网络首发时间: 2019-04-19 16:49

稀有金属 2021,45(02),129-136 DOI:10.13373/j.cnki.cjrm.xy18120028

2195铝锂合金力学性能和组织与冷热变形过程的相关性

章润喆 马云龙 刘丹阳 宁红 李劲风 蔡超

中南大学材料科学与工程学院

北京宇航系统工程研究所

中南大学轻质高强结构材料重点实验室

宁夏大学化学与化工学院

摘 要：

采用力学性能测试、光学显微镜（OM）、X射线衍射（XRD）及透射电镜（TEM），研究了5 mm厚度2195铝锂合金挤压板材和后续2 mm厚度冷轧薄板T8时效态力学性能及微观组织的演化。研究结果表明：相同T8时效（3.8%预变形，148℃，38 h）条件下，5 mm厚度挤压板材抗拉强度及屈服强度比后续2 mm厚度冷轧薄板高45～55 MPa，伸长率基本相当。不同方式成形的2195铝锂合金织构组成有明显区别；固溶处理后，挤压板材主要由变形织构（Brass织构及S织构，总体积分数达66%）以及部分Cube织构（体积分数为18%）组成，而冷轧薄板变形织构含量较少（仅含Brass织构，体积分数为14%）但出现了大量的R取向织构（R-Cube及R织构，总体积分数达21%）。2195铝锂合金T8时效主要强化相为T1相（Al₂CuLi）和θ'相（Al₂Cu）；与2 mm厚度冷轧薄板相比，相同T8时效状态下，5 mm厚度挤压板材T1相尺寸较大，但数密度降低且分布不均匀。

关键词：

2195铝锂合金;力学性能;微观组织;变形过程;

中图分类号： TG146.21

作者简介：章润喆（1994-），男，湖南湘潭人，硕士研究生，研究方向：铝锂合金，E-mail:m17307483121@163.com；;*李劲风，教授，电话：13278861206,E-mail:lijinfeng@csu.edu.cn;

收稿日期：2018-12-29

基金：国家自然科学基金项目（51741107）资助;

Dependence of Mechanical Properties and Microstructures of 2195 Al-Li Alloy on Cold and Hot Deformation Process

Zhang Runzhe Ma Yunlong Liu Danyang Ning Hong Li Jinfeng Cai Chao

School of Materials Science and Engineering,Central South University

Beijing Institute of Astronautical Systems Engineering

National Key Laboratory of Science and Technology on High Strength Structural Materials,Central South University

