镍基718合金因力学性能优异、易加工且耐腐蚀性能好，被广泛用于制备反应堆燃料组件格架弹簧。格架弹簧是燃料组件核心部件，用于夹持定位、固定燃料棒，其服役环境十分苛刻。一方面，格架弹簧承受较大服役应力；另一方面，格架弹簧直接暴露于反应堆堆芯强中子辐照环境，基体内会产生大量结构损伤导致合金发生力学性能变化。同时，格架弹簧还直接与一回路高温高压水腐蚀环境接触。在上述腐蚀、应力、辐照等苛刻环境作用下格架弹簧存在发生开裂的风险，可能导致燃料棒破损，对核电站的安全性、可靠性和经济性产生重要影响。近年来国内外已经陆续出现因镍基718合金格架弹簧SCC导致的燃料棒破损现象。由于国内外对格架弹簧SCC机理的认知不足，目前尚不能解释格架弹簧发生SCC的根本原因。
密歇根大学Wang等[6]和Ning等[7]研究认为，δ相由于活性Nb含量高可以优先氧化，且会加剧局部变形的程度，会降低合金SCC的性能；Ning等[7]还认为γ′/γ″相沉淀相与δ相类似的作用。Garzarolli等[8,9]通过堆内研究表明，含有δ相的镍基718合金的抗辐照促进应力腐蚀开裂(IASCC)性能优于不含有δ相的镍基718合金。Leonard等[10]研究表明，裂纹在晶界δ相附近处停止扩展。因此，关于δ相对镍基718合金SCC的影响目前还存在争议。Wang等[6]和Demetriou等[11]的拉伸试验研究表明，镍基718合金中的碳化物和氮化物在高温氩气环境和常温空气环境中会开裂，且镍基718合金中碳化物富含活性Nb元素。因此，碳化物和氮化物也可能对镍基718合金SCC性能产生影响，亟需开展相关研究。
基于上述现状，本研究以两个厂家提供的标准工艺镍基718合金带材为对象，研究碳化物、氮化物等组织结构对镍基718合金SCC裂纹萌生行为的影响，以期为揭示镍基718合金格架弹簧SCC的机理提供参考。
1 实验方法
试验材料为两个厂家提供的沉淀硬化态镍基718合金带材(分别命名为带材A和B)，均按照核电商用标准热处理工艺进行处理，即1080 ℃/47 s + 718 ℃/8 h + 621 ℃/8 h，两种镍基718合金带材的化学成分如表1所示。将10 mm × 10 mm × 1 mm的两种镍基718合金样品采用金相嵌镶粉进行热镶嵌。镶嵌后的样品采用#100，#400，#1000，#2000，#5000的SiC砂纸湿磨，并用1 μm的金刚石抛光膏进行机械抛光，随后使用王水侵蚀15 s，侵蚀后的样品置于酒精和去离子水中依次清洗，并在空气中干燥。采用Zeiss Axio Observer. Z1m光学显微镜观察合金的组织结构，采用FEI XL30的扫描电子显微镜(SEM)进一步观察组织结构。
表1 两种镍基718合金化学成分 (mass fraction / %)
Table 1
| Strip | C | Si | Mn | P | S | Cr | Ni | Mo | Al | Ti | Nb + Ta | Co | Fe |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| A | 0.009 | 0.05 | 0.26 | 0.003 | 0.001 | 18.28 | 53.4 | 3.07 | 0.51 | 1.03 | 5.11 | 0.0018 | Bal. |
| B | 0.01 | 0.04 | 0.26 | 0.006 | 0.001 | 18.08 | 53.6 | 3.06 | 0.52 | 1.02 | 5.04 | 0.002 | Bal. |
高温空气拉伸实验试样长60 mm，厚度为0.266 mm，具体试样尺寸如图1所示。实验温度为315 ℃，拉伸速率为2.5 × 10-4 s-1。
图1
慢应变速率拉伸(SSRT)实验样品尺寸与高温拉伸实验样品相同。SSRT实验样品一面为原始表面，另一面采用#100，#400，#1000，#2000，#5000的SiC砂纸打磨后，使用1 μm的金刚石抛光膏进行机械抛光，随后采用0.02 μm的氧化硅精细抛光悬浮液在绒布上抛光2 h，使析出相凸显，最后采用酒精超声、去离子水清洗表面，晾干备用。
SSRT实验包含高温氮气环境SSRT实验和高温高压水环境SSRT实验。高温氮气环境SSRT实验在320 ℃氮气环境中采用SCC试验机开展，拉伸速率为3 × 10-7 s-1，在10%应变中止实验，取出样品进行SEM观察分析。高温高压水SSRT实验在320 ℃、15.5 MPa的1200 mg/L B(以H3BO3形式)+ 2.2 mg/L Li (以LiOH·H2O形式)动态循环水溶液中采用SCC试验机开展。升温前，通入高纯氮气除氧直至溶解氧浓度低于5 μg/L，并持续通入氮气。在温度和压力稳定后，开始SSRT实验。实验采用间断式慢应变速率拉伸的方法，拉伸速率为3 × 10-7 s-1，在不同应变中止实验，取出样品进行SEM观察分析；设置的中止应变分别为10%、15%，最后拉断样品。每次取出样品后，采用SEM原位跟踪观察裂纹萌生和扩展情况，并选取特定区域统计裂纹数量、长度等信息。样品拉断后，进行断口观察。
2 实验结果
2.1 组织结构
图2
图2
两种镍基718合金带材组织结构形貌
Fig.2
Microstructures of A (a) and B (b) types of nickel based 718 alloy strips
图3
表2 图3中3个位置处EDS分析结果 (mass fraction / %)
Table 2
| Position | Ni | Cr | Fe | Nb | C | Ti | N |
|---|---|---|---|---|---|---|---|
| Location 1 | 1.82 | 0.78 | 0.86 | 69.76 | 17.58 | 9.20 | - |
| Location 2 | 3.37 | 1.83 | 1.41 | 4.12 | 5.36 | 62.85 | 21.05 |
| Location 3 | 49.35 | 17.94 | 17.46 | 4.82 | 6.43 | 1.01 | - |
2.2 力学实验及慢应变速率拉伸实验结果
2.2.1 力学性能结果
表3 两种镍基718合金带材力学性能
Table 3
| Alloy | Rp0.2 / MPa | A / % | |
|---|---|---|---|
| Strip A | 1089 | 1280 | 18.5 |
| Strip B | 1112 | 1299 | 19 |
