于兴福
, 周金华
YU Xingfu, ZHOU Jinhua
中图分类号: TG146.1
文章编号: 1005-3093(2016)06-0465-08
通讯作者:
收稿日期: 2016-02-25
网络出版日期: 2016-06-25
版权声明: 2016 《材料研究学报》编辑部 《材料研究学报》编辑部
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摘要
采用定向凝固炉制备出含有不同V含量的柱状晶合金, 进行了高温蠕变性能测试和微观组织形貌观察, 研究合金化元素V对镍基柱状晶合金微观组织及高温蠕变性能的影响。结果表明: 低V含量的合金中碳化物主要以棒状形式存在, 而高V合金中碳化物主要以块状形式存在, 棒状碳化物中的V含量低于块状碳化物。随V含量的增加, 合金中碳化物由棒状向块状转变, 当V含量达到1.04%(质量分数)时, 碳化物的形貌已经完全转变成块状。铸态合金经固溶处理后, 合金中的碳化物发生溶解, 低V合金中的棒状碳化物溶解较多, 而高V合金中碳化物溶解较少。在980℃/216 MPa和760℃/725 MPa条件下低V柱状晶合金持久寿命较短, 高V合金持久寿命较长, 提高合金中的V含量有利于提高合金的蠕变寿命。高V合金与低V合金相比, 时效和蠕变期间在晶界析出M23C6型碳化物数量少。蠕变期间合金中的裂纹主要在晶内MC型碳化物及晶界析出的M23C6型碳化物处萌生和扩展。
关键词：
Abstract
The effect of alloying element V on the microstructure and high temperature creep properties of a diretional solidified nickel-base superalloy was researched by varying the amount of V addition. The alloy composition was set in a controlled fixedness except for element V, and then the effect of V can be assessed. The alloys were directionally solidified by utilizing Bridgeman casting process and followed by appropriate post heat treatment. The microstructure and high temperature creep properties of the prepared alloys were then characterized. The results shown that, with lower amount of V addition, the alloying V existed mainly in acicular type carbides rather than in blocky type carbides; while for the higher addition of V, the shape of vanadium carbides in the alloys transition from acicular type to blocky type. When the V content reaches 1.04 % in the alloy, the shape of carbides has been transformed into blocky type completely. After solid solution treatment, the carbides in the alloy were dissolved partly, and the acicular carbides in the alloy with lower V content dissolved much more than the blocky carbides in high V content alloy. Under the condition of 980℃/216 MPa and 760℃/725 MPa, the stress rupture life is shorter for the alloy with lower V content, but loger for that with higher V content. The amount of M23C6 type carbide precipitated at grain boundaries in the alloy with higher V content is less than that with lower V content during creep test and aging heat treatment. During creep test, cracks initiate and grow mainly at the MC type carbide within grains or the M23C6 type carbides precipitated at the grain boundaries. Therefore, the increase of V content is of benefit for the improvement of creep life of the alloy.
