TA1纯(chun)钛(tai)平(ping)面(mian)各向(xiang)异(yi)性变(bian)形行为及(ji)机制(zhi)研(yan)究(jiu)

引言(yan)

纯(chun)钛因(yin)具(ju)有耐腐蚀(shi)性能好、比强(qiang)度高(gao)以及可(ke)加工性(xing)好(hao)等优点(dian),被(bei)广(guang)泛应(ying)用于(yu)汽车(che)、航空(kong)航(hang)天、军(jun)事和(he)生物(wu)工程(cheng)等领域[1-4]。其中,以α钛为基(ji)的(de)工(gong)业(ye)纯钛(tai)是(shi)一种(zhong)滑(hua)移(yi)系较少、对称(cheng)性(xing)较(jiao)差的密排六方(Hexagonal Close-Packed,HCP)晶(jing)体结构(gou)金(jin)属。经过轧制形变处(chu)理的(de)钛材在(zai)组织(zhi)结构(gou)上(shang)会(hui)产生晶(jing)体的择(ze)优取向即织构,使得轧制(zhi)钛(tai)材表(biao)现出明显(xian)的平(ping)面各向(xiang)异性。NASIRI-ABARBEKOHH等[5]研(yan)究了不(bu)同轧制(zhi)变(bian)形量对多晶(jing)纯(chun)钛(tai)力(li)学(xue)性(xing)能各(ge)向(xiang)异(yi)性(xing)的(de)影(ying)响,研究(jiu)结果(guo)表明(ming),中(zhong)低(di)变形(xing)量下(xia)纯(chun)钛的微(wei)观组(zu)织细(xi)化(hua)和机械孪晶对(dui)拉(la)伸(shen)性能的(de)各向异(yi)性(xing)行为影响(xiang)较(jiao)小,而在高(gao)变形量(>50%)下,基(ji)面织构的分(fen)裂分(fen)布以及(ji)机(ji)械纤维(wei)化对(dui)力(li)学性(xing)能的(de)各向异性(xing)响应(ying)影响显著,并(bing)且(qie)与中低程(cheng)度变(bian)形(xing)相(xiang)比(bi),轧(ya)向(xiang)(Rolling Direction,RD)试样(yang)具有比横向(xiang)(Transeverse Direction,TD)试样(yang)更(geng)大(da)的(de)强(qiang)度。迄(qi)今(jin)为(wei)止,针(zhen)对钛(tai)合金的(de)力学(xue)性能[6-9]、晶粒(li)织构(gou)[10-12]以及微(wei)观组(zu)织形貌(mao)[13-17]已(yi)经开(kai)展(zhan)了(le)大量工(gong)作(zuo)与研究(jiu),表(biao)明(ming)钛(tai)合金(jin)构件(jian)微(wei)观(guan)组织的(de)变化对其力(li)学(xue)性(xing)能具(ju)有(you)显(xian)著(zhu)影(ying)响(xiang)。

作(zuo)为(wei)一(yi)种(zhong)高性能(neng)金(jin)属结(jie)构材料(liao),钛(tai)及其(qi)合(he)金(jin)在(zai)服役(yi)过程(cheng)中(zhong)不(bu)可(ke)避(bi)免地承受(shou)压缩载荷(he)。许(xu)峰等[18]研究(jiu)了纯钛在(zai)准静态压(ya)缩条件下(xia)的(de)微观(guan)组织(zhi)及其变形机制,发现(xian)纯(chun)钛在准(zhun)静(jing)态压(ya)缩下的(de)变形机制在不同(tong)的应(ying)变(bian)阶(jie)段由(you)不(bu)同的(de)机制主(zhu)导(dao),微(wei)观(guan)组织(zhi)演变经历了形变孪晶细(xi)化(hua)晶(jing)粒、孪晶达到饱和以及(ji)晶粒破(po)碎细(xi)化3个(ge)阶(jie)段(duan)。KIMD等(deng)[19]在室(shi)温(wen)下(xia)对沿RD、TD和法向(Nnomal Drection,ND)的(de)纯钛(tai)圆(yuan)柱试(shi)样进(jin)行(xing)了(le)准静态(tai)单(dan)轴(zhou)压缩试验(yan),同(tong)时采用本(ben)构方法对纯钛的(de)变(bian)形(xing)特性进(jin)行(xing)了(le)分(fen)析,结(jie)果(guo)表明(ming)不同(tong)加载方(fang)向(xiang)的流(liu)变应力曲线与(yu)应变(bian)硬化率存在明(ming)显(xian)差异(yi)。陈(chen)志永(yong)等(deng)[20]分别沿(yan)热轧(ya)TB2钛合(he)金板(ban)的RD、TD以(yi)及板(ban)材(cai)轧(ya)制(zhi)平面内轧向成45°这3个(ge)方向(xiang)进行了压(ya)缩(suo)试验,并基(ji)于(yu)微(wei)观(guan)晶(jing)体塑(su)性变(bian)形理论(lun),定性讨(tao)论了(le)热(re)轧TB2钛(tai)合金(jin)板(ban)屈(qu)服强(qiang)度的各向异(yi)性,结果(guo)发(fa)现TD方(fang)向屈(qu)服(fu)强度和流(liu)变(bian)应力最(zui)大。

