Acta Metallurgica Sinica, 2017, 53(6): 684-694
doi: 10.11900/0412.1961.2016.00495
一种高硼定向凝固合金的初熔行为及其对力学性能的影响

Incipient Melting Behavior and Its Influences on the Mechanical Properties of a Directionally Solidified Ni-Based Superalloy with High Boron Content
张洪伟1,2, 秦学智2, 李小武1, 周兰章2,

摘要:

系统研究了高硼DZ444定向凝固合金的初熔行为及其对力学性能的影响。结果表明,在铸态合金中,枝晶间包含大量γ/γ′共晶、MC碳化物和由硼化物、Ni5Hf及η相组成的“团聚相”。在固溶处理期间,团聚相周围受硼化物显著影响的γ基体首先发生初熔。硼化物不是初熔的形核点,但是对初熔的形成具有关键作用。较高的B含量,使得合金具有较低的初熔温度,介于1160~1170 ℃之间,明显低于正常合金。提升温度或延长保温时间,初熔现象变得更加严重。采用水淬方式,初熔倾向于凝固为典型的γ枝晶和大量细小的沉淀相颗粒;而采用空冷方式时,初熔依次凝固为团聚相、γ基体和γ/γ′共晶相,团聚相形貌与铸态时没有明显差异。完整热处理时,固溶温度由1210 ℃提升到1230 ℃,初熔略微增加,而当温度达1250 ℃时,初熔区尺寸和面积分数剧烈增大,对合金造成严重破坏。由于温度较低,合金的高、低温时效对初熔组织影响不是很大。随着初熔区尺寸和面积分数增加,初熔区消耗了大量的固溶强化元素,同时初熔区内部易萌生大量微裂纹,从而使合金的拉伸性能稍有下降,持久性能显著降低。

关键词: 定向凝固高温合金 ; 固溶处理 ; 初熔 ; 团聚相 ; 力学性能

Abstract:

A new directionally solidified Ni-based superalloy is developed for industrial gas turbine applications, which has high strength and excellent hot corrosion resistance at high temperatures. The high strength of the alloy is primarily derived from precipitation hardening by ordered L12 γ′ phase. To achieve a uniform distribution of precipitated γ′ particles for optimized mechanical properties, the suitable heat treatments are used. However, the heat treatment temperature in Ni-based superalloys is limited by the problem of incipient melting. Incipient melting microstructrue evolution during heat treatment has been hardly reported. Therefore, the behaviors of incipient melting and its effect on mechanical properties in the new directionally solidified superalloy DZ444 with high boron have been investigated in this work. The results show that some interdendritic regions of the as-cast DZ444 sample exhibit many of γ′/γ eutectic, MC carbides and multi-phase eutectic-like constituent which are composed of boride, Ni5Hf and η phases. During solution treatments, incipient melting does not occur in boride or Ni5Hf phase with low melting point firstly, but appears in γ matrix around multi-phase eutectic-like constituent which is affected significantly by borides. Compared to DZ444 alloy with the normal boron content, incipient melting occurs at the lower temperature in the range between 1160 ℃ and 1170 ℃. Incipient melting can occur significantly with the increase of the solid solution temperature or time. Incipient melting consists of typical γ dentrites and a lot of tiny precipitation particles after the water quenching (WQ) method following solution treatment. However, incipient melting forms multi-phase eutectic-like constituent, γ matrix and γ′/γ eutectic successively during air cooling (AC) following solution treatment, and the morphology of multi-phase eutectic-like constituent is similar to that of as-cast alloy. Firstly, a so-called incipiently melted circle (IMC) forms around multi-phase eutectic-like constituent; with the increase of the solid solution temperature or time, IMC extends inwards which makes γ matrix and multi-phase eutectic-like constituent in this circle melt successively. Finally, a incipiently melted pool forms gradually. Incipient melting is limited to the IMC below 1200 ℃ and the area of incipient melting changes with temperature or time correspondingly. However, incipiently melted region (IMR) expands outwards continuously which makes γ matrix outside the incipiently melted circle melt when the temperature is higher than 1210 ℃. Especially, IMR swallows up plenty of γ matrix, and many matrix islands, regions unmelted, exist in IMR above 1250 ℃ which destroys the continuity of the matrix significantly. The incipient melting has a minor effect on the tensile properties, nevertheless, decreases the creep-rupture properties remarkably. The degradation of mechanical properties mainly results from the increasing of the incipient melting area fraction and size.

Key words: directionally solidified superalloy ; solid solution treatment ; incipient melting ; multi-phase eutectic-like constituent ; mechanical property

定向凝固镍基高温合金需要热处理后使用,在热处理过程中,凝固期间析出的强化相溶解或大部分溶解,并重新析出均匀细小、弥散分布的强化相,降低或消除合金铸态下存在的元素偏析,特别是合金凝固后期的共晶相及周围区域与基体的成分偏差引起的偏析[1,2]。共晶前沿区域聚集了低熔点相或低熔点组分。有文献[3~9]报道共晶前沿发现了Cr-Mo硼化物和Ni-Zr等低熔点相。低熔点相的形成使合金在热处理过程中容易发生初熔,对合金的力学性能产生重要影响[10~13]

