Solidification microstructure in a supercooled binary alloy

The maximum undercooling has achieved 270 K for Ni82Cu18 alloy by using molten glass purification combined with cyclic superheating technology. The infrared thermometer is used to record the temperature change of the whole alloy liquid during solidification and recalescence. Figure 2 shows the recalescence processes under several typical undercooling, and Figure 3 shows the maximum recalescence temperatures under different undercooling.

Fig. 2

Fig. 3

Figures 2 and 3 show that with the increase of undercooling, there is a gradual decrease in the rapid solidification time, the maximum recalescence temperature, and the slow solidification time after recalescence. When the undercooling is large enough, the slow solidification process can hardly be seen. Li [9] pointed out in the dendrite super-heating remelting mechanism that the latent heat of crystallization released during rapid solidification at small undercooling makes the temperature rise above the solid line and the primary dendrites remelt and refine due to superheat. We can see that the first grain refinement occurs in Ni82Cu18 alloy at ΔT = 76 k, and the microstructure is filled with a large number of fine equiaxed grains, as shown in Figure 5c.

The solid-phase temperature of Ni82Cu18 alloy is 1,392°C. When ΔT = 142 K, the maximum recalescence temperature just reaches the solid-phase line temperature (Figure 2), which indicates that the critical hypercooling is just reached at this time [18]. When ΔT > 142 K, the maximum recalescence temperature does not exceed the solid-phase line temperature (Figure 3), and the latent heat of crystallization released during the solidification process is not enough to make the dendrites remelted. However, when ΔT > 170 K, the second grain refinement occurs in Ni82Cu18 alloy, and a large number of fine equiaxed grains appear again in the microstructure, as shown in Figure 5h. This indicates that grain refinement at this time is not related to dendrite remelting. At present, the stress-induced recrystallization mechanism has attracted much attention. The grain refinement mechanism at large undercooling is further demonstrated below.

The heat released by the alloy during rapid solidification is converted into light signals, which can be captured using a high-speed video camera. The solidification process can be effectively presented by using the brightness of the solidification interface in the image [19]. The continuous solidification process of Ni82Cu18 alloy under different undercooling is shown in Figure 4, where the bright areas represent the condensed solids and the dark areas represent the undercooled melts. At small undercooling, the solidification process starts at the interface where contact is established between the alloy melt and B₂O₃, and the solidification front is a plane with a small angle, as shown in Figure 4a, b. Since most of the impurities are distributed at the interface and the quartz tube wall, the solidification velocity at the interface and on the quartz tube wall is obviously faster than that inside the undercooled melt. With the increase of the undercooling, the purification effect becomes gradually obvious. The solidification front becomes very smooth and the solidification position appears randomly, as shown in Figure 4c–e, and the phenomenon of nonuniform solidification velocity does not exist in different positions. We can see from Figure 4 that the solidification of the alloy always starts at one point and migrates from one side to the other, rather than crystallization growth at multiple locations. With the increase of undercooling, the solidification front becomes smooth, the solidification position is not fixed, and the solidification velocity of each part becomes uniform gradually.

Fig. 4

By analyzing the microstructure morphology of all samples, we found that the following transformation process exists in the microstructure of Ni82Cu18 alloy, as shown in Figure 5. The grain size of Ni82Cu18 alloy under different undercooling was studied systematically by drawing lines and measuring dendritic trunk length, as shown in Figure 6. Therewere four characteristic ranges in undercooled Ni82Cu18 alloy:

ΔT < 60 K. When the undercooling is in the range of 0–60 K, the microstructure is occupied by coarse dendrites surrounded by the remelted secondary dendrite arms, and there is no obvious directivity between the dendrites, as shown in Figure 5a. The grain size in this undercooling range is very large, as shown in Figure 6.

