金属学报, 2025, 61(12): 1829-1844 DOI: 10.11900/0412.1961.2024.00075

研究论文

选区激光熔化GH4169高温合金的微观组织和力学性能

孙勇飞1,2, 向超,2, 张涛2, 吴文伟1,2, 邹志航2, 刘金鹏2, 孙桂芳,2,3, 蒲吉斌4, 韩恩厚2,5

1 广州大学 物理与材料科学学院 广州 510006

2 广东腐蚀科学与技术创新研究院 广州 510530

3 东南大学 机械工程学院 南京 211189

4 中国科学院宁波材料技术与工程研究所 海洋关键材料全国重点实验室 宁波 315201

5 华南理工大学 材料科学与工程学院 广州 510641

Microstructures and Mechanical Properties of GH4169 Superalloy Manufactured by Selective Laser Melting

SUN Yongfei1,2, XIANG Chao,2, ZHANG Tao2, WU Wenwei1,2, ZOU Zhihang2, LIU Jinpeng2, SUN Guifang,2,3, PU Jibin4, HAN En-Hou2,5

1 School of Physics and Materials Science, Guangzhou University, Guangzhou 510006, China

2 Institute of Corrosion Science and Technology, Guangzhou 510530, China

3 School of Mechanical Engineering, Southeast University, Nanjing 211189, China

4 State Key Laboratory of Advanced Marine Materials, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China

5 School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, China

通讯作者: 向超,cxiang@icost.ac.cn,主要从事高性能金属材料增材制造技术研究; 孙桂芳,gfsun@seu.edu.cn,主要从事特殊环境氛围(水下)激光增材制造理论、技术研究

收稿日期: 2024-03-12   修回日期: 2024-05-11  

基金资助: 广东腐蚀科学与技术创新研究院青年创新基金项目(E1551601)
海洋关键材料全国重点实验室开放课题项目(2024Z02)

Corresponding authors: XIANG Chao, Tel:(020)22309456, E-mail:cxiang@icost.ac.cn; SUN Guifang, professor, Tel:(025)52090501, E-mail:gfsun@seu.edu.cn

Received: 2024-03-12   Revised: 2024-05-11  

Fund supported: Youth Innovation Fund of Institute of Corrosion Science and Technology(E1551601)
Opening Project of State Key Laboratory of Advanced Marine Materials(2024Z02)

作者简介 About authors

孙勇飞,男,1998年生,硕士生

摘要

GH4169材料广泛应用于航空航天、核电和石油化工等领域,传统加工方式难以满足复杂结构零件的高性能及快速制造需求,选区激光熔化(SLM)提供了一种新的制造方法。SLM成型过程中的高温度梯度和高冷却速率与传统制造方法显著不同,有必要对热处理后SLM成型GH4169高温合金的微观组织演变和力学性能进行研究。本工作采用SLM技术制备GH4169高温合金,分析讨论了打印态以及直接时效和固溶时效热处理后GH4169高温合金的微观组织和力学性能,通过不同热处理状态合金的相结构分析,建立显微组织与力学性能的构效关系。结果表明,打印态合金组织主要以γ基体和Laves相为主;直接时效后基体内部有γ′/γ″相析出;固溶时效后Laves相发生溶解,有大量尺寸小于1 μm的δ相析出,并均匀分布在晶内和晶界,基体内部γ′/γ″相分布更加均匀。SLM成型GH4169高温合金的显微硬度为311 HV,室温抗拉强度为961 MPa,屈服强度为649 MPa。热处理后,合金的硬度和强度均显著提升,其中固溶时效态合金的显微硬度为518 HV,室温抗拉强度为1393 MPa,屈服强度为1233 MPa,高于对应的锻件材料。550、650和750 ℃高温拉伸性能结果表明,热处理态样品在650 ℃的高温强度满足相应锻件标准。由于SLM成型过程中的温度梯度大、冷却速率高,制备的GH4169试样具有精细的胞状和柱状枝晶结构,晶粒尺寸小,位错密度高,热处理后有γ′/γ″强化相析出,由细晶强化、位错强化和沉淀强化作用机制共同提高了GH4169高温合金的力学性能。