School of Chemistry and Chemical Engineering,Ningxia University

Abstract：

Al-Li alloys are suitable for application in aerospace,due to their low density,high specific strength and stiffness and low fatigue crack growth rate.Among them,2195 Al-Li alloy is the main structural materials of rocket cryogenic fuel tank.It is usually used as plate,and there are few reports on the properties of extruded profiles.In addition,the properties and structures of Al-Li alloys are highly dependent on the proccessing.To widen the application of 2195 Al-Li alloy,the mechanical properties and structures of its extruded profiles were investigated.Meanwhile,the mechanical properties and structures of its cold-rolled thin sheet were investigated,and the influence mechanism of processing was clarified.The 2195 Al-Li alloy ingot with chemical composition of Al-4.11 Cu-1.03 Li-0.4 Mg-0.4 Ag-0.12 Zr(%,mass fraction) was prepared through melting and casting.After homogenization annealing,it was extruded to a plate with 5 mm thickness at 460℃A part of the extruded plate was annealed and then rolled to a sheet with a thickness of 2 mm through cold-rolling.For simplification,the extruded plate with 5 mm thickness was denoted as E-plate,and the cold-rolled sheet with 2 mm thickness was referred to as CR-sheet.After solutionization at 505℃ for 60 min,T8 aging at 148℃ for 38 h following 3.8% pre-stretch was applied to the E-plate and CR-sheet.The mechanical properties of the T8 aged E-plate and CR-sheet were measured through an MTS 858 test machine.The grain structures were investigated through optical microscope(OM).Their textures were measured by an X-ray diffractometer(XRD,Brucker D8 Discovery).Meanwhile,the aging precipitates in the T8 aged samples were observed through a Tecnai G² 20 transmission electron microscopy(TEM).The results were as follows:(1) After the same T8 aging,the average tensile strength and yield strength of the T8 aged E-plate were about 591 MPa and 555 MPa,respectively,which were 45-55 MPa higher than those of the T8 aged CR-sheet.Their elongation is almost equivalent in the range of 11%-12%.(2) The recrystallization degree in the solutionized E-plate and CR-sheet was different.The CR-sheet was highly recrystallized during the solutionization process due to its high energy storage.The energy storage was much low in the E-plate,the solutionized E-plate therefore was not recrystallized and composed of fiber-like grains.(3) The texture component of the solutionized 2195 Al-Li alloy with different deformation modes was different.Corresponding to un-recrystallized grain structures,deformation textures(Brass and S textures) with a high volume fraction of about 66% and cube textures with volume fraction of about 18% existed in the solutionizaed E-plate.However,the volume fraction of deformation textures(Brass and S textures) was decreased to about 14%,and a large number of R-oriented textures(R-Cube and R textures,volume fraction of about 21%) appeared in the solutionized CR-sheet,which corresponded to high recrystallization of the solutionized CR-sheet.(4) The main aging precipitates were T1(Al₂CuLi) and θ'(Al₂Cu) in both the T8 aged Eplate and T8 aged CR-sheet,but their distribution was different.It was interesting that the average number density of T1 and θ' precipitates in the TEM foil of the T8 aged E-plate was lower,but their average size was larger than that of the T8 aged CR-sheet.In addition,the T1 precipitates were distributed evenly in the T8 aged Cr-sheet,but unevenly in the T8 aged E-plate.The deformation process impacted the mechanical properties and structures of 2195 Al-Li alloy.The extrusion at high temperature was beneficial to retain deformation texture and reduce recrystallization degree in the solutionized state,and therefore enhanced the mechanical properties of the T8 aged state.However,the cold-rolling increased the recrystallization degree and decreased deformation textures in the solutionized state,and therefore lowered the mechanical properits of the T8 aged state.

Keyword：

2195 Al-Li alloy; tensile property; microstructure; texture; deformation process;

Received： 2018-12-29

锂是最轻的金属元素，在铝合金中每加入1%锂，可使铝合金密度降低3%，刚度提高6% ^[1] 。新型铝锂合金具有密度低、弹性模量高、比强度高和比刚度高、疲劳裂纹扩展速率低和低温性能好等特点 ^[2,3] ，是航空航天领域最理想的结构材料，具有广泛的应用前景。

变形过程的不同使得铝合金组织（晶粒组织、时效析出相）有着明显区别，进而影响铝合金的性能。张永皞等 ^[4] 研究发现，3104及1050铝合金板材冷轧变形后主要由S取向的织构组成，而热轧变形后则具有明显的铜型织构特征。不同织构组分对铝合金力学性能的影响也有区别，如{111}面织构可以明显降低制耳效应，提高深冲压性能，同时适量的再结晶织构可以降低合金的各向异性 ^[5,6,7] 。研究发现 ^[8,9,10] ,T1相是Al-Cu-Li合金的主要强化相，其强化效果与分布特征（尺寸、数密度等）有直接关系，并建立数学模型对其进行了定量分析，但并未结合变形过程综合讨论时效析出相与力学性能的关系。

2195铝锂合金板材是航天运载器低温燃料贮箱主要的结构材料，对于其轧制板材的组织性能及应用已有大量报道 ^[11,12] ，但尚无其挤压型材的组织、性能及相关应用报道。本文进行了2195铝锂合金挤压及后续冷轧两种变形过程的组织与性能研究，一方面阐明2195铝锂合金挤压件的组织与性能特征，为拓宽2195铝锂合金的应用奠定基础；另一方面揭示变形过程对2195铝锂合金性能的影响及机制。

1 实验

2195铝锂合金成分如表1所示。金属配料完成后放入坩埚中在电阻炉中熔炼，随后在模具中浇注成型圆柱形铸锭（Φ90 mm）。铸锭经均匀化处理，在460℃保温8 h之后挤压成厚度为5 mm，宽度为70 mm的带板；挤压时，挤压速度为5 m·min^-1，挤压比为19.35。而后，部分5 mm厚度挤压带板经中间退火后冷轧至2 mm厚度薄板。5 mm厚度挤压带板以及2 mm厚度冷轧薄板经505℃固溶60 min、冷水淬火后，再进行T8人工时效处理。T8时效处理工艺为：3.8%拉伸预变形，时效温度148℃，时效时间38 h。