目(mu)前(qian),对(dui)于(yu)钛及(ji)钛合金的(de)研(yan)究(jiu)主(zhu)要集(ji)中在(zai)RDTD平面内的各(ge)向异(yi)性压(ya)缩变形(xing)[21-24],而(er)对(dui)于(yu)RDND平(ping)面(mian)内(nei)的(de)各向(xiang)异(yi)性压(ya)缩行(xing)为少(shao)有研(yan)究。本(ben)文(wen)利用万能材料试(shi)验机对RD-ND平面(mian)内不(bu)同取(qu)样(yang)方(fang)向的(de)冷轧态(tai)工业纯(chun)钛板(ban)TA1进(jin)行了(le)压缩试(shi)验(yan),研究(jiu)了(le)RD-ND平(ping)面(mian)内(nei)TA1纯钛压缩(suo)变(bian)形力(li)学响应的各向异性行(xing)为(wei)和机(ji)制(zhi),以期(qi)为(wei)纯(chun)钛(tai)板(ban)TA1的加工(gong)、组织(zhi)优化和应(ying)用(yong)提供(gong)参考价值(zhi)。

1、试验

试验(yan)原(yuan)材料为30mm厚(hou)的(de)TA1退(tui)火(huo)态工业纯钛板(ban),利(li)用二辊轧(ya)机(ji)在(zai)室(shi)温(wen)下(xia)以每道(dao)次(ci)0.3~0.5mm的(de)压(ya)下(xia)量(liang)轧至(zhi)7.5mm厚(hou),得到轧(ya)制(zhi)变形(xing)量为75%的(de)冷轧TA1纯(chun)钛板(ban)。用电(dian)火花(hua)线(xian)切割机切成Φ3mm×3mm的圆柱试(shi)样,圆(yuan)柱试样轴向(xiang)与轧制(zhi)方向(xiang)的夹角依(yi)次为0°、45°和90°,分(fen)别(bie)记(ji)作(zuo)RD-0°、RD-45°和(he)RD-90°,如图1a所示(shi)。在室温下采(cai)用(yong)万能材料(liao)试(shi)验机对(dui)圆柱(zhu)试(shi)样(yang)沿轴(zhou)向压缩(suo),如(ru)图1b所(suo)示(shi),应(ying)变(bian)速率为(wei)1×10-2s^-1。每个(ge)条件(jian)重复测(ce)试(shi)3次,以确保测(ce)量(liang)结(jie)果的可重(zhong)复性。将试(shi)验后(hou)的(de)试样回(hui)收(shou),沿轴线将其(qi)切开(kai),对(dui)样品(pin)进(jin)行(xing)打(da)磨(mo)抛(pao)光以(yi)及(ji)腐(fu)蚀(shi)(Kroll试剂:HF∶HNO₃∶H₂O=3∶6∶91),利用(yong)光(guang)学(xue)显(xian)微镜(jing)(Optical Microscope,OM)获(huo)得截面的(de)微观组织形(xing)貌(mao),同时(shi)采(cai)用(yong)高(gao)分辨电子(zi)背散(san)射(she)衍(yan)射(she)Electron Back scatter Diffraction,EBSD)技(ji)术(shu)对(dui)压(ya)缩后(hou)试(shi)样(yang)的(de)微(wei)观(guan)组(zu)织进(jin)行分析,以充分(fen)揭示(shi)其组织变化(hua)特(te)征和微观机(ji)理。

2、结(jie)果(guo)与(yu)讨论

2.1轧制后(hou)的(de)组(zu)织形(xing)貌

图(tu)2为75%轧(ya)制变形量的(de)TA1板材ND、RD以(yi)及(ji)TD面(mian)的(de)金(jin)相组织图。从(cong)图2可(ke)以看出,轧(ya)制(zhi)加(jia)工(gong)引起(qi)晶粒剧烈变形,使(shi)得(de)晶粒沿着(zhe)轧制(zhi)方(fang)向完全拉长(zhang)成(cheng)长条(tiao)状,这种非均质微观组织(zhi)主要(yao)由(you)滑(hua)移系和变(bian)形(xing)孪(luan)生(sheng)引(yin)起(qi)的拉长型(xing)晶粒组(zu)成,称(cheng)为(wei)机(ji)械(xie)纤维化[5]。由于(yu)轧制(zhi)变形(xing)量较(jiao)大,在(zai)小(xiao)角(jiao)度晶界和(he)变(bian)形(xing)孪晶(jing)的(de)分(fen)割(ge)下晶(jing)界变得(de)模(mo)糊不(bu)清(qing),同时产生了许多(duo)小(xiao)角度(du)晶(jing)粒和(he)变(bian)形晶粒(li),并且(qie)部分(fen)变(bian)形晶粒发(fa)生了一(yi)定(ding)程(cheng)度(du)的(de)细化(hua),形(xing)成细(xi)晶(jing)区。