Sidhu等[10]对定向凝固高温合金Rene80的研究结果表明,初熔是由合金凝固后期的γ/γ′共晶周围区域的Cr-Mo硼化物、Ni-Ti相和Ni-Zr相引起的。郑运荣等[11,12]对含Hf定向凝固合金DZ22初熔的研究表明,Ni5Hf相的熔化是合金初熔的主要原因。初熔体积分数超过3%的合金,无论持久或高温拉伸性能都明显下降,在拉伸或持久试样断口上都可以看到沿初熔区的γ/γ′共晶边缘断裂的特征。郑运荣等[11,12]和陈荣章等[13]通过调节合金的成分和热处理制度,消除或减少了合金的低熔点相,从而有效控制合金的初熔。Jahangiri等[14]对铸造镍基高温合金IN939的研究结果表明,通过调整合金热处理制度,可以有效地避免合金发生初熔,从而使合金的热加工性能得到改善。

DZ444合金是一种低成本、抗热腐蚀定向凝固镍基高温合金,在K444等轴晶合金基础上设计开发而成。该合金的使用温度较K444提高30 ℃左右,力学性能与国外同类型合金MGA1400DS相当,优于GTD111DS和IN792DS等合金,主要用于制造现代船舶和地面燃气轮机中工作温度不高于930 ℃的涡轮工作叶片等高温零部件[15,16]。由于该合金包含微量B元素(0.01%, 质量分数,下同)和少量Hf元素(0.5%),在偏析较严重的情况下,理论上有形成低熔点相的倾向性,因此本工作向合金中添加较高含量的B (0.09%),考察合金在固溶处理期间的初熔规律、初熔与B或硼化物的关系以及对合金力学性能的影响,对铸造镍基高温合金的化学成分设计和热处理工艺选择具有一定的指导意义。

1 实验方法

实验合金的化学成分(质量分数,%)为:C 0.07,B 0.09,Cr 15.8,Co 10.7,W 5.3,Mo 2.0,Al 3.0,Ti 4.7,Ta 0.5,Hf 0.5,Ni余量,这相当于在DZ444合金中加入了过量的B。在真空感应炉中冶炼母合金,然后在定向凝固炉中制备定向凝固试棒。从试棒上切取小块试样,在不同温度(1150、1160、1170、1180、1190和1200 ℃)下进行不同时间(5、10和30 min)的固溶处理,分别采用水淬(WQ)和空冷(AC) 2种方式冷却试样。

将试棒分为3组,分别在1210、1230和1250 ℃ 3个温度下固溶处理2 h,接着空冷至室温,然后进行两级时效(1080 ℃、4 h、AC和850 ℃、24 h、AC)处理。根据由低到高的固溶温度,以上3个完整热处理制度对应表示为HT1、HT2和HT3。热处理后,在AG-X250kN型电子拉伸试验机上测试试棒在900 ℃下的拉伸性能,在RJ-50型持久试验机上测试试棒在930 ℃、275 MPa下的持久性能。

使用GX51型光学显微镜(OM)、配有能谱仪(EDS)的JEM 6340型场发射扫描电镜(FESEM)和Tecnai G2 F20透射电镜(TEM)观察显微组织并测定成分,使用EPMA-1610型电子探针仪(EPMA)测定元素面分布。试样腐蚀采用电解方法:一种电解液(A)是体积比为1∶3∶5的HNO3+HCl+甘油溶液,腐蚀掉的是γ′相;另一种电解液(B)是10%H2CrO4 (10 g CrO3+90 mL H2O)水溶液,腐蚀掉的是γ基体。TEM样品采取双喷电解减薄工艺制备,双喷液为10%HClO4+90%C2H5OH (体积分数)。采用软件Image Pro. Plus 6.0统计初熔区尺寸和面积分数,结果为至少30张图片的平均值。

2 结果与分析
2.1 合金的铸态组织

图1给出了高硼DZ444合金的典型铸态组织。

图1 高硼DZ444合金的典型铸态组织

Fig.1 SEM (a~d) and TEM (e, f) images of as-cast microstructures of DZ444 alloy with high boron (specimens of Figs.1a and b were etched by electrolyte A which removes the γ’ precipitates in the matrix , specimens of Figs.1c and d were etched by electrolyte B which removes the γ matrix in the alloy)
(a) dendrite morphology (b) γ/γ’ eutectic (c) borides
(d) multi-phase (generally including γ, η, boride and Ni5Hf phases) eutectic-like constituent
(e) TEM image of boride (Inset shows SAED pattern of boride)
(f) TEM image of η and Ni5Hf phases (Insets show SAED patterns of Ni5Hf and η)

在低倍SEM像(图1a)中,铸态合金呈现典型的枝晶状组织,枝晶间区域存在着大量的γ/γ′共晶,像珠链一样包围在枝晶周围。在较高倍SEM像中,γ/γ′共晶为葵花状,从心部向外,γ′相尺寸逐渐增大,尤其是最外围γ′相,尺寸比心部大得多。共晶周围经常会有硼化物、Ni5Hf、η相或MC碳化物等相出现(图1b~f)。MC碳化物形貌为块状,有2种形式:一种富Ti和W,呈灰色,可表示为MC(1);另一种主要富Hf,亮白色,表示为MC(2)。硼化物多呈骨架状,也可分为2类:其中一类的成分比值(W+Mo)/Cr约为1,亮灰色;另一类的(W+Mo)/Cr约为2.6,白色(图1c和表1)。相对于硼化物,蜂窝状或块状Ni5Hf相数量少、尺寸小,因此不易观察到(图1d)。在高温合金中,硼化物和Ni5Hf相均为低熔点相。如在K5或K19H合金中,硼化物(M3B2)约于1200~1220 ℃开始熔化,而Ni5Hf相的熔化温度范围约为1135~1160 ℃[17]图1d中,γ相与硼化物、Ni5Hf和η相等在γ/γ′共晶前沿团聚析出,本文称之为“团聚相”。而在一些文献[3,4,18]里,把它们称为多相共晶状析出物(multi-phase eutectic-like constituent)。

