Evaluation Method for Creep Properties of Directional Solidification Nickel-Based Alloys Based on Small Samples
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摘要:
在热处理态定向凝固镍基合金棒材上制取工字型非标准小试样和棒状蠕变标准试样,并进行了不同温度（800,850,900,980 ℃）下的应力松弛试验和不同应力（标准试样为250,350,450 MPa,小试样为250,350,400,450 MPa）下的蠕变持久试验,研究了标准试样和小试样的松弛蠕变和持久蠕变行为,建立了小试样松弛蠕变速率与标准试样持久寿命的关系。结果表明：小试样和标准试样的持久蠕变曲线均单调递增直至试样断裂,持久蠕变速率曲线均均呈“U”字型,两者的最小持久蠕变速率接近；小试样和标准试样的高温松弛蠕变曲线均单调递减,温度归一化后两者松弛蠕变速率分布在一条线上；持久蠕变和应力松弛试验后标准试样和小试样中γ'相均沿垂直于应力方向聚集粗化,属于N型筏化；标准试样持久寿命与小试样松弛蠕变速率存在线性关系,利用小试样松弛蠕变速率预测的持久寿命和标准试样实际持久寿命接近。
Abstract:The I-shaped non-standard small samples and the bar creep standard samples were sampled from the directional solidification nickel-based alloy bar in heat treatment state. The stress relaxation tests at different temperatures (800,850,900,980 ℃) and creep rupture tests under different stresses (250,350,450 MPa for standard samples and 250,350,400,450 MPa for small samples) were carried out. The relaxation creep and creep rupture behavior of standard and small samples were studied, and the relationship between the relaxation creep rate of small samples and the creep rupture life of standard samples was established. The results show that the creep rupture curves of both small and standard samples increased monotonically until samples fracturing, and the creep rupture rate curves were U-shaped curves. The minimum creep rupture rates of the two samples were close. The relaxation creep curves of both small and standard samples at high temperature decreased monotonically, and after temperature normalization, the relaxation creep rates of both distributed on one line. After creep rupture and stress relaxation tests, the γ′ phases in standard and small samples aggregated and coarsened along the direction perpendicular to the stress, which belonged to N-type raft. There was a linear relationship between the persistent life of standard sample and the relaxation creep rate of small sample, and the creep rupture life predicted by using the relaxation creep rate of small sample was close to the actual creep rupture life of standard sample.
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0. 引言
航空发动机涡轮叶片长时间在高温、高压环境下服役,其叶片性能会发生劣化,从而影响飞机的整体安全,因此研究发动机涡轮叶片性能变化对于飞机安全服役和减少维护成本具有重要意义[1-5]。通过对标准试样进行蠕变持久试验获取的最小蠕变速率和持久寿命是材料安全服役评估的重要参数。然而,一些设备由于形状受限等原因难以满足标准试样制备条件,只能制取小尺寸试样（小试样）。小试样可以保留服役构件的服役状态[6-7],主要分为两类：一类是将标准试样等比例或非等比例缩小的标准棒状小试样[8],目前已有比较完全的试验体系；另一类是非标准小试样,能针对特殊构件进行试验[9]。WOODFORD等[10]在某燃气轮机叶片不同区域制取了标准试样和小试样,发现试样尺寸会影响蠕变强度的测试结果。庄法坤等[7]研究发现,不同类型小试样的蠕变曲线差异较大。此外,传统的蠕变持久试验耗时较长,而高温下基于应力松弛测试获取的松弛蠕变性能与持久蠕变性能并无本质区别,因此可以采用短时应力松弛试验实现材料持久蠕变性能的低成本快速检测[11-14]。BOSE等[15]通过应力松弛测试数据预测了高温下1CrMoV钢的蠕变持久寿命,具有较好的预测效果。研究人员构建了可以将金属材料松弛蠕变速率转化为最小蠕变速率的模型,模拟得到的最小蠕变速率与实际最小速率相近,这验证了模型的准确性[16-18]。