另外一(yi)部(bu)分(fen)则通过轧(ya)制(zhi)过(guo)程(cheng)中(zhong)发(fa)生的晶粒破(po)碎而生成亚微米(mi)级亚结(jie)构颗(ke)粒(li)。ND面(mian)的(de)晶(jing)粒在轧(ya)制过程(cheng)中受(shou)到(dao)的(de)沿RD方向(xiang)以(yi)及(ji)TD方向的拉(la)力不均(jun)匀(yun),造成(cheng)晶粒形(xing)变不(bu)均(jun)匀,由图(tu)2a可看(kan)出(chu),ND面(mian)的晶(jing)粒碎化严(yan)重(zhong),晶粒沿(yan)RD方(fang)向拉长(zhang)程度(du)比(bi)TD方向剧(ju)烈;由图2b与(yu)图(tu)2c可(ke)看出(chu)RD面(mian)与(yu)TD面的(de)晶粒经过室温轧(ya)制(zhi)后(hou)呈(cheng)长条(tiao)状(zhuang),晶粒之间(jian)近乎(hu)平行(xing)且(qie)晶(jing)界(jie)多(duo)为(wei)平直状,此外(wai)还混(hun)有(you)大量的细(xi)小(xiao)晶(jing)粒分(fen)布在长条晶(jing)界(jie)处(chu)。因(yin)此,经(jing)过大变形量轧(ya)制(zhi)加工(gong)后(hou),TA1工业纯钛板内(nei)部(bu)组织呈现(xian)出(chu)各(ge)向(xiang)异性。在(zai)进(jin)行轧(ya)制后(hou)的(de)TA1工(gong)业纯钛板压缩试(shi)验时,需(xu)要考(kao)虑不(bu)同(tong)晶体(ti)取(qu)向(xiang)的协调(diao)变(bian)形机(ji)制存(cun)在(zai)差(cha)异(yi),使得不同取(qu)向的(de)组(zu)织演(yan)变和(he)力(li)学响应产(chan)生不(bu)一(yi)致(zhi)性(xing),从(cong)而(er)导致轧制(zhi)TA1工业(ye)纯钛(tai)板在(zai)压(ya)缩(suo)性(xing)能(neng)上表(biao)现(xian)出(chu)各(ge)向(xiang)异(yi)性。对(dui)冷轧TA1纯(chun)钛(tai)板(ban)进(jin)行(xing)电(dian)解抛(pao)光(guang)处理(li),利(li)用EBSD对TD面进行解(jie)析,扫(sao)描(miao)步(bu)长为0.7μm,如图(tu)3所(suo)示。其(qi)中(zhong),残(can)余应力(li)较为(wei)集中的部位以及(ji)尺寸(cun)小(xiao)于扫(sao)描(miao)步(bu)长(zhang)的(de)晶粒呈现出(chu)白色(se)的零分辨(bian)率区域。

当塑性变形达到(dao)一定程度时(shi),由于(yu)晶粒发(fa)生(sheng)扭(niu)转(zhuan),大(da)部分(fen)晶粒的某(mou)一位向与变形方(fang)向趋于(yu)一致(zhi),称为织(zhi)构或择优(you)取向(xiang)。α纯(chun)钛(tai)为密(mi)排六方结(jie)构(gou),在室(shi)温下(xia)的晶(jing)格常数a=0.295,c=0.468,其(qi)轴比(bi)c/a=1.587,较(jiao)理想(xiang)HCP轴比(bi)(1.633)要(yao)小(xiao),因(yin)此,可(ke)能(neng)发(fa)生(sheng)的(de)滑(hua)移(yi)方(fang)式包括{0001}基(ji)面(mian)滑(hua)移(yi)、{10-10}棱柱面(mian)滑(hua)移(yi)以及{10-11}棱(leng)锥(zhui)面(mian)滑(hua)移。

在EBSD取向图(tu)中,不同颜色(se)代(dai)表不(bu)同(tong)晶体(ti)取(qu)向(xiang)的(de)晶(jing)粒。由(you)图(tu)3可(ke)知(zhi),TA1工(gong)业(ye)纯(chun)钛板经(jing)经冷(leng)轧后(hou)形(xing)成严(yan)重的轧制织(zhi)构,大部(bu)分晶体(ti)c轴(zhou)由TD向ND偏转,其(qi)极(ji)大(da)密(mi)度(du)最(zui)高可达19.9。

