鎳基單晶高溫合金具有優(yōu)異的使役性能，是制造先進(jìn)航空發(fā)動(dòng)機(jī)渦輪和導(dǎo)向葉片的關(guān)鍵材料[1,2] 但是，服役溫度的不斷提高和苛刻的服役環(huán)境使葉片發(fā)生高溫氧化和熱腐蝕損傷[3,4] 為了解決這一問(wèn)題，通常將高溫防護(hù)涂層涂覆到葉片等部件上，以減緩葉片基體的退化[5~7]

Pt-Al涂層可明顯提高合金的抗氧化和熱腐蝕性能，廣泛用于航空發(fā)動(dòng)機(jī)單晶渦輪葉片 在長(zhǎng)時(shí)高溫服役過(guò)程中，單晶渦輪葉片的涂層/基體間化學(xué)組分差異以及“熱-力耦合”、互擴(kuò)散反應(yīng)等因素，使基體、涂層及基體/涂層界面發(fā)生組織退化和性能降低[8~11] 將其在高溫下長(zhǎng)時(shí)熱暴露，可較好地模擬葉片材料組織退化 研究表明[12, 13]，隨著熱暴露時(shí)間的延長(zhǎng)合金基體中的γ'相遵循Ostwald熟化理論發(fā)生粗化 隨著熱暴露溫度的提高γ'相粗化越發(fā)嚴(yán)重，甚至形成筏排化 另外，在長(zhǎng)時(shí)熱暴露過(guò)程中γ基體可能析出TCP相和碳化物退化或分解[14~16]

Yuan等[17~20]研究了不同熱暴露時(shí)間、溫度及熱力耦合后涂層體系的組織演變 結(jié)果表明：Al元素的向內(nèi)擴(kuò)散和Ni元素的向外流失使共格排列的γ/γ'相結(jié)構(gòu)遭到破壞并促進(jìn)TCP相的生成，降低了合金的高溫力學(xué)性能 為了模擬渦輪葉片的服役工況，預(yù)先對(duì)含涂層試樣進(jìn)行熱暴露然后開(kāi)展蠕變?cè)囼?yàn) Alam等[11,21,22]發(fā)現(xiàn)，隨著熱暴露時(shí)間的延長(zhǎng)涂層體系發(fā)生退化，裂紋在界面萌生并沿γ'/β相界、孔洞等缺陷“雙向擴(kuò)展”，使合金的蠕變性能降低 Han等[23,24]針探究了含滲鋁涂層的K403鎳基高溫合金渦輪葉片的服役損傷機(jī)理，發(fā)現(xiàn)涂層在高溫下長(zhǎng)時(shí)服役過(guò)程中發(fā)生組織退化、產(chǎn)生表面損傷和孔洞 多種損傷的積累加速涂層的剝落，惡化了體系的服役表現(xiàn) 但是，目前針對(duì)長(zhǎng)時(shí)服役后Pt-Al涂層/單晶高溫合金體系的研究，較多的是圍繞基體組織退化、涂層退化和性能惡化，關(guān)于服役溫度對(duì)涂層/基體界面演化的影響的研究還比較少[25~27] 鑒于此，本文研究在不同溫度(850℃、1000℃)下抗熱腐蝕單晶高溫合金/Pt-Al涂層體系的長(zhǎng)時(shí)服役行為，以及界面及界面附近的微觀組織演化規(guī)律，以深入了解微觀結(jié)構(gòu)演變與互擴(kuò)散反應(yīng)之間的關(guān)聯(lián)

1 實(shí)驗(yàn)方法1.1 實(shí)驗(yàn)用材料

實(shí)驗(yàn)用抗熱腐蝕鎳基單晶高溫合金DD413的名義成分，列于表1 用真空感應(yīng)爐熔煉母合金，用傳統(tǒng)高速凝固Bridgman法(High Rate Solidification，HRS)制備單晶試棒 用電子背散射衍射技術(shù)(Electron Backscattered Diffraction，EBSD)確定單晶試棒的晶體取向，其晶體取向與<001>生長(zhǎng)方向的取向差均小于10°

