史蒲英，劉向宏，王濤，王凱旋，李瑤，張豐收，何衛(wèi)鋒，李應(yīng)紅

激光沖擊近β型鈦合金的室溫拉伸和高周疲勞性能及其斷裂機理

史蒲英1,2，劉向宏2，王濤2，王凱旋2，李瑤2，張豐收2，何衛(wèi)鋒1，李應(yīng)紅1

（1.西安交通大學(xué) 機械工程學(xué)院 航空發(fā)動機研究所，西安 710049；2.西部超導(dǎo)材料科技股份有限公司，西安 710018）

提高近β鈦合金的強度和高周疲勞性能。對兩相區(qū)固溶時效熱處理后的TB6和Ti55531鈦合金鍛態(tài)材料表面進行了激光沖擊強化（LSP），并對強化前后的試樣進行室溫拉伸和高周疲勞試驗。采用掃描電鏡（SEM）對拉伸和高周疲勞斷口進行了觀察和分析。與未LSP的樣品相比，LSP后TB6和Ti55531鈦合金的抗拉強度（m）分別提高了25 MPa和25 MPa，提高比例分別為2.26%和2.02%；屈服強度（p0.2）分別降低了48 MPa和30 MPa，降低比例分別為4.58%和2.54%；斷面收縮率（）、延伸率（）和彈性模量略有提升。在低應(yīng)力水平下，LSP后Ti55531合金的疲勞壽命高于TB6合金，而在高應(yīng)力水平下，TB6合金具有略高于Ti55531合金的疲勞壽命。經(jīng)過和未經(jīng)LSP的TB6和Ti55531鈦合金的拉伸斷裂模式均為微孔聚集型韌性斷裂與沿晶脆性斷裂混合的斷裂模式，表面激光沖擊不改變其拉伸斷裂模式，近β鈦合金在不同應(yīng)力狀態(tài)的疲勞壽命差異與材料顯微組織差異導(dǎo)致的疲勞裂紋萌生和擴展速率不同有關(guān)。

激光沖擊（LSP）；近β型鈦合金；拉伸性能；高周疲勞；斷裂機理

近β型鈦合金具有強度高、斷裂韌性好、抗疲勞性能優(yōu)異、淬透深度大等特點，被廣泛用于飛機起落架、發(fā)動機短艙接頭、直升機旋翼系統(tǒng)中央件、連接件等關(guān)鍵承力零部件制造[1-5]。TB6（名義成分為Ti- 10V-2Fe-3Al）和Ti55531（Ti-5Al-5Mo-5V-3Cr-1Zr）鈦合金是2種典型的近β鈦合金[6-7]。在兩相區(qū)（α+β相區(qū)）固溶和時效熱處理（STA）過程中，可以實現(xiàn)近β型鈦合金良好的組織–性能匹配，使其擁有優(yōu)異的綜合性能[8-11]。但TB6合金中固有的Fe元素偏析易導(dǎo)致β斑的形成，β斑的存在嚴(yán)重降低材料的塑性和疲勞性能，因而在一定程度上限制了其在大型構(gòu)件中的應(yīng)用[12-13]。而Ti55531合金的發(fā)明，則很好地規(guī)避了Fe元素偏析問題，且該合金在與TB6近似的組織狀態(tài)下，具有更高的靜強度和更寬廣的塑韌性匹配窗口。激光沖擊強化（LSP）是近年來興起的一種材料表面強化方式[14-17]，被認(rèn)為可以有效提高材料的抗高周疲勞性能[18-22]。其通過誘導(dǎo)殘余壓應(yīng)力（CRS）、顯微組織改變和位錯密度的增加來實現(xiàn)的[23-24]。通常認(rèn)為，材料的抗拉強度和疲勞極限之間存在一定的關(guān)聯(lián)。一般情況下，材料抗拉強度越高，其疲勞極限也越高[25]。因此，可以通過提高材料的抗拉強度來提高其疲勞性能。

本文以2種近β型鈦合金TB6和Ti55531為研究對象，對材料表面進行激光沖擊強化，通過對比LSP前后材料的拉伸和疲勞性能，結(jié)合斷口觀察與分析，揭示激光沖擊強化對近β型鈦合金性能及斷裂機理的影響。