Keywords：
钒(V)是Ni基高温合金中加入的一种非常特殊的强化元素, 与广泛应用于高温合金中的元素Cr、Co、Ti、Al、Mo、W等相比, 尽管加入V的合金数量较少, 但是V在等轴晶合金、柱状晶合金和单晶合金中均有应用[1-3]。已经在航空发动机上得到应用的知名高温合金中如RR2000、René125、René150等均含有V元素。在含合金元素V的高温合金中, 美国合金中V的加入量在0.6%~5.5%(质量分数)之间, 而俄罗斯合金中V的加入量在0.3%~0.8%之间。虽然美、俄、法的合金中都有V元素加入, 但其在合金中的主要强化作用方式及机理的研究, 公开报道的甚少。MC型碳化物是Ni基高温合金凝固中最常出现的碳化物类型, 其形状主要为不规则的块状及发达程度不等的汉字体状[4-13], 其形貌主要取决于合金成分及冷凝条件。V形成的碳化物与镍基高温合金中加入的Ti、Ta、Nb、Zr等碳化物相比, 按照TaC>NbC>ZrC>TiC>VC的顺序稳定性依次降低。镍基合金中碳化物主要存在形式有MC、M6C、M23C6和M7C3等几种方式, V在碳化物中含量的变化是否会影响碳化物的存在方式, 以及是否会影响热处理及服役期间MC型碳化物向M23C6碳化物转变仍需探讨。本文以镍基柱状晶合金为基础, 研究了V对合金微观组织及高温蠕变性能的影响。本研究结果可为镍基高温合金的设计提供参考, 设计出的合金可应用于航空发动机及燃气轮机制造行业[14, 15]。
实验用镍基柱状晶高温合金熔炼及制备均在沈阳中航动力精密铸造科技有限公司进行, 合金熔炼采用VIM-0.2型工业生产用真空感应炉熔炼, 漏气率≤0.4 Pa/min, 极限真空度为0.06 Pa; 当真空度≤0.67 Pa时开始熔化合金, 合金精炼真空度≤1.3 Pa。熔炼用坩埚采用MA-150型镁铝质坩埚, 并经高温烧结而成。熔炼后合金经陶瓷过滤网过滤, 过滤网材质为Al2O3, 孔隙大小为8~12PPI。熔炼后的合金浇铸成ϕ80 mm铸锭。定向凝固试棒制备采用25 kg定向凝固真空炉, 浇注温度及铸型预热温度为1520℃, 拉晶速率为6 mm/min。为了研究V对合金组织及性能的影响, 定向凝固制备柱状晶期间调整合金中V元素含量。对制备出的定向试棒进行化学成分分析, 其化学成分如表1所示。其中V元素含量分别为0.62%、0.96%和1.04%。
表1 定向凝固合金棒的主要化学成分
Table 1 Nominal composition of bars (%, mass fraction)
| C | Cr | Al | Mo | Ti | Co | V | B | Ni | |
|---|---|---|---|---|---|---|---|---|---|
| Alloy 1 | 0.147 | 9.03 | 5.22 | 3.05 | 4.67 | 10.23 | 0.62 | 0.015 | Bal. |
| Alloy 2 | 0.145 | 9.01 | 5.26 | 3.04 | 4.67 | 10.24 | 0.96 | 0.016 | Bal. |
| Alloy 3 | 0.146 | 9.00 | 5.28 | 3.06 | 4.65 | 10.23 | 1.04 | 0.015 | Bal. |
制备出柱状晶合金试棒后, 采用的热处理制度为: 1220℃×4 h; 空冷+980℃×16 h, 空冷。将热处理后的试棒加工成图1所示的蠕变试样。将试样置于GTW1504型蠕变试验机中进行980℃/216 MPa和760℃/725 MPa下的蠕变性能测试, 绘制出合金的蠕变曲线。为了确定蠕变期间碳化物的演化特征, 在760℃/500 MPa条件下进行了长时间持久性能测试。对不同状态合金的微观组织采用扫描电镜进行观察, 确定元素V对合金微观组织的影响, 金相试样采用的化学腐蚀液为: 20 g CuSO4+5 ml H2SO4+100 ml HCl+80 ml H2O, 微观组织观察采用S-3400N型扫描电镜。为了确定合金中的碳化物相, 对Alloy1试样采用电解萃取法获得碳化物和γ'相的混合物, 采用XRD-7000S型衍射分析仪进行碳化物相组成分析。
采用扫描电镜观察不同V含量铸态合金中的碳化物形貌如图2所示。
图2 镍基柱状晶合金中碳化物形貌
Fig.2 Morphologies of carbides in nickel-base columnar crystal superalloy (a) Alloy1, (b) Alloy2, (c) Alloy3
图2a为V含量0.62%合金中碳化物的形貌, 合金中的碳化物主要以长条棒状形式存在。图2b为V含量0.96%合金中的碳化物形貌, 合金中的碳化物以长条棒状和块状方式存在, 部分块状碳化物呈现为黑色。图2c为V含量1.04%合金中的碳化物形貌, 合金中不存在长条状碳化物, 存在黑色块状碳化物, 在基体中同时也存在一些弥散分布的白色颗粒状碳化物。