Table 1

表1

表1DD413合金名義成分(%，質(zhì)量分?jǐn)?shù))

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) 的制度進(jìn)行熱處理，然后用電火花線切割機(jī)沿<001>方向?qū)⒃嚢羟谐芍睆綖?4 mm、厚度為2 mm的試片(試片表面的法線為<001>生長(zhǎng)方向) 將試片進(jìn)行倒角(R=1)處理后，用800#砂紙進(jìn)行磨拋

1.2 涂層的制備

將試片吹砂和清洗后用電鍍法在試片表面沉積一層厚度約為4 μm的Pt層，然后放入VGQ-80型真空爐進(jìn)行1080℃/4 h的退火處理 退火處理后，用高溫低活度滲鋁工藝進(jìn)行滲鋁，滲鋁劑為Fe-Al粉和活化劑NH4Cl的混合粉末 將爐腔抽成真空狀態(tài)后充入氬氣，反復(fù)幾次確保將爐腔內(nèi)的空氣排盡后即可加熱，滲鋁結(jié)束后試樣隨爐自然冷卻至室溫

1.3 長(zhǎng)期熱暴露實(shí)驗(yàn)

將試片裝入坩堝并置于馬弗爐中，分別在 850℃、1000℃進(jìn)行長(zhǎng)期熱暴露實(shí)驗(yàn)，熱暴露時(shí)間分別為50 h、100 h、300 h、600 h、1800 h、3600 h 在每個(gè)時(shí)間段各取3個(gè)樣品用于觀察顯微組織

1.4 微觀組織表征

用X'Pert PRO型X射線衍射儀(XRD)進(jìn)行分析涂層的物相 用配有能譜儀(EDS)和背散射電子(BSE)的Tescan MIRA 3型掃描電鏡(SEM)，觀察金相樣品的微觀形貌、組織和成分(腐蝕液為4 g CuSO4+12 mL HCl+20 mL H2O)

用FEI T20型透射電鏡并使用選區(qū)衍射技術(shù)(SAD)鑒定互擴(kuò)散區(qū)及二次反應(yīng)區(qū)中的物相 透射樣品的制備：沿<001>方向切取厚度約為500 μm的薄片樣品(如圖1所示)，依次將基體面和涂層面研磨(為避免研磨導(dǎo)致涂層脫落，僅針對(duì)涂層面采用細(xì)砂紙進(jìn)行簡(jiǎn)單研磨)至約50 μm(使互擴(kuò)散區(qū)/二次反應(yīng)區(qū)位于樣品的中間層)，然后將樣品沖成直徑為3 mm的圓片 用Tenupol-5電解雙噴減薄儀對(duì)樣品進(jìn)行減薄，以此獲得易于觀察的薄區(qū) (雙噴的參數(shù)為：電壓20 V，雙噴液為10%高氯酸+90%酒精溶液，溫度為-20℃~-25℃，冷卻介質(zhì)為液氮)



圖1涂層/基體截面的示意圖

Fig.1Diagram of cross section of coating/substrate

1.5 圖像分析

使用Image Pro Plus圖像分析軟件測(cè)量互擴(kuò)散區(qū)、二次反應(yīng)區(qū)的厚度 為確保數(shù)據(jù)的準(zhǔn)確性，選取3張視場(chǎng)大小相同SEM圖像進(jìn)行厚度測(cè)量，且測(cè)量點(diǎn)不少于50個(gè) 統(tǒng)計(jì)互擴(kuò)散區(qū)中的高亮粒子的尺寸及體積分?jǐn)?shù)，視場(chǎng)大小相同且每張圖像中的粒子數(shù)量不少于200個(gè) 因?yàn)槲龀鱿喽酁椴灰?guī)則圖形，借助等效直徑

Req=S/π(1)