2.2.2 慢应变速率拉伸实验结果
图4
图4
高温高压水环境镍基718合金A种带材SSRT实验原位SEM观察及EDS分析结果
Fig.4
In-situ SEM observation and EDS analysis of A type of 718 alloy strips during SSRT experiment in high-temperature and high-pressure water environment, showing the presence of cracks at different strains (a-c) and compositions of (Nb, Ti)C (d) in Fig.4a2 and TiN (e) in Fig.4a3 precipitation phases
图5
图5
高温高压水环境镍基718合金B种带材SSRT实验原位SEM观察及EDS分析结果
Fig.5
In-situ SEM observation and EDS analysis of B type of 718 alloy strips during SSRT experiment in high-temperature and high-pressure water environment, showing the presence of cracks at different strains (a-c) and compositions of (Nb, Ti)C (d) in Fig.5a2 and TiN (e) in Fig.5a3 precipitation phases
表4 两种带材裂纹信息统计
Table 4
| Materials | Strain | Crack density #cracks/mm2 | Crack length per unit area / μm·mm-2 |
|---|---|---|---|
| Strip A | 10% | 7 | 44 |
| 15% | 48 | 356 | |
| Strip B | 10% | 33 | 253 |
| 15% | 145 | 1314 |
图6
图6
高温高压水环境中两种镍基718合金带材SSRT实验典型断口形貌
Fig.6
Typical fracture morphologies of A type (a) and B type (b) of 718 alloy strips after SSRT experiment in high-temperature and high-pressure water environment
综上，断口形貌的结果不能比较两种带材的SCC敏感性，但从不同应变下裂纹长度和密度的统计结果知，带材A的SCC敏感性小于带材B。
3 分析与讨论
3.1 裂纹萌生特征分析
图7
图7
320 ℃下氮气环境中10%应变后镍基718合金B种带材表面TiN析出相SEM观察及能谱分析
Fig.7
SEM observation and EDS analysis of TiN phase on the surface of B type of 718 alloy strips after 10% strain during SSRT test in nitrogen environment at 320 ℃
图8
图8
320 ℃下氮气环境中10%应变后镍基718合金B种带材表面(Nb,Ti)C析出相SEM观察及EDS分析
Fig.8
SEM observation (a) and EDS analysis (b) of (Nb,Ti)C phase on the surface of B type of 718 alloy strips after 10% strain during SSRT test in nitrogen environment at 320 ℃
表5 两种带材裂纹特征信息统计
Table 5
| Materials | Characteristics of original organizational structure | Crack density at different positions | ||
|---|---|---|---|---|
| Density of precipitated phases #cracks/mm2 | Strain | Crack density at grain boundaries / #cracks·mm-2 | Crack density at precipitated phases / #cracks·mm-2 | |
| Strip A | 130 | 10% | 3 | 4 |
| 15% | 40 | 8 | ||
| Strip B | 363 | 10% | 21 | 12 |
| 15% | 120 | 45 | ||
对比两种带材的裂纹特征信息，在10%和15%应变下，带材B的晶界和析出相处裂纹密度均大于带材A。对两种带材原始组织结构中析出相的密度进行统计，结果表明带材A中析出相密度为130个/mm2，带材B中析出相密度为363个/mm2。因此，带材B中析出相密度大于带材A，这是带材B的SCC敏感性大于带材A的原因之一。
3.2 析出相对镍基718合金SCC的影响
图9
图9
带材A在拉断以及带材B在应变10%时(Nb, Ti)C析出相SEM像
Fig.9
SEM images of (Nb, Ti)C precipitates for fractured strip A (a) and strip B with 10% strain (b)
图10
图10
带材B表面(Nb, Ti)C析出相SEM观察及EDS分析
Fig.10
SEM observation and EDS analysis of (Nb, Ti)C precipitates on the surface of strip B
4 结论
(1) 核电用标准热处理工艺的镍基718合金在除氧模拟核电一回路水环境中以晶界开裂为主，氧化物后的(Nb, Ti)C与TiN在应力的作用下也发生开裂。
(2) 在SSRT实验过程中，随着应变的增加，晶界和(Nb, Ti)C析出相处的裂纹沿着晶界扩展，TiN析出相处的裂纹存在向晶界扩展的趋势。因此，(Nb, Ti)C与TiN会降低镍基718合金的抗SCC性能。
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