where ΔH mix is the heat of mixing, ΔS mix is the entropy of mixing, and T is the absolute temperature. Usually, the entropy increases after mixing. Therefore, if ΔH mix < 0, the ΔG mix < 0, and the mixing reaction can occur spontaneously from a thermodynamic point of view. On the other hand, if ΔH mix > 0 the sign (positive or negative) of ΔG mix depends on the temperature. If the temperature is adequately controlled to make the enthalpy term larger than the entropy term, then ΔG mix > 0, and we can avoid the mixture of the two elements. Here we dip an A-B binary alloy precursor into a metallic melt consisting of element C. If the heat of mixing between elements B and C is negative, i.e., ΔH mix, B−C < 0 and if the heat of mixing between elements A and C is positive, i.e., ΔH mix, A−C > 0, then by controlling temperature adequately only element B dissolves from the precursor into the C melt; since element A is rejected from the C melt, it is expected to self-organize into a porous structure by surface diffusion in the same manner as that of the ordinary dealloying method in an aqueous solution . Figure 8.1 shows a schematic of this novel dealloying method that involves the selective dissolution of B atoms (orange) in the C atom melt (pink) and surface diffusion of the remaining A atoms (yellowish green). Figure 8.2 summarizes this “triangle” relationship in terms of the heat of mixing among elements A, B, and C required for the dealloying reaction in a metallic melt. We have to calculate the accurate value for the heat of mixing by considering the temperature and chemical composition for designing the dealloying reaction. However, this is sometimes complicated. The heat of mixing between the transition metals, and the transition metals and metalloids can be obtained from the table in Boer and Perrifor , the values of which are approximately calculated by the Miedema model, and that of other metals can be obtained from the table constructed by Takeuchi et al. . In our study, we first identify the candidates for elements A, B, and C from the tables in Boer and Perrifor  and Takeuchi and Inoue  and we then confirm the relationships A-B and B-C (mixture) and A-C (separation) by the related binary phase diagrams. Here, we summarize the preparation procedures for nanoporous metals by dealloying in a metallic melt, as they are schematically shown in Fig. 8.3.
Surface Improvement for Biocompatibility of Ti-6Al-4V by Dealloying in Metallic Melt
Schematic of the dealloying method using a metallic melt, where atom B (orange) dissolves into a melt composed of C atoms (pink), and the remaining atom A (yellowish green) self-organizes into a porous structure by surface diffusion
Triangle relationship of the enthalpies of mixing among elements A, B, and C for dealloying in a metallic melt
Schematic of the process of porous metal preparation using dealloying in a metallic melt
Selection of A-B-C elements, which satisfy the triangle relationship of the heats of mixing. (tables of values of heat of mixing and equilibrium phase diagrams can be used).
Preparation of the A–B alloy precursor.
Selective dissolution of element B from the A–B precursor into the C metal melt (formation of the porous structure).
Removal of the C element by etching with an acid or alkaline solution (the remaining A component must be inert in the solution).
The Ti-6Al-4V alloy, which consists of both α-Ti and β-Ti phases, is one of the promising biomedical materials among Ti alloys. However, the Al and V in this alloy are known to be cytotoxic elements. We attempted selective removal of the toxic element(s) from the surface of the Ti-6Al-4V alloy using dealloying with a metallic melt. In this section, we demonstrate the selected removal of Al as the first step for improving the biocompatibility of this alloy. Based on the triangle relationship of values of heat of mixing, the Mg melt can be used due to the negative enthalpy of mixing with Al and the positive enthalpy of mixing with both Ti and V. This relationship is illustrated in Fig. 8.4. Figure 8.5 and Table 8.1 exhibit SEM images and the corresponding results of EDX analysis of the Ti-6Al-4V surface dealloyed in a Mg melt at 1,148 K for 0.3–7.2 ks, respectively. An increasing immersion time resulted in the coarsening of the porous structure on the surface. Similarly, an increase in the immersion temperature from 1,048 to 1,148 K under the fixed immersion time of 1.2 ks resulted in the coarsening of the porous structure on the surface. It has been generally observed that the morphology and chemical composition of the dealloyed sample depend on the immersion time and temperature of the melt during dealloying treatment . An increased immersion time up to 1.2 ks at 1,148 K resulted in a slight decrease in Al concentration. However, a further increase in the immersion time resulted in an increase of Al content. This is probably due to the dissolution of Ti into the Mg melt, which is suggested by the observed concentration decrease in Ti with immersion time that became dominant after 1.2 ks. To confirm dissolution of Ti, a cp-Ti rod was immersed into a Mg melt at 1,184 K for 1.8 ks in a carbon crucible. The mass loss, which is defined by (mass loss) = (mass of initial cp-Ti) − (mass of treated cp-Ti), was estimated to be ~6 mg (Fig. 8.6). Therefore, dissolution of Ti was confirmed to occur in the Mg melt, although the heat of mixing between them is positive. Interestingly, it is found that the mass loss of cp-Ti is well suppressed when a Ti crucible is used, as shown in Fig. 8.6