Different undercooling degrees of Cu55Ni45 alloy were obtained by the combination of molten glass purification and cyclic superheating, and the maximum undercooling degree reached 284 K. The microstructure of the alloy was observed by metallographic microscope, and the evolution of microstructure was studied systematically. There are two occasions of grain refinement in the solidification structure of the alloy: one occurs in the case of low undercooling, and the other occurs in the case of high undercooling. Electron backscatter diffraction (EBSD) technology was used to analyze the rapid solidification structure under high undercooling. The features of flat polygonal grain boundary, high proportion of twin boundary, and large proportion of large angle grain boundary indicate recrystallization. The change in microhardness of the alloy under different undercooling degrees was studied by microhardness tester. It was found that the average microhardness decreased sharply at high undercooling degrees, which further confirmed the recrystallization of the solidified structure at high undercooling degrees.
- solidification structure
- rapid solidification
- grain refinement
The properties of materials are related not only to the composition but also closely to the existing form of structure. Because of its excellent mechanical properties, materials with fine and uniform grain structure are often directly cast into engineering application components or used as initial raw materials for further machining. Therefore, the means to make the material grain refinement and study the mechanism of grain refinement has been the focus of research [1,2,3].
There are many techniques for grain refinement, one of which is deep undercooling rapid solidification . The deep undercooling technology includes the deep undercooling of bulk metals, and examples of this technology include the molten glass purification method , the circulation superheating method , and the combination of molten glass purification and circulation superheating method . Another technique is the deep undercooling of small metal droplets, and examples of this technology include the emulsification thermal analysis method , the falling tube method , and the containerless electromagnetic suspension melting method . It is generally believed that only the undercooling degree of alloy or metal melt is required; the grain refinement will occur in the rapid solidification process and subsequent solidification stage.
To study the relationship between grain refinement mechanism and undercooling, scholars put forward the following theories to explain the grain refinement mechanism: (1) The dynamic nucleation mechanism proposed by Horvey  is mainly confirmed in pure nickel and some nickel base alloys. (2) Critical velocity theory, proposed by Willnecker . (3) Recrystallization mechanism [13, 14]. (4) Dendrite remelting and fracture mechanism, etc. The dendrite remelting effect includes two aspects; one is driven by solid–liquid interfacial tension proposed by Karma ; the other is caused by chemical superheating proposed by Li . It is generally believed that the grain refinement under low undercooling is caused by dendrite remelting and fracture in the process of recalescence and subsequent stages. However, there is no unified view on the causes of grain refinement under large undercooling, and there are still many disputes. Recrystallization mechanism is one of the mechanisms that may refine the grains under high undercooling. More and more people begin to verify it from the experimental point of view [17,18,19]. The recrystallization mechanism suggests that the recrystallization is induced by stress . When the undercooling degree of undercooled melt is greater than the critical undercooling degree, the proportion of primary solid phase formed in the early stage of rapid solidification will continue to increase and a continuous dendrite network will be formed. Solidification shrinkage and thermal strain induced liquid flow between dendrites will lead to stress accumulation in the primary dendrite skeleton. With the increase of undercooling, when the accumulated stress exceeds the strength limit of the alloy, the primary dendrite skeleton will undergo plastic deformation and stress fracture. The broken dendrite fragments store strain energy and provide driving force for subsequent recrystallization, which makes the grains refined into fine and uniform equiaxed grains.
In this paper, the microstructural evolution of Cu55Ni45 alloy has been systematically studied by means of the combination of molten glass purification and cyclic superheating. Two grain refinement phenomena have been found under low undercooling and high undercooling. The electron backscatter diffraction (EBSD) technique was used to characterize the grain refinement structure. The hardness test of the alloy showed that the sharp decrease of microhardness under high undercooling was the indirect evidence of recrystallization.
Pure copper (99.99%) and pure nickel (99.99%) weighed in a certain proportion are smelted in-situ in a vacuum arc melting furnace under an argon atmosphere to prepare Cu55Ni45 master alloy. The alloy was remelted at least three times to ensure that it was well-mixed, and 4 g sample was cut from each original ingot for experiments. The alloy sample and quartz tube were cleaned in the ultrasonic cleaner for 30 min to remove the surface impurities. After the alloy sample and quartz tube were dried, the sample was put into the tube, and the treated B2O3 purification agent was added. Finally, the quartz tube was put into the high frequency induction coil, the vacuum of the experimental furnace was pumped to 3 × 10−3Pa, and argon was backfilled to 5 × 10−2Pa. An infrared thermometer with a response time of 1 ms was used to record the temperature curve during the whole solidification process. First, the temperature was slowly raised to 750°C for 15 min to melt B2O3 and fully cover the alloy. Second, when the temperature was higher than the melting point of the alloy from 100 K to 200 K and kept for 15 min, B2O3 purifier could fully absorb impurities to inhibit heterogeneous nucleation. And finally, the cyclic super-heating method is used until the ideal degree of undercooling is obtained. The experimental diagram is shown in Figure 1.