2.2压缩力学(xue)性能

沿(yan)RD-0°、RD-45°和(he)RD-90°试样的压缩真(zhen)应力-真(zhen)应(ying)变曲(qu)线(xian)如(ru)图4a所(suo)示(shi)。由(you)图可知,不(bu)同取向(xiang)圆柱试样的(de)塑(su)性(xing)变(bian)形流(liu)动应力具有显著(zhu)差异,从(cong)RD-0°到(dao)RD-90°,随着压缩方(fang)向(xiang)与(yu)轧(ya)制向夹角(jiao)的(de)增(zeng)加(jia),对(dui)应(ying)的塑(su)性(xing)变(bian)形流动(dong)应(ying)力(li)随之(zhi)增加(jia)。

为(wei)了(le)探(tan)究(jiu)轧制TA1纯钛(tai)板(ban)材在(zai)压(ya)缩变形过程中(zhong)的塑(su)性变形能(neng)力,基于(yu)压(ya)缩真(zhen)应(ying)力-真(zhen)应变曲(qu)线计算了应(ying)变硬化率(θ=dσ/dε)[25]。如(ru)图(tu)4b所示(shi),RD-0°与(yu)RD-45°具有明显的(de)应(ying)变(bian)硬(ying)化(hua)行(xing)为,而(er)RD90°试(shi)样(yang)的(de)应(ying)变(bian)硬(ying)化(hua)率敏(min)感(gan)性则相对(dui)较低。

其(qi)中(zhong),RD-0°和RD-45°试(shi)样(yang)均(jun)经过(guo)初始硬(ying)化阶段和(he)硬(ying)化(hua)效应趋缓(huan)阶段,当(dang)试(shi)样(yang)经(jing)过均(jun)匀(yun)弹(dan)性变形阶段达到屈(qu)服(fu)点后,流动应力呈(cheng)缓慢(man)上升(sheng)趋势(shi),表现(xian)出(chu)良好(hao)的塑(su)性(xing)变(bian)形(xing)能(neng)力。应变硬(ying)化行(xing)为大致(zhi)可(ke)划(hua)分(fen)为3个阶段(duan):在(zai)Ⅰ阶(jie)段,试样(yang)初(chu)始的(de)应(ying)变(bian)硬化率(lv)较高,随(sui)着(zhe)应变的进(jin)一(yi)步增(zeng)加,RD-0°和(he)RD-45°试(shi)样的(de)应变(bian)硬(ying)化率迅速降(jiang)低,但(dan)是(shi)仍然保(bao)持为(wei)正值。随(sui)着应(ying)变的(de)增加(jia),试样(yang)由(you)非均(jun)匀变形(xing)向均匀变形(xing)转变(bian),从(cong)而(er)造成(cheng)应(ying)变硬化率迅(xun)速(su)下降(jiang)。而RD-90°试(shi)样(yang)的(de)初始强(qiang)度(du)较高(gao),因(yin)此在Ⅰ阶段应(ying)变(bian)硬化(hua)率下(xia)降幅度相对(dui)较缓。

同时,RD-90°试样在(zai)Ⅰ阶(jie)段(duan)结(jie)束时降(jiang)至负(fu)值,表(biao)明其承(cheng)载能力(li)降低,此(ci)时的(de)加(jia)工(gong)硬化(hua)能力(li)已跨(kua)过(guo)极(ji)限(xian)。

Ⅱ阶(jie)段(duan)为(wei)试(shi)样(yang)均(jun)匀塑性(xing)变形阶(jie)段(duan),其中(zhong)RD-0°试(shi)样(yang)的(de)应(ying)变硬化率(lv)小(xiao)幅度上(shang)升,而(er)RD-45°试样(yang)的应(ying)变(bian)硬化率(lv)保持恒定(ding)。进(jin)入(ru)Ⅲ阶段(duan),RD-0°试(shi)样应(ying)变硬化(hua)率再次(ci)下降,值(zhi)得(de)注意的是,RD-90°试样在(zai)应(ying)变(bian)达到0.3时(shi)已(yi)提(ti)前进入(ru)Ⅲ阶(jie)段(duan),说明RD-90°试(shi)样(yang)已(yi)失去承(cheng)载能(neng)力。

2.3微观(guan)组(zu)织变化

将(jiang)压缩(suo)后的样品(pin)沿(yan)轴线切开(kai),并对截(jie)面进(jin)行金(jin)相组织(zhi)观(guan)察(cha)与表征(zheng),探(tan)讨压缩载荷(he)对轧制(zhi)TA1纯(chun)钛圆(yuan)柱试样(yang)微(wei)观组(zu)织(zhi)的影响(xiang)。图(tu)5a~图(tu)5c为(wei)压缩(suo)变形(xing)后RD-0°、RD-45°以及(ji)RD-90°圆(yuan)柱(zhu)试(shi)样(yang)在(zai)沿45°最(zui)大剪(jian)切(qie)方(fang)向(xiang)上(shang)的金相(xiang)组(zu)织(zhi)图。图(tu)5d是整个截面(mian)变(bian)形组(zu)织(zhi)流(liu)动示(shi)意(yi)图(tu)。

