Phase Transformations in Fe-Al-Mn-C, Fe-Al-Mn-Si-C and Fe-Al-Ni- C Alloys

• Main Authors: Hwang, Chih-Neng, 黃志能
• Other Authors: Liu Tzeng-Feng
• Format: Others
• Language: zh-TW
• Online Access: http://ndltd.ncl.edu.tw/handle/2qpvaf

碩士 === 國立交通大學 === 材料科學與工程研究所 === 85 === The phase transformations in the Fe-8.0Al-31.5Mn-1.05C, Fe-9Al-30Mn-2.0C, Fe-9.0Al-30.5Mn-(0-2.5)Si-1.2C, Fe-8Al-5Ni-2C, have been examined by means of OM, STEM and EDS. As a result the following conclusions have been presented.(1) When the Fe-8.0 Al-31.5Mn-1.05C alloy was solution heat-treated and quenched, the microstructure was single-phase austenite. After being aged at 550℃, a γ→γ+κ-phase transition had occurred on the grain boundaries and twin boundaries. This result is quite different from that observed by other workers in the Fe-(4.9-11) wt pct Al-(28.5-34.3) wt pct Mn-(0.50-1.62)wt pct C alloys. In their studies, they found that when the alloy was aged at temperatures ranging from 550℃to 650℃, aγ→α+β-Mn, γ→ α+β-Mn+κand γ→ β-Mn reaction occurred on the grain boundaries.(2) In the as-quenched condition, the microstructure of Fe-9Al-30Mn-2.0C alloy was a mixture of (γ+κ''- carbide). When the alloy was aged at temperatures ranging from 550 ℃ to 750 ℃, fine κ''-carbides existing in the as- quenched austenite matrix grew preferentially along <100> directions and a γ→γ+κ-phase transition had occurred on the grain boundaries. When the aging temperature was in the range from 850℃ to 950℃, both coarse κ'' and fine κ''- carbides could be observed within the austenite matrix . In the grain boundaries, a γ→ γ+coarse κ-carbide + fine κ- carbide transition had occurred on the grain boundaries. It is worthy to note that the κ-phase carbide has L''12-type structure at 650℃ or below and has an L12 type structure at 750℃ or above. These results are quite different from those reported by other workers in various Fe-Al-Mn-C alloys.(3) The effects of Si addition on the as-quenched microstructure of Fe-9.0Al-30.5Mn-1.2C alloy have been investigated. The results indicated that Si addition would enhance the κ''- carbides to precipitate coherently within the austenite matrix during rapid quenching by spinodal decomposition. With increasing the Si content to 1.5wt.%, (α+ D03) phases were also formed on the austenite grain boundaries in addition to the formation of the κ''-carbides within the austenite matrix. The D03 phase was formed during quenching by a α→B2→ D03 continuous ordering transition. (4) In the as-quenched condition, the microstructure of the alloy was austenite phase containing fineκ''-carbides. Theκ'' carbides were formed during quenching by a spinodal decomposition. When the as- quenching alloy was aged at 550℃for short times, theκ''- carbides grew along <100> directions. After prolong aging at this temperature, aγ→α+B2+κ reaction occurred on the austenite grain boundary. This result is different from that observed by Choo and Kim in the same alloy.