Mo含量對FeCoCrNiMox高熵合金等離子噴焊層組織與性能的影響

張乾坤，唐俊，肖逸鋒，杜萌萌，吳靚，錢錦文，賀躍輝

Mo含量對FeCoCrNiMo高熵合金等離子噴焊層組織與性能的影響

張乾坤1a,1b，唐俊1a,1b，肖逸鋒1a,1b，杜萌萌1a,1b，吳靚1a,1b，錢錦文1a,1b，賀躍輝2

（1.湘潭大學 a.機械工程學院 b.焊接機器人與應(yīng)用技術(shù)湖南省重點實驗室，湖南 湘潭 411105；2.中南大學 粉末冶金國家重點實驗室，長沙 410083）

研究不同Mo元素添加量對FeCoCrNiMo（=0、0.5、1、1.5）高熵合金等離子噴焊層組織和性能的影響，以期望獲得一種高硬度、耐腐蝕的噴焊層，用于改善傳統(tǒng)工模具表面防護與使用壽命的問題。采用等離子噴焊技術(shù)在Q235A低碳鋼表面制備了不同Mo含量的高熵合金噴焊層，通過X射線衍射儀（XRD）、光學顯微鏡（OM）、掃描電子顯微鏡（SEM）、能量色散X射線光譜儀（EDS）表征其微觀組織與相結(jié)構(gòu)，借助顯微硬度計和電化學工作站對噴焊層的硬度和耐腐蝕性能進行測試。隨著Mo含量從0逐漸增加到1.5，噴焊層的晶界胞狀枝晶組織（枝晶內(nèi)為白色富Mo相，枝晶間為灰色富Fe、Ni相）逐漸增加，合金微觀組織變得細??；噴焊層的硬度由204.4HV0.2增加至706.8HV0.2；噴焊層在3.5%NaCl溶液中呈現(xiàn)出明顯的鈍化行為，腐蝕電位由?0.753 V增大到?0.412 V，腐蝕電流密度由1.23×10?4A/cm2減小到3.80×10?6A/cm2，點蝕電位由?0.642 V增大到?0.371 V，具有優(yōu)異的耐腐蝕性能。所設(shè)計的FeCoCrNiMo合金及相應(yīng)的等離子噴焊工藝，滿足對噴焊層高耐磨以及耐腐蝕性的要求，有望應(yīng)用于傳統(tǒng)工模具的表面防護與修復(fù)。

等離子噴焊；高熵合金噴焊層；微觀組織；顯微硬度；耐腐蝕性；模具修復(fù)

等離子噴焊是表面改性技術(shù)中一種十分重要且發(fā)展迅速的方法，其優(yōu)點是具有高的能量密度，熱量集中且電弧穩(wěn)定，能夠獲得低稀釋率的致密涂層[1-2]。等離子噴焊熱量集中，冷卻速度快，可使合金粉末與基體充分形成熔池，且形成均勻細小的組織。熔池在氬氣和氮氣的雙重保護下，在加熱和反應(yīng)過程中都隔絕了空氣，在一定程度上避免了噴焊層元素受到氧化和燒損[3]。等離子噴焊工藝常常被應(yīng)用于零件表層修復(fù)與強化以及壽命優(yōu)化[4]，在工業(yè)生產(chǎn)中占據(jù)重要的地位[5-6]。

