解朋朋，曹宇鵬,2,3，花國(guó)然，楊聰，朱鵬飛

激光沖擊E690高強(qiáng)鋼Ostwald熟化現(xiàn)象的試驗(yàn)研究

解朋朋1，曹宇鵬1,2,3，花國(guó)然1，楊聰1，朱鵬飛1

（1.南通大學(xué) 機(jī)械工程學(xué)院，江蘇 南通 226019；2.南通理工學(xué)院 3D打印技術(shù)研究所，江蘇 南通 226001；3.南通中遠(yuǎn)海運(yùn)船務(wù)工程有限公司，江蘇 南通 226006）

研究功率密度對(duì)激光沖擊E690高強(qiáng)鋼表面Ostwald熟化現(xiàn)象的影響。根據(jù)理論分析激光沖擊金屬材料與產(chǎn)生調(diào)幅分解的內(nèi)在聯(lián)系，提出因激光沖擊強(qiáng)化產(chǎn)生 Ostwald熟化現(xiàn)象所需要的條件。使用場(chǎng)發(fā)式透射電鏡（TEM）獲取激光沖擊E690高強(qiáng)鋼試樣表面微觀組織結(jié)構(gòu)和選區(qū)電子衍射花樣，觀測(cè)不同功率密度的TEM形貌相中晶粒尺寸的變化特征，以及Ostwald熟化現(xiàn)象驗(yàn)證。通過TEM形貌像可以看出，E690高強(qiáng)鋼基材是由鐵素體層與滲碳體層交替重疊組成的珠光體形貌，在激光沖擊強(qiáng)化作用下，發(fā)生了晶粒細(xì)化，薄層滲碳體逐漸消失，電子衍射花樣逐漸呈圓環(huán)狀變化。當(dāng)激光功率密度上升至4.07 GW/cm2時(shí)，持續(xù)細(xì)化的材料發(fā)生粗化，出現(xiàn)調(diào)幅分解組織，選區(qū)電子衍射花樣中出現(xiàn)衛(wèi)星斑，E690高強(qiáng)鋼表面發(fā)生了Ostwald熟化現(xiàn)象。當(dāng)激光功率密度達(dá)到5.09 GW/cm2，E690高強(qiáng)鋼表層產(chǎn)生了納米晶。較弱和較強(qiáng)的功率密度都不能使脫溶物到達(dá)發(fā)生Ostwald熟化機(jī)制的臨界半徑，Ostwald熟化現(xiàn)象與納米晶相鄰出現(xiàn)。

激光沖擊強(qiáng)化；Ostwald熟化；E690高強(qiáng)鋼；晶粒細(xì)化；微結(jié)構(gòu)

當(dāng)脫溶沉淀從晶粒中析出的時(shí)候，由于系統(tǒng)中眾多的第二相顆粒導(dǎo)致界面的大量存在，使材料中界面能維持在較高的水平，而沒有達(dá)到最低的能量狀態(tài)，一些具有高能的因素使得小于臨界面積的脫溶物逐漸消融，大于臨界面積的粒子逐漸長(zhǎng)大，導(dǎo)致界面面積的減小，這種現(xiàn)象一般稱為粗化或者Ostwald熟化現(xiàn)象[1-4]。激光沖擊強(qiáng)化作為一種具有強(qiáng)大潛力與應(yīng)用前景的材料表層改性技術(shù)，是通過高能激光照射材料表面涂覆的吸收層，利用高壓等離子體產(chǎn)生爆轟波的力學(xué)效應(yīng)作用在材料表層，高應(yīng)變率使材料發(fā)生晶粒細(xì)化甚至形成納米晶，從而提高材料的硬度、抗磨損等性能[5-9]。筆者課題組前期開展了對(duì)E690高強(qiáng)鋼一系列的研究，通過仿真與試驗(yàn)相結(jié)合的方式研究了激光沖擊前后殘余應(yīng)力的變化，進(jìn)行了不同功率密度激光沖擊E690微結(jié)構(gòu)的研究，從位錯(cuò)組態(tài)與晶粒細(xì)化角度研究了激光與材料相互作用形式，并且還開展了激光沖擊微造型對(duì)減摩潤(rùn)滑的研究，并證明了激光沖擊微造型提高了E690高強(qiáng)鋼的摩擦學(xué)性能[10-12]。

