| Summary: | 碩士 === 國立臺灣科技大學 === 材料科技研究所 === 97 === We studied the phase transformations of a high manganese steel in the aging process. The composition of the steel is Fe-20.1 Mn-0.5 C (wt%). The alloy is a single phase of FCC at high temperature 1100℃. The wicker-shape precipitates in the FCC matrix are martensites which identified as either HCP or FCC twins.
When the aging temperature was below 700℃, we observed two kinds of carbides coexist along FCC grain boundaries. One is M23C6 and the other M3C carbide. When the aging temperature was below 550℃, two kinds of pearlites with either M3C or M23C6 carbides both appeared within FCC grains.
We observed the initial stage of the growing pearlite composing layer structure of M3C carbide and ferrite. M3C carbide first nucleated and grew along FCC grain boundaries. The carbon content of the FCC matrix near the M3C carbide was depleted. This situation causes the nucleation of ferrite grains in this region. Thus, ferrite formed lamella grains with M3C carbide. Besides the ferrite grain, the M3C carbide formed for the same reason for high carbon content of the FCC matrix near the BCC grain. Therefore, alternating layers of M3C and ferrite grains form pearlite layer structure.
In addition to the layers of M3C carbide in the pearlite structure, we also observed layers of M23C6 carbide, too. The orientation relationships between the M23C6 carbide and BCC grain are the K-S orientation relationships. The pearlite-like structures are composed of M23C6 carbide and BCC grains.
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