鐵-9鋁-30錳-(1.0~1.3)碳合金於熱滾軋狀態下之顯微結構與其機械性質之研究

Abstract

本篇論文主要利用光學顯微鏡(OM)、掃描式電子顯微鏡(SEM)、X光繞射儀(XRD)以及穿透式電子顯微鏡(TEM)來觀察Fe-9wt.%Al-30wt.%Mn-1.0wt.%C (1.0C)和Fe-9wt.%Al-30wt.%Mn-1.3wt.%C(1.3C)合金於熱滾軋狀態下(不經任何熱處理)之顯微結構，並探討其機械性質與破壞機制。 1.0C合金於熱滾軋狀態下，其顯微結構為單一沃斯田鐵相，無任何析出物產生，此時合金之最大抗拉強度為973MPa、降伏強度為660MPa而伸長率則為51.4%，而破裂模式則為具有許多酒窩狀凹洞的延性破裂。而1.3C合金除了沃斯田鐵母相外，還可觀察到具有L'12結構的細微κ'-碳化物析出，其體積比約為34%、平均顆粒大小則約為22nm，而由於沃斯田鐵母相內產生許多奈米尺度的κ'-碳化物，使最大抗拉強度與降伏強度提升至1070MPa與973MPa，而延伸率仍保有38.2%，合金破裂模式仍為延性破裂的酒窩狀凹洞。 1.3C合金於25℃至-196℃範圍內進行衝擊測試，其保有穩定的沃斯田鐵相。而衝擊值隨著溫度下降而下降，無明顯轉脆溫度。合金於25℃及-50℃衝擊，其破裂面皆為酒窩狀凹洞之延性破斷面；而於-100℃衝擊時，破裂面主要為延性破裂伴隨著些許的劈裂平面；當測試溫度降至-196℃時，破裂面則為穿晶破裂為主之平坦多面體，但仍被延性之酒窩狀凹洞包圍著，因此其仍擁有不錯之62焦耳衝擊值。合金於25℃衝擊時，其破裂機制為平面滑移；-196℃衝擊時，其破壞機制為以機械雙晶來調節變形。

The microstructures of Fe-9wt.%Al-30wt.%Mn-1.0wt.%C (1.0C) and Fe-9wt.%Al-30wt.%Mn-1.3wt.%C (1.3C) alloys in the as-hot rolled condition (without heat treatment) were studied by using optical microscopy (OM), scanning electron microscopy (SEM), X-Ray diffraction (XRD) and transmission electron microscopy (TEM). Besides, the mechanical properties and fracture mechanisms were also investigated. The microstructure of the as-hot rolled 1.0C alloy was single austenite without existing any precipitate. At this condition, the alloy possessed ultimate tensile strength (UTS) of 973 MPa, yield strength (YS) of 660 MPa with 51.4% elongation. For the 1.3C alloy, its microstructure was austenite phase containing fine κ'-carbides. The volume fraction of the κ'-carbides was around 34% with average particle size of 22nm. Due to the precipitated of fine κ'-carbides, the 1.3C alloy had higher UTS and YS of 1070MPa and 973 MPa, respectively; the elongation was 38.2%. Regardless of the 1.0C alloy or 1.3C alloy, both the fracture modes were ductile dimple fractures. After Charpy impact test, 1.3C alloy showed stable austenite phase within all test temperatures ranging from 25℃ to -196℃. The impact value of the alloy decreased with decreasing temperature. The fracture morphologies were ductile dimples at 25℃ and -50℃. At -100℃, the morphology changed to dimple fracture containing small fraction of flat facets. When the test temperature decreased to -196℃, it showed transgranular brittle fracture. However, the flat facets were still surrounded by ductile dimples; therefore, the impact value was still 62 joule. From TEM examination, 1.3C alloy showed typical planar glide configurations at 25℃, while it revealed extensive mechanical twins at -196℃.