氦離子注入對(duì)Ti6Al4V合金組織和力學(xué)性能的影響

陳善華,吳 勇,G.Schumacher(成都理工大學(xué)材料與化學(xué)化工學(xué)院,成都60059;成都理工大學(xué)

地質(zhì)災(zāi)害防治與地質(zhì)環(huán)境保護(hù)國(guó)家重點(diǎn)實(shí)驗(yàn)室,成都610059; 3亥姆霍茲材料與能源研究中心,德國(guó)柏林14109)

氦離子注入對(duì)Ti6Al4V合金組織和力學(xué)性能的影響

陳善華1,吳 勇2,G.Schumacher3(1成都理工大學(xué)材料與化學(xué)化工學(xué)院,成都610059;2成都理工大學(xué)

地質(zhì)災(zāi)害防治與地質(zhì)環(huán)境保護(hù)國(guó)家重點(diǎn)實(shí)驗(yàn)室,成都610059; 3亥姆霍茲材料與能源研究中心,德國(guó)柏林14109)

在室溫和400～700℃條件下,采用1×1017ions/cm2注入劑量和200keV加速電壓對(duì)Ti6Al4V合金進(jìn)行了氦離子注入。分別采用納米硬度儀和X射線衍射方法對(duì)Ti6A l4V合金的氦離子注入表面層(≤700nm)進(jìn)行了納米硬度、彈性模量測(cè)試和物相分析。結(jié)果表明:在室溫至600℃范圍內(nèi),氦離子注入溫度越高,Ti6A l4V合金注入層的硬度也越高,而其彈性模量則變小。氦離子注入溫度為700℃時(shí),Ti6A l4V合金發(fā)生了軟化,其彈性模量也有所提高。氦離子注入引起的硬化現(xiàn)象與點(diǎn)缺陷和Ti6A l4V合金中β相的析出有關(guān),而軟化現(xiàn)象則與β相的粗化和γ2TiH相的形成有關(guān)。

Ti6A l4V合金;氦離子注入;納米硬度;彈性模量;物相分析

鈦合金具有優(yōu)良的低活化特性、室溫和高溫強(qiáng)度、耐液態(tài)金屬腐蝕、抗中子輻照腫脹等性能,已成為聚變反應(yīng)堆重要的候選結(jié)構(gòu)材料[1]。研究表明[2-4],鈦合金的輻照效應(yīng)與輻照溫度、離子類(lèi)型、離子通量、離子劑量和能量及材料的組織結(jié)構(gòu)有關(guān)。雖然人們對(duì)航空、航天、艦船、化工、石油、機(jī)械等諸多領(lǐng)域廣泛使用鈦合金的結(jié)構(gòu)和性能已經(jīng)開(kāi)展了深入研究,但關(guān)于不同溫度和氦離子輻照條件下的鈦合金組織與性能變化的研究報(bào)道卻很少。為此,本工作以國(guó)際熱核實(shí)驗(yàn)堆包層模塊和真空容器的柔性支承結(jié)構(gòu)候選材料Ti6A l4V合金[5]為研究對(duì)象,在室溫和400～700℃條件下,采用1×1017ions/cm2注入劑量和200keV加速電壓對(duì)Ti6A l4V合金進(jìn)行了氦離子注入,并對(duì)其表面氦離子注入層硬度、彈性模量和物相變化等進(jìn)行了研究。

1 實(shí)驗(yàn)

1.1 樣品制備及測(cè)試方法

Ti6A l4V合金樣品由德國(guó)AAB公司提供。試樣尺寸為10mm×10mm×2mm。氦離子注入前,試樣進(jìn)行了機(jī)械打磨和拋光。氦離子注入在德國(guó)Rossendo rf研究中心下屬離子束物理及材料科學(xué)研究所的離子束微探針實(shí)驗(yàn)裝置上進(jìn)行,加速電壓為200keV,注入劑量為1×1017ions/cm2,注入溫度分別為室溫,400, 500,600℃和700℃。

