納米氧化鋁彌散強(qiáng)化銅的放電等離子體燒結(jié)動(dòng)力學(xué)及機(jī)制

蔣少文，程立金，劉耀，劉紹軍, 3

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納米氧化鋁彌散強(qiáng)化銅的放電等離子體燒結(jié)動(dòng)力學(xué)及機(jī)制

蔣少文1，程立金2，劉耀1，劉紹軍1, 3

(1. 中南大學(xué) 粉末冶金研究院，長(zhǎng)沙 410083； 2. 華中科技大學(xué) 材料成形與模具技術(shù)國(guó)家重點(diǎn)實(shí)驗(yàn)室，武漢 430074； 3. 中南大學(xué) 深圳研究院，深圳 518057)

利用經(jīng)典熱壓模型，系統(tǒng)研究納米氧化鋁顆粒彌散強(qiáng)化銅的放電等離子燒結(jié)(SPS)致密化過程與機(jī)理。結(jié)果表明，放電等離子燒結(jié)初期，氧化鋁彌散強(qiáng)化銅的致密化過程由晶界滑移和晶界擴(kuò)散共同控制。隨保溫時(shí)間延長(zhǎng)，燒結(jié)機(jī)制轉(zhuǎn)變?yōu)橛删Ы缁扑鲗?dǎo)。燒結(jié)后期致密化主要以塑性變形的方式進(jìn)行。納米氧化鋁顆粒抑制了銅的燒結(jié)致密化，導(dǎo)致材料的密度降低。抑制機(jī)理為氧化鋁顆粒阻礙晶界和位錯(cuò)運(yùn)動(dòng)，導(dǎo)致晶界滑移和塑性變形的激活能提高，從而增大致密化抗力。在外力和納米氧化鋁顆粒的共同作用下，塑性變形的主要形式為孿生。

氧化鋁彌散強(qiáng)化銅；放電等離子燒結(jié)；燒結(jié)動(dòng)力學(xué)；孿生；致密化

氧化鋁彌散強(qiáng)化銅是一種具有高強(qiáng)高導(dǎo)的高性能復(fù)合材料，廣泛應(yīng)用于汽車、航空等領(lǐng)域，受到眾多學(xué)者的關(guān)注和研究[1?3]。在銅基體中添加增強(qiáng)相氧化鋁顆?？墒蛊淞W(xué)性能極大地提高[4]，且納米氧化鋁顆粒的彌散分布對(duì)材料的電、熱學(xué)性能不會(huì)有明顯的削弱作用[5?6]。銅基體和納米氧化鋁顆粒間的界面結(jié)構(gòu)決定氧化鋁彌散強(qiáng)化銅材料的性能[7]。高致密度是彌散強(qiáng)化銅材料中金屬與陶瓷顆粒間界面結(jié)合良好的保證。然而，納米氧化鋁顆粒對(duì)銅的燒結(jié)致密化有較強(qiáng)的阻礙作用[8]。一般可通過2種方式來提高氧化鋁彌散強(qiáng)化銅的密度，一種是在簡(jiǎn)單燒結(jié)后進(jìn)行熱擠壓，使材料發(fā)生較大的塑性變形[9?10]；另一種是采用特殊燒結(jié)工藝來制備彌散強(qiáng)化銅材料，如微波燒結(jié)[11]和放電等離子燒結(jié)(spark plasma sintering，SPS)等。在過去的幾十年，放電等離子燒結(jié)技術(shù)因集熱場(chǎng)、力場(chǎng)和電場(chǎng)于一身，能快速制備高密度的塊狀金屬、陶瓷和復(fù)合材料而被關(guān)注[12?14]。許多學(xué)者已采用SPS制備了性能良好的氧化鋁彌散強(qiáng)化銅復(fù)合材料[15?17]，但關(guān)于該材料的SPS致密化機(jī)理卻鮮有研究。ZHANG等[18]研究納米晶銅的SPS動(dòng)力學(xué)時(shí)發(fā)現(xiàn)，低溫?zé)o宏觀壓力狀態(tài)下主要發(fā)生蒸發(fā)凝聚和燒結(jié)頸形成與長(zhǎng)大，隨后在溫度和壓力的共同作用下快速完成致密化。LEON等[19]研究了氧化鋁彌散強(qiáng)化銅SPS的致密化過程，發(fā)現(xiàn)低溫階段主要通過顆粒重排機(jī)制實(shí)現(xiàn)燒結(jié)致密化，400~700 ℃則由塑性變形和擴(kuò)散機(jī)制主導(dǎo)致密化進(jìn)程。LEON的研究很有意義，但僅通過致密化曲線來判定燒結(jié)機(jī)理的方法并不嚴(yán)謹(jǐn)，而且納米氧化鋁顆粒對(duì)燒結(jié)機(jī)制和微觀組織結(jié)構(gòu)的影響尚未明確。RAJAN等[20]研究了Y2O3顆粒對(duì)Fe-9Cr-1Mo鐵素體鋼的SPS致密化的影響，發(fā)現(xiàn)Y2O3顆粒的加入可促進(jìn)鐵素體鋼的致密化。因此，正確理解氧化鋁顆粒彌散強(qiáng)化銅的SPS燒結(jié)致密化過程以及納米氧化鋁顆粒對(duì)燒結(jié)機(jī)制的影響，對(duì)改善燒結(jié)工藝具有重要的指導(dǎo)作用。同時(shí)，氧化鋁彌散強(qiáng)化銅作為一種典型的金屬基復(fù)合材料，其SPS燒結(jié)動(dòng)力學(xué)以及研究動(dòng)力學(xué)的方法對(duì)其它金屬基復(fù)合材料的相關(guān)研究也有很好的參考意義。本文作者采用SPS制備納米氧化鋁彌散強(qiáng)化銅，利用經(jīng)典熱壓模型研究燒結(jié)過程中的燒結(jié)動(dòng)力學(xué)問題，重點(diǎn)研究納米氧化鋁顆粒對(duì)銅燒結(jié)動(dòng)力學(xué)及機(jī)制的影響。