定向凝固镍基合金是一种无横向晶界的柱状合金,具有耐高温、耐腐蚀和抗氧化等优点[19],在以其为主要材料的涡轮叶片上制取标准试样比较困难。为此,作者在热处理态定向凝固镍基合金棒材上制备了工字型小试样和棒状蠕变标准试样,并进行了不同温度下的应力松弛试验和不同应力下的蠕变持久试验,研究了标准试样和小试样的松弛蠕变和持久蠕变行为,建立了小试样松弛蠕变速率与标准试样持久寿命的关系,以期为将小试样松弛蠕变信息转化成标准试样持久蠕变信息提供参考。
1. 试样制备与试验方法
试验材料为热处理态定向凝固镍基合金棒材,由沈阳金属所提供,最终热处理工艺为1 220 ℃×2 h+1 120 ℃×2 h+850 ℃×24 h,采用XRF-1800型X射线荧光分析仪测试得到其化学成分（质量分数/%）为13.44Cr,9.67Co,4.85Ti,3.26Al,4.17W,1.63Mo,3.24Ta,余Ni。在试验合金上制取金相试样,经磨抛,用由5 mL浓盐酸（HCl质量分数为36%）+2 g CuSO4+23.5 mL H2O组成的溶液腐蚀5~10 s后,采用SU5000型热场式扫描电镜（SEM）观察显微组织。由图1可见：试验合金由共晶组织、MC型碳化物和γ'相组成,MC型碳化物主要为块状和条状,γ'相呈立方体状（边长约为0.2μm）均匀分布在基体上。经image pro plus软件统计可得,γ'相的面积分数和平均等效直径分别为51%,231μm。
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图 1 试验合金的显微组织Figure 1. Microstructures of test alloy: (a) at low magnification and (b) at high magnification根据GB/T 2039-2012并综合考虑实验室仪器、夹具以及涡轮叶片形状等因素,在试验合金上制取棒状蠕变标准试样和工字型小试样,尺寸如图2所示。采用RD-50型微控电子式通用蠕变持久试验机进行持久蠕变试验和应力松弛试验,使用TG115型光栅测微传感器测试蠕变应变。持久蠕变试验温度为900 ℃,温度误差范围在±1 ℃内,标准试样的试验应力分别为250,350,450 MPa,小试样的试验应力分别为250,350,400,450 MPa；应力松弛试验温度分别为800,850,900,980 ℃,由于应变超过弹性极限后对松弛蠕变规律变化影响较小[20],而小试样在试验过程中受到挂片型过渡夹具变形和试样尺寸形状的影响,应变为5%才到达弹性极限,因此将标准试样初始应变设为2%,小试样初始应变设为7%,应变速率均为8×10-5 s-1。采用SU5000型热场式扫描电镜观察持久蠕变和应力松弛试验后小试样和标准试样的显微组织。
在应力松弛试验加载阶段,应变随时间延长而增加,达到初始应变后随试验进行,试样初始应变中有一部分弹性应变转化为松弛蠕变应变,应力松弛试验过程中总应变保持不变,标准试样在拉应力方向的各类应变之和为恒值,即松弛试验初始总应变ε0为
(1) 式中：εe为弹性应变；εp为塑性应变,在松弛蠕变过程中不变；εSRT为松弛蠕变应变；σ为应力；E为弹性模量；C1为常数。
将式（1）对时间t进行微分得到应力松弛通用模型,即
(2) 式中：
为塑性应变速率,即松弛蠕变速率；
为应力变化速率。
2. 试验结果与讨论
2.1 持久蠕变行为
由图3可知：小试样和标准试样的持久蠕变曲线和持久蠕变速率曲线均相似,持久蠕变曲线单调递增直至试样最终断裂,持久蠕变速率曲线则呈“U”字型；随着应力增加,小试样和标准试样的持久寿命均缩短,持久蠕变速率均增大；相同应力下小试样的断裂应变和持久寿命均大于标准试样。
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图 3 不同应力下小试样和标准试样的持久蠕变曲线和持久蠕变速率曲线Figure 3. Creep rupture curves (a) and creep rupture rate curves (b) of small and standard samples under different stresses由图4可知：随着应力增加,小试样和标准试样的最小持久蠕变速率均增大,且两者接近,说明可以使用小试样代替标准试样进行持久蠕变试验以获取试验合金的最小持久蠕变速率；小试样和标准试样持久寿命不同,但两者Monkman-Grant关系曲线平行,持久寿命和最小持久蠕变速率在对数坐标轴下呈良好线性关系,拟合可得
(3) 式中：tr为持久寿命；
为最小持久蠕变速率；C2为常数。
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图 4 小试样和标准试样的最小持久蠕变速率-应力曲线和Monkman-Grant关系曲线Figure 4. Minimum creep rupture rate-stress curves (a) and Monkman-Grant relationship curves (b) of small and standard samples2.2 松弛蠕变行为
由图5可见：小试样和标准试样的高温松弛蠕变曲线均单调递减,小试样的初始应力和残余应力均大于标准试样；随着试验温度升高,小试样和标准试样的初始应力和残余应力减小。
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图 5 标准试样和小试样在不同温度下的松弛蠕变曲线Figure 5. Relaxation creep curves of standard (a) and small (b) samples at different temperatures根据式（2）将松弛蠕变曲线转化为松弛蠕变速率-应力曲线。由图6（a）可知：小试样和标准试样的松弛蠕变速率接近,且均随应力增加或温度升高而增加。将不同温度下标准试样和小试样的松弛蠕变速率-应力曲线进行温度归一化,公式为
(4) 式中：PLM为温度归一化参数；T为温度。
