Acta Metallurgica Sinica, 2017, 53(6): 695-702
doi: 10.11900/0412.1961.2016.00508
形变诱导GH3625合金热挤压管材δ相的析出行为

Precipitation Behavior of δ Phase of Deformation Induced GH3625 Superalloy Hot-Extruded Tube
丁雨田, 高钰璧, 豆正义, 高鑫, 刘德学, 贾智

摘要:

采用XRD、SEM、EDS和Image-Pro Plus金相分析等方法测定了GH3625合金热挤压管材在不同冷变形量ε下经过800 ℃时效后δ相的析出含量,研究了冷变形对δ相的析出规律及析出动力学的影响。结果表明,δ相首先在形变孪晶界、晶界以及变形带上形核并析出,随后在晶内形核长大,并且随冷变形量的增加,δ相在变形带上析出量增加;随冷变形量的增加,δ相的形貌从针状向棒状或颗粒状转变;随着时效时间的延长,δ相的平均尺寸不断增大,长大规律符合LSW理论。当时效温度为800 ℃时,δ相的析出含量与时效时间的关系满足Avrami方程,且随冷变形量的增加,δ相的含量增加,时间指数n减小,δ相析出速率α增加,冷变形促进δ相的析出。Nb的溶质拖曳与δ相的钉扎共同作用抑制晶粒长大。ε=35%时,合金的硬度随保温时间的延长而增加,ε≥50%时硬度未发生明显变化。

关键词: GH3625高温合金 ; 冷变形 ; δ相 ; 析出行为 ; 动力学

Abstract:

GH3625 alloy is a wrought nickel-based superalloy mainly used in aeronautical, aerospace, chemical, nuclear, petrochemical, and marine applications industry due to its good mechanical properties, processability, weldability and resistance to high-temperature corrosion on prolonged exposure to aggressive environments. However, in medium and high temperature environment during long-term service, the γ'' is a metastable phase, easily transformed into stable δ phase, or δ phase directly formed in the γ matrix so that alloy performance was deteriorated, leading to the result of alloy failure. At the present work, mass fraction of δ phase in GH3625 superalloy hot-extruded tube cold deformed to different reductions and then aged at 800 ℃ for different times, were measured by XRD. The effect of cold deformation on the law and kinetics of δ phase precipitation was investigated by SEM, EDS and Image-Pro Plus metallographic analysis. The results show that δ phase first precipitates at the deformation twin and grain boundaries as well as deformation bands, and then precipitates in the grains. The amount of δ phase at the deformation bands increases with the increase of cold deformation. The morphologies of δ phase change gradually from needles to spheroids or rodlike with increasing cold deformation. With the extend of ageing time, the average size of δ phase increases which grows according to LSW theory. At 800 ℃, the relationship between the precipitation content of δ phase and ageing time follows Avrami equation. As cold deformation increases, the content of δ phase increases, the time index n decreases, whereas the δ phase precipitation rate increases. Cold deformation promotes the precipitation of δ phase. The solute drags of Nb in soild solution and pinning of δ phase inhibits the grain growth during ageing process of cold deformed GH3625 superalloy hot-extruded tube. The hardness of the alloy increases with the extension of the holding time at ε =35% but no obvious change at ε ≥50%.

Key words: GH3625 superalloy ; cold deformation ; δphase ; precipitation behavior ; kinetics

GH3625镍基变形高温合金以析出体心四方晶体结构的金属间相γ''-Ni3Nb (DO22结构),与γ'相[Ni3(Al, Ti, Nb)]共同起沉淀强化作用[1~4]。该合金在中高温(600~900 ℃)环境中工作时仍具有较高的力学性能和良好的耐蚀性以及抗氧化性,其合金管材被广泛应用于航空航天、燃气轮机和核电设备等领域,是航空、航天、核能、石油以及化工领域关键零件的制造材料[5~7]。GH3625合金管材在中高温环境中长期服役时,由于合金组织中γ''是亚稳相,容易转变成正交结构的稳定相δ-Ni3Nb (DOa结构),或者直接从γ基体中析出δ-Ni3Nb相,使合金性能退化,进而导致合金失效[8,9]。因此,研究冷变形对GH3625合金热挤压管材δ相的析出规律及动力学行为的影响十分重要。