表1 2195铝锂合金化学成分  下载原图

Table 1 Chemical composition of 2195 Al-Li alloy (%,mass fraction)

采用MTS810材料试验机进行时效后2195铝锂合金的室温拉伸性能测试，挤压板材拉伸试样平行段长度为55 mm，平行段宽度为12.5 mm；冷轧薄板拉伸试样平行段长度为32 mm，平行段宽度为8 mm，拉伸速度为1 mm·min^-1。采用Leica DMILM光学显微镜（OM）对金相组织样品进行观察，样品观察前经打磨、抛光及阳极覆膜处理。采用Brucker D8 Discovery型X射线衍射仪（XRD）测定固溶处理后板材的织构组分，测得{111}，{200}和{220}方向的不完全极图，然后利用Bunge函数级数展开法计算取向分布函数（ODF）。采用粒子群算法对织构进行定量分析，角度偏差在15°以内视为同种织构。采用Tecnai G²20型透射电镜（TEM）对时效处理后合金的微观组织进行观察。TEM试样首先利用打磨机机械减薄至0.08～0.10 mm，再采用双喷电解减薄仪制备。电解溶液（体积分数）为25%硝酸+75%甲醇混合溶液，工作温度为-25～-30℃，工作电压为15～20 V，工作电流为80～95m A。TEM观察时加速电压为200 k V。TEM观察后采用Image J软件对时效析出相的数量、尺寸进行统计分析。

2 结果与讨论

2.1 力学性能

图1所示为挤压板材及冷轧薄板T8态时效的纵向力学性能。由图1可见，挤压板材的平均屈服强度为555 MPa，抗拉强度为591 MPa，伸长率为11.9%。

T8时效处理后，冷轧薄板力学性能明显低于挤压板材，其平均屈服强度为513 MPa，抗拉强度为553 MPa，伸长率为11.4%。挤压板材的T8态时效屈服强度及抗拉强度均高于冷轧薄板45～55MPa，而伸长率差别很小。

图1 2195铝锂合金挤压板材及冷轧薄板T8态时效纵向力学性能

Fig.1 Longitudinal mechanical properties of extruded and cold-rolled 2195 Al-Li alloy

2.2 金相显微组织与织构

2195铝锂合金纵截面金相照片如图2所示。图2(a）为挤压板材挤压态金相照片，可以明显观察到晶粒均沿挤压方向拉长，呈细长纤维状。图2(b）为挤压板材固溶态的金相照片，经固溶处理后晶粒沿厚度方向稍有长大，但基本还是维持原有纤维状形貌。图2(c）为冷轧薄板固溶处理后的金相照片，虽然晶粒仍表现为一定的长条状，但其长厚比明显降低，合金发生了明显的再结晶。

图3所示为固溶处理后2195铝锂合金挤压板材和冷轧薄板的取向分布函数（ODF）全图。挤压板材固溶处理后主要含有大量的Brass织构{110}<211>,S织构{123}<634>以及部分Cube织构{001}<110>，体积分数分别为21.98%,44.04%和17.78%。由其存在较强的Brass织构以及S织构可知，挤压板材固溶处理后处于明显的非再结晶状态。与挤压板材不同的是，冷轧薄板固溶处理后Brass织构和Cube织构的体积分数明显降低，分别为13.70%和2.23%，而S织构则彻底消失。另外，合金中出现了Goss织构{011}<100>和剪切再结晶织构（包括R-Cube织构{001}<110>和R织构{124}<211>），体积分数分别为6.34%,16.14%和5.00%。图4所示为挤压板材和冷轧薄板固溶处理后各织构组分体积分数，挤压板材固溶处理后变形织构（Brass织构及S织构）体积分数（约66%）远高于冷轧薄板，且未检测到剪切再结晶织构（包括R-Cube织构及R织构）。织构分布密度可以反映织构各组分的强度。固溶处理后，挤压板材存在非常强烈的Brass织构及S织构，最大强度级别分别达到23及18，而冷轧薄板Brass织构最大强度F(g)_max级别仅为4，且未检测出S织构（图5）。

