鐵-30wt.%錳-9wt.%鋁-1.5 wt.%碳合金鑄件顯微結構與其機械性質之研究

Abstract

本篇論文主要利用光學顯微鏡(OM)、掃描式電子顯微鏡(SEM)以及穿透式電子顯微鏡(TEM)來探討Fe-30wt.%Mn-9wt.%Al-1.5wt.%C合金鑄件於鑄造狀態、固溶狀態和450℃時效狀態之顯微結構，並探討其機械性質與破壞行為。 Fe-30wt.%Mn-9wt.%Al-1.5wt.%C合金鑄件在鑄造狀態下其顯微結構在晶粒內為樹枝狀的γ和 γ⁄κ–碳化物共晶，且在晶界上也有κ-碳化物。經過1100℃ 36小時固溶處理後，晶粒內鑄造成分偏析和晶界上的κ-碳化物完全消除，在淬火狀態下即有緻密奈米級的κ'-碳化物在γ基地內均勻整合析出，而κ'-碳化物是在淬火過程中藉由spinodal decomposition相分解反應所產生，具有L'12結構，其體積比為36%，平均顆粒大小則約為8~10nm。固溶處理後經450℃ 12小時以內時效處理，緻密奈米級的κ'-碳化物沿˂100˃方向性成長，其體積比成長為46%，平均顆粒大小則約為12~14nm，晶界上無明顯析出物，但經450℃ 15小時時效處理後，晶界上有明顯κ-碳化物產生。 Fe-30wt.%Mn-9wt.%Al-1.5wt.%C合金鑄件經1100℃ 36小時固溶處理後，其機械性質為降伏強度(YS):600MPa，抗拉強度(UTS):720MPa，伸長率(El):66% ;經450℃ 12小時時效處理後其機械性質提升至YS:920 MPa，UTS:970MPa，而El:29%。固溶處理及450℃ 12小時時效處理後其破斷面皆為酒窩狀組織，破壞行為皆屬於典型的延性破壞;450℃ 15小時時效處理後破斷形態為延性破壞與沿晶脆性破壞共同主導。

The microstructures of Fe-30wt.%Mn-9wt.%Al-1.5wt.%C cast alloy in as-cast,solution heat-treatment (SHT) and aging conditions were investigated by using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy(TEM). Besides,the mechanical properties and tensile-test fracture mechanisms were also investigated. The microstructure of the Fe-9wt.%Al-30wt.%Mn-1.5wt.%C alloy in the as-cast alloy isγdendrite and γ⁄κ–carbideeutectic. After being solution -heat-treated (SHT) at 1100℃ for 36 h and then quenched,the casting segregation would disappear completely. In the as-quenched condition, an extremely high density of nano-sized κ'-carbides was formed within the γmatrix by spinodal decomposition during quenching. The fineκ'-carbide has an ordered L'12 structure.The average size and volume fraction of the κ'-carbides in the as-quenched alloy are about 8~10 nm and 36 %,respectively. After being aged at 450 ℃ for 12 h, the nano-sizedκ'-carbides grew along the ˂100˃ directions and no grain boundary precipttates could be detected.The average size and volume fraction of the κ'-carbides are about 12~14 nm and 46 %, respectively. After being aged at 450 ℃ for 15 h, it was found that coarseκ-carbides started to appear on the grain boundaries. The mechanical property of the as-quenched alloy was yield stress(YS): 600MPa, ultimate tensile strength(UTS): 720MPa, elongation(El): 66%; After being aged at 450 ℃ for 12 h, the alloy could possess maximum strength (YS: 920 MPa, UTS: 970 MPa) with a good elongation of 29%. The fractographs of both the as-quenched and aged at 450℃ for 12 h alloys indicate that the fractured behavior belong to ductile fracture with fine and deep dimples. However, the fractograph of the aged at 450℃ for 15 h alloy shows the breaking formation was contributed by ductile and intergranular brittle failure simultaneously.