1 實(shí)驗(yàn)

1.1 樣品制備

實(shí)驗(yàn)采用SPS工藝制備氧化鋁質(zhì)量分?jǐn)?shù)為0.65%的彌散強(qiáng)化銅復(fù)合材料。所用原料為霧化銅粉(純度＞99.7%，平均粒徑約74 μm)和納米氧化鋁粉(純度＞99.9%，平均粒徑約30 nm)。具體過程如下：首先按比例稱量銅粉和氧化鋁粉末，利用行星球磨機(jī)球磨混合12 h。球磨介質(zhì)為無水乙醇，球磨轉(zhuǎn)速為300 r/min?；旌虾蟮臐{料經(jīng)干燥后，在氫氣氣氛中還原2 h，還原溫度250 ℃。為了對(duì)比純銅和彌散強(qiáng)化銅SPS致密化的差別，將純銅粉在同樣條件下進(jìn)行球磨、干燥和還原處理。將還原后的粉末裝入石墨模具內(nèi)，在SPS裝置(FCT Systeme GmbH)中進(jìn)行燒結(jié)，得到純銅和氧化鋁顆粒彌散強(qiáng)化銅復(fù)合材料樣品。燒結(jié)溫度分別為475，500和525 ℃，壓力30 MPa，真空環(huán)境下保溫15 min。

1.2 性能檢測(cè)

采用阿基米德排水法測(cè)定所有樣品的密度。利用球差透射電鏡(TECNAI G2 60-300, FEI)表征其微觀結(jié)構(gòu)，透射電鏡分析樣品經(jīng)機(jī)械預(yù)減薄至約30 μm后，進(jìn)行小角度離子減薄得到薄區(qū)。

SPS裝置能監(jiān)測(cè)燒結(jié)過程中樣品的即時(shí)高度變化。為排除升溫過程中模具膨脹對(duì)結(jié)果的影響，設(shè)置一組空燒實(shí)驗(yàn)來獲取升溫狀態(tài)下模具的膨脹量。因此，升溫過程中樣品的即時(shí)高度可用式(1)計(jì)算。

式中：f是樣品的最終高度，f是燒結(jié)結(jié)束時(shí)樣品的高度變化值。為了分析整個(gè)燒結(jié)過程中每個(gè)時(shí)刻的致密化行為，需要得到樣品在燒結(jié)時(shí)的即時(shí)相對(duì)密度。假設(shè)燒結(jié)過程中石墨模具的徑向截面面積不變，則樣品的即時(shí)相對(duì)密度與最終相對(duì)密度f之比等于樣品的即時(shí)高度與最終高度f之比，整理可得到下式[21]：

