长时热暴露对一种抗热腐蚀单晶高温合金/Pt-Al涂层体系微观组织演化的影响

镍基单晶高温合金具有优异的使役性能，是制造先进航空发动机涡轮和导向叶片的关键材料[1,2] 但是，服役温度的不断提高和苛刻的服役环境使叶片发生高温氧化和热腐蚀损伤[3,4] 为了解决这一问题，通常将高温防护涂层涂覆到叶片等部件上，以减缓叶片基体的退化[5~7]

Pt-Al涂层可明显提高合金的抗氧化和热腐蚀性能，广泛用于航空发动机单晶涡轮叶片 在长时高温服役过程中，单晶涡轮叶片的涂层/基体间化学组分差异以及“热-力耦合”、互扩散反应等因素，使基体、涂层及基体/涂层界面发生组织退化和性能降低[8~11] 将其在高温下长时热暴露，可较好地模拟叶片材料组织退化 研究表明[12, 13]，随着热暴露时间的延长合金基体中的γ'相遵循Ostwald熟化理论发生粗化 随着热暴露温度的提高γ'相粗化越发严重，甚至形成筏排化 另外，在长时热暴露过程中γ基体可能析出TCP相和碳化物退化或分解[14~16]

Yuan等[17~20]研究了不同热暴露时间、温度及热力耦合后涂层体系的组织演变 结果表明：Al元素的向内扩散和Ni元素的向外流失使共格排列的γ/γ'相结构遭到破坏并促进TCP相的生成，降低了合金的高温力学性能 为了模拟涡轮叶片的服役工况，预先对含涂层试样进行热暴露然后开展蠕变试验 Alam等[11,21,22]发现，随着热暴露时间的延长涂层体系发生退化，裂纹在界面萌生并沿γ'/β相界、孔洞等缺陷“双向扩展”，使合金的蠕变性能降低 Han等[23,24]针探究了含渗铝涂层的K403镍基高温合金涡轮叶片的服役损伤机理，发现涂层在高温下长时服役过程中发生组织退化、产生表面损伤和孔洞 多种损伤的积累加速涂层的剥落，恶化了体系的服役表现 但是，目前针对长时服役后Pt-Al涂层/单晶高温合金体系的研究，较多的是围绕基体组织退化、涂层退化和性能恶化，关于服役温度对涂层/基体界面演化的影响的研究还比较少[25~27] 鉴于此，本文研究在不同温度(850℃、1000℃)下抗热腐蚀单晶高温合金/Pt-Al涂层体系的长时服役行为，以及界面及界面附近的微观组织演化规律，以深入了解微观结构演变与互扩散反应之间的关联

1 实验方法1.1 实验用材料

实验用抗热腐蚀镍基单晶高温合金DD413的名义成分，列于表1 用真空感应炉熔炼母合金，用传统高速凝固Bridgman法(High Rate Solidification，HRS)制备单晶试棒 用电子背散射衍射技术(Electron Backscattered Diffraction，EBSD)确定单晶试棒的晶体取向，其晶体取向与<001>生长方向的取向差均小于10°

Table 1

表1

表1DD413合金名义成分(%，质量分数)

Table 1Nominal composition of DD413 alloy (mass fraction, %)

Alloy	C	Cr	Co	W	Mo	Al	Ti	Ta	Ni
DD413	0.07	12	9	3.8	1.9	3.6	4.1	5	Bal.