After cutting, inlaying, and polishing, the cooled samples were etched with 20 ml HCl and 20 ml HNO3 mixed acid solution. Then, the morphology of undercooled microstructure was observed by metallographic microscope (OM, Leica DM2500M) and the pictures were taken. To observe the grain and grain boundary characteristics of the samples, the samples were polished with Al2O3 colloidal suspension for 10 h on a vibration polishing machine, and then the samples were detected by EBSD. Finally, the microhardness of all samples was measured by microhardness tester (HMV-2T), and the load was 490.3 mN for 15 s. It was ensured that at least 40 points were tested for each sample, and also that these were evenly distributed on the whole sample surface in a cross shape; and the average value was taken as the final hardness value.
Different undercooling degrees of Cu55Ni45 alloy were obtained by the combination of molten glass purification and cyclic superheating, and the temperature curve of alloy undercooling experiment was recorded by infrared thermometer. A sample was obtained every 10 K undercooling, and finally lots of samples with an undercooling range of 47 K–284 K were obtained. In the whole range of undercooling, the following microstructure transformation occurs:
0 K < Δ 54 K ≤ Δ 96 K ≤ Δ Δ
0 K < Δ
54 K ≤ Δ
96 K ≤ Δ
Through the analysis of microstructure in the whole undercooling range (47–284 K), we found that Cu55Ni45 experienced the evolution process of “coarse dendrite – equiaxed crystal – oriented fine dendrite – equiaxed crystal,” the grain refinement occurred at low and high undercooling, and the morphology of the two grain refinement structures was very different. Based on the analysis of the high undercooling of experimental samples, the microstructure with undercooling of 227 K, 274 K, and 284 K are polygonal equiaxed crystals, and there are a large number of annealing twins and a high proportion of large angle grain boundaries. It can be inferred that the microstructure with undercooling >284 K also has the same microstructure characteristics as above due to the second grain refinement.
The BCT  model can describe the dendrite precipitation process in undercooled melt and the evolution of four undercooling components with initial undercooling degree. The dendrite tip undercooling degree (Δ
In the evolution process of microstructure morphology of Cu55Ni45 alloy: when undercooling Δ
The solidification structure of Cu55Ni45 alloy undergoes first grain refinement at the undercooling degree of 54–96 K, and complete refinement occurs at Δ
Figure 3 shows several typical solidification recalescence curves of Cu55Ni45 alloy. With the increase of undercooling, the rapid solidification time becomes shorter and shorter, and the maximum recalescence temperature
The grain refinement structure of Cu55Ni45 alloy with low undercooling Δ
Here, we consider the grain boundary orientation difference less than 15°as small angle grain boundary. The solidification microstructure of Cu55Ni45 alloy with equiaxed grains, curved grain boundaries, and no annealing twins (Figure 2C). The red lines in Figure 4A represent small angle grain boundaries, and a large number of small angle grain boundaries can be clearly seen. In Figure 4B, different colors indicate different grain orientations, and the same or similar colors indicate the same orientation. A large number of grains with the same or similar orientations can be observed, and high-strength textures can be observed in Figure 4C.
Figure 4D is obtained by statistical calculation of grain boundary orientation difference, in which the small angle grain boundary orientation difference is as high as 69%. The mechanism of dendrite overheat remelting fracture proposed by Li  well explains the phenomenon of grain refinement at low undercooling. When the undercooled melt releases a large amount of latent heat of crystallization due to rapid solidification, the temperature of the system rises rapidly and exceeds the solidus temperature, resulting in the primary dendrite superheating remelting fracture, which refines into smaller equiaxed grains in the subsequent slow solidification stage.
The primary solid remelting fraction of alloy liquid during rapid solidification has a great relationship with undercooling . In agreement with results presented in previous studies, Li  proposed a formula for remelting fraction of primary solid phase:
The calculated results are in good agreement with the experimental results. The maximum remelting fraction is about 90% at the undercooling of about 75 K. The results show that the trend of dimensionless remelting fraction curve increases first and then decreases: the remelting fraction of Cu55Ni45 dendrite is very small at very low undercooling, and the corresponding microstructure is coarse dendrite. When the undercooling is 54–96 K, the dendrite remelting fraction is at a relatively high value, and a large number of equiaxed grains and proportions and relatively small dendrites can be observed in the corresponding microstructure. At moderate undercooling, the remelting fraction decreases, and the microstructure of the alloy in the undercooling range of 128–227 K presents dendrite. With the increase of undercooling, the dendrite shows strong orientation and gradually decreasing dendrite spacing. At high undercooling of 227 K, the remelting fraction has been significantly reduced, and the microstructure morphology is equiaxed grains with flat polygon. The proportion of remelted dendrites is very small, which is not enough to cause grain refinement. This shows that the grain refinement mechanism employed at high undercooling is a different one.