关键词: 选区激光熔化; GH4169; 热处理; 微观组织; 力学性能

Abstract

GH4169 materials are widely used in aerospace, nuclear power, petrochemical, and other industries. However, conventional processing methods fail to meet the demands of high-performance and rapid manufacturing for complex structural parts. Therefore, selective laser melting (SLM) has been adopted as a new rapid manufacturing technology to address these demands. The high-temperature gradient and rapid cooling rate generated during SLM result in a considerably different microstructure in the GH4169 alloy compared with those produced via conventional melting and forging methods. Consequently, heat treatment is an essential post-processing step to enhance the precipitation strengthening of the GH4169 superalloy. Thus, it is critical to examine the microstructure and mechanical properties of the SLM-formed GH4169 alloy after heat treatment. This study focuses on fabricating the GH4169 alloy using SLM technology and investigates the microstructure and mechanical properties of the as-built, directly aged, and solution-aged GH4169 alloy specimens. Results reveal that the as-built structures primarily comprise a γ matrix and Laves phase. After direct aging, the γ′/γ″ phase precipitates within the matrix. After solution aging, the Laves phase completely dissolves. Moreover, the δ phase precipitates with a size of < 1 μm become abundant and uniformly distributed within grains and grain boundaries. Simultaneously, the γ′/γ″ phase precipitate within the matrix, resulting in a more homogeneous distribution. The microhardness, tensile strength, and yield strength at room temperature (25 oC) of the SLM GH4169 alloy are 311 HV, 961 MPa, and 649 MPa, respectively. Heat treatment substantially improves the hardness and strength of the material. In the solution-aged state, the microhardness reaches 518 HV, with a tensile strength of 1393 MPa and a yield strength of 1233 MPa at room temperature. Notably, these static mechanical properties surpass those of the corresponding forged materials. The tensile properties at 550, 650, and 750 oC indicate that the elevated strength of the heat-treated samples at 650 oC complies with relevant forging standards. Owing to the substantial temperature gradient and rapid cooling rate during the SLM forming process, the GH4169 sample exhibits a refined cellular and columnar dendritic structure, small grain size, and high dislocation density. Subsequent heat treatment induces γ′/γ″ phase precipitation, enhancing the mechanical properties of the GH4169 alloy through fine crystal strengthening, dislocation strengthening, and precipitation strengthening.

Keywords: selective laser melting; GH4169; heat treatment; microstructure; mechanical property

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孙勇飞, 向超, 张涛, 吴文伟, 邹志航, 刘金鹏, 孙桂芳, 蒲吉斌, 韩恩厚. 选区激光熔化GH4169高温合金的微观组织和力学性能[J]. 金属学报, 2025, 61(12): 1829-1844 DOI:10.11900/0412.1961.2024.00075

SUN Yongfei, XIANG Chao, ZHANG Tao, WU Wenwei, ZOU Zhihang, LIU Jinpeng, SUN Guifang, PU Jibin, HAN En-Hou. Microstructures and Mechanical Properties of GH4169 Superalloy Manufactured by Selective Laser Melting[J]. Acta Metallurgica Sinica, 2025, 61(12): 1829-1844 DOI:10.11900/0412.1961.2024.00075

GH4169高温合金能够在恶劣环境中承受高机械和热负荷[1],是涡轮叶片和燃气轮机叶片等高温部件中使用最广泛的镍基高温合金。然而,传统方法制备的GH4169合金存在切削加工困难、刀具磨损严重等问题,此外,具有复杂几何形状、精确尺寸公差和更高机械性能的GH4169零件的成型生产仍然是一个挑战[2, 3]。选区激光熔化(selective laser melting,SLM)技术作为近十年发展最快的金属高精度增材制造(additive manufacturing,AM)技术之一[4],为快速成型高精度GH4169零件提供了一个新的解决方案。SLM成型过程有着极高的冷却速率(105~108 K/s)[5],高于电子束选区熔化(electron beam selective melting,EBSM) (104~106 K/s)和定向能量沉积(directed energy deposition,DED) (103~104 K/s)[6]工艺,成型合金晶粒细小,可显著强化成型件的力学性能,所制备的零部件具有尺寸精度高、表面质量好、性能优异等特点[7],在航空航天[8]、船舶工业[9]及医疗器械[10]领域得到了一定的应用,同时在汽车工业、国防安全等国民经济和国家安全核心领域也具有重要的应用前景[6]

与传统工艺不同,SLM过程中熔池经历的超快加热和超快冷却过程导致基体内强化相来不及析出,这是打印态SLM-GH4169合金力学性能低于铸锻件GH4169合金的重要原因[11]。激光束与粉末、熔池与粉末床之间存在相互作用,具有复杂的非平衡物理冶金和热物理特性,导致打印过程中产生大量热应力[12],而通过优化工艺参数缓解SLM成型过程导致的残余应力较为困难,并且制造时间和成本增加[13]。此外,SLM成型方式为逐层加工,材料打印过程中受到不同的热循环,由于基板和粉末散热效率不一致,实际打印过程中同一层加工幅面也不一定相同,层间温度和冷却时间不同导致微观组织自下而上呈现异质结构[14]。SLM成型具有熔池熔道精细结构,与传统制造差异较大,因此需要采取合适的热处理调控微观结构和应力,进而优化合金的力学性能[15]