高熵合金（HEAs）作為一種新型涂層材料，由于其熱力學上的高熵效應(yīng)、動力學上的遲滯擴散效應(yīng)、結(jié)構(gòu)上的晶格畸變效應(yīng)與性能上的雞尾酒效應(yīng)等四大效應(yīng)[7]，使其具有較傳統(tǒng)涂層材料不同的性能特點。該合金可以同時具有高硬度、高耐磨性、高耐腐蝕性和高界面相容性等性能[8]。同時，還可以通過相組成預(yù)測和相圖模擬對合金元素進行優(yōu)化設(shè)計，具有很好的研究價值。Pan等[9]通過激光熔覆等方法成功在Q235低碳鋼表面制備了FeCoNiCrCu高熵合金涂層，涂層具有單一的FCC結(jié)構(gòu)，顯微硬度達到375HV。在該成分的基礎(chǔ)上添加Si、Mn和Mo元素，涂層的相結(jié)構(gòu)沒有明顯變化，但顯微硬度卻提高了20%，這就是由于高熵合金的晶格畸變效應(yīng)所導(dǎo)致的。Qiu等[10]通過對Al2CrFeCoCuTiNi高熵合金涂層的耐蝕性研究發(fā)現(xiàn)，隨著Ni含量的添加，高熵合金涂層在1 mol/L NaOH溶液中的耐蝕性呈現(xiàn)先升高后下降的趨勢，但是涂層整體的耐蝕性都遠優(yōu)于Q235低碳鋼基材。此外，在模擬酸性和海洋環(huán)境中，Co1.5CrFeNi1.5Ti0.5Mo（=0、0.1、0.5、0.8）高熵合金表現(xiàn)出良好的耐腐蝕性，表明添加Mo可以提高合金在NaCl溶液中的點蝕性[11]。與此同時，Mo的添加大大提高了CoCrFeNiW1?xMo（=0、0.5）高熵合金涂層在3.5%NaCl溶液中的耐蝕性[12]。研究普遍認為Mo可以提高合金在Cl?環(huán)境中的耐蝕性，多種耐蝕機制被提出，如Mo離子兼并到鈍化膜中可以提高鈍化膜的穩(wěn)定性，阻礙Cl?擊穿鈍化膜，提高鈍化膜的再鈍化能力[13-15]或者降低點蝕內(nèi)部合金的活化溶解[16-20]。本文擬研究Mo含量對FeCoCrNiMo高熵合金組織與耐腐蝕性的影響。

目前，制備高熵合金涂層的方式以激光熔覆為主，等離子噴焊工藝的研究較少。本文采用FeCoCrNiMo高熵合金粉末作為涂層材料，采用等離子噴焊技術(shù)制備高熵合金等離子噴焊層，研究不同Mo含量對焊層組織和耐腐蝕性的影響。

1 實驗

1.1 材料

實驗采用Q235A低碳鋼作為基體材料，用激光切割機設(shè)備切取150 mm×100 mm×8 mm的板材，并用角磨機、300# SiC砂紙、丙酮以及超聲波清洗儀對基材的表面進行嚴格的打磨、除油、除氧化層等處理。涂層粉末選用純度99.9%以上的Co、Cr、Fe、Ni、Mo粉末，經(jīng)球磨機械合金化后真空預(yù)燒結(jié)制備成粒，粒度為–160～ +200目。高熵合金粉末化學成分如表1所示。本次實驗主要研究不同Mo元素對高熵合金等離子噴焊層組織與性能的影響，Mo元素的添加比例為0、0.5、1、1.5（物質(zhì)的量比）。

表1 高熵合金FeCoCrNiMo的化學成分

Tab.1 Chemical composition of FeCoCrNiMox HEAs wt.%

1.2 方法

噴焊采用DML-VO3BD等離子噴焊設(shè)備，焊接參數(shù)如表2所示，等離子噴焊后焊層連續(xù)且致密，沒有出現(xiàn)宏觀氣孔和裂紋，焊后變形小。

表2 等離子噴焊工藝參數(shù)

Tab.2 Parameters of plasma spray welding

利用D/max 2550 XRD X射線衍射儀對噴焊層的組織進行物相分析。采用電火花切割機切取金相、電鏡以及電化學試樣。金相試樣的規(guī)格為15 mm×15 mm× 8 mm，電鏡試樣的規(guī)格為8 mm×8 mm×8 mm，電化學試樣的規(guī)格為10 mm×10 mm×8 mm。金相試樣打磨拋光后，選用王水作為腐蝕液腐蝕10～15 s，在金相顯微鏡（OM，4XFZ）下觀察組織，并拍攝組織圖片。電鏡試樣經(jīng)拋光后，采用型號FEG Quanta 250掃描電子顯微鏡（SEM）和能量色散x射線光譜儀（EDS）對高熵合金等離子噴焊層的顯微組織和成分進行分析。

采用HZr-1000型顯微硬度計測試試樣的顯微硬度，試驗力為9.807 N，加載15 s。對每個噴焊層區(qū)域測試5組數(shù)據(jù)，并取其平均值作為該試樣的顯微硬度值。

采用CS350電化學工作站（三電極體系）進行耐腐蝕性能測試，考慮到工模具材料在服役過程中的腐蝕環(huán)境以及常見腐蝕試劑的選擇，本實驗選取3.5%NaCl溶液作為近似的腐蝕液進行模擬，其中用鉑片電極作為輔助電極，利用魯金毛細管將參比電極和工作電極連接起來，參比電極為氧化汞，采用動電位極化曲線掃描法進行測試。同時，選取Q235A低碳鋼作為標準試樣作為參照，進一步研究該高熵合金等離子噴焊層的耐腐蝕性能。