采用排水法測(cè)定燒結(jié)樣品的最終密度，計(jì)算出最終相對(duì)密度f，依據(jù)式(2)即可計(jì)算出燒結(jié)過程中樣品的即時(shí)相對(duì)密度。

2 結(jié)果與討論

通常，常見的燒結(jié)過程可根據(jù)燒結(jié)行為的不同分為2個(gè)階段：燒結(jié)致密化階段和晶粒長(zhǎng)大階段[22?23]。在燒結(jié)致密化階段，隨燒結(jié)時(shí)間延長(zhǎng)，燒結(jié)體的密度不斷提高，不會(huì)發(fā)生明顯的晶粒長(zhǎng)大現(xiàn)象。當(dāng)達(dá)到一定密度時(shí)，燒結(jié)致密化速率變慢，晶粒開始長(zhǎng)大，燒結(jié)進(jìn)入晶粒長(zhǎng)大階段。本文以研究燒結(jié)致密化階段為主，依據(jù)升溫過程中樣品的致密化速率變化曲線來大致區(qū)分燒結(jié)過程的2個(gè)階段。圖1所示為400~800 ℃升溫過程中氧化鋁彌散強(qiáng)化銅的相對(duì)密度及致密化速率隨溫度的變化曲線。由圖可知，溫度低于400 ℃時(shí)致密化過程已經(jīng)開始。隨溫度升高，樣品的密度不斷提高。值得注意的是，致密化速率在525 ℃附近出現(xiàn)最大值，表明在此溫度下致密化速率最快，即525 ℃為致密化和晶粒長(zhǎng)大2個(gè)階段的臨界溫度點(diǎn)，在低于525 ℃溫度下樣品主要處于致密化階段，在高于525 ℃時(shí)晶粒長(zhǎng)大現(xiàn)象較明顯。因此本文選取致密化階段的475，500和525 ℃這3個(gè)溫度點(diǎn)研究彌散強(qiáng)化銅的放電等離子燒結(jié)動(dòng)力學(xué)。

圖2所示為彌散強(qiáng)化銅和純銅樣品分別在475，500和525 ℃這3個(gè)溫度點(diǎn)的燒結(jié)致密化曲線，保溫時(shí)間為15 min。由圖可見，隨保溫時(shí)間延長(zhǎng)，樣品的密度增加。在保溫開始階段，燒結(jié)致密化速率非?？?；約3 min后，致密化曲線變得平緩。整個(gè)燒結(jié)過程中，燒結(jié)溫度越高，樣品密度越高。更重要的是，在相同條件下燒結(jié)，純銅的相對(duì)密度顯著高于彌散強(qiáng)化銅的相對(duì)密度。由此可推斷，氧化鋁顆粒的加入，阻礙了彌散強(qiáng)化銅燒結(jié)致密化的進(jìn)行，降低了材料的密度。

圖1 升溫過程中Al2O3彌散強(qiáng)化銅的相對(duì)密度及致密化速率隨溫度的變化曲線

圖2 純銅和Al2O3彌散強(qiáng)化銅在不同溫度下的等溫?zé)Y(jié)致密化曲線

SPS和熱壓均為有壓燒結(jié)，外界宏觀壓力是燒結(jié)驅(qū)動(dòng)力的主要來源[24]。而且，SPS和熱壓的燒結(jié)機(jī)制相似[25]。由于燒結(jié)上的共性，已有不少學(xué)者利用熱壓模型研究SPS燒結(jié)機(jī)理[26?27]。當(dāng)宏觀壓力是主要的燒結(jié)驅(qū)動(dòng)力時(shí)，燒結(jié)致密化速率(dd)用下式表示[28]：

式中：0為樣品的初始相對(duì)密度。在等溫?zé)Y(jié)過程中，溫度保持不變，且由于燒結(jié)時(shí)間較短且溫度較低，晶粒長(zhǎng)大不顯著，假設(shè)晶粒大小也是常數(shù)，式(3)可簡(jiǎn)化為：

