Acta Metallurgica Sinica, 2017, 53(4): 423-432
doi: 10.11900/0412.1961.2016.00291
Ru对一种高Cr镍基单晶高温合金凝固组织的影响

Effect of Ru on the Solidification Microstructure of a Ni-Based Single Crystal Superalloy with High Cr Content
宁礼奎, 佟健, 刘恩泽, 谭政, 纪慧思, 郑志

摘要:

以一种新型高Cr镍基单晶高温合金为基础,调整Ru的添加量,通过对3种不同Ru含量合金铸态组织的观察,研究了Ru对合金相析出特征与元素分布规律的影响。结果表明,随着Ru含量(0、1.5%、3%,质量分数)的增加,合金一次枝晶间距与二次枝晶间距逐渐减小,(γ+γ′)共晶含量先增后降,γ′相尺寸逐渐变小;3%的Ru添加使合金凝固组织中析出β-NiAl相,该相除Ni、Al基本组成元素外,还包含一定量的Cr、Co和Ru;Ru对合金中其它元素具有典型的“逆分配”作用,β-NiAl相的析出降低了Ru对其它元素“逆分配”的影响程度;Ru提高了正偏析元素Ta、Al和负偏析元素Re的偏析程度,降低了正偏析元素Mo、Cr的偏析程度。

关键词: 单晶高温合金 ; Ru ; -NiAl相 ; 逆分配 ; 偏析

Abstract:

Ni-based single crystal superalloys have been widely used in manufacturing the critical components of aero-engines, such as turbine blades and vanes. Improvements in phase stability on the addition of Ru are well known in the field of Ni-based superalloy development. Cr is beneficial to hot co-rrosion resistance of Ni-based superalloys. Generally, superalloys which used under easy corrosion conditions should contain high levels of Cr. Early researches about the influence of Ru on solidification microstructures in Ni-based single crystal alloys are mostly focused on low-Cr systerms (<6%). Since Cr has complex interactions with Ru, it is meanful to study the effects of Ru on solidification microstructures in high-Cr (>10%) Ni-based single crystal superalloy systems. The materials used in this work are Ni-based single crystal superalloy with high Cr content. Three superalloys by changing Ru addition (0, 1.5%, 3%, mass fraction) were designed. By observing the as-cast structure, the effect of Ru on the elements distribution and the precipitation characters of different phases in these alloys were studied. It is found that as the Ru content increases, the primary and secondary dendrite arm spacings decrease gradually; the volume fraction of (γ+γ′) eutectic increases firstly and then decreases; the γ′ size is reduced progressively. The addition of 3%Ru leads to the formation of β-NiAl phase, which contain a certain amount of Cr, Co and Ru except the basic elements Ni and Al. The typical "reverse partitioning" of other alloying elements is exhibited with the addition of Ru, while the formation of β-NiAl phase can reduce the "reverse partitioning" of other alloying elements. The addition of Ru could enhance the segregation of positive segregation elements Ta, Al and negative segregation element Re while reduce the segregation of positive segregation elements Mo and Cr.

Key words: single crystal superalloy ; Ru ; -NiAl phase ; reverse partitioning ; segregation

镍基单晶高温合金具有优良的高温性能,是目前制造先进航空发动机涡轮叶片的主要材料。为了适应航空发动机涡轮前进口温度不断提高的要求,镍基单晶高温合金的合金化程度不断提高[1~10],致使合金组织稳定性降低的趋势不断增大。研究发现,Ru能够抑制单晶高温合金中拓扑密排(TCP)相的析出[11~14],进而可以对高温合金产生稳定化的作用。

目前,以Ru添加为标志的第四、五代镍基单晶高温合金已成为国际上该领域的研究热点,其成分特点是含有较高的W、Mo、Ta、Re等难熔元素,以保证合金具有较高的承温能力。在此前提下,为了兼顾合金的组织稳定性,Cr选择了相对较低的加入量(<6%,质量分数,下同)。所以,关于Ru在镍基单晶高温合金中的作用大多基于低Cr (<6%)体系合金研究而来,并且发现Ru对不同单晶高温合金相析出特征与元素分布等的影响暂无统一性的结论,甚至出现截然相反的结果[15~21],原因是Ru与其它元素间存在明显的交互作用,进而影响了其作用形式与规律。例如,Cr可以削弱Ru对其它元素的反偏析作用,Ru同样也可以改变Cr对其它元素偏析的影响趋势[16,22]

