CM247LC高溫合金單晶凝固過程中MC碳化物 對γ/γ′共晶反應(yīng)的影響

CM247LC高溫合金單晶凝固過程中MC碳化物 對γ/γ′共晶反應(yīng)的影響Influence of MC Carbides on the Formation of γ/γ′ Eutectics during Single Crystal Directional Solidification of Superalloy CM247LC

在0.2 mm/min的抽拉速率下對高溫合金CM247LC進(jìn)行了單晶定向凝固實(shí)驗(yàn)，并對液淬的糊狀區(qū)內(nèi)組織進(jìn)行了觀察分析。結(jié)果顯示，合金凝固時首先是γ枝晶作為領(lǐng)先相長出，然后初生MC碳化物在γ枝晶尖端稍后的位置開始形成，在凝固最后階段γ/γ′共晶體從液體中析出。值得注意的是，γ/γ′共晶反應(yīng)是以化學(xué)成分和晶體結(jié)構(gòu)都完全不同的MC碳化物作為異質(zhì)形核核心，而不是依附在形核條件更好的γ相上。檢測結(jié)果表明，在MC碳化物基底上形成的γ/γ′共晶體具有雜亂的晶體取向，與單晶γ相基體的晶向完全不同。這說明，宏觀上為單晶體的高溫合金鑄件實(shí)際上可能含有許多微小雜晶晶粒，使得鑄件的單晶性受到影響。

The structure formation of superalloys is very complicated because of their multicomponent composition and multiphase transition processing. Duo to the limitation of some pre-conditions, the structure formation can’t be accurately determined by thermodynamic calculation method. Knowledge about the structure is critical for the design of the following heat treatment process. In this work, a single crystal (SC) sample of superalloy CM247LC was directional solidified in a labor Bridgman-furnace with a velocity of 0.2 mm/min and quenched, to investigate the solidification sequence including MC carbide and γ/γ′-eutectic. It was observed that the γ-phase is firstly formed in the form of dendrites; it is then followed by the homogeneously precipitation of MC carbides from the liquid behind dendrite tips. Near the end of solidification the interdendritic residual liquid transits into γ/γ′-eutectics. It is interesting to found that the γ/γ′ eutectics don’t nucleate on the existing γ-phase, but preferably on the MC carbides which have completely different chemical composition and crystal structure. The result of EBSD examination indicates that the γ/γ′ eutectics formed on the MC substrates have random crystal orientations compared to the SC γ-matrix, exhibiting the misoriented multi-crystal microstructure in the so called “single crystal” superalloy casting.

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