衡量不規(guī)則形狀析出相的演變規(guī)律[28] 式中Req為粒子等效直徑(Equivalent diameter)，S為粒子面積

2 結(jié)果和討論2.1 Pt-Al涂層原始截面的形貌

圖2給出了熱暴露前DD413合金表面Pt-Al涂層的XRD譜 由圖2可見(jiàn)，涂層由單相β-(Ni,Pt)Al相組成 圖3a給出了DD413合金/Pt-Al涂層的截面形貌，可見(jiàn)涂層分為兩個(gè)區(qū)域：外層(Outer coating，OC)由單一β相構(gòu)成，厚度為(20±1 μm)；內(nèi)層為互擴(kuò)散區(qū)(Interdiffusion of zone，IDZ)，以β-(Ni,Pt)Al相為基，彌散分布著析出相(富Cr、Ta、W、Mo等)，厚度為(14±0.5 μm)，主要受互擴(kuò)散反應(yīng)的影響，在涂層/基體間生成的一個(gè)連續(xù)區(qū)域[29](如圖3a所標(biāo)注) 圖3b給出了涂層區(qū)域紅色方框的EDS定量分析結(jié)果：涂層的成分為Ni-35.3Pt-16.1Al-4.6Cr-4.2Co(質(zhì)量分?jǐn)?shù)，%)



圖2原始態(tài)Pt-Al涂層的XRD譜

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



圖3原始態(tài)Pt-Al涂層/合金截面的形貌和形貌中紅色方框?qū)?yīng)EDS能譜

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

圖4a給出了DD413合金/Pt-Al涂層蝕刻后的BSE圖 受蝕刻的影響，涂層組織中出現(xiàn)些許裂紋 可以看出，在IDZ區(qū)下方出現(xiàn)零散分布的基體擴(kuò)散區(qū)(Substrate diffusion zone，SDZ)(圖4a虛線所示) 此外，靠近互擴(kuò)散區(qū)的基體γ'相發(fā)生了筏化(筏化方向平行于試樣表面)，其厚度為5±0.8 μm，如圖4b所示 γ'形筏主要是預(yù)先的噴砂處理和后續(xù)的涂層熱加所致[29] 遠(yuǎn)離涂層基體內(nèi)部的γ'相為正方體結(jié)構(gòu)，如圖4c所示 互擴(kuò)散區(qū)中有兩種不同襯度的相析出，細(xì)節(jié)如圖4a中的插圖所示；具體成分如圖4d、e所示，較亮的析出相為富Ta和Ti的MC碳化物，另一種較暗的相為富Cr相 SAD分析結(jié)果(圖5)表明，富Cr相為σ-TCP相 使用圖像分析軟件統(tǒng)計(jì)和計(jì)算了互擴(kuò)散區(qū)中MC碳化物的尺寸和體積分?jǐn)?shù)(σ-TCP相與基體襯度差異較小，因此沒(méi)有統(tǒng)計(jì))，結(jié)果表明：MC碳化物的平均尺寸為(0.13±0.006) μm，體積分?jǐn)?shù)為(7.3±0.7)%



圖4Pt-Al涂層/合金截面蝕刻后的BSE圖像和EDS能譜

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)



圖5Pt-Al涂層/合金的TEM圖像及選區(qū)衍射花樣(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 涂層/基體界面微觀組織的演化

涂層-基體間的化學(xué)組分明顯不同，在長(zhǎng)時(shí)熱暴露過(guò)程中持續(xù)進(jìn)行的互擴(kuò)散，誘發(fā)了界面附近微觀組織的演化 隨著熱暴露時(shí)間的延長(zhǎng)和熱暴露溫度的提高，涂層/基體界面微觀結(jié)構(gòu)的演化愈加劇烈 在此，分析幾個(gè)關(guān)鍵組織損傷參量(IDZ、SRZ (Secondary reaction of zone，SRZ))，用厚度和富集難熔元素析出相的尺寸、體積分?jǐn)?shù)量化表征界面微觀組織的演化規(guī)律