相对于其他镍基合金,GH4169高温合金SLM成型性较好,相对密度可达99.8%以上[16]。由于平行于构建方向(build direction,BD)具有最大冷却梯度,柱状晶粒倾向于沿着构建方向择优生长。一般将平行于构建方向定义为XOZ面,垂直于构建方向定义为XOY[17]。Ni等[18]发现,与XOY面试样相比,XOZ面试样具有较低的极限抗拉强度,但断后延伸率更高,热处理后各向异性性能差异减小。GH4169高温合金的热处理工艺主要包括热等静压[19]、均匀化、固溶以及时效处理,热处理温度、时间[20]、冷却方式以及不同组合工艺对微观组织有显著影响,从而影响性能。研究[21]发现,在1000~1240 ℃温度范围内,晶粒尺寸随着均匀化温度的升高而持续增加,晶粒形态由柱状向等轴状转变,SLM成型过程中的高密度位错缠结逐渐消失。在825~1000 ℃固溶温度范围内,Laves相初步溶解并伴随δ相的生成,适量的δ相会在晶界上起到钉扎作用,从而提高晶界强度,同时抑制晶粒长大,但过量δ相析出会诱导裂纹萌生和扩展,导致力学性能下降[3]。同时,Ni等[22]发现,时效处理促进了强化相的完全析出,直接时效后合金强度相较于打印态提高了24.8%。Huang等[23]根据扩散理论,得出固溶温度与时间的经验公式,并通过调整固溶处理的冷却速率和时效时间来控制强化相的数量,实现强度和延展性的匹配。Švec等[24]对常规轧制GH4169高温合金和SLM-GH4169高温合金进行相同热处理,发现热处理后SLM-GH4169高温合金的力学性能与轧制态相当。SLM-GH4169高温合金优异的力学性能与其精细的微观组织密切相关。文献[25~27]依据传统制造中常用的均匀化退火工艺消除偏析,采用较高温度固溶而忽略SLM成型过程中精细组织的优势,研究[28~30]发现,正是这种精细的组织结构,使其裂纹扩展、腐蚀、疲劳以及蠕变等性能较传统材料有显著优势。此外关于SLM-GH4169高温合金热处理工艺的研究大多根据形貌和成分来判断析出相,而从晶体结构及与基体的位相关系分析其析出规律的研究鲜见报道。了解晶体结构与位相关系有助于掌握析出相的析出规律,为工艺优化和热处理调控微观组织和力学性能提供理论依据。

本工作研究了SLM成型GH4169高温合金试样的微观组织和力学性能,选取了尽可能保留SLM成型态独特微观结构的热处理方案,对不同热处理状态的相结构进行表征,主要包括γ基体、γ′/γ″相、Laves相和δ相,分析SLM成型GH4169高温合金的相转变规律和强化机理,定性描述了析出相与力学性能的构效关系,为SLM成型GH4169高温合金的实际应用提供理论和数据支撑。

1 实验方法

实验用GH4169高温合金粉末的化学成分(质量分数,%)为:Ni 51.45,Cr 19.19,Nb 5.21,Mo 2.94,Ti 1.00,Al 0.60,O 0.018,N 0.007,Fe余量。粉末形貌和粒径分布如图1所示。可见,粉末球形度良好,有少量卫星粉,粒度分布呈单峰分布。粉末的D10为21.24 μm,D50为35.14 μm,D90为56.05 μm (其中,D10D50D90表示粉末累计粒度分布中10%、50%、90%所对应的粒径)。

图1

图1   GH4169高温合金粉末形貌及粒度分布

Fig.1   Morphology (a) and particle size distribution (b) of GH4169 superalloy powders (D10, D50, and D90 are the particle sizes of the powder at 10%, 50%, and 90% cumulative distribution, respectively)


采用FS273M型SLM设备制备实验试样。在高纯Ar气保护下进行打印,铺粉层厚40 μm,激光功率270 W,扫描速率800 mm/s,扫描间距0.12 mm,基板预热100 ℃,层间旋转角度67°,扫描策略为条带扫描,条带宽度10 mm。测试块尺寸为10 mm × 10 mm × 10 mm,拉伸棒成型尺寸为直径13 mm、长80 mm,摆放方向与构建方向平行。试棒加工尺寸如图2所示,依据GB/T 228.1—2021《金属材料 拉伸试验 第1部分:室温试验方法》进行拉伸实验。采用RGQ1400-50高温气氛马弗炉依据AMS5663标准进行热处理,设置直接时效(direct aging,DA)和固溶+时效(solution aging,SA) 2种热处理,并与打印态(as build,AB)进行对比,具体热处理工艺参数如表1所示。

图2

图2   拉伸试棒加工尺寸

Fig.2   Dimensions of tensile test specimens (unit: mm)


表1   热处理工艺参数

Table 1  Parameters of heat treatment process

StateSolutionAging
ABNoneNone
DANone720 oC for 8 h, furnace
cooling to 620 oC;
620 oC for 8 h, air cooling
SA980 oC for 1 h,720 oC for 8 h, furnace
air coolingcooling to 620 oC;
620 oC for 8 h, air cooling