分別采用Philips XL 30型環(huán)境掃描電鏡、Olym2 pus2OLS21100型激光掃描聚焦顯微鏡和Philips X’Pert Pro MRD型X射線衍射儀進(jìn)行氦離子注入前后Ti6A l4V合金樣品表面微觀組織觀察和物相分析。采用CSEM型納米硬度計(jì),對(duì)氦離子注入前后Ti6A l4V試樣進(jìn)行了納米硬度和彈性模量測(cè)試。實(shí)驗(yàn)前,采用標(biāo)準(zhǔn)程序,通過(guò)測(cè)試SiO2納米硬度(0.1～100mN范圍內(nèi))對(duì)金剛石壓頭進(jìn)行了校正。實(shí)驗(yàn)時(shí),加載和卸載時(shí)間均為30s,最大載荷時(shí)保持2s。為了消除樣品缺陷、組織或者晶體學(xué)取向引起的不均勻性,對(duì)每個(gè)樣品選取15個(gè)位置進(jìn)行測(cè)試,壓入深度分別為50,100,150,…,700nm。因此,每個(gè)樣品共有210組硬度和彈性模量數(shù)據(jù)。

1.2 X射線掠入射角αi的確定

采用TRIM 96程序計(jì)算氦離子注入深度分布。輸入?yún)?shù)和計(jì)算結(jié)果分別見(jiàn)表1和圖1(a)。

表1 TRIM 96模擬程序輸入?yún)?shù)Table 1 Input parametersof the simulation p rogram TRIM 96

圖1 He離子注入深度分布(a)和X射線入射角αi與其穿透深度h(αi)的關(guān)系(b)Fig.1 Helium depth p rofile of irradiated Ti6Al4V sample(a)and dependence of the penetration dep th on the grazing angle of incidenceαi(b)

設(shè)介質(zhì)的質(zhì)量密度、原子序數(shù)、線質(zhì)量吸收系數(shù)和原子量分別為ρ,Z,μ和A,re,N,ρe,NA分別為經(jīng)典電子半徑、晶胞中單位體積的電子數(shù)、物質(zhì)中電子的平均密度和Avogadro常數(shù),X射線的全反射臨界角αc、吸收參數(shù)β和入射深度h(αi)可由式(1)～(3)計(jì)算[9]:

將相關(guān)參數(shù)代入式(1),(2)可得:αc=5.134× 10-3,即0.293°,β=1.1×10-6。入射角αi與穿透深度h(αi)的關(guān)系如圖1(b)所示。選取入射角αi=2°,X射線的穿透深度h(αi)(682nm)和氦粒子濃度峰位置(725nm)大致相同。

2 結(jié)果與討論

2.1 氦離子注入前的Ti6Al4V合金組織

圖2為氦離子注入前的Ti6A l4V合金背散射掃描電鏡圖,白亮部分為β相?？梢钥闯?Ti6A l4V合金的原始組織是由等軸狀α相,晶界一次βI相和部分αII+βII二次晶粒組成的雙態(tài)組織。α相晶粒尺寸約為4～10μm,βII約為3μm×0.5μm。β相體積分?jǐn)?shù)為9%。

圖2 氦離子注入前的Ti6A l4V合金背散射掃描電鏡圖Fig.2 Backscattered SEM micrograph of as2received Ti6A l4V alloy

2.2 氦離子注入Ti6Al4V合金的物相分析

圖3(a)為T(mén)i6A l4V合金的GIXRD分析結(jié)果?？梢钥闯?氦離子注入后β2Ti衍射峰有所增強(qiáng),這與氦離子注入提高原子的擴(kuò)散能力,從而導(dǎo)致β相沉淀析出有關(guān)[2,10,11]。700℃氦離子注入樣品的β2Ti衍射峰較窄,并在2θ=39.5°處出現(xiàn)一個(gè)多余峰,說(shuō)明700℃氦離子注入樣品中β相晶粒尺寸有所增大且存在新物相。由于該譜圖上沒(méi)有出現(xiàn)其他衍射峰,因而很難確定這個(gè)新物相,也無(wú)法判斷該新相是否與氦離子注入有關(guān)。因此,本工作分別對(duì)所有樣品的正反兩面都進(jìn)行了常規(guī)XRD和GIXRD分析。在700℃氦離子注入樣品的反面衍射譜圖中36.9°,39.5°和44.6°處存在較強(qiáng)的衍射峰分別為γ2TiH相的(002),(111)和(200)衍射峰(圖3(b))。因此,700℃氦離子注入樣品中出現(xiàn)的γ2TiH相與氦離子注入本身無(wú)關(guān),而是反映了注入溫度對(duì)Ti6A l4V合金試樣相變的影響。研究表明[12-16],鈦合金中通常都含有一定量的氫,應(yīng)變或者離子注入都可以誘發(fā)氫化物的形成。700℃氦離子注入時(shí),Ti6A l4V合金中的氫擴(kuò)散和聚集能力增強(qiáng),在熱應(yīng)力的作用下,α2Ti相將通過(guò)馬氏體相變機(jī)制轉(zhuǎn)變?yōu)棣?TiH[15]。

