Characteristics, Mechanism and Criterion of Channel Segregation in NbTi Alloy via Numerical Simulations and Experimental Characterizations.

Baohui Zhu,Yanchang Liu,Paixian Fu,Dianzhong Li,Yun Chen,Hongwei Liu,Zhenzhen Chen,Yikun Luan,Yanfei Cao,Xiuhong Kang

doi:10.3390/ma14040796

Abstract

Channel segregation (CS) is the most typical defect during solidification of NbTi alloy. Based on numerical simulation and experimental characterizations, we deeply elucidated its characteristics, formation mechanism, effecting factor and prediction criterion. According to acid etching, industrial X-ray transmission imaging, 3D X-ray microtomography and chemical analysis, it was found that in a casing ingot, by He cooling, finer grain size, weaker segregation and slighter CS can be obtained compared with air-cooled ingot. The simulation results of macrosegregation show that CS is caused by the strong natural convection in the mushy zone triggered by the thermo-solutal gradient. Its formation can be divided into two stages including channel initiation and growth. In addition, due to the stronger cooling effect of the He treatment, the interdendritic flow velocity becomes smaller, consequently lowering the positive segregation and CS and improving the global homogenization of the final ingot. Finally, to predict the formation of CS, the Rayleigh number model was proposed and its critical value was found to be 15 in NbTi alloy for the first time. When it is lower than the threshold, CS disappears. It provides an effective tool to evaluate and optimize the solidification parameters to fabricate the homogenized NbTi ingot in engineering practice.

Highlights

As the common material of low-temperature superconductors, NbTi alloy has been widely used in high-energy physics, controlled thermonuclear fusion, energy storage and magnetic levitation considering its superior machinability and high critical current density [1,2,3]
The positive segregation in the top, the negative segregation in the bottom and channel segregation in both sides of the ingot body are reproduced, which are totally consistent with the typical types of macrosegregation in the casing ingot
Nb-47 wt.% Ti alloy are elucidated in detail, and the main conclusions are summarized below

Summary

Introduction

As the common material of low-temperature superconductors, NbTi alloy has been widely used in high-energy physics, controlled thermonuclear fusion, energy storage and magnetic levitation considering its superior machinability and high critical current density [1,2,3]. Owing to the large density difference between Nb and Ti, the severe Ti segregation occurs during solidification of ingots, which always causes the breakage of NbTi superconducting wire [4,5]. In NbTi alloy, channel segregation (CS, namely freckle, chimney) is the most common type of macrosegregation during solidification. It cannot be eliminated by the subsequent deforming and heat treatment processes considering its large scale of width in ~mm and length in ~cm. Due to the thermal-solutal distribution feature of the ingot, CS is usually located in the 1/3–2/3 radius and, it cannot be cut off by punching or peeling

Methods

Results

Conclusion