同時(shí)，金屬材料中Ostwald熟化吸引了世界上眾多學(xué)者對(duì)其進(jìn)行探索研究。楊洪波等[13]研究了GCr15軸承鋼中滲碳體球化的長(zhǎng)大機(jī)制，結(jié)果顯示，滲碳體球化長(zhǎng)大是由于Ostwald機(jī)制形成的。尹鴻翔等[14]通過原子探針層析技術(shù)和高分辨透射電子顯微技術(shù)對(duì)鐵素體中銅析出相進(jìn)行了研究，根據(jù)時(shí)效時(shí)長(zhǎng)的增長(zhǎng)，銅析出相發(fā)生了粗化現(xiàn)象。Jiang等[15]綜合研究了Ti-14Cu合金相對(duì)于Ti2Cu相在長(zhǎng)期等溫?zé)岜┞逗蟮拇只袨?，觀察到合金在穩(wěn)定粗化階段受Ostwald熟化機(jī)制控制，隨著穩(wěn)定粗化，Ti-14Cu合金中高體積Ti2Cu相的存在增加了變形期間的有效滑移長(zhǎng)度，并降低了塑性。Badykaa等[16]對(duì)時(shí)效過程中鑄造奧氏體不銹鋼的鐵素體相發(fā)生調(diào)幅分解進(jìn)行了研究，并且對(duì)比了不同元素析出相對(duì)調(diào)幅分解速度的影響。激光沖擊加載的時(shí)間極短（納秒級(jí)），而載荷極大（吉帕級(jí)），試樣在激光沖擊處理后發(fā)生了無需形核的調(diào)幅分解，進(jìn)而誘發(fā)了Ostwald熟化現(xiàn)象。相對(duì)于由熱處理引起Ostwald熟化現(xiàn)象，激光沖擊強(qiáng)化引起的沖擊相變極為復(fù)雜，研究其誘發(fā)的Ostwald熟化現(xiàn)象對(duì)激光沖擊強(qiáng)化技術(shù)具有一定的理論意義。

本文對(duì)激光沖擊E690高強(qiáng)鋼晶粒尺寸影響過程進(jìn)行分析，觀察到晶粒尺寸不隨著激光功率密度的增加而細(xì)化。在4.07 GW/cm2時(shí)，觀察到了E690高強(qiáng)鋼發(fā)生調(diào)幅分解，并導(dǎo)致Ostwald熟化現(xiàn)象的產(chǎn)生。通過理論與試驗(yàn)相結(jié)合，探究Ostwald熟化產(chǎn)生所需要的激光沖擊能量。通過場(chǎng)發(fā)式透射電鏡對(duì)不同功率密度下試樣TEM形貌像和選區(qū)電子衍射進(jìn)行分析，探究不同功率密度沖擊后的E690高強(qiáng)鋼中材料微觀結(jié)構(gòu)的變化，并驗(yàn)證了Ostwald熟化現(xiàn)象的存在。通過TEM形貌像驗(yàn)證Ostwald熟化現(xiàn)象前后試樣表面晶粒尺寸變化，為科學(xué)研究激光沖擊E690表層微觀結(jié)構(gòu)變化，優(yōu)化海工平臺(tái)裝備性能提供理論基礎(chǔ)。

1 調(diào)幅分解與“Ostwald熟化”關(guān)系分析

激光作用在材料過程中具有納秒級(jí)時(shí)間、超高的應(yīng)變率與極高壓力的特點(diǎn)。在高應(yīng)變率影響下，塑性變形使材料溫度升高，沖擊波與金屬材料彼此作用過程中，晶體缺陷增加使組織不穩(wěn)定性升高，容易受到其他因素影響發(fā)生調(diào)幅分解，導(dǎo)致其Gibbs能變化，Δ為[17-18]：

