根據(jù)Hall-Petch公式可推測(cè)具有均質(zhì)結(jié)構(gòu)金屬材料的力學(xué)性能[1, 2] 納米材料[3, 4]的強(qiáng)度和硬度都非常高，但是很難生成極其細(xì)化的晶胞以存儲(chǔ)更多的位錯(cuò)，因此往往很早就失穩(wěn)進(jìn)入了頸縮階段 粗晶材料的均勻延伸性好，加工硬化能力強(qiáng)，但是其強(qiáng)度較低 近年來(lái)，朱運(yùn)田、盧柯等[5~10]提出了異質(zhì)結(jié)構(gòu)材料的概念，即在金屬材料中引入一個(gè)異質(zhì)區(qū)域以同時(shí)得到納米材料的高強(qiáng)度和粗晶材料的高延展性，從而實(shí)現(xiàn)金屬材料強(qiáng)度-塑性的良好匹配

異質(zhì)結(jié)構(gòu)材料，分為異質(zhì)層狀結(jié)構(gòu)材料、梯度結(jié)構(gòu)材料、核殼結(jié)構(gòu)材料、雙相結(jié)構(gòu)材料以及多模態(tài)結(jié)構(gòu)材料[9] 在異質(zhì)結(jié)構(gòu)材料的塑性變形過(guò)程中，異質(zhì)區(qū)的變形是不均勻的 在軟質(zhì)區(qū)產(chǎn)生背應(yīng)力[11]，在硬質(zhì)區(qū)產(chǎn)生前應(yīng)力[12]，其共同作用產(chǎn)生異質(zhì)變形誘導(dǎo)強(qiáng)化，增強(qiáng)應(yīng)變硬化，有助于提高異質(zhì)結(jié)構(gòu)材料的全局屈服強(qiáng)度并保持延展性 異質(zhì)變形誘導(dǎo)應(yīng)力和應(yīng)變硬化的微觀機(jī)理研究尚處于前沿階段，梯度結(jié)構(gòu)層中界面影響區(qū)的形成和演變尚不十分清楚 科研人員以數(shù)字圖像相關(guān)法開展塑性變形過(guò)程的力學(xué)研究[13, 14]，為梯度結(jié)構(gòu)材料的力學(xué)行為的解釋和變形機(jī)理的闡述提供了直觀的表征支持 本文研究Cu-4.5%(質(zhì)量分?jǐn)?shù))Al合金梯度結(jié)構(gòu)材料的加工硬化行為，探究室溫下準(zhǔn)靜態(tài)拉伸優(yōu)異的屈服強(qiáng)度-均勻延伸性組合的微觀機(jī)理，通過(guò)表征表面機(jī)械研磨處理(Surface mechanical attrition treatment，SMAT)后合金板的梯度層由表層到芯部形成的微觀組織的差異并結(jié)合異質(zhì)變形誘導(dǎo)強(qiáng)化等理論，系統(tǒng)說(shuō)明高強(qiáng)度、高塑性潛在的微觀塑性變形的力學(xué)行為，以較低的加工成本換取極佳的強(qiáng)塑性匹配，探索梯度結(jié)構(gòu)材料的工業(yè)化生產(chǎn)和應(yīng)用方式

1 實(shí)驗(yàn)方法

實(shí)驗(yàn)用銅鋁合金為純銅中固溶了4.5%的Al元素 將熱鍛和冷鍛成厚度為4 mm的銅鋁合金板放置在溫度為923 K的管式真空爐中退火2 h，制備出均質(zhì)結(jié)構(gòu)的粗晶銅鋁合金板 再將其在液氮下表面機(jī)械研磨[7] 2 min，制備出具有雙面梯度結(jié)構(gòu)的銅鋁合金板

使用型號(hào)為SHIMADZU Universal Tester的力學(xué)試驗(yàn)機(jī)進(jìn)行單軸拉伸測(cè)試，應(yīng)變速率為5×10-4 s-1 拉伸試樣標(biāo)距部分的尺寸為15 mm×5 mm×4 mm(圖1a)，用240#到2000#砂紙將打磨電火花線切割殘留痕跡的截面 數(shù)字圖像相關(guān)法(Digital image correlation，DIC)與拉伸試驗(yàn)同步進(jìn)行，圖1b給出了斑點(diǎn)與分析概覽