按照1220℃/2 h+1250℃/4 h(Air cooling；AC)+1080℃/4 h(AC) 的制度进行热处理，然后用电火花线切割机沿<001>方向将试棒切成直径为14 mm、厚度为2 mm的试片(试片表面的法线为<001>生长方向) 将试片进行倒角(R=1)处理后，用800#砂纸进行磨抛

1.2 涂层的制备

将试片吹砂和清洗后用电镀法在试片表面沉积一层厚度约为4 μm的Pt层，然后放入VGQ-80型真空炉进行1080℃/4 h的退火处理 退火处理后，用高温低活度渗铝工艺进行渗铝，渗铝剂为Fe-Al粉和活化剂NH4Cl的混合粉末 将炉腔抽成真空状态后充入氩气，反复几次确保将炉腔内的空气排尽后即可加热，渗铝结束后试样随炉自然冷却至室温

1.3 长期热暴露实验

将试片装入坩埚并置于马弗炉中，分别在 850℃、1000℃进行长期热暴露实验，热暴露时间分别为50 h、100 h、300 h、600 h、1800 h、3600 h 在每个时间段各取3个样品用于观察显微组织

1.4 微观组织表征

用X'Pert PRO型X射线衍射仪(XRD)进行分析涂层的物相 用配有能谱仪(EDS)和背散射电子(BSE)的Tescan MIRA 3型扫描电镜(SEM)，观察金相样品的微观形貌、组织和成分(腐蚀液为4 g CuSO4+12 mL HCl+20 mL H2O)

用FEI T20型透射电镜并使用选区衍射技术(SAD)鉴定互扩散区及二次反应区中的物相 透射样品的制备：沿<001>方向切取厚度约为500 μm的薄片样品(如图1所示)，依次将基体面和涂层面研磨(为避免研磨导致涂层脱落，仅针对涂层面采用细砂纸进行简单研磨)至约50 μm(使互扩散区/二次反应区位于样品的中间层)，然后将样品冲成直径为3 mm的圆片 用Tenupol-5电解双喷减薄仪对样品进行减薄，以此获得易于观察的薄区 (双喷的参数为：电压20 V，双喷液为10%高氯酸+90%酒精溶液，温度为-20℃~-25℃，冷却介质为液氮)



图1涂层/基体截面的示意图

Fig.1Diagram of cross section of coating/substrate

1.5 图像分析

使用Image Pro Plus图像分析软件测量互扩散区、二次反应区的厚度 为确保数据的准确性，选取3张视场大小相同SEM图像进行厚度测量，且测量点不少于50个 统计互扩散区中的高亮粒子的尺寸及体积分数，视场大小相同且每张图像中的粒子数量不少于200个 因为析出相多为不规则图形，借助等效直径

Req=S/π(1)

衡量不规则形状析出相的演变规律[28] 式中Req为粒子等效直径(Equivalent diameter)，S为粒子面积

2 结果和讨论2.1 Pt-Al涂层原始截面的形貌

图2给出了热暴露前DD413合金表面Pt-Al涂层的XRD谱 由图2可见，涂层由单相β-(Ni,Pt)Al相组成 图3a给出了DD413合金/Pt-Al涂层的截面形貌，可见涂层分为两个区域：外层(Outer coating，OC)由单一β相构成，厚度为(20±1 μm)；内层为互扩散区(Interdiffusion of zone，IDZ)，以β-(Ni,Pt)Al相为基，弥散分布着析出相(富Cr、Ta、W、Mo等)，厚度为(14±0.5 μm)，主要受互扩散反应的影响，在涂层/基体间生成的一个连续区域[29](如图3a所标注) 图3b给出了涂层区域红色方框的EDS定量分析结果：涂层的成分为Ni-35.3Pt-16.1Al-4.6Cr-4.2Co(质量分数，%)



图2原始态Pt-Al涂层的XRD谱

Fig.2XRD diffraction pattern of original Pt-Al coating



图3原始态Pt-Al涂层/合金截面的形貌和形貌中红色方框对应EDS能谱

Fig.3(a) Cross section morphology of original Pt-Al coating/Alloy and (b) EDS energy spectrum corresponding to red box in (a)