Fig. 5

Fig. 6

It can be seen from Figures 5 and 6 that two grain refinements occur in Ni82Cu18 alloy at different undercooling. The microstructural transformation of “coarse dendrite – equiaxed grain – directional fine dendrite – equiaxed grain” occurred in Ni82Cu18 alloy in the undercooling range. Compared with the first grain refinement, the grain boundary of equiaxed grain in the second grain refinement is straighter and the grain size is smaller, and there are annealing twins that the former does not have. This may be due to more accumulated stress at large undercooling, which causes recrystallization of the structure at the later stage of solidification [13]. The dendrite morphology in the range of 100 K < ΔT < 170 K is also quite different from that in the small undercooling. In the following sections, the microstructure transformation mechanism is further explained based on the BCT model, EBSD technique, TEM characterization, and microhardness.

The solidification microstructure of alloys with different undercooling degrees is quite different. The solidification microstructure of the alloy with small undercooling shows coarse dendrites, while the microstructure with large undercooling is very fine directional dendrites. This indicates that the factors controlling dendrite growth at different undercooling have changed. By analyzing the evolution process of different components against the initial undercooling, the microstructure transformation during the dendrite precipitation can be effectively explained. Figure 7 shows the evolution of component undercooling, dendrite tip radius, and dendrite growth velocity according to the BCT model, taking place resultant to the initial under-cooling in Ni82Cu18 alloy.

Fig. 7

As shown in Figure 7a, when ΔT < 40 K, the solute undercooling ΔTc is greater than the other three. This indicates that the dendrite growth of Ni82Cu18 alloy is mainly affected by solute diffusion, resulting in a very slow dendrite growth velocity (Figure 7b). In the range of 0–40 K, the solute undercooling gradually increases and the dendrite tip radius also increases. This directly results in the coarse-dendrite microstructure (Figure 8a). In the range of 40–60 K, the thermal undercooling ΔTt gradually exceeds the solute undercooling ΔTc, but the ΔTt and ΔTc are still in the same order of magnitude, which indicates that both thermal and solute diffusion control the dendrite growth of Ni82Cu18 alloy in this range. When ΔT = 48 K, there is a maximum value of solute undercooling ΔTc, so the dendrite tip radius also increases to a maximum value. Therefore, in the range of 40 K < ΔT <60 K, the dendrite growth velocity is still relatively slow and the dendrites in the microstructure are not only coarser but also more directional, as shown in Figure 8b.

Fig. 8

As the undercooling continues to increase, the thermal undercooling ΔTt and kinetic undercooling ΔTk rapidly increase, resulting in a rapid decrease of the dendrite tip radius and a rapid increase of the dendrite growth velocity. When ΔT > 100 K, thermal diffusion plays a dominant role in dendrite growth, and the microstructure is well-developed directional dendrites (Figure 8c). When ΔT > 170 K, the dendrite tip radius R does not decrease basically, but dendrites have been replaced by equiaxed grains in microstructure, as shown in Figure 5g, h. The following part will further explore the microstructure transformation mechanism at large undercooling.

The stress-induced recrystallization mechanism [13] indicates that the accumulated stress at large undercooling causes plastic strain in the microstructure. The plastic strain as the driving force causes recrystallization of the solidified structure in the late stage of recalescence. EBSD technology was used to characterize the microstructure at 210 K to obtain the orientation information and grain boundary characteristics of grains, as shown in Figures 9 and 10.

Fig. 9

Fig. 10

Recrystallization, driven by stored energy, replaces the original deformed grains by nucleation and growth of new grains. Therefore, under the driving force, the grain boundaries must have migrated. After recrystallization, the crystallographic orientations of adjacent grains must be different, with high-angle grain boundaries and stable annealing twins, and this has been confirmed in the study by Wang [16]. It can be seen from Figure 9b, c that the grain refinement does not introduce new texture, and the microstructure presents random distribution. Figure 10a shows the corresponding grain boundary character distribution. The high-angle grain boundaries are black (the misorientation angle is >15°) and the small-angle grain boundaries are green (the misorientation angle is in the range of 2°–15°). At large undercooling, the proportion of high angle grain boundaries is about 90%, and the proportion of twin boundaries reaches 13.6% (Figure 10b). The local misorientation can qualitatively determine the deformation degree and dislocation density of the microstructure, as shown in Figure 10c. Large areas of blue and small amounts of green and red indicate that the microstructure contains a small amount of dislocations. Most of the values in local misorientation distribution (Figure 10d) are small, which also indicates the low plastic strain in the microstructure. Figure 10e, f can more intuitively represent the recrystallization distribution. Where the recrystallized grains account for 35%, the substructures with a few dislocations exceed 50%, and the deformed grains are only 11%.