由于W、Mo、Hf和Ta等难熔元素的扩散系数较低,它们在合金液中的偏析较为严重[19~22]。凝固后期,γ/γ′共晶的大量析出加剧这些元素的偏析,使得它们以硼化物、Ni5Hf或η相等形式团聚析出。由于这些元素在不同部位的偏析程度不同,导致合金中MC碳化物和硼化物出现了不同的类型(表1)。分布于γ/γ′共晶周围的这些相,凝固时消耗了大量强化元素(W、Mo、Ti和Al等),同时由于它们熔点较低,热处理过程中容易发生初熔,因此对合金的力学性能很不利。图2为各相化学成分的EPMA面扫描,清晰地显示了合金元素在各相及周围基体中的分布情况。由图可见,团聚相周围的基体明显贫Cr,而富Ni、Al和Ti等元素,这与硼化物,尤其是Boride(1)富Cr而贫Ni、Al和Ti正好相反。因此,可以推断,团聚相周围基体的元素分布特点,可能与硼化物析出时发生的元素互扩散有密切关系。

图2 团聚相形貌的BSE像和元素面分布

Fig.2 BSE (backscattered electron) image (a) and EPMA elemental mapping results of multiphase eutectic-like constituent for Cr (b), Mo (c), W (d), B (e), C (f), Al (g), Co (h), Ni (i), Hf (j), Ti (k) and Ta (l)

2.2 固溶温度和时间对初熔的影响

铸态试样在1150和1160 ℃保温不同时间(5、10和30 min)后,没有发生初熔现象。在1170 ℃处理5 min,也未发生初熔(图3a),但是当时间达到10 min时,团聚相周围出现一个几乎封闭的初熔圈(incipiently melted circle,IMC,内容物实际为细小的γ/γ′共晶相),将η相、硼化物和Ni5Hf等包围在里面(图3b)。可见,初熔并未像一些文献[11~13,23]描述的那样发生在硼化物或Ni5Hf等低熔点相处,而是首先出现在受硼化物显著影响的团聚相周围的基体内。这说明初熔虽然不是由硼化物直接导致,但是与硼化物的析出有着密切联系。γ/γ′共晶圈的形成,一方面印证了团聚相周围富Ni、Al和Ti等γ′相形成元素的情况,另一方面也证实了硼化物对初熔形成的关键性贡献。在1170 ℃延长处理时间至30 min时,初熔圈向团聚相方向侵蚀扩展,使得圈内基体大部分熔化。但是,由于处理温度相对较低,各团聚相本身(如η相、硼化物和Ni5Hf相等)并没有表现出明显的熔化迹象(图3c)。Rene80合金在1160 ℃[10]、IN738合金在1155 ℃ [3]及IN939合金在1150~1160 ℃[14]保温一定时间后,都会发生初熔现象,而且都发生在类似于本合金一样的团聚相(Cr-Mo硼化物、Ni-Ti相或Ni-Zr相等)周围。

当处理温度从1180 ℃增加到1200 ℃、保温时间从5 min延长到30 min时,初熔圈内的基体首先熔化,接着各团聚相先后溶解或熔化,最后整个初熔圈内的原始组织都变成了金属液,即初熔池形成(图4)。水淬的冷却方式保留了试样在高温时的状态,能很好地显示初熔区(incipiently melted region,IMR)内未熔组织与初熔组织的差异,前者虽未熔化,但里面会溶有部分团聚相溶解时释放出的大量W、Mo和Hf等元素,近似于过饱和的γ固溶体相,后者则为快凝组织,主要由水淬时形成的各种新沉淀相组成(图4b和c)。在1200 ℃下保温30 min后,初熔区内的原始组织完全熔化为金属液。水淬时,这些金属液快速凝固,形成细小的枝晶状组织[24~28]:除了星形的枝晶核心,还有几个较为粗大完整的枝晶,其一次和二次枝晶臂清晰可见,而其它区域则被一些针状或絮状沉淀物覆盖。初熔区内原始组织复杂,往往包含硼化物、Ni5Hf、η相、γ/γ′共晶和γ基体相等,众多相在高温下熔合到一起,成分重新分配,最后在水淬时形成γ相枝晶和大量的细小沉淀物。由于尺寸小,且混杂在一起,这些沉淀物很难被鉴定和区分开来(图4d)。

表1 铸态合金中“团聚相”的化学成分
Table 1 Compositions of various phases around γ/γ′ eutectics in the as-cast alloy (mass fraction / %)
Phase Al Ti Cr Co Ni Mo Hf Ta W
MC(1) 0.6 47.9 2.0 1.2 6.5 7.0 3.3 0.2 31.3
MC(2) 0.6 12.4 4.2 2.9 13.4 2.5 48.4 4.4 11.1
η 3.1 8.9 4.2 7.5 63.3 0.6 8.9 1.5 1.9
Boride(1) 0.2 2.2 42.1 3.4 10.2 15.9 0 0 26.1
Boride(2) 0.3 2.7 23.8 2.9 11.6 18.6 1.0 0 39.2
Ni5Hf 0.7 2.6 3.7 8.0 56.3 0.8 18.9 5.9 3.1

