在高温合金的定向凝固过程中，溶质元素在固相和液相间的再分配使铸件各部位的成分和组织不均匀，表现为柱状晶以枝晶的形态生长，先凝固的枝晶干区域(固相)往往富集Re、W、Mo等难熔元素，后凝固的枝晶间区域(液相)富集Al、Ti、Ta等γ'相形成元素，使枝晶间γ'相的尺寸明显比枝晶干的大[1,2]。随着凝固过程的进行固相的体积分数不断增大，剩余液相中Al、Ti、Ta等元素的含量不断提高，达到共晶成分后剩余液相以共晶的形式析出[3,4]。因此，为了提高合金的成分和组织的均匀性以制备出符合性能要求的合金零件，需要进行进一步热处理[5~7]。定向凝固高温合金的热处理包括固溶处理和时效处理，固溶处理温度应高于γ'相溶解温度以避免发生初熔。固溶处理温度还应该低于合金的初熔温度即枝晶间共晶熔化温度，因为W、Mo等难熔元素倾向于偏聚在固相。随着凝固过程中固相的不断增加液相中难熔元素的含量不断降低，共晶作为最后析出的相其中的难熔元素含量最低，因此熔点最低[8]。为了在时效热处理后得到均匀分布的细小γ'相，固溶处理应该使粗大的γ'相全部或大部分溶解，降低或消除铸态合金元素偏析[9~11]。同时，在高温条件下未溶解的共晶往往是合金的主要裂纹源，因此固溶处理还应该使共晶全部或大部分溶解[12,13]。随着高温合金的发展其中的Re、W、Ta等难熔元素的含量提高，枝晶间共晶数量的增加和偏析的严重使固溶处理温度提高和保温时间延长[14~16]。现有固溶处理研究的大多是固溶处理后合金中共晶溶解情况和元素的均匀化，对固溶处理时组织演化的研究较少[17~19]。但是，研究人员在高温合金固溶过程中发现了多种反常现象。Hedge等[20]研究发现，当固溶处理温度低于γ'相溶解温度时，随着保温时间的延长共晶的稳定性提高和体积分数增加；当温度高于γ'相的溶解温度时，在高温长时间保温使元素发生上坡扩散。刘心刚等[21]发现，随着固溶处理温度的提高共晶中Al、Ta、Cr、Co等元素的偏析加重，并认为与合金元素的上坡扩散有关。张少华等[22]和Lee等[23]在固溶处理时，也发现共晶体积分数先降低后增加的反常现象。这表明，固溶处理时组织的演化过程具有不确定性。
DZ125L合金国内是目前性能最高的第一代定向凝固高温合金，已用于生产航空发动机涡轮叶片，常用的固溶处理工艺是1220℃/2 h，属于不完全固溶处理[24]。同时，DZ125L合金作为采用低偏析技术发展成的无铪合金，通过控制合金中P、B、Zr和Si的含量明显减少了主元素的凝固偏析。高温合金固溶处理的目的，是溶解部分或全部析出相以及降低或消除元素偏析。由于DZ125L合金的低偏析特性，其固溶处理的重点是析出相γ'和γ/γ'共晶的溶解。鉴于此，本文研究DZ125L合金在更高温度下固溶处理过程中的组织演变，并根据组织演变的特点和Ti、Ta元素的均匀化时间研究DZ125L合金在1230℃固溶时出现γ'和γ/γ'共晶相面积分数反常增大的原因。
1 实验方法
用500 kg真空感应熔炼(VIM)浇注DZ125L母合金锭并按照HB 5220分析母合金成分，结果列于表1。用25 kg定向凝固设备制备DZ125L合金定向凝固试棒，其直径为16 mm长度为220 mm。定向凝固工艺参数：抽拉速率6 mm/min，双区温度1500℃。沿[001]方向在试棒上切取直径为16 mm厚度为5 mm的试样，用于初熔和固溶处理。为了消除升温速率对合金相变温度的影响，对试样采用同一升温方式进行初熔和固溶处理：高温炉炉温升至1200℃后将试样放入炉内，并以8~10℃/min的升温速率随炉升至指定温度，随后保温。初熔实验温度为1250~1300℃，保温时间为15 min，冷却方式均为水冷；固溶处理温度为1230~1260℃，保温时间为1~8 h，冷却方式均为空冷。
表1 DZ125L母合金的成分
Table 1
| Elements | C | Cr | Co | W | Mo | Ta | Al | Ti | B | Ni |
|---|---|---|---|---|---|---|---|---|---|---|
| Content | 0.087 | 9.01 | 10.0 | 7.02 | 2.01 | 3.50 | 5.13 | 2.25 | 0.0072 | Bal. |
按照GB/T 13298-2015标准对试样进行热镶嵌、研磨、抛光和腐蚀(化学和电解)。所用化学腐蚀试剂为4 g CuSO4 +20 mL HCl +20 mL H2O，电解腐蚀试剂为15 g C6H8O7 +5 g (NH4)2SO4 +500 mL H2O。观察样品中γ'相时采用电解腐蚀，其他样品均采用化学腐蚀。
