摘要
为了研究新型合金在高温状态下的晶粒长大行为,进行了Haynes 282合金在973~1 373 K条件下保温0~10 800 s的晶粒长大实验,通过实验分析了保温时间和保温温度对其晶粒尺寸的影响,研究了晶粒尺寸在其实验条件下的演变规律。由此构建了符合Haynes 282合金晶粒长大的数学模型,该模型的建立能够有效表征Haynes 282合金在高温下的晶粒长大行为。
关键词
Haynes 282合金是一种新型镍基高温合金,在高温下具有杰出的抗氧化、耐腐蚀、耐辐射性能及较高的蠕变强度,且其焊接性能优
目前对于高温合金晶粒长大行为的研究主要集中于三个方面:第一是高温合金晶粒长大机理的研究,如CHEN
为了研究Haynes 282新型高温合金在高温环境下的晶粒长大规律,拟为其晶粒尺寸及力学性能控制提供理论依据,本文在宽泛的工艺参数范围内开展晶粒长大试验,并从微观组织形貌和平均晶粒尺寸的角度定性、定量地研究Haynes 282合金晶粒长大规律及保温时间和保温温度对Haynes 282合金晶粒尺寸的影响规律。
Haynes 282合金化学成分如

图1 Haynes 282合金原始金相照片
Fig.1 The original metallograph of Haynes 282 alloy
试验设备为电磁感应加热的数控金属熔炼炉。将试样清洗、干燥后,每3件为一组放入炉膛,关闭炉门,按照

图2 Haynes 282合金晶粒长大试验示意图
Fig.2 The diagrammatic process for the grain growth tests of Haynes 282 alloy
通过晶粒长大试验,获得Haynes 282合金在不同温度下保温不同时间后的微观组织如
使用Nano Measurer软件统计各试验条件下3个试样的平均晶粒尺寸,将其平均值作为该条件下的平均晶粒尺寸,统计结果如
由

图3 不同温度下Haynes 282合金平均晶粒尺寸随保温时间演变情况
Fig.3 The average grain size evolution of Haynes 282 alloy with holding time at different holding temperatures
通过将晶粒尺寸-保温时间关系拟合并对保温时间求导,可得到如

图4 Haynes 282合金在不同保温温度下晶粒长大速度随保温时间的变化关系
Fig.4 The variation relationship of the grain growth rate of Haynes 282 alloy with holding time at different holding temperatures
由

图5 不同保温时间下Haynes 282合金晶粒尺寸与保温温度的关系
Fig.5 The relationship between the average grain size of Haynes 282 alloy and holding temperature at different holding times
用于表征等温条件下晶粒长大后晶粒尺寸的模型主要有两种:其一是如
(1) |
(2) |
式中,dA和d0分别表示保温后的平均晶粒尺寸和初始晶粒尺寸,t和T分别表示保温时间和温度,R代表普适气体常数,R=8.314 J/(mol/K),Qa表示晶粒长大激活能,k和n为常数。
研究结果表明,初始晶粒尺寸对于晶粒长大过程有显著影
(3) |
式中,m、k和n为需要求解的常数。
将
(4) |
假设m已知,保温温度T一定时,有:
(5) |
保温时间t一定时,有:
(6) |
通过回归方法可以通过式(
为验证模型的预测精度,将973、1 073、1 173、1 273、1 373 K条件下的试验数据作为模型计算基础数据,1 223 K条件下的试验数据作为测试组,不参与模型计算,作为最终计算模型的验证数据组。为确定晶粒长大模型中m值,假设取m值为区间[1,10]的整数,按式(
(7) |
式中,S和Y分别代表平均晶粒尺寸试验值和晶粒长大模型预测值,N为样本数量。
经求解,不同m值所对应的模型常数值和均方误差如
由

(a) MSE1与m值

(b) MSE2与m值
图6 均方误差MSE1及MSE2和m值的关系
Fig.6 The relationships between the mean square errors MSE1,MSE2 and m-values

(a) m1=2.2

(b) m2=4.88
图7 和-1/RT值的关系
Fig.7 The relationships between and -1/RT

(a) m1=2.2

(b) m2=4.88
图8 和ln t值的关系
Fig.8 The relationships between and ln t
综上所述,计算所得Haynes 282合金晶粒长大模型为:
(8) |
使用构建的晶粒长大模型预测973~1 373 K保温温度及3 600~10 800 s保温时间范围内Haynes 282合金晶粒尺寸,预测值与试验值的比较如

图9 晶粒尺寸实测值及计算值的比较
Fig.9 The comparisons between the experimental and predicted grain sizes
根据预测数据,可计算出型T≤1 173 K及T>1 173 K时对应MSE1=0.963,MSE2=2.048。m1=2.2,m2=4.88时MSE1、MSE2值较m1、m2取其他值时更小,进一步验证了m1=2.2,m2=4.88时模型精度最高。
由
(9) |
(10) |
式中,S和Y分别表示试验值和预测值,和分别表示试验值和预测值的平均值,i和N分别表示试验值和预测值对象的序号和总数。

图10 晶粒尺寸预测结果与试验结果的相关性
Fig.10 The correlation relationship between the predicted and experimental grain sizes
(1)在高温条件下保温时,Haynes 282合金晶粒会出现不同程度的长大。在973~1 273 K条件下保温,晶粒正常长大,在1 373 K条件下保温晶粒出现异常长大现象。
(2)Haynes 282合金在特定的温度下保温时,晶粒随着保温时间的增加逐渐长大,但晶粒长大速度逐渐降低;保温时间一定时,保温温度越高,Haynes 282合金晶粒尺寸越大。Haynes 282合金在973~1 173 K内保温晶粒尺寸及长大速度均处于较低水平,而在1 173~1 373 K内保温晶粒尺寸及晶粒长大速度均显著提高。
(3)根据Haynes 282合金晶粒长大规律构建了分段式晶粒长大改进模型,其预测值最大相对误差5.21%,平均相对误差2.04%,相关系数0.996 1,具有较高的预测精度。
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