圖1拉伸樣品尺寸、數(shù)字圖像相關(guān)法斑點(diǎn)及觀測(cè)區(qū)域以及微觀表征區(qū)域

Fig.1Size of tensile text samples (a), DIC speckle and observation area (b) and the area of microscopic characterization (c)

用儀器型號(hào)Optic Microscope (OM) Leica DM 5000的光學(xué)顯微鏡對(duì)樣品進(jìn)行微觀表征 制備所用樣品時(shí)，依次用400#、800#、2000#、5000#砂紙將其打磨，然后將研磨拋光的表面腐蝕余額1 min，腐蝕液是用氯化鐵5 g+50 mL稀鹽酸+100 mL水配制；將樣品研磨拋光后，再用離子拋光儀拋光以去除機(jī)械損傷層用于掃描電鏡觀察，所用儀器是型號(hào)為FE-SEM, NOVA Nano SEM 450并帶有電子背散射衍射(Electron backscattered diffraction，EBSD)信號(hào)探頭的掃描電鏡，其操作電壓為250 KV；用于透射電鏡(Transmission electron microscope，TEM)觀察的樣品，需要沿深度取寬約1000 μm的梯度截面，用2000#砂紙將其機(jī)械減薄至50 um，再鑲嵌在雙聯(lián)銅環(huán)上離子減薄使其有良好的可觀察薄區(qū)，離子減薄參數(shù)為5 keV 8°處理2 h、4 keV 5°處理0.5 h、2.5 keV 3°處理2 h，透射電鏡的型號(hào)為JEM-2100 Plus TEM，操作電壓為200 kV 微觀表征區(qū)域的選區(qū)示意圖，在圖1c中給出

2 結(jié)果和討論

Cu-4.5%Al的合金板在液氮溫度下SMAT處理2 min，為了維持材料整體的連續(xù)性，在其表面層產(chǎn)生了沿深度呈梯度遞減分布的幾何必要位錯(cuò)(Geometrically necessary dislocations，GNDs)(圖2a) 這符合金屬材料的塑性變形理論，統(tǒng)計(jì)同一深度的平均局部取向差(Kernel average misorientation，KAM)值并基于應(yīng)變梯度理論[15, 16]，其GND的值為

ρGND=2KAMaveμb(1)

式中ρGND是GNDs的值，KAMave為局部取向差的平均值，μ為EBSD的掃描步長(zhǎng)，b為位錯(cuò)的柏氏矢量



圖2梯度層的微觀結(jié)構(gòu)

Fig.2Microstructure of gradient layer (a) KAM map of the depth ~300 μm depth, characterized by EBSD, (b) The GND distribution map with depth, calculated by the average KAM statistics in (a), (c~e) TEM bright field image of ~30 μm, ~153 μm, ~246 μm depth, respectively

圖2b表明，統(tǒng)計(jì)出來(lái)的GND密度沿深度呈梯度降低，在~50 μm處趨勢(shì)放緩最終在~250 μm深度處趨于平穩(wěn) 這表明，處理后的樣品加工影響深度可能比~250 μm更大，其最表面的GND約為6.8×1013 m-2，芯部區(qū)域的GND約為2.4×1013 m-2，表層和芯部GND的差距約2倍 該成分的層錯(cuò)能(Stacking fault energy，SFE)很低只有12 mJ/m2(純銅的層錯(cuò)能有78 mJ/m2)，加工溫度對(duì)微觀結(jié)構(gòu)的影響各不相同[17]，生成位錯(cuò)的臨界分切應(yīng)力隨溫度降低而增大，生成層錯(cuò)(Stacking faults，SFs)的臨界分切應(yīng)力隨溫度的降低而降低[18] 在此加工溫度下占主導(dǎo)地位的變形機(jī)制由層錯(cuò)取代了位錯(cuò)[19, 20]，從而促進(jìn)了{(lán)111}晶面族層錯(cuò)的形成 用TEM表征了~30 μm、~153 μm、~246 μm 3個(gè)深度，觀察到極其細(xì)小的層錯(cuò)密度沿深度梯度遞減，而梯度層中的位錯(cuò)也從靠近表層的大量林位錯(cuò)(圖2c)演變成靠近芯部纏結(jié)的位錯(cuò)(圖2d)，在芯部甚至能觀察到單個(gè)分布的位錯(cuò)(圖2e)，證實(shí)了微觀結(jié)構(gòu)隨深度變化呈現(xiàn)缺陷密度梯度特征