Note: ABas build, DA—direct ageing, SA—solution ageing

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采用D8 ADVANCE型X射线衍射仪(XRD)及Jade 6.5软件进行物相分析,XRD测试条件为:CuKα,衍射波波长λ = 0.15418 nm,加速电压40 kV,加速电流40 mA,扫描范围2θ = 30°~100°,扫描速率4°/min。金相试样利用Kallings试剂(50 mL HCl + 50 mL H2O + 2.5 g CuCl2)进行腐蚀,采用Axio Imager. M2m光学显微镜(OM)进行金相组织观察。采用Apreo 2S场发射扫描电镜(SEM)观察试样的微观组织及断口形貌,加速电压为20 kV,加速电流1.6 nA,工作距离10 mm。使用JXA-iHP200F 电子探针显微分析仪(EPMA)进行微区成分分析,加速电压为20 kV,加速电流20 nA,测试步长0.0256 μm。使用配备BRUKER电子背散射衍射(EBSD)探头的GeminiSEM 460型SEM进行EBSD分析,加速电压为20 kV,加速电流0.2 nA,扫描步长1 μm,并使用OIM软件对晶粒尺寸进行分析。使用QATM Qness 60A+全自动Vickers硬度计进行硬度测试,加载载荷200 g,保压时间15 s,每个试样测5个点取平均值。使用Z250SH电子万能材料试验机进行室温拉伸实验,参照GB/T 228.1—2021《金属材料 拉伸试验 第1部分:室温试验方法》标准进行,试样标距25 mm,屈服前应变速率为0.00025 s-1,屈服后应变速率为0.0067 s-1直至拉断,每个试样进行至少3次重复拉伸测试,取3次结果平均值作为拉伸性能数据。使用Z100SH电子万能材料试验机进行高温拉伸实验,参照GB/T 228.2—2015《金属材料 拉伸试验 第2部分:高温试验方法》标准进行,屈服前应变速率为0.00007 s-1,屈服后应变速率为0.0014 s-1。采用Gatan 955.AH离子减薄仪制备透射电镜(TEM)样品,使用Thermo Fisher SCIENTIFIC Talos F200X TEM进行显微组织观察和能谱(EDS)分析,加速电压200 kV。

2 实验结果与讨论

2.1 物相分析

图3为GH4169高温合金AB、DA和SA试样的XRD谱。AB、DA和SA试样主要由奥氏体相组成,其中(111)晶面衍射峰强度最高。插图为2θ在46°~48°的局部放大图,其中SA试样有δ相的(211)晶面衍射峰,但衍射峰强度较弱,说明δ相的相对含量较少。根据图3计算γ相衍射峰的晶面间距,结果如表2所示。热处理后晶面间距减小,这是由于热处理后γ′/γ″相析出,γ′/γ″相与γ基体的衍射峰角度相近,但γ′/γ″相的晶胞参数小于γ基体[31],所以热处理后晶面间距减小。在相同外应力的作用下,晶面间距减小,位错运动更难进行[32]

图3

图3   GH4169高温合金打印态(AB)、直接时效(DA)和固溶+时效(SA)试样的XRD谱

Fig.3   XRD spectra of GH4169 superalloy specimens in AB, DA, and SA states (Inset is a partially magnified spectra)


表2   AB、DA和SA试样中奥氏体晶面间距 (nm)

Table 2  Intergranular spacings of austenite in AB, DA, and SA states

IntergranularABDASA
spacing
(111)0.207970.207880.20765
(200)0.180240.180020.17994
(220)0.127290.127280.12711
(311)0.108560.108540.10840
(222)0.103880.103690.10375

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2.2 微观组织分析

图4为GH4169高温合金AB、DA和SA试样显微组织的OM像。可以看出,SLM成型GH4169试样的组织致密,无裂纹,但有少量气孔等缺陷。AB试样的XOY面显示出明显的熔道结构(图4a),不同层熔道之间角度与设置的层间旋转角度67°一致,XOZ面显示出明显的熔池结构(图4d)。DA试样的熔道和熔池结构依然清晰可见,内部深色刻蚀组织增多。SA试样中的熔道、熔池结构基本消失,难以区分XOYXOZ面。

图4

图4   GH4169高温合金AB、DA和SA试样显微组织的OM像

Fig.4   OM images of GH4169 superalloy specimens in different states at the XOY surface (a-c) and XOZ surface (d-f)

(a, d) AB state (b, e) DA state (c, f) SA state


图5为GH4169高温合金AB、DA和SA试样显微组织的SEM像。由图5ad可以看到,SLM成型GH4169试样的组织均匀、细小,具有胞状和柱状晶组成。如图5be所示,DA试样的胞状和柱状结构依然存在,内部有颗粒状析出物,平均尺寸约20 nm。图5cf显示,SA试样中的胞状和柱状结构基本消失,在晶内和晶界均有片状或针状析出物,长度约1 μm,这些片状析出物呈一定角度(约70°)均匀分布在基体中,与立方晶体{111}中晶面夹角为70.53°相接近[33],基体内部有与DA试样相似的颗粒状析出物。

图5

图5   GH4169高温合金AB、DA和SA试样显微组织的SEM像

Fig.5   Low (a-c) and high (d-f) magnified SEM images of GH4169 superalloy specimens in different states