图2 2195铝锂合金纵截面金相照片

Fig.2 Longitudinal sectional OM images of 2195 Al-Li alloy

(a) Extruded plate with 5 mm thinkness;(b) Solutionizedplate with 5 mm thinkness;(c)Solutionized sheet with 2 mmthickness

图3 固溶处理后挤压板材和冷轧板材ODF图

Fig.3 ODF images of solutionized 2195 Al-Li alloy

(a)Extruded plate with 5 mm thickness;(b)Cold-rolled sheetwith 2 mm thickness

2.3 时效析出相

图6所示为2195铝锂合金T8态时效挤压板材和冷轧薄板[112]_Al晶带轴选区衍射谱（SAED）及相应TEM暗场（dark field,DF）像照片。T8态时效挤压板材的SAED谱中存在着非常明显的T1(Al₂CuLi）相斑点（图6(a）），相应TEM暗场像中可观察到大量的T1相（图6(b））。同样，T8态时效冷轧薄板中也观察到大量T1相（图6(c,d）），但其数密度高于挤压板材。

图4 2195铝锂合金挤压板材及冷轧薄板固溶处理后各织构组分体积分数

Fig.4 Volume fraction of different textures in extrueded plate and cold-rolled sheet after solution treatment

图5 2195铝锂合金挤压板材及冷轧薄板固溶处理后各织构组分最大分布密度

Fig.5 Maximum density of different textures in extrueded plate and cold-rolled sheet after solution treatment

图6 2195铝锂合金T8时效[112]Al晶带轴SAED谱及TEM暗场像照片

Fig.6[112]_AlSAED patterns and TEM DF images of T8 aged2195 Al-Li alloy

(a)SAED pattern of extruded plate;(b)DF image of extrudedplate;(c)SAED pattern of cold-rolled sheet;(d)DF image ofcold-rolled sheet

图7所示为2195铝锂合金T8时效挤压板材和冷轧薄板[001]_Al晶带轴SAED谱及相应TEM暗场像照片。SAED谱中均存在θ'（Al₂Cu）相芒线（图7(a,c）），相应TEM暗场像中均可以观察到相互垂直的θ'相（图7(b,d）），但T8时效冷轧薄板的θ'相数密度明显较高。

为更精确表征析出相的分布特征，对T8态时效挤压板材和冷轧薄板T1相以及θ'相的尺寸及数密度进行了统计。分别从<112>_Al和<001>_Al方向观察的TEM暗场像中各选取3张照片进行T1相和θ'相统计，而后计算其平均值，其结果如图8所示。T8态时效挤压板材的T1相及θ'相平均尺寸均大于冷轧薄板（图8(a）），而平均数密度均低于冷轧薄板（图8(b））。

另外，由图6(b,d）可知，T8态时效挤压板材与冷轧薄板中T1相分布存在很大差异。为更直观反映这种差异，选取图6中数密度大和数密度小的样本区域（图中标记为M和S）进行统计，样本区域面积为0.16μm²，结果如表2所示。由表2可知，挤压板材的T1相分布密集区域M1和稀疏区域S1数密度分别为320#·μm^-2和75#·μm^-2，而冷轧薄板的区域M2和S2数密度分别为356和202#·μm^-2。上述结果表明，T8态时效挤压板材T1相数密度的极差及与均值的差异都更大，这反映出其分布不均匀性高于冷轧薄板。

图7 2195铝锂合金T8时效[001]Al晶带轴SAED谱及TEM暗场像照片

Fig.7[001]_AlSAED patterns and TEM DF images of T8 aged2195 Al-Li alloy

(a)SAED pattern of extruded plate;(b)DF image of extrudedplate;(c)SAED pattern of cold-rolled sheet;(d)DF image ofcold-rolled sheet

图8 2195铝锂合金T8时效态析出相平均尺寸及数密度

Fig.8 Average length and number density of T1 andθ'precip-itates in T8 aged 2195 Al-Li alloy

