Phase Transformation in Fe-Al-Mn-C and Fe-Al-Mn-C-Si Alloys

• Main Authors: Hwang Lang-Kung, 黃仁谷
• Other Authors: Liu Tzeng-Feng
• Format: Others
• Language: en_US
• Online Access: http://ndltd.ncl.edu.tw/handle/21821249836384094113

碩士 === 國立交通大學 === 材料科學(工程)研究所 === 81 === The microstructures of five austenitic Fe-Al-Mn-C-(Si) The microstructure of an Fe-8.5Al-4.5Ni-1.6C alloy, prepared by conventional casting, has been investigated by using optical microscopy (OM), transmission electron microscopy (TEM) and energy- dispersive X-ray spectrometer (EDS). In the as-quenched condition, the microstructure of the present alloy was austenite phase containing (Fe,Ni)3AlC carbides (k'-carbide). The k' -carbides having an L'12-type structure were formed within the austenite matrix during quenching by spinodal decomposition. This feature is quite different from that observed by other workers in various Fe-Al-Mn-C alloys. When the present alloy was aged at 550 C for short times, fine k'- carbides existing in the as-quenched alloy had grown along <100> directions, and no precipitates could be detected on the grain boundary. However, prolonged the aging time at 550 C, coarse (Fe,Ni)3AlC carbides ( k-carbide ) started to appear on the grain boundaries. Subsequently, these coarse k-carbides grew into the adjacent grains through a y ?+ k-carbide transition. With further aging at 550 C, a spherical B2-type particles were started to form within the ferrite region. The orientation relationship between k-carbide and (?+ B2) phases was determined to be (111)k-carbide // (110)?B2, and [110]k- carbide// [001](?B2). Consequently, the phase decomposition of y ( ?+ k-carbide ) ( ?+ B2 + k- carbide ) would continue to occur during aging , and finally the stable microstructure of the present alloy aged at 550 C should be a mixture of Widmanstdtten-like k-carbide, ferrite phase and spherical B2- type precipitates dispersing within the ferrite phase. It is noted here that the B2 phase has never been observed by other workers in various Fe-Al-C, Fe-Al-Mn-C, Fe-Al-Ni-C and Fe-Al-( Mn,Ni)-C alloys.