在圆(yuan)柱(zhu)试(shi)样的压(ya)缩过(guo)程中(zhong),试样(yang)内(nei)部承(cheng)受(shou)着(zhe)轴(zhou)向压(ya)缩(suo)载(zai)荷(he),随(sui)着轴(zhou)向(xiang)应力的不(bu)断(duan)增(zeng)大,圆柱试(shi)样底(di)面横截(jie)面(mian)积(ji)持续增(zeng)加(jia)。RD-0°试样(yang)在(zai)压缩过程中,轧(ya)制(zhi)形(xing)成的长条(tiao)状(zhuang)晶粒被(bei)反向(xiang)压(ya)缩,与压(ya)缩(suo)加载(zai)方(fang)向(xiang)平行(xing)的(de)长(zhang)条(tiao)晶粒(li)将(jiang)发生(sheng)不(bu)均匀(yun)的变(bian)形过程(cheng),如(ru)图5a所示(shi)。圆柱(zhu)试(shi)样(yang)中心发生了(le)剧(ju)烈的(de)塑性(xing)流(liu)动,长条状晶粒(li)发(fa)生(sheng)扭转,由中(zhong)心(xin)向四(si)周倾(qing)斜至(zhi)垂直(zhi)于(yu)轴(zhou)向应(ying)力方(fang)向。同时,晶(jing)粒在(zai)变(bian)形(xing)与(yu)扭转的过(guo)程中(zhong),各晶粒转动(dong)并不一(yi)致(zhi),致(zhi)使晶格发生(sheng)畸(ji)变(bian),从而(er)诱发形变(bian)孪晶(jing)与二次孪晶形成。随着(zhe)压(ya)下量(liang)的(de)增加(jia),形(xing)变(bian)孪(luan)晶和(he)二(er)次(ci)孪(luan)晶(jing)的(de)体(ti)积(ji)分数(shu)持(chi)续升(sheng)高。孪(luan)晶界(jie)有望(wang)分割(ge)晶(jing)粒,从(cong)而降(jiang)低(di)有效(xiao)滑(hua)移距(ju)离(li),提高(gao)流(liu)动(dong)应力(li),因此,RD-0°试(shi)样(yang)的应(ying)变(bian)硬(ying)化(hua)率增大,表(biao)明其承受压缩(suo)载荷的(de)能力(li)增强(qiang)。从图5b可(ke)见(jian),RD45°试样塑(su)性变形过程均(jun)匀(yun),剪切变(bian)形易(yi)沿晶界发(fa)生(sheng)。长条(tiao)状晶(jing)粒在主(zhu)切(qie)应(ying)力的(de)作(zuo)用(yong)下,向(xiang)横(heng)向偏转(zhuan)程(cheng)度(du)比(bi)向上(shang)下基(ji)面(mian)偏(pian)转程(cheng)度大,当主(zhu)切应(ying)力处于极值状态(tai)时(shi)会(hui)激(ji)活(huo)45°滑(hua)移(yi)面(mian),位错沿(yan)着45°最大(da)剪切面运(yun)动(dong),从而(er)减缓材料内部(bu)的应(ying)力(li)集(ji)中(zhong),使(shi)材料具有(you)较(jiao)高的(de)韧性和延(yan)展性。然而(er),法向(xiang)应力(li)增(zeng)大(da)了滑(hua)移(yi)的阻(zu)力,造成(cheng)流变(bian)应力增加(jia)。因此(ci),RD-45°试(shi)样(yang)塑性(xing)变(bian)形(xing)流(liu)动应力比(bi)RD-0°试(shi)样大(da)。而(er)对(dui)于RD-90°试(shi)样(图(tu)5c),其(qi)长条(tiao)状(zhuang)晶(jing)与压缩加载(zai)方(fang)向垂直(zhi),在压(ya)缩(suo)过(guo)程(cheng)中(zhong)由轧制(zhi)形(xing)成(cheng)的长条(tiao)状(zhuang)晶粒(li)持续(xu)拉长。

在变(bian)形初(chu)期,由于(yu)应(ying)变(bian)硬化作用(yong),RD-90°试样(yang)具(ju)有(you)较高的(de)强度。当(dang)位错运(yun)动到(dao)晶(jing)界(jie)附(fu)近时,受晶界的阻碍,位(wei)错更容易在晶界处发(fa)生塞积(ji),导致(zhi)变(bian)形抗力(li)提(ti)升,造(zao)成(cheng)后续(xu)塑(su)性变(bian)形(xing)困难(nan)。随着(zhe)变形(xing)量的(de)进(jin)一(yi)步(bu)增(zeng)大(da),拉长(zhang)晶(jing)粒(li)将触及(ji)强(qiang)度(du)极限并被(bei)拉(la)断,导(dao)致(zhi)试(shi)样(yang)承(cheng)载能(neng)力(li)降低(di),呈(cheng)现(xian)出(chu)应变(bian)软化现(xian)象。