對(duì)SMAT處理前后的拉伸試樣進(jìn)行準(zhǔn)靜態(tài)單軸拉伸測(cè)試(圖3a)，短時(shí)SMAT處理將Cu-4.5%Al合金的屈服強(qiáng)度從~90 MPa提高到~170 MPa，均勻延伸率從~54%降低至~45%，在真應(yīng)力應(yīng)變曲線(圖3a)背景的金相上可觀察到加工后腐蝕液對(duì)位錯(cuò)滑移帶的點(diǎn)蝕痕跡[21, 22] 加工后屈服強(qiáng)度的提高可以歸因于層錯(cuò)，對(duì)梯度層~30 μm深度處捕捉了一張放大數(shù)倍的TEM明場(chǎng)像(圖3b)，可以發(fā)現(xiàn)a2<110>全位錯(cuò)沿{111}面滑移會(huì)分解成2個(gè)a6<211>肖克利不全位錯(cuò) 由于Cu-4.5%Al合金的層錯(cuò)能較低，兩個(gè)不全位錯(cuò)之間的層錯(cuò)寬度增大，因此異號(hào)不全位錯(cuò)很難碰到而湮滅，因此在梯度層留下了大量的層錯(cuò)(圖3c) 在相同晶面上產(chǎn)生的層錯(cuò)堆疊在一起，形成了有一定原子層寬度的納米孿晶[23, 24](Nano twins，NTs)，從面缺陷演變成了體缺陷；在不同晶面上產(chǎn)生的層錯(cuò)和層錯(cuò)之間發(fā)生反應(yīng)，兩個(gè)拓展位錯(cuò)在各自滑移面相向移動(dòng) 當(dāng)每個(gè)拓展位錯(cuò)中的一個(gè)肖克利不全位錯(cuò)運(yùn)動(dòng)到滑移面的交截線時(shí)，位錯(cuò)反應(yīng)產(chǎn)生了一個(gè)新的純?nèi)行臀诲e(cuò)，將這兩個(gè)可動(dòng)的拓展位錯(cuò)在此處固定住，此時(shí)的混合位錯(cuò)組態(tài)稱為面角位錯(cuò)(Lomer-cottrell dislocation，L-C dislocation)(圖3b)，由三個(gè)不全位錯(cuò)和兩片層錯(cuò)所構(gòu)成 對(duì)于面心立方晶體(Face centered cubic，F(xiàn)CC)結(jié)構(gòu)的金屬加工硬化起重大作用，能有效釘扎和塞積位錯(cuò) 由此可知，屈服強(qiáng)度大大提高的直接原因，可歸于低層錯(cuò)能合金在液氮下塑性應(yīng)變產(chǎn)生的由層錯(cuò)組成的微觀結(jié)構(gòu)以及層錯(cuò)對(duì)位錯(cuò)的塞積[25, 26]



圖3SMAT處理前后的真應(yīng)力-應(yīng)變曲線對(duì)比(背景為梯度層的金相顯微圖)、在梯度層~30 μm深度處的TEM明場(chǎng)像以及拓展位錯(cuò)的形成-全位錯(cuò)分解為不全位錯(cuò)

Fig.3Comparison of true stress-strain curves before and after SMAT treatment (a, metallographic image with gradient layer in the background); TEM bright field image at gradient layer ~30 μm depth (b); Extended dislocation formation-decomposition of full dislocations into partial dislocations (c)

為此，圖4給出了準(zhǔn)靜態(tài)單軸拉伸測(cè)試試樣掃描的斷口形貌，圖4a和4b分別給出了梯度結(jié)構(gòu)試樣和粗晶試樣的斷口 斷口芯部的韌窩很大且均勻(圖4a2、4b2)，表明芯部的塑性變形充分且穩(wěn)定 從斷口邊緣可觀察到，粗晶試樣的表層和芯部特征相同，而梯度試樣的特征就有所不同 斷口邊緣從表及芯存在過(guò)渡的形貌(圖4a3)，表明材料變形的非均質(zhì)的特性 梯度層雖然有韌窩，但不均勻且較淺(圖4a4)，說(shuō)明單一梯度層在頸縮階段前SMAT處理的影響 晶胞的加工硬化的潛力降低了[27]，不能繼續(xù)均勻變形，失去了和芯部同步塑性變形的能力 梯度層失穩(wěn)進(jìn)入頸縮階段，使試樣提前進(jìn)入頸縮階段[28]