2.3 在850℃熱暴露界面微觀組織的演化

圖6a給出了在850℃熱暴露50 h后DD413合金/Pt-Al涂層的截面BSE圖像 與原始態(tài)組織(圖3a)比較，熱暴露50 h后互擴(kuò)散區(qū)內(nèi)MC碳化物的尺寸略微增大(約為0.17 μm)，其體積分?jǐn)?shù)由7.3%增大到8.7%；溫度和互擴(kuò)散等因素的影響使σ-TCP相也發(fā)生了不同程度的溶解或長(zhǎng)大 由于其與基體相襯度差異較小，無(wú)法量化表征其演化規(guī)律 受涂層/基體互擴(kuò)散反應(yīng)的影響，也觀察到SRZ，其形成在互擴(kuò)散區(qū)下方，由γ相、γ'相、TCP相及三相轉(zhuǎn)換產(chǎn)物所形成的區(qū)域[30]，如圖8所示 此時(shí)，IDZ、SRZ分別增大到17.7 μm和11.1 μm 同時(shí)，MC相的體積分?jǐn)?shù)和尺寸變化可能受到噴砂產(chǎn)生的表面再結(jié)晶的影響——晶界和位錯(cuò)為(Ta、Ti)元素的擴(kuò)散提供了通道[31]，涂層/基體間的化學(xué)勢(shì)梯度也使互擴(kuò)散區(qū)中MC碳化物的長(zhǎng)大



圖6在850℃熱暴露不同時(shí)間后Pt-Al涂層/合金截面的BSE圖像

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



圖7850℃熱暴露3600 h后IDZ中的塊狀析出相的TEM圖像及選區(qū)衍射花樣(SAD)

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



圖8在850℃熱暴露不同時(shí)間后蝕刻的Pt-Al涂層/合金截面的BSE圖像

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

圖6b給出了在850℃熱暴露600 h后DD413合金/Pt-Al涂層的截面BSE圖像 對(duì)比結(jié)果表明，互擴(kuò)散區(qū)內(nèi)大顆粒MC的尺寸進(jìn)一步增大，而小顆粒MC卻溶解了，其平均尺寸增長(zhǎng)至0.24 μm，體積分?jǐn)?shù)降至5.5%；尺寸較小的σ-TCP相也在互擴(kuò)散區(qū)中溶解 IDZ和SRZ分別增至22.9 μm和27.9 μm MC碳化物和σ相尺寸的變化不僅受涂層/基體間化學(xué)梯度的影響[32]，還受析出相曲率的影響[33]，使大顆粒析出相進(jìn)一步長(zhǎng)大和一部分小顆粒析出相在互擴(kuò)散區(qū)中溶解

當(dāng)熱暴露時(shí)間延長(zhǎng)至1800 h(如圖6c所示)時(shí)，伴隨著互擴(kuò)散反應(yīng)的進(jìn)行MC碳化物逐漸在互擴(kuò)散區(qū)中溶解，尺寸減小至0.15 μm，體積分?jǐn)?shù)降至4.7% 此時(shí)IDZ的厚度趨于穩(wěn)定(21.8±1.3) μm，SRZ繼續(xù)增長(zhǎng)至34.5 μm 當(dāng)熱暴露3600 h時(shí)只有少量的MC碳化物分布在互擴(kuò)散區(qū)(3.1±0.3)%，且在界面組織中生成了新的塊狀析出相(如圖6d黑色方框所示，該析出相富集Cr元素；SAD鑒定該相為M23C6相，如圖7所示)；SRZ則繼續(xù)增長(zhǎng)，厚度達(dá)到46.7 μm 值得注意的是，隨著熱暴露時(shí)間延長(zhǎng)到3600 h，伴隨MC碳化物與σ-TCP相的溶解M23C6碳化物也在界面組織中析出