V含量为1.04%的铸态和热处理态合金中的γ'相形貌如图3所示。图3a为铸态合金中的γ'相形貌, 合金中的γ'相呈立方体形, 单个γ'相尺寸为0.5 μm左右。图3b为热处理态合金中的γ'相形貌, 合金中γ'相大小不均匀, 部分区域γ'相与铸态相比略有长大, 部分区域γ'相发生了细化, 个别γ'相产生了粗化, 粗化后的γ'相尺寸为2 μm左右。
图3 合金中γ'相形貌
Fig.3 Morphologies of γ' phase in the nickel-base superalloy (a) as-cast, (b) after heat treatment
图4为固溶处理后合金中碳化物部分溶入基体后的组织形貌, 图4a为含0.62%V合金中碳化物的溶解情况, 可见合金中的棒状碳化物已经发生溶解并断开, 呈细小短棒状。图4b为含0.96%V合金中碳化物形貌, 合金中的棒状碳化物溶解断开, 但是黑色块状碳化物仍然存在, 图4c为含1.04%V合金中碳化物的组织形貌, 存在于铸态合金基体中的白色颗粒状碳化物已经完全溶入基体, 而黑色块状碳化物仍然存在, 且溶解较少。
图4 固溶处理后合金中碳化物形貌
Fig.4 Morphologies of carbides in nickel-base superalloy after solution (a) Alloy1, (b) Alloy2, (c) Alloy3
图5为经过完全热处理后合金碳化物在晶界析出的形貌。由图可见, 含有较低V的合金, 其晶界析出碳化物数量较多, 并且连接成一体, 而含V较高合金晶界析出碳化物数量少, 晶界碳化物以颗粒状分布于晶界。
图5 完全热处理后合金中晶界析出的碳化物形貌
Fig.5 Carbides precipitated from grain boundary after fully heat treatment (a) Alloy1, (b) Alloy3
采用EDAX能谱分析仪对V含量为0.62%合金中块状碳化物和棒状碳化物进行成分分析, 结果如图6所示。图6a和图6b为棒状碳化物形貌及能谱分析曲线, 由能谱分析曲线可以确定棒状碳化物中主要碳化物形成元素为Ti、Mo、V、Cr。由块状碳化物形貌及能谱分析曲线(图6c和图6d)知, 块状碳化物中的主要碳化物形成元素包括Ti、Mo、V, 也包括少量的Cr, 但是与棒状碳化物相比Cr含量有所降低。
图6 碳化物形貌及EDAX能谱分析曲线
Fig. 6 Morphologies of carbides and EDAX, (a) morphology of rod like carbides, (b) energy spectrum curve of rod like carbide, (c) morphology of bulk like carbides, (d) energy spectrum curve of bulk like carbide
采用电解萃取法对V含量为0.62%的铸态合金进行电解萃取, 测定出萃取物的XRD谱如图7所示。通过分析可以确定合金中的碳化物主要以MC型碳化物存在, 主要成分为(Mo0.54, Ti0.46)C, 同时也存在M23C6型碳化物, 主要成分为Cr23C6。结合能谱分析可以确定, 合金中的块状碳化物主要为MC型碳化物, 而棒状碳化物是MC型碳化物和M23C6型碳化物的组合体。
不同V含量柱状晶合金在980℃/216 MPa实验条件下的蠕变曲线如图8所示。在980℃/216 MPa条件下, 含0.62%V合金的蠕变断裂寿命为42 h, 断裂伸长率为25%; 含0.96%V合金的蠕变断裂寿命为57 h, 断裂伸长率为26%; 含1.04%V合金的蠕变断裂寿命为83 h, 断裂伸长率为25%。可见980℃/216 MPa下随着合金中元素V含量的提高, 合金的蠕变寿命增加, 合金的断裂伸长率变化不大。
图8 980℃/216 MPa条件下不同V含量合金的蠕变曲线
Fig.8 Creep curves of various V content alloys under the condition of 980℃/216 MPa
在760℃/725 MPa条件下测定出含V量0.62%和0.96%的两种合金的蠕变曲线如图9所示。该条件下, 蠕变初始阶段两种合金的初始应变量相当, 稳态蠕变阶段含0.96%V合金的应变速率为0.041%/h, 含0.62%V合金的应变速率较高为0.205%/h。含0.96%V合金的蠕变寿命达到245 h仍未断裂, 而含0.62%V合金在该条件下的蠕变寿命为93 h, V的添加可以提高合金在中温条件下的蠕变持久性能。
图9 760℃/725 MPa条件下不同V含量合金的蠕变曲线
Fig.9 Creep curves of different V content alloys under the condition of 760℃/725 MPa