式中：0為母相的平均成分；Δ為激光沖擊引起的成分起伏；(0)為摩爾Gibbs能。

用三階泰勒公式將(0+Δ)和(0－Δ)展開，最終可得：

式中：(2)(0)、(4)(0)分別為Δ的二階導(dǎo)數(shù)和四階導(dǎo)數(shù)。

分析式（2）可知，系統(tǒng)的Gibbs能與(2)(0)的取值相關(guān)，若(2)(0)>0，則系統(tǒng)的Gibbs能上升；反之則減小。

在激光沖擊作用下，塑性應(yīng)變能向熱能轉(zhuǎn)變會(huì)使材料各相Gibbs能增加，直到合金的Gibbs能曲線具有負(fù)曲率時(shí)發(fā)生調(diào)幅分解。在激光沖擊中形成調(diào)幅分解時(shí)不需要形核，這種相變不因界面的產(chǎn)生和遷移而發(fā)生，而是均勻分布在合金中。在E690高強(qiáng)鋼表面晶粒調(diào)幅分解后期，材料處于較高能態(tài)，同時(shí)其調(diào)幅分解產(chǎn)物（脫溶物）的析出會(huì)使E690高強(qiáng)鋼具有更大的界面能。為減小材料整體內(nèi)部能量，小尺寸的顆?？梢赃M(jìn)入一些粗大的粒子中，導(dǎo)致后者產(chǎn)生尺寸增大現(xiàn)象，即發(fā)生了“奧斯特瓦爾德熟化（Ostwald Ripening）”[18-19]。當(dāng)E690高強(qiáng)鋼在激光沖擊加載過程中發(fā)生調(diào)幅分解時(shí)，將出現(xiàn)許多細(xì)小脫溶物，滿足了Ostwald熟化發(fā)生條件，相鄰晶粒尺寸差異大，界面能較大，將發(fā)生Ostwald熟化。

2 試驗(yàn)方案設(shè)計(jì)

E690高強(qiáng)鋼作為本次試驗(yàn)材料，其力學(xué)性能與元素組成（質(zhì)量分?jǐn)?shù)）為：C ≤1.72%，Si ≤0.45%，Mn ≤1.24%，Cr ≤0.74%，屈服強(qiáng)度為690 MPa，抗拉強(qiáng)度為835 MPa。通過線切割裝置將E690高強(qiáng)鋼加工成50 mm×50 mm×5.5 mm試樣，使用240#—1200#砂紙對(duì)試樣正反兩面研磨至厚度為5 mm。吸收層使用150 μm厚的鋁箔，約束層為去離子水。

激光沖擊試驗(yàn)使用ND:YAG固體激光器（SGR系列，Beamtech公司，中國(guó)），激光沖擊的具體參數(shù)：脈寬為10 ns，波長(zhǎng)為1 064 nm，光斑直徑為5 mm，分別采用3、3.89、5.43、8、10 J的能量，對(duì)應(yīng)激光功率密度分別為1.53、1.98、2.77、4.07、5.09 GW/cm2，搭接率為70%，沖擊次數(shù)為1次，沖擊區(qū)域以光斑中心構(gòu)成的20 mm×20 mm正方形，沖擊區(qū)域與光斑搭接方案如圖1所示。

先用分析純乙醇浸泡試樣，隨后利用超聲清洗并冷風(fēng)風(fēng)干。E690高強(qiáng)鋼線切割后，從試樣基體側(cè)預(yù)減薄，然后經(jīng)凹坑研磨，最后進(jìn)行離子減薄，制成TEM薄膜試樣。使用透射電子顯微電鏡（Tecnai G2 F20，F(xiàn)EI公司，美國(guó)）觀察試樣表層的微觀形貌和選區(qū)電子衍射。

圖1 激光沖擊區(qū)域與光斑搭接方案

3 TEM形貌像分析

3.1 E690高強(qiáng)鋼基體

E690高強(qiáng)鋼基材結(jié)構(gòu)的TEM形貌如圖2所示。根據(jù)圖2可以看出，E690高強(qiáng)鋼基體結(jié)構(gòu)是由薄層滲碳體和薄層鐵素體2種相組成的混合物，亦稱片狀珠光體。珠光體中清晰可見板條狀鐵素體和薄層滲碳體交替排列，2種相分布距離在160～500 nm。