Sundararaman等[10]将Inconel 625合金在750 ℃下保温100 h后,发现其组织中有少量的δ相析出,而在700 ℃下进行类似的热处理时,则没有观察到δ相的析出;当时效温度升高到800 ℃时,该合金组织中析出了大量的δ相。邸新杰等[11]将Inconel 625熔敷合金经850 ℃焊后热处理,在基体γ相中析出大量的针状δ相呈网格分布,且其附近出现了贫γ''相区域。陈名浩和何银秋[12]采用X射线衍射对GH625合金δ相定量分析,发现δ相析出峰的温度为800 ℃,尤其是时效200 h以上,析出的粒度较大的δ相是引起韧性下降的主要原因。赵新宇[13]研究了GH625合金的冷变形及其对力学性能的影响,发现冷变形量为18%的合金在回复过程中析出大量的δ-Ni3Nb相,硬度有所降低,但δ相的析出抑制了再结晶的发生,因而持久寿命很高。但已有的报道鲜少对形变诱导GH3625合金热挤压管材δ相的析出规律及动力学行为进行系统研究。

本工作对形变诱导GH3625合金热挤压管材中δ相的析出规律及动力学进行研究,计算得到δ相析出动力学曲线,了解δ相的析出规律,通过调节冷变形量来控制δ相的含量、分布及形态,为提高GH3625合金的持久性能和疲劳性能提供理论指导。

1 实验方法

本实验GH3625热挤压管化学成分(质量分数,%)为:C 0.042,Cr 21.77,Ni 60.63,Co 0.19,Mo 8.79,Al 0.21,Ti 0.40,Fe 3.68,Nb 3.75,Si 0.12,Mn 0.2,S 0.0006,P 0.006,Cu 0.06。试样从热挤压管上切取,经1150 ℃、1 h、空冷固溶处理后机加工成直径6 mm、长9 mm的圆柱试样,在应变速率为0.1 s-1条件下进行变形量ε为35%、50%和65%的室温压缩,随后进行时效处理,时效温度为800 ℃,保温时间分别为25、50、75和100 h,随后空冷。采用线切割方法将冷变形和时效处理后试样沿轴向中心剖开,进行机械研磨和抛光,用3 mL HNO3+5 mL H2SO4+90 mL HCl混合溶液化学腐蚀1~3 min。

采用Axiovert 40 MAT光学金相显微镜(OM)、Quanta FEG 450热场发射扫描电镜(SEM)、能谱仪(EDS)及Image-Pro-Plus金相分析软件,观测合金显微组织中δ相析出的形貌、分布和数量;用D8 Advance型X射线衍射仪(XRD)测定GH3625合金的XRD谱,CuKα,波长λ=0.154056 nm,管电压40 kV,管电流40 mA,衍射角范围20°≤2θ ≤100°,步长0.02°。利用微机对重叠峰进行分离,并计算奥氏体γ相、δ相和NbC衍射峰的积分强度。为提高点阵常数的测量精度,衍射峰的位置利用Si粉末标样进行校正,用Nelson-Riley函数外推法计算奥氏体点阵常数。

借助Image-Pro-Plus金相分析软件测量SEM像中δ相的平均尺寸,测量数目大于总数目的2/3取平均值;在Axiovert 40 MAT OM上观察合金的显微组织,并按照GB/T 6394-2002测定合金平均晶粒尺寸;用FRC-3e型洛氏硬度计测量合金的硬度,测量3个不同点的硬度取平均值。

图1 GH3625合金中相析出的温度-时间-转变曲线[14,15]

Fig.1 Time-temperature-transformation diagram of the phases in GH3625 superalloy[14,15]

图1中GH3625合金中相析出的温度-时间-转变曲线[14,15]中可以得到,800 ℃加热温度下,在奥氏体γ中析出γ''δM6C、M23C6和NbC相。本工作采用X射线定量相分析方法测定析出过程中δ相的含量,计算公式为[16]

W γ W δ = ρ γ ρ δ 1 n i n ( I i γ / R i γ ) 1 m i m ( I i δ / R i δ ) (1)

W NbC W δ = ρ NbC ρ δ 1 k i k ( I i NbC / R i NbC ) 1 m i m ( I i δ / R i δ ) (2)

W γ + W δ + W NbC = 1 (3)

R = 1 ν 2 P F 2 φ ( θ ) e - 2 M (4)