2 結(jié)果與分析

2.1 XRD分析

圖1為添加不同含量Mo的高熵合金等離子噴焊層的XRD圖譜?？梢钥闯觯醇覯o的合金由于四元相互固溶，形成單一的FCC固溶體相。而隨著Mo含量的增加，逐漸形成了以Mo為主的FCC1固溶體相，主要成分可能為Ni-Co-Cr-Mo；還有以Fe、Ni為主的FCC2固溶體相，主要成分可能為Ni-Cr-Fe，同時也伴隨有少量的(Fe,Ni)固溶體新相的形成。這可能是因為合金粉末在高能等離子束的作用下快速升溫熔化，并以較高的冷卻速率冷卻，使得大量的過飽和固溶體被保留下來。當合金粉末中的Mo含量增加時，噴焊層中FCC1相的衍射峰強度增加，說明Mo含量的增加有利于促進FCC1固溶體相的凝固和生長。

圖1 噴焊層的XRD圖譜

2.2 噴焊層的組織分析

圖2為不同試樣噴焊層表層的顯微組織?？梢钥闯觯哽睾辖饑姾笇咏M織致密且宏觀分布均勻，基體與涂層結(jié)合面無開裂、夾雜、氣孔等缺陷。從圖2a可知，未加Mo的高熵合金噴焊層組織以等軸晶的形式存在，為Co、Cr、Fe、Ni四元在噴焊過程中形成的FCC固溶體[21]。從圖2b、圖2c、圖2d可知，當高熵合金噴焊層加入少量Mo元素時，顯微組織開始細化，出現(xiàn)大量的胞狀晶和少量的樹枝晶；當高熵合金噴焊層中的Mo含量不斷增加時，顯微組織中出現(xiàn)大量的樹枝晶，生長方向各異，組織進一步細化。這是由于在等離子噴焊的過程中，熔化和冷卻速率都較快，有效抑制了晶粒的生長。同時，由于Mo具有較高的熔點，成為非平衡凝固過程中形核的核心[22]，優(yōu)先形核，高的Mo含量提高了形核率，使得晶粒細化[23]。

圖2 噴焊層表層顯微組織圖

圖3為噴焊層的背散射電子顯微組織圖。表3為FeCoCrNiMo高熵合金噴焊層的EDS分析結(jié)果。從圖3和表3可知，顯微組織呈現(xiàn)出典型的胞狀枝晶組織結(jié)構(gòu)，枝晶內(nèi)和枝晶間區(qū)域存在明顯的圖像明暗差異，根據(jù)背散射電子的成像原理，原子序數(shù)越大，所產(chǎn)生的背散射電子越多，所以亮白色組織主要是富Mo相；灰色區(qū)域以富Fe、Ni為主。當Mo含量增加時，熔池中凝固產(chǎn)生的富Mo相增多，形成了以枝晶為特征的共晶組織[24]。隨著Mo含量的增加，噴焊層中的白色枝晶組織逐漸增加，而灰色晶間組織逐漸減少，見圖3b、圖3c和圖3d。根據(jù)之前的XRD以及EDS分析可以做出判斷，當Mo含量增加所產(chǎn)生的雙相結(jié)構(gòu)，一種為白色富Mo相，其成分可能為Ni-Co- Cr-Mo；另一種為灰色富Fe、Ni相，其成分可能為Ni-Cr-Fe，同時也產(chǎn)生了少量的(Fe,Ni)固溶體新相。