文獻(xiàn)[34]指出，隨著熱暴露時(shí)間的延長(zhǎng)高溫合金基體中碳化物發(fā)生演變MC+γ→M23C6(或M6C)+ γ'，但是這種反應(yīng)在涂層組織中難以發(fā)生 由此可以推斷，MC碳化物的溶解和M23C6的析出可能受到涂層由β→γ/γ'的相變(如圖13a)和σ-TCP相溶解的影響 隨著γ'相體積分?jǐn)?shù)的增大更多的γ'相形成元素(Ta、Ti)逐漸被吸收[35]而使MC碳化物溶解，而σ-TCP的溶解釋放出的γ相形成元素Cr卻被拒絕在外 同時(shí)，C元素在晶界上擴(kuò)散更快和C原子與Cr原子之間結(jié)合能力較強(qiáng)[37]，促進(jìn)了M23C6的生成



圖9在850℃熱暴露3600 h后的TEM圖像、選區(qū)衍射花樣(SAD)和針狀相對(duì)應(yīng)的EDS能譜

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)



圖10在1000℃長(zhǎng)時(shí)熱暴露不同時(shí)間后Pt-Al涂層/合金截面的BSE圖像:

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



圖11在850℃和1000℃熱暴露后IDZ、SRZ厚度的演化趨勢(shì)

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



圖12在850℃和1000℃熱暴露后IDZ中MC碳化物的演化趨勢(shì)

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



圖13長(zhǎng)期熱暴露后Pt-Al/DD413體系的XRD譜

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，在長(zhǎng)期熱暴露過(guò)程中界面附近及二次反應(yīng)區(qū)中的γ/γ'微觀結(jié)構(gòu)也受到了明顯的影響 為了更清楚的了解涂層/基體界面演化情況，對(duì)DD413合金/Pt-Al涂層截面進(jìn)行了蝕刻 圖8給出了蝕刻后DD413合金/Pt-Al涂層截面的BSE圖像及近界面附近基體γ/γ'相的局部放大圖 與標(biāo)準(zhǔn)熱處理態(tài)γ/γ'組織相比表明，隨著熱暴露時(shí)間的延長(zhǎng)界面下方立方狀γ'相依次發(fā)生球化、獨(dú)立γ'相粒子相互聯(lián)接呈筏形轉(zhuǎn)變(筏化方向垂直于試樣表面)以及基體通道寬度進(jìn)一步增大等演化，如圖8a~d中的插圖所示

同時(shí)，在長(zhǎng)時(shí)熱暴露過(guò)程中難熔元素出現(xiàn)局部過(guò)飽和，針狀相在界面下方析出 圖9給出了針狀TCP相的TEM圖像、選區(qū)衍射花樣(SAD)和EDS能譜，分析結(jié)果表明針狀相為σ相 對(duì)比分析發(fā)現(xiàn)，隨著熱暴露時(shí)間的延長(zhǎng)σ析出相的含量明顯提高

2.4 在1000℃熱暴露界面微觀組織的演化

圖10a給出了在1000℃熱暴露50 h后DD413合金/Pt-Al涂層的截面BSE圖像 與標(biāo)準(zhǔn)態(tài)(圖3a)相比，MC碳化物的尺寸增至0.26 μm，體積數(shù)上升至7.5%，σ-TCP相彌散分布在互擴(kuò)散區(qū)中；IDZ和SRZ分別增長(zhǎng)至20.4 μm和14.9 μm 圖10b給出了在1000℃熱暴露600 h后DD413合金/Pt-Al涂層的截面BSE圖像 可以看出，部分MC碳化物溶解在互擴(kuò)散區(qū)中，其體積分?jǐn)?shù)只占4.5%，MC碳化物的尺寸也減小到0.21 μm，伴隨著部分σ-TCP相的溶解M23C6也在互擴(kuò)散區(qū)中析出 IDZ和SRZ分別增長(zhǎng)到24.8 μm和37.1 μm