圖3 氦離子注入Ti6Al4V的GIXRD譜圖(a)及700℃氦離子注入樣品表面的GIXRD,XRD和背面XRD譜圖(b)Fig.3 GIXRD pattern of the specimens unimplanted and implanted at different temperatures,respectively(a)and GIXRD,XRD patterns of the Ti6Al4V specimen implanted at 700℃taken from the implanted and backside surfaces(b)

2.3 氦離子注入Ti6Al4V合金試樣表面形貌變化

氦離子注入Ti6A l4V合金表面的激光掃描共聚焦顯微形貌觀察表明,室溫至600℃氦離子注入樣品的表面形態(tài)大致相同(圖4(a))。700℃氦離子注入Ti6A l4V合金試樣的表面形貌卻明顯不同,具有表面浮凸特征(圖4(b))。定量測(cè)試表明,室溫,400,500, 600℃和700℃氦離子注入Ti6A l4V合金試樣的表面粗糙度分別為0.3381,0.5960,0.5035,0.6210μm和0.9582μm。因此,700℃氦離子注入Ti6A l4V合金試樣表面粗糙度較大。

2.4 氦離子注入前后納米硬度和彈性模量隨壓入深度的變化

圖4 氦離子注入樣品表面形貌(a)400℃;(b)700℃Fig.4 Laser scanning confocalmicroscopy p rojections of Ti6Al4V surfaces after implantation (a)400℃;(b)700℃

不同溫度條件下,氦離子注入Ti6A l4V前后的加卸載曲線如圖5所示。雖然卸載曲線的坡度大致相同,但從其相應(yīng)的放大部分圖5(a),(b)仍然可以看出差別。與氦離子注入前相比,400～700℃氦離子注入后,樣品的恢復(fù)深度變大,而接觸剛度和彈性模量變小;室溫和400℃氦離子注入后的樣品恢復(fù)深度幾乎相同,且小于500℃和600℃注入后的恢復(fù)深度; 700℃注入氦離子后的恢復(fù)深度最小;與其他注入溫度相比,在相同壓入深度時(shí),600℃氦離子注入后的樣品具有最大載荷;當(dāng)溫度升高到700℃時(shí),所有壓入深度下的載荷都明顯降低。圖6(a)為氦離子注入前后的Ti6A l4V合金樣品的硬度與壓入深度之間的關(guān)系。由于壓痕深度為50nm處的測(cè)量值波動(dòng)較大,因而沒(méi)有標(biāo)出?？梢钥闯?在壓痕深度為100nm處,所有樣品的納米硬度值最大,而它們的最大彈性模量則出現(xiàn)在150～200nm處。這種硬度和彈性模量值隨壓痕深度變化的現(xiàn)象稱(chēng)為壓痕尺寸效應(yīng)(Indentation size effect)[17,18]。雖然對(duì)Si和NiTi等材料以及He+2 Ni3+雙離子束注入樣品也有類(lèi)似結(jié)果[18,19],但由于壓痕尺寸效應(yīng)涉及材料的彈性、塑性變形機(jī)制及表面效應(yīng)和氦離子濃度分布不均勻性等多種因素的共同作用,很難對(duì)其進(jìn)行定量解釋。因此,圖7(a),(b)分別比較了壓痕深度為500～700nm時(shí)的硬度變化ΔH和楊氏模量變化ΔE與氦離子注入溫度的關(guān)系?？梢钥闯?當(dāng)注入溫度低于600℃時(shí),平均硬度值隨注入溫度的升高而連續(xù)上升。硬化峰出現(xiàn)在600℃氦離子注入溫度處,這是因?yàn)榈陀?00℃氦離子注入時(shí),Ti6A l4V合金中的結(jié)構(gòu)缺陷比較穩(wěn)定,隨著離子注入溫度升高, β沉淀相增加,對(duì)位錯(cuò)運(yùn)動(dòng)的阻礙增強(qiáng)的緣故[10,11,20]。700℃氦離子注入時(shí),合金的硬化效應(yīng)幾乎觀察不到,彈性模量也有所提高,這可能與高溫下合金中的氫擴(kuò)散和聚集能力增強(qiáng),并導(dǎo)致Ti6Al4V合金中β沉淀相粗化、位錯(cuò)結(jié)構(gòu)回復(fù)和γ2TiH硬度較低有關(guān)[10,11,15,21]。

