Abstract
The microstructure of second-phase particles in the Ti-55 alloy (Ti-5Al-4Sn-2Zr-1Mo-0.25Si-1Nd) was studied by scanning electron microscopy, transmission electron microscopy (TEM), and high-resolution electron microscopy (HREM) observations. The second-phase particles in the conventional ingot-cast Ti-55 alloy of 1 to 15 µm in diameter and uniform distribution in matrix were observed, where the majority of these particles are elliptical. The mean free path between the particles is about 46 µm, and the volume fraction (pct) is 2.35. The second-phase particles typically contain Nd, Sn, and O in substantial amounts, and the content of Nd is the largest in the three elements. The elements Ti, Al, Zr, Mo and Si are depleted in the particles. The second-phase particle consists of either a dark or bright matrix and some small dark blocks dispersed within the matrix. Dark blocks match SnO (orthorhombic, a=0.500 nm, b=0.572 nm, and c=1.120 nm), and the matrix consists of a nanocrystalline phase with a stoichiometric Nd3Sn structure having a space group of Pm3m and lattice parameter of a=0.344 nm. The grain size of the nanocrystalline Nd3Sn phase is about 3 to 15 nm. The melting range of the second-phase particle is estimated to be 1042 °C to 1600 °C. The microstructure of the second-phase particles in the quenched Ti-55 alloy was also studied. Fine and uniform dispersoids (6 to 15 nm in diameter) were observed in the as-quenched state. Some lenslike particles occur at the grain boundaries, other elliptical particles appear within the grains, and some particles within the grains form rows which are parallel to the advancing liquid-solid interface. After annealing at 980 °C (1 to 10 hours), of the as-quenched Ti-55 alloy, coarse particles are 17 to 42 nm in average diameter, and the growth of the particles is very slow. The dispersoids in the as-annealed Ti-55 alloy are identified as nanocrystalline Nd5Sn (orthorhombic, Pnmn, a=0.814 nm, b=1.732 nm, and c=0.814 nm) intermetallic compound, and the interface between the Nd5Sn4 phase and the matrix is a typical high-angle grain boundary.
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