(a, d) AB state (b, e) DA state (c, f) SA state


为分析胞状柱状组织以及析出相的元素成分,对AB、DA和SA试样进行EPMA测试,图6为EPMA面扫描结果。可见,AB试样的胞状和柱状组织均有Nb、Ti、Mo的偏聚(图6a),偏聚位置在胞状和柱状边界,Ni、Cr、Fe分布均匀。DA试样中依然有Nb元素的偏析(图6b),但偏析程度较AB试样有所减弱,其他元素分布均匀。SA试样中没有胞状和柱状组织(图6c),片状、短棒状和针状析出物有Ni、Nb、Ti的偏聚,同时可见Cr、Fe的贫瘠,Mo元素由于含量较少,没有明显偏析。

图6

图6   GH4169高温合金AB、DA和SA试样的EPMA面扫描结果

Fig.6   EPMA surface mappings of GH4169 superalloy specimens in different states

(a) AB state (b) DA state (c) SA state


图7为GH4169高温合金AB、DA和SA试样的反极图(IPF)面分布图,图7a~cXOY面,图7d~fXOZ面。热处理后晶粒形貌变化不明显,晶粒随机取向,无明显织构。SLM成型样品晶粒与锻件均匀等轴晶粒[34]不同,其中XOY面晶粒呈等轴状,XOZ面晶粒呈柱状,并与构建方向平行,XOY面和XOZ面大晶粒周围均分布有细小的晶粒。

图7

图7   GH4169高温合金AB、DA和SA试样的反极图(IPF)面分布图

Fig.7   Inverse pole figure (IPF) maps of GH4169 superalloy specimens in AB (a, d), DA (b, e), and SA (c, f) states (BD—building direction)

(a-c) XOY surface (d-f) XOZ surface


由于晶粒形状差异较大,采用等效圆面积统计不能反映晶粒尺寸变化的真实情况,因此采用横向和纵向截距法分别统计晶粒尺寸,结果如表3所示。可见,平均晶粒尺寸为10~20 μm,热处理后晶粒尺寸变化不大,其中XOZ面SA试样晶粒尺寸增大,可能与热处理过程中发生再结晶晶粒长大有关[26]

表3   GH4169高温合金AB、DA和SA试样的晶粒尺寸

Table 3  Grain size (intercept lengths) of GH4169 superalloy specimens in AB, DA, and SA states

SurfaceStateTransverse size / μmLongitudinal size / μmAspect ratio
XOYAB10.078.351.21
DA11.879.341.27
SA11.069.751.13
XOZAB9.6916.660.58
DA9.4016.010.59
SA11.0319.710.56

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2.3 力学性能分析

对不同状态样品进行Vickers硬度测试,结果表明,与AB试样(硬度322.2 HV0.2)相比,DA试样的硬度最高,平均值达到523.2 HV0.2,提高了62%左右;而经过SA处理后的硬度为518.0 HV0.2,提高了60%左右。可见,热处理后试样的硬度有较大提高。

表4为GH4169高温合金的室温拉伸性能。从表4可以看出,热处理态GH4169高温合金的屈服强度、抗拉强度及断后延伸率与锻件[35]相当,均高于AMS 5663对锻造件的标准要求。DA试样的屈服强度及抗拉强度最高,分别比AB试样提高了约96%和50%,而断后延伸率下降幅度为54%。SA试样较DA试样强度有所下降,但断后延伸率上升幅度约20%。

表4   GH4169高温合金的室温拉伸性能

Table 4  Room-temperature tensile properties of GH4169 superalloy specimens

StateYield strength / MPaTensile strength / MPaElongation after fracture / %
AB654 ± 11961 ± 233.2 ± 1.6
DA1283 ± 241443 ± 115.0 ± 1.8
SA1238 ± 61393 ± 218.0 ± 1.8
Forged[35]1192138019.1
AMS 5663 standards1034127612.0

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图8为GH4169合金AB、DA和SA试样的室温拉伸断口形貌。低倍观察时均可见台阶解离面,DA和SA试样断面较AB试样更加平整,AB试样有空洞分布在中心区域。高倍下观察时均可见韧窝形成,断裂方式为韧性断裂,AB试样的韧窝较浅,2种热处理态试样的拉伸断口韧窝较深且密集,说明在拉伸过程中承受较高的载荷,表现出较高的抗拉强度及屈服强度,这可归因于晶粒内析出相与位错之间复杂的交互作用。图8ef显示,热处理态试样内部均发现有裂纹,在晶界附近的裂纹由于应力集中被撕裂,DA试样有明显剪切韧窝(图8e),一般分布在试样边缘由剪切应力切断[36]

图8

图8   GH4169高温合金AB、DA和SA试样室温拉伸断口形貌的SEM像

Fig.8   Low (a-c) and high (d-f) magnified SEM images showing the tensile fracture morphologies at room temperature of GH4169 superalloy specimens

(a, d) AB state (b, e) DA state (c, f) SA state


图9为GH4169高温合金AB、DA和SA试样的高温拉伸工程应力-应变曲线。550 ℃高温拉伸曲线均出现锯齿现象,锯齿流变现象一般认为发生了Portevin-Le Chatelier (PLC)效应或动态应变时效(dynamic strain aging,DSA)[37,38],从图中可以判断锯齿为C型锯齿[39,40],PLC效应本质上是一种溶质原子与位错相互作用的过程,受溶质原子扩散运动控制,多数合金[41,42]在一定应变速率和温度区间内会发生PLC效应。锯齿流变现象会使高温合金表面质量降低,影响力学性能[41]