利(li)用氩离子抛光(guang)技术对(dui)压(ya)缩变(bian)形(xing)后(hou)的(de)RD-0°、RD-45°和RD-90°圆柱(zhu)试样(yang)截面(mian)进行抛光(guang)处理(li),采(cai)用EBSD对圆柱试(shi)样截(jie)面(mian)中心区域进(jin)行(xing)表(biao)征(图(tu)6),扫描步(bu)长(zhang)为0.2μm。在压缩(suo)变形过(guo)程中,经轧(ya)制(zhi)形成(cheng)的(de)长(zhang)条状晶(jing)粒(li)进(jin)一(yi)步细(xi)化(hua)成(cheng)细(xi)晶和(he)超(chao)细晶(jing),由(you)图(tu)6a~图(tu)6c可知,试(shi)样在(zai)变(bian)形过程中(zhong)受到(dao)沿轴(zhou)向(xiang)施(shi)加的应力,并产(chan)生(sheng)了(le)较(jiao)大(da)的压缩应(ying)变(bian),导致晶粒发生(sheng)扭转(zhuan),从(cong)而(er)引起(qi)晶(jing)粒(li)间的取向(xiang)差异,根(gen)据取样(yang)方(fang)向的不(bu)同,形(xing)成了(le)明(ming)显不(bu)同的(de)织构(gou)。同(tong)时可(ke)以(yi)看出,在压(ya)缩变形(xing)过程(cheng)中(zhong),塑(su)性(xing)变(bian)形沿剪切方(fang)向高度(du)集中,相应的晶粒细(xi)化(hua)严重。

如(ru)图6a所(suo)示(shi),RD-0°取向的(de)压缩(suo)试(shi)样(yang)中,由(you)轧制所(suo)形成(cheng)的{11-20}孪晶(jing)主要分(fen)布在长条(tiao)晶(jing)粒(li)内(nei)部(bu)以(yi)及(ji)个(ge)别(bie)发(fa)生扭转的(de)粗大(da)晶(jing)粒中。在压(ya)缩(suo)过(guo)程(cheng)中(zhong),由于轴(zhou)向(xiang)应力与RD方(fang)向一致(zhi),存(cun)在于长条状晶粒内(nei)的(de)轧制(zhi)孪晶会跟(gen)随母(mu)晶粒一同(tong)被反(fan)向压缩(suo),使得部(bu)分(fen)轧(ya)制(zhi)孪晶(jing)逐(zhu)渐恢复(fu)成(cheng)与母晶粒取向一致的纤(xian)维织(zhi)构(gou)。

图(tu)6d为(wei)RD-0°试(shi)样(yang)压缩后(hou)在RD-ND坐(zuo)标(biao)系(xi)下(xia)的极(ji)图,由图可(ke)知(zhi),样品(pin)中出(chu)现(xian)3种(zhong)主(zhu)要(yao)的织(zhi)构组(zu)分(fen)。其中,长条状(zhuang)晶(jing)粒(li)经过反(fan)向(xiang)压缩(suo)后大(da)部(bu)分(fen)c轴(zhou)向(xiang)TD倾斜,如图(tu)6d中A类(lei)型织构(gou)所示,最大织(zhi)构强度达到27.3。由(you)于在(zai)压(ya)缩(suo)过程(cheng)中(zhong)晶(jing)粒扭(niu)转(zhuan)程(cheng)度不(bu)同,与(yu)A类(lei)型(xing)织构较(jiao)为接(jie)近的(de)B类型织(zhi)构滑(hua)移(yi)方(fang)式(shi)皆属于(yu){0001}基面滑(hua)移(yi),少部分晶(jing)粒(li)的滑移方(fang)式为{10-11}棱(leng)锥(zhui)面滑(hua)移(yi),即C类型(xing)织(zhi)构(gou)。同时(shi),随着应(ying)变(bian)的增加,轧制孪晶逐(zhu)渐(jian)愈合(he),而二(er)次(ci)孪(luan)晶(jing)在变(bian)形后期开(kai)始(shi)形核(he),促(cu)使(shi)部(bu)分(fen)晶粒细化(hua)。

