Refining Mechanism of 7075 Al Alloy by In-Situ TiB2 Particles

The nucleation undercooling of TiB2/7075 Al matrix composites, the microstructure observed after solidification at different cooling rate, and the size and distribution of TiB2 particles were investigated. The experimental results have shown that the grain sizes of TiB2/7075 Al matrix composites firstly decreased, then increased, and finally decreased again with the increase of TiB2 content. The nucleation undercooling of TiB2/7075 Al matrix composites first increased, then decreased, and finally increased again with the increase of TiB2 content when the cooling rates was 5 and 10 °C/min respectively, but kept decreasing with the increase of TiB2 content at a cooling rate of 20 °C/min. The melting and solidification process showed no significant change with the decrease of cooling rate in 9.0% TiB2/7075 Al matrix composites. Most small particles can act as heterogeneous nucleus, which induced grain growth and were captured into the grain by the solid/liquid interface. At the same time, most of the larger particles and a minority of the small TiB2 particles are pushed into the grain boundary; locating in the grain boundary can hinder the Al atoms from diffusing during the solidification process and restrain α-Al phase growth. The influence of particles shifted from dominating by locating to dominating by nucleation as the quantity of TiB2 particles increased.


Introduction
TiB 2 particles can improve the strength and modulus of aluminum alloy due to high hardness and modulus, and have been widely used in aluminum alloy as reinforcement and grain refiner, especially in the aeronautical industry as structural materials [1][2][3][4][5]. The effects of reactive conditions such as reaction temperature and the proportion of reactants on the refining effect of TiB 2 particles have been studied extensively [6][7][8][9][10][11][12]. However, until now, there has been no consensus on the exact mechanism for grain refinement involving the addition of Al-Ti-B-type grain refiners. Some calculations suggested that only less than one percent of TiB 2 particles nucleated a grain [13], but it strongly depended on two conditions in order to obtain efficient grain refinement: that a sufficient number of potential nuclei must be present in the melt, and a large fraction of the potential nuclei must be activated and effective [14]. A free-growth model based on the assumption of isothermal melt has been presented to predict the grain size on grain refinement by Maxwell and Greer et al. [15][16][17]. Greer et al. investigated the effects of the hypothetical size distributions of TiB 2 particles on as-cast grain size in Al alloys, and found that of the size distribution are preferable for an excellent grain refiner [18][19][20]. The particle density and the nucleation undercooling of the refining particles were two main parameters to control the transition from dendrites to equiaxed growth [21][22][23]. However, the density of effective particles cannot be easily estimated. In this paper, we present the nucleation undercooling of TiB2/7075 Al matrix composites by using differential scanning calorimetry (DSC), compare to the microstructure after solidification, and then investigate the size and distribution of TiB2 particles. Finally, the refining mechanism of the composites is discussed.

Experimental
K2TiF6 (mass > 97%) and KBF4 (mass > 97%) mixture were put into molten 7075 Al alloy with the composition Al-5.52Zn-2.36Mg-1.51Cu-0.18Si-0.26Fe-0.15Mn-0.25Cr (in mass %) at 850 °C. After being stirred at the same temperature for 30 min and degassed by using C2Cl6, 3.0% (in mass, similarly hereafter) and 4.5%, 6.0% and 9.0% TiB2/7075 Al matrix composites were prepared. The molten TiB2/7075 Al composites at 720 °C were casted in a graphite mold 15 mm in diameter. The liquidus and solidus temperatures of composites were measured by NETZSCH Q100 differential scanning calorimetry (DSC) with weight of about 8-9 mg, and the heating and cooling rates were 1 and 5, 10 and 20 °C/min between 400 °C and 700 °C, respectively. Samples were held for 3 min under the protection of argon when the temperature reached 400 °C with the same heating rate, then all samples were heated to 700 °C with the different heating rate (1 and 5, 10 and 20 °C/min, respectively). Finally, the temperature was decreased to 400 °C with the different cooling rate (1 and 5, 10 and 20 °C/min, respectively) after holding for 3 min again. After sanding and polishing, the specimens were etched in a solution of mixed acids (1% HF + 1.5% HCl + 2.5% HNO3 + 95% H2O).
The microstructures of composites were tested by ZEISS SUPRA55 scanning electron microscope (SEM) and CIKONG 4XCE optical microscope (OM). The grain size and area of the primary solid phase were analyzed statistically by a quantitative image analysis system (Image-Pro plus 6.0).