V含量0.62%合金中碳化物主要以棒状为主(图2a), 对棒状、块状碳化物和基体采用EDAX进行能谱分析, 得到棒状、块状碳化物和基体成分如表2中的Alloy1所示, 合金中棒状碳化物中V含量为0.56%, 基体中V含量为0.36%, 而块状碳化物中V含量为4.6%。对V含量为1.04%合金中的块状碳化物(图2c)和基体采用EDAX进行能谱分析, 得到块状碳化物和基体成分如表2中Alloy3所示。合金中块状碳化物中含V量为6.96%, 而基体中含V量为0.79%。
表2 合金中各相的化学成分
Table 2 Chemical composition of phases in the alloy (%, mass fraction)
| Co | Cr | Ti | Al | Mo | V | Ni | ||
|---|---|---|---|---|---|---|---|---|
| Alloy1 | Rod like carbide | 10.68 | 9.97 | 6.94 | 4.45 | 4.48 | 0.56 | Bal. |
| Bulk like carbide | 0.09 | 2.24 | 57.1 | -- | 32.8 | 4.60 | Bal. | |
| Matrix | 11.03 | 9.26 | 4.66 | 4.36 | 2.16 | 0.36 | Bal. | |
| Alloy3 | Bulk like carbide | -- | 1.87 | 66.5 | -- | 21.8 | 6.96 | Bal. |
| Matrix | 10.60 | 8.92 | 4.95 | 4.76 | 2.83 | 0.79 | Bal. | |
通过分析认为, 随合金中V含量的增加, 碳化物和基体中的V含量也相应增加, 特别是基体和碳化物中的V存在一定的分配关系, 随基体中V含量的增加碳化物中的V含量也相应增加。
完全热处理后, 合金中部分碳化物溶入基体, 当合金中的V含量较低时, 形成的棒状碳化物更容易溶入基体内, 而V含量较高的块状碳化物, 制约了碳化物向基体内的溶入, 另外块状碳化物与基体接触面积小, 延长了碳化物的溶入时间。溶入基体中的碳元素在合金中为长程扩散元素, 可通过晶界扩散。热处理时效及高温蠕变期间, 溶入的碳元素将以M23C6型碳化物的形式在晶内和晶界析出[16-20]。
合金中大量存在的MC型碳化物及晶界析出M23C6型碳化物对合金起第二相强化作用, 但是在高温蠕变期间MC型碳化物具有向更稳定的M23C6型碳化物转变的趋势。合金蠕变期间MC型碳化物分解转变为M23C6型碳化物的形貌如图10所示。图10a为980℃/216 MPa蠕变57 h后, 合金中碳化物的转变特征, 图10b为760℃/500 MPa蠕变1100 h后合金中的碳化物转变特征。
图10 蠕变期间碳化物的分解
Fig.10 Decomposition of the carbide during creep (a) creep 57 h under the condition of 980℃/216 MPa, (b) creep 1100 h under the condition of 760℃/500 MPa
由于MC型碳化物与基体之间存在变形不协调性, M23C6型碳化物又是一种强度较低的碳化物, 因此经过一段时间的蠕变变形后, 碳化物又成为合金蠕变期间裂纹萌生的主要区域, 持久断裂后观察到合金中裂纹在MC型碳化物和M23C6碳化物处萌生及扩展的形貌如图11所示。图11中黑色箭头为晶粒内MC型碳化物处产生裂纹及裂纹扩展形貌, 白色箭头为M23C6碳化物处产生裂纹及扩展形貌。
1. 在含V镍基柱状晶合金中, 棒状碳化物中的V含量低于块状碳化物中的V含量, 随合金V含量的增加, 碳化物由棒状逐渐向块状转变, 当V含量达到1.04%时, 碳化物形貌已经完全转变成块状。
2. 固溶处理期间合金中碳化物会发生溶解, V含量低的合金中的棒状碳化物会溶解断开, 而V含量较高的合金中碳化物溶解较少。
3. V含量为0.62%合金在980℃/216 MPa下的持久寿命短, 而V含量为1.04%的合金持久寿命长。
4. 提高合金中的V含量, 有利于V进入基体相及γ'相, 提高固溶强化效果。同时也提高合金碳化物中的V含量, 使碳化物在热处理期间更稳定, 溶解进入基体中碳化物数量减少, 进而导致时效和蠕变期间晶界析出M23C6型碳化物数量减少, 提高合金的高温蠕变寿命。
The authors have declared that no competing interests exist.
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Some effects of carbon in the production of single crystal superalloy castings , |