用蔡司金相显微镜(OM)和FEI-F50型场发射扫描电镜(SEM)研究合金的组织和相成分。用EPMA-1610型电子探针 (EPMA) 研究合金主元素的分布规律，EPMA的加速电压为20 kV，工作电流为20 nA，束斑直径为1 μm，在枝晶干和枝晶间随机各测量5点，取其平均值，分析元素的偏析行为。
使用ImageJ软件统计各析出相体积分数和每个γ'相的面积，根据γ'相面积计算其等效尺寸：
方形γ'相
球形γ'相
式中S为γ'相的面积；a为等效尺寸。
2 实验结果
2.1 铸态合金的组织特征
图1给出了DZ125L合金的铸态组织。从图1a可见，铸态合金具有典型的枝晶形貌，灰色区域为枝晶干，白色区域为枝晶间，合金一次枝晶间距约为520 μm，二次枝晶间距约为63 μm。除γ基体外合金中还有γ'、γ/γ'共晶和碳化物等析出相，且γ/γ'共晶和碳化物主要分布在枝晶间。从图1b、1c可以观察到枝晶干和枝晶间γ'相尺寸相差较大，枝晶干γ'相尺寸约300 nm，面积分数为35.5%，枝晶间γ'相尺寸约660 nm，面积分数为35.3%。此外，在枝晶间γ基体通道上还析出了少量纳米级的细小γ'相(图1c中方框所示)，约60 nm。按照尺寸大小依次将上述三种γ'相称为γ
图1
图1
铸态DZ125L合金的微观组织
Fig.1
Microstructure of the as-cast DZ125L superalloy (a) dendritic structure, (b) γ' phase at dendrite core, (c) γ' phase at interdendrite, (d) γ/γ' eutectic, (e) carbide
表2 铸态样品γ/γ'共晶和碳化物的成分
Table 2
| Elements | Al | Ti | Cr | Co | Ni | Mo | Ta | W | C |
|---|---|---|---|---|---|---|---|---|---|
| Eutectic | 5.37 | 3.72 | 8.85 | 9.37 | 61.68 | 0.40 | 5.36 | 5.26 | - |
| Carbide | 0.05 | 15.91 | 1.47 | 0.95 | 4.70 | 1.86 | 48.89 | 13.51 | 12.65 |
2.2 相转变温度
图2
图3
图3
DZ125L合金在不同温度下的初熔组织
Fig.3
Microstructure of the as-cast DZ125L superalloy at 1280℃ (a), 1290℃ (b), 1300℃ (c) keeping for 15 mins and then quenched in water
2.3 在固溶处理过程中组织的演化
2.3.1 碳化物形貌的变化
图4a给出了在固溶过程中碳化物形貌的变化。可以看出，与铸态合金的草书状形貌明显不同(图1e)，固溶后碳化物变为颗粒状或短棒状，但是其位置分布仍保持草书形状。EDS分析结果(图4c~f)表明，碳化物仍为富Ta和富Ti的MC型，即在固溶过程中MC型碳化物只发生形态变化，其类型不变。其原因是，相同体积的球形沉淀相的表面积比草书状沉淀相小得多。即草书状碳化物具有较高的表面积和体积比，为热处理过程中草书状碳化物分解成球形提供了高驱动力[25]。同时，MC型碳化物与γ基体存在着取向关系。但是两相的点阵常数相差较大，使界面出现较大的晶格错配。有研究表明，较大的晶格错配促进高温合金液凝固生成小表面积和体积比的碳化物[26]，故较大的晶格错配也为草书状碳化物分解为颗粒状提供了驱动力。因此，固溶处理时，因界面能和应变能减小，MC型碳化物由草书状转变为颗粒状或短棒状。
图4
图4
固溶处理后碳化物的形貌
Fig.4
Morphologies and EDS mapping results of MC carbide after solution heat treatment (1250℃ for 2 h) (a) and (b) morphologies of MC carbide, (c) Ti, (d) Ta, (e) Cr, (f) W
2.3.2 γ′相
图5
图5
在不同条件下固溶处理后合金的微观组织
Fig.5
Microstructure of DZ125L superalloy after solution heat treatment (a-1) to (a-4) 1230℃ from 1 h to 8 h, (b-1) to (b-4) 1230℃ from 1 h to 8 h, (c-1) to (c-4) 1250℃ from 1 h to 8 h, (d-1) to (d-3) 1260℃ from 1 h to 4 h
