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Title

鎳矽基介金屬合金添加硼、碳後之環境脆化研究

A Study on the Environmental Embrittlement of the Ni-Si Based Intermetallic Alloys Doped with Boron and Carbon

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[[abstract]]摘要 鎳矽介金屬合金(Ni3Si,有序之面心立方L12結構)為一種耐中、高溫及耐特殊酸腐蝕的材料,特別在高溫時,其優異的抗腐蝕性、抗氧化、及降伏強度隨溫度上昇而增加的特性,使其應用在高溫環境之潛力有優於其他合金;但由於該合金的常溫脆性及熱加工性相當差,以致於Ni3Si基合金目前僅可使用於低張力之鑄造零件上,如Hastelloy D.;而常溫脆性乃由於脆弱之晶界結合性所導致脆性之沿晶破裂,故若能有效增加晶界之契合性,大幅提昇其延性,當可突破其應用限制。 鎳-19矽-3鈮(Ni-19Si-3Nb)基介金屬合金,經由前人的研究結果,應用微量合金法(次元素添加量小於600ppm)以化學計量方式同時添加微量硼與碳元素,在氬氣保護下,進行電弧熔煉及墜落式鑄造成鑄塊胚料;鑄胚經以真空熱處理爐1080℃、四小時均質化與700℃、十小時時效處理後,一部份製成標距平行區3mm×2 mm×20 mm之拉伸試片,分別於大氣(14000ppm H2O vapor)真空、水氣(850ppm H2O vapor)各種氣氛下,分別進行25℃、500℃、600℃、700℃及800℃之拉伸試驗,另一部份試片則取樣進行GDS、Auger、DTA、EDS各種分析,另外藉由其金相組織、破斷面之組織,與拉伸試驗之結果比對,以探討在各種環境下影響鎳矽基合金之延性及破裂模式之主要因素。 由本實驗之結果發現,當使用微量合金法添加300ppm硼與200ppm碳於Ni-19Si-3Nb基介金屬合金時,可以有效改善水氣所引發之氫脆現象,其室溫下最大抗拉強度(UTS)由780MPa增加至1160MPa,延伸率也由5.2%提升增加到11.8%,同時亦發現添加硼與碳使其脆性轉換溫度由前人研究中的500℃提升至700℃。

[[abstract]]Abstract Intermetallic compounds based on Ni3Si (order 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 temperature, good specific strength in comparison with other materials, increasing yield strength as a function of temperature. However, binary Ni3Si, similarity the Ni3Al suffers from grain boundary embrittlement at room temperature. In addition, the Ni3Si intermetallic alloy also exhibits, brittle behavior at mid-high temperature due to the environment embrittlement. Therefore, how to improve its grain boundary adhesion and resistance of environment embrittlement will be the critical points 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 under5×10-5 torr at 1080℃ for 4 hours and 700℃ for 10 hours respectively. The main part of heat-treated ingots where machined into the tensile test specimen with the gauge dimension of 3mm×2mm×20mm. Then the specimen where tensile test under different atmosphere, [such as air (14000ppm water vapor), vacuum (2×10-4 torr), and pure water vapor (850ppm)] at different temperature (25℃, 500℃, 600℃, 700℃, 800℃). Both of results of atmosphere-controlled tensile test, the metallography microstructure, and the fracture surface observation were summarized to investigate the effect of environment embrittlement on the ductility and fracture mode for the Ni-19Si-3Nb, and Ni-19Si-3Nb-0.15B-0.1C alloys. According to the result of this study, the microalloying of 300ppm boron and 200ppm carbon into the Ni-19Si-3Nb base alloy exhibits an effective improvement on overcoming the environment embrittlement by water vapor. The positive effect results in improving ultimate tensile strength and elongation from 780MPa, 5.2% to 1160MPa, 11.8% at room temperature in air, as well as increasing the brittle transition temperature from 500℃ to 700℃.

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