鐵-9wt.%鋁-30wt.%錳-1.3wt.%碳合金於熱軋延狀態 之高溫下機械性質與其顯微結構之研究

Abstract

本篇論文是研究鐵-9wt.%鋁-30wt.%錳-1.3wt.%碳合金於熱軋延後未經熱處理過程於573K、773K以及973K下進行拉伸試驗，再透過光學顯微鏡、掃描式電子顯微鏡、X光繞射儀以及穿透式電子顯微鏡來觀察拉伸前後的顯微結構變化，並分析探討其高溫下之機械性質與破斷模式。 本合金成分於熱軋延後淬火狀態下之母相為完全沃斯田鐵相，而其中均勻分布奈米尺度之L'12結構之Ƙ'-碳化物，其體積比為32%、平均顆粒大小約為26nm，且為史賓諾多相分解之方式在母相中析出，並未發現明顯析出物於晶界上，可大幅增加合金常溫之機械性質。在573K下 其降伏強度、抗拉強度以及延伸率分別為462MPa、679 MPa和62.5%，在773K下則為374MPa、604MPa和43%， 973K下為296MPa、312MPa與46%，在573K可以發現晶界上有異質析出之Ƙ-碳化物受溫度與應力影響成長為較大顆粒，且使晶界附近缺乏碳原子，以致晶界附近之Ƙ'-碳化物消失，但因晶界上之異質析出之Ƙ'-碳化物而使晶界得以抵抗溫度，以致強度不會大幅下降。在973K下，異質析出之Ƙ-碳化物更加成長，但仍無法抵抗高溫。在不同溫度下合金皆在破斷面上出現酒窩狀凹洞，為延性破裂之特徵。

The mechanical properties and microstructures of the as-hot-rolled Fe-9wt. %Al-30wt. %Mn-1.3wt. %C alloy after being tensile tested at 573K, 773K and 973K were investigated by using optical microscopy, scanning electron microscopy, X-Ray diffraction and transmission electron microscopy, respectively. The mechanical properties and fracture modes were also investigated. The microstructure of the alloy in the as-hot-rolled condition was full austenite phase containing a high density of Ƙ'-carbides with L'12 structure (ordered face-centered cubic). The volume fraction of the Ƙ'-carbides was about 32% and the average particle size is about 26nm. The Ƙ'-carbides were formed by spinodal decomposition during quenching in the matrix, and with no precipitates were found on the grain boundaries. At 573K, the ultimate tensile strength (UTS), yield strength (YS) and elongation(El) is 679MPa, 462MPa and 62.5%, respectively 604MPa, 374MPa and 43% at 773K, 312MPa, 296MPa and 46% at 973K, respectively. The heterogeneous Ƙ-carbides were not only grown up on the grain boundaries but also consumed the carbon atoms besides at 573K. Additionally, the heterogeneous Ƙ-carbides can help the grain boundaries to resist the high-temperature fracture. Therefore, The mechanical properties will not decrease at high-temperature significantly. The heterogeneous Ƙ-carbides’ size was bigger at 773K than that at 973K, but it can no resistance high-temperature fracture. The dimple fractures were found on the fracture surfaces at each experimental temperature. It is the characteristic of ductile fracture mode of the as-hot-rolled alloy tested at high-temperature.