1 試驗

本研究用TB6和Ti55531鈦合金材料來自于西部超導(dǎo)材料科技股份有限公司生產(chǎn)的鍛制棒材。本研究TB6鈦合金主元素的化學(xué)成分（質(zhì)量分?jǐn)?shù)）為：V 9.0%～ 11.0%，F(xiàn)e 1.6%～2.2%，Al 2.6%～3.4%，Ti余量。Ti55531合金主元素的化學(xué)成分（質(zhì)量分?jǐn)?shù)）為：Al 4.0%～ 6.0%，Mo 4.5%～6.0%，Cr 2.0%～3.6%，F(xiàn)e 0.2%～0.5%，Zr 0.3%～2.0%，Ti余量。2種材料在兩相區(qū)固溶和時效熱處理后的顯微組織如圖1所示?？梢钥吹?，經(jīng)固溶時效熱處理后，2種材料均呈現(xiàn)等軸組織。等軸和棒狀初生α相彌散分布在轉(zhuǎn)變的β基體上。TB6材料的初生α相含量約為10%，Ti55531材料的初生α含量約為20%，TB6材料的初生α相尺寸和β晶粒尺寸均略大于Ti55531合金。2種材料的拉伸性能見表1。

圖1 兩相區(qū)固溶時效熱處理后TB6和Ti55531合金的顯微組織

表1 固溶時效熱處理后TB6和Ti55531合金的拉伸性能

Tab.1 Tensile properties of TB6 and Ti55531 alloy after solution and aging treatment

切取自2種鈦合金鍛棒的試樣坯在兩相區(qū)固溶和時效熱處理后，分別按照GB/T 228.1和HB5278加工拉伸試樣和高周疲勞試樣，并對加工樣品表面進行激光沖擊強化。樣品強化區(qū)域示意圖如圖2所示。激光沖擊強化采用西安天睿達光電技術(shù)股份有限公司的YS100-R200A型激光沖擊設(shè)備進行。激光沖擊過程采用單路沖擊，激光能量為(5±0.2) J，光斑直徑為(2.6±0.1) mm，光斑搭接率為50%，激光脈寬為18～ 20 ns，脈沖上升沿小于6 ns，約束層為水，強化次數(shù)為3次。采用X射線衍射儀對激光沖擊強化后的樣品表面進行殘余應(yīng)力測試。采用Zwick ZAVu-A型顯微維氏硬度儀按照GB/T 4340.1進行樣品維氏硬度測試。采用MTS拉伸試驗機進行室溫拉伸試驗，拉伸速率為0.015 mm/min。采用QBG–100型高頻試驗機進行高周疲勞試驗，高周疲勞試驗選用max=590 MPa、= ?1和max=1 040 MPa、=0.5兩個應(yīng)力水平和應(yīng)力比組合進行。采用ZEISS掃描電鏡對拉伸和疲勞斷口形貌進行觀察。

圖2 拉伸和高周疲勞試樣及LSP強化區(qū)域示意圖

2 結(jié)果及分析

2.1 樣品表面殘余應(yīng)力和維氏顯微硬度

試驗前對拉伸試樣激光沖擊區(qū)域的表面殘余應(yīng)力進行了測試，TB6試樣的表面殘余應(yīng)力為?639.06 MPa，Ti55531試樣的表面殘余應(yīng)力為?588.24 MPa。可見，經(jīng)過LSP 2種材料表面均產(chǎn)生了殘余壓應(yīng)力，TB6試樣表面的殘余壓應(yīng)力大于Ti55531試樣。測試了強化前后2種材料的維氏硬度，維氏硬度測試結(jié)果見圖3?？梢钥闯?，本研究Ti55531材料的初始硬度高于TB6樣品的硬度。經(jīng)LSP處理，2種材料的表面硬度均有所提高（如圖3a所示），LSP后Ti55531和TB6材料的表面硬度分別達到452HV0.1/10和400HV0.1/10，較LSP前分別增加了11.3%和12.1%。根據(jù)深度方向的硬度測試結(jié)果可以看出（如圖3b所示），2種材料的硬度在深度方向逐漸減小，TB6-LSP樣品和Ti55531- LSP樣品分別在900 μm和700 μm深度處，硬度與基體材料硬度一致。研究表明，顯微硬度的提高可歸因于激光表面噴丸過程中高強度沖擊波引起的材料嚴(yán)重塑性變形[24-25]，而塑性變形區(qū)域內(nèi)存在大量高密度位錯，導(dǎo)致材料的硬度升高。