3.2 不同功率密度激光沖擊下E690高強(qiáng)鋼TEM形貌像分析與選區(qū)電子衍射標(biāo)定

E690高強(qiáng)鋼經(jīng)過激光功率密度為 1.53 GW/cm2沖擊后的 TEM 形貌像和對(duì)應(yīng)的電子衍射花樣如圖3所示。由圖3a可知，經(jīng)過1.53 GW/cm2的激光沖擊加載后，材料的滲碳體區(qū)域明顯減少，鐵素體和滲碳體兩相邊界逐漸模糊，但整體依舊呈現(xiàn)基體中類似的兩相相互疊加而成的層狀混合物，并且局部區(qū)域的晶粒開始出現(xiàn)細(xì)化現(xiàn)象。對(duì)該位置進(jìn)行選區(qū)電子衍射，然后利用特征平行四邊形法則[20-21]對(duì)電子衍射花樣進(jìn)行標(biāo)定，如圖3b所示。對(duì)比分析標(biāo)定的結(jié)果可知，可以確定該區(qū)域呈現(xiàn)出珠光體與微量奧氏體的復(fù)相疊加，其發(fā)生衍射的晶面中晶帶軸指數(shù)為[111]方向。衍射花樣標(biāo)定后可以看出，晶粒呈現(xiàn)出體心立方晶格，結(jié)合形貌像可以判斷該衍射區(qū)域存在鐵素體。對(duì)另一套衍射花樣標(biāo)定分析可以確定衍射晶面中晶帶軸指數(shù)為[125]方向，表明此處晶粒呈現(xiàn)面心立方晶格。結(jié)合圖3a和沖擊相變的相關(guān)理論可推知，該衍射區(qū)域存在殘余奧氏體。此外，圖3b中僅有少數(shù)衍射斑向圓弧狀變化，說明在1.53 GW/cm2的激光加載下，E690高強(qiáng)鋼晶粒細(xì)化不明顯。

圖2 E690高強(qiáng)鋼基體組織TEM形貌像

圖3 激光功率密度1.53 GW/cm2時(shí)的TEM形貌和選區(qū)電子衍射圖

E690高強(qiáng)鋼經(jīng)過激光功率密度為1.98 GW/cm2沖擊后的TEM形貌像和對(duì)應(yīng)的電子衍射花樣如圖4所示。由圖4a可知，在1.98 GW/cm2的激光功率密度作用下，激光沖擊的高應(yīng)變率作用形成的馬氏體組織相互擠壓，致使原本形態(tài)改變，區(qū)域內(nèi)位錯(cuò)分布均勻，原先的滲碳體薄層基本消失，剩下的滲碳體聚集在馬氏體晶界處。圖4b為圖4a中的選取電子衍射圖，標(biāo)定分析該選區(qū)為2種相的疊加，其中晶帶軸指數(shù)為[011]方向的衍射斑點(diǎn)表明此處晶粒為體心立方晶格，可以判斷出該區(qū)域鐵素體經(jīng)激光沖擊形成了BCC（體心立方）結(jié)構(gòu)位錯(cuò)型馬氏體。馬氏體的晶體結(jié)構(gòu)常為BCC、BCT（體心四方）結(jié)構(gòu)。在塑性變形的過程中，F(xiàn)CC結(jié)構(gòu)的奧氏體既可以轉(zhuǎn)變成BCC結(jié)構(gòu)的馬氏體，也可以轉(zhuǎn)變成BCT結(jié)構(gòu)的馬氏體，并且它們之間可以相互轉(zhuǎn)化和共存[22]。晶帶軸指數(shù)為[125]的衍射斑點(diǎn)表現(xiàn)為面心立方晶格，且衍射斑亮度較暗，可推知此選區(qū)仍然存在微量的殘余奧氏體。與功率密度1.53 GW/cm2激光沖擊處理后試樣的TEM形貌像相比，1.98 GW/cm2激光沖擊處理后，試樣表面的珠光體形貌基本消失，滲碳體聚集在馬氏體晶界處，位錯(cuò)明顯增殖。

E690高強(qiáng)鋼經(jīng)過激光功率密度為2.77 GW/cm2沖擊后的TEM形貌像和對(duì)應(yīng)的電子衍射花樣如圖5所示。觀察圖5a可知，位錯(cuò)分布明顯增殖，E690高強(qiáng)鋼表層晶粒繼續(xù)保持細(xì)化趨勢(shì)，此時(shí)有更多的滲碳體組織融進(jìn)晶體內(nèi)部，此時(shí)晶粒尺寸分布在200 nm以內(nèi)。圖5b為圖5a的電子衍射圖，標(biāo)定分析該選區(qū)為2個(gè)體心立方的衍射斑點(diǎn)，且晶帶軸指數(shù)為[100]方向，可以判斷這是由2個(gè)馬氏體晶粒組成的。2套電子衍射花樣的角度為6.4°，表明這2個(gè)晶粒經(jīng)過劇烈塑性應(yīng)變后形成了取向差。此外，通過圖5b可以看出，衍射斑有不斷向圓環(huán)狀演化的趨勢(shì)。與功率密度1.98 GW/cm2激光沖擊處理后試樣的TEM形貌像相比，經(jīng)2.77 GW/cm2激光沖擊處理后，E690高強(qiáng)鋼試樣表層的晶粒進(jìn)一步細(xì)化。

