熔融鉛與鉛錫合金穿透鎳基材晶界行為之研究

Abstract

晶界穿透行為是固體和固體之間的表面能與液體和固體之間表面能差異，當 液態金屬侵入固態金屬的晶界，在晶界上形成微米級之金屬薄膜，因而導致固態金屬之延展性、機械強度的嚴重劣化，而產生脆性破壞。本實驗乃藉由熔點低之純鉛與鉛錫合金作為液態金屬，純鎳板、電鍍鎳以及無電鍍鎳磷合金三種鎳基材作為固態金屬。三種鎳基材的差別為晶粒大小以及磷的添加，純鎳板之晶粒最大，電鍍鎳則較小，無電鍍鎳磷合金中 ，由於磷以插入型原子存在於鎳的晶格中，扭曲鎳的晶格，使無電鍍鎳磷合金為非晶質結構，不具有晶界。 實驗在360oC下進行液固反應，採用大量體積之液態金屬(約12g)，以確保有足夠之液態金屬提供晶界穿透，並且在鎳基材與鉛錫合金反應時，不會因為形成介金屬化合物而改變液態金屬的成分組成。 本研究結果包含三個部分。第一部分探討三種鎳基材與純鉛反應後，晶界穿透之深度及形貌。實驗結果顯示，純鎳板與電鍍鎳之晶界穿透深度差不多，而無電鍍鎳磷合金之穿透深度最深。由於非晶質合金並沒有晶界，代表鎳磷合金在高溫時應由非晶質結構轉為結晶結構，導致晶界穿透發生。比較三種鎳基材之晶界穿透形貌，純鎳板及電鍍鎳皆有形成微米級之金屬薄層，並且基材與液態金屬的界面都變得粗糙，而無電鍍鎳磷合金並未形成微米級金屬薄層，而是形成Ni-P-Pb相，與液態金屬的介面仍然維持平整。 第二部分則是探討無電鍍鎳磷合金的晶界穿透區，以XRD分析以及EPMA分析，推得Ni-P-Pb相應為Ni3P與Pb所構成。代表無電鍍鎳磷合金在高溫時會先轉為Ni與Ni3P之結晶結構使液態鉛得以穿透，且Ni3P為穩定的介金屬化合物，故能維持鎳磷合金的厚度，並保持著與液態鉛相接處的平整界面。最後利用維氏硬度分析其機械性質，得知無電鍍鎳磷合金在高溫時效時硬度會提升，但形成Ni-P-Pb相後硬度會大幅下降。 第三部分則是探討三種鎳基材與95Pb5Sn反應後，晶界穿透之深度及形貌。錫會與鎳形成介金屬化合物，使晶界穿透的速率減緩。以95Pb5Sn為液態金屬時，晶界穿透深度變為純鎳板最大，其次為電鍍鎳，而鎳磷合金幾乎觀察不到晶界穿透現象。三種基材皆有生成介金屬化合物，並在介金屬與基材間皆有孔洞產生，孔洞生成原因在純鎳板與電鍍鎳應為晶界穿透，而鎳磷合金則為Kirkendall void。

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.