式中,WNbCWγWδ分别为NbC、γδ相的质量分数;I NbCI γI δ分别为NbC、γδ相衍射峰的积分强度(峰面积);ρNbCργρδ分别为NbC、γδ相的密度;mnk分别为选用δγ、NbC相衍射峰个数;ν为单位晶胞体积;F为合金成分、δ相成分和NbC计算的结构因数;P为多重性因数;φ(θ )为角因数;e-2M为温度因数,其中M=Bsin2θ /λ2,为一个与原子偏离其平衡位置的均方位移有关的常数,γ相、δ相和NbC均使用B=0.40。

2 实验结果与分析
2.1 GH3625合金管材中相的含量

图2为不同冷变形量及时效制度下GH3625合金热挤压管材的XRD谱。可以看出,GH3625合金热挤压管材在800 ℃保温25~100 h后奥氏体γ中主要析出δ相和NbC。奥氏体γ相的点阵常数为0.3572 nm,NbC的点阵常数为0.4470 nm,δ相的点阵常数在不同的制度下有较小的变化,a=0.5106 nm,b=0.4251 nm,c=0.4556 nm,与Sundararaman[10]的测试结果相近。在定量分析过程中,选用的奥氏体衍射峰为(111)γ、(200)γ、(220)γ、(311)γ、(222)γ,δ相衍射峰为(201)δ、(020)δ、(012)δ、(211)δ,NbC衍射峰为(111)NbC。采用X射线定量分析方法测定不同冷变形量及时效制度下δ相的质量分数,结果列于表1。由表可知,δ相的含量随冷变形量的增加和保温时间的延长而增加。

图2 不同冷变形量及时效制度下GH3625合金热挤压管材XRD谱

Fig.2 XRD spectra of GH3625 superalloy hot-extruded tube under different cold reductions (ε) and ageing times (t) (a) ε=35% (b) ε=50% (c) ε=65%

2.2 形变对相析出规律的影响

图3为冷变形GH3625合金热挤压管材中δ相的SEM像及EDS分析。由图可知,冷变形影响δ相析出的位置,在ε =35%时,δ相首先在晶界及形变孪晶界上形核并析出(图3a),随后在晶内形核并长大;ε≥50%时,δ相首先在变形孪晶界、晶界及变形带上形核并长大(图3b),随后在晶内形核并析出(图3c),并且随着ε的增加,δ相在变形带上析出的含量增多。这是因为在小变形量时,由于变形产生的位错密度较低,δ相在晶界(孪晶界)或晶内析出。随着ε的增加,位错密度提高,δ相在变形带上析出,使Nb在缺陷处的非平衡偏聚程度提高,使得δ相的含量随ε的增加而增多[17]。同时,冷变形程度越高,冷变形产生的超空位越多,Nb原子的非平衡偏聚程度随形变量的增加越严重,这样在位错胞壁和位错墙位的Nb含量越高,因而降低了δ相形核的临界自由能,促进了δ相析出[18]。由图3d可知,晶界处Nb的偏析程度大于基体中的偏析程度,此结果与Liu等[18]的研究结果相近;同时,测定的图3中1、2和3点处Nb的含量(质量分数)分别为6.0%、8.1%和11.3%,这说明,随着ε的增加,Nb在缺陷处的含量增加,从而验证了δ相的析出是由Nb原子的非平衡偏聚引起的。

表1 不同冷变形量及时效制度下GH3625合金管材δ相的质量分数
Table 1 Mass fraction of δ phase in GH3625 superalloy tubes under different cold reductions and ageing times
ε / % Mass fraction / %
25 h 50 h 75 h 100 h
35 1.58 1.77 1.95 2.15
50 1.73 1.89 2.12 2.24
65 1.88 2.07 2.28 2.34

表1 不同冷变形量及时效制度下GH3625合金管材δ相的质量分数

Table 1 Mass fraction of δ phase in GH3625 superalloy tubes under different cold reductions and ageing times

图3还可看出,冷变形还影响δ相的析出形貌。ε=35%时,析出的δ相为针状(图3a);ε=50%时,析出的δ相主要为棒状或颗粒状,还有少量的针状(图3b);ε=65%时,析出的δ相为短棒状或颗粒状(图3c)。这是因为在冷变形量较小的情况下,冷变形产生的位错组态为位错胞、平面状滑移位错和位错带,位错胞型的位错密度较低,Nb在奥氏体中均匀分布,δ相一旦形核将沿与奥氏体取向关系生长,其形貌为针状;在大的冷变形条件下,冷变形产生的位错结构为位错胞和位错墙,也就是说奥氏体被位错墙所分割,位错墙具有较高的位错密度,并且有Nb的非平衡偏聚使Nb在位错胞和位错墙处富集,δ相一旦在位错壁和位错墙处形核,则不可能沿奥氏体的取向关系长入基体,因此δ相的形貌为短棒状或颗粒状[19~21]