如图(tu)6b所(suo)示(shi),在RD-45°试样中,由(you)于(yu)晶界方(fang)向与剪切方(fang)向(xiang)平行(xing),在剪应力(li)的作(zuo)用(yong)下(xia),位(wei)于晶(jing)界(jie)附(fu)近(jin)的位错与晶(jing)界相(xiang)互(hu)作(zuo)用(yong),长(zhang)条(tiao)状(zhuang)晶(jing)粒之间(jian)沿剪切方向(xiang)形成细(xi)小的晶粒。同(tong)时(shi),极(ji)大(da)密度(du)点向(xiang)试(shi)样45°最(zui)大剪(jian)切(qie)面(mian)两侧(ce)偏(pian)离(li),如图6e所示(shi),由(you)于晶粒择优(you)取(qu)向高(gao)度一致,其(qi)最大织构强(qiang)度(du)可(ke)达(da)69.9;而(er)处于最大剪(jian)切(qie)面以(yi)外(wai)的(de)晶(jing)粒在(zai)正应(ying)力(li)的作(zuo)用下(xia)依(yi)旧(jiu)会(hui)发生(sheng)扭(niu)转(zhuan),表现(xian)形式(shi)为位错(cuo)在(zai){10-10}<1-210>滑移(yi)系上发(fa)生滑(hua)移。RD-90°试样(yang)塑(su)性(xing)变(bian)形能力(li)相对较(jiao)弱,位错(cuo)滑(hua)移及(ji)孪生都较为(wei)困难(nan)。由(you)图(tu)6c可看出(chu),具有(you)高取(qu)向差(cha)角的(de)孪晶(jing)界(jie)可以阻碍(ai)可动(dong)位(wei)错的(de)运动(dong),从而提高(gao)应变(bian)硬化(hua)率,导致流变应力的(de)增(zeng)加。在(zai)压(ya)缩(suo)载荷作用(yong)下,位(wei)于(yu)长条状晶粒(li)内部(bu)的(de)轧制(zhi)孪(luan)晶(jing)会(hui)随着(zhe)应变(bian)的增(zeng)加(jia)与母晶(jing)粒(li)一(yi)同(tong)被压缩(suo)变(bian)短(duan),而(er)孪(luan)晶在剪应(ying)力(li)作用下将长(zhang)条状晶粒分割成(cheng)独立且(qie)不均(jun)匀(yun)的(de)细(xi)小(xiao)碎晶(jing)。同(tong)时(shi),随(sui)着(zhe)应变的(de)增(zeng)加(jia),位(wei)错(cuo)在(zai)晶(jing)界处不断偏聚,形成(cheng)拉长(zhang)的胞(bao)状(zhuang)亚(ya)结构(gou),当应(ying)变量(liang)超过(guo)一定程(cheng)度后,亚(ya)晶(jing)的(de)胞壁越来越(yue)薄(bao),直(zhi)至(zhi)亚(ya)晶粒坍塌(ta)破(po)碎(sui)成(cheng)超(chao)细晶。如(ru)图(tu)6f所示(shi),RD-90°试(shi)样为(wei)TD偏(pian)向(xiang)ND的双峰织(zhi)构(gou)。在压(ya)缩过程(cheng)中(zhong),由(you)于位错滑移(yi)与孪生(sheng)都较为(wei)困(kun)难(nan),且轧制孪(luan)晶受(shou)剪(jian)应力的(de)影(ying)响(xiang)特(te)征(zheng)不再明(ming)显,从(cong)而控(kong)制(zhi)了最终的织构演变(bian)。其极(ji)大(da)密度(du)点偏离(li)重(zhong)心(xin)更(geng)贴(tie)近于(yu)ND,最(zui)大(da)织(zhi)构(gou)强度(du)为(wei)23.1。轧制(zhi)后(hou)的TA1纯(chun)钛(tai)在(zai)沿不同的(de)方向(xiang)压缩(suo)时表(biao)现出明(ming)显(xian)不(bu)同(tong)的塑性变(bian)形行(xing)为(wei)和(he)组织演(yan)变(图(tu)4~图6),这(zhe)种差(cha)异(yi)与(yu)压(ya)缩后(hou)晶粒平均(jun)取向(xiang)或(huo)几(ji)何(he)必(bi)需(xu)位错(cuo)密度(du)(Geometrically Necessary Dislocation,GND)变(bian)化(hua)密(mi)切(qie)相(xiang)关(guan)。且平(ping)均(jun)取(qu)向角差(cha)(Kernel average misorientation,KAM)与(yu)几何必(bi)需(xu)位错(cuo)密(mi)度呈正相关,通(tong)过(guo)KAM可(ke)以定(ding)量计算(suan)出(chu)GND密(mi)度(du)[26]。

ρ_GND=2θ_KAM-ave/μb(1)

式(shi)中:ρ_GND为局(ju)域(yu)点(dian)的GND密度;θ_KAM-ave为所(suo)选(xuan)区域的(de)平均取(qu)向角(jiao)差;μ为(wei)扫(sao)描步(bu)长;b为Burgers矢量大(da)小(xiao)。