Grain refinement occurs when undercooling of Cu55Ni45 alloy is greater than 227 K. In contrast with the microstructure of the first grain refinement, the second grain refinement has smaller grain size, flat polygonal grain boundary, and annealing twins – which are not found in the first grain refinement . According to the remelting fraction calculated by the dendrite remelting formula, as shown in Figure 5, the proportion of dendrite remelting is very small under high undercooling, which is not enough to refine the grains. Stress induced recrystallization mechanism  is one of the potential mechanisms to refine the grains of the alloy. Stress induced recrystallization indicates that with the increase of the initial undercooling, the volume fraction of dendrite in the primary solid phase formed during rapid solidification will increase, and a continuous dendrite network will gradually form in the alloy liquid. Accordingly, the dendrite skeleton also begins to have strength, and then presents deformation resistance. When the undercooling degree of the undercooled melt is high, the accumulated stress will reach the strength limit of the alloy, and the dendrite framework will undergo plastic deformation, resulting in stress fracture. The broken dendrite fragments store the strain energy, which provides the driving force for the subsequent recrystallization behavior.
EBSD technology was used to characterize the grain refinement structure of Cu55Ni45 alloy with high undercooling Δ
The microstructure of Δ
Based on the solidification shrinkage and thermal shrinkage of undercooled melt during rapid solidification, Liu  proposed a stress accumulation model, which can semi-quantitatively estimate the stress accumulation in the solidification structure during rapid solidification. The specific formula is as follows:
Based on the above equation, the stress accumulation of undercooled Cu55Ni45 alloy melt during rapid solidification can be obtained, which is shown in Figure 7. The diagram can explain the transformation of microstructure from oriented fine dendrites to polygonal equiaxed grains (Figure 7B, 7C).The stress accumulated in the solidified structure of Cu55Ni45 alloy is very small at low undercooling, but quite large at high undercooling. With the continuous increase of undercooling, the accumulated stress will exceed the strength limit of the alloy, the fine dendrites will break up, and the fine polygonal equiaxed grains will be formed again in the subsequent recrystallization stage.
The dendrite and grain sizes were measured by measuring the dendrite trunk length and drawing lines, respectively. Figure 8A shows the evolution relationship between the average grain size and undercooling. All samples were tested on microhardness tester, as shown in Figure 8B, which shows the evolution relationship between microhardness and undercooling.
It is found that the solidification microstructure of Cu55Ni45 alloy is characterized by directional dendrite with a large undercooling range. In addition, the grain size of the alloy perceptibly decreases twice, and the grain size of the two refinements is also different. The average grain size of the alloy reaches 23 μm at the undercooling Δ
The maximum undercooling of Cu55Ni45 alloy is ascertained as 284 K by the combination of molten glass purification and cyclic superheating. Grain refinement occurs twice in the whole undercooling range of the alloy. EBSD technology is used to characterize the grain refinement structure to elaborate the grain refinement mechanism under high undercooling. The main conclusions are as follows:
The grain refinement of Cu55Ni45 alloy occurs at low undercooling and high undercooling. The morphology of grain refinement under low undercooling is spherical equiaxed grains with curved grain boundaries, while the morphology under high undercooling is polygonal grains with flat grain boundaries. Under high undercooling, the grain refinement structure, high proportion of large angle grain boundary and random grain boundary orientation, and a large number of annealing twins indicate that the microstructure recrystallizes under high undercooling, and the microhardness decreases sharply, which also confirms the mechanism of stress-induced recrystallization. In the solidification-recalescence curve, it is found that the maximum recalescence temperature of the low undercooling curve in the rapid solidification stage exceeds the solidus temperature of the alloy, which also explains the dendrite superheating remelting fracture mechanism under low undercooling. The results show that the grain size of the refined structure at high undercooling is smaller than that at low undercooling.
The grain refinement of Cu55Ni45 alloy occurs at low undercooling and high undercooling. The morphology of grain refinement under low undercooling is spherical equiaxed grains with curved grain boundaries, while the morphology under high undercooling is polygonal grains with flat grain boundaries.
Under high undercooling, the grain refinement structure, high proportion of large angle grain boundary and random grain boundary orientation, and a large number of annealing twins indicate that the microstructure recrystallizes under high undercooling, and the microhardness decreases sharply, which also confirms the mechanism of stress-induced recrystallization. In the solidification-recalescence curve, it is found that the maximum recalescence temperature of the low undercooling curve in the rapid solidification stage exceeds the solidus temperature of the alloy, which also explains the dendrite superheating remelting fracture mechanism under low undercooling.
The results show that the grain size of the refined structure at high undercooling is smaller than that at low undercooling.