然而,由于Cr是决定合金抗热腐蚀性能水平高低的关键元素,在易腐蚀条件下服役的合金中,Cr含量必须保持在较高的水平。因此,有必要研究Ru对高Cr体系镍基单晶高温合金的影响。此外,鉴于合金的凝固组织不仅与其热处理制度的复杂程度密切相关,而且可以影响合金的力学性能、环境性能及组织稳定性等。所以,研究Ru对高Cr体系镍基单晶高温合金凝固组织相析出特性与元素分布规律的影响,对以后该类合金的成分设计与应用具有重要的参考价值和指导意义。

1 实验方法

实验合金为一种正在研发的新型高强抗热腐蚀类镍基单晶高温合金,其主要化学成分(质量分数,%)为:Al 3.0~6.0,Co 6.0~9.0,(Re+W+Mo+Ta) 12.0~16.0,Cr 10.0~14.0,Ni余量。在载体合金中分别加入1.5%和3.0%的Ru,通过VIM-F25型真空感应熔炼炉熔炼3种母合金,浇注成直径为83 mm的母合金锭。用螺旋选晶法和定向凝固技术将不含Ru (0Ru)、1.5% (1.5Ru)及3%Ru (3Ru)的3种母合金锭分别在工业用大型双区加热ZGD-002型真空高梯度定向炉上制备单晶试棒。单晶抽拉速率为6 mm/min,试样取向控制在[001]方向,偏离度控制在7°以内。

用于微观组织观察的试样均取自试棒中部,将3种不同Ru含量的铸态试样进行线切割,经磨平、抛光后,用4 g CuSO4+20 mL HCl+20 mL H2O溶液进行化学腐蚀或者用10 mL H3PO4+90 mL H2O溶液电解腐蚀。通过Axiovert 200 MAT光学金相显微镜进行低倍金相组织观察,利用INSPECT F50型扫描电子显微镜(SEM)进行高倍微观组织结构观察,合金中元素在枝晶干和枝晶间的偏析情况通过EPMA-1610型电子探针(EPMA)进行定量分析。在TECNAI G2 20型透射电镜(TEM)上观察合金中相的微观结构。通过图像分析软件(SISC-IAS)计算每种合金的枝晶间距,同时结合扫描图像测量合金中γ'相的尺寸,计算测量不同视场至少30张图像的平均值。

将直径2.8 mm、厚2 mm的样品置入SETSYS Evolution18综合热分析仪中,采用快速加热方式,将试样加热到900 ℃少许停留,然后以5 ℃/min升温速率,从900 ℃加热至1400 ℃,测试合金差热分析(DSC)曲线。

2 实验结果与讨论
2.1 Ru对合金铸态组织的影响

2.1.1 Ru对枝晶组织的影响 图1为不同Ru含量铸态单晶试棒的横向与纵向金相组织。可以看出,3种铸态单晶高温合金均呈现出典型的树枝晶组织,枝晶偏析明显。4 g CuSO4+20 mL HCl+20 mL H2O腐蚀剂腐蚀的是γ'相,枝晶干处γ'相尺寸较小,不易被腐蚀,呈亮衬度;枝晶间处γ'相尺寸较大,容易被腐蚀,因而呈暗衬度。此外,3种合金一次枝晶干生长方向均为<001>,平行度较高,一次枝晶干与二次枝晶臂分布均匀。经测量,3种合金的一次枝晶间距为240~260 μm,二次枝晶间距为40~65 μm。综上所述,Ru含量对单晶高温合金枝晶间距有明显影响,随着Ru含量的增加,一次枝晶间距与二次枝晶间距均呈降低的变化趋势,如图2所示。