表1 铸态合金中“团聚相”的化学成分

Table 1 Compositions of various phases around γ/γ′ eutectics in the as-cast alloy (mass fraction / %)

使用电解液A腐蚀试样,可以很好地显示初熔区的整体形貌、尺寸和熔池中原始组织的熔化程度等信息,但是无法分辨各团聚相在初熔过程中的变化情况(图3和4)。使用电解液B对试样进行腐蚀,试样能很好地显示各团聚相(包括η相、硼化物和Ni5Hf相等)的尺寸、形貌和分布等信息,如图5所示。

图3 高硼DZ444合金在1170 ℃保温不同时间+水淬处理后初熔区SEM像

Fig.3 SEM images of IMR in the DZ444 alloy with high boron solid solution treated at 1170 ℃ for 5 min (a), 10 min (b) and 30 min (c) and then cooled by WQ (Specimens were etched with electrolyte A, IMR—incipiently melted region, IMC—incipiently melted circle, WQ—water quenching)

图4 高硼DZ444合金在不同温度保温不同时间+水淬后初熔区SEM像

Fig.4 SEM images of IMR in the DZ444 alloy with high boron solid solution treated at different temperatures for different times and then cooled by WQ (Specimens were etched with electrolyte A)
(a) 1180 ℃, 5 min (b) 1190 ℃, 10 min (c) 1200 ℃, 10 min (d) 1200 ℃, 30 min

合金经1190 ℃以下温度不同时间处理后,初熔区内各团聚相都未出现明显的熔化迹象(图5a和b)。而在1200 ℃,处理时间仅为10 min时,大部分团聚相就已消失,它们或熔化,或溶解进了过饱和γ的基体中(图5c);时间达30 min时,团聚相完全消失,之后在WQ过程中重新形成大量细小的η相、硼化物和Ni5Hf相等颗粒(图5d)。由此可见,团聚相的熔化温度应在1190 ℃和1200 ℃之间。一些团聚相首先熔化,变成高温金属液,侵蚀未熔化的相,并使其熔化或溶解,直至团聚相全部熔化,这就是初熔池形成的典型过程。水淬冷却时,初熔池中的金属液快速凝固成一些细小的沉淀相,经鉴定为硼化物、Ni5Hf和η相等,与铸态时的团聚相种类基本一致。在DZ22和K19H合金中,团聚相的熔化首先从Ni5Hf低熔点相开始,接着硼化物和γ/γ′共晶接连熔化,最后形成初熔池[11,12]。在本实验合金中,团聚相包含的种类更多,分布更复杂,因此它们的熔化顺序难以确定。

图5 高硼DZ444合金在不同温度保温不同时间+水淬后的初熔区SEM像

Fig.5 SEM images of IMR in the DZ444 alloy with high boron solid solution treated at different temperatures for different times and then cooled by WQ (Specimens were etched with the electrolyte B)
(a) 1180 ℃, 5 min (b) 1190 ℃, 10 min (c) 1200 ℃, 10 min (d) 1200 ℃, 30 min

图6 高硼DZ444 合金在不同温度保温不同时间+空冷后的初熔区SEM像

Fig.6 SEM images of IMR in the DZ444 alloy with high boron solid solution treated at different temperatures for different times and then cooled by AC (Specimens were etched with electrolyte B, AC—air cooling)
(a) 1180 ℃, 5 min (b) 1190 ℃, 10 min (c) 1200 ℃, 10 min (d) 1200 ℃, 30 min

图7 不同固溶处理后初熔区的SEM像(使用的腐蚀液为电解液B)

Fig.7 Low (a~c) and high (d~f) magnified SEM images of IMR in the specimens solution treated at 1210 ℃(a, d), 1230 ℃ (b, e) and 1250 ℃ (c, f) for 2 h and then followed by AC (Specimens were etched with electrolyte B)

相对于WQ方式,AC方式使合金液获得了足够长的凝固时间,因此,沉淀相有充足的时间形核和长大(图6c和d),其最终尺寸要比WQ时大得多(图5c和d)。有趣的是,在AC方式下,新沉淀相的形貌和尺寸与原始团聚相(图1d)相比几乎没有什么差异。导致这个现象的主要原因有2个:(1) 初熔区与周围基体相对独立,其化学成分在初熔发生前后理论上是一致的;(2) 这里的AC冷却方式,与实验合金制备凝固时的冷却方式(空冷)可以认为是相同的。在这2个条件下,初熔的凝固组织与原始组织(铸态组织)基本保持了一致。

2.3 初熔组织在热处理期间的演变及其对力学性能的影响

DZ444合金(B含量为0.01%)的标准热处理工艺为1210 ℃、2 h、AC+1080 ℃、4 h、AC+850 ℃、24 h、AC。在本实验中,对B含量为0.09%的DZ444合金进行相似的热处理,但是固溶温度选取1210、1230和1250 ℃ 3个温度,以研究初熔在热处理期间的变化及其对力学性能的影响。

图7为高硼DZ444合金经不同温度固溶处理后的微观组织,使用的腐蚀液为电解液B。这种腐蚀方法主要显示初熔区内的团聚相,因此图中所标示的初熔并非其全貌,而是其区域内团聚相所占的面积。由低倍SEM像(图7a~c)可见,随着固溶温度的升高,团聚相的分布和形貌特点变化不大,但是尺寸明显增大。团聚相是初熔最重要的组成部分,它的尺寸增大,暗示了初熔区尺寸的增加,即初熔边界向周围基体的侵蚀和扩展。高倍SEM像(图7d~f)显示,初熔区内团聚相主要为硼化物、Ni5Hf和η相等,与图6中的团聚相基本相同。在图7d中,团聚相周围存在一个初熔圈,将团聚相包围在中间,这个析出物圈可能是初熔的真正边界,边界与团聚相之间的区域为γ基体相[12]