圖3 噴焊層的背散射電子顯微組織圖

表3 FeCoCrNiMo高熵合金噴焊層的EDS分析

Tab.3 EDS analysis of spray welding layer of FeCoCrNiMox HEAs

2.3 顯微硬度分析

圖4為噴焊層的截面顯微組織分布圖。圖5為基體到噴焊層的截面硬度分布圖，噴焊層的厚度可達2 mm左右。將位于0～2 mm處噴焊層的顯微硬度取平均值，得到噴焊層的平均顯微硬度。由圖5可知，隨著Mo元素的不斷增加，噴焊層的硬度不斷增加，Mo0、Mo0.5、Mo1、Mo1.5合金的平均顯微硬度分別為204.4、422.7、589.8、706.8HV0.2。其中Mo1.5的顯微硬度最高，可達770.3HV0.2，其平均顯微硬度約為基板（約180HV0.2）硬度的4倍。噴焊層顯微硬度高可能歸因于以下3點：（1）Mo（0.136 3 nm）元素的原子半徑顯著大于Co、Cr、Fe和Ni[25]，較大的原子尺寸差異導(dǎo)致合金內(nèi)部出現(xiàn)嚴重的晶格畸變，由于高熵合金的晶格畸變效應(yīng)，導(dǎo)致了噴焊層具有高硬度；（2）由于高熵合金的延遲擴散效應(yīng)[26]以及Mo元素與其他四元的熔點差異，使得噴焊層在冷卻過程中獲得大量以Mo為主的過飽和固溶體，并彌散分布于基體之中，阻礙位錯運動，導(dǎo)致硬度上升；（3）等離子噴焊過程中，Mo元素優(yōu)先形核，抑制晶體生長，提高整體形核率，細化組織，使得噴焊層保留了大量的細小晶粒，而細小的組織產(chǎn)生的晶粒細化效應(yīng)也是硬度逐漸增加的原因之一[27]。

圖4 噴焊層的截面顯微組織分布圖

圖5 基體到噴焊層的截面顯微硬度分布圖

2.4 噴焊層的耐蝕性

圖6為Q235A基材和FeCoCrNiMo（=0、0.5、1、1.5）高熵合金噴焊層在3.5%NaCl溶液中的動電位極化曲線。表4為4種試樣與基板在NaCl溶液中對應(yīng)的腐蝕參數(shù)。將4種高熵合金噴焊層與基板對比發(fā)現(xiàn)，噴焊層具有更高的腐蝕電位corr和更小的腐蝕電流corr。之后比較4種不同Mo含量的噴焊層發(fā)現(xiàn)，隨著Mo元素的增加，其表面耐腐蝕性能逐漸增加。其中Mo1.5具有最高的腐蝕電位corr和最小的腐蝕電流corr。同時發(fā)現(xiàn)，隨著Mo元素的增加，陽極極化鈍化區(qū)變寬，點蝕電位p也隨之增加，噴焊層的抗點蝕性能增加，展現(xiàn)出很好的整體耐腐蝕能力。噴焊層耐腐蝕能力的提高可能歸因于3個原因：（1）Mo、Cr、Ni元素可以在合金表面形成一層保護鈍化膜來提高耐腐蝕性[28-31]，且Mo離子可以提高鈍化膜的穩(wěn)定性，阻礙Cl?擊穿鈍化膜，提高鈍化膜的在鈍化能力或者降低點蝕內(nèi)部合金的活化溶解[13-15]，所以Mo、Cr、Ni元素本身就具備很好的耐腐蝕能力，可以有效地提高合金整體的耐腐蝕性能；（2）Mo和Cr在高電位下過鈍化溶解產(chǎn)生MoO42?和CrO42?，在這些離子的共同吸附作用下，可以提高活性位點的再鈍化能力，從而提高合金的抗點蝕性能[9-10]；（3）噴焊層中分布的組織細小，有利于致密鈍化膜的形成，從而提高整體耐腐蝕性。

圖6 Q235A基材和FeCoCrNiMox（x=0、0.5、1、1.5）高熵合金噴焊層在3.5%NaCl溶液中的塔菲爾曲線

表4 Q235A基材和FeCoCrNiMo（=0、0.5、1、1.5）高熵合金噴焊層在3.5%NaCl溶液中的電化學參數(shù)

Tab.4 Electrochemical parameters of Q235A substrate and FeCoCrNiMo x(x=0, 0.5, 1, 1.5) HEAs layer in the 3.5wt.%NaCl solution

基于以上研究可以確定Mo1和Mo1.5高熵合金噴焊層在微觀組織、顯微硬度以及耐腐蝕性等各方面都表現(xiàn)優(yōu)異。但是由于其噴焊工藝性較差，即Mo元素含量達到1.5時，易產(chǎn)生Mo的揮發(fā)，且焊接應(yīng)力增大，在噴焊過程中噴焊層結(jié)合面易出現(xiàn)宏觀開裂和表面龜裂，改進Mo1.5的工藝性能和在此基礎(chǔ)上進行更高Mo含量的研究是下一步的研究重點。

3 結(jié)論

1）采用等離子噴焊工藝在Q235A低碳鋼表面制備了不同Mo含量的高熵合金等離子噴焊層，所得噴焊層組織致密且無夾雜、氣孔等缺陷。未添加Mo元素的物相為FCC單相固溶體，含有Mo元素的物相包含2種不同的FCC相以及少量的(Fe,Ni)固溶體新相。

