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Transformation Mechanism of (γ + γ') and the Effect of Cooling Rate on the Final Solidification of U720Li Alloy

Zhao Guangdi Yang Guoliang Liu Fang Xin Xin Sun Wenru

文摘	The transformation mechanism of (γ + γ') was studied by analyzing the microstructure and elemental distribution of the U720Li samples heated at 1250 °C and cooled at the rates in the range of 1-100 °C/s. Although the (γ + γ') is deemed to be formed by a eutectic reaction and has been called eutectic (γ + γ'),it was found in the present study that the (γ + γ) precipitation begins with a peritectic reaction of (L + γ) → γ',and develops by the eutectic reaction of L → (γ + γ'). The energy for the γ' nucleation is low because the interfacial energy for the γ/γ' interface is about one-tenth of the solid/liquid interface, and hence, the nucleation rate is high and the fine structure of (γ + γ') is formed at the initial precipitation stage. The γ and γ' in (γ + γ') tend to grow into a lamellar structure because it is difficult for them to nucleate directly from the residual liquids, and hence, the γ' precipitates naturally tend to grow divergently direction of the regions rich in Al and Ti, forming a fan-like structure of the (γ + γ'). As a result, the γ' precipitates will coarsen finally because the space between them is enlarged. The solidification of the final residual liquids is a diffusion dependent process. When cooled at a higher rate, a higher degree of super cooling is reached and finally the solidification is finished by the pseudo-eutectic reaction of L → (γ + boride) and L → (γ + η), which can absorb Zr and B. When cooled at a rate low enough,most of the residual liquids are consumed by the (γ + γ') growth due to the sufficient diffusion, and the boride and Zrbearing phase are precipitated at a quasi-equilibrium state. Under this condition, Ti is depleted at the (γ + γ') growth front. However, the η-Ni_3Ti phase is formed there occasionally due to the boride precipitation, because the compositions of the two phases are complementary.
来源	Acta Metallurgica Sinica(English Letters) ,2017,30(9):887-894 【核心库】
DOI	10.1007/s40195-017-0566-7
关键词	U720Li ; (γ + γ') formation mechanism ; Cooling rate ; Solidification
地址	Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016
语种	英文
文献类型	研究性论文
ISSN	1006-7191
学科	金属学与金属工艺
文献收藏号	CSCD:6065929

参考文献共 17 共1页

引证文献 4 篇

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