圖3 LSP前后TB6和Ti55531材料的維氏硬度

2.2 拉伸性能與斷裂機理

2.2.1 拉伸性能

LSP前后TB6和Ti55531材料的室溫拉伸性能見表1和表2，LSP前后2種材料拉伸性能的比較如圖4所示。根據(jù)表1、表2和圖4可知，經(jīng)過LSP，TB6和Ti55531合金的抗拉強度（m）、延伸率（）、斷面收縮率（）有不同程度的增加，而屈服強度（p0.2）下降較明顯。其中，TB6合金的抗拉強度增加值為25 MPa，增加幅度為2.26%，屈服強度降低值為48 MPa，降低幅度為4.58%；Ti55531合金的抗拉強度增加值為25 MPa，增加幅度為2.02%，屈服強度降低值為30 MPa，降低幅度為2.54%。抗拉強度增加和屈服強度下降與激光沖擊強化過程在材料表面引入的殘余壓應(yīng)力，導(dǎo)致材料表面和近表面組織與應(yīng)力狀態(tài)發(fā)生變化有關(guān)[14,17-19]。研究發(fā)現(xiàn)，激光沖擊過程會在材料近表面幾十到幾百微米深度范圍內(nèi)形成壓應(yīng)力層，壓應(yīng)力值隨深度的增加而逐漸減小[15,17,19]。圖5為LSP處理后TB6材料的OM和EBSD圖像。可以看到，OM圖像中存在明顯的襯度差異區(qū)域，距LSP表面深度150～200 μm，在距離表面約50 μm內(nèi)，初生α相形貌與其他區(qū)域存在顯著差異，這與EBSD觀察到的組織差異類似，推斷均因LSP過程產(chǎn)生的殘余壓應(yīng)力所致。而在材料內(nèi)部，為平衡該壓應(yīng)力，則會出現(xiàn)拉應(yīng)力區(qū)域。材料拉伸過程中，內(nèi)部拉應(yīng)力水平最高的微區(qū)在外加應(yīng)力尚未達到材料屈服極限時就提前發(fā)生屈服，導(dǎo)致材料屈服強度降低[28]。隨著應(yīng)變增加，其他微區(qū)也相繼發(fā)生屈服，導(dǎo)致材料不斷屈服并最終斷裂。因此，激光沖擊強化試樣內(nèi)部殘余應(yīng)力的不均分布是影響拉伸屈服強度的主要原因。此外，激光沖擊過程會導(dǎo)致材料表面形成梯度組織層，梯度組織帶來的微區(qū)變形不協(xié)調(diào)也是導(dǎo)致微區(qū)提前屈服的原因[29]。而LSP帶來的抗拉強度增加，是因為經(jīng)過強化的表面層在拉伸過程中可以抵抗更高的應(yīng)力，這一點從LSP樣品斷口表面更劇烈的變形可以看出。

表2 LSP后TB6和Ti55531鈦合金拉伸試驗結(jié)果

Tab.2 Tensile test results of TB6 and Ti55531 titanium alloys after LSP

圖4 激光沖擊強化前后TB6和Ti55531拉伸性能比較

2.2.2 拉伸斷口觀察與斷裂機理分析

經(jīng)過和未經(jīng)LSP處理的TB6和Ti55531材料的拉伸斷口宏觀形貌如圖6所示?？梢钥闯?，經(jīng)過激光沖擊處理的試樣斷口均存在明顯頸縮，斷口起伏明顯，如圖6a和圖6b所示。而未處理的試樣，頸縮不明顯，斷口與正應(yīng)力呈45°方向斷裂，斷口相對平齊，如圖6c和圖6d所示。這說明經(jīng)過LSP的樣品在斷裂前經(jīng)歷了更大的塑性變形，材料抵抗變形的能力更強，剛度更高，這與LSP后2種合金拉伸樣品具有更大的延伸率（）、斷面收縮率（）和彈性模量值（）一致。