图8 不同固溶处理后初熔区的SEM像(使用的腐蚀液为电解液A)

Fig.8 SEM images of IMR in the specimens solution treated at 1210 ℃ (a), 1230 ℃ (b) and 1250 ℃ (c) for 2 h and then followed by AC (Specimens were etched with electrolyte A)

使用电解液A腐蚀试样,可以清晰地观察初熔的整体形貌(图8)。由图8a和b可以看出,初熔区从内到外依次可以细分为3个区域,即团聚相、γ基体和γ/γ′共晶,这也可能是初熔的凝固顺序。形成机理是:随着熔池的逐渐冷却,团聚相首先从过饱和金属液中析出、长大,接着γ基体大量生成,并向周围液体释放一定量的γ′相形成元素,再加上剩余合金液中本来就富含Ni、Al和Ti等元素(图2),因此很容易满足共晶形成条件,并导致γ/γ′共晶最后形成。可见,初熔的边界实际上是γ/γ′共晶与合金基体的界面。随着温度的提高,边界上的γ′相不断粗化、连接,逐渐形成一条连续的链,使得边界明显粗化;同时,从边界形貌推断,其多处向合金基体侵蚀突入,暗示初熔区正在向外扩展,尺寸增大(图8a和b)。当温度达1250 ℃时,初熔尺寸剧烈增大,也不再由团聚相、γ基体和边界3个典型区域构成,而是在熔化区域内出现了多个“基体岛”。初熔在向基体侵蚀生长时,基体的局部区域熔点较高,不发生熔化,因而被已熔化的液体包围,最后形成所谓的基体岛。其实,在1230 ℃时就有基体岛出现,但是数量少,尺寸小,不容易观察到。

表2 热处理时固溶温度对初熔的影响
Table 2 Volume fraction and size changes of IMR with different temperatures
Solution temperature / ℃ Area fraction / % Size / μm
1210 5.3 49.8
1230 7.1 52.0
1250 18.6 88.5

表2 热处理时固溶温度对初熔的影响

Table 2 Volume fraction and size changes of IMR with different temperatures

统计表明,当温度从1210 ℃升至1230 ℃时,初熔区变化不是很大,但是当温度升至1250 ℃时,其尺寸和面积分数都显著增大(表2)。原因可能是,在前2个温度下,初熔主要发生在初熔圈内部,而在1250 ℃下,初熔圈(或边界)向外推移,使得初熔区大大扩展。可见,当温度高于1250 ℃时,初熔对合金微观组织连续性的破坏是严重的。

图9 固溶和高温时效后的初熔区组织

Fig.9 Morphology of IMR after solution treatment at 1210 ℃, 2 h, AC and then high temperature ageing at 1080 ℃, 4 h, AC (Specimens were etched with the electrolyte B)

研究结果表明,在高温时效(1080 ℃、4 h、AC)期间,初熔区的尺寸、形貌和分布等不发生明显变化。但是,团聚相中蜂窝状的Ni5Hf相消失,取而代之的是富Hf的颗粒状MC(2)碳化物(图9)。根据文献[29]推断,Ni5Hf相发生了分解反应,反应式为Ni5Hf+γ(C)→MC(2)+γ,即Ni5Hf相与γ基体中的C反应形成MC(2)碳化物和包覆在其周围的γ层。与高温时效相比,由于处理温度进一步下降,初熔在低温时效(850 ℃、24 h、AC)过程中的变化更加不明显,可以忽略不计。因此,可以认为,经高、低温时效处理后,初熔区的数量、尺寸和形貌等基本与固溶后保持一致,即初熔产生于固溶期间,主要受固溶温度和时间等参数的影响,而与之后的高、低温时效关系不大。

经完整热处理(HT1、HT2和HT3)后,测试了合金在900 ℃下的拉伸性能和在930 ℃、275 MPa条件下的持久性能,结果列于表3。可以看出,随着固溶温度的提高,合金的拉伸性能(包括强度和塑性)略微下降,而持久性能则显著降低,尤其是经HT3处理后,持久性能下降到只有HT2的一半。导致上述结果的主要原因有2方面:一方面,随着固溶温度的提高,初熔区的尺寸和面积分数增大(表2),初熔区消耗了大量的W、Mo等固溶强化元素,从而使固溶强化效果减弱,合金的力学性能降低。另一方面,在力学性能测试过程中,初熔区内部存在着很多相界面,裂纹很容易萌生于初熔区内部各相与基体界面上,从而使合金力学性能降低。从图10可以看出,裂纹主要萌生于初熔区内部。由于初熔区内部骨架状、条状的硼化物与基体的界面结合力较低,在高温拉伸和持久实验中,温度和外力可以直接导致硼化物/基体界面裂纹的形成,甚至硼化物本身会开裂形成裂纹源。初熔区内部富Hf的MC型碳化物及η相均为脆性相,几乎没有塑形变形承受能力,也容易从碳化物及η相与基体交界处产生裂纹。而且裂纹沿着初熔区快速扩展,加速合金的断裂过程,从而导致合金的力学性能降低。对比表2和3发现,初熔是影响力学性能的最重要因素:初熔区数量和尺寸增加时,持久寿命几乎直线下降。