In the microstructure of ΔT = 210 K, high-angle grain boundaries and twin boundaries are in high proportion. The deformation degree of the microstructure is low, and most of the microstructure characterized by deformation is substructures with a few dislocations and recrystallized grains with stability and low energy. This indicates that the recovery and recrystallization have occurred in the microstructure after the critical undercooling [17].

According to the stress-accumulation model [13], the primary solid fraction is higher at large undercooling, and the interaction between flowing melt and primary dendrites results in stress accumulation. When the stress exceeds the yield strength of the alloy, the plastic strains occur in the dendrites and the microstructure is in an unstable and high-energy state. At the late stage of recalescence, the migration of grain boundaries has occurred in the microstructure, driven by the stored deformation energy, and new undistorted equiaxed grains are formed gradually. The solid fraction in the recalescence process is: (2) fSR=CPΔH(TR−Tn) f_S^R = {{{C_P}} \over {\Delta H}}\left( {{T_R} - {T_n}} \right) The stress accumulation can be expressed as follows: (3) σS=160μ⋅|a→|(fRS)2tfλ22×(1−fS(x→))fS(x→)⋅{|a→|⋅βS[fS(x→)−fcoh+1(1−fS)−1(1−fcoh)+ 2ln[1−fS(x→)1−fcoh]]−2(1+βS)αtherΔT0ΔT3GSQ[fS(x→)]} {\sigma _S} = {{160\mu \cdot \left| {\overrightarrow a } \right|} \over {{{\left( {f_R^S} \right)}^2}{t_f}\lambda _2^2}} \times {{\left( {1 - {f_S}\left( {\overrightarrow x } \right)} \right)} \over {{f_S}\left( {\overrightarrow x } \right)}} \cdot \left\{ {\matrix{ {\left| {\overrightarrow a } \right| \cdot {\beta _S}\left[ {\matrix{ {{f_S}\left( {\overrightarrow x } \right) - {f_{coh}} + {1 \over {\left( {1 - {f_S}} \right)}} - {1 \over {\left( {1 - {f_{coh}}} \right)}}} \cr { + \;2\ln \left[ {{{1 - {f_S}\left( {\overrightarrow x } \right)} \over {1 - {f_{coh}}}}} \right]} \cr } } \right]} \cr { - {{2\left( {1 + {\beta _S}} \right){\alpha _{ther}}\Delta {T_0}\Delta T} \over {3{G_S}}}Q\left[ {{f_S}\left( {\overrightarrow x } \right)} \right]} \cr } } \right\} The evolution of solid fraction and stress accumulation with initial undercooling of Ni82Cu18 alloy according to Eqs (2) and (3) is shown in Figure 11a, b.

Fig. 11

When the undercooling is small, the solidification velocity is small, the solid fraction is small, and the rapid solidification process cannot form the continuous dendrites network, so the stress cannot be accumulated between dendrites. When ΔT = 80 K, the primary solid fraction approaches 30%, and the continuous dendrites network began to form. The flowing melt and dendrite began to interact to accumulate stress between dendrites, as shown in Figure 11b. With the increase of solid fractions, the accumulated stress also increases. After critical undercooling ΔT^∗, the stress increases almost linearly. At the large undercooling, there is the larger stress in the microstructure (Figure 11b), while the microhardness drops sharply after the critical undercooling (Figure 12a), and a large number of hexagonal equiaxed grains appear in the microstructure (Figure 12b). This indicates that at large undercooling, the microstructure is in a stable state with low energy, and the plastic strains caused by stress as stored deformation energy promote the recrystallization of the microstructure, which can also be found in the rapid quenching and hypercooling alloy experiment of Yang [15], which concerns the undercooling experiment. This is strong evidence for “stress-induced recrystallization.”

Fig. 12