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微量合金法添加硼與碳對鎳-19矽-3鈮基介金屬合金微結構及機械行為之研究

Study on the Microstructure Evolution and Mechanical Behaviors of the Ni-19Si-3Nb Based Intermetallic Alloys Doped with Boron and Carbon by using Microalloying Methods

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[[abstract]]鎳矽介金屬合金 (Ni3Si,有序之面心立方 L12結構)為一種有別於其他金屬之特殊結構,尤其在高溫環境時,其優良的抗侵蝕、抗氧化,及降伏強度隨溫度之上升而升高的特性 (最大之降伏強度發生於 700℃),使此合金在高溫應用上優於其他金屬;然而,鎳矽二元合金相似於鎳鋁合金,在室溫環境下有著脆性破壞的現象;以至於如何增加晶界之契合性使合金提升延性,達到多用途的應用成為尚待解決的問題之一。 經由前人實驗研究之結果,以鎳-19矽-3鈮(Ni-19Si-3Nb)基介金屬合金,應用微量合金法(次元素添加量約小於600 ppm)以化學計量之方式同時添加微量硼與碳元素,在氬氣氣氛保護之下,使用電弧熔煉、墜落式鑄造法製造成鑄塊胚料,續以真空退火爐進行鑄塊 1080℃、四小時均質化與700℃、十小時時效化處理,而後取樣進行成份分析驗證;並藉由其組織、破斷面之光學顯微鏡、掃瞄式電子顯微鏡及穿透式電子顯微鏡觀察,與拉伸試驗、微硬度之機械行為等變化,以獲得改善該合金材料脆性之最佳微量合金硼與碳元素之添加值。 本實驗之研究結果證實,當添加微量硼與碳元素各為300 ppm及200ppm時,顯示該合金有最佳之機械性質組合;其極限拉伸應力 (UTS)由原 727 MPa 增加至1224 MPa,延率從 4.3﹪提升至12﹪。由於添加微量元素、均質化與時效熱處理後,對合金之相變化、相含量與相結構並無太大影響,但造成析出物之形態鈍化、呈現粒狀析出散佈之較具延性與強度之微結構,並證實該合金在添加硼與碳元素後,對其機械行為確有明顯之改善效果。

[[abstract]]Intermetallic compounds based on Ni3Si (ordered fcc L12 structure) offer a number of interesting properties for structural applications in aggressive environments at elevated temperature. These interesting properties include the excellent corrosion and oxidation resistance at both ambient and elevated temperatures, good specific strength in comparison with other materials, increasing yield strength as a function of temperature, and a peak yield strength occurring at about 700℃. However, similar to the Ni3Al, binary Ni3Si suffers from grain boundary embrittlement at room temperature. Therefore, how to improve its grain boundary adhesion will be the critical point of increasing the ductility and widespread the applications of Ni-Si intermetallic alloys. According to the pre-studies, we select the Ni-19Si-3Nb alloy as the based alloy to investigate the effect of adding boron and carbon by means of arc melting and drop casting under argon atmospheres, and then homogenizing and aging in a vacuum annealing furnace of 5×10-5 torr at 1080℃ for 4 hours and 700℃ for 10 hours respectively. The evolution of microstructure change and mechanical properties of the Ni-19Si-3Nb-xB-yC alloys were characterized by x-ray diffraction, DTA, SEM, TEM, EPMA, microhardness test, Vickers hardness test, and tensile tests. The results of x-ray diffraction, DTA and TEM could not resolve any change of phase content for the Ni-19-Si-3Nb alloy with different boron and carbon content. But, the consequences of the fracture behavior of the Ni-19Si-3Nb alloys with different carbon and boron additions exhibits significant change from brittle mode to ductile mode. The optimum additions of carbon and boron for improving the mechanical properties of the based alloy will occur at the combination of 200-ppm carbon and 300-ppm boron addition. Their elongation will increase from originally 4.3﹪to nearly 12﹪, the ultimate tensile stress (UTS)from 727 MPa to 1224 MPa, and the yield stress also from 705 MPa to 840 MPa. Meanwhile, the morphology of precipitates transformed a lot of blunt-edged equiaxed precipitates existing in the grain boundary and in the matrix. According to the fracture surface observed, they reveal that an obviously fracture mode change from intergranular to transgranular due to the grain boundary strengthening by increasing the boron and carbon additions in this alloys. Meanwhile, these results can supply an information for researching into the grain boundary strengthening mechanism in the future.

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