固溶处理时合金中γ′相的面积分数和尺寸均发生变化，如图6所示。图6a给出了DZ125L合金固溶处理时γ′相的面积分数。与图5给出的结果相对应，在1230℃固溶处理时，随着保温时间的延长γ′相的面积分数先减小、后增加、再减小；而在1240℃和1250℃处理时，γ′相的面积分数持续减小。在1230℃保温1 h约有81%的γ′相溶解，在1240℃和1250℃保温1 h约有99%的γ′相溶解。这表明，保温1 h后合金的γ′相显著溶解。但是随着保温时间的延长γ′相面积分数的减小变慢，因为随着γ′相溶解数量的增加基体中γ′相形成元素的浓度提高而基体和γ′相的浓度梯度降低，减缓了γ′相的溶解。提高固溶温度使γ′相面积分数减小的速率提高，因为随着温度的提高元素的扩散加快[27]，基体溶解度的提高[28]驱动了γ′相溶解。同时，规定固溶处理后剩余γ′相面积分数低于铸态的5%(γ′相面积分数低于3.5%)为完全固溶处理，由图6a可知，在1230℃保温8 h和在1240℃、1250℃、1260℃保温1 h后γ′相面积分数均低于3.5%，表明在1230℃保温8 h和在1240℃、1250℃、1260℃保温1 h后试样经过了完全固溶处理，1230~1260℃又可称为合金的完全固溶处理温度。从图6b可以看出，在1230℃固溶时γ
图6
图6
在不同条件下固溶处理后γ'相的面积分数和γ
Fig.6
Area fraction of γ' phase and average size of γ
图7给出了DZ125L合金在1230~1250℃固溶处理时γ′相的尺寸分布。铸态合金中γ′相的尺寸具有双峰特性，左边高峰为枝晶干γ
图7
图7
在不同条件下固溶处理后γ'相的尺寸分布
Fig.7
Evolution of γ' phase size distribution as a fuction of time at (a) 1230℃, (b) 1240℃, (c) 1250℃
在1230℃固溶时，枝晶干γ′相的溶解速率明显比枝晶间高。其原因，一方面是γ′相的溶解受扩散控制且枝晶干γ
与铸态合金中的γ
图8
图8
固溶处理后γ'相的形貌
Fig.8
Morphologies of γ' phase after solution heat treatment: (a) 1240℃ for 1 h, (b) 1250℃ for 1 h
时效时，γ′相的长大和粗化的主要方式有两种：Ostwald熟化和相邻颗粒合并[32,33]。其中Ostwald熟化为小尺寸γ′相溶解，溶质原子扩散至大尺寸γ′相周围，促进γ′相长大。参考Ostwald熟化理论，在1240~1250℃固溶1 h和2 h时，枝晶干和枝晶间的γ′相均大量溶解，此时枝晶间未溶的γ
2.3.3 γ/γ′共晶形貌的变化
在固溶过程中γ/γ′共晶形貌发生了明显变化。从图9可见，γ/γ′共晶的溶解主要是通过内部γ′相的合并粗化进行的。按照不同部位的变化特点，可将γ/γ′共晶的溶解分为共晶冠γ′相的粗化、共晶芯γ′相的粗化和溶入基体。首先是共晶冠γ′相的粗化，如图9a、9b所示，随着固溶保温时间的延长铸态合金共晶冠上较大的γ′相相互合并形成一个整体，即多个γ′相合并为一个γ′相。与此同时，γ/γ′共晶周围出现了γ′相贫化区，说明共晶冠γ′相的粗化长大是由Ostwald熟化和相邻颗粒合并两种机制控制的。共晶冠形成整体后此γ′相继续扩大，逐渐合并共晶芯上的细小γ′相，共晶芯部的面积分数减小，γ′相尺寸增大。当共晶冠和共晶芯γ′相全部合并后γ/γ′共晶就变为γ′相，如图9c、e所示。最后，与枝晶间的其它γ′相相同直接溶入基体或分裂后溶入基体。由图9d可见，γ/γ′共晶溶解时向周围释放Al、Ti等γ′相形成元素，促进了γ′相的长大。
图9
图9
固溶处理后γ/γ'共晶形貌
Fig.9
Morphologies of γ/γ' eutectics after solution heat treatment (a) 1230℃ for 1 h, (b) 1230℃ for 2 h, (c) and (d) 1240℃ for 4 h, (e) 1250℃ for 2 h