表3 不同热处理制度下高硼DZ444合金在900 ℃下的拉伸性能及930 ℃、275 MPa下的持久性能
Table 3 Tensile properties at 900 ℃ and stress-rupture properties at 930 ℃, 275 MPa of DZ444 alloy with high B content under different heat treatments
Heat treatment Tensile property Stress-rupture property
σb / MPa σ0.2 / MPa δ / % φ / % t / h δ / %
HT1 707 457 29.8 40.3 55 30.6
HT2 690 460 27.3 35.0 44 27.8
HT3 665 447 24.5 32.3 22 13.5

Note: σb—tensile strength, σ0.2—yield strength, δ—elongation, φ—area reduction, t—stress-rupture life

表3 不同热处理制度下高硼DZ444合金在900 ℃下的拉伸性能及930 ℃、275 MPa下的持久性能

Table 3 Tensile properties at 900 ℃ and stress-rupture properties at 930 ℃, 275 MPa of DZ444 alloy with high B content under different heat treatments

图10 性能测试过程中裂纹在初熔区域形成的SEM像

Fig.10 SEM images of cracks mainly initiated in IMR of tensile ruptured specimens at 900 ℃ (HT1) (a) and after stress-rupture test (HT3) (b) (Specimens were etched with electrolyte A)

3 结论

(1) 在高硼DZ444合金铸态组织中,枝晶间区域分布着大量的γ/γ′共晶、MC碳化物和由γ相、硼化物、Ni5Hf及η相组成的“团聚相”。固溶处理时,初熔首先发生在团聚相周围受硼化物显著影响的γ基体内。硼化物不是初熔的形核点,但是对初熔的形成具有关键作用。

(2) 较高的B含量,使得合金具有较低的初熔温度(在1160~1170 ℃之间)。提升温度或延长保温时间,初熔更加严重。采用水淬冷却方式,初熔凝固为典型的γ枝晶和大量细小的沉淀相颗粒,而采用空冷方式时,初熔从内到外依次凝固为团聚相、γ基体和γ/γ′共晶,团聚相形貌与铸态时没有明显差异。

(3) 完整热处理时,固溶温度由1210 ℃提高到1230 ℃,初熔略微增加;而当温度达1250 ℃时,初熔吞噬大量周围基体,初熔区尺寸和面积分数剧烈增大,对合金的微观组织连续性造成严重破坏。由于温度较低,合金的高、低温时效对初熔组织的影响不是很大。

(4) 随着初熔区尺寸和面积分数的增加,初熔区消耗了大量的固溶强化元素,使合金的固溶强化效果降低,同时初熔区内部易萌生大量微裂纹,从而使合金的拉伸性能稍有下降,持久性能则显著降低。

The authors have declared that no competing interests exist.