图3 冷变形GH3625合金热挤压管材中δ相的SEM像及EDS

Fig.3 SEM images (a~c) and EDS scaned along the line shown in Fig.3a (d) of δ phase in cold deformed GH3625 superalloy hot-extruded tube ageing at 800 ℃ for 75 h under ε =35 % (a), ε =50% (b) and ε =65% (c)

图4 不同保温时间下冷变形GH3625合金热挤压管材中δ相的SEM像

Fig.4 SEM images of δ phase in cold deformed GH3625 superalloy hot-extruded tube (ε=65%) ageing at 800℃ for 25 h (a), 50 h (b), 75 h (c) and 100 h (d)

图4为不同保温时间下冷变形GH3625合金热挤压管材中δ相的SEM像。随着保温时间延长,GH3625合金热挤压管材中δ相的平均尺寸不断增大,其含量也不断增加。表2为实验中测得的合金管材中δ相的平均尺寸(平均长度 l ̅ 和平均宽度 w ̅ ),分析可知,δ相的 l ̅ w ̅ 都与t 1/3成线性关系(图5),符合LSW理论[22,23],即:

d ̅ 3 - d ̅ 0 3 = kt (5)

式中, d ̅ 为时效后析出相颗粒的平均直径,μm; d ̅ 0 t=0时析出相颗粒的平均直径,μm;k为析出相的长大速率,μm3/h;t为保温时间,h。

表2中的数据代入式(5),得到δ l ̅ w ̅ 的长大速率分别为2.11×10-1 μm3/h和2.82×10-4 μm3/h,其长度方向的长大速率比宽度方向更为显著。

2.3 形变对GH3625合金热挤压管材相的动力学影响

图6为冷变形GH3625合金管材在800 ℃时效时δ相质量分数与时效时间的关系。由图可知,在冷变形量一定的条件下,随着时效时间的延长,δ相的含量增加,最后达到平衡状态。平衡态时δ相的含量Ws取决于加热温度,在800 ℃时效时,Ws大约为3%。在时效时间一定的条件下,随冷变形量的增加,δ相的含量增加。

表2 不同保温时间下δ相的平均尺寸
Table 2 Average sizes of δ phase ageing for different holding times at 800 ℃ (ε=65%)
Holding time / h l̅/ μm w̅ / μm
25 1.462 0.204
50 1.854 0.260
75 2.205 0.326
100 2.536 0.377

Note:l̅, w̅—average length and average width of δ phase

表2 不同保温时间下δ相的平均尺寸

Table 2 Average sizes of δ phase ageing for different holding times at 800 ℃ (ε=65%)

图5 不同冷变形量下在800 ℃时δ相的平均尺寸与时效保温时间的关系

Fig.5 Relationship between average sizes of δ phase and the ageing holding times at 800 ℃

图6 冷变形GH3625合金热挤压管材在800 ℃时效温度下δ相的含量与时效时间的关系

Fig.6 Relationship between δ phase content (Wδ) and ageing time of cold deformed GH3625 superalloy hot-extruded tube at 800 ℃

图7 lg[-ln(1-Wδ/Ws)]与lgt的关系

Fig.7 Relationship between lg[-ln(1-Wδ/Ws)] and lgt (Wsδ phase content at equilibrium state)

表3 δ相析出动力学参数
Table 3 Parameters for precipitation kinetics of δ phase
ε / % α / s-n n
35 1.144×10-2 0.364
50 1.693×10-2 0.342
65 2.463×10-2 0.322

Note: α—precipitation velocity of δ phase, n—time index

表3 δ相析出动力学参数

Table 3 Parameters for precipitation kinetics of δ phase

在一定时效温度下,δ相的含量Wδ与等温时间t的关系可用Avrami方程[24~26]表示:

W δ = W s [ 1 - exp ( - α t n ) ] (6)

式中,αδ相的析出速率;n是时间指数,取决于δ相的形核和长大机制。根据图6的实验结果绘制出 lg [ - ln ( 1 - W δ / W s ) ] 与lgt的关系曲线(图7)。通过线性回归分析,求出δ相的析出动力学参数,列于表3。