The mean grain size was denoted by the average gain diameter

The Microstructures of TiB2/7075 Al Matrix Composites with Different Cooling Rate
The microstructures of 3.0% and 4.5%, 6.0% and 9.0% TiB2/7075 Al matrix composites after being poured into the graphite mold are pictured in Figure 1. The average grain sizes of 3.0% and 4.5% TiB2/7075 Al matrix composites were 69 μm and 50 μm, respectively. However, larger dendrite grains could be found after adding 6.0% TiB2; the 9.0% TiB2/7075 Al matrix composite was also mainly composed of the dendrite grains smaller than that of 6.0% TiB2/7075 Al matrix composites.  The microstructures of the samples after their solidification inside the DSC crucible have been compared. Figures 2-4 show the microstructures of TiB2/7075 Al matrix composites with cooling rate of 20 °C/min, 5 °C/min and 1 °C/min from 700 °C to 400 °C respectively. The grain sizes of TiB2/7075 Al matrix composites firstly decrease, then increase, and finally decrease with the increase of TiB2 content. The mean grain sizes of 3.0% and 4.5% TiB2/7075 Al matrix composites reached 107 μm and 66 μm, respectively, with a cooling rate of 20 °C/min in Figure 2. The grain sizes of TiB2/7075 Al matrix composites increased with the decrease of cooling rate in Figures 3 and 4, and the mean grain sizes of 3.0% and 4.5% TiB2/7075 Al matrix composites reached 236 μm and 165 μm, respectively, with cooling rate of 1 °C/min in Figure 4. Large dendrites can be found in 6.0% TiB2/7075 Al matrix composites. Rosette grains in 9.0% TiB2/7075 Al matrix composites are also very strongly apparent in Figure 4.  The mean grain sizes of 3.0% and 4.5% TiB 2 /7075 Al matrix composites reached 107 µm and 66 µm, respectively, with a cooling rate of 20 • C/min in Figure 2. The grain sizes of TiB 2 /7075 Al matrix composites increased with the decrease of cooling rate in Figures 3 and 4, and the mean grain sizes of 3.0% and 4.5% TiB 2 /7075 Al matrix composites reached 236 µm and 165 µm, respectively, with cooling rate of 1 • C/min in Figure 4. Large dendrites can be found in 6.0% TiB 2 /7075 Al matrix composites. Rosette grains in 9.0% TiB 2 /7075 Al matrix composites are also very strongly apparent in Figure 4.  Figure 2. The grain sizes of TiB2/7075 Al matrix composites increased with the decrease of cooling rate in Figures 3 and 4, and the mean grain sizes of 3.0% and 4.5% TiB2/7075 Al matrix composites reached 236 μm and 165 μm, respectively, with cooling rate of 1 °C/min in Figure 4. Large dendrites can be found in 6.0% TiB2/7075 Al matrix composites. Rosette grains in 9.0% TiB2/7075 Al matrix composites are also very strongly apparent in Figure 4.

The Undercooling of TiB 2 /7075 Al Matrix Composites with Different Cooling Rate
The solidus temperatures of TiB 2 /7075 Al matrix composites decreased with the increase of heating rate; the liquidus temperatures of TiB 2 /7075 Al matrix composites were the opposite, but the solidus and liquidus temperatures of 9.0% TiB 2 /7075 Al matrix composites showed no remarkable change, as seen in Figure 5 and Table 1. T L and T s are the liquidus and solidus temperature, respectively; T I and T P are the initial solidification temperature and peak temperatures of solidification, respectively. The nucleation needs to satisfy the condition with ∆T > ∆T N [22], where ∆T N is nucleation undercooling. The solidus temperatures of TiB 2 /7075 Al matrix composites are on the decline, but the liquidus temperatures of TiB 2 /7075 Al matrix composites first decrease, then increase, and finally decrease again with the increase of TiB 2 content. The liquidus and solidus temperatures of 7075 Al alloy were 637.7 • C and 475.0 • C, respectively, with heating rate of 10 • C/min [24]. The gap between the liquidus and solidus temperatures was 162.7 • C, but the maximum and minimum temperature differences of

The Undercooling of TiB2/7075 Al Matrix Composites with Different Cooling Rate
The solidus temperatures of TiB2/7075 Al matrix composites decreased with the increase of heating rate; the liquidus temperatures of TiB2/7075 Al matrix composites were the opposite, but the solidus and liquidus temperatures of 9.0% TiB2/7075 Al matrix composites showed no remarkable change, as seen in Figure 5 and Table 1. TL and Ts are the liquidus and solidus temperature, respectively; TI and TP are the initial solidification temperature and peak temperatures of solidification, respectively. The nucleation needs to satisfy the condition with N T T    [22],   The initial solidification temperatures of 3.0% and 4.5% TiB 2 /7075 Al matrix composites decrease with the increase of cooling rate, but 6.0% and 9.0% TiB 2 /7075 Al matrix composites are the opposite. The peak temperatures of solidification in TiB 2 /7075 composites all decreased with the increase of cooling rate. The initial solidification temperatures of TiB 2 /7075 Al matrix composites also firstly decrease, then increase, and finally decrease again with the increase of TiB 2 content. The peak temperatures of solidification in TiB 2 /7075 Al matrix composites firstly increase, then decrease with the increase of TiB 2 content at cooling rate of 10 and 20 • C/min, respectively, but keep decreasing with a cooling rate of 5 • C/min. The nucleation undercooling of TiB 2 /7075 Al matrix composites firstly increase, then decrease, and finally increase with the increase of TiB 2 content at a cooling rate of 5 and 10 • C/min, respectively, but the nucleation undercooling of TiB 2 /7075 Al matrix composites keep decreasing at a cooling rate of 20 • C/min.

