Grain Boundary Penetration of Various Types of Ni layers by Molten Pb and PbSn alloy
Date Issued
2012
Date
2012
Author(s)
Chang, Chia-Yuan
Abstract
Grain boundary penetraion is caused by difference between solid-solid surface energy and solid-liquid surface energy. When , liquid metal would penetrate into grain boundaries of solid phase and form micrometric film. Grain boundary penetration effect would decrease mechanical property, toughness and lead to embrittlement. In experiment, Pb and PbSn alloy, which have low melting point, are used as liquid metal. Ni foil, electroplating Ni and electroless Ni-P alloy are used as solid metal. The differences between three types of Ni substrates are grain size and addition of P. The grain size of Ni foil is the largest, and grain size of electroplating Ni is smaller. Electroless Ni-P alloy is amorphous structure without grain boundary.
The reaction of solid-liquid was taken at 360oC, and the large volume of liquid metal was used to assure the composition of liquid metal didn’t change even though intermetallic compound formed during reaction.
The subjects of the research proposal include three parts. The first part of study investigates the penetration length and morphology of reaction of three types Ni substrate and pure Pb. The result reveals the penetration length of Ni foil and electroplating Ni are similar, whereas the penetration length of Ni-P alloy is the deepest. In light of no grain boundary in amorphous structure, the result implicates the amorphous structure transforms into crystalline structure at high temperature. It’s the reason why Ni-P alloy can be penetrated by molten Pb. Comparing morphology of penetration of three types of Ni substrates, both pure Ni and electroplating Ni have micrometric film and rough interface, but in Ni-P alloy, Ni-P-Pb phase is formed instead of micrometric film, and interface is still complete.
The second part of study investigates Ni-P-Pb phase in Ni-P alloy. XRD and EPMA analysis are used to deduce the Ni-P-Pb phase is composed of Ni3P and Pb. It implicates electroless Ni-P alloy would transform into crystalline structure at high temperature, which can be penetrated by molten Pb. Because Ni3P is steady intermetallic compound, it can keep Ni-P alloy thickness and complete interface. Finally, Vickers Hardness is used to analyze mechanical property. It reveals hardness of Ni-P alloy would increase at high temperature, but enormously decrease when Pb penetrate.
The third part of study investigates the penetration length and morphology of reaction of three types Ni substrate and 95Pb5Sn. Sn and Ni would form intermetallic compound, which could reduce penetration velocity. Among the three kinds of Ni, the penetration of molten 95Pb5Sn into the Ni foil is most pronounced, and followed by electroplating Ni. However, the penetration effect was hardly observed in the reaction of molten 95Pb5Sn and Ni-P alloy. Three types of Ni substrates would form intermetallic compound, and the voids are formed between intermetallic compound and substrate in all types of Ni substrate. The voids formed in Ni foil and electroplating Ni are caused by grain boundary penetration effect, whereas the voids formed in electroless Ni-P alloy are caused by Kirkendall void.
Subjects
grain boundary penetration
PbSb alloy
intermetallic compound
electroplating Ni
electroless Ni
Type
thesis
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