图1 3种不同Ru含量合金的铸态组织

Fig.1 Microstructures of alloys along horizontal (a~c) and longitudinal (d~f) in as-cast 0Ru alloy (a, d), 1.5Ru alloy (b, e) and 3Ru alloy (c, f)

图2 Ru含量与枝晶间距的关系

Fig.2 Relationships of primary and secondary dendrite arm spacings with the content of Ru (PDAS—primary dendrite arm spacing, SDAS—secondary dendrite arm spacing)

枝晶间距是表征合金铸态组织的重要特征参数,对单晶高温合金的性能起着决定性的影响。小尺寸枝晶间距可获得细小的枝晶组织,降低合金元素的偏析程度,进而提高单晶高温合金的力学性能。Kurz-Fisher (KF)模型[23]的一次枝晶间距(λ1)可以用下式描述:

λ 1 = 4.3 ( D L ΓΔ T 0 k ) 1 4 G - 1 2 V - 1 4 (1)

其中,

k = 1 - Δ T 0 T m - T S (2)

式中,DL是液相扩散系数,主要与合金的成分有关; Γ 是Gibbs-Thomson系数;ΔT0是结晶温度间隔;k是等效溶质分凝系数,与合金的固相线和液相线温度有关;G是温度梯度,V是晶体生长速率,二者均与定向炉设备、模壳及浇注工艺参数等因素有关;TS是液相线温度;Tm是纯Ni的熔点。图3为3种合金的升温DSC曲线,由该曲线可以得到合金的固相线温度和液相线温度,具体如表1所示。可以看出,Ru含量的变化对该体系合金的固相线温度和液相线温度均未产生显著影响。由于不同Ru含量合金的冶炼均由同一定向炉拉制而成,并且选取了相同的抽拉工艺参数。因此,Ru含量的变化主要影响了式(1)中的DL。由于Ru是一种原子尺寸较大的难熔元素,随着合金中Ru含量的增加,阻碍了浇注前合金液内部自身的扩散,因此降低了DL,最终导致一次枝晶间距不断降低。

图3 3种不同含Ru量合金的升温DSC曲线

Fig.3 DSC heating thermograms of 0Ru (a), 1.5Ru (b) and 3Ru (c) alloys

2.1.2 Ru对(γ+γ')共晶的影响 图4为3种不同Ru含量合金的典型铸态SEM像。显然,随着Ru含量的增加,合金典型铸态组织变化很大。3种合金铸态组织均主要由γγ'、(γ+γ')共晶及少量的碳化物组成,由于合金中的碳化物极少,因此未在图中标示。合金中的(γ+γ')共晶分为两种类型,一种呈典型的葵花状,从形貌上可以分为两部分,即共晶芯部的筛网状结构和共晶帽处的冠状结构。随着Ru含量的增加,冠状结构和粗大的共晶γ'相尺寸逐渐减小,含量逐渐减少。另一种为板状的共晶组织,其形貌和EDS分析结果如图5所示。可以看出,该类共晶在枝晶间呈光板状析出,其二次电子像的灰度与γ'相类似,图5a中A区域EDS分析结果表明,该相为大块状的γ'相。观察发现,1.5Ru合金中的光板状共晶数量较0Ru合金多,而3Ru合金中的光板状共晶数量明显降低。

图4 3种不同含Ru量合金的典型铸态SEM像

Fig.4 SEM images of as-cast 0Ru (a), 1.5Ru (b) and 3Ru (c) alloys

在3Ru合金的铸态组织中,除上述组成相之外,另生成一种黑色相,如图4c所示。可以看出,该相主要以块状或条状分布于枝晶间,大部分与(γ+γ')共晶依附共存,个别单独存在于枝晶间,见图6a。图6b为图6a中A区域的EDS分析结果。可以看出,该相主要由Ni、Al组成,此外,还包含一定量的Cr、Co和Ru。对3Ru合金中新相进行TEM分析,其形貌像和SAED谱如图7所示。可以看出,该相的边缘呈现出明显的锯齿状,经SAED分析得知,该相属于以NiAl为基的β相,即β-NiAl相,表现为B2型有序结构。