2）Mo元素在合金中可以起到細化晶粒、提高組織致密度的作用。噴焊層組織隨著Mo含量的增加，由等軸晶轉(zhuǎn)變?yōu)榘麪罹г俎D(zhuǎn)變?yōu)闃渲?，組織更加均勻致密。高倍顯微鏡下，噴焊層組織具有胞狀枝晶組織特征，當Mo含量增加時，枝晶內(nèi)的白色富Mo相增多，而枝晶間的灰色富Fe、Ni相減少。

3）Mo元素能夠有效地提高FeCoCrNiMo高熵合金噴焊層的硬度和耐腐蝕性能。=1.5時，高熵合金的平均硬度取得最大值706.8HV0.2，且此時高熵合金的耐腐蝕性也是最佳的，電化學腐蝕電位?0.412V，點蝕電位為?0.371 V，腐蝕電流密度為3.80×10?6A/cm2。

4）Mo1和Mo1.5高熵合金噴焊層在微觀組織、顯微硬度以及耐腐蝕性等各方面都表現(xiàn)優(yōu)異，但噴焊工藝性較差。改進Mo1.5的工藝性能和在此基礎(chǔ)上更高Mo含量的研究是下一步的研究重點。

[1] 趙洪運, 田澤, 賀文雄, 等. Q235鋼表面等離子噴焊鈷基自熔性高溫合金工藝分析[J]. 焊接學報, 2017, 38(2): 47-50, 56, 3.

ZHAO Hong-yun, TIAN Ze, HE Wen-xiong, et al. Proc-ess Research of Co-Based Coating on Q235 Steel by PTAW[J]. Transactions of the China Welding Institution, 2017, 38(2): 47-50, 56, 3.

[2] 徐紅勇. 等離子噴焊硬質(zhì)增強耐磨層組織與性能研究[D]. 長春: 吉林大學, 2016.

XU Hong-yong. Microstructures and Properties of Hard Phases Reinforced Wear-Resistant Layers Prepared by Plasma Spray Welding[D]. Changchun: Jilin University, 2016.

[3] 王光, 王洪福, 趙文, 等. Ni60-Cr3C2-WC/TiC等離子堆焊層耐磨性能的研究[J]. 粉末冶金工業(yè), 2019, 29(6): 23-27.

WANG Guang, WANG Hong-fu, ZHAO Wen, et al. Study on the Wear Resistance of Ni60-Cr3C2-WC/TiC Plasma Surfacing Layer[J]. Powder Metallurgy Industry, 2019, 29(6): 23-27.

[4] 鄧德偉, 陳蕊, 張洪潮. 等離子堆焊技術(shù)的現(xiàn)狀及發(fā)展趨勢[J]. 機械工程學報, 2013, 49(7): 106-112.

DENG De-wei, CHEN Rui, ZHANG Hong-chao. Present Status and Development Tendency of Plasma Transferred Arc Welding[J]. Journal of Mechanical Engineering, 2013, 49(7): 106-112.

[5] 汪錫恒, 宋琪, 潘德成, 等. MoSi2對等離子噴焊鈷基合金層組織和耐磨性的影響[J]. 熱處理, 2021, 36(3): 1-6.

WANG Xi-heng, SONG Qi, PAN De-cheng, et al. Effect of MoSi2on Microstructure and Wear Resistance of Cobalt- Base Alloy Coating Prepared by Plasma Spray Welding Technology[J]. Heat Treatment, 2021, 36(3): 1-6.

[6] BOTTO D, LAVELLA M. High Temperature Tribological Study of Cobalt-Based Coatings Reinforced with Diffe-rent Percentages of Alumina[J]. Wear, 2014, 318(1-2): 89-97.

[7] 趙雪柔, 呂煜坤, 石拓. 高熵合金相形成理論研究進展[J]. 材料導(dǎo)報, 2019, 33(7): 1174-1181.

ZHAO Xue-rou, LYU Yu-kun, SHI Tuo. Advances in the Study of Phase Formation Theory of High Entropy All-oys[J]. Materials Reports, 2019, 33(7): 1174-1181.

[8] 王成, 李先芬, 劉昊, 等. 激光熔覆Co1.5CrFeNi1.5Ti0.75Nb高熵合金的組織和性能研究[J]. 精密成形工程, 2021, 13(2): 81-86.

WANG Cheng, LI Xian-fen, LIU Hao, et al.Microstruc-ture and Properties of Laser Cladding Co1.5CrFeNi1.5Ti0.75NbHigh-Entropy Alloy[J]. Journal of Netshape Forming Engi-neering, 2021, 13(2): 81-86.