冷变形量对δ相析出动力学的影响可借助于nα值的变化来反映。从表3可以看出,在时效温度一定的条件下,随冷变形量增加,时间指数n降低,而δ相析出速率α增加。这说明,随冷变形量的增加,δ相析出速率增大,析出的结束时间缩短,说明冷变形促进了δ相的析出。

2.4 相对合金晶粒尺寸及硬度的影响

图8为GH3625合金热挤压管材晶粒尺寸与冷变形量与时效时间的关系。由图可知,随着冷变形量的增加和保温时间的延长,GH3625合金热挤压管材的晶粒尺寸逐渐减小。这是由于随着冷变形量的增加和保温时间的延长,Nb原子的非平衡偏聚程度加大,Nb原子的含量在位错处增加,Nb的溶质原子拖曳[27]δ析出相钉扎[28]共同作用阻碍晶界在晶体组织中的迁移,从而对晶粒长大产生明显的抑制作用。

图8 冷变形量和时效时间对GH3625合金热挤压管材晶粒尺寸的影响

Fig.8 Effect of cold deformation and ageing time on grain size of GH3625 superalloy hot-extruded tube

图9为GH3625合金热挤压管材硬度与冷变形量与时效时间的关系。由图可知,当ε =35%时,合金的硬度随保温时间的延长而增加;当ε ≥50%时,合金的硬度随保温时间的延长并未发生明显变化。对于冷变形量为35%的时效试样来说,其硬度的提高是由于冷变形量较小时,在晶界及孪晶界上形核析出的δ相对晶界(孪晶界)的钉扎作用;对于冷变形量达到50%以上,合金的硬度未发生明显下降是由于合金在时效过程中析出了δ相,消耗了基体中起固溶强化作用的Nb元素,其数量的减少使合金硬度降低。此外,随着冷变形量的增加,δ相的含量增多,而且其形貌由针状转变为颗粒状或短棒状,颗粒状的δ相对位错起阻碍作用,起到弥散强化的作用,从而使合金硬度提高。综上2种因素的综合作用,使合金的硬度未发生明显变化。

图9 冷变形量和时效时间对GH3625合金热挤压管材硬度的影响

Fig.9 Effect of cold deformation and ageing time on hardness of GH3625 superalloy hot-extruded tube

3 结论

(1) 冷变形影响δ相析出位置,δ相首先在晶界及形变孪晶界上形核析出,随后在晶内形核析出,随着变形量的增加,δ相在变形带上形核析出,并且析出含量增多;同时,冷变形影响δ相的析出形貌,随着冷变形量的增加,δ相的形貌由针状转变为短棒状或颗粒状;随着保温时间的延长,δ相的平均尺寸不断增大,其长大规律符合LSW理论。

(2) 冷变形GH3625合金管材在800 ℃时效过程中δ相析出质量分数与时效时间的关系符合Avrami方程,随着冷变形量的增加,时间指数n降低,析出速率α增加。冷变形促进了δ相的析出。

(3) Nb的溶质拖曳与δ相的钉扎共同作用抑制晶粒长大;ε=35%时,其合金的硬度随保温时间的延长而增加,ε≥50%时硬度未发生明显变化。

The authors have declared that no competing interests exist.

参考文献

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Addiert man zur freien Energie einen geeignet definierten elastischen Energieterm, dann erh01lt man eine neue Funktion für die freie Energie, die in einem begrenzten Konzentrationsbereich alle Eigenschaften eines thermodynamischen Potentials für koh01rente Prozesse besitzt. Sie kann z.B. zur Bestimmung des entsprechenden Phasendiagramms nach der üblichen Tangentenmethode benutzt werden. Hier wird sie verwendet, um die koh01rente Keimbildung in isotropen Festk02rpern zu untersuchen, insbesondere in der N01he der Metastabilit01tsgrenze, wo diese Keimbildung nicht mehr klassisch ist.
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关键词(key words)
GH3625高温合金
冷变形
δ相
析出行为
动力学

GH3625 superalloy
cold deformation
δphase
precipitation behavior
kinetics

作者
丁雨田
高钰璧
豆正义
高鑫
刘德学
贾智

DING Yutian
GAO Yubi
DOU Zhengyi
GAO Xin
LIU Dexue
JIA Zhi