图(tu)7分(fen)别为(wei)经(jing)过压缩变形后RD-0°、RD-45°和(he)RD-90°试样(yang)的(de)KAM分布(bu)图,统(tong)计(ji)得(de)出(chu)局部晶(jing)粒KAM平均值(zhi)mka分别为1.65°、1.15°和(he)1.53°。在压(ya)缩变形过(guo)程中(zhong),RD-0°试样形成(cheng)了大(da)量的压(ya)缩孪(luan)晶(jing)与(yu)二(er)次(ci)孪(luan)晶,孪晶(jing)界的(de)存(cun)在(zai)可作(zuo)为(wei)位(wei)错(cuo)运(yun)动(dong)的障(zhang)碍(ai),有助于应变(bian)硬化(hua)能力的增(zeng)强。如图7a所(suo)示,大(da)晶粒中的(de)GND密(mi)度(du)比(bi)晶界(jie)处的(de)GND密(mi)度小(xiao),这(zhe)是(shi)因(yin)为(wei)位(wei)错(cuo)滑移(yi)在孪(luan)晶(jing)界面处的(de)排(pai)斥作用导(dao)致(zhi)位(wei)错塞积和高(gao)度(du)缠结(jie),从(cong)而使(shi)得(de)RD-0°试样局(ju)部平(ping)均(jun)KAM值较(jiao)高(gao),试(shi)样总GND密度也(ye)更(geng)高,因(yin)此,在提升(sheng)材(cai)料强度(du)的(de)同时(shi)使(shi)圆柱试(shi)样(yang)承(cheng)受(shou)压(ya)缩载荷(he)的能力(li)提高。RD-45°试(shi)样中的位(wei)错(cuo)易在45°最大滑移面上(shang)运(yun)动,在滑移(yi)面(mian)上(shang)相(xiang)对运动(dong)的(de)异号位错滑(hua)移(yi)相互(hu)抵消(xiao),减缓了材料内(nei)部的应(ying)力(li)集中,导(dao)致(zhi)GND释放(fang),因(yin)此(ci)其(qi)平均KAM值(zhi)低(di)于(yu)RD-0°和RD-90°试样(yang)。图7c为(wei)RD-90°试(shi)样KAM分(fen)布图,在(zai)轧(ya)制拉长晶(jing)粒(li)的基(ji)础上(shang),经(jing)压缩变(bian)形(xing)处(chu)理(li),RD-90°试样(yang)的长条(tiao)状(zhuang)晶粒得(de)以(yi)进(jin)一步延伸,从(cong)而使得(de)平均(jun)KAM值在(zai)轧制处(chu)理(li)的(de)基(ji)础上(shang)不断(duan)提升。然(ran)而(er),RD-90°试(shi)样的平(ping)均(jun)KAM要稍(shao)微低(di)于RD-0°试样,这(zhe)是(shi)因为随(sui)着应(ying)变的(de)逐(zhu)渐(jian)增加,位(wei)错和轧(ya)制孪晶在剪应(ying)力的(de)作(zuo)用(yong)下将(jiang)长(zhang)条状晶粒分割(ge)成独立且不(bu)均匀(yun)的细小(xiao)碎(sui)晶(jing),应力(li)得(de)以释放,形(xing)成(cheng)应(ying)变软(ruan)化,因此(ci)RD-90°试(shi)样的(de)平均KAM稍(shao)微(wei)低(di)于(yu)RD-0°试(shi)样。

3、结(jie)论(lun)

(1)取(qu)向对(dui)材料(liao)的(de)塑性变(bian)形流动(dong)应力(li)有明(ming)显地(di)影(ying)响(xiang)。从RD-0°到RD-90°试样,当(dang)压缩(suo)方(fang)向与(yu)轧(ya)制(zhi) 方(fang)向的(de)夹(jia)角(jiao)增(zeng)大(da)时(shi),材料(liao)相(xiang)应的(de)塑(su)性变(bian)形流(liu)动应(ying)力(li)也增大。

(2)取(qu)向对(dui)材(cai)料的(de)应变(bian)硬(ying)化(hua)能(neng)力有重(zhong)要影响(xiang)。沿(yan)RD-0°与RD-45°压(ya)缩(suo)时,材料的(de)应(ying)变(bian)硬(ying)化(hua)能力(li)较高,而沿RD-90°压(ya)缩时,材(cai)料(liao)中(zhong)因(yin)易(yi)形成(cheng)裂(lie)纹(wen)而(er)失(shi)效。

(3)取(qu)向对材料的塑(su)性(xing)变(bian)形(xing)行为(wei)具(ju)有影响(xiang)。RD0°、RD-45°与RD-90°试样(yang)在(zai)变形(xing)初期均以位错(cuo)滑(hua)移为 主(zhu),随着变形量(liang)的增(zeng)加(jia),不(bu)同取(qu)向试(shi)样的(de)塑性变(bian)形有所不(bu)同(tong)。RD-0°试样(yang)的(de)塑性变形以(yi)位(wei)错滑移和(he)形成(cheng)孪(luan)晶(jing)为主;RD-45°试(shi)样(yang)的塑性变(bian)形(xing)以(yi)位(wei)错滑移为主(zhu);RD90°试(shi)样的(de)塑性(xing)变(bian)形以(yi)位错(cuo)滑(hua)移(yi)和晶(jing)粒(li)破(po)碎为主。

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