如圖10c所示，當(dāng)熱暴露時(shí)間持續(xù)到1800h時(shí) MC碳化物只占1.2%(此時(shí)，MC碳化物幾乎全部溶解在互擴(kuò)散區(qū)中，存在較大的計(jì)算誤差 所以，在此之后沒(méi)有給出與MC碳化物尺寸相關(guān)的數(shù)據(jù)) 除了C碳化物，在互擴(kuò)散區(qū)中還發(fā)現(xiàn)尺寸較大的TCP相和M23C6碳化物(分別在圖10c中標(biāo)出)；IDZ和SRZ的厚度分別為23.5 μm和49.3 μm 圖10d給出了在1000℃熱暴露3600 h后DD413合金/Pt-Al涂層的截面BSE圖像 可以看出，β→γ/γ'相變產(chǎn)生的體積收縮使涂層表面出現(xiàn)起伏，Al損耗比在850℃熱暴露更加嚴(yán)重；此時(shí)的互擴(kuò)散區(qū)以γ/γ'為基，彌散分布著MC、M23C6碳化物和σ-TCP IDZ和SRZ的厚度分別為23.1 μm和62.3 μm

與在850℃熱暴露相比，在1000℃熱暴露時(shí)合金/涂層界面微觀組織的演化更快 在1000℃長(zhǎng)時(shí)熱暴露使互擴(kuò)散區(qū)的厚度和二次反應(yīng)的厚度都比在850℃熱暴露時(shí)大 對(duì)比分析發(fā)現(xiàn)，MC碳化物的溶解和M23C6的析出明顯加速；同時(shí)，在1000℃熱暴露時(shí)TCP相的尺寸也比在850℃熱暴露時(shí)略大，如圖11、12所示 出現(xiàn)以上現(xiàn)象的原因是：一方面，β-(Ni,Pt)Al向γ/γ'相的轉(zhuǎn)變加速，如圖13b所示 互擴(kuò)散區(qū)中Ta、Ti元素被γ'相吸收[35]使MC碳化物溶解 同時(shí)，σ-TCP相在高溫下不穩(wěn)定而易發(fā)生分解，閑置下來(lái)的Cr與C結(jié)合也加快了M23C6的形成[36]，如圖10b所示 另一方面，高溫使大量的Al元素?cái)U(kuò)散進(jìn)入基體，Ni元素向外流失使反應(yīng)

γ+[Al]→γ'(2)

γ→[Ni]+γ'(3)

更加劇烈 這使更多的γ'相在界面下方生成，而γ相中元素的擴(kuò)散速率比γ'相的擴(kuò)散速率高1-2數(shù)量級(jí)[37]，因此γ'相限制了元素的向外擴(kuò)散 同時(shí)，γ'相比Cr、Co、Mo、W等難熔元素的溶解性較差，使更多的難熔元素在界面偏聚并以針狀相的形式在界面下方析出 文獻(xiàn)[38]指出，針狀TCP相限制了基體中元素向外擴(kuò)散，減少了難熔元素在互擴(kuò)散區(qū)中的聚集 雖然γ'相和TCP相在一定程度上限制了基體元素向外流失，但是與Ta、Ti等元素相比，來(lái)自基體深處的Ni更容易向外補(bǔ)充[10] Ni元素不斷向外擴(kuò)散，使難熔元素在互擴(kuò)散區(qū)中的局部溶解能力提高 因此，在1000℃長(zhǎng)期熱暴露過(guò)程中，涂層相變和涂層/基體的互擴(kuò)散加速了互擴(kuò)散區(qū)中MC碳化物的溶解和M23C6碳化物的析出