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Advance in research of carbon effect on single crystal Ni-base Superalloy ,
碳在镍基单晶高温合金中作用研究的进展 ,URL 摘要
综述了碳对镍基单晶高温合金显 微组织、缺陷、合金元素偏析行为、相稳定性以及拉伸性能、蠕变性能、疲劳性能等力学行为的影响及机理。总结得出,随碳含量增加,合金初熔点逐渐降低,合金 中共晶数量和尺寸减少,一次碳化物逐渐增多。除W和Ta外,碳含量变化对其它合金元素的凝固偏析没有明显影响。随含碳量增大,合金蠕变寿命显著降低,最小 蠕变速率增大。微量碳的添加提高疲劳寿命,屈服应力随含碳量增加而降低。比较并分析了碳对合金组织结构和力学性能的各种影响机制,并指出今后的研究方向。
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Strengthening mechanisms in polycrystalline multimodal Nickel-base superalloys ,
Polycrystalline γ - γ ′ superalloys with varying grain sizes and unimodal, bimodal, or trimodal distributions of precipitates have been studied. To assess the contributions of specific features of the microstructure to the overall strength of the material, a model that considers solid-solution strengthening, Hall–Petch effects, precipitate shearing in the strong and weak pair-coupled modes, and dislocation bowing between precipitates has been developed and assessed. Cross-slip-induced hardening of the Ni 3 Al phase and precipitate size distributions in multimodal microstructures are also considered. New experimental observations on the contribution of precipitate shearing to the peak in flow stress at elevated temperatures are presented. Various alloys having comparable yield strengths were investigated and were found to derive their strength from different combinations of microconstituents (mechanisms). In all variants of the microstructure, there is a strong effect of antiphase boundary (APB) energy on strength. Materials subjected to heat treatments below the γ ′ solvus temperature benefit from a strong Hall–Petch contribution, while supersolvus heat-treated materials gain the majority of their strength from their resistance to precipitate shearing.
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Effect of carbon content on the as-cast microstructures in a nickel base single crystal superalloy ,碳对一种单晶镍基高温合金铸态组织的影响 ,
研究了不同碳含量对一种单晶镍基高温合金的铸态组织的影响.结果 表明:随着碳含量的增加,合金的初熔温度逐渐降低,合金中的共晶数量和尺寸也减少.随着碳含量的增加,合金中的碳化物数量逐渐增多,碳化物的形态从斑点状 变为斑点状和骨架状相结合的网状结构.随着碳含量的增加,一次枝晶间距变化较大,但是二次枝晶间距变化不大,W和Al元素的偏析降低,Ta和Mo元素的偏 析增大.
