Melting of Pb clusters without free surfaces

Melting behaviors of Pb clusters without free surfaces have been studied using molecular dynamics and the Sutton-Chen (SC) potential. Two Pb clusters $({\mathrm{Pb}}^N,$ with $N=201\mathrm{}$ and 249) are selected and each of them has been embedded in the core of an ${\mathrm{Al}}^{4033}$ cubo-octahedron, but with different orientation relationship with Al coatings. For the ${\mathrm{Pb}}^{201}$ cluster, where a parallel cube-cube relationship exists with the Al lattice, semicoherent interfaces can be formed and the core can be superheated up to 750 K, in comparing to the equilibrium melting point of $615\pm 10\mathrm{K}$ for bulk Pb predicted using the SC potential. On further elevating the temperature, a simultaneous melting of both the Pb core and the outer Al coating will occur. For the ${\mathrm{Pb}}^{249}$ cluster without a parallel orientation relationship with Al, no effective semicoherent interface can be formed, and the core premelts at about 500 K without breaking the crystalline structure of Al shells. The melting point of the Pb inclusions can be predicted using thermodynamic relations. For different Pb-Al interfaces, two melting mechanisms exist: one is the homogeneous melting which leads to a melting point elevation and the other is the heterogeneous melting which can induce a melting point depression.