图10给出了固溶处理时DZ125L合金中γ/γ′共晶的面积分数。从图10可见，在1230℃固溶处理时，随保温时间的延长γ/γ′共晶的面积分数先减小、后略微增大、再减小。在固溶过程中γ/γ′共晶数量增加的这一反常现象，在单晶高温合金中也出现过[20,22,23]。在1240℃和1250℃固溶，随着保温时间的延长γ/γ′共晶的面积分数持续减小。保温时间为1 h时γ/γ′共晶的面积分数显著减小，继续保温后γ/γ′共晶的面积分数减小的速率降低。提高固溶温度，使γ/γ′共晶面积分数减小的速率提高。与图6a的结果对比发现，固溶时γ/γ′共晶面积分数的变化规律与γ′相基本相同。据此可以推测，引起1230℃ γ/γ′共晶面积分数反常增加的原因与γ′相相同，即枝晶干γ′相快速溶解使γ′相形成元素向枝晶间扩散，促进了γ/γ′共晶的形核和长大。
图10
图10
在不同条件下固溶处理后γ/γ'共晶的面积分数
Fig.10
Area fraction of γ/γ' eutectic after solution heat treatment
式中C(x)为x位置处偏析元素的浓度；
式中
图11
为了进一步验证这一结论，测定了在1230℃保温1~4 h后枝晶干和枝晶间γ基体中Ta元素的含量，即
在1230℃固溶处理时γ基体中Ta元素的偏析系数k随时间的变化曲线，如图12所示。固溶前，DZ125L合金中的Ta元素偏析在枝晶间，此时CTa, γ,dc小于CTa, γ,id；保温1 h后枝晶干γ′相完全溶解使CTa, γ,dc迅速增大，当CTa, γ,dc大于CTa, γ,id时枝晶干γ基体中的Ta向枝晶间扩散使CTa, γ,dc减小，和CTa, γ,id增大，但是CTa, γ,dc的增大速度仍大于CTa, γ,id，k值的增加；保温2 h时Ta元素继续向枝晶间扩散，使CTa, γ,dc减小、CTa, γ, id增大和k值减小；保温4 h时Ta元素从枝晶干γ基体向枝晶间的扩散已经完成，此时枝晶间γ′相持续溶解使CTa, γ,id增大，当CTa, γ,id大于CTa, γ,dc时Ta元素又从枝晶间向枝晶干扩散，使CTa, γ,id减少、CTa, γ,dc增大，当CTa, γ,dc的增大速度高于CTa, γ,id时k值增加。
图12
图12
在1230℃ γ基体中Ta元素的偏析系数与时间的关系
Fig.12
Segregation coefficient curve of Ta element in γ matrix at 1230℃
综合上述结果，在1230℃固溶2 h时γ′和γ/γ′共晶相的面积分数增加这一反常现象，与枝晶干和枝晶间γ′相的溶解不同步有关。这种不同步使枝晶干中的Ta元素通过γ基体快速扩散至枝晶间，从而引起γ′和γ/γ′共晶相形核和长大。发生这种现象的机制，可借助图13分析和解释。固溶前(图13a)枝晶干的析出相有γ
图13
图13
在1230℃固溶处理后枝晶间γ'和γ/γ'共晶相数量反常增加的机制示意图
Fig.13
Shematic illustration of the abnormal increase in area fraction of γ' phase and γ/γ' eutectic during solution heat treatment at 1230℃
4 结论
DZ125L合金的完全固溶处理温度区间为1230~1260℃。在固溶过程中MC碳化物的形貌由草书状转变为颗粒状或短棒状。保温时间一定时，随着固溶温度的提高γ′和γ/γ′共晶相的面积分数减小。固溶温度为1230℃时，随着保温时间的延长γ′、γ/γ′共晶相的面积分数和枝晶间γ′相平均尺寸按先减小、后增加、再减小规律变化；固溶温度为1240℃或1250℃时，随着保温时间的延长γ′和γ/γ′共晶相面积分数减小，枝晶间γ′相的平均尺寸增加。固溶处理时枝晶干γ′相的快速溶解使合金中Ta元素由枝晶干向枝晶间扩散，引起在1230℃固溶处理时γ′和γ/γ′共晶相的面积分数反常增大。
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Influence of W content on the microstructure of nickel base superalloy with high W content
