AN Hongen, Nancy Julius Siambun, Bih-Lii Chuab, Melvin Gan Jet Hong, LI Hongtao

(1. Faculty of Engineering, University Malaysia Sabah, Kota Kinabalu 88400, Malaysia; 2. College of Mechanical and Electronic Engineering, Huanghe Jiaotong University, Jiaozuo 454950, China; 3. Henan Engineering Technology Research Center on Intelligent Manufacturing Technology and Equipment, Jiaozuo 454950, China)

Abstract: Taking a Cu55Ni45 alloy as experimental alloy systems, we systematically studied the method of obtaining deep undercooling of alloy melt. Stable deep undercooling of alloy melt was obtained by the combination of molten glass purification and cyclic overheating. Combined with the nucleation and growth mechanism of undercooled melt, the microstructure evolution and grain refinement mechanism of the alloy were systematically studied in a wide undercooling range. The grain refinement solidification structure under large undercooling was analyzed by EBSD technology. Combined with the typical characteristics of recrystallization in metallographic pictures, it was finally confirmed that the grain refinement was caused by recrystallization.

Key words: microstructure refinement; undercooling; Cu-Ni alloys

1 Introduction

For a long time, the solidification structure of single-phase solid solution alloy has been deeply studied. Under the condition of deep undercooling, single-phase alloys generally solidify in the form of dendrites. When the initial undercooling degree of the alloy melt is greater than a certain critical value, the solidification structure changes from developed dendrites to fine equiaxed crystals, and the grain size decreases by about 2 orders of magnitude[1-10]. Kattamiset alfound that CuNi alloy presents developed dendrites when undercooling is less than 175 K and fine equiaxed crystals when undercooling is greater than 175 K. Recently, some researchers[4,11-14]found that the grain refinement phenomenon of columnar dendrite to equiaxed crystal also exists under small undercooling.

During the solidification of undercooled melt, most of them go through an obvious recalescence process. In this rapid recalescence process, the primary dendrites will be damaged to varying degrees due to strong thermal shock and stress shock, which will affect the final solidification structure. The common grain refinement phenomenon in undercooled single-phase alloy is a typical reflection. After investigating the relationship between chemical overheating and initial undercooling of Ni-Cu single-phase alloy melt, researcher had pointed out that the first grain refinement in Ni-Cu single-phase alloy is caused by dendrite remelting, and the second grain refinement is caused by recrystallization during solidification. After systematically studying the solidification law of deep undercooled Ni-Cu-(Cu) single-phase alloy, it was found that the single-phase Ni-Cu-(Cu) alloy also experienced two grain refinement during solidification. The first grain refinement under low undercooling is controlled by “fusing recrystallization” mechanism, while the grain refinement under high undercooling is controlled by “breaking recrystallization” mechanism. Karmaet alpointed out that whether the dendrite is broken depends on the dendrite breaking time ΔTbuand recalescence platform time ΔTplis determined by the size relationship. If ΔTbu＜ΔTpl, the dendrites are remelted by the solidification latent heat released by recalescence to form fine granular crystals ΔTbu＞ΔTpl, it is still dendrite. In this chapter, taking Cu55Ni45 single-phase alloy as a typical representative, the solidification structure evolution and grain refinement of the alloy under different undercooling degrees were deeply studied. At the same time, combined with the study of other Cu-Ni single-phase alloys, the deep undercooling rapid solidification law of single-phase alloys in Cu Ni peritectic system was discussed.

2 Experimental

The melt was purified by the combination of molten glass wrapping and cyclic overheating. The purification agent is B2O3glass. The purified glass used was fired from analytical pure B2O3. First, an appropriate amount of B2O3was put in a ceramic crucible and fully dehydrated at low temperature, then we heated it to 1 073 K for 5 h, and then cooled it with the furnace. The fired glass was broken and placed in the dryer for use. After mechanical grinding and derusting, the Cu and Ni block with purity of 99.8% was immersed in 10% HCl solution to fully remove oil stains and other impurities on the surface. After cleaning with anhydrous alcohol, it was dried quickly to prevent surface oxidation. In order to ensure the rapid fusion of the alloy, the weight of each fusion was controlled at about 5 g. During the experiment, the compacted Ni block was placed at the bottom of the quartz crucible, and then the pure Cu block was placed, which is covered with a certain amount of fired B2O3glass as the purifying agent and protective agent. Firstly, the pure Cu block was heated with low power, and the glass was melted by the heat of pure Cu and fully degassed, so that the molten glass was completely covered on the alloy surface to avoid the oxidation of Cu and Ni. Then increasing the power, after all Cu and Ni were melted, heat preservation, cooling, reheating and cooling cycles were implemented. After three cycles, we took out the sample. Finally, we broke all the alloys prepared for many times into small pieces, mixed them together, put them into a large crucible and melted them again to ensure uniform composition of the master alloy. The melting and purification of the experimental alloy were carried out in the same quartz crucible. In order to ensure that the cooling conditions of the sample are roughly the same, the weight of the sample is 5 g. The melting process is as follows: firstly, Cu-Ni master alloy and pure Cu block are placed in the quartz crucible, and then covered with a certain amount of fired B2O3glass as the purifying agent. Then, the induction furnace is heated and the cyclic overheating and molten glass purification process are implemented. In order to obtain large undercooling, the purification of alloy melt and deep undercooling rapid solidification are carried out simultaneously in vacuum high frequency induction furnace. Its micro purification technology includes the following three links: vacuum induction melting + molten glass purification + circulating superheat purification. The alloy liquid is purified by the thermal cycle method of heating, melting, overheating, heat preservation, and cooling. The adjustable process parameters are: vacuum degree, overheating temperature, melting rate, holding time, and cycle times.