[9] ZHANG Hui, PAN Ye, HE Yi-zhu. Synthesis and Charac-terization of FeCoNiCrCu High-Entropy Alloy Coating by Laser Cladding[J]. Materials & Design, 2011, 32(4): 1910- 1915.

[10] QIU Xing-wu, LIU Chun-ge. Microstructure and Proper-ties of Al2CrFeCoCuTiNiHigh-Entropy Alloys Prepared by Laser Cladding[J]. Journal of Alloys and Compounds, 2013, 553: 216-220.

[11] CHOU Y L, YEH J W, SHIH H C. The Effect of Molyb-denum on the Corrosion Behaviour of the High-Entropy Alloys Co1.5CrFeNi1.5Ti0.5Moin Aqueous Environments[J]. Corrosion Science, 2010, 52(8): 2571-2581.

[12] SHANG Cai-yun, AXINTE E, SUN Jun, et al. CoCrFeNi (W1–xMo) High-Entropy Alloy Coatings with Excellent Mechanical Properties and Corrosion Resistance Prepared by Mechanical Alloying and Hot Pressing Sintering[J]. Materials & Design, 2017, 117: 193-202.

[13] HENDERSON J D, LI Xue-jie, SHOESMITH D W, et al. Molybdenum Surface Enrichment and Release during Transpassive Dissolution of Ni-Based Alloys[J]. Corro-sion Science, 2019, 147: 32-40.

[14] PARDO A, MERINO M C, COY A E, et al. Pitting Corro-sion Behaviour of Austenitic Stainless Steels-Combining Effects of Mn and Mo Additions[J]. Corrosion Science, 2008, 50(6): 1796-1806.

[15] YI J Z, HU H X, WANG Z B, et al. On the Critical Flow Velocity for Erosion-Corrosion of Ni-Based Alloys in a Saline-Sand Solution[J]. Wear, 2020, 458-459: 203417.

[16] LV Jin-long, LIANG Tong-xiang, WANG Chen. Surface Enriched Molybdenum Enhancing the Corrosion Resis-tance of 316L Stainless Steel[J]. Materials Letters, 2016, 171: 38-41.

[17] SONG Dan, HAO Jun, YANG Fa-lin, et al. Corrosion Behavior and Mechanism of Cr-Mo Alloyed Steel: Role of Ferrite/Bainite Duplex Microstructure[J]. Journal of Alloys and Compounds, 2019, 809: 151787.

[18] DONG Xin, SUN Qi, ZHOU Ya-nan, et al. Influence of Microstructure on Corrosion Behavior of Biomedical Co- Cr-Mo-W Alloy Fabricated by Selective Laser Melting[J]. Corrosion Science, 2020, 170: 108688.

[19] PISAREVSKII L A, FILIPPOV G A, LIPATOV A A. Eff-ect of N, Mo, and Si on Local Corrosion Resistance of Unstabilized Cr-Ni and Cr-Mn-Ni Austenitic Steels[J]. Metallurgist, 2016, 60(7): 822-831.

[20] MOON J, HA H Y, PARK S J, et al. Effect of Mo and Cr Additions on the Microstructure, Mechanical Properties and Pitting Corrosion Resistance of Austenitic Fe-30Mn- 10.5Al-1.1C Lightweight Steels[J]. Journal of Alloys and Compounds, 2019, 775: 1136-1146.

[21] CAI Yang-chuan, SHAN Meng-die, CUI Yan, et al. Mic-rostructure and Properties of FeCoCrNi High Entropy Alloy Produced by Laser Melting Deposition[J]. Journal of Alloys and Compounds, 2021, 887: 161323.

[22] 苗國策. 等離子噴焊Ni基WC增強耐磨涂層的研究[D]. 哈爾濱: 哈爾濱工業(yè)大學, 2013: 1-78.

MIAO Guo-ce. Study on Plasma Spray Welding of Ni- Based Wc Enhanced Wear-Resistance Coating[D]. Harbin: Harbin Institute of Technology, 2013: 1-78.

[23] 李明喜, 顧鳳麟, 李殿凱. 碳化硅增強鈷基合金等離子噴焊層組織與力學性能[J]. 焊接學報, 2015, 36(12): 35-38, 114.

LI Ming-xi, GU Feng-lin, LI Dian-kai. Microstructure and Mechanic Properties of SiC Reinforced Cobalt-Based Alloy Coatings Deposited by Plasma Transferred Arc Welding[J]. Transactions of the China Welding Institution, 2015, 36(12): 35-38, 114.