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The microstructures of base/modified RR2072 SX superalloys and their effects on creep properties at elevated temperatures ,
<h2 class="secHeading" id="section_abstract">Abstract</h2><p id="">Investigation into the addition of minor grain boundary strengthening elements (C, Hf and B) on the properties of an experimented single crystal nickel superalloy has been undertaken. The alloys were subjected to a variety of conditions including thermal exposure at 950°C and creep over 850–1050°C. Detailed microstructural analysis has revealed the presence of script and blocky MC carbides in the eutectic regions. On prolonged exposure σ precipitation occurred close to the MC carbides in the eutectic areas. A significant observation with regard to creep is that the principal crack initiation sites, namely casting pores, are greatly reduced owing to the precipitation of the MC carbides during solidification. This improves the intermediate temperature rupture life, although the presence of retained eutectic regions due to incomplete solidification degrades the high temperature performance of the modified alloys. The morphology of the carbides is a strong function of composition, with the presence of significant levels of Hf leading to the more blocky morphology, which is less prone to fracture during creep.</p>
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| [6] |
Microstructure and phase stability in INCONEL alloy 740 during creep ,
<h2 class="secHeading" id="section_abstract">Abstract</h2><p id="">Phase transformations in a creep-rupture tested specimen of nickel-base superalloy 740 were characterized by electron microscopy. During creep at 816 °C, specimens developed chromium carbides and a complex silicide, the fcc G-phase, along grain boundaries, and an hcp eta phase, having a Widmanstätten pattern of laths, across grains and grain boundaries.</p>
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| [7] |
Formation of MC-γ/γ′ eutectic fibers and their effect on stress rupture behavior in D/S Mar-M247LC superalloy , |
| [8] |
Investigation of the solidification behavior of a new Ru-containing cast Ni-base superalloy with high W content ,
<h2 class="secHeading" id="section_abstract">Abstract</h2><p id="">The solidification process of a new cast Ni-base superalloy containing 3 wt.% Ru and high W content was investigated and compared with a Ru-free superalloy. The results show that Ru is a microstructure stabilizer for high W superalloy due to its retarding effect on the formation of harmful large blocky M<sub>6</sub>C carbides. Another Ru effect is to change the phase precipitation temperatures between the solidus and liquidus.</p>
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| [9] |
Effect of alloying chemistry on MC carbide morphology in modified RR2072 and RR2086 SX superalloys ,
The effect of minor additions (C and Hf) on the morphology of MC carbides in SX superalloys RR2072 and RR2086 has been investigated. It was found that the blocky MC particles contain a higher Hf level and have a larger MC/matrix interfacial misfit than the needle MC carbides. A simple calculation of lattice parameters indicates that the higher Hf level in MC carbides makes them have a larger interfacial misfit, and that the large misfit energy makes them tend to be blocky.
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| [10] |
Research on MC type carbide in nickel-base superalloy powders during rapid solidification , |
| [11] |
Influence of withdrawal rate on microstructure of blade shaped directionally solidified DZ125 superalloy ,
As a precipitation hardened unidirectionally solidified Ni-based superalloy, DZ125 has been widely applied as structure materials in advanced aeroengine for gas turbine blades and vanes. In present, the paper on the influence of solidification parameters on microstructures have been largely published, but unfortunately, few of them have focused on giving a direct comparison between directional solidification characteristics in liquid metal cooling (LMC) and high rate solidification (HRS). In this paper, the influences of processing parameters on microstructures of blade shaped castings prepared both by LMC and HRS technique were studied. The results show that the dendrite structure and gamma' precipitate in castings prepared by the same method are refined with elevated withdrawal rate; in the same solidification conditions, the LMC castings have finer dendrite structure and gamma' precipitate than HRS, the larger the disparity between primary dendrite arm spacings in LMC and HRS castings, the thicker wall thickness is or the higher the withdrawal rate is. It is found that higher temperature gradient in front of solid/liquid interface can be obtained by LMC, and its variation with elevated withdrawal rate, however, is smaller than that by HRS. The gamma+gamma' eutectic fraction is lower for LMC castings than HRS castings except for withdrawal rate of 70 mu m/s, only at which more severe segregation than HRS's occurs. Chinese script type MC flakes between dendrites in HRS castings are larger at withdrawal rate of 110 mu m/s.