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采用水淬法测定出铸态及采用不同温度阶段处理后的K417G合金γ+γ′共晶相的初始熔化温 度,对铸态合金及水淬处理后的合金的微观组织采用扫描电镜进行观察,确定出阶段热处理对合金中γ+γ′共晶相的初始熔化温度的影响.研究结果表明:铸态 K417G合金中的γ+γ′共晶相主要由细小的γ+γ′共晶组织和粗大的γ′相组成,在共晶相的前沿存在少量的硼化物共晶组织.铸态K417G合金中的 γ+γ ′共晶组织的熔化主要发生在共晶相的前沿,熔化后出现大量的白色组织相.电子探针对熔化区的分析表明,熔化后形成的激冷组织主要为γ+γ′共晶,熔化区主 要富集B、Mo和Cr元素.铸态合金中的γ+γ′共晶的初熔温度为1 220~1 230℃,而经1 090℃、1 100℃和1 110℃阶段处理10h后合金中的γ+γ′共晶的初熔温度升高至1 280~1 290℃.
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The solidification path of single-crystal nickel-base superalloys containing minor carbon was investigated under various laser rapid directional solidification (LRDS) conditions. By controlling the solidification rate, LRDS processing can provide the evidence whether some diffusion-controlled phase transformations occur because such transformations will be suppressed under high cooling rates. Results show that the solidification path and final solidification microstructure depend upon the cooling rate; the microstructure without γ-γ′ eutectic can be obtained as long as the cooling rate is high enough. A peritectic transformation in carbon-containing single-crystal superalloys was first experimentally verified by controlling the cooling rate during LRDS processing.
DOI:10.1016/j.scriptamat.2016.08.039      URL     [本文引用:1]
[25] Wang H F, Su H J, Zhang J, et al.Effect of melt thermal history on solidification behavior and microstructural characteristics of a third-generation Ni-based single crystal superalloy[J]. J. Alloys Compd., 2016, 688: 430
The melt superheating treatment is performed on a third-generation Ni-based single crystal superalloy during directional solidification, aiming to investigate the influence of melt thermal history (melt superheating temperature, superheating time and cooling rate) on solidification behavior and microstructural characteristics. The results show that the nucleation undercooling increases nonlinearly with the increase of melt superheating temperature from 1450 to 1780°C, but decreases when the melt superheating temperature further increases up to 1800°C. The nucleation undercooling first has little change and then increases sharply with increasing the melt superheating time. The critical superheating time is obtained to be about 15min. Moreover, it is found that the higher the cooling rate is, the larger the nucleation undercooling and liquidus-solidus range are. Additionally, the segregation is reduced, and the dendrite and γ′ phase are obviously refined when the superheating temperature increases from 1500 to 1700°C. The influence mechanism of melt superheating treatment on the nucleation undercooling and solidification microstructure is discussed, which is helpful for optimizing the process parameters and improving the metallurgical quality of Ni-based single crystal superalloy.
DOI:10.1016/j.jallcom.2016.07.031      URL     [本文引用:]
[26] Shi Z X, Dong J X, Zhang M C, et al.Solidification characteristics and segregation behavior of Ni-based superalloy K418 for auto turbocharger turbine[J]. J. Alloys Compd., 2013, 571: 168
The solidification characteristics and segregation behavior of K418 alloy were investigated by the DSC measurement and the isothermal solidification quenching process. Phase transformation temperatures, together with the solid fraction with temperature were measured by experiments and comparatively calculated by Thermo-Calc. It is demonstrated that the solidification begins with the formation of primary γ dendrite and terminates with the γ/γ′ eutectic reaction, being accompanied by the formation of MC carbide and M(C, B) carboboride. During the solidification, interdendritic region is significantly enriched with Nb, Ti, Zr, B, C and Mo, which results in the formation of carbide, carboboride and γ/γ′ eutectic. The slow increase of solid fraction at the final stage of solidification is also caused by the strong segregation of the positive segregation elements in interdendritic area. In the case of K418, equilibrium model is shown to be a valuable tool to predict the solidification sequence and phase transition temperatures. Scheil calculation can be used to investigate the segregation behavior during solidification.
DOI:10.1016/j.jallcom.2013.03.241      URL     [本文引用:]
[1] Zhang J, Zhang A B, Tan Y N, et al.Effect of Re on microstructure and properties of directionally solidified superalloy[J]. J. Aeronaut. Mater., 2010, 30(3): 24
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(张俊, 张爱斌, 谭永宁. Re对定向凝固Ni基高温合金组织及性能的影响[J]. 航空材料学报, 2010, 30(3): 24)
采用真空感应熔炼和定向凝固重熔技术制备了不同Re含量的Ni基 高温合金试样,分析了合金微观组织,测试了热处理后其室温拉伸和高温持久性能.结果表明:Re主要分布于γ基体中,在γ’强化相中分布很少.随着Re含量 的增加,γ+γ’共晶体积分数略有增多,枝晶杆γ’细化明显.热处理后γ’立方化程度增加,共晶相含量明显减少.Re的加入显著提高了合金的事温拉伸屈服 强度和高温持久寿命,但室温和高温塑性有所降低.Re主要通过固溶在γ基体中来阻碍γ’粗化,增大错配度,从而提高合金综合性能.
DOI:10.3969/j.issn.1005-5053.2010.3.006      Magsci     URL    
[2] Zhou P J, He X M.Influences of minor alloying elements on the eutectic volume of Ni-Base superalloy[J]. Foundry, 2012, 61: 868
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(周鹏杰, 何向明. 微量元素对镍基高温合金共晶数量的影响[J]. 铸造, 2012, 61: 868)
利用光学显微镜、扫描电子显微 镜、定量金相分析、差示扫描量热仪等研究了微量元素对一种镍基高温合金共晶数量的影响。结果表明,添加B、Zr、Y等元素都使合金的共晶数量有了明显的增 加,而且同时加入两种微量元素对共晶数量的影响更加显著。分析表明,Zr、Y等元素由于具有较大的原子半径,阻碍了元素的扩散,导致糊状区偏析加剧而促进 共晶组织的形成。而添加B增大了凝固区间,从而促进元素偏析导致共晶数量增加。
[3] Ojo O A, Richards N L, Chaturvedi M C.On incipient melting during high temperature heat treatment of cast Inconel 738 superalloy[J]. J. Mater. Sci., 2004, 39: 7401
ABSTRACT The cause and the nature of the limiting incipient melting, which should aid development of appropriate higher solution heat treatment schedule, was investigated. The lamellar morphology of the solidification product suggests that it was formed by eutectic transformation involving at least ternary and quaternary eutectic reactions. The results show that the constituent phases of the terminal solidification product in the alloy could respond differently to heat treatment. Eutectic melting of the terminal solidification constituents during fabrication welding will contribute to sub-solidus intergranular liquation, which is known to reduce resistance to HAZ cracking.
DOI:10.1023/B:JMSC.0000048761.32712.eb      URL     [本文引用:3]