图5 1.5Ru合金中板状共晶形貌及EDS分析

Fig.5 Morphology of block eutectic in the 1.5Ru alloy (a) and EDS analysis of zone A in Fig.5a (b)

图6 3Ru合金中新相的形貌及其EDS分析

Fig.6 Morphology of new phase in 3Ru alloy (a) and EDS analysis of zone A in Fig.6a (b)

图7 3Ru合金中新相TEM像和SAED谱

Fig.7 TEM image of new phase and SAED pattern (inset) in 3Ru alloy

表1 3种不同Ru含量的合金在DSC升温曲线上获得的相变温度
Table 1 Phase transformation temperatures of DSC heating thermograms for alloys with different contents of Ru(℃)
Alloy TS TL ΔT0
0Ru 1334 1381 47
1.5Ru 1336 1381 45
3Ru 1335 1380 45

Note:TS—solidus temperature, TL—liquidus temperature, ΔT0—solidification range

表1 3种不同Ru含量的合金在DSC升温曲线上获得的相变温度

Table 1 Phase transformation temperatures of DSC heating thermograms for alloys with different contents of Ru(℃)

利用EPMA对3Ru合金中β-NiAl相进行元素面扫分析,结果如图8所示。由于采用背散射电子对其组织形貌进行观察,所以相中所含原子序数决定了衬度的明暗度。可以看出,β-NiAl相衬度颜色较深,呈黑色,周围颜色较浅的为γ'相。β-NiAl相中除Ni、Al基本组成元素外,另发现Ru、Co和Cr元素明显富集,而Mo、Re和Ta 3种元素明显贫化。对比β-NiAl相与其包覆层的γ'相,可以看出,β-NiAl相中Al、Ru、Cr 3种元素含量相对较多,Ni和Ta 2种元素含量相对较少,其它元素含量相当。排除其它元素在β-NiAl相和γ'相中对Al元素的替代作用,由于Al元素在β-NiAl相中的原子分数(50%)要明显大于γ'相(25%),因此Al在β-NiAl相中的含量要高于γ'相。反之,Ni在β-NiAl相中的含量要低于γ'相;由于Ta是γ'相的主要形成元素之一,因此Ta在γ'相中的含量要高于β-NiAl相。以上结果表明,β-NiAl相的析出占用了大量的γ'相形成元素Al和抗热腐蚀元素Cr,因此会对合金的力学性能和环境性能造成影响,必须予以重视,需要通过适当的热处理工艺使其退化溶解。

图8 3Ru合金中β-NiAl相的SEM像及元素面分布

Fig.8 SEM image and EPMA elemental mapping results of β-NiAl phase in 3Ru alloy

表2列出3种不同Ru含量合金中(γ+γ')共晶与β-NiAl相的体积分数。可以看出,(γ+γ')共晶含量随Ru含量的增加表现为先增后降的变化趋势,并且3Ru合金中的(γ+γ')共晶含量低于0Ru合金。虽然3Ru合金中(γ+γ')共晶含量降低,但(γ+γ')共晶与β-NiAl相二者总含量要高于前两种合金。

利用EPMA定量分析测得各合金的(γ+γ')共晶成分,结果见表3。可以看出,Ru含量不同的合金,其(γ+γ')共晶组成有很大差异。0Ru合金中Al与Ni含量最高,W、Re、Mo含量最低。但对于3Ru合金,Al与Ni含量最低,Re、W、Mo、Cr及Co含量最高。随着合金中Ru含量的增加,(γ+γ')共晶中Ru含量也逐渐增加。可见,Ru的添加,不仅改变了单晶高温合金中(γ+γ')共晶相的含量,而且改变了其共晶相的组成。