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Nickel base superalloys are widely used in the preparation of hot end parts for aircraft engines because of their good comprehensive mechanical properties, oxidation resistance and structural stability. It's strengthened mainly by solid solution strengthening, γ' phase strengthening and carbide strengthening. High alloying is one of the main methods to improve the solid solution strengthening level of nickel base superalloys, where the element W is an efficient alloying element with low price. The control of the W content is extremely important for high W content nickel base superalloys. However, there are few reports on the influence of W content on the microstructure of high W alloy. According to this background, by means of OM, SEM observation and XRD analysis, the influence of W content on the solidified microstructure in nickel base superalloy have been investigated in this work. Results show that when the W content is about 14% (mass fraction, the same below), there is no α-W phase being precipitated in the alloy. While as the content of W is higher than 16%, α-W could be precipitated during the solidification. On another hand, the grain size of the alloy decreases from 1.04 mm to 0.17 mm and the volume fraction of eutectic increases from 6% to 10% with the increase of the W content. While the content of W has no obvious effect on the sizes and morphologies of γ' phase in the dendrite and inter-dendrite areas. During solidification, the α-W phase will be first precipitated due to its higher precipitation temperature, and the shrinkage of the residual liquid phase may cause the shift and growth of the α-W to the core of the liquid phase. The α-W could be as the core of the heterogeneous nucleation to reduce the critical nucleation energy, which is the main reason that the grain size of the 18%W alloy is smaller. During the growth of the dendrites with various orientations, the concentration of Al and Ti in the residual liquid phase may have a higher concentration gradient to cause the occurrence of eutectic transformation, which is the main reason that there is a higher volume fraction of eutectic in 18%W alloy.
