Inconel 718高溫時效相變態行為之研究

Abstract

本研究是針對榮剛材料科技股份有限公司所研發Inconel 718鎳基超合金預鍛棒材進行時效處理，利用顯微結構分析技巧，探討Inconel 718經高溫時效熱處理之相變態行為，尋求最佳熱處理製程參數，在熱機過程中利用δ相阻礙基地γ晶界移動，以達細化晶粒的效果。 將Inconel 718鎳基超合金在溫度900℃，分別經1.5、5、10和50小時之時效熱處理後，隨即空冷至室溫，並對不同時效時間的試片進行顯微結構分析。經1.5小時時效熱處理後，γ晶界初始生成針狀δ相組織，而γ晶粒內開始析出顆粒狀γ”相；經5小時時效熱處理後，γ”相大量析出而成長，且相同方位關係之γ”相會互相結合，而成對稱的透鏡狀組織，同時γ晶粒內亦有許多γ”相已逐漸轉變成δ相；經過10小時的長時間時效後，針狀δ相已由晶界向晶粒內成長且緊密排列，大部分的δ相析出物已經橫跨整個晶粒的範圍，而在晶粒內部互相交錯成長；經50小時時效熱處理後，晶粒內部已無法觀察到γ”相而，晶界與晶粒內部形成完全的穩態δ相組織。 由微硬度變化趨勢與顯微組織變化，經時效處理10小時，於顯微組織中觀察到階梯狀的晶界，顯示時效處理10小時所得之於晶界上與晶粒內適量析出的δ相，已扮演抑制晶粒成長與晶界滑移的效果，在此條件下Inconel 718具有最優異的硬度值；然而時效處理50小時，過量的δ相析出導致γ”相消失，不利於Inconel 718的機械性質。

This research aims to investigate the microstructure variation with different aging conditions in the Inconel 718 Nickel-based superalloy produced by the GMTC. In this thesis, the aging behavior of Inconel 718 at high temperature by microstructure analysis will be discussed. Furthermore, the optimization parameter of heat treatment for future Thermal-Mechanical Process will be obtained so that grain refinement can be achieved by hindering the migration of grain boundary. To study the microstructure variation with different aging conditions, Inconel 718 was aging treated for 1.5, 5, 10 and 50 hours at 900℃ and then air-cooling immediately. After aging treatment for 1.5 hours, needle shaped δ phase starts to nucleate at grain boundaries and γ’’ phase precipitates in the saturated matrix. After aging treatment for 5 hours, a large amount of γ’’ phase precipitates in the γ matrix and the precipitated γ’’ in the prior stage grows into a twice size. In this stage, several γ’’ phase which possess the same orientation with matrix starts to link head-to-head with each other and form a symmetrical lens shaped product. Large amount of γ’’ phase was transfer to δ phase in the grains at this stage. After aging treatment for 10 hours, the boundary-precipitated needle shaped δ phase grows into the matrix and arrange closely by further nucleation and growth. Most of δ needles grow across the whole spans of grains and inevitably intersect with each other. Approximately all γ’’ phase transforms into stable δ phase at the grain boundaries and in the matrix, after aging treatment for 50 hours. Investigating the relationship between the variation of Hv hardness and the change in microstructure, the highest Hv hardness (~261 HV) can be achieved among all experimental conditions by forming appropriate amounts of δ phase precipitation at grain boundary and matrix after aging treatment for 10 hours. The bugled grain boundary in the microstructure can be found. That means δ phase acts as an important role hindering the growth of grains. However, excess precipitation of δ phase sacrificing the amounts of γ’’ phase is a detrimental microstructure to the mechanical property in Inconel 718.