Abstract
本實驗之目的在以合金法來改善 B2 結構之 NiAl 介金屬化合物的機械性 質。本實驗選擇以鑄造法來配製合金，並且僅在 1273K下做4 小時退火熱 處理以消除鑄造應力。實驗中，首先以鐵元素取代 B2-NiAl中之鋁元素來 改變其微結構及組成，並改變鐵含量來觀察其影響，合金之化學式為 Ni-25Al-xFe。 結果發現適量的添加鐵可將原先NiAl中之單相結構改變成 主要為 ordered bcc + fcc雙相結構之樹枝狀組織。在此樹枝狀組織中， 樹枝為 ordered bcc相而樹枝間主要為雙相之共晶組成。由於此樹枝間區 域可有效承受應變，所以此樹枝狀結構之合金可明顯提昇其室溫斷裂應變 。在 Ni-25Al-xFe合金系列中，其室溫機械性質之改變可以用平面應變及 平面應力條件之理論加以解釋之；而高溫機械性質之改變則是樹枝劈裂強 度與介面破壞強度兩者競爭之結果。為求進一步提昇合金之性質，本實驗 以 Ni-25Al-27.5Fe為母合金來添加鉻或鈮，並觀察其影響。結果顯示鉻 可增加樹枝間區域之體積百分比，從而增加室溫之斷裂應變。適量添加 鈮 (1 at.%)則可大幅提昇其機械性質，這是因為 (1)鈮原子可固溶強化 兩相之基地、(2) 維持樹枝間內之析出物以及 (3)增進介面強度所致。本 實驗中，所鑄造之(Ni-25Al-27.5Fe)-Cr 及 (Ni-25Al-27.5Fe)99Nb1等合 金在均質化熱處理後，其室溫斷裂應變均可達10%以上，尤其是 (Ni- 25Al-27.5Fe)99Nb1 合金之抗拉強度及降伏強度亦因鈮元素之強化效應而 分別達到 1330 MPa 及 632 MPa。因此可大幅提昇其應用潛力。 To improve the mechanical properties of B2-structured NiAl intermetallic compound by macroalloying method is the objective of this study. The original alloys were prepared by arc-melting and drop-casting, followed by annealing at 1273 K for 4 hours for stress relief. In this study, iron was selected to substitute aluminum in the B2-NiAl for investigating its effects on the microstructure and mechanical properties. The formula of this series is Ni- 25Al-xFe. The results reveal that the suitable additions of Fe can change the NiAl from a single- phased granular structure to a multi-phased dendritic one. Because the major plastic deformation could be supported by the fcc-phase in the interdendrite, the alloys with such a microstructure could possess better mechanical properties. In the Ni-25Al- xFe alloy system, the variation of the room- temperature tensile properties with Fe-content could be successfully explained by considering the conditions of plain strain or plain stress. In contrast, the variation of elevated- temperature tensile properties is a result from the competition of the cleavage strength of the dendrites with the fracture strength of the interdendrite. In addition, Cr and Nb were added to the Ni-25Al-27.5Fe alloy in order to further improve the mechanical properties. The results indicate that increasing chromium-content could increase the volume fraction of the interdendritic regions and subsequently enhance the room- temperature elongation. Furthermore, the mechanical properties of the Ni-25Al-27.5Fe alloy can be also remarkably improved by suitable additions of niobium, the (Ni-25Al-27.5Fe)99Nb1 alloy has an ultimate tensile strength of 1330 MPa and a yield stress of 632 MPa. All of these improvements let the alloys have a high potential for structural application.