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| [12] |
Microstructural evolution of directionally solidified Ni-base superalloy DZ125 under planar growth ,
<p>By using liquid metal cooling method, the Ni-based superalloy DZ125 was directionally solidified under planar interface growth condition of drawing rate of 1.5 <em>μ</em>m/s. The microstructures at different solidified fractions were examined by OM and SEM. The results showed that the solid/liquid interface is planar and the microstructure evolution undergoes three stages: the <em>γ</em> phase was formed as solidified fraction (<em>f</em><sub>s</sub>) was not more than 0.26, and the fine HfC phase sparsely distributed in <em>γ</em> matrix; the coupled <em>γ</em>/<em>M</em>C growth appeared as <em>f</em><sub>s</sub> ranged from 0.26 to 0.86, and the morphology of <em>M</em>C was fibrous or plate-like shapes; the <em>γ</em>/<em>γ´</em> eutectic was obtained as <em>f</em><sub>s</sub> was more than and equaled 0.86 0.86, in which the octahedral <em>M</em>C was precipitated simultaneously. EPMA was used to determine the solute distribution along the longitudinal direction of the sample. The contents of Al, Ti, Ta and Mo increased with <em>f</em><sub>s</sub> increasing, while the contents of W, Cr and Co decreased. The analysis showed that the microstructure transformation is attributed to macro-scale non-uniform solute distribution which resulted from solute redistribution during directional solidification.</p>
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| [13] |
Morphological evolution of MC carbide in K465 superalloy , |
| [14] |
Microstructural characterization of the white etching layer in nickel-based superalloy ,
Microstructural characterization of the white etching layer (WEL) formed during milling in a fine-grained IN100 Ni-based superalloy was conducted. The microstructure of the layer depended on milling parameters, and under typical machining conditions, where moderate surface speed was used, the white layer exhibited nanostructure character. Fast surface speed produced partial amorphization of the outermost layer. Limited notched low cycle fatigue (LCF) testing was performed, and it was demonstrated that the fatigue properties deteriorated significantly in the specimens where WEL was present in the notch-root surface.
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| [15] |
Creep rupture behavior of a directionally solidified nickel base superalloy ,
The creep-rupture behavior of the directionally solidified (DS) nickel-base superalloy DZ17G has been investigated over a wide stress range of 60 to 950 MPa at high temperature (923 to 1323 K). In this article, the detailed creep deformation and fracture mechanisms at constant load have been studied. The results show that all creep curves exhibit a short primary and a dominant accelerated creep stage, which results in higher ductility of DS superalloy DZ17G compared to the conventionally cast alloy. From the creep parameters and transmission electron microscopy (TEM) observations, it is suggested that the dominant creep deformation mechanism has a change from gamma prime particles shearing by matrix dislocations in high stress region to dislocation climb process in low stress region. It is found that the fracture mode of DS superalloy DZ17G is transgranular, and it is controlled by the propagation rate of creep cracks initiated at both surface and inner microstructure discontinuities. The creep rupture data follows the Monkman-Grant relationship under all the explored test conditions.
