Phase transformation in an Fe-9.0AI-29.5Mn-1.2Si alloy
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INTRODUCTION
THE
phase diagrams and phase transformations of Fe-A1-Mn ternary alloys have been extensively studied by many workers, it-61 Six different kinds of phases, namely, a-Fe (body-centered cubic), y-Fe (face-centered cubic), /3-Mn (A13), FeAI3, MnA16 (D2h), and a CsC1type (B2) structure, were observed and identified. Much of the previous work was comprehensively reviewed by Rivlin in 1983. tvl In order to improve the corrosion resistance and hightemperature oxidation resistance, chromium or silicon has been added to the Fe-A1-Mn alloys, t~-~41 Based on these studies, it can be generally concluded that the addition of a small amount of chromium or silicon can effectively improve these properties. However, up to 1985, little information concerning the microstructure of the Fe-A1-Mn-Cr and Fe-A1-Mn-Si alloys has been reported. In 1985, the present workers performed transmission electron microscopy observations on the phase transformations of the Fe-9.1AI-29.9Mn-2.9Cr alloy. [15-191 Ill the as-quenched condition, the microstructure of the alloy consisted of austenite (3,) and ferrite (a) phases. No precipitates could be detected in the as-quenched specimen. However, when the as-quenched alloy was aged at temperatures ranging from 350 ~ to 525 ~ both ordered D03-type and a-Mn (A12) precipitates were formed within the ferrite matrix. 1~5,~6JThe a-Mn precipitate has a dodecahedral shape which is bounded by six sets of {1113} crystallographic planes, t~7~Neither the ordered D03 n o r the a-Mn phases have been found by other researchers in the Fe-A1-Mn alloy system before. As the aging temperature was increased to 550 ~ or above, /3-Mn precipitates having a typical Widmansffttten morphology were observed and identified,t~81The orientation relationship and axis/angle pair of rotation between the/3-Mn precipitate and the ferrite matrix were determined to be (001)~ff(012)~_Mn, (010)~ff (02i)t3.M., (100)~ff (100)t~_M., and [100]/26 deg, respectively,tlg~ T.F. LIU, Professor, Institute of Materials Science and Engineering, and G.C. UEN, C.Y. CHAO, Y.L. LIN and C.C. WU, Graduate Students, Institute of Mechanical Engineering, are with the National Chiao Tung University, Taiwan, Republic of China. Manuscript submitted July 25, 1990. METALLURGICAL TRANSACTIONS A
Recently, we have also made transmission electron microscopy observations on the phase transformations of an Fe-9.0A1-29.5Mn-l.2Si alloy. Based on our present study, it is found that extremely fine DO3 particles were formed within the ferrite matrix by a continuous ordering transition during quenching. After being aged at temperatures ranging from 550 ~ to 950 ~ a complex phase transition (a + D03 + L phase) ~ (a + I303 +/3-Mn) (B2 + D03 + f l - M n ) ~ (B2 +/3-Mn) ~ (a + fl-Mn) (a + 7) --* a has occurred within the (a + I303) region, where the L phase has a monoclinic structure with lattice parameters a = 0.656 nm, b = 0.797 nm, c = 0.637 nm, and/3 = 109.4 deg. The L phase has never been observed in various Fe-A1-Mn, Fe-A1-Si, Fe-Mn-Si, and Mn-A1-Si alloy systems
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