圖4梯度結(jié)構(gòu)試樣和粗晶試樣的斷口特征

Fig.4Observation of fracture features of gradient-structure sample (a) and coarse-grain sample (b)

SMAT處理前后拉伸試樣斷口形貌的差異表明，剪切帶過(guò)早的形核并集中使梯度層的加工硬化不能保持，為此分析了拉伸過(guò)程中剪切帶的演變 結(jié)果表明，材料的加工硬化并不是整體同步進(jìn)行，而是加工硬化更強(qiáng)的區(qū)域?qū)?yīng)力分散到加工硬化較弱的區(qū)域[13, 29] 觀察了粗晶試樣DIC從屈服后到~30%應(yīng)變階段，如圖5所示，剪切帶的形核在邊緣產(chǎn)生(圖5a的0.78%應(yīng)變點(diǎn))，并不是在邊緣立即深化導(dǎo)致微裂紋產(chǎn)生，加工硬化將應(yīng)力分散到芯部(圖5a的4.54%應(yīng)變點(diǎn)) 從圖5a可見，早期沿Y軸產(chǎn)生的剪切帶與同一Y軸高度上的應(yīng)變大致相近，較突出的應(yīng)變集中區(qū)通過(guò)應(yīng)力分散將附近應(yīng)變較小的區(qū)域加工硬化(見圖5a的29.95%應(yīng)變點(diǎn))，最終在頸縮階段前試樣整體達(dá)到加工硬化能力的最大值，不能繼續(xù)加工硬化保持均勻的塑性變形 此時(shí)剪切帶從試樣邊緣區(qū)域深化產(chǎn)生微裂紋，試樣進(jìn)入不穩(wěn)定變形的頸縮階段，直至斷裂 依據(jù)均勻塑性變形時(shí)體積恒定原則，沿Y軸正應(yīng)變較大的區(qū)域在X軸收縮的負(fù)應(yīng)變更多 在全局上，均勻分布的剪切帶，是試樣保持較好均勻塑性變形必不可少的



圖5粗晶試樣的剪切帶演變

Fig.5Evolution of shear bands of CG sample (a) plastic strain distribution along the Y-axis at each global strain; (b) plastic strain distribution along the X-axis at each global strain

具有梯度結(jié)構(gòu)試樣(圖6)，其剪切帶形核也從邊緣開始，而梯度層中的剪切帶形核比粗晶試樣更明顯 隨著塑性變形的進(jìn)行，表層內(nèi)很集中的應(yīng)變區(qū)(圖6a)從表層傳遞到芯部 從圖6a中9.61%應(yīng)變圖像可見，芯部的變形量比表層多，因?yàn)樵嚇觿倧那筮M(jìn)入塑性變形階段，梯度層內(nèi)的缺陷密度梯度較高，表層與芯部之間的彈性-塑性階段相互作用引起的長(zhǎng)程內(nèi)應(yīng)力導(dǎo)致了過(guò)渡界面的高應(yīng)力集中[13, 14] 芯部?jī)?nèi)較高的加工硬化使強(qiáng)度提高從而降低了流應(yīng)力差異，粗晶芯部穩(wěn)定了剪切帶并阻止了向芯部傳遞 這導(dǎo)致在梯度層形核的剪切帶先向標(biāo)距段未加工硬化的部分傳遞(圖6a的0.34%應(yīng)變點(diǎn))，標(biāo)距段全局都加工硬化后梯度層的剪切帶核才開始向芯部擴(kuò)散，造成應(yīng)變局域化(圖6a的9.61%應(yīng)變點(diǎn)) 其微觀機(jī)理是，表層與芯部存在應(yīng)變差異 為了維持材料的整體連續(xù)性，在芯部開動(dòng)的弗蘭克-里德位錯(cuò)源產(chǎn)生了大量的幾何必要位錯(cuò)并向梯度層發(fā)射(圖7a) 但是，梯度層靠近表層的區(qū)域有高密度的幾何必要位錯(cuò)，因此從芯部發(fā)射過(guò)來(lái)的幾何必要位錯(cuò)在梯度層的過(guò)渡界面塞積[12]，有序排列塞積的幾何必要位錯(cuò)使靠近表層的梯度層產(chǎn)生前應(yīng)力 前應(yīng)力使梯度層在承受的流應(yīng)力之外附加了來(lái)自芯部的應(yīng)力，梯度層也反饋背應(yīng)力施加在芯部 芯部加工硬化的強(qiáng)度加上背應(yīng)力，可分擔(dān)芯部承受的部分流應(yīng)力 因此，芯部變相地被背應(yīng)力強(qiáng)化了 梯度層與芯部之間的異質(zhì)結(jié)構(gòu)誘導(dǎo)強(qiáng)化行為使剪切帶在整個(gè)標(biāo)距部分均勻分布，芯部發(fā)生的幾何必要位錯(cuò)彌補(bǔ)了一部分表層晶胞產(chǎn)生位錯(cuò)能力的不足，從而使試樣在塑性變形過(guò)程中在保持了較高的均勻延伸性的同時(shí)使屈服強(qiáng)度大大提高



