| Summary: | 碩士 === 國立清華大學 === 材料科學工程學系 === 100 === This work studied the effect of heat treatment on precipitation of secondary carbides and their effects on the properties of a new cobalt base superalloy–X alloy. The X alloy is strengthened by carbide precipitations. The as-cast alloy have three kinds of primary carbides: M(Cr, Co)7C3 and M(Cr, Co, W, Mo)6C in the interdendrite region, and Chinese script-like NbC in the dendrite. Since M7C3 carbides are stable only in the high temperature range, heat treatment at 1000 ˚C for 4 hours would cause the dissolution of the M7C3 carbides and the precipitation of secondary carbides M23C6 and M6C. When aged at 1106 ˚C for 4 hours, the precipitation of secondary carbides M7C3 and M6C were observed. If solution heat treated at 1220 ˚C for 12 hours, the area fraction of Chinese script NbC phase would increase and some of the NbC arms extend to touch the interdendrite eutectic area. By the following aging treatment at 1000 ˚C for 4 hours, wider precipitate free zone and coarser secondary carbides in less number were observed that would detrimental to the corrosion resistance and mechanical properties.
The precipitation of the secondary carbides increases the strength and wear resistance of the alloy. However, the precipitation of the secondary carbides decreases the corrosion resistance of X alloy in 3.5wt% NaCl (pH=7) and 0.5 M (1 N) H2SO4 aqueous solution at ambient temperature by reducing the Mo and Cr content in the matrix. The corrosion resistance of X alloy can be improved by the 1200 ˚C / 2 h solution heat treatment following the 1106 ˚C / 4 h aging. When tested in 1 M (1 N) KOH aqueous solution at ambient temperature, all the carbides in the alloy were preferentially attacked no matter how it was heat treated.
|
|---|
