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Acta Metall Sin  2020, Vol. 56 Issue (3): 301-310    DOI: 10.11900/0412.1961.2019.00287
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Formation of Sliver Defects in Single CrystalCastings of Superalloys
MA Dexin1,2,WANG Fu3,XU Weitai1,XU Wenliang3,ZHAO Yunxing1()
1. Wedge Central South Research Institute, Shenzhen 518045, China
2. Powder Metallurgy Research Institute, Central South University, Changsha 410083, China
3. School of Mechanic and Electronic Engineering, Xi’an Jiao Tong University, Xi’an 710049, China
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Abstract  

In the casting components of superalloys with increasing content of the refractory elements the occurrence of sliver defects becomes rather frequent. In comparison to the other grain defects such as stray grains and freckles, the sliver defect was less well understood and its formation mechanism is still unclear. In the present work, the origins of the sliver defects in the single crystal (SC) castings were investigated. In the metallographic detection, sliver grains can be identified to be miss-alignments of isolated, individual primary dendrite on the SC matrix. The sliver defects originated from the tearing of the existing dendrite stems in the mush zone, revealing a clear starting point of sliver defect. The cracks of the dendrites were filled by the interdendritic residual melt, which finally solidified into γ′-stripes. The tearing of some existing dendrite stems can be attributed to the adhesion of shell mold that hinders the shrinkage of the columnar dendrites on the casting surface. The second reason for the dendrite tearing is the insertion of oxide residues which significantly weakens the strength of the dendrite stems. Due to the support of the neighboring columnar dendrites, the tilting of the broken dendrites is limited, so that the grain boundary between a sliver and the matrix structure has normally a low angle. The structure and formation mechanisms of sliver defects were discussed in comparison with other defects such as stray grains, freckles and low angle grain boundaries. The corresponding methods were proposed to avoid sliver defects in production of SC superalloy castings.

Key words:  sliver      superalloy      single crystal      grain defect     
Received:  02 September 2019     
ZTFLH:  TG146  
Fund: Supported by National Natural Science Foundation of China(91860103);Shenzhen Peacock Plan(20150128085205453)
Corresponding Authors:  Yunxing ZHAO     E-mail:  sunwq@wedge.com.cn

Cite this article: 

MA Dexin,WANG Fu,XU Weitai,XU Wenliang,ZHAO Yunxing. Formation of Sliver Defects in Single CrystalCastings of Superalloys. Acta Metall Sin, 2020, 56(3): 301-310.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2019.00287     OR     https://www.ams.org.cn/EN/Y2020/V56/I3/301

AlloyCrCoWMoAlTiTaHfReNi
DD57.027.525.031.526.23-6.510.153.01Bal.
CMSX-46.499.716.410.635.601.016.520.102.97Bal.
Table 1  Nominal composition of used alloys (mass fraction / %)
Fig.1  Sliver defects originate at a Pt pinning wire (a) and at a convex hull of blades (b)
Fig.2  The sliver defect (a),OM images (b~d) and SEM image (e) showing the microstructure at its origin
Fig.3  OM image of sliver defect in another example (a) and the magnification of its origin (b)
Fig.4  Macroscopic photo of a sliver defect at blade surface (a) and magnifications (b, c) showing oxide as the sliver origin
Fig.5  Sliver defect (a), and the corresponding OM images (b~d) and SEM image (e) at its origin
Fig.6  EDS mapping analyse on the specimen section showing in Fig.5eColor online(a) Al (b) O (c) Ni (d) Ti (e) W (f) Re
Fig.7  Schematic of sliver formation mechanism A, showing the dendrite growth into mold hole (a), the dendrite deformation (b) and crack (c) caused by vertical shrinkage force (FV)Color online
Fig.8  Schematic of sliver formation mechanism B, showing the oxide insertion into dendrite trunk (a), the stress concentration (b) and crack at the oxide (c)Color online
Fig.9  Schematic of sliver formation caused by lateral shrinkage force in normal (FN) and tangential direction (FT)(a) dendrite growth into mold hole (b) rotation of the broken dendrite
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