經(jīng)過和未經(jīng)LSP處理的TB6和Ti55531材料的拉伸斷口微觀形貌如圖7所示。可以看出，經(jīng)過和未經(jīng)LSP處理的2種鈦合金材料斷口均呈暗灰色，經(jīng)過LSP處理的斷口表面處變形和撕裂痕跡明顯，如圖7a2、圖7a3、圖7c2、圖7c3所示，斷面上的起伏也更明顯。但斷口均存在明顯的撕裂棱與解理刻面，說明經(jīng)過和未經(jīng)表面LSP處理的TB6和Ti55531合金均呈現(xiàn)韌性斷裂和沿晶斷裂混合的斷裂特征，表面LSP對2種材料的微觀斷裂機制沒有明顯影響。

圖5 TB6合金LSP試樣表面的應(yīng)力影響區(qū)域OM和EBSD圖像

圖6 TB6和Ti55531拉伸試樣和斷口宏觀形貌

圖7 LSP強化前后TB6和Ti55531拉伸斷口顯微組織形貌

2.3 高周疲勞性能與斷裂機理

2.3.1 高周疲勞性能

經(jīng)過LSP處理的TB6和Ti55531樣品高周疲勞試驗結(jié)果見表3?？梢钥闯觯诓煌瑧?yīng)力狀態(tài)下，2種材料的疲勞壽命存在差異。隨著應(yīng)力水平的增加，TB6和Ti55531合金的疲勞壽命均降低。在相對較低的應(yīng)力水平（=590 MPa，= ?1）下，強度更高的Ti55531合金具有更高的疲勞壽命，兩者的疲勞壽命分別達到7.52×104循環(huán)周次和2.646×105循環(huán)周次。而在較高的應(yīng)力水平（=1 050 MPa，=0.5）下，2種材料具有近似的疲勞壽命，TB6合金的疲勞壽命為7.42×104循環(huán)周次，而Ti55531合金的疲勞壽命為4.12×104循環(huán)周次。這與材料疲勞裂紋擴展過程中裂紋萌生和擴展速率差異及其在疲勞壽命構(gòu)成中的比例相關(guān)。高強韌鈦合金的疲勞行為研究表明[30-31]，疲勞裂紋萌生壽命占整體壽命的絕大部分。高周疲勞過程中，疲勞裂紋萌生壽命甚至可能高達疲勞總壽命的90%。鈦合金疲勞微裂紋萌生機制主要有[33-34]：表面駐留滑移帶開裂；晶界、亞晶界、相界、孿晶界等界面處堆積位錯致使應(yīng)力集中引起界面開裂，以及相內(nèi)滑移帶界面處位錯堆積引起裂紋萌生等。不同的顯微組織特征對鈦合金的高周疲勞裂紋萌生有顯著的影響[35]。對于TB6和Ti55531等亞穩(wěn)定β鈦合金，細(xì)小晶粒的雙態(tài)組織比晶粒粗大的片層組織有更高的高周疲勞裂紋萌生抗力。在低應(yīng)力水平下，裂紋萌生占疲勞壽命的絕大部分，強度高且晶粒細(xì)小的等軸晶組織，具有更好的抗裂紋萌生能力。而在高應(yīng)力水平下，疲勞裂紋擴展壽命是材料疲勞壽命的重要組成部分。晶粒粗大的細(xì)片層組織具有更高的抗疲勞裂紋擴展能力。