Effect of TiB 2 Content on the Nucleation of 7075 Al Alloy
According to the free growth model [15,16], two basic parameters are the density of effective particle and the degree of nucleation undercooling, which have an effect on dendrites to equiaxed growth in refined alloys. It is assumed that the nucleation is so potent that the initial aluminum nucleus can easily form a thin coating on the surface of the TiB 2 particles, and that the undercooling for free growth ∆T f g is the critical factor for grain initiation. Quested [26] also found the undercooling for free growth ∆T f g and the nucleant particle diameter d are simply related by where σ is the solid-liquid interfacial free energy, and ∆S V is the entropy of fusion per unit volume. The grain sizes of TiB 2 /7075 composites firstly decrease, then increase, and finally decrease with the increase of TiB 2 content from Figures 1-4, existing inversely proportional to the nucleation undercooling at cooling rates of 5 and 10 • C/min, respectively, by Equation (2). It can be found that the nucleation undercooling of TiB 2 /7075 composites firstly increase, then decrease, and finally increase with the increase of TiB 2 content at cooling rates 5 and 10 • C/min, respectively, from Table 1. However, it cannot truly reflect the effect of TiB 2 content on the nucleation of 7075 Al alloy at large cooling rates such as 20 • C/min, so the nucleation undercooling of TiB 2 /7075 composites keep growing with the increase of TiB 2 content. The nucleation undercooling of 3.0% and 4.5%, 6.0% and 9.0% TiB 2 /7075 composites at a cooling rate of 5 • C/min are 1.9 • C and 3.0 • C, 1.6 • C and 4.6 • C, respectively. The nucleation undercooling of 3.0% and 4.5%, 6.0% and 9.0% TiB 2 /7075 composites at a cooling rate of 10 • C/min are 5.5 • C and 7.3 • C, 3.3 • C and 4.6 • C, respectively. The gap of nucleation undercooling in TiB 2 /7075 composites at cooling rates of 5 • C/min and 10 • C/min are 3.6 • C and 4.3 • C, 1.7 • C and 0 • C, respectively, with the increase of TiB 2 content. The change of the nucleation undercooling reflects the change of the critical nucleation radius of particles. The large change of nucleation undercooling with cooling rate means a wide range in particle diameter; the small change means a narrow range of particle diameter. That is to say, the nucleation undercooling of 9.0% TiB 2 /7075 composites has almost no change with changing cooling rate, which means the range of particle diameters is relatively narrow. Figure 6 shows the SEM images of as-cast TiB 2 /7075 composites. Most of the TiB 2 particles appear to be located on α-Al grain boundaries; however, small agglomerates of TiB 2 particles could be found in the grains of 6.0% and 9.0% TiB 2 /7075 composites. TiB 2 particles are distributed in the grain boundary area; most particles were about 600-1000 nm in 3.0% and 4.5% TiB 2 /7075 composites in Figure 7a,b, and a minority of small TiB 2 particles could also be found. The size of large TiB 2 particles will decrease, the numbers of which will increase with increasing TiB 2 particles, but the number of small TiB 2 particles will also increase, because the heat of reaction of in-situ synthesis will also increase to refine the particles. More small TiB 2 particles in 6.0% TiB 2 /7075 composites dispersed near large TiB 2 particles in Figure 7c. Large TiB 2 particles are distributed in the grain boundary, and a large number of scattered small TiB 2 particles with size of about 400 nm were distributed in the grain near the grain boundary in Figure 7d. Decreasing of the size of large TiB 2 particles will narrow the range of the size distribution; for example, the range of particle diameter changed from 1000-50 nm to 600-50 nm with the increase of TiB 2 content, resulting in the change of nucleation undercooling.
