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

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Effect of Heat Treatment on High Temperature Stress Rupture Strength of Brazing Seam for Nickel-base Superalloy

In order to enhance the high-temperature stress rupture strength of brazing seam by heat treatment, it was diffusion treated, then solution heat treated, and finally aging treated. The microstructure of brazing seam especially morphology of gamma ' phase and boride was observed and the strength of brazing seam was measured in this process. The results show that heat treatment can enhance high-temperature stress rupture strength by improving the microstructure of brazing seam. The strength of brazing seam after solution heat treatment decreases in comparison with that only after diffusion treatment while aging treatment after solution heat treatment increases the strength of brazing seam.

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.

Effect of grain size on high-temperature oxidation behavior of Cu-80Ni alloy

The thermogravimetric analysis of binary Cu 80Ni alloys prepared respectively by conventional casting(CA) and mechanical alloying(MA) techniques and presenting widely different grain sizes was performed at 800 ℃ in air in order to study the effect of grain size change on the oxidation behavior of a solid solution alloy. The results show that the kinetic curves for the oxidation of the two alloys are complex and deviate from the parabolic rate law and usually are not composed of a single stage. Mixed scales were produced on the CACu 80Ni alloy surface, which consists of a mixture of copper and nickel oxides. However, oxide scale for MACu 80Ni alloy is mainly composed of a thick compact and continuous inner layer of nickel oxide. The reduction in the alloy grain size speeds up the diffusion of the more reactive component nickel from the alloy to alloy/oxide scale interface and completes the transition from a mixed scale to continuous scale of nickel oxide.

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.

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。

CMQL和UEV下Incone1690镍基合金切削力和温度分析

低温微量润滑(CMQL)可以使切削阶段达到更优的切削摩擦性能,提升刀具耐磨性能并获得更优减磨效果;超声椭圆振动(UEV)能够提高难加工合金的切削性能。在超声椭圆振动基础上设置低温微量润滑条件,并通过实验分析其切削效果。研究结果表明:采用UEV和CCMQL模式时,切削力与进给力降低比例依次为62.3%与75.0%,在低温微量润滑过程能够实现更低温度控制效果,有效防止工件的热软化问题。普通切削时温度都接近445℃。UEV可以获得更低温度,同时运用UEV和CMQL处理方式,温度下降了16.1%,采用UEV和CCMQL方式时,温度下降19.5%。通过降低喷嘴角度的方式,获得更低切削雾滴温度,刀尖应力出现降低实现刀具保护,促进刀具寿命延长。该研究有助于改善合金切削工艺,对提高切削质量提供了一个思路。

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。

Phenomenon and Mechanism of High Temperature Low Plasticity in High-Cr Nickel-based Superalloy

Cr is the most important element in nickel-based alloys to prevent high temperature oxidation and corrosion. However, high-Cr content will lead to a decline of hot workability which limits the addition of Cr for most nickel-based superalloys. In order to add more Cr into Ni-based alloy for improving high temperature oxidation and corrosion resistance, the poor hot workability of high-Cr alloy must be first solved. Deformation characteristic of a high-Cr nickel-based alloy （40 wt% Cr） under hot compression conditions at 800-1200 ℃ has been investigated by using a Gleeble 3500 machine, and the microstructural evolution during hot working process has been observed by optical microscopy and scanning electron microscopy. The results show that a high-temperature low-plasticity （HTLP） region exists in this high-Cr nickel-based alloy. This phenomenon can be attributed to its non-uniform interdendritic microstructure at high temperatures. These results can explain the poor hot workability of high-Cr nickel-based alloy.

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。

NbC纤维增强共晶镍基合金的超高梯度定向凝固组织研究

采用液态金属冷却定向凝固工艺制备NbC纤维增强高温共晶镍基合金。利用SEM、XRD、EDS和EPMA等测试方法,对NbC纤维增强高温共晶镍基合金不同区域的显微组织及形成机理进行分析,随后分别改变抽拉速率(2、7、12μm/s)和熔体温度(1 550、1 580、1 600℃),研究工艺参数对合金组织的影响。根据组织类型及形态不同,可将定向凝固高温共晶镍基合金分为启动区与复合区两个区域,启动区以大块状的富Nb和富W碳化物为主,而位于启动区之上的复合区主要分布定向排列的NbC纤维。定向凝固过程中NbC纤维的生长可划分为3个阶段,即启动阶段、多取向竞争阶段和稳定阶段。随抽拉速率增大,纤维体积分数增大,纤维中C、Nb和W质量分数降低,且Nb含量下降更显著。随熔体温度升高,纤维体积分数、横截面积以及间距皆增大,纤维中C、Nb和W质量分数升高,且W含量上升更显著。

高温氧化对粉末高温合金微观结构和疲劳性能影响研究

为了探究涡轮盘用粉末高温合金表面氧化对其低周疲劳性能的影响,分别针对第三代镍基粉末高温合金的粗晶(Coarse grain,CG)和细晶(Fine grain,FG)材料开展氧化时间对其疲劳性能影响机理的研究。通过在700℃空气环境下开展不同时长的高温预氧化实验和低周疲劳(Low-cycle fatigue,LCF)实验,使用SEM和EDS表征LCF断口、表面氧化层结构成分及其强化相形貌变化,揭示LCF裂纹萌生机理。实验结果表明,氧化层厚度随氧化时间而增加,氧元素以氧化侵入的形式进入基体;相同氧化时间下,CG抗氧化性能优于FG;疲劳裂纹萌生于氧化侵入和亚表面夹杂物等应力集中部位,在实验温度下LCF寿命受氧化作用和夹杂物共同影响;高温氧化作用下氧化层呈现分层结构,外层为NiO,中间层为含有Cr_(2)O_(3),TiO_(2)等复杂氧化物及尖晶石相(NiCr_(2)O_(4))的混合层,内层为Al_(2)O_(3);CG二次γ’相氧化后平均尺寸增加,FG二次γ’相平均尺寸没有明显的变化,但两种组织氧化后在晶界附近均观察到长条状的二次γ’相,表明高温氧化作用下晶界优先遭到破坏。