表3 TB6和Ti55531材料的高周疲勞性能測試結(jié)果

Tab.3 High cycle fatigue properties of TB6 and Ti55531 alloy

2.3.2 高周疲勞斷口觀察與斷裂機理分析

圖8為經(jīng)LSP處理的TB6和Ti55531材料疲勞斷口形貌。可以看出，在不同應(yīng)力狀態(tài)下，2種材料的疲勞均起源于樣品表面，且均為單源疲勞。在本研究中激光沖擊強化并未實現(xiàn)材料的疲勞源從表面向近表面轉(zhuǎn)移，這與強化后樣品表面不完整性及強化參數(shù)的適宜性相關(guān)（對試樣表面觀察發(fā)現(xiàn)，LSP后的樣品表面粗糙度增加，且存在少量燒蝕坑）。對2種材料疲勞擴展區(qū)微觀形貌的觀察可知，疲勞裂紋沿晶和穿晶擴展，并在擴展區(qū)形成明顯的疲勞臺階和疲勞條帶。普遍認(rèn)為，高應(yīng)力狀態(tài)下，材料的疲勞壽命主要取決于長裂紋的擴展壽命。對距離裂紋源0.5 mm處的疲勞條帶寬度統(tǒng)計發(fā)現(xiàn)，Ti55531合金的疲勞條帶平均寬度為0.2 μm，TB6合金的疲勞條帶平均寬度為0.16 μm。TB6合金的疲勞裂紋擴展速率低于Ti55531合金，這與高應(yīng)力水平下，TB6合金具有略長的疲勞壽命一致。進一步分析表明，含有等軸αp相的等軸/雙態(tài)組織在疲勞過程中會產(chǎn)生較為分散的孔洞，這些孔洞可能成為裂紋源?？偟膩碚f，由于疲勞損傷的影響因素很多，且高強韌鈦合金具有特殊的成分及組織特征，所以高強韌鈦合金的疲勞裂紋萌生機制也異常復(fù)雜。

圖8 LSP處理的TB6和Ti55531材料疲勞斷口形貌

a.1,a.2 and b.1,b.2 lower stress state (=590 MPa,= ?1) a.3,a.4 and b.3,b.4 higher stress state (=1 050 MPa,=0.5)

3 結(jié)論

1）激光沖擊強化（LSP）導(dǎo)致近β型鈦合金TB6和Ti55531的室溫抗拉強度升高，屈服強度降低。LSP后TB6合金的抗拉強度較LSP前增加25 MPa，增加幅度為2.26%，屈服強度降低48 MPa，降低幅度為4.58%。LSP后Ti55531合金的抗拉強度較LSP前增加25 MPa，增加幅度為2.02%，屈服強度降低30 MPa，降低幅度為2.54%。LSP過程在材料表面引入的不均勻分布?xì)堄鄩簯?yīng)力和形成的表面梯度組織微區(qū)變形不協(xié)調(diào)是導(dǎo)致材料屈服強度下降的主要原因。

2）經(jīng)過和未經(jīng)LSP的TB6和Ti55531鈦合金的拉伸斷裂模式為微孔聚集型韌性斷裂與沿晶脆性斷裂混合的斷裂模式，本研究過程中LSP未改變材料的拉伸斷裂模式。

3）經(jīng)過LSP的TB6和Ti55531材料在不同載荷條件下均呈現(xiàn)單源疲勞，疲勞裂紋均起始于樣品表面。在低應(yīng)力水平下，抗拉強度更高的Ti55531合金具有較TB6鈦合金更高的疲勞壽命。而在高應(yīng)力水平下，晶粒更粗大的TB6鈦合金的高周疲勞壽命略高于Ti55531合金，這與2種材料顯微組織差異導(dǎo)致的疲勞裂紋萌生和擴展速率不同及其對疲勞壽命的貢獻有關(guān)。

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Tensile and High Cycle Fatigue Properties and Fracture Mechanism of Near β Titanium Alloy Strengthened by Laser Shock Peening

1,2,2,2,2,2,2,1,1

(1. School of Mechanical Engineering, Xi’an Jiaotong University, Xi’an 710049, China; 2. Western Superconducting Technologies Co., Ltd., Xi'an 710018, China)