圖3 純銅(a)和彌散強(qiáng)化銅(b)的ln[(1/ρ)?(dρ/dt)]與ln(pa)關(guān)系曲線

根據(jù)阿倫尼烏斯方程，致密化速率(d/d)和燒結(jié)溫度之間存在以下關(guān)系[26]：

式中：是理想氣體常數(shù)；a為表觀激活能。

圖4 純銅(a)和彌散強(qiáng)化銅(b)的ln[(1/ρ)(dρ/dt)(pa)?nT]與T?1的關(guān)系曲線

圖5 純銅的暗場(chǎng)像照片和孿晶衍射花樣標(biāo)定

圖6所示為525 ℃燒結(jié)15 min的彌散強(qiáng)化銅的高角環(huán)形暗場(chǎng)像和EDX分析。圖6(a)顯示彌散強(qiáng)化銅中也分布著與純銅中相似的孿晶，晶粒尺寸較小，數(shù)量較多。除此之外，晶界處還彌散分布著許多納米級(jí)顆粒。EDX分析結(jié)果表明，納米顆粒的主要元素包括Cu、Al和O?？梢酝茢啵植荚诰Ы缣幍募{米顆粒為氧化鋁顆粒。眾所周知，納米氧化鋁顆粒通過釘扎晶界而阻礙晶界的移動(dòng)[31]。另外，位錯(cuò)線移動(dòng)時(shí)無法直接越過第二相顆粒[32?33]，但在外力作用下，位錯(cuò)線可以環(huán)繞第二相發(fā)生彎曲，形成位錯(cuò)環(huán)并導(dǎo)致晶格畸變能增加。在這2種機(jī)制的共同作用下，彌散強(qiáng)化銅發(fā)生晶界滑移和塑性變形所需能量比純銅高(見圖4)，燒結(jié)致密化過程被抑制。同時(shí)，在外力作用和納米氧化鋁顆粒的阻礙下，彌散強(qiáng)化銅塑性變形的主要形式為孿生。

圖6 彌散強(qiáng)化銅的HADF-STEM照片和EDX分析

3 結(jié)論

1) 氧化鋁彌散強(qiáng)化銅的放電等離子燒結(jié)初期，燒結(jié)致密化過程由擴(kuò)散和晶界滑移共同控制。隨保溫時(shí)間延長(zhǎng)，燒結(jié)機(jī)制轉(zhuǎn)變?yōu)榫Ы缁?。燒結(jié)后期致密化主要以塑性變形的方式進(jìn)行。

2) 氧化鋁彌散強(qiáng)化銅放電等離子燒結(jié)時(shí)，晶界滑移和塑性變形所需的激活能分別為115.4和155.6 kJ/mol。

3) 在相同燒結(jié)條件下，納米氧化鋁顆粒會(huì)抑制銅的燒結(jié)致密化，導(dǎo)致樣品密度降低。抑制機(jī)理為氧化鋁顆粒阻礙晶界和位錯(cuò)運(yùn)動(dòng)，導(dǎo)致晶界滑移和塑性變形的激活能提高，從而增大致密化抗力。

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(編輯 湯金芝)

Sintering kinetics and mechanism of nano Al2O3particles dispersion strengthened copper by spark plasma sintering

JIANG Shaowen1, CHENG Lijin2, LIU Yao1, LIU Shaojun1, 3

(1. Powder Metallurgy Research Institute, Central South University, Changsha 410083, China; 2. State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, China; 3. Shenzhen Research Institute, Central South University, Shenzhen 518057, China)

The effects of nano Al2O3particles on Al2O3dispersion strengthened copper prepared by spark plasma sintering (SPS) were studied systematically by using the hot-pressing sintering model. The results show that the densification process is dominated by grain boundary diffusion and sliding in the early stage of sintering, followed by the grain boundary sliding. And the plastic deformation occurs in the last stage of sintering. The density of the copper strengthened by Al2O3particles decreases. The Al2O3particles existing along the grain boundaries can inhibit the densification of copper because the particles can block the movement of grain boundaries and dislocation, which indicates that the densification process required higher activation energy. The deformation mode is mainly twinning, which is resulted from co-existence of the shear stress and the pinning of Al2O3particles.

Al2O3dispersion strengthened copper; spark plasma sintering (SPS); sintering kinetics; twin; densification

TG146

A

1673-0224(2018)04-354-07

深圳市基礎(chǔ)研究計(jì)劃資助項(xiàng)目(JCY201110100，JCYJ20140509142357196)

2017?04?07；

2018?04?24

劉紹軍，研究員，博士。電話：0731-88876135；E?mail: liumatthew@csu.edu.cn