图4a给出了DD413合金/Pt-Al涂层蚀刻后的BSE图 受蚀刻的影响，涂层组织中出现些许裂纹 可以看出，在IDZ区下方出现零散分布的基体扩散区(Substrate diffusion zone，SDZ)(图4a虚线所示) 此外，靠近互扩散区的基体γ'相发生了筏化(筏化方向平行于试样表面)，其厚度为5±0.8 μm，如图4b所示 γ'形筏主要是预先的喷砂处理和后续的涂层热加所致[29] 远离涂层基体内部的γ'相为正方体结构，如图4c所示 互扩散区中有两种不同衬度的相析出，细节如图4a中的插图所示；具体成分如图4d、e所示，较亮的析出相为富Ta和Ti的MC碳化物，另一种较暗的相为富Cr相 SAD分析结果(图5)表明，富Cr相为σ-TCP相 使用图像分析软件统计和计算了互扩散区中MC碳化物的尺寸和体积分数(σ-TCP相与基体衬度差异较小，因此没有统计)，结果表明：MC碳化物的平均尺寸为(0.13±0.006) μm，体积分数为(7.3±0.7)%



图4Pt-Al涂层/合金截面蚀刻后的BSE图像和EDS能谱

Fig.4BSE images and EDS spectrum of Pt-Al coating/alloy section after etched: (a) sectional image of Pt-Al coating; (b, c) enlarged image of coating/substrate interface image of γ/γ'; (d, e) are the corresponding MC and σ-TCP EDS spectra in (a)



图5Pt-Al涂层/合金的TEM图像及选区衍射花样(SAD)

Fig.5TEM image and selected area diffraction pattern (SAD) of Pt-Al coating/alloy (a) TEM image of interdiffusion region of original coating; (A) and (B) selected area diffraction (SAD) corresponding to (a)

2.2 涂层/基体界面微观组织的演化

涂层-基体间的化学组分明显不同，在长时热暴露过程中持续进行的互扩散，诱发了界面附近微观组织的演化 随着热暴露时间的延长和热暴露温度的提高，涂层/基体界面微观结构的演化愈加剧烈 在此，分析几个关键组织损伤参量(IDZ、SRZ (Secondary reaction of zone，SRZ))，用厚度和富集难熔元素析出相的尺寸、体积分数量化表征界面微观组织的演化规律

2.3 在850℃热暴露界面微观组织的演化

图6a给出了在850℃热暴露50 h后DD413合金/Pt-Al涂层的截面BSE图像 与原始态组织(图3a)比较，热暴露50 h后互扩散区内MC碳化物的尺寸略微增大(约为0.17 μm)，其体积分数由7.3%增大到8.7%；温度和互扩散等因素的影响使σ-TCP相也发生了不同程度的溶解或长大 由于其与基体相衬度差异较小，无法量化表征其演化规律 受涂层/基体互扩散反应的影响，也观察到SRZ，其形成在互扩散区下方，由γ相、γ'相、TCP相及三相转换产物所形成的区域[30]，如图8所示 此时，IDZ、SRZ分别增大到17.7 μm和11.1 μm 同时，MC相的体积分数和尺寸变化可能受到喷砂产生的表面再结晶的影响——晶界和位错为(Ta、Ti)元素的扩散提供了通道[31]，涂层/基体间的化学势梯度也使互扩散区中MC碳化物的长大



图6在850℃热暴露不同时间后Pt-Al涂层/合金截面的BSE图像

Fig.6BSE images of Pt-Al coating/alloy section after long-term thermal exposure at 850℃ for different time (a) 50 h; (b)600 h; (c) 1800 h; (d) 3600 h



图7850℃热暴露3600 h后IDZ中的块状析出相的TEM图像及选区衍射花样(SAD)

Fig.7TEM image and selected area diffraction patterns (SAD) of block precipitated in IDZ after heat exposure at 850℃ for 3600 h



图8在850℃热暴露不同时间后蚀刻的Pt-Al涂层/合金截面的BSE图像

Fig.8BSE images of Pt Al coating/alloy section etched after thermal exposure at 850℃ for different time (a) 50 h; (b) 600 h; (c) 1800 h; (d) 3600 h