[27] Zhang Y J, Huang Y J, Yang L, et al.Evolution of microstructures at a wide range of solidification cooling rate in a Ni-based superalloy[J]. J. Alloys Compd., 2013, 570: 70
The evolution of microstructures at a wide range of solidification cooling rate in a Ni-based superalloy was investigated by employing conventional casting, spray casting and melt spinning processes. Depending on solidification cooling rate, microstructures sequentially show planar, cellular, dendritic, the dendritic growth suppressed features (in melt spinning process) and the shapes of γ′ precipitates progressively exhibit irregular (planar and cellular growth), cuboidal (dendritic growth) and spherical (dendritic growth suppressed) patterns. Moreover, in dendritic growth conditions, γ′ shapes experience irregular cuboidal, regular cuboidal and near cuboidal patterns with increasing cooling rate. γ′ precipitates in dendritic cores show more and less regular morphology than those in interdendritic regions in lower and higher cooling rate range, respectively. The size scale of γ′ precipitates decreases with solidification cooling rate in cellular and dendritic growth conditions and γ′ precipitates are obviously smaller in crystallizing (cellular or dendritic) cores than those in interval regions.
DOI:10.1016/j.jallcom.2013.03.085      URL     [本文引用:]
[28] Wang F, Ma D, Zhang J, et al.Effect of solidification parameters on the microstructures of superalloy CMSX-6 formed during the downward directional solidification process[J]. J. Cryst. Growth, 2014, 389: 47
61Single-crystal CMSX-6 was processed by a novel directional solidification process.61The microstructures were characterized as functions with increasing withdrawal rate.61Some functional relationships did not agree with that in LMC and Bridgman processes.61The comparison of PDAS theoretical models with the experimental data was conducted.61The microsegregation will be suppressed with the increasing withdrawal rate.
DOI:10.1016/j.jcrysgro.2013.11.084      URL     [本文引用:1]
[29] Zheng Y R, Cai Y L.Phase transformations in hafnium-bearing cast nickel-base superalloys [A]. Superalloys[C], 1980: 465
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[4] Seo S M, Kim I S, Lee J H, et al.Eta phase and Boride formation in directionally solidified Ni-base superalloy IN792+Hf[J]. Metall. Mater. Trans., 2007, 38A: 883
A series of directional solidification experiments have been conducted to elucidate the formation mechanism of eta and Cr-rich phases in the Ni-base superalloy IN79202+02Hf. Both eta and Cr-rich phases were found to be the final solidification products developed from the remaining liquid after γ/γ′ eutectic reaction. The (Ti02+02Ta02+02Hf)/Al ratio in the residual liquid played a significant role in the nucleation of eta phase. During the solidification of γ/γ′ eutectic, the continual increase of (Ti02+02Ta02+02Hf)/Al ratio in the residual liquid eventually led to the completion of γ/γ′ eutectic reaction and caused the nucleation of eta phase. The results of electron probe microanalysis and transmission electron microscopy revealed that the Cr-rich phase was Cr, Mo, and W containing M 5 B 3 and M 3 B 2 type borides. The formation of these boride phases was found to be strongly influenced by the formation of γ/γ′ eutectic. Because of the limited solubility of Cr, Mo, and W in γ′ phase, these elements were enriched in the residual liquid during the solidification of γ/γ′ eutectic. In addition, boron would preferentially segregate into liquid due to its very limited solubility in both γ and γ′ phases so that the possibility of boride formation in the residual liquid ahead of the γ/γ′ eutectic was increased. A modified Scheil model was adopted to explain the influence of solidification rate on the formation of eta phase and borides, and the results were discussed.
DOI:10.1007/s11661-007-9090-0      URL     [本文引用:1]
[5] Shulga A V.Boron and carbon behavior in the cast Ni-base superalloy EP962[J]. J. Alloys Compd., 2007, 436: 155
The results of investigations of boron and carbon segregations, boride and carbide phases forming in various elements of the structure of ingots of the Ni-base superalloy EP962 have been presented. The methods of light metallography (LM), scanning electron microscopy (SEM), X-ray spectral microanalysis (XRSMA), high sensitive track autoradiography on boron via the nuclear reaction 10B(n, α) 7, Li and activation autoradiography on carbon via the reaction 12C(d, n) 13N were used. Boron segregation and precipitation of boride phase in interdendritic regions and on the grain boundaries were established. Composition of boride phase was found to be (Ni 0.32Cr 0.22Nb 0.14Mo 0.11Co 0.13W 0.02Al 0.02Ti 0.01)B 0.67, which is of M 3B 2 type. Formation of the non-equilibrium γ′–γ eutectic in interdendritic regions causes microliquation of Cr, Nb, Mo, Co, W and B in the crown of γ′–γ eutectic that results in formation of boride particles in the crown of γ′–γ eutectic. Carbon was found mainly forming carbide particles in interdendritic regions. Composition of carbide particles was found by the XRSMA as (Nb 0.61Ti 0.22Ni 0.10Cr 0.02W 0.01Hf 0.01)C.
DOI:10.1016/j.jallcom.2006.07.051      URL     [本文引用:]
[6] Wu B P, Li L H, Wu J T, et al.Effect of boron addition on the microstructure and stress-rupture properties of directionally solidified superalloys[J]. Int. J. Miner. Metall. Mater., 2014, 21: 1120
这研究集中于硼增加的效果,在到 0.03wt% 的 0.0007wt% 的范围,在微观结构和方向性地团结的 superalloy 的压力破裂性质上。与在当演员组合金增加硼内容,最容易溶解的 / 的部分有增加,块硼化物在 / 附近猛抛最容易溶解。在 0.03wt% 的一个高硼内容,有薄片状的硼化物的降水。在热处理之上,好块硼化物趋于与增加硼内容在谷物边界猛抛。总的来说,方向性地团结的 superalloy 的破裂生活显著地与硼的名字的内容的增加被改进。然而,当硼内容超过 0.03wt% 时,破裂生活减少。
DOI:10.1007/s12613-014-1017-3      URL     [本文引用:]
[7] Zhen B L, Zhang S J.The phase composition and the rules of the phase precipitation in Hf-bearing nickel base superalloys[J]. Cent. Iron Steel Res. Inst. Tech. Bull., 1981, (1): 65
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(甄宝林, 张绍津. 加铪镍基合金相的组成和析出规律[J]. 钢铁研究总院学报, 1981, (1): 65)
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关键词(key words)
定向凝固高温合金
固溶处理
初熔
团聚相
力学性能

directionally solidified ...
solid solution treatment
incipient melting
multi-phase eutectic-like...
mechanical property

作者
张洪伟
秦学智
李小武
周兰章

ZHANG Hongwei
QIN Xuezhi
LI Xiaowu
ZHOU Lanzhang