(a)Length;(b)Number density

表2 不同局部区域T1相数密度  下载原图

Table 2 Number density of T1 precipitates in different regions

2.4 分析与讨论

上述研究表明，2195铝锂合金挤压板材固溶处理后，纵截面晶粒沿挤压方向拉长成纤维状分布呈未再结晶特征；而冷轧薄板固溶处理后发生明显的再结晶。这种晶粒组织特征与变形后的储能有关。挤压过程中发生动态回复及动态再结晶，变形后合金储能较低再结晶驱动力小，固溶处理时不易发生静态再结晶，仍然保持其原有的纤维组织形貌 ^[13] 。而薄板在冷轧过程中累积了大量储能，提高了固溶处理时的静态再结晶驱动力，因而固溶后形成了大量细小的再结晶晶粒 ^[14] 。

Jata等 ^[15] 研究发现，层错能较高的铝锂合金在高温变形时更容易进行动态回复，使得变形带边界附近的位错可以在较低的能量状态下重新排列，有利于形成具有带状倾向的变形织构。而冷轧过程中，不发生动态回复及动态再结晶，弥散第二相周围引入大量局部内应力，使得相邻晶粒旋转，产生细小且取向凌乱的亚晶结构及弱织构 ^[16] 。而且，冷轧过程的非均匀剪切变形机制使得晶粒取向趋于随机分布，形成易于再结晶的取向，如（001）和（111）取向，促进了固溶时变形织构向再结晶织构的转变 ^[17,18] 。所以，冷轧薄板固溶处理时，储能释放发生再结晶，形成了大量的R-Cube织构和R织构（总体积分数约21%），而Brass织构及S织构体积分数显著减少。

挤压变形过程中，温度高，容易发生动态回复和动态再结晶。因此与冷轧薄板相比，挤压板材中位错密度较低，储能较小。因而固溶处理时静态再结晶驱动力小，静态再结晶程度很低，大部分位错及小角度晶界不均匀地保留在固溶体内部。但薄板在冷轧过程中，不发生动态回复和动态再结晶，变形后位错密度高，储能较大，固溶处理时静态再结晶程度高使得固溶处理后位错密度显著降低。上述原因导致挤压板材固溶处理后位错密度较高且分布不均匀，而冷轧薄板固溶后位错密度较低。由于位错促进T1相形核及溶质原子扩散 ^[19] ，因而挤压板材时效时T1相析出不均匀且部分T1相生长速率较高，导致T1相尺寸较大。

铝合金的强度与Schmid因子密切相关，Schmid因子越大，位错越容易滑移，从而屈服强度越低。文献表明，来自于[112]轴的Brass织构与[634]轴的S织构对铝合金多数滑移系的Schmid因子值偏小，对合金强化作用大 ^[20] 。2195铝锂合金挤压板材固溶后变形织构（Brass及S织构）总体积分数大于冷轧薄板，这是相同T8热处理工艺条件下挤压板材强度高于冷轧薄板的重要因素之一。冷轧薄板固溶处理后，形成了较多R取向的剪切再结晶织构（R织构和R-Cube织构），削弱了晶粒取向的一致性，对铝合金的强度有不利的影响 ^[21,22] 。

另外，即使挤压板材固溶并时效后T1相密度小于冷轧薄板，但其强度并没有降低，反而提高，这与T1相的形貌特征有关。T1相沿基体{111}_Al面呈片状析出；其直径（即<112>_Al方向观察时的长度）越大，T1相三个变体在{111}_Al滑移面上的有效距离越小，强化效果明显增加 ^[23] 。这也导致挤压板材的强度高于冷轧薄板。

3 结论

1.经相同T8态时效（3.8%预拉伸变形，148℃，38 h）后，5 mm厚度挤压板材抗拉强度及屈服强度比后续2 mm厚度冷轧薄板高45～55 MPa，伸长率基本相当。

2.不同方式成形的2195铝锂合金织构组分有明显区别。固溶处理后，5 mm厚度挤压板材主要由变形织构（Brass织构、S织构，总体积分数为66%）以及部分Cube织构（体积分数为18%）组成；后续2 mm厚度冷轧薄板的Brass织构含量较少，未发现S织构，但出现了大量的R取向织构（R-Cube及R织构，总体积分数为21%）。

3.2195铝锂合金T8时效主要强化相为T1相（Al₂Cu Li）和θ'相（Al₂Cu）。与2 mm厚度冷轧薄板相比，相同T8时效状态下，5 mm厚度挤压板材T1相的尺寸较大，但数密度降低且分布不均匀。

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