E690高強(qiáng)鋼經(jīng)過激光功率密度為4.07 GW/cm2沖擊后的TEM形貌像和對(duì)應(yīng)的電子衍射花樣如圖6所示。從圖6a可以觀察到，經(jīng)過此次沖擊波加載后，材料中的第二相顆粒增多，一些滲碳體在晶粒內(nèi)形成偏聚，晶粒尺寸分布在200～300 nm，沒有持續(xù)細(xì)化。圖6b為對(duì)應(yīng)的選區(qū)電子衍射花樣，根據(jù)標(biāo)定可以看出，該區(qū)域仍然為2套標(biāo)準(zhǔn)的體心立方晶格，晶帶軸指數(shù)為[111]方向，判斷此處為仍然是2個(gè)馬氏體晶粒組成。與功率密度2.77 GW/cm2激光沖擊處理后試樣的TEM形貌像相比，4.07 GW/cm2激光沖擊處理后，試樣表面的晶粒呈小角度晶界向大角度轉(zhuǎn)化的趨勢(shì)，2個(gè)晶粒的取向角差增大到7.4°。在此功率密度下進(jìn)一步觀察TEM形貌像，失穩(wěn)分解組織發(fā)展成為均勻分散的兩相結(jié)構(gòu)，典型的明暗相間的波紋組織消失，如圖6c所示。該形貌像特征表明該區(qū)域發(fā)生調(diào)幅分解。對(duì)該選區(qū)進(jìn)行電子衍射分析（如圖6d所示），選區(qū)內(nèi)出現(xiàn)了衛(wèi)星斑，由形貌像和選取電子衍射表明，激光在4.07 GW/cm2功率密度下調(diào)幅分解長(zhǎng)大[23-26]。

圖4 激光功率密度1.98 GW/cm2時(shí)的TEM形貌像和選區(qū)電子衍射圖

圖5 激光功率密度 2.77 GW/cm2時(shí)的TEM形貌像和選區(qū)電子衍射圖

圖6 激光功率密度4.07 GW/cm2時(shí)的TEM形貌像、選區(qū)電子衍射圖、調(diào)幅分解和衛(wèi)星斑

E690高強(qiáng)鋼經(jīng)過激光功率密度為5.09 GW/cm2沖擊后的TEM形貌像和對(duì)應(yīng)的電子衍射花樣如圖7所示。觀察圖7a可知，晶粒尺寸都在100 nm以內(nèi)，表明E690高強(qiáng)鋼晶粒細(xì)化至納米級(jí)。根據(jù)圖7b可以看出，其衍射花樣為連續(xù)的同心環(huán)，說明晶粒在經(jīng)受5.09 GW/cm2的強(qiáng)激光加載后，形成了分布均勻，取向隨機(jī)的納米晶[11,27]。

3.3 E690高強(qiáng)鋼表層Ostwald熟化驗(yàn)證

隨著激光能量的增大，其晶粒尺寸不斷減小。當(dāng)激光功率小于2.77 GW/cm2時(shí)，雖存在少量小晶粒，但在試樣不同區(qū)域的TEM形貌像中并未觀察到調(diào)幅分解和Ostwald熟化現(xiàn)象，說明界面能并不能支撐細(xì)小晶粒的遷移。當(dāng)激光功率密度到達(dá)5.09 GW/cm2時(shí)，E690高強(qiáng)鋼在極高塑性變形情況下形成細(xì)小均勻的納米晶。細(xì)小脫溶物溶入較大的顆粒是發(fā)生Ostwald熟化現(xiàn)象的前提。由此可推知，當(dāng)激光功率為5.09 GW/cm2時(shí)，試樣表面的納米晶不滿足Ostwald熟化現(xiàn)象的條件。當(dāng)功率密度為4.07 GW/cm2的激光沖擊加載后，試樣表面的TEM形貌像中觀察到了失穩(wěn)分解及Ostwald熟化，且只在該功率密度激光沖擊處理后試樣表面TEM形貌像中觀察到了失穩(wěn)分解及Ostwald熟化。