图9

图9   GH4169高温合金AB、DA和SA试样的高温工程应力-应变曲线

Fig.9   High-temperature engineering stress-strain curves of GH4169 superalloy specimens in AB, DA, and AS states


550 ℃下拉伸性能与室温性能趋势相一致,DA试样的强度最高,AB试样的断后伸长率最高,但整体强度较室温时下降了10%~14%,断后伸长率减少了3%~4%。650 ℃高温拉伸时锯齿现象消失,DA和SA试样的强度高于锻件650 ℃高温拉伸性能标准要求,与室温强度相比下降20%,断后伸长率低于锻件标准要求(具体数值见表5)。值得注意的是,AB试样在650 ℃下的拉伸性能与550 ℃相比有所提高。当测试温度达到750 ℃时,3种状态拉伸曲线趋于一致,呈明显的脆性断裂,强度和塑性显著下降。由于γ″为亚稳态相[43,44],在高于650 ℃时会转变为δ相,γ″共格强化减弱。此外,750 ℃会促进O2沿晶界扩散[45],裂纹扩展速率更快,造成力学性能恶化。

表5   GH4169高温合金的高温拉伸性能

Table 5  High-temperature tensile properties of GH4169 superalloy specimens

Temperature / oCStateYield strength / MPaTensile strength / MPaElongation after fracture / %
550AB582 ± 20836 ± 129.9 ± 2.7
DA1100 ± 661263 ± 111.9 ± 0.1
SA1111 ± 281195 ± 114.0 ± 0.6
650AB677 ± 4863 ± 1030.9 ± 1.4
DA949 ± 81158 ± 19.1 ± 0.6
SA999 ± 41129 ± 19.7 ± 0.8
AMS 5663 standard862100012
750AB656 ± 36837 ± 257.8 ± 1.0
DA635 ± 5825 ± 67.8 ± 1.1
SA684 ± 6850 ± 14.9 ± 0.8

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图10为GH4169高温合金AB、DA和SA试样高温拉伸断口形貌的SEM像。可以看出,550 ℃时拉伸断口与室温拉伸断口相似,均有解理台阶,但整体更加平整。650 ℃时拉伸断口突起和凹坑增加,断面相较于550 ℃不平整,DA试样内部有裂纹且有分布不均的凹坑(图10e)。750 ℃时拉伸断口形貌趋于一致,具有分布均匀的凹坑,并在凹坑边界发现裂纹。随着温度的升高,断裂方式由韧性穿晶断裂转变为脆性沿晶断裂。

图10

图10   GH4169高温合金AB、DA和SA试样高温拉伸断口形貌的SEM像

Fig.10   SEM images showing the high-temperature tensile fracture morphologies of GH4169 superalloy specimens at 550 oC (a, d, g), 650 oC (b, e, h), and 750 oC (c, f, i)

(a-c) AB state (d-f) DA state (g-i) SA state


2.4 热处理对析出相演变的影响

为进一步明确析出相结构及其演变规律,对试样进行TEM观察。图11为AB、DA和SA试样胞状结构的TEM明场像。可见,AB样品中有大量位错(图11a),并在胞状边界缠绕,位错的滑移使其具有较高的断后伸长率,在胞壁节点处有不规则块状析出相。DA样品中的胞状组织依然存在(图11b),胞内有细小的析出相生成,由于胞内析出相的阻挡,位错滑移较难进行,强度大幅提高,但塑性下降。SA样品中的胞状组织完全消失(图11c),位错不明显,在晶内和晶界有片状或短棒状析出相生成,位于边界的析出相可作为裂纹萌生的起点[46],同时基体内存在细小的析出相。

图11

图11   GH4169高温合金AB、DA和SA试样胞状结构的TEM明场像

Fig.11   TEM bright-field images of cellular structures of GH4169 superalloy specimens in AB (a), DA (b), and SA (c) states


图12a~c左上角为GH4169高温合金AB、DA和SA试样高角环形暗场(HAADF)像,HAADF像为原子序数衬度像。可以看出,析出相的位置有高原子序数元素偏析,经过热处理后,偏析依然存在。根据TEM-EDS分析结果,AB试样的胞状组织有Nb、Ti、Mo的偏聚(图12a),偏聚位置在胞状边界,同时有Cr、Fe的贫瘠。DA试样的胞状结构依然存在(图12b),元素偏析更加明显,胞状边界Nb、Ti、Mo偏聚加剧,Ni、Cr、Fe贫化显著,与前文EPMA偏析减弱结果不一致,原因可能在于EPMA测试步长(0.0256 μm)较大。SA试样的胞状结构消失(图12c),针状和短棒状析出物有Ni、Nb、Ti的偏聚,同时有Cr、Fe、Mo、Al的贫瘠。