[24] CUI Gang, HAN Bin, ZHAO Jian-bo, et al. Microstruc-ture and Tribological Performance of Sulfurizing Layer Prepared on the Laser Cladding Co-Based Alloy Coating[J]. Surface and Coatings Technology, 2017, 331: 27-34.

[25] CHUANG Ming-hao, TSAI M H, WANG W R, et al. Microstructure and Wear Behavior of AlCo1.5CrFeNi1.5TiHigh-Entropy Alloys[J]. Acta Materialia, 2011, 59(16): 6308-6317.

[26] 邱星武, 張云鵬. 粉末冶金法制備CrFeNiCuMoCo高熵合金的組織與性能[J]. 粉末冶金材料科學與工程, 2012, 17(3): 377-382.

QIU Xing-wu, ZHANG Yun-peng. Microstructure and Properties of CrFeNiCuMoCo High-Entropy Alloy Prepa-red by Powder Metallurgy[J]. Materials Science and Engi-neering of Powder Metallurgy, 2012, 17(3): 377-382.

[27] ZUO Ting-ting, YANG Xiao, LIAW P K, et al. Influence of Bridgman Solidification on Microstructures and Mag-netic Behaviors of a Non-Equiatomic FeCoNiAlSi High- Entropy Alloy[J]. Intermetallics, 2015, 67: 171-176.

[28] ZHANG Xiang-rong, SHOESMITH D W. Influence of Temperature on Passive Film Properties on Ni-Cr-Mo Alloy C-2000[J]. Corrosion Science, 2013, 76: 424-431.

[29] MOURA V S, LIMA L D, PARDAL J M, et al. Influence of Microstructure on the Corrosion Resistance of the Duplex Stainless Steel UNS S31803[J]. Materials Charac-terization, 2008, 59(8): 1127-1132.

[30] POTGIETER J H, OLUBAMBI P A, CORNISH L, et al. Influence of Nickel Additions on the Corrosion Behaviour of Low Nitrogen 22% Cr Series Duplex Stainless Steels[J]. Corrosion Science, 2008, 50(9): 2572-2579.

[31] PARDAL J M, DA SILVA M R, BASTOS I N, et al. Influence of Tempering Treatment on Microstructure and Pitting Corrosion Resistance of a New Super Ferritic- Martensitic-Austenitic Stainless Steels with 17%Cr[J]. Corrosion Engineering, Science and Technology, 2016, 51(5): 337-341.

Effect of Mo Content on the Microstructure and Properties of High-entropy alloy Coatings Produced by Plasma Transferred Arc Welding

1a,1b,1a,1b,1a,1b,1a,1b,1a,1b,1a,1b,2

(1. a. School of Mechanical Engineering, b. Key Laboratory of Welding Robot and Application Technology of Hunan Province, Xiangtan University, Hunan Xiangtan 411105, China; 2. State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China)

Plasma spray welding is a very important and rapidly developing method in surface modifica-tion techno-logy. It can strengthen the surface properties without changing the matrix materials, such as improving the wear resistance,corrosion resistance, high temperature oxidation resistance and service life of materials. At the same time, as a new coating material, high entropy alloys (HEAs) has different performance characteristics from traditional coating materials because of its four effects: high entropy effect in thermody-namics, hysteresis diffusion effect in dynamics, lattice distortion effect in structure and cocktail effect in performance. The alloy can have high hardness, high wear resistance, high corrosion resistance and high inter-facial compatibility at the same time. At the same time, the alloy elements can be optimized by phase compo-sition prediction and phase diagram simulation, which has good research value. In this paper, FeCoCrNiMo high entropy alloy powder was used as coating material, and high entropy alloy plasma spray welding layer was prepared by plasma spray welding technology. The effects of different Mo content on the microstructure and corrosion resistance of the welding layer were studied.