3 Results and discussion

Within the undercooling range that can be achieved in the experiment, Cu55Ni45 alloys have undergone three obvious structural transformations, as shown in Fig.5. The solidification structure of undercooled alloy can be divided into four categories through four characteristic critical undercooling degrees: 54, 96, and 227 K. Within the range of small undercooling (ΔT＜54 K), the solidified structure is an ordinary dendritic structure with coarse primary and secondary dendrite arms, which is called small undercooling dendrite in this paper. With the increase of undercooling, the solidification structure changes into irregular granular crystals in a certain undercooling range (54 K＜ΔT＜ 96 K). In this paper, the granular crystals formed in the range of small undercooling are called the first type of granular crystals. With the continuous increase of undercooling, the solidified structure changes into dendrite again in the undercooling range (96 K＜ΔT＜227 K). Compared with small undercooled dendrites, the cross-sectional dimensions of primary and secondary arms of dendrites in this undercooling range are significantly smaller, which is called large undercooled dendrites.

Fig.1 Microstructure images of Cu55Ni45 alloy under different undercooling degrees: (a)ΔT=47 K; (b)ΔT=68 K; (c)ΔT=70 K; (d)ΔT=91 K; (e)ΔT=187 K; (f)ΔT=227 K; (g)ΔT=272 K; (h)ΔT=284 K

When the undercooling degree of the melt rises above 227 K, the solidification structure is obviously broken, and transforms into regular granular crystal again when ΔT* = 227 K. Its two-dimensional crosssection structure is similar to the annealing structure of Fe-C alloy. In this paper, it is defined as the second type of granular crystal.

With the gradual increase of the initial undercooling, for Cu55Ni45 alloy, when the undercooling is greater than 227 K, the primary coarse dendrite breaks again, that is, the second type of microstructure refinement occurs, showing irregular arrangement of fine equiaxed crystals. When the undercooling further increases, the microstructure of the alloy completely changes into uniform and fine equiaxed grains, accompanied by a large number of annealing twins. However, when the initial undercooling of the melt exceeds 240 K, the solidification structure is still fine equiaxed crystal, but the grain morphology changes obviously. We can clearly see that this structure is all fine equiaxed crystals with flat grain boundaries, most of them show polygonal morphology, the grain size distribution is very uniform, and contains a large number of annealing twins. The microstructure of the alloy sample was characterized by back scattering electron microscopy (EBSD) (see Fig.2). As shown in Fig.2(c), the polar diagram of the microstructure shows a random orientation. The causes of random orientation are as follows: during rapid solidification, the stress fracture of the primary dendrite network in the mushy region and the remelting fracture of dendrites after glow cause the random orientation of the rapidly solidified structure of the alloy. The recrystallization and grain growth of the deformed dendrites can not make the final microstructure appear new texture. The twin boundary fraction in the microstructure of the alloy exceeds 22% (see Fig.2(d)), and most of the grain boundaries belong to large angle grain boundaries.

Fig.2 EBSD characterization of Cu55Ni45 alloy with undercooling of 272 K: (a) Grain boundary diagram ofmicrostructureof 272 K undercooling alloy; (b) Grain boundary orientation diagram; (c) Pole diagram of (b); (d) Distribution diagram of grain boundary orientation difference of (b)

4 Conclusions

In this paper, we studied the solidification structure of typical binary single phase Cu55Ni45 alloy in detail under different undercooling. By EBSD testing of the refined grain structure, there is evidence of recrystallization under high undercooling conditions, and the solidification morphology of the undercooled melt is also analyzed. The results show that there are two kinds of grain refinement in the alloy. The first kind occurs in the low undercooling range and the second kind occurs above the critical undercooling. The two kinds of grain refinement come from completely different physical mechanisms. The main conclusions of this paper are shown as follows:

a) In the range of undercooling, the two alloy systems have similar solidification structure transformation process, there are twice grain refinement structure under the conditions of low undercooling and high undercooling respectively. The whole process of solidification structure transformation can be summarized as: coarse dendrites →fine equiaxed crystals →dense dendrites →fine equiaxed crystals.

b) The mechanism of grain refinement was also different. We proved that dendrite remelting caused by chemical overheating was the main cause of grain refinement under the condition of small undercooling. With the help of EBSD analyse, recrystallization has been proved to be the main mechanism leading to grain refinement under high undercooling.

c) The high stress generated in the rapid solidification process not only promotes the deformation and fragmentation of the dendrite network, but also stores it in the dendrite fragments in the form of strain energy as the driving force for high temperature recovery and recrystallization, so as to further refine the grains.