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

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℃.