W含量对一种高W镍基高温合金显微组织的影响
[J].通过OM、SEM和XRD对高W镍基高温合金进行组织观察与分析，研究了W含量对镍基高温合金凝固组织的影响规律。结果表明，当W含量为14% (质量分数，下同)时，镍基合金中无α-W相析出。当W含量高于16%时，合金凝固期间可析出α-W，并且随W含量提高，合金的晶粒尺寸由1.04 mm减小至0.17 mm，共晶含量由6%增至10%；W含量对在枝晶间/枝晶干内的γ'相尺寸及形态无明显影响。由于α-W的析出温度较高，在凝固期间首先析出，并在残余液相收缩作用下，α-W向液相核心处发生转移并长大；同时α-W可作为异质形核的核心，降低枝晶形核的临界形核功，使18%W合金获得较小的晶粒尺寸。此外，在不同取向枝晶汇聚生长的作用下，残余液相中Al、Ti等元素形成较高的浓度梯度而发生共晶转变，这是18%W合金中共晶含量较高的主要原因。
Microstructure evolution and mechanical properties of disk superalloy under multiplex heat treatment
[J].
Effects of heat treatment on the microstructure and mechanical properties of Ni3Al-based superalloys: A review
[J].
Research progress on solution treatment regimes of Ni based single crystal superalloy
[J].
镍基单晶高温合金固溶处理制度的研究进展
[J].
Incipient melting phase and its dissolution kinetics for a new superalloy
[J].
Dissolution behavior and kinetics of γ' phase during solution treatment in powder metallurgy nickel-based superalloy
[J].
Effects of homogenization treatment on microstructure of as-cast GH625 alloy
[J].
均匀化处理对铸态GH625合金组织的影响
[J].利用光学显微镜、扫描电镜和能谱分析等研究了采用真空感应熔炼和保护气氛电渣重熔工艺生产的GH625合金的铸态组织和均匀化热处理后的微观组织。结果表明：采用双联工艺生产的GH625合金电渣锭存在枝晶和元素偏析，其中Nb、Mo、Ti元素为主要偏析元素；均匀化热处理后枝晶消除明显，块状及条状碳化物碎化，棱角钝化；相同均匀化热处理工艺下Ti、Mo、Nb达到均匀化的难度依次加大；GH625合金最佳的均匀化热处理的工艺为1200℃保温24 h。
Microstructure and homogenization process of as-cast GH4169D alloy for novel turbine disk
[J].
High-cycle fatigue behavior of K416B Ni-based casting superalloy at 700℃
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K416B镍基铸造高温合金的700℃高周疲劳行为
[J].
Effect of heat treatment on microstructure and stress rupture life of Ni-base single crystal superalloy
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热处理对镍基单晶高温合金微观组织和高温持久性能的影响
[J].
Heat treatment process for single-crystal superalloy used in aeroengines
[J].
航空发动机用单晶铸造高温合金热处理工艺
[J].
Solution heat treatment response of a third generation single crystal Ni-base superalloy
[J].
Investigation on remelting solution heat treatment for nickel-based single crystal superalloys
[J].
Investigation on the homogenization treatment and element segregation on the microstructure of a γ/γ'-cobalt-based superalloy
[J].
Investigation on a ramp solution heat treatment for a third generation nickel-based single crystal superalloy
[J].
Effect of solution heat treatment on microstructure and segregation of carbon-containing nickel-based single crystal AM3 superalloy
[J].
固溶热处理对含碳镍基单晶高温合金AM3组织和偏析的影响
[J].
Designing homogenization-solution heat treatments for single crystal superalloys
[J].
Microstructural evolution of a third-generation single crystal superalloy DD33 during solution treatment
[J].The microstructural evolution of a third-generation single crystal superalloy DD33 during solution treatment has been investigated by optical microscope (OM), Scanning electron microscope (SEM) and electron probe microanalysis (EPMA). The results show that the γ' particles in interdendritic regions of the alloy grew up obviously during isothermal exposure at 1310℃, and fully dissolved after heat treated at 1310℃/2 h. No significant dissolution of (g +γ') eutectic could be observed when solution treated at temperatures below 1320℃. The cursive -script like MC carbides dissolved gradually and then transform into granular MC particles during solution treatment, which promoted the coarsening of γ' precipitates in the interdendritic regions. The segregation of elements Al, Ta, Cr and Co in the eutectic γ' was enhanced with the increasing temperature, which may be induced by up-hill diffusion.