图6b给出了在850℃热暴露600 h后DD413合金/Pt-Al涂层的截面BSE图像 对比结果表明，互扩散区内大颗粒MC的尺寸进一步增大，而小颗粒MC却溶解了，其平均尺寸增长至0.24 μm，体积分数降至5.5%；尺寸较小的σ-TCP相也在互扩散区中溶解 IDZ和SRZ分别增至22.9 μm和27.9 μm MC碳化物和σ相尺寸的变化不仅受涂层/基体间化学梯度的影响[32]，还受析出相曲率的影响[33]，使大颗粒析出相进一步长大和一部分小颗粒析出相在互扩散区中溶解

当热暴露时间延长至1800 h(如图6c所示)时，伴随着互扩散反应的进行MC碳化物逐渐在互扩散区中溶解，尺寸减小至0.15 μm，体积分数降至4.7% 此时IDZ的厚度趋于稳定(21.8±1.3) μm，SRZ继续增长至34.5 μm 当热暴露3600 h时只有少量的MC碳化物分布在互扩散区(3.1±0.3)%，且在界面组织中生成了新的块状析出相(如图6d黑色方框所示，该析出相富集Cr元素；SAD鉴定该相为M23C6相，如图7所示)；SRZ则继续增长，厚度达到46.7 μm 值得注意的是，随着热暴露时间延长到3600 h，伴随MC碳化物与σ-TCP相的溶解M23C6碳化物也在界面组织中析出

文献[34]指出，随着热暴露时间的延长高温合金基体中碳化物发生演变MC+γ→M23C6(或M6C)+ γ'，但是这种反应在涂层组织中难以发生 由此可以推断，MC碳化物的溶解和M23C6的析出可能受到涂层由β→γ/γ'的相变(如图13a)和σ-TCP相溶解的影响 随着γ'相体积分数的增大更多的γ'相形成元素(Ta、Ti)逐渐被吸收[35]而使MC碳化物溶解，而σ-TCP的溶解释放出的γ相形成元素Cr却被拒绝在外 同时，C元素在晶界上扩散更快和C原子与Cr原子之间结合能力较强[37]，促进了M23C6的生成



图9在850℃热暴露3600 h后的TEM图像、选区衍射花样(SAD)和针状相对应的EDS能谱

Fig.9(a) TEM image after thermal exposure at 850℃ for 3600 h and selected area diffraction pattern (SAD), (b) EDS energy spectrum corresponding to needle phase in (a)



图10在1000℃长时热暴露不同时间后Pt-Al涂层/合金截面的BSE图像:

Fig.10BSE images of Pt-Al coating/alloy section after long-term thermal exposure at 1000℃ for different time (a) 50 h; (b)600 h; (c)1800 h; (d) 3600 h



图11在850℃和1000℃热暴露后IDZ、SRZ厚度的演化趋势

Fig.11Evolution trend of thickness of IDZ (a) and SRZ (b) after 850℃ and 1000℃ thermal exposure



图12在850℃和1000℃热暴露后IDZ中MC碳化物的演化趋势

Fig.12Evolution trend of MC carbide in IDZ after thermal exposure at 850℃ and 1000℃ (a) Average size of MC carbide; (b) Volume fraction of MC carbide



图13长期热暴露后Pt-Al/DD413体系的XRD谱

Fig.13XRD pattern of Pt-Al / DD413 system after long-term thermal exposure (a) 850℃/0-3600 h; (b) 1000℃/0-3600 h

另外，在长期热暴露过程中界面附近及二次反应区中的γ/γ'微观结构也受到了明显的影响 为了更清楚的了解涂层/基体界面演化情况，对DD413合金/Pt-Al涂层截面进行了蚀刻 图8给出了蚀刻后DD413合金/Pt-Al涂层截面的BSE图像及近界面附近基体γ/γ'相的局部放大图 与标准热处理态γ/γ'组织相比表明，随着热暴露时间的延长界面下方立方状γ'相依次发生球化、独立γ'相粒子相互联接呈筏形转变(筏化方向垂直于试样表面)以及基体通道宽度进一步增大等演化，如图8a~d中的插图所示