與在850℃熱暴露相比，在1000℃長(zhǎng)時(shí)熱暴露后涂層和界面的損傷隨著熱暴露時(shí)間的延長(zhǎng)愈加嚴(yán)重 圖14a~d分別給出了蝕刻后在1000℃熱暴露50 h、600 h、1800 h、3600 h后Pt-Al涂層的截面圖像和界面γ/γ'相的放大圖像 可以看出，界面下方的γ'相互相聯(lián)接形成筏化結(jié)構(gòu)，但是仍存在部分獨(dú)立的立方狀γ'相 隨著熱暴露時(shí)間的延長(zhǎng)筏化層不斷深入，如圖14a~d中的插圖所示 大量的難熔元素釋放出來(lái)生成了TCP相，SAD分析結(jié)果表明針狀相為σ-TCP，如圖15所示 另外，在高溫下隨著熱暴露時(shí)間的延長(zhǎng)氧化反應(yīng)和互擴(kuò)散反應(yīng)持續(xù)進(jìn)行，使Al元素的損耗加劇和發(fā)生β-(Ni,Pt)Al→γ'→γ轉(zhuǎn)變 隨著界面組織中γ相體積分?jǐn)?shù)的增大γ'相形成元素Ta和Ti則處于游離狀態(tài)，加之C較快的擴(kuò)散使其與Ta、Ti重新結(jié)合并以二次MC的形式在界面組織中析出，如圖15b~d界面組織中的高亮相



圖14Pt-Al涂層/合金截面蝕刻后在1000℃長(zhǎng)時(shí)熱暴露不同時(shí)間后的BSE圖像

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



圖15在1000℃/3600 h后的TEM形貌、選區(qū)衍射花樣(SAD)和與針狀相對(duì)應(yīng)的EDS能譜

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)

在不同溫度下近涂層γ/γ'相也發(fā)生了不同程度的退化，其原因是：(1)在高溫下合金基體(FCC)的熱膨脹系數(shù)略大于β-(Ni,Pt)Al涂層(BCC)，噴砂處理使界面應(yīng)變能無(wú)法釋放，涂層相對(duì)基體產(chǎn)生了平行于界面的壓應(yīng)力[32]，使界面下方的γ'相形筏 (2) 在1000℃熱暴露時(shí)涂層中的Al與O2的劇烈反應(yīng)支撐了表面氧化層的生成，與在850℃熱暴露相比，由β到γ/γ'的轉(zhuǎn)變明顯加速，如圖13所示 而β相的晶粒尺寸略大于γ'相，在熱暴露過(guò)程中伴隨相變反應(yīng)的進(jìn)行涂層的體積收縮，出現(xiàn)在涂層中的應(yīng)力使界面γ'相筏化層不斷深入[39] (3)涂層/基體互擴(kuò)散反應(yīng)：高溫下，劇烈的反應(yīng)(3、4)使界面附近γ'相的體積分?jǐn)?shù)增大，影響γ/γ'相的錯(cuò)配度[38]，進(jìn)一步使界面下方的γ'相形筏出現(xiàn)差異

綜上所述，在不同溫度下長(zhǎng)期熱暴露后界面下方發(fā)生γ'相筏化的原因，是涂層相變和涂層/基體間持續(xù)進(jìn)行的互擴(kuò)散反應(yīng)

3 結(jié)論

(1) DD413合金/Pt-Al涂層在不同溫度下熱暴露的前期，MC碳化物的體積分?jǐn)?shù)和尺寸發(fā)生不同程度的增大，隨著熱暴露時(shí)間的延長(zhǎng)MC碳化物和σ-TCP相在互擴(kuò)散區(qū)內(nèi)逐漸溶解；熱暴露3600 h后少量MC碳化物和部分σ-TCP相分布在互擴(kuò)散區(qū)中，并有M23C6在界面組織中析出 隨著熱暴露溫度的提高界面組織的退化嚴(yán)重，使以上進(jìn)程明顯加速

(2) 長(zhǎng)時(shí)熱暴露后SRZ出現(xiàn)在界面下方，熱暴露溫度為1000℃時(shí)SRZ的厚度和σ-TCP相的尺寸均比在850℃熱暴露時(shí)大

(3) 長(zhǎng)時(shí)熱暴露后近涂層基體立方狀γ'相依次發(fā)生球化和相互聯(lián)接成筏形轉(zhuǎn)變 隨著熱暴露溫度提高到1000℃近涂層基體γ'相的損傷愈加嚴(yán)重，界面下方的部分γ'已形筏(平行于試樣表面)，且隨著熱暴露時(shí)間的延長(zhǎng)筏化層厚度不斷增大，并向基體內(nèi)部延伸

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