High entropy alloy spray welding layers with different Mo content were prepared on the surface of Q235A low carbon steel by plasma spray welding technology. The microstructure and phase structure were characterized by X-ray diffraction (XRD), optical microscope (OM), scanning electron microscope (SEM) and energy dispersive X-ray spec-trometer (EDS). The hardness and corrosion resistance of the spray welded layer were tested by microhardness tester and electrochemical workstation. The results show that with the gradual increase of Mo content x from 0 to 1.5, the grain boundary cellular dendrite structure of spray welding layer gradually increases and the microstructure of the alloy becomes fine. By means of XRD and energy spectrum analysis, the dendrite is white Mo rich phase, and the dendrite is gray Fe and Ni rich phase. This is because the alloy powder rapidly heats up and melts under the action of high-energy plasma beam and cools at a higher cooling rate, so that a large number of supersaturated solid solutions are retained. When the Mo content in the alloy powder increases, the diffraction peak intensity of FCC1 phase in the spray welding layer increases, indicating that the increase of Mo content is conducive to promoting the solidification and growth of FCC1 solid solution phase. With the increase of Mo content, the hardness of spray welding layer increases from 204.4HV0.2 to 706.8HV0.2. This is because (1) the atomic radius of Mo (0.136 3 nm) element is significantly larger than that of Co, Cr, Fe and Ni. The large atomic size difference leads to serious lattice distortion in the alloy. Due to the lattice distortion effect of high entropy alloy, the spray welding layer has high hardness. (2) Due to the delayed diffusion effect of high entropy alloy and the melting point difference between Mo and other quaternary elements, a large number of supersaturated solid solutions dominated by Mo are obtained in the spray welding layer during cooling, which are dispersed in the matrix, hinder the dislocation movement and lead to the increase of hardness. (3) In the process of plasma spray welding, Mo element gives priority to nucleation, inhibits crystal growth, improves the overall nucleation rate and refines the structure, so that a large number of fine grains are retained in the spray welding layer, and the grain refinement effect produced by the fine structure is also one of the reasons for the gradual increase of hardness. At the same time, from the analysis of corrosion resistance, the content of Mo increases and the spray welding layer is 3.5wt.%. The corrosion potential increased from ?0.753 v to ?0.412 v, and the corrosion current density increased from 1.23×10?4a/cm2reduced to 3.80×10?6a/cm2, pitting potential increased from ?0.642 v to ?0.371 v. This is because (1) Mo, Cr and Ni elements can form a protective passivation film on the alloy surface to improve the corrosion resistance, and Mo ions can improve the stability of the passivation film, hinder Cl-breakdown of the passivation film, improve the passivation ability of the passivation film or reduce the activation and dissolution of the alloy in pitting corrosion. Therefore, Mo, Cr and Ni elementsthemselves have good corrosion resistance, It can effectively improve the overall corrosion resistance of the alloy; (2) MoO42?and CrO42?ions produced by overpassivation and dissolution of Mo and Cr at high potential can improve the re passiva-tion ability of active sites under the joint adsorption of these ions, so as to improve the pitting corrosion resistance of the alloy; (3) The microstructure distributed in the spray welding layer is fine, which is conducive to the formation of dense passive film, so as to improve the overall corrosion resistance.

In conclusion, the designed FeCoCrNiMoalloy and corresponding plasma spray welding process meet the requi-rements for high wear resistance and corrosion resistance of spray welding layer, and are expected to be applied to the surface protection and repair of traditional tools and dies.

plasma transferred arc welding; high-entropy alloy coatings; microstructure; microhardness; corrosion resis-tance; mold repair

TG174.442

A

1001-3660(2022)11-0279-08

10.16490/j.cnki.issn.1001-3660.2022.11.026

2021–08–18；

2022–02–19

2021-08-18；

2022-02-19

國家自然科學基金項目（51771237、51704257）；湖南省聯(lián)合基金項目（2019JJ60019）；青年科學基金項目（51704257）

The National Natural Science Foundation of China (51771237, 51704257); the Joint Fund of Hunan Province (2019JJ60019); the Youth Science Foundation (51704257)

張乾坤（1987—），男，博士研究生，主要研究方向為粉末冶金材料、金屬材料及超硬材料。

ZHANG Qian-kun (1987-), Male, Doctor student, Research focus: powder metallurgy, metals and superhard materials.

肖逸鋒（1975—），男，博士，教授，主要研究方向為表面工程技術(shù)和異種合金焊接等。

XIAO Yi-feng (1975-), Male, Ph. D., Professor, Research focus: surface engineering technology and dissimilar alloy welding, etc.

張乾坤, 唐俊, 肖逸鋒, 等. Mo含量對FeCoCrNiMo高熵合金等離子噴焊層組織與性能的影響[J]. 表面技術(shù), 2022, 51(11): 279-286.

Zhang Qian-kun, Tang Jun, Xiao Yi-feng, et al. Effect of Mo Content on the Microstructure and Properties of High-entropy alloy Coatings Produced by Plasma Transferred Arc Welding[J]. Surface Technology, 2022, 51(11): 279-286.

責任編輯：萬長清