第三代单晶高温合金DD33固溶处理中组织的演变
[J].通过金相显微镜(OM)、扫描电镜(SEM)和电子探针(EPMA)等手段研究了第三代单晶高温合金DD33在固溶处理过程中组织的演变。结果表明: 在第三代单晶高温合金DD33中, 枝晶间γ'相在1310℃保温时明显长大, 而在1320℃/2 h固溶后完全溶解; (g+γ')共晶在高于1320℃时显著溶解; MC碳化物在固溶处理过程中发生溶解, 由草书体状向颗粒状转变, 促进枝晶间γ'相的长大; 随着固溶温度的提高共晶γ'相中Al、Ta、Cr、Co等元素的偏析加剧, 这一现象可能是上坡扩散引起的。
Investigation on eutectic dissolution behavior of single crystal superalloy by differential scanning calorimetry
[J].Ni-based single crystal (SX) superalloys are used for the production of blades in gas turbines and aircraft engines because of their superior mechanical performance at high temperatures. To improve the temperature capabilities of modern SX superalloys, specific refractory elements are added to the alloys. This leads to micro-segregation in alloys, requiring a complex heat treatment process to eliminate γ/γ′ eutectic. Therefore, the dissolution process of γ/γ′ eutectic must be understood. In this study, a second-generation Ni-based SX superalloy was used to investigate the effect of extended holding time at 1290oC and 1300oC on the γ′ phase dissolving temperature (Tγ′) and γ/γ′ eutectic phase-melting temperature (Tγ/γ′), respectively. The method involves measuring the differential heating curves of as-cast and as heat-treated samples using DSC. The results showed that Tγ′ and Tγ/γ′ increased at a holding time of 2 h. However, with an increase in the holding time, the temperature increase was not obvious. The volume fraction of γ/γ′ eutectic decreased with the extended holding at 1300oC, while the volume fraction of γ/γ′ eutectic increased after holding at 1290oC for 8 h. This abnormal phenomenon was confirmed by the metallographic experiments. The analyses showed that the increase in the eutectic volume fraction was due to the incomplete dissolution of coarse γ′ phase at the inter-dendritic region, which resulted in the diffusion of Ta element from dendrite core to the inter-dendritic region, promoting eutectic growth.
单晶高温合金共晶溶解行为的差热分析
[J].选用一种第二代单晶高温合金，基于差示扫描量热技术(DSC)，采用对比法测量了铸态和完全热处理态样品的升温DSC曲线，研究了保温过程中单晶合金中γ′相、γ/γ′共晶相的相变温度变化规律。结果表明，1290和1300℃保温过程中，随着保温时间的延长，γ′相溶解温度和γ/γ′共晶相熔化温度先显著提高，然后缓慢增加。1300℃保温过程中，γ/γ′共晶体积分数随保温时间延长而逐渐降低。而1290℃保温过程中，随保温时间延长，共晶体积分数出现了先降低后增加的反常现象，这与金相实验方法相吻合。分析表明，枝晶间粗大γ′相未完全溶解，造成枝晶轴Ta元素向枝晶间扩散，促使共晶长大，从而使共晶体积分数增加。
Microstructural changes by heat treatment for single crystal superalloy exposed at high temperature
[J].
Microstructural evolution of a carbon modified single crystal Ni-base superalloy
[J].
The microstructures of base/modified RR2072 SX superalloys and their effects on creep properties at elevated temperatures
[J].
