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Acta Metall Sin  2017, Vol. 53 Issue (4): 423-432    DOI: 10.11900/0412.1961.2016.00291
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Effect of Ru on the Solidification Microstructure of a Ni-Based Single Crystal Superalloy with High Cr Content
Likui NING(),Jian TONG,Enze LIU,Zheng TAN,Huisi JI,Zhi ZHENG
Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
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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      β-NiAl phase      reverse partitioning      segregation     
Received:  08 July 2016     
Fund: Supported by National Natural Science Foundation of China (No.51501193)

Cite this article: 

Likui NING,Jian TONG,Enze LIU,Zheng TAN,Huisi JI,Zhi ZHENG. Effect of Ru on the Solidification Microstructure of a Ni-Based Single Crystal Superalloy with High Cr Content. Acta Metall Sin, 2017, 53(4): 423-432.

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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)
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)
Fig.3  DSC heating thermograms of 0Ru (a), 1.5Ru (b) and 3Ru (c) alloys
Fig.4  SEM images of as-cast 0Ru (a), 1.5Ru (b) and 3Ru (c) alloys
Fig.5  Morphology of block eutectic in the 1.5Ru alloy (a) and EDS analysis of zone A in Fig.5a (b)
Fig.6  Morphology of new phase in 3Ru alloy (a) and EDS analysis of zone A in Fig.6a (b)
Fig.7  TEM image of new phase and SAED pattern (inset) in 3Ru alloy
Alloy TS TL ΔT0
0Ru 1334 1381 47
1.5Ru 1336 1381 45
3Ru 1335 1380 45
Table 1  Phase transformation temperatures of DSC heating thermograms for alloys with different contents of Ru(℃)
Fig.8  SEM image and EPMA elemental mapping results of β-NiAl phase in 3Ru alloy
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)
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
Table 2  Volume fractions of (γ+γ') eutectic and β-NiAl phase in three alloys(%)
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
Table 3  Comparison of chemical composition of (γ+γ') eutectic in three alloys (mass fraction / %)
Fig.10  Partition coefficients of elements between γ and γ' phase in three alloys
Fig.11  Segregation coefficients (K) of elements in three alloys
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