同时，在长时热暴露过程中难熔元素出现局部过饱和，针状相在界面下方析出 图9给出了针状TCP相的TEM图像、选区衍射花样(SAD)和EDS能谱，分析结果表明针状相为σ相 对比分析发现，随着热暴露时间的延长σ析出相的含量明显提高

2.4 在1000℃热暴露界面微观组织的演化

图10a给出了在1000℃热暴露50 h后DD413合金/Pt-Al涂层的截面BSE图像 与标准态(图3a)相比，MC碳化物的尺寸增至0.26 μm，体积数上升至7.5%，σ-TCP相弥散分布在互扩散区中；IDZ和SRZ分别增长至20.4 μm和14.9 μm 图10b给出了在1000℃热暴露600 h后DD413合金/Pt-Al涂层的截面BSE图像 可以看出，部分MC碳化物溶解在互扩散区中，其体积分数只占4.5%，MC碳化物的尺寸也减小到0.21 μm，伴随着部分σ-TCP相的溶解M23C6也在互扩散区中析出 IDZ和SRZ分别增长到24.8 μm和37.1 μm

如图10c所示，当热暴露时间持续到1800h时 MC碳化物只占1.2%(此时，MC碳化物几乎全部溶解在互扩散区中，存在较大的计算误差 所以，在此之后没有给出与MC碳化物尺寸相关的数据) 除了C碳化物，在互扩散区中还发现尺寸较大的TCP相和M23C6碳化物(分别在图10c中标出)；IDZ和SRZ的厚度分别为23.5 μm和49.3 μm 图10d给出了在1000℃热暴露3600 h后DD413合金/Pt-Al涂层的截面BSE图像 可以看出，β→γ/γ'相变产生的体积收缩使涂层表面出现起伏，Al损耗比在850℃热暴露更加严重；此时的互扩散区以γ/γ'为基，弥散分布着MC、M23C6碳化物和σ-TCP IDZ和SRZ的厚度分别为23.1 μm和62.3 μm

与在850℃热暴露相比，在1000℃热暴露时合金/涂层界面微观组织的演化更快 在1000℃长时热暴露使互扩散区的厚度和二次反应的厚度都比在850℃热暴露时大 对比分析发现，MC碳化物的溶解和M23C6的析出明显加速；同时，在1000℃热暴露时TCP相的尺寸也比在850℃热暴露时略大，如图11、12所示 出现以上现象的原因是：一方面，β-(Ni,Pt)Al向γ/γ'相的转变加速，如图13b所示 互扩散区中Ta、Ti元素被γ'相吸收[35]使MC碳化物溶解 同时，σ-TCP相在高温下不稳定而易发生分解，闲置下来的Cr与C结合也加快了M23C6的形成[36]，如图10b所示 另一方面，高温使大量的Al元素扩散进入基体，Ni元素向外流失使反应

γ+[Al]→γ'(2)

γ→[Ni]+γ'(3)

更加剧烈 这使更多的γ'相在界面下方生成，而γ相中元素的扩散速率比γ'相的扩散速率高1-2数量级[37]，因此γ'相限制了元素的向外扩散 同时，γ'相比Cr、Co、Mo、W等难熔元素的溶解性较差，使更多的难熔元素在界面偏聚并以针状相的形式在界面下方析出 文献[38]指出，针状TCP相限制了基体中元素向外扩散，减少了难熔元素在互扩散区中的聚集 虽然γ'相和TCP相在一定程度上限制了基体元素向外流失，但是与Ta、Ti等元素相比，来自基体深处的Ni更容易向外补充[10] Ni元素不断向外扩散，使难熔元素在互扩散区中的局部溶解能力提高 因此，在1000℃长期热暴露过程中，涂层相变和涂层/基体的互扩散加速了互扩散区中MC碳化物的溶解和M23C6碳化物的析出