2.1.3 Ru对γ'相的影响 图9分别是3种不同Ru含量合金的枝晶干与枝晶间γ'相组织。可以看出,枝晶干处的γ'相较细小,形状基本呈较规则的立方体形或蝶形,而枝晶间区域的γ'相比较粗大,形状不规则。由于凝固过程中枝晶干和枝晶间存在微观区域的凝固偏析,使枝晶间区域富集了大量Al、Ta等γ'相形成元素,导致枝晶间区域的溶质过饱和度大于枝晶干区域,因而枝晶间区域γ'相形核较早,形核率较大,而且长大速度较快,从而造成枝晶间区域γ'相尺寸较大。此外,不同合金枝晶干和枝晶间对应的γ'相尺寸也不均匀,差异较大,其组织形态亦呈现出明显的差异。对比可见,0Ru合金的γ'相最为粗大,1.5Ru合金次之,3Ru合金γ'相尺寸最小。这说明,合金元素Ru对单晶高温合金γ'相组织有较大影响,即随着Ru含量的增加,枝晶干和枝晶间的γ'相都越来越细小,γ'相形态逐渐规则。

图9 3种不同Ru含量合金的铸态γ'相形貌

Fig.9 Morphologies of γ' phase in the dendrite core (a~c) and interdendritic region (d~f) in as-cast 0Ru alloy (a, d), 1.5Ru alloy (b, e) and 3Ru alloy (c, f)

表2 3种不同Ru含量合金中(γ+γ')共晶和β-NiAl相的体积分数
Table 2 Volume fractions of (γ+γ') eutectic and β-NiAl phase in three alloys(%)
Alloy Eutectic β-NiAl Eutectic+β-NiAl
0Ru 0.45 - 0.45
1.5Ru 0.75 - 0.75
3Ru 0.39 0.99 1.38

表2 3种不同Ru含量合金中(γ+γ')共晶和β-NiAl相的体积分数

Table 2 Volume fractions of (γ+γ') eutectic and β-NiAl phase in three alloys(%)

根据30个视场测定了3种合金中枝晶干γ'相的平均尺寸。经测定,0Ru合金中γ'相的平均尺寸较大,约为292 nm。随着Ru的加入,1.5Ru合金中γ'相尺寸急剧减小,约为222 nm,减少约24%。进一步增加Ru含量,3Ru合金中γ'相尺寸继续减小,约为218 nm,但降幅较小。上述结果表明,合金中的Ru含量显著影响着γ'相尺寸,与无Ru合金相比,Ru添加可强烈抑制合金凝固后析出γ'相的长大;增加Ru含量,γ'相尺寸进一步降低,但幅度较小。

表3 3种合金中(γ+γ')共晶相成分的比较
Table 3 Comparison of chemical composition of (γ+γ') eutectic in three alloys (mass fraction / %)
Alloy Re Mo Ru W Cr Co Al Ta Ni
0Ru 0.260 0.574 - 2.073 6.378 6.698 7.889 10.371 65.757
1.5Ru 0.361 0.640 1.264 2.208 5.619 6.663 7.667 11.937 63.641
3Ru 0.576 0.951 2.507 2.623 8.238 7.442 6.883 10.966 59.814

表3 3种合金中(γ+γ')共晶相成分的比较

Table 3 Comparison of chemical composition of (γ+γ') eutectic in three alloys (mass fraction / %)

铸态单晶高温合金中的γ'相有两种,即共晶γ'相和次生γ'相。其中,次生γ'相是合金凝固后由过饱和γ基体中脱溶沉淀而来的,该相的形核、长大过程与基体γ相的过饱和度息息相关。γ'相脱溶沉淀的本质为原子的迁移过程,即Re、Mo、W等γ相形成元素向γ相迁移,而Al、Ti、Ta等γ'相形成元素向γ'相迁移,其过程受元素的扩散能力控制。Ru是一种原子尺寸较大的元素,主要以间隙原子的形式固溶于γ相基体中,对次生γ'相的形成主要产生两方面的影响。一方面,Ru的添加增加了γ'相脱溶沉淀前基体γ相的过饱和度,提高了γ'相脱溶沉淀的形核驱动力,进而增加了γ'相的形核率;另一方面,Ru的原子尺寸较大,添加Ru降低了各元素的扩散能力,延缓了γ'相脱溶沉淀过程中的原子迁移,进而降低了γ'相的长大速度。综上所述,随着Ru含量的增加,降低了合金铸态组织中的γ'相尺寸。

