有色金属科学与工程  2018, Vol. 9 Issue (4): 35-39
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长期时效温度对一种单晶高温合金组织和拉伸性能的影响[PDF全文]
史振学 , 杨万鹏 , 刘世忠 , 王效光     
北京航空材料研究院先进高温结构材料重点实验室, 北京 100095
摘要:在定向凝固炉中采用螺旋选晶法制备了一种单晶高温合金试棒,标准热处理后分别在980 ℃,1 070 ℃,1 100 ℃和1 140 ℃长期时效处理500 h,研究了不同温度长期时效后合金的显微组织和1 100 ℃的拉伸性能.结果表明合金在不同温度长期时效后,γ′相发生粗化或筏排化.随着时效温度增加,γ′相粗化或筏排化程度增加,γ′相体积分数减少,γ相基体通道变宽.在980 ℃长期时效500 h后,无TCP相析出;1 070 ℃,1 100 ℃,1 140 ℃长期时效500 h后,有针状TCP相析出.随着长期时效温度增加,TCP相含量增加.随着时效温度升高,合金拉伸强度降低,拉伸延伸率先增加后在1 140 ℃时效时又降低.长期时效后γ′相粗化或筏排化、γ′相含量减少和TCP相析出是合金拉伸强度降低的主要原因.
关键词单晶高温合金    长期时效    拉伸性能    组织    
Effect of long-term aging temperature on the microstructure and tensile properties of a single crystal superalloy
SHI Zhenxue , YANG Wanpeng , LIU Shizhong , WANG Xiaoguang     
Science and Technology on Advanced High Temperature Structural Materials Laboratory, Beijing Institute of Aeronautical Materials, Beijing 100095, China
Abstract: A nickel-based single crystal superalloy was casted using the directional solidification furnace. The long term aging of the alloy was performed at 980 ℃, 1 070 ℃, 1 100 ℃ and 1 140 ℃ for 500 h, respectively. The microstructure and tensile properties at 1 100 ℃ of the alloy after long term aging at different temperature were investigated. The results show that the γ′ phase directional coarsening or rafting and the γ matrix channel widen can be observed in the alloy after long term aging. The coarsening extent of γ′ phases increased and the volume fraction of γ′ phase decreased and space of γ matrix channel obviously increased with rise of aging temperature. No TCP phase was found in the alloy after aging at 980 ℃ for 500 h. The needle shaped TCP phase precipitated after long term aging at 1 070 ℃, 1 100 ℃ and 1 140 ℃ for 500 h. The volume fraction of TCP phase increased with rise of aging temperature. With rise of aging temperature, the yield strength and ultimate tensile strength all decreased with rise of aging temperature, while the elongation increased at first and decreased afterward when aging at 1 140 ℃.
Key words: single crystal superalloy    long term aging    tensile properties    microstructure    

单晶高温合金在高温下具有良好的力学、抗氧化和耐热腐蚀等性能,是先进地面燃机和航空发动机涡轮叶片的关键材料[1-4].为提高涡轮叶片的承温能力,单晶高温合金中W、Mo、Ta、Re等高熔点元素含量显著提高[5-9],但在高温状态下长期服役时容易导致TCP相的析出,从而降低合金的力学性能[5-7].因此高温下的组织稳定性是合金研制和应用需考察的重要指标之一[10-16].作者研究了一种新型镍基单晶高温合金在不同温度长期时效后显微组织和拉伸性能,可为合金的研制和安全使用提供科学依据.

1 试样制备和试验方法

试验材料为Ni-Cr-Co-Mo-W-Ta-Nb-Re-Ru-Al-Hf系镍基单晶高温合金.在真空定向凝固炉中采用螺旋选晶法制备单晶高温合金试棒.采用X射线衍射仪测试单晶试棒的晶体取向,单晶试棒的晶体取向偏离度保持在7°以内.试棒按1 310 ℃/2 h + 1 320 ℃/3 h +1 330 ℃/4 h + 1 345 ℃/6 h/空冷+ 1120 ℃/4 h/空冷+870 ℃/32 h/空冷进行完全热处理后,分别在980 ℃,1 070 ℃,1 100 ℃,1 140 ℃长期时效500 h.加工成拉伸性能试样,测试合金1 100 ℃的拉伸性能.采用扫描电镜研究合金不同条件下的显微组织.

2 结果与分析 2.1 长期时效后组织

图 1所示为合金在不同温度长期时效500 h后的显微组织.由图 1可以看出,980 ℃长期时效500 h后,γ′相的尺寸明显地增加,但仍保持原来的立方体形态,γ相基体通道宽度稍有增加. 1 070 ℃长期时效500 h后,γ′相已经沿[001]、[100]或[010]方向长大合并,形状为长条形或L形,γ相基体通道宽度明显增加. 1 100 ℃长期时效500 h后,γ′相合并长大后尺寸增加,γ相基体通道宽度进一步增加. 1 140 ℃长期时效500 h后,γ′相合并程度增加,γ相基体通道宽度显著增加.

图 1 合金不同温度长期时效500 h后γ′组织 Fig. 1 Morphologies of γ′ phase of the alloy after long term aging time for 500h at different temperature

γ′相的粗化本质上是合金元素在γ相和γ′相中扩散和重新分布的过程.对合金未时效组织的γ相和γ′相进行了能谱分析,结果见表 1.由表 1看出,γ′相中含有较多的Al, Hf, Nb, Ta,而γ相含有较多的Cr, Co, W, Re, Ru, Mo.长期在高温状态下,γ′相形成元素Al、Ta、Hf、Nb由γ相向γ′相中扩散,促使γ′相长大;γ基体相形成元素Cr, Co, W, Mo, Re, Ru由γ′相向γ相中扩散,使基体通道宽度增加. γ′相的筏排化示意图[17]图 2所示.