圖5 氦離子注入Ti6A l4V合金的載荷2壓入深度曲線(a)接觸深度及其局部放大圖; (b)最大壓痕深度及其局部放大圖Fig.5 Comparison of indentation force versus indentation depth curves for various Ti6Al4V samples in the unirradiated condition (original)and after implantation at room temperature, 400,500,600℃and 700℃ (a)contact depths and their enlarged view;(b)maximum displacements and their enlarged view

圖6 氦離子注入Ti6A l4V合金樣品的納米硬度(a)和彈性模量(b)與壓痕深度的關(guān)系Fig.6 Hardness(a)and elastic modulus(b)of Ti6Al4V samples unirradiated and irradiated at room temperature, 400,500,600℃and 700℃as a function of indentation depth between 100nm and 700nm

圖7 氦離子注入溫度與Ti6A l4V合金納米硬度變化ΔH(a)和彈性模量變化ΔE(b)的關(guān)系Fig.7 Changes in nano2indentation hardness(a)and elastic modulus(b)in Ti6A l4V subjected to helium imp lantation at room temperature,400,500,600℃and 700℃

3 結(jié)論

(1)當(dāng)注入溫度由室溫升至600℃時(shí),氦離子注入層的納米硬度增加,而彈性模量則降低。然而,注入溫度為700℃時(shí),氦離子注入層的納米硬度大幅下降,彈性模量則有所升高。

(2)在本工作所述離子注入條件下,Ti6A l4V合金的硬化效應(yīng)與合金中β相的析出、離子注入缺陷以及γ2TiH相的形成有關(guān)。室溫至600℃氦離子注入時(shí),α相中的β彌散相含量逐漸增加,導(dǎo)致其硬度升高。700℃氦離子注入時(shí),合金的硬度降低可能與較軟γ2TiH相的形成、β相變粗和位錯(cuò)結(jié)構(gòu)的回復(fù)等有關(guān)。

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Effects of Helium Imp lantation on M icrostructure and Mechanical Properties of Ti6A l4V A lloy

CHEN Shan2hua1,WU Yong2,G.Schumacher3
(1 College of M aterials and Chemistry&Chemical Engineering,Chengdu University of Technology,Chengdu 610059,China;2 State Key Labo rato ry of Geohazard Prevention& Geoenvironment Protection,Chengdu U niversity of Technology,Chengdu 610059,China;
3 Helmholtz Centre Berlin for M aterials and Energy,Berlin 14109,Germany)

Ti6A l4V specimens w ere helium2ion imp lanted w ith constant fluence of 1×1017ions/cm2at 200keV w ithin the temperature range f rom room temperature to 700℃.Nano2hardness and elastic modulus of the unimp lanted and irradiated specimensw eremeasured by means of the nano2indentation technique and analyzed using the Oliver2Pharr method.The indentation dep th of all samp les was 700 nm,w hich w as comparable in magnitude to the ion range in the irradiated specimens.The phases in the surface layer of the Ti6A l4V specimens were investigated by the grazing incidence X2ray diffrac2 tion(GIXRD).The results indicate that w ith the imp lantation temperature increasing from room tem2 perature to 600℃,the nano2hardness and elastic modulus of the irradiated specimens increase and de2 crease,respectively.The imp lantation at 700℃,however,caused softening and slight increase of the elastic modulus of the Ti6A l4V alloy.The hardening and reduction of the elastic modulus of the Ti6A l4V alloy under imp lantation conditions is related to the point defects and dispersed obstacles of β2p recipitates.The softening and slight increase of elastic modulus of helium2irradiated Ti6A l4V at 700℃m ight be a result of the coarsening ofβ2p recipitates and the newγ2TiH phase fo rmed inα2 phase.

Ti6A l4V alloy;helium imp lantation;nano2hardness;elastic modulus;phase identification

TG156.99;TG142.3

A

100124381(2010)0520025205

教育部科學(xué)技術(shù)研究重點(diǎn)項(xiàng)目(205139)

2009205218;

2010203215

陳善華(1965—),男,博士,教授,主要從事材料結(jié)構(gòu)與性能研究,聯(lián)系地址:成都市成華區(qū)二仙橋東三路1號(hào)成都理工大學(xué)材料與化學(xué)化工學(xué)院(610059),E2mail:chenshanhua@cdut.cn