功率密度為4.07 GW/cm2的激光沖擊加載后，試樣的TEM形貌像如圖8所示，其中圖8b為圖8a的暗場(chǎng)像。激光沖擊促使E690高強(qiáng)鋼試樣因調(diào)幅分解內(nèi)部產(chǎn)生沉淀相差異的不均勻結(jié)構(gòu)，因成分梯度導(dǎo)致材料組織內(nèi)應(yīng)力的差異，從而導(dǎo)致系統(tǒng)Gibbs能增高[19,28-29]。觀察圖8可知，試樣表面因調(diào)幅分解生成了眾多細(xì)小的脫溶顆粒。為了減小系統(tǒng)能量，以及維持脫溶物與基體界面間的濃度平衡，細(xì)小脫溶晶粒a1、b1、c1、d1、e1、f1沿濃度梯度逐漸向大晶粒L移動(dòng)。具體到粒子而言，伴隨著小粒子不斷向大晶粒L移動(dòng)，導(dǎo)致小粒子脫溶物消失，以及大粒子L的尺寸長(zhǎng)大。由此可知，在功率密度4.07 GW/cm2激光沖擊加載后，試樣表面產(chǎn)生了Ostwald熟化現(xiàn)象，與前文分析相符。

圖7 激光功率密度5.09 GW/cm2時(shí)的TEM形貌像和選區(qū)電子衍射圖

圖8 功率密度4.07 GW/cm2下E690高強(qiáng)鋼表面典型Ostwald熟化TEM像

4 結(jié)論

1）對(duì)E690高強(qiáng)鋼激光沖擊過程中調(diào)幅分解現(xiàn)象的發(fā)生進(jìn)行分析，探究了材料表層發(fā)生Ostwald熟化現(xiàn)象與激光功率密度之間的關(guān)系。通過對(duì)試驗(yàn)結(jié)果進(jìn)行分析證明，激光沖擊E690高強(qiáng)鋼其表面存在Ostwald熟化現(xiàn)象，且激光沖擊E690高強(qiáng)鋼表面納米化與Ostwald熟化相鄰出現(xiàn)。

2）對(duì)激光加載后的E690高強(qiáng)鋼形貌像進(jìn)行分析，晶粒尺寸在4.07 GW/cm2功率密度下增大，其中明暗相間的條紋組織以及選區(qū)電子衍射存在衛(wèi)星斑表明，在此功率密度下發(fā)生了調(diào)幅分解，進(jìn)而使 E690高強(qiáng)鋼材料表面發(fā)生Ostwald熟化現(xiàn)象。

3）E690高強(qiáng)鋼經(jīng)過激光沖擊后，其表層材料在高應(yīng)變率作用下使得Ostwald熟化現(xiàn)象和表面納米化現(xiàn)象相鄰出現(xiàn)，但Ostwald熟化轉(zhuǎn)變成納米晶過程有待進(jìn)一步探究。此外，激光沖擊波沿材料深度方向衰減，E690高強(qiáng)鋼截面組織是否會(huì)發(fā)生Ostwald熟化現(xiàn)象也尚需考察。

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Experimental Study on Ostwald Ripening of E690 High Strength Steel Treated by Laser Shock Peening

1,1,2,3,1,1,1

(1. College of Mechanical Engineering, Nantong University, Jiangsu Nantong 226019, China; 2. 3D Printing Technology Research Institute, Nantong Institute of Technology, Jiangsu Nantong 226001, China; 3. Nantong COSCO Shipyard Co. Ltd, Jiangsu Nantong 226006, China)

Laser shock peening is a surface modification technology with great potential and application prospect. Given the extremely short loading time (ns) and extremely large load (GPa) of laser shock peening, samples undergoing laser shock treatment exhibited spinodal decomposition without nucleation, which induced Ostwald ripening phenomenon. Compared with the Ostwald ripening phenomenon caused by heat treatment, the impact phase transition caused by laser shock peening is more complex. Therefore, researching the Ostwald ripening phenomenon induced by laser shock peening has certain theoretical significance for laser shock strengthening technology.

