样品在二元氯化物熔盐中的腐蚀速率较低、但存在明显晶间腐蚀;在三元氯化物熔盐中腐蚀速率更高,而晶间腐蚀较为轻微。通过表面改性提高了镍基合金在氯化物熔盐环境中的耐蚀性,可为太阳能热发电储热材料的金属部件选择提供全新的思路。

Ti-44Al-6Nb-2Fe合金低温超塑性及高温拉伸组织演化

为改善高Nb TiAl基合金的热变形能力,本研究利用Fe部分替代高Nb TiAl基合金中的Nb,熔炼制备新型高Nb含Fe的Ti-44Al-6Nb-2Fe合金。首先对合金铸锭进行包套锻造,再对锻造合金在800~1 000℃、1×10^(-4)s^(-1)初始应变速率下进行高温拉伸实验,对合金的高温拉伸变形行为和组织演化行为进行研究。结果表明,合金中含有较多B2相,占30%~40%,随高温拉伸温度升高,合金动态再结晶作用增强,位错减少,晶粒尺寸增大,晶粒长径比降低,这些组织演化特征与合金的应力-应变状态和热变形能力提升相对应。合金在800℃和850℃拉伸变形时,B2相为硬脆相,B2相及B2/γ相界面处的应力集中导致该区域形成大量裂纹及孔洞(800℃时孔洞百分比为14.5%),延伸率较低(800℃时延伸率为123.1%),甚至发生提早断裂,试样断裂处颈缩现象明显;合金在900℃拉伸变形时,应变速率敏感指数m=0.284,延伸率明显提高(900℃时延伸率为163.0%),试样断裂处仍有颈缩现象;合金在950℃和1 000℃拉伸变形时,B2相转化为“软”相,发挥了很好的应力-应变协调作用,B2相与γ相之间发生了广泛的晶界滑动,组织中几乎无裂纹孔洞出现,延伸率陡然提高(950℃时延伸率为307.9%),试样断裂处无颈缩现象,合金具备超塑性,但该合金在TiAl基合金中属于低温超塑性。较高的B2相含量及其所发挥的优异的应力-应变协调作用是该合金具备低温超塑性的重要原因和特色之处。

长时热暴露对镍铁基高温合金GH2070P组织和性能的影响

对650℃参数锅炉用关键候选材料GH2070P镍铁基高温合金管在675℃进行1000、3000 h和5000 h的高温热暴露试验,对合金热暴露后的力学性能和微观组织演变情况进行分析。分析发现,GH2070P合金管在675℃高温热暴露1000 h后,晶内析出γ′相析出物,晶界析出M23C6型碳化物,合金室温拉伸强度、屈服强度和硬度明显增高,冲击韧性下降;随着热暴露时间继续增加至5000 h,γ′相尺寸发生粗化,γ′相的粗化符合LSW粗化理论,M23C6型碳化物尺寸略有增加,但仍断续分布在晶界处,未形成网状结构,合金室温拉伸强度、屈服强度和硬度稳定保持在较高的水平,冲击韧性下降速度减缓。

AlCrCuFeNb_(x)NiTi高熵合金的摩擦磨损及高温氧化性能

采用真空电弧熔炼方法制备了AlCrCuFeNb_(x)NiTi高熵合金,并研究了铸态合金的组织、摩擦磨损性能和高温氧化性能。结果表明:Nb元素的添加促进了Laves相的形成,使AlCrCuFeNiTi合金的相结构由L2_(1)相、BCC相和FCC相转变为AlCrCuFeNb_(0.5)NiTi合金的L2_(1)相、Laves相和FCC相。Nb元素的添加会降低AlCrCuFeNiTi合金的耐磨性能,磨损速率从AlCrCuFeNiTi合金的1.572×10^(-5) mm^(3)/(N·m)提高到AlCrCuFeNb_(0.5)NiTi合金的4.483×10^(-5) mm^(3)/(N·m),合金的室温磨损机制均为粘着磨损。合金的氧化动力学遵循抛物线速率定律,Nb元素对AlCrCuFeNb_(x)NiTi合金高温氧化性能的影响随着温度的改变而改变,800和900℃时能提高其高温氧化性能,但1000℃时Laves相的内氧化现象严重导致其高温氧化性能恶化。

国产镍基合金C-HRA-1以及C-HRA-2在超临界水中的氧化性能研究

针对超(超)临界电站锅炉高温受热面材料蒸汽侧氧化问题,该文开展国产镍基合金C-HRA-1在620~650℃/25 MPa以及C-HRA-2在650℃/25 MPa超临界水环境的氧化实验研究。利用精密电子天平、扫描电子显微镜(scanning electron microscope,SEM)、X射线衍射(X-ray diffraction,XRD)、X射线光电子能谱(X-ray photoelectron spectroscopy,XPS)以及原子力显微镜(atomic force microscope,AFM)等对材料氧化动力学、氧化膜微观形貌以及氧化膜物相组成进行分析。同时,开展600℃/25 MPa超临界水环境氧同位素标记实验,探究镍基合金C-HRA-1在超临界水中的氧化机理。结果可知,C-HRA-1与C-HRA-2氧化后的主要物相为Cr_(2)O_(3)和NiCr_(2)O_(4),NiO、CoO、MoO_(3)以及NiFe_(2)O_(4)也被检测到;合金中阳离子向外扩散导致了氧化膜的生长。国产镍基合金C-HRA-1在超临界水中表现出较高的抗氧化性能。