2.2 Ru对合金元素在 /两相中分配比的影响

合金组元在γγ'两相中呈现为典型的不均匀分布状态,对γ'相的形筏会产生显著的影响,并最终影响合金的高温力学性能和组织稳定性。通常定义分配比R来描述合金元素在γ /γ'两相中的分配特点:R=C /Ciγ' (其中,CCiγ'分别为i元素在γγ'相中的成分)。

图10为3种合金中各元素分配比的直方图。为了区别不同分配类型的元素,当元素iγ相中的含量大于在γ'相中的含量时,分配比设为正,反之设为负。可以看出,Ta、Al、Ni的分配比均为负值,说明其在γ'相中富集,偏聚程度Ta>Al>Ni;而W、Co、Mo、Cr、Re、Ru等元素的分配比均为正值,说明这些元素向γ相中富集,从偏聚程度看Re的富集最为严重,Cr、Mo、Ru次之,W和Co的富集相对较轻。添加1.5%Ru后,除W、Ni 2种元素分配比变化不明显外,1.5%Ru明显降低了Re、Cr、Mo、Co向γ相的偏聚,同时也降低了Al、Ta向γ'相的偏聚,即γ基体形成元素更多地向γ'相分配;相反,γ'相形成元素更多地向γ基体分配,这与文献[23]中提到的“逆分配”现象一致。当合金中Ru含量继续增加到3%时,各元素在γ基体和γ'相中的分配类型没有改变,W、Ni 2种元素的分配比大小依然无明显变化,然而与添加1.5%Ru不同的是,其它元素在各自偏聚相中的富集程度发生逆向趋势改变,即3%Ru的添加增加了Re、Cr、Mo、Ru、Co向γ相富集,同时也增加了Al、Ta向γ'相的富集,这与文献[24]中提到的“逆分配”现象相悖。

元素在γ /γ'两相中的分配比大小主要受每种元素在γ基体和γ'相中的固溶度影响。一方面,3Ru合金中各元素依然受Ru的“逆分配”作用的影响;另一方面,如前文所述,在γ'相脱溶沉淀前,3Ru合金中有β-NiAl相析出,其析出温度高于γ'相脱溶沉淀的温度,该相的析出占用了大量的Ni,间接增加了γ基体的过饱和度,提高了难熔元素在γ基体中的含量。由于该富Al相主要在枝晶间区域形成,因此可以推测,枝晶间区域的γ基体过饱和度提高幅度较枝晶干区域更为显著。此外,γ'相属于金属间化合物,除Ni、Co、Al、Ta外,对其它难熔元素的溶解度较小,以Re为例,仅有百分之十几的Re会进入γ'相中[25,26]。在γ'相脱溶过程中,绝大多数的难熔元素会选择遗留在γ基体中。所以,β-NiAl相的析出直接影响各元素在γ基体和γ'相中的含量,表现为上文中正分配比的元素更倾向偏聚于γ基体,负分配比的元素更倾向偏聚于γ'相。综上所述,3Ru合金中各元素的分配比变化是上述2方面影响因素综合作用的结果。

2.3 Ru对偏析系数的影响

由于枝晶偏析的影响,使凝固合金枝晶干处富集提高合金熔点的元素,贫化降低合金熔点的元素,而枝晶间区域元素的分布规律与其恰恰相反。随着合金化程度的日趋提高,这种在凝固过程中形成的合金成分微观偏析日趋严重,严重降低了合金微观区域组织与性能的均匀性,影响合金的使用寿命。通常将枝晶干处元素含量与枝晶间处元素含量的比值定义为偏析系数[27]K=Cd/Ci,在本实验中,Cd为元素在枝晶干的平均成分,Ci为元素在枝晶间的平均成分。