The research has shown that the grain size varied from 73.58 µm to 12.75 µm, and then increased to 23.11 µm with the increase of reaction holding time [27]. At the same time, the mean size of TiB 2 particles also varied from 570 nm to 430 nm, and then increased to 530 nm. Quested [20] also found the same tendencies-that the grain sizes of Al matrix composites firstly decreased and then grew again with decreasing of the average diameter of TiB 2 particles, whereas the minimum grain size and the corresponding size of TiB 2 particles were different in our research. TiB 2 particles have the strongest nucleation abilities when the mean size of TiB 2 particles is 250-500 nm, as shown in Figure 1 of [20]. TiB 2 particles smaller than 600 nm can be found in the grain near the grain boundary in Figure 8a,c,d; most of the large TiB 2 particles and parts of small TiB 2 particles are distributed in the boundary area of grain in Figure 8b.
Materials 2017, 10, 132 7 of 11 a wide range in particle diameter; the small change means a narrow range of particle diameter. That is to say, the nucleation undercooling of 9.0% TiB2/7075 composites has almost no change with changing cooling rate, which means the range of particle diameters is relatively narrow. Figure 6 shows the SEM images of as-cast TiB2/7075 composites. Most of the TiB2 particles appear to be located on α-Al grain boundaries; however, small agglomerates of TiB2 particles could be found in the grains of 6.0% and 9.0% TiB2/7075 composites. TiB2 particles are distributed in the grain boundary area; most particles were about 600-1000 nm in 3.0% and 4.5% TiB2/7075 composites in Figure 7a,b, and a minority of small TiB2 particles could also be found. The size of large TiB2 particles will decrease, the numbers of which will increase with increasing TiB2 particles, but the number of small TiB2 particles will also increase, because the heat of reaction of in-situ synthesis will also increase to refine the particles. More small TiB2 particles in 6.0% TiB2/7075 composites dispersed near large TiB2 particles in Figure 7c. Large TiB2 particles are distributed in the grain boundary, and a large number of scattered small TiB2 particles with size of about 400 nm were distributed in the grain near the grain boundary in Figure 7d. Decreasing of the size of large TiB2 particles will narrow the range of the size distribution; for example, the range of particle diameter changed from 1000-50 nm to 600-50 nm with the increase of TiB2 content, resulting in the change of nucleation undercooling.
The research has shown that the grain size varied from 73.58 μm to 12.75 μm, and then increased to 23.11 μm with the increase of reaction holding time [27]. At the same time, the mean size of TiB2 particles also varied from 570 nm to 430 nm, and then increased to 530 nm. Quested [20] also found the same tendencies-that the grain sizes of Al matrix composites firstly decreased and then grew again with decreasing of the average diameter of TiB2 particles, whereas the minimum grain size and the corresponding size of TiB2 particles were different in our research. TiB2 particles have the strongest nucleation abilities when the mean size of TiB2 particles is 250-500 nm, as shown in Figure 1 of [20]. TiB2 particles smaller than 600 nm can be found in the grain near the grain boundary in Figure 8a,c,d; most of the large TiB2 particles and parts of small TiB2 particles are distributed in the boundary area of grain in Figure 8b.    The nucleation at low cooling rates can best reflect the influence of particles on microstructures.
