简介概要

时效制度对2A97铝锂合金强韧性的影响

来源期刊:中国有色金属学报2011年第3期

论文作者:钟申 郑子樵 廖忠全 蔡彪

文章页码:546 - 553

关键词:2A97铝锂合金;微观组织;时效制度;断裂韧性

Key words:2A97 Al-Li alloy; microstructure; aging treatment; fracture toughness

摘    要:通过拉伸试验及平面应变断裂韧性测试,研究不同时效制度对2A97合金强韧性的影响,并利用透射电镜和扫描电镜对合金的微观组织及断口形貌进行观察。结果表明:2A97合金在135 ℃单级时效时断裂韧性较好,但强度偏低;双级时效可促进强度的提高,但第二级时效温度过高时(175 ℃),断裂韧性迅速下降,经过(135 ℃, 36 h)+(150 ℃, 18 h)时效,可得到较好的强韧性匹配,其σb、σ0.2和KIC 分别为547 MPa、489 MPa和28.9 MPa·m1/2。2A97合金中的主要强化相为δ ′相和T1相。δ ′相易共面滑移而引起应力集中,对合金韧性不利;T1相能有效地阻止位错滑移,对合金强度有显著提高。2A97合金在时效初期以穿晶和沿晶混合型断裂为主,断口表现出明显的分层特征,具有较好的韧性;随时效时间的延长,析出相的体积分数、尺寸以及临界分切应力增大,所造成的应力集中更明显,同时晶界平衡相增多,晶界无沉淀带加宽,沿晶断裂比例增多,合金韧性急剧下降。

Abstract: The effect of ageing treatment on the strength and fracture toughness of 2A97 alloy was investigated by tensile test and plane-strain fracture toughness experiments. The microstructures and fracture morphologies were observed and analyzed by TEM and SEM. The results indicate that the 2A97 alloy obtains a preferable fracture toughness of in single aging at 135 ℃,but its strength is relatively low. The duplex aging enhances the strength, while the fracture toughness deteriorates at an excessive temperature as 175 ℃. The 2A97 alloy can obtain the superior combination of the strength and toughness after (135 ℃, 36 h)+(150 ℃, 18 h) aging treatment, with the σb, σ0.2and KIC of 547 MPa, 489 MPa and 28.9 MPa·m1/2, respectively. The δ′ and T1 phases are the major precipitates in the 2A97 alloy. The δ′ phase may induce the stress localization and thus deteriorate the toughness, while the T1 phase can resist the slip of dislocation effectively and significantly enhance the strength. At the early stage, it primarily presents a hybrid of transgranular and intergranular fractures with obvious delamination crackings at the fracture surfaces, which indicates a high level of toughness. The volume fraction, size and critical resolved shear stress of precipitations increase with prolonging the aging time, and lead to more severe stress localizations. In additions, the more equilibrium phases at grain boundaries, the wider PFZs together with the greater proportional of intergranular fractures result in a sharply reduce of fracture toughness.



详情信息展示

文章编号:1004-0609(2011)03-0546-08

时效制度对2A97铝锂合金强韧性的影响

钟 申, 郑子樵, 廖忠全, 蔡 彪

(中南大学 材料科学与工程学院,长沙 410083)

摘  要: 通过拉伸试验及平面应变断裂韧性测试,研究不同时效制度对2A97合金强韧性的影响,并利用透射电镜和扫描电镜对合金的微观组织及断口形貌进行观察。结果表明:2A97合金在135 ℃单级时效时断裂韧性较好,但强度偏低;双级时效可促进强度的提高,但第二级时效温度过高时(175 ℃),断裂韧性迅速下降,经过(135 ℃, 36 h)+(150 ℃, 18 h)时效,可得到较好的强韧性匹配,其σb、σ0.2和KIC 分别为547 MPa、489 MPa和28.9 MPa·m1/2。2A97合金中的主要强化相为δ ′相和T1相。δ ′相易共面滑移而引起应力集中,对合金韧性不利;T1相能有效地阻止位错滑移,对合金强度有显著提高。2A97合金在时效初期以穿晶和沿晶混合型断裂为主,断口表现出明显的分层特征,具有较好的韧性;随时效时间的延长,析出相的体积分数、尺寸以及临界分切应力增大,所造成的应力集中更明显,同时晶界平衡相增多,晶界无沉淀带加宽,沿晶断裂比例增多,合金韧性急剧下降。

关键字: 2A97铝锂合金;微观组织;时效制度;断裂韧性

中图分类号: TG14612 ; TG113       文献标志码:A

Effects of aging treatment on strength and fracture toughness of 2A97 aluminum-lithium alloy

ZHONG Shen, ZHENG Zi-qiao, LIAO Zhong-quan, CAI Biao

(School of Materials Science and Engineering, Central South University, Changsha 410083, China)