图12

图12   GH4169高温合金AB、DA和SA试样胞状结构的高角环形暗场(HAADF)像及EDS面扫描图

Fig.12   High-angle annular dark field (HAADF) images and EDS mappings of cellular structure of GH4169 superalloy specimens in different states

(a) AB state (b) DA state (c) SA state


图13bc沿[100]、[110]带轴的选区电子衍射(SAED)花样可以确定,AB样品中的基体相为fcc结构,未发现γ′/γ″析出相。对胞状组织节点处析出相进行标定,如图13d~f所示。析出相呈不规则形状,并且尺寸不均,通过标定确定析出相为hcp结构,分别从基体[110]带轴和析出相[112¯0]带轴观察,确定该相为Laves相,并得出基体与Laves相的取向关系:[110] γ // [101¯0]Laves,[11¯2] γ // [112¯0]Laves,与文献[33]结果一致。Laves相是一种与基体γ形成的共晶相,结构式为AB2,结合图12 EDS结果,可以确定其分子式为(Ni, Cr, Fe)2(Nb, Mo, Ti),由于Laves相为拓扑密排相,位错难以绕过或切过,容易形成应力集中,Laves相一般被认为是GH4169中的有害相[22],尺寸较大的Laves相在拉伸过程中极易发生断裂,尺寸较小的Laves相由于与基体割裂,导致微孔形成,成为裂纹萌生的起源,对力学性能产生不利影响[33]

图13

图13   AB、DA、SA试样基体和Laves相的TEM明场像及其SAED花样

Fig.13   TEM bright-field images of matrix and Laves phase, and corresponding SAED patterns of the circle areas (insets) of samples

(a-f) γ matrix bright-field image (a) and SAED patterns with [100] axis (b) and [110] axis (c); Laves phase bright-field image (d) and SAED patterns with [101¯0] axis (e) and [112¯0] axis (f) of sample in AB state (g-j) γ matrix bright-field images under [100] axis (g) and [110] axis (i), and corresponding SAED patterns (h, j) of sample in DA state (k-o) γ matrix bright-field image under [100] axis (k) and SAED pattern (l), intragranular γ matrix bright-field image under [110] axis (m) and SAED pattern (n), and grain boundary γ matrix bright-field image under [110] axis (o) and SAED pattern (p) of sample in SA state


DA处理后,如图13g所示,在基体内部有细小的短棒状析出相,在[100]带轴下获得具有γ′γ″的超点阵,经过标定,基体与γ′/γ″相完全共格,与基体取向关系为:[100] γ // [100] γ′ // [100] γ″,一般认为沉淀析出过程中,会先析出γ′相,然后随着时间的延长,γ′周围有片状γ″相析出,当γ′尺寸合适时,γ″会在γ′相周围6个面成立方体式包覆[47]。若γ′尺寸小于临界尺寸,由于Nb元素的消耗,会形成单个或层状γ″相。同时在晶界交界处发现Laves相,说明时效过程并不能使Laves相溶解,测试过程中发现DA试样中某些Laves相与基体的取向关系发生偏转,这可能与γ′/γ″析出相的形成导致基体与Laves相界面发生改变有关。

SA处理后,如图13k、m、o所示,与DA试样相同,在基体析出细小的γ′/γ″相,并与基体完全共格,同时发现在晶内和晶界有大量短棒状析出相,980 ℃固溶温度在δ相析出的温度区间内[22]δ相为正交结构,结构式为A3B,晶胞参数a、b、c均不相等,δ相有12种变体[48],在基体[110]带轴下观察到基体与δ相的取向关系:[110] γ // [100] δ,[110] γ // [102] δ。结合图12 EDS结果,可以确定为Ni3(Nb, Ti)。黄文普[3]通过差示扫描量热仪(DSC)测量Laves相初始溶解温度为825 ℃,远低于传统制造1140~1160 ℃[49]的初熔温度。此固溶温度下Laves相有更快溶解动力[50],低温溶解现象与SLM成型GH4169的高位错密度和精细微观组织密切相关。在Laves相溶解过程中,由于SLM成型高冷却速率导致组织中枝晶间距十分细小,初始偏析程度较低,扩散距离较短,同时δγ″两相的析出降低了基体中Nb元素含量,进一步促进了Laves相的溶解,因此实验中并未发现明显的Laves相残留。

在进行TEM观察时,注意到DA和SA试样析出的γ′/γ″有所不同,图14b~df~h分别为DA和SA试样中γ′/γ″衍射斑点暗场像。可见,SA试样较DA试样的γ′/γ″含量更高且分布更加均匀,表明固溶处理使元素分布更加均匀。根据文献[51]对γγ′γ″的模拟SAED结果,可以得出斑点暗场像分别为[001] γ′ 和[010] γ″ (图14bf),[100] γ″ (图14cg),及[001] γ′ 和[001] γ″ (图14dh)。γ″为体心四方结构,且c / a ≈ 2[52],存在3种变体,与基体形成共格结构,是主要强化相,成分为Ni3Nb,长时间时效会逐渐转变为稳定δ[44]γ′相由于尺寸较小(约20 nm)分布在γ″相之间,一般认为相较于圆片状γ″相,近球形颗粒为γ′[53]