We take the highest available value of  = 158 mJ/m 2 from contact-angle measurements, 6 3 1.112 10 / V S J K m    [16], it can be seen that the nucleant particle diameter of 4.5% and 9.0% TiB2/7075 Al matrix composites at a cooling rate of 5 °C/min is 188 nm and 123 nm, respectively, by Equation (2). From Figure 8e,f, more TiB2 particles are smaller than 200 nm (white dot in the picture) in the grain of 9.0% TiB2/7075 Al matrix composites, compared to 4.5% TiB2/7075 Al matrix composites. Comparing Figures 6-8, we come to the conclusion that most particles smaller than 600 nm can act as heterogeneity nuclei, which will induce grain growth, and then TiB2 particles will be captured into the grain by solid/liquid interface. At the same time, most particles larger than 600 nm and a minority of small TiB2 particles are pushed into the grain boundary by the solid/liquid interface; locating in the grain boundary can hinder the Al atoms from diffusing during the solidification of the 7075 Al alloy and restrain α-Al phase growth. The mean size of TiB2 particles is relatively larger in 3.0% TiB2/7075 composites, which particles value a major role locating in the grain boundary. With increasing of TiB2 particles, the number of TiB2 particles smaller than 600 nm will increase, resulting in the increase of the capability of heterogeneous nucleus, and nucleation is the main function of TiB2 particles at contents below 4.5%. That is to say, the parts of TiB2 particle valve a major role locating in the grain boundary, and others become the nucleus of α-Al phase. The influence of particles shifted from dominating by locating to dominating by nucleation as the quantity of TiB2 increased, which causes a change from dendrites to uniform rosette grain, and again to large dendrites. Meanwhile, small agglomerates of TiB2 particles could also be found in TiB2/7075 f In the grain The nucleation at low cooling rates can best reflect the influence of particles on microstructures. We take the highest available value of σ = 158 mJ/m 2 from contact-angle measurements, ∆S V = 1.112 × 10 6 J/Km 3 [16], it can be seen that the nucleant particle diameter of 4.5% and 9.0% TiB 2 /7075 Al matrix composites at a cooling rate of 5 • C/min is 188 nm and 123 nm, respectively, by Equation (2). From Figure 8e,f, more TiB 2 particles are smaller than 200 nm (white dot in the picture) in the grain of 9.0% TiB 2 /7075 Al matrix composites, compared to 4.5% TiB 2 /7075 Al matrix composites. Comparing Figures 6-8, we come to the conclusion that most particles smaller than 600 nm can act as heterogeneity nuclei, which will induce grain growth, and then TiB 2 particles will be captured into the grain by solid/liquid interface. At the same time, most particles larger than 600 nm and a minority of small TiB 2 particles are pushed into the grain boundary by the solid/liquid interface; locating in the grain boundary can hinder the Al atoms from diffusing during the solidification of the 7075 Al alloy and restrain α-Al phase growth. The mean size of TiB 2 particles is relatively larger in 3.0% TiB 2 /7075 composites, which particles value a major role locating in the grain boundary. With increasing of TiB 2 particles, the number of TiB 2 particles smaller than 600 nm will increase, resulting in the increase of the capability of heterogeneous nucleus, and nucleation is the main function of TiB 2 particles at contents below 4.5%. That is to say, the parts of TiB 2 particle valve a major role locating in the grain boundary, and others become the nucleus of α-Al phase. The influence of particles shifted from dominating by locating to dominating by nucleation as the quantity of TiB 2 increased, which causes a change from dendrites to uniform rosette grain, and again to large dendrites. Meanwhile, small agglomerates of TiB 2 particles could also be found in TiB 2 /7075 composites with the increase of TiB 2 content, because the tiny particles have a larger surface energy, even in 4.5% TiB 2 /7075 composites.

Conclusions
(1) The grain sizes of TiB 2 /7075 Al matrix composites firstly decreased, then increased, and finally decreased again with the increase of TiB 2 content. The grain sizes of TiB 2 /7075 Al matrix composites increased with decreasing cooling rate, but the changes were not very noticeable from 20 • C/min to 5 • C/min. The mean grain size of 3.0% and 4.5% TiB 2 /7075 Al matrix composites could reach 236 µm and 165 µm, respectively, at a cooling rate of 1 • C/min. (2) The liquidus temperatures of TiB 2 /7075 Al matrix composites firstly decreased, then increased, and finally decreased again with the increase of TiB 2 content. The initial solidification temperatures and the nucleation undercooling of TiB 2 /7075 Al matrix composites firstly increased, then decreased, and finally increased again with the increase of TiB 2 content. The melting and solidification process showed no significant change with the decrease of cooling rate in 9.0% TiB 2 /7075 Al matrix composites. A large change of the nucleation undercooling with cooling rate meant a wide range in particle diameter in TiB 2 /7075 composites, so a small change meant narrow range. (3) Most small particles can act as heterogeneous nuclei, which induced the grain growth and were captured into the grain by solid/liquid interface. At the same time, most of larger particles and a minority of small TiB 2 particles were pushed into the grain boundary, locating in the grain boundary can hinder the Al atoms from diffusing during the solidification of the 7075 Al alloy and restrain α-Al phase growth. The influence of particles shifted from dominating by locating to dominating by nucleation as the quantity of TiB 2 particles increased due to the decrease of the average diameter of TiB 2 particles.