Based on the theoretical analysis of the internal relationship between laser shock metal materials and spinodal decomposition, the conditions required for Ostwald ripening due to laser shock strengthening are put forward. The surface microstructure and selected electron diffraction patterns of E690 high-strength steel samples following laser shock were observed by using field-induced transmission electron microscopy (TEM). The variation characteristics of microstructure in TEM morphology of the sample surface following laser shock treatment at different power densities were obtained, and the Ostwald ripening phenomenon was verified. The specific experimental process and parameters are as follows: E690 high strength steel was cut into 50 mm×50 mm×5.5 mm rectangular block, which was then designed as the sample, and 240-1200# sandpaper was applied to grind the front and back sides of the sample until the thickness was 5 mm. The machine used for laser shock test was Nd: YAG (SGR series, beamtech company, China). The absorption layer was 150 μm thick aluminum foil, and the constraint layer was deionized water. The specific parameters of laser shock were: pulse width 10 ns, wavelength 1,064 nm and spot diameter 5 mm. Using 3, 3.89, 5.43, 8 and 10J laser energy respectively, the corresponding laser power densities were 1.53, 1.98, 2.77, 4.07 and 5.09 GW/cm2. The lap rate was 70%, and the impact times was once. The laser shock area of E690 high strength steel was cut by wire, and the cut sample was pre-thinned from the substrate side, then it was ground in a recess and finally ion-thinned, eventually made into the TEM sample. The micro morphology and selected area electron diffraction of the sample surface were observed by transmission electron microscopy (TECNAI G2 F20, FEI, USA).

As can be seen from the TEM image, the matrix of E690 high strength steel was pearlite morphology formed by alternating overlap of ferrite layer and cementite layer; when the laser power density was weak, the E690 high strength steel material continued to refine under the action of laser shock peening. In the meantime, the thin layer cementite gradually melted into ferrite and disappeared, and the electron diffraction pattern gradually changed into a ring shape. E690 high strength steel gradually changed from pearlite to martensite. However, when the laser power density increased to 4.07 GW/cm2, the continuously refined material was coarsened, the spinodal decomposition structure appeared. The satellite spots appeared in the selected area electron diffraction pattern, and the Ostwald ripening occurred on the surface of E690 high strength steel; when the laser power density reached 5.09 GW/cm2, geometric dislocations divided the whole large grain into finer grains, nanocrystals were produced on the surface of E690 high strength steel.

In conclusion, when the laser power density was 4.07 GW/cm2, Ostwald ripening occurred on the surface of E690 high strength steel due to spinodal decomposition; neither weak nor strong power density can make the precipitate reach the critical radius of Ostwald ripening mechanism; in the experiment, the laser power density required for Ostwald ripening phenomenon is close to the laser power density required for nanocrystals.

laser shock peening; Ostwald ripening; E690 high strength steel; grain refinement; microstructure

2021-09-07；

2021-11-15

XIE Peng-peng (1997-), Male, Postgraduate, Research focus: laser processing.

曹宇鵬（1981—），男，博士，副教授，主要從事激光加工檢測(cè)技術(shù)的研究。

CAO Yu-peng (1981-), Male, Doctor, Associate professor, Research focus: laser processing and testing technology research.

解朋朋, 曹宇鵬, 花國(guó)然, 等.激光沖擊E690高強(qiáng)鋼Ostwald熟化現(xiàn)象的試驗(yàn)研究[J]. 表面技術(shù), 2022, 51(9): 371-378.

TN249

A

1001-3660(2022)09-0371-08

10.16490/j.cnki.issn.1001-3660.2022.09.000

2021–09–07；

2021–11–15

國(guó)家自然科學(xué)基金（51505236，51979138，52109106）；江蘇省博士后科研資助計(jì)劃（2021K606C）；國(guó)家重點(diǎn)研發(fā)計(jì)劃（2019YFB2005300）；國(guó)家高技術(shù)船舶科研項(xiàng)目（工信部裝函[2019]360號(hào)）

Fund：The National Natural Science Foundation of China (51505236, 51979138, 52109106); The Jiangsu Planned Projects for Postdoctoral Research Funds (2021K606C); The National Key Research and Development Program of China (2019YFB2005300); National High-tech Ship Scientific Research Project of China (MIIT [2019]360)

解朋朋（1997—），男，碩士研究生，主要研究方向?yàn)榧す饧庸ぁ?/p>

XIE Peng-peng, CAO Yu-peng, HUA Guo-ran, et al. Experimental Study on Ostwald Ripening of E690 High Strength Steel Treated by Laser Shock Peening[J]. Surface Technology, 2022, 51(9): 371-378.

責(zé)任編輯：劉世忠