表 1 合金未时效组织中γ′相和γ相的能谱/(质量分数,%) Table 1 Chemical composition of γ′ phase and γ matrix in the alloy before aging/(mass fraction, %)
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图 2 合金筏排化示意 Fig. 2 A mode describing the process of forming raft of the alloy

长期时效温度是影响合金元素扩散的主要因素,长期时效温度越高,扩散系数越大,合金元素的扩散速度越快,γ′相尺寸就越大.在980 ℃长期时效温度下,元素扩散相对较慢,γ′相的长大速率相对较小,其形貌仍然呈现立方化形态.在1 070 ℃、1 100 ℃、1 140 ℃长期时效温度下,γ′相合并长大速率显著增加,不再保持立方化形态,逐渐转变为筏排化形貌.

对不同温度长期时效后的γ′相体积分数和γ相基体通道宽度进行定量分析,结果见图 3.由图 3可以看出,随着长期时效温度的增加,γ′相体积分数降低,γ相基体通道宽度增加.这是因为随着γ相基体通道宽度的增加,γ相的体积分数增加,因而γ′相的体积分数相对减少.

图 3 长期时效温度对γ′相体积分数和γ基体通道宽度的影响 Fig. 3 Effect of long term aging temperature on the volume fraction of γ′ phase and space of γ matrix channel

对合金TCP相的析出情况进行了分析,结果如图 4所示.研究发现,980 ℃长期时效500 h后,合金中没有发现TCP相析出. 1 070 ℃长期时效500 h后,有少量细针状的TCP相析出.合金1 100 ℃长期时效后,TCP相析出量稍有增加.合金1 140 ℃长期时效500 h后,TCP相析出量明显增多. TCP相与基体有明显的晶体取向关系,这是因为TCP相的晶体结构非常复杂,析出时沿着基体一定的平面析出并沿一定的方向长大[18, 19].TCP相的能谱分析结果见表 2.由表 2可以看出,不同温度时效析出的TCP相化学成分基本相同,都含有大量的Co、W和Re等元素.

图 4 合金不同温度长期时效500 h后析出的TCP相 Fig. 4 TCP phase precipitates in the alloy after long term aging time for 500 h at different temperature

表 2 合金不同温度长期时效后析出TCP相的能谱/(质量分数,%) Table 2 Chemical composition of TCP phase in the alloy after aging at different temperature/(mass fraction, %)
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2.2 长期时效后拉伸性能

合金在不同温度长期时效后1 100 ℃的拉伸性能见图 5,每个数据为3个试样的平均值.由图 5可以看出,合金长期时效后,合金的拉伸强度都降低.随着合金的长期时效温度升高,拉伸强度逐渐减小,而延伸率先增加后在1 140 ℃长期时效后减小.

图 5 合金不同温度长期时效后1 100 ℃的拉伸性能 Fig. 5 The tensile properties of the alloy at 1 100 ℃ after long term aging at different temperature

合金的组织决定其性能,长期时效带来的组织变化主要为γ′相的粗化或筏排化和TCP相的析出.单晶高温合金主要由基体γ和沉淀强化相γ′组成.强化相γ′的大小、形貌和体积分数对合金的拉伸强度有明显影响.随着长期时效温度增加,合金中γ相的含量增加,而γ′相含量减少,其强化作用降低.在单晶高温合金的高温变形过程中,位错首先在γ基体中移动.随着长期时效温度增加,基体通道宽度的增加,导致位错很容易在基体中运动.位错运动到相界面受到γ′相的阻碍,不同柏氏矢量的位错相遇发生反应,在相界面上形成位错网.位错网能够有效地阻碍位错向γ′相中运动,因此位错网的密度越大,合金就越难变形,其拉伸强度越大.随着合金中γ′相的尺寸变大,γ/γ′相界面面积减小,形成位错网的密度变小,对后续位错剪切通过γ′相的阻碍作用变小[20, 21],其拉伸强度变小.因而长期时效后形成粗大的筏排组织不能有效地阻止高温拉伸变形.以上这些原因导致长期时效后合金的屈服强度和抗拉强度的降低.

TCP相的析出也是合金1 070 ℃,1 100 ℃,1 140 ℃长期时效后高温拉伸强度降低的原因. TCP相较脆,破坏了基体的连续性. TCP相中含有大量的合金强化元素,它的析出导致周围基体中强化元素含量的降低,使周围基体强度降低.因此高温拉伸过程中显微裂纹容易TCP相处形成和长大,最终导致试样的断裂,如图 6所示.同时,1 140 ℃长期时效试样拉伸断裂的过早发生使合金的延伸率较低.

图 6 合金1 140 ℃长期时效500 h拉伸断裂试样断口附近的显微组织 Fig. 6 Microstructure near the fracture surface of tensile ruptured specimen after aging at 1 140 ℃ for 500 h

3 结论

1)随着长期时效温度增加,γ′相粗化或筏排化程度增加,γ′相体积分数减少,γ相基体通道变宽.在980 ℃长期时效500 h后,无TCP相析出;在更高温度长期时效后,有针状TCP相析出.随着长期时效温度增加,TCP相含量增加.

2)随着长期时效温度增加,合金的拉伸强度降低,拉伸延伸率先升高后降低.长期时效后γ′相粗化或筏排化、γ′相含量减少和TCP相析出是合金拉伸强度降低的主要原因.

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