图14

图14   DA和SA试样的TEM明场像和[001]带轴SAED花样,及试样斑点TEM暗场像

Fig.14   TEM bright-field images (a, e) and SAED patterns with [001] axis (insets), and corresponding TEM dark-field images of spots 1-3 indicated by the arrows (b-d, f, g) of specimens in DA (a-d) and SA (e-h) states


2.5 析出相与力学性能的关系

GH4169高温合金的微观组织演变如图15所示。由于SLM成型属于快速熔化并凝固的过程(图15a),AB试样具有精细的胞状和柱状枝晶结构,在凝固过程中,强化相来不及析出,在枝晶间生成大量的富含Nb、Ti、Mo等强化元素的有害Laves脆性相,导致力学性能较差。而枝晶内部存在高密度位错,没有析出相的阻碍,位错滑移相对容易进行,具有较高的塑性(断后延伸率约为33%)。DA处理后位错被γ′/γ″析出相截断(图15b),位错的开动更加困难,力学性能大幅提高。由于γ′/γ″析出相形成以及原子扩散的影响,Laves相与基体共格关系减弱,但DA态温度不能全部溶解Laves相,故DA处理后试样塑性较差(断后延伸率约为15%)。SA处理后胞状和柱状结构消失(图15c),晶界显现,δ相呈短棒状在基体内均匀析出,主要沿胞状和柱状边界以及晶界分布。同时,γ′/γ″强化相均匀分布在基体中,δ相和γ″相的析出不断消耗Nb元素,促进Laves相发生溶解,以至观察不到明显的Laves相。δ相是GH4169高温合金中的稳定相,不提供强度[54,55]。由于Laves相的溶解,导致SA试样较DA试样强度有所下降,但塑性有所增加(断后延伸率约为18%)。SLM成型GH4169高温合金在650 ℃下保持较好的力学性能,但较室温性能有所下降。550 ℃具有锯齿流变现象。650 ℃时AB试样强度提高,这与此温度下γ′/γ″析出有关。当温度到达750 ℃,亚稳态的γ″会转变为δ[44],力学性能急剧恶化,实际应用场景中应避免服役温度过高。980 ℃固溶时在胞状和柱状边界均匀析出δ相,间接保留了胞状和柱状结构,同时溶解Laves相,在未来应用环境具有一定潜在优势。

图15

图15   GH4169高温合金AB、DA和SA试样的微观组织演变示意图

Fig.15   Schematics of microstructure evolution of GH4169 superalloy specimens in different states

(a) AB state (b) DA state (c) SA state


SLM成型合金有较传统加工更加精细的结构,热处理后室温拉伸性能高于锻造件要求,对于强度要求高的构件可进行直接时效以获得最高强度,对于需要强韧性匹配的构件可选择固溶时效来调控强度和塑性。相较于传统加工长时间高温度的均匀化退火,SLM成型合金由于具有精细的微观结构,热处理工艺可以简化,以降低能耗节约成本。通过选择合适的热处理,权衡打印时间和热处理时间,减少总时间和成本是未来的发展方向。

3 结论

(1) SLM成型GH4169试样在XOY面的宏观组织呈现交错排布的熔道特征,在XOZ面呈现为“鱼鳞状”熔池形貌,具有激光逐层加工特征;直接时效处理后熔池熔道依然清晰可见,而固溶时效后熔池熔道消失。SLM成型晶粒的XOY面呈等轴状,XOZ面呈柱状,沿构建方向定向生长,平均晶粒尺寸在10~20 μm,热处理后晶粒尺寸变化不明显。

(2) 打印态合金组织主要由γ基体和Laves相组成,同时有大量的位错,基体与Laves相的取向关系为:[110] γ // [101¯0]Laves,[11¯2] γ // [112¯0]Laves;直接时效态主要由γ基体、γ′/γ″相和Laves相组成,γ′/γ″析出相与基体完全共格,取向关系为:[100] γ // [100] γ′ // [100] γ″;固溶时效态合金组织主要由γ基体、γ′/γ″相和δ相组成,Laves相发生溶解,在晶内和晶界析出δ相,基体与δ相的取向关系为:[110] γ // [100] δ,[110] γ // [102] δ

(3) 由于SLM快速冷却的特点,合金中形成胞状和柱状的枝晶组织,枝晶间有Nb、Ti等强化元素的偏聚,形成脆性Laves相,强度较低;直接时效处理后有颗粒状γ′/γ″相析出,硬度和强度显著提高,元素偏析有所缓解但仍存在Laves相,塑性较差;固溶时效后γ′/γ″相析出更加均匀,在晶界和晶粒内部析出片状或短棒状δ相,Laves相发生溶解。热处理后试样的室温拉伸性能均达到锻件水平。高温力学性能相较于室温性能有所下降,550 ℃具有锯齿流变现象;650 ℃强度满足相应锻件要求,塑性有所不足;750 ℃力学性能急剧恶化。热处理后SLM成型GH4169高温合金具有与锻件媲美的力学性能,但应注意服役环境温度。

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