Microstructures and Mechanical Properties in an Fe-9Al-30Mn-2C Alloy

• Main Authors: Liu, Liang-Yen, 劉亮延
• Other Authors: Chao, Chuen-Guang
• Format: Others
• Language: zh-TW
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/89305603005156626702

碩士 === 國立交通大學 === 材料科學與工程系所 === 97 === Abstract The purpose of this study is to examine the microstructural development of the Fe-9Al-30Mn-2C alloy after solution-heat-treated (SHT) and aged at various temperatures and different timeperiods by means of optical microscopy (OM) , scanning electron microscopy (SEM) and transmission electron microscopy (TEM) . Besides , the effect of the microstructural development on the mechanical properties has also been investigated . The as-guenched microstructure of the Fe-9Al-30Mn-2C alloy was austenite phase containing fine (Fe,Mn)3 AlC carbides . The fine (Fe,Mn)3 AlC carbides having an L′12 structure were formed by spinodal decomposition during quenching . No precipitate could be observed on the grain boundaries . The as-guenched alloy had good combination of ultimate tensile strength (UTS) of 160 ksi with an excellent 55％ elongation . The fracture mechanism of the as-guenched alloy was typically ductile dimple fracture . After aged at low temperatures (450℃) , the fine (Fe,Mn)3 AlC carbides grew within the austenite matrix . With increasing the aging time to 18 hours , optimal precipitation hardness effect could be achieved . The UTS was 240ksi , with suitable 10％ elongation . The fracture surface was a mixture of small dimples and cleavage river patterns . Therefore , the fracture mechanism was transferred from typically ductile fracture to a mixture of ductile and cleavage fracture . It is worthwhile to note that the mechanism properties and fracture behavior of the austenitic Fe-Al-Mn-C alloy with C＞1.3wt.％ aged at low temperatures (450℃) have never been investigated by other workers before . With raising the aging temperatures , the UTS peak value could be achieved , but the UTS peak value was poorer than that aged at 450℃ . For instance , when the alloy was aged at 550℃ for 6 hours , the UTS peak value was 196 ksi . Microstructural observation reveals that after age at higher temperature (550℃) , (Fe,Mn)3AlC carbides started to occur on the austenite grain boundaries . With increasing aging times , the κ carbides grew continuously . Therefore , the strength and the elongation of the alloy droped drastically . The aged specimen exhibited a cleavage fracture surface with obvious cleavage river patterns .