| Summary: | 碩士 === 國立臺灣科技大學 === 機械工程系 === 96 === We studied the phase transformation of an alloy which was undergone solution heat-treatment at a temperature of 1100oC and followed by aging processes at low temperatures ranging from 900 to 500oC. After the heat treatment, water quenching was utilized. The composition of the alloy is Fe-20wt%Mn-2wt%Al-0.4wt%C.
Needle-like martensites were dispersed within the austenitic matrix after the solution heat treatment. The martensites were composed of HCP structure and FCC twin. Shockley partial dislocations were believed to play an important role for the formation of needle-like martensite. The result is similar to that observed in the alloy with a composition of Fe-21wt%Mn-0.5wt%C. Thus, it is apparent that the addition of Al has no effect on the formation of needle-like martensite. Al lowers the stacking fault energy of the alloy. The movement of Shockley partial dislocation results in the formation of stacking fault. However, the different type of stacking faults could form HCP structure or FCC twins.
Both M23C6 carbide and pearlite-like structure precipitated at the FCC grain boundaries after the aging processes at temperatures below 600oC. The M23C6 carbide has a cubic-to-cubic orientation relationship with the FCC matrix, and its lattice constant was 1.96 nm, three times larger than the matrix. The pearlite-like structure is comprised of lamellar structure of ferrite and M23C6 carbide. In the pearlite-like structure, the Mn content was far richer in M23C6 carbide than that in ferrite. Thus, the M23C6 carbide is rich in Mn.
The upper limit temperature of the pearlite transformation was found between 600 to 550oC. The nose temperature of the TTT curves of the alloy is believed to be around 525 oC.
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