圖6梯度結(jié)構(gòu)試樣的剪切帶演變

Fig.6Evolution of shear bands of GS sample (a) plastic strain distribution along the Y-axis at each global strain; (b) plastic strain distribution along the X-axis at each global strain



圖7GND塞積的示意圖,在軟區(qū)中誘導(dǎo)背應(yīng)力，在硬區(qū)中誘導(dǎo)前應(yīng)力[12]以及有限元模擬平均應(yīng)變?yōu)?.2%應(yīng)力和應(yīng)變沿厚度方向的變化[30]

Fig.7Schematics of a GND pile-up, inducing back stress in the soft domain, which in turn induces forward stress in the hard domain[12] (a) andstress and strain variations along thickness direction on 0.2% average strain by finite element simulation (FEM)[30] (b)

異質(zhì)結(jié)構(gòu)誘導(dǎo)的強(qiáng)化行為，不只是在強(qiáng)-弱的過(guò)渡界面上通過(guò)前-背應(yīng)力實(shí)現(xiàn)的 雙面梯度結(jié)構(gòu)的強(qiáng)-弱-強(qiáng)的雙面結(jié)構(gòu)分布推遲了應(yīng)變局域化[30](如圖7b)，有助于應(yīng)力均勻分布和避免應(yīng)變集中 應(yīng)力應(yīng)變的均勻分布，使試樣的延展性提高

3 結(jié)論

將Cu-4.5%Al合金板材進(jìn)行機(jī)械研磨后，在其表面形成了厚度約為250 μm的梯度結(jié)構(gòu)層，從表層到芯部的位錯(cuò)結(jié)構(gòu)分別為林位錯(cuò)、纏結(jié)的位錯(cuò)、單個(gè)的位錯(cuò)，層錯(cuò)結(jié)構(gòu)在靠近表層區(qū)域(~30 μm)生成了納米孿晶 Cu-4.5%Al合金的拉伸試樣經(jīng)準(zhǔn)靜態(tài)單軸拉伸后，其屈服強(qiáng)度提高了~80 MPa，均勻延伸率降低9% 屈服強(qiáng)度提高的直接原因，是梯度層中的納米孿晶和面角位錯(cuò)對(duì)可動(dòng)位錯(cuò)的存儲(chǔ)、塞積的作用；間接原因，是異質(zhì)結(jié)構(gòu)誘導(dǎo)強(qiáng)化產(chǎn)生的額外的強(qiáng)化作用 均勻延伸性降低程度很低，可以歸因于雙面梯度結(jié)構(gòu)對(duì)剪切帶均勻分布的貢獻(xiàn)，梯度層與芯部之間的異質(zhì)結(jié)構(gòu)誘導(dǎo)強(qiáng)化行為使剪切帶在整個(gè)標(biāo)距段均勻分布，在芯部發(fā)生的幾何必要位錯(cuò)彌補(bǔ)了一部分表層晶胞產(chǎn)生位錯(cuò)能力的不足，從而使試樣在塑性變形過(guò)程中在保持了一定均勻延伸性的同時(shí)大大提高了屈服強(qiáng)度、推遲了應(yīng)變局域化，從而使標(biāo)距段繼續(xù)加工硬化，避免了在較早階段失穩(wěn)進(jìn)入頸縮階段

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