Abstract: The effect of ageing treatment on the strength and fracture toughness of 2A97 alloy was investigated by tensile test and plane-strain fracture toughness experiments. The microstructures and fracture morphologies were observed and analyzed by TEM and SEM. The results indicate that the 2A97 alloy obtains a preferable fracture toughness of in single aging at 135 ℃,but its strength is relatively low. The duplex aging enhances the strength, while the fracture toughness deteriorates at an excessive temperature as 175 ℃. The 2A97 alloy can obtain the superior combination of the strength and toughness after (135 ℃, 36 h)+(150 ℃, 18 h) aging treatment, with the σb, σ0.2and KIC of 547 MPa, 489 MPa and 28.9 MPa·m1/2, respectively. The δ′ and T1 phases are the major precipitates in the 2A97 alloy. The δ′ phase may induce the stress localization and thus deteriorate the toughness, while the T1 phase can resist the slip of dislocation effectively and significantly enhance the strength. At the early stage, it primarily presents a hybrid of transgranular and intergranular fractures with obvious delamination crackings at the fracture surfaces, which indicates a high level of toughness. The volume fraction, size and critical resolved shear stress of precipitations increase with prolonging the aging time, and lead to more severe stress localizations. In additions, the more equilibrium phases at grain boundaries, the wider PFZs together with the greater proportional of intergranular fractures result in a sharply reduce of fracture toughness.

Key words:2A97 Al-Li alloy; microstructure; aging treatment; fracture toughness

铝锂合金具有低密度、高比强度和比刚度等优点,已成为航空航天领域最具应用潜力的结构材料之   一[1-3]。2A97铝锂合金是作者课题组与国内相关单位合作自主研发的新型第三代铝锂合金,具有比强度高、可焊性好及耐疲劳损伤性能优异等特点,有望用于飞机框梁结构及机翼下壁板和燃料贮箱等航空航天结构件,具有广泛的应用前景[4]

对航空航天承力结构件来说,强度和断裂韧性是极其重要的性能指标要求。众所周知,材料的强度和断裂韧性与其微观组织结构密切相关,合理的时效制度可以改善析出相的尺寸和分布,是调控合金微观组织的重要方法之一。因此,了解时效过程中显微组织的变化及其对该合金性能的影响,优化其时效工艺,从而改善其强韧性匹配,对推动2A97合金在航空和航天工程中的应用有重要意义。由于2A97是我国自主研发的一个新合金,有关时效制度对该合金断裂韧性的影响尚未开展系统研究[5-9]。本文作者研究不同时效制度对2A97合金的常规力学性能、断裂韧性及微观组织的影响,讨论微观组织与合金强度和韧性之间的关系,并期望通过优化时效工艺以获得最佳强韧性能匹配,为该合金在航天航空领域的应用奠定基础。

1  实验

试验用料为半工业化生产的25 mm厚2A97铝锂合金热轧板材。板材在铝加工厂的辊底式喷淋淬火炉中进行固溶和淬火,淬火后立即进行2.5%的预拉伸变形,并在135 ℃预时效36 h。预时效后的板材经剪切后,按表1所示的时效制度进行后续时效处理。沿厚板L方向截取圆棒状试样在CSS-44100万能电子拉伸机上进行拉伸性能测试。从T-L方向取紧凑拉伸(CT)试样测定其断裂韧性,试样尺寸按ASTM E399-08标准制定(W/B=2,B=20 mm)。在MTS 810材料试验机上进行平面应变断裂韧性KIC测定,每测试点取3个有效数据,试验结果取平均值。TEM观察在TecnaiG220透射电子显微镜上进行,加速电压为200 kV;SEM断口观察在FEI Quanta 200环境扫描电镜上进行,加速电压为15 kV。

2  结果

2.1  拉伸性能及断裂韧性

2A97合金预拉伸板经(135 ℃, 36 h)预时效后,分别在135、150和175 ℃进一步时效不同时间后,拉伸性能及断裂韧性如图1和表2所示。由此可知,预时效后2A97合金具有较好的塑性和断裂韧性(δ5=13.7 %,KIC=33.1 MPa·m1/2),但强度偏低,其抗拉强度和屈服强度分别为522和435 MPa。后续时效处理使合金强度得到进一步的提高,在135 ℃继续时效时,随时效时间增加,合金强度缓慢上升,经96 h后达到峰值,对应抗拉强度和屈服强度分别为567和514 MPa;而在双级时效处理时,合金时效响应速率加快,强度提高,合金在150与175 ℃进行第二级时效时,分别经72和24 h达到峰值,对应的抗拉强度分别为588 和597 MPa,屈服强度分别为568和562 MPa。由此可见,双级时效与单级时效相比,时效响应速率更快,合金峰值强度更高。

由表2可知,随着时效的进行,合金的断裂韧性逐渐降低。合金在135 ℃继续时效时,随着时效时间的延长,断裂韧性下降不大,时效96 h后,其KIC为24 MPa·m1/2);在双级时效制度下,随第二级时效温度的提高,合金的断裂韧性下降更快。第二级时效温度


表1  2A97合金后续时效制度

Table 1  Subsequent heat treatment schedule of 2A97 alloys

表2  不同时效制度下2A97合金断裂韧性

Table 2  Fracture toughness of 2A97 alloy under different aging conditions

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