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图 6 不同温度下小试样和标准试样的松弛蠕变速率-应力曲线和温度归一化曲线Figure 6. Relaxation creep rate-stress curves (a) and stress-temperature normalization parameter curves (b) of small and standard samples at different temperatures由图6（b）可知,温度归一化后标准试样和小试样的松弛蠕变速率吻合性良好,分布在一条曲线上。通过进行多项式拟合,可得松弛主曲线方程为
(5) 由图7可知：900 ℃下持久蠕变和应力松弛试验后标准试样和小试样中的γ'相演变趋势一致,均沿垂直于应力方向聚集粗化,属于N型筏化；持久蠕变试验后标准试样和小试样的γ'相面积分数分别为39%,36%,均出现边缘圆化；应力松弛试验后标准试样和小试样的γ'相面积分数分别为47%,45%,纵横比的最小值分别为0.105,0.079。
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图 7 900 ℃下持久蠕变和应力松弛试验后标准试样和小试样的显微组织Figure 7. Microstructures of standard (a-b) and small (c-d) samples after creep rupture (a,c) and stress relaxation tests (b,d) at 900 ℃2.3 松弛与持久蠕变的关系
为了建立松弛蠕变速率与最小持久蠕变速率的转化关系,将松弛蠕变速率归一化的参数记为PSRT,最小持久蠕变速率归一化的参数记为PCRT。由图8（a）可见,标准试样和小试样的PSRT与PCRT呈线性相关,其关系式为
(6) 式中：B为拟合参数。
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图 8 标准试样和小试样的PSRT-PCRT曲线和预测持久寿命-实际持久寿命曲线Figure 8. PSRT-PCRT curve (a) and predicted creep rupture life-actual creep rupture life curve (b) of small and standard samples将B代入式（5）可得
(7) 由于标准试样和小试样的松弛蠕变速率相近,可以相互代替,联立式（3）和式（7）可得标准试样持久寿命与小试样松弛蠕变速率的关系式为
(8) 由式（5）可得某一温度和应力下小试样的松弛蠕变速率,代入式（8）可进一步预测标准试样的持久寿命。将预测持久寿命作为横坐标,标准试样的实际持久寿命为纵坐标作图,如图8（b）所示,可见预测持久寿命和标准试样实际持久寿命接近,说明可以通过此方法解决传统持久蠕变试验方法由于试样数量、尺寸和试验时间在评估持久寿命时受限的问题。
3. 结论
（1）热处理态定向凝固镍基合金棒材小试样和标准试样的持久蠕变曲线均单调递增直至试样断裂,持久蠕变速率曲线则均呈“U”字型,两者的最小持久蠕变速率接近。
（2）小试样和标准试样的高温松弛蠕变曲线均单调递减,温度归一化后,两者的松弛蠕变速率吻合性好,分布在一条曲线上。
（3）持久蠕变和应力松弛试验后标准试样和小试样中γ'相的演变趋势一致,均沿垂直于应力方向聚集粗化,属于N型筏化。
（4）建立了松弛蠕变速率与最小持久蠕变速率的转化关系和标准试样持久寿命与小试样松弛蠕变速率的线性关系,利用小试样松弛蠕变速率预测的持久寿命和标准试样实际持久寿命接近。
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图 1 试验合金的显微组织
Figure 1. Microstructures of test alloy: (a) at low magnification and (b) at high magnification
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图 3 不同应力下小试样和标准试样的持久蠕变曲线和持久蠕变速率曲线
Figure 3. Creep rupture curves (a) and creep rupture rate curves (b) of small and standard samples under different stresses
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图 4 小试样和标准试样的最小持久蠕变速率-应力曲线和Monkman-Grant关系曲线
Figure 4. Minimum creep rupture rate-stress curves (a) and Monkman-Grant relationship curves (b) of small and standard samples
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图 5 标准试样和小试样在不同温度下的松弛蠕变曲线
Figure 5. Relaxation creep curves of standard (a) and small (b) samples at different temperatures
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图 6 不同温度下小试样和标准试样的松弛蠕变速率-应力曲线和温度归一化曲线
Figure 6. Relaxation creep rate-stress curves (a) and stress-temperature normalization parameter curves (b) of small and standard samples at different temperatures
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图 7 900 ℃下持久蠕变和应力松弛试验后标准试样和小试样的显微组织
Figure 7. Microstructures of standard (a-b) and small (c-d) samples after creep rupture (a,c) and stress relaxation tests (b,d) at 900 ℃
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