Grain Refining Efficiency and the Role of Alloying Elements in Determining the Nucleation Potency of LaB 6 in Aluminum A
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https://doi.org/10.1007/s11837-019-03970-2 2019 The Minerals, Metals & Materials Society
ALUMINUM AND MAGNESIUM: CASTING TECHNOLOGY AND SOLIDIFICATION
Grain Refining Efficiency and the Role of Alloying Elements in Determining the Nucleation Potency of LaB6 in Aluminum Alloys LIJUN JING,1 TAO LU,1 and YE PAN1,2 1.—School of Materials Science and Engineering, Jiangsu Key Laboratory of Advanced Metallic Materials, Southeast University, Nanjing 211189, China. 2.—e-mail: [email protected]
This work discloses the refinement mechanism of LaB6 in three binary aluminum alloys via grain refinement experiments, model calculation, and cooling curve analysis. The refining efficacy of LaB6 in aluminum was influenced by an interfacial relationship which can be controlled by alloying with different solute elements. Al atoms attach directly onto the surface of the LaB6 substrate during solidification. Because the lattice parameter of a-Al increased after alloying with larger metal atoms, the lattice mismatch between LaB6 and Al matrix decreased. This investigation reveals that the nucleating behavior of LaB6 can be influenced by adding solute Si, Mg, and Cu, and thereby offers a pathway for controlling the content of aluminum alloys.
INTRODUCTION Grain refinement is a practical method to improve the mechanical properties and metallurgical quality of aluminum castings. In recent decades, much work has been focused on the mechanism of grain refinement in aluminum and its alloys.1–4 A series of refinement theories have been proposed, such as the carbide/boride theory,5 the phase diagram theory,6 and the solute theory.7,8 Although there are still some unaccountable results, it is generally accepted that both nucleant particles and solute elements contribute to the grain refinement phenomenon.9,10 Hence, the characteristic of both nucleant particles and solute elements should be taken into consideration when investigating refinement phenomena. According to traditional solute theory, the growth restriction factor, Q, is utilized to identify the effect of solute elements in aluminum on the final grains.11–13 Easton et al.12 indicated that Q was a measure of how rapidly the constitutionally undercooled zone was formed at the earliest stages of growth. It was found that grain size is related to solute content by a semi-empirical relationship: dgs ¼ a þ b=Q
ð1Þ
where dgs is the grain size, a is a constant related to the maximum number of particles that can be successfully activated as nucleants, b is another constant related to the nucleation potency of heterogeneous particles, and Q is the growth restriction factor which equals mC0(k 1). StJohn et al.14 developed the interdependence theory on the basis of Eq. 1. The interdependence theory links nucleant selection and grain growth together and assumes that grain formation is the result of the interdependence between nucleation and growth. StJohn indicated that Eq. 1 can be expressed as: D zDTn ð2Þ þ xSd dgs ¼ xnfz þ xSd ¼ 5:6 vQ where xnfz represents a nucleation-free zone in
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