通过EPMA分别测量合金中枝晶干与枝晶间的平均成分,可以获得各元素的偏析系数。元素的偏析系数小于1,定义为正偏析元素,说明元素聚集于枝晶间区域,正偏析系数越小,元素在枝晶间的含量越高;元素的偏析系数大于1,定义为负偏析元素,说明元素聚集于枝晶干区域,负偏析系数越大,元素在枝晶干区域的含量越高。

图11为3种不同Ru含量合金中各元素的偏析系数K。可以看出,Ta、Al、Mo、Cr元素的K都小于1,这表明以上元素均属于正偏析元素;而Co、W、Re、Ru元素的K都大于1,这表明以上元素均属于负偏析元素;Ni的K约为1,这表明Ni是一种几乎无偏析的元素;Ta、W、Re偏析明显。0Ru合金中主元素的偏析程度都不大,合金中正偏析元素Ta、Al、Mo、Cr的偏析系数介于0.5~1.0之间,负偏析元素中W、Re偏析比较大,介于1.5~2.0之间。随着合金中加入Ru元素以后,各元素的偏析情况发生了明显改变。Ru的加入增加了正偏析元素Ta、Al和负偏析元素Re的偏析程度,降低了正偏析元素Mo、Cr的偏析程度,对Ni、Co、W 3种元素的偏析程度影响不大,Ru自身表现为微弱的负偏析现象。值得注意的是,当合金中Ru含量增加至3%时,Re显著增加了向枝晶干偏析的程度。

图10 3种不同Ru含量合金中各元素在γ /γ'相中的分配比

Fig.10 Partition coefficients of elements between γ and γ' phase in three alloys

图11 3种不同Ru含量合金中各元素的偏析系数K

Fig.11 Segregation coefficients (K) of elements in three alloys

凝固过程中元素偏析的本质是溶质扩散的再分配。凝固初期,γ相优先生成,形成枝晶干区域,因此富集较多的高熔点元素。而凝固后期形成的枝晶间区域,则富含较多的低熔点元素。由于Ru与Cr之间存在显著的交互作用,因此在不同Cr含量的合金体系中,Ru对其它元素的偏析影响也会有差异。此外,Kearsey等[28]发现Ru可以降低合金内元素的偏析程度;Feng等[15]发现Ru加重了Re元素向枝晶干的偏析程度;金涛[20]在研究Ru含量变化对一种含Re单晶高温合金时发现,1.5%的Ru增加了Re元素向枝晶干的偏析程度,进一步增加Ru含量至3%时,Re元素向枝晶干的偏析程度降低了,几乎与0Ru合金中Re的偏析程度相当。以上研究结果表明,Ru对单晶合金内各主元素的偏析程度影响,不仅受合金体系及元素间交互作用的影响,而且与Ru的添加量也有联系。

3 结论

(1) 随着Ru含量的增加,合金一次枝晶间距与二次枝晶间距逐渐减小,(γ +γ')共晶含量先增后降,γ'相尺寸逐渐变小。除此之外,3%的Ru添加使合金凝固组织中析出β -NiAl相,该相除Ni、Al基本组成元素外,还包含一定量的Cr、Co与Ru。

(2) Ru是一种弱偏聚于γ相的元素,对合金中其它元素具有典型的“逆分配”作用。随着Ru含量的增加,合金中β -NiAl相的析出降低了Ru对其它元素“逆分配”的影响程度。

(3) Ru是一种弱偏聚于枝晶干的元素。随着Ru含量的提高,增加了正偏析元素Ta、Al和负偏析元素Re的偏析程度,降低了正偏析元素Mo、Cr的偏析程度。

The authors have declared that no competing interests exist.

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关键词(key words)
单晶高温合金
Ru
-NiAl相
逆分配
偏析

single crystal superalloy
Ru
-NiAl phase
reverse partitioning
segregation

作者
宁礼奎
佟健
刘恩泽
谭政
纪慧思
郑志

NING Likui
TONG Jian
LIU Enze
TAN Zheng
JI Huisi
ZHENG Zhi