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| [16] |
Decomposition of primary MC carbide and its effects on the fracture behaviors of a cast Ni-base superalloy ,
Primary MC degeneration in long-term thermally exposed cast Ni-base superalloy K452 was investigated in detail. It is shown that the primary MC degeneration is a diffusion-controlled process where various transformation products are sequentially present within the reaction region during exposure. The primary MC degeneration produces a DM or SM microstructure on the MC/γ interface at the initiation of exposure, which hinders the diffusion of alloying elements via its ordered γ′ layers and leads to the presence of the η and α-(W, Mo) phases, respectively during the intermediate and late exposure. In addition, the primary MC deterioration is examined to be detrimental to the long-term thermally exposed alloy due to its contribution to the cracking incidence especially when the MC is present at the grain boundaries.
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| [17] |
Morphological evolution of MC carbide in K465 superalloy ,
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| [18] |
Effect of long-term thermal exposures on microstructures and mechanical properties of directionally solidified CM247LC alloy ,
A directionally solidified CM247LC alloy was exposed at 871 °C and 982 °C for 1000 h, 5000 h, and 10000 h under free stress in order to study the effect of microstructural degradation on the creep properties. None of the specimens exposed at temperatures up to 10000 h produced any kind of topologically close-packed phases because of the excellent phase stability of CM247LC alloy. The plate-like M 6 C carbide was formed only at exposure of 982 °C for 10000 h through a decomposition reaction between γ and MC. Moreover, an M 23 C 6 carbide layer was observed between the M 6 C and the matrix. The exposure at 982°C for 5000 h and 10000 h had a spontaneous rafting of γ′ under free stress, while the exposure at 871 °C for 1000 h, 5000 h, and 10000 h had a non-rafted structure. The spontaneous rafted structure resulted in a drastic decrease in creep life. A 3-dimensional morphology of γ′ in the as-crept specimens, which were pre-exposed at 982 °C for 5000 h and 10000 h, had a non-rafted structure. This microstructural feature proves that the significant decrease in creep life of the specimen resulted from a loss of coherency between γ and γ′.
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| [19] |
Influence of service-induced microstructural changes on the aging kinetics of rejuvenated Ni-based superalloy gas turbine blades ,
<a name="Abs1"></a>Rejuvenation of Ni-based superalloy gas turbine blades is widely and successfully employed in order to restore the material microstructure and properties after service at high temperature and stresses. Application of hot isostatic pressing (HIP) and re-heat treatment can restore even a severely overaged blade microstructure to practically “as-new” condition. However, certain service-induced microstructural changes might affect an alloy’s behavior after the rejuvenated blades are returned to service. It was found that advanced service-induced decomposition of primary MC carbides, and the consequent changes of the <i>γ</i>-matrix chemical composition during the rejuvenation, can cause a considerable acceleration of the aging process in the next service cycle. The paper will discuss the influence of the previous microstructural deterioration on the aging kinetics of rejuvenated gas turbine blades made from IN-738 and conventionally cast GTD-111 alloys.
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| [20] |
Mechanism of primary MC carbide decomposition in Ni-base superalloys ,
Three industrial gas turbine blades made of conventionally cast (CC) IN-738 and GTD-111 and directionally solidified GTD-111 Ni-base superalloys were examined after long-term exposures in service environments. All three blades exhibit similar, service-induced microstructural changes (MCs) including ′ coarsening and coalescence, excessive secondary MCprecipitation, and primary MC degeneration, regardless of the chemical composition and the grain size. Special attention was paid to the primary MC decomposition. It is shown that the primary MC decomposition occurs by carbon diffusion out of the carbide into the + ′ matrix, resulting in the formation of Cr-rich MCcarbides near the initial carbide/matrix interface. A transition zone is shown to develop between the original MC core and its perimeter, demonstrating the gradual outward diffusion of carbon and a slight inward increase in nickel concentration. The hexagonal Ni(TiTa) -phase was also found in the MC transition zone and on the MC-′ interface. The primary MC decomposition can be expressed by the reaction MC + ′→ MC+ . Finally, it is shown that the grain-boundary (GB) MC decomposes more rapidly than that in the grain interiors. This is consistent with the more rapid GB diffusion that leads to the acceleration of the MC diffusional decomposition processes.
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