Preparation method and diffusion mechanism of Fe-Al coating on Q235 low carbon steel by pack aluminizing
[J].The Fe-Al coating, with compactness, stiffness, and continuity, could be prepared on Q235 low carbon steel by pack aluminizing. The phase structure, morphology, composition, and hardness of the prepared coating were characterized by XRD, SEM, EDS, and micro-hardness tester respectively. Results indicate that the Fe-Al coating is composed of Fe2Al5 and FeAl3 phases, whilst, the coating fabricated at 750℃ is particularly rich in Fe2Al5 phase. With the rising temperature, the thickness of Fe-Al coating increases, whereas the micro-hardness decreases. As a result of aluminizing for different time, the formed coatings are composed of the two phases Fe2Al5 and FeAl3 as well. However, with the increasing aluminizing time, the content of FeAl3 phase decreases, while the micro-hardness of the coating decreases slightly. Finally, a diffusion mechanism related with the formation of Fe-Al coating is proposed based on the comprehensive analysis on the thermodynamics and kinetics of pack aluminizing process.
包埋渗铝法制备Fe-Al渗层及其扩散机制
[J].包埋渗铝法可在钢基体表面制备出一层致密、坚固、连续的Fe-Al渗层，以改善基体性能。本文在不同温度和不同时间下对Q235低碳钢进行包埋渗铝，形成Fe-Al渗层，采用X射线衍射、扫描电镜及能谱分析等方法研究了渗铝层的物相结构、表面及截面形貌和成分，采用显微硬度仪测量了截面硬度。结果表明，不同渗铝温度下获得的渗铝层，主要含有Fe<sub>2</sub>Al<sub>5</sub>和FeAl<sub>3</sub>两相，且750℃得到的渗层存在较多Fe<sub>2</sub>Al<sub>5</sub>相；随着渗铝温度升高，Fe-Al渗层厚度增加，Al原子扩散系数增大，但显微硬度降低；不同渗铝时间下制备的渗铝层，物相仍以Fe<sub>2</sub>Al<sub>5</sub>和FeAl<sub>3</sub>为主，但随着渗铝时间延长，FeAl<sub>3</sub>含量减少，且Al原子扩散系数变大，渗层显微硬度略有降低。在进一步分析Fe-Al渗层形成的热力学与动力学基础上，总结了渗铝层形成的扩散机制。
Analysis on the factors affecting the performance of 6061aluminum alloy extruded profiles
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6061铝合金挤压型材性能影响因素分析
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Coarsening and dissolution of γ' precipitates during solution treatment of AD730TM Ni-based superalloy: mechanisms and kinetics models
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Effect of elastic interaction energy on the distribution of coherent precipitate particles in nickel-base alloys
[A].
γ' precipitate splitting in nickel-based superalloys: a 3-D finite element analysis
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The mechanism of morphogenesis in a phase-separating concentrated multicomponent alloy
[J].What determines the morphology of a decomposing alloy? Besides the well-established effect of the nucleation barrier, we demonstrate that, in a concentrated multicomponent Ni(Al,Cr) alloy, the details of the diffusion mechanism strongly affect the kinetic pathway of precipitation. Our argument is based on the combined use of atomic-scale observations, using three-dimensional atom-probe tomography (3D APT), lattice kinetic Monte Carlo simulations and the theory of diffusion. By an optimized choice of thermodynamic and kinetic parameters, we first reproduce the 3D APT observations, in particular the early-stage transient occurrence of coagulated precipitates. We then modify the kinetic correlations among the atomic fluxes in the simulation, without altering the thermodynamic driving force for phase separation, by changing the vacancy-solute interactions, resulting in a suppression of coagulation. Such changes can only be quantitatively accounted for with non-zero values for the off-diagonal terms of the Onsager matrix, at variance with classical models.
Evolution of γ' phases of the fourth generation powder metallurgy superalloy FGH4102 during long-term aging
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第四代粉末高温合金FGH4102长期时效过程中γ'相演变规律
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Influence of Ti content on microstructure, mechanical properties and castability of directionally solidified superalloy DZ125L
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Diffusion and interface control of reactions in alloys
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Modelling of the microsegregation in CMSX-4 superalloy and its homogenisation during heat treatment
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