Inconel 718 合金之δ相於熱處理與熱機處理之析出研究

Abstract

Inconel 718鎳基超合金在高溫擁有良好的機械性質與抗腐蝕性，但在部分的應用需要靠特殊製程來達成所需的顯微組織。例如渦輪引擎中的轉盤與軸承，其需要在400˚C左右能承受較大的流變應力以及抗疲勞性質，因此超細晶Inconel 718的製程被發展出來。有別於一般標準的Inconel 718製程，超細晶Inconel 718製程為利用高溫針狀析出之非契合δ相(Ni3Nb)，其溶解溫度超過Inconel 718再結晶溫度，在高溫加工發生再結晶時，形成阻礙晶粒成長的第二相，因而達到晶粒細化的效果。因此對於δ相的觀察以及熱加工製程參數的選擇對於IN-718晶粒細化非常重要。 本研究對於Inconel 718高溫熱處理析出之δ相作晶體結構研究，利用光學顯微鏡與電子顯微鏡觀察δ相的析出機制。δ相與基地的方位關係經由繞射圖形可以判定，利用TEM影像也可觀察到δ相成長與其交錯的情形。δ相成長由小段δ相聯結成針狀或板狀析出物，某些繞射圖形顯示δ相內部有部分含有γ”相的晶體結構。因此過時效δ相的析出和介穩γ”相有關。 並對880˚C、900˚C 和920˚C分別作5、10和15小時熱處理，利用影像分析軟體觀察不同熱處理條件下，δ相的析出量與析出速率。發現在900˚C時效將能在短時效時間得到大量δ相。隨後選擇兩種析出δ相的熱處理參數進行熱壓縮實驗以觀察晶粒細化的效果。其中，打斷針狀δ相以得到均勻細小顆粒分佈的壓縮量為60%，在1100˚C進行熱壓縮觀察到許多再結晶但δ相完全固溶。而當熱壓縮溫度在1025˚Ｃ時部分區域發生再結晶，大部分δ相固溶。熱壓縮溫度為950˚C時則未觀察到再結晶，幾無δ相固溶。本研究提供之晶粒細化製程建議為得到均勻δ相分布幫助再結晶晶粒細化之較適溫度為980~990˚C，此溫度以防止δ相快速固溶；壓縮量為60%並且應變率選擇應使之有足夠時間進行再結晶，若選擇高應變速率，則壓縮後需繼續持溫使之完成再結晶。 關鍵字: Inconel 718、delta相、熱處理、熱機處理、TEM

Nickel-based superalloy Inconel 718 is largely used due to its good mechanical properties and good corrosion resistance. For gas turbine engine disks and shafts, fatigue resistance, particularly low cycle fatigue resistance, may in fact be the limiting factor in the establishment of the useful operating lives of such components. Ultra fine grain 718 has been developed for this application. Overaged δ phase which are stable above recrystallization temperature used as pinning phase during recrystallization. Studying on δ phase and choosing good parameter for thermal mechanical process are important for grain refining Inconel 718. The research work used optical microscopy and transmission electron microscopy to observe the precipitation and crystallography of δ phase precipitated during high temperature aging in Inconel 718. The orientation relationship between matrix (γ) and δ phase was determined by several diffraction patterns. TEM images shows that the growth of needle-shaped or plate δ phase was connected by periods of small part δ phase. δ. Some TEM observation shows that γ” crystal structure existed inside the needle-shape precipitation. The precipitation and growth of overaged δ phase associated with metastable γ” phase. Thermal process results shows Inconel 718 aged at 900˚C precipitate larger amount of δ phase than other aging temperature. Select two thermal process precipitating δ phase for thermal mechanical process. The results was that after 60% compression the δ phase broken as globular shape and has uniform distribution. The compression at 1100˚C has apparent recrystallization and all δ phase been solid solute; compression at 1025˚C shows some recrystallization and almost all δ phase been solid solute. At 900˚C no recrystallization occurs and δ phase conserve during compression. The work results providing the good thermal mechanical process temperature for ultra fine grain 718 was 980~990˚C. 60% compression for uniform distribution and globular δphase.