No abstract