与在850℃热暴露相比，在1000℃长时热暴露后涂层和界面的损伤随着热暴露时间的延长愈加严重 图14a~d分别给出了蚀刻后在1000℃热暴露50 h、600 h、1800 h、3600 h后Pt-Al涂层的截面图像和界面γ/γ'相的放大图像 可以看出，界面下方的γ'相互相联接形成筏化结构，但是仍存在部分独立的立方状γ'相 随着热暴露时间的延长筏化层不断深入，如图14a~d中的插图所示 大量的难熔元素释放出来生成了TCP相，SAD分析结果表明针状相为σ-TCP，如图15所示 另外，在高温下随着热暴露时间的延长氧化反应和互扩散反应持续进行，使Al元素的损耗加剧和发生β-(Ni,Pt)Al→γ'→γ转变 随着界面组织中γ相体积分数的增大γ'相形成元素Ta和Ti则处于游离状态，加之C较快的扩散使其与Ta、Ti重新结合并以二次MC的形式在界面组织中析出，如图15b~d界面组织中的高亮相



图14Pt-Al涂层/合金截面蚀刻后在1000℃长时热暴露不同时间后的BSE图像

Fig.14BSE images of Pt-Al coating/alloy section after long-term thermal exposure at 1000℃ for different time (a) 50 h; (b)600 h; (c) 1800 h; (d) 3600 h



图15在1000℃/3600 h后的TEM形貌、选区衍射花样(SAD)和与针状相对应的EDS能谱

Fig.15(a) 1000℃/3600 h, TEM image and selected area diffraction pattern (SAD) and (b) EDS energy spectrum corresponding to needle phase in (a)

在不同温度下近涂层γ/γ'相也发生了不同程度的退化，其原因是：(1)在高温下合金基体(FCC)的热膨胀系数略大于β-(Ni,Pt)Al涂层(BCC)，喷砂处理使界面应变能无法释放，涂层相对基体产生了平行于界面的压应力[32]，使界面下方的γ'相形筏 (2) 在1000℃热暴露时涂层中的Al与O2的剧烈反应支撑了表面氧化层的生成，与在850℃热暴露相比，由β到γ/γ'的转变明显加速，如图13所示 而β相的晶粒尺寸略大于γ'相，在热暴露过程中伴随相变反应的进行涂层的体积收缩，出现在涂层中的应力使界面γ'相筏化层不断深入[39] (3)涂层/基体互扩散反应：高温下，剧烈的反应(3、4)使界面附近γ'相的体积分数增大，影响γ/γ'相的错配度[38]，进一步使界面下方的γ'相形筏出现差异

综上所述，在不同温度下长期热暴露后界面下方发生γ'相筏化的原因，是涂层相变和涂层/基体间持续进行的互扩散反应

3 结论

(1) DD413合金/Pt-Al涂层在不同温度下热暴露的前期，MC碳化物的体积分数和尺寸发生不同程度的增大，随着热暴露时间的延长MC碳化物和σ-TCP相在互扩散区内逐渐溶解；热暴露3600 h后少量MC碳化物和部分σ-TCP相分布在互扩散区中，并有M23C6在界面组织中析出 随着热暴露温度的提高界面组织的退化严重，使以上进程明显加速

(2) 长时热暴露后SRZ出现在界面下方，热暴露温度为1000℃时SRZ的厚度和σ-TCP相的尺寸均比在850℃热暴露时大

(3) 长时热暴露后近涂层基体立方状γ'相依次发生球化和相互联接成筏形转变 随着热暴露温度提高到1000℃近涂层基体γ'相的损伤愈加严重，界面下方的部分γ'已形筏(平行于试样表面)，且随着热暴露时间的延长筏化层厚度不断增大，并向基体内部延伸

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1

2012