Advanced aircraft has put forward the requirements of lightweight, high strength and high reliability for the materials used in its key structural parts. Near-β titanium alloy has the characteristics of high strength, good fracture toughness, excellent fatigue resistance, large quenching depth and so on, is widely used in the manufacture of key bearing parts such as aircraft landing gear, helicopter rotor system central parts and connectors. TB6 (nominal composition Ti-10V-2Fe-3Al) and Ti55531 (Ti-5Al-5Mo-5V-3CR-1Zr) are two typical near-β titanium alloys. Laser shock strengt-hening (LSP) is a surface treatment that can improve the fatigue life by inducing residual compressive stress (CRS), which cause change of microstructure and increase of dislocation density on the surface of the material, thus reduce the probability of fatigue crack initiation and propagation at surface.

In this study, laser shock processing (LSP) was carried out on the surface of as-forged TB6 and Ti5553 titanium alloy after solution aging treatment in two-phase region. X-ray diffractometer and Vickers hardness tester were used to measure the residual stress and Vickers micro-hardness on the surface and along the depth of the sample. The results display that surface residual stress of TB6 sample is ?639.06 MPa, and that of Ti55531 sample is ?588.24 MPa. The surface hardness of Ti55531 and TB6 after LSP reaches 452HV0.1/10 and 400HV0.1/10, respectively, the value increases by 11.3% and 12.1% respectively compared with that before LSP. The hardness of the TB6-LSP sample and Ti55531-LSP sample at the depth of 900 μm and 700 μm respectively is consistent with the hardness of the matrix material.

Tensile tests at room temperature and high cycle fatigue tests were carried out on the specimens before and after laser shock processing. The tensile and high cycle fatigue fractures were observed and analyzed by scanning electron microscopy (SEM). Compared with the sample without LSP, the tensile strength (m) of LSP’ed TB6 and Ti55531 titanium alloys increased by 25 MPa (2.26%) and 25 MPa (2.02%), respectively. The yield strength (p0.2) decreased by 48 MPa (4.58%) and 30 MPa (2.54%), respectively. The percentage reduction of area (), elongation () and elastic modulus () were slightly improved. Fatigue life of Ti55531 alloy after LSP is higher than that of TB6 alloy at low stress level, while the fatigue life of TB6 alloy is slightly higher than that of Ti55531 alloy at high stress level.

The increase of micro-hardness can be attributed to the severe plastic deformation of the material caused by the high- intensity shock wave in the process of laser surface shot peening, and there are a lot of high-density dislocations in the plastic deformation region, which leads to the increase of the hardness of the material. The tensile fracture modes of TB6 and Ti55531 titanium alloys with and without LSP are microporous aggregated ductile fracture and inter-granular brittle fracture. Surface laser shock processing does not change the tensile fracture mode of the alloys in this study. The difference of fatigue life of near β titanium alloy under different stress states is related to the difference of fatigue crack initiation and propagation rate caused by the difference of material microstructure.

laser shock processing (lsp); near β titanium alloy; tensile properties; high cycle fatigue; fracture mechanism

V261.8

A

1001-3660(2022)10-0058-08

10.16490/j.cnki.issn.1001-3660.2022.10.007

2022–07–27；

2022–09–15

2022-07-27；

2022-09-15

陜西省重點研發(fā)計劃項目（2020GY-259）

Key Research and Development Program of Shaanxi Province (2020GY-259)

史蒲英（1984—），女，博士生，高級工程師，主要研究方向為鈦合金材料組織性能關(guān)系、表面工程等。

SHI Pu-ying (1984-), Female, Ph. D. student, Senior engineer, Research focus: the relationship between microstructure and proper-ties of titanium alloy materials, surface engineering, etc.

史蒲英, 劉向宏, 王濤, 等. 激光沖擊近β型鈦合金的室溫拉伸和高周疲勞性能及其斷裂機理[J]. 表面技術(shù), 2022, 51(10): 58-65.

SHI Pu-ying, LIU Xiang-hong, WANG Tao, et al. Tensile and High Cycle Fatigue Properties and Fracture Mechanism of Near β Titanium Alloy Strengthened by Laser Shock Peening[J]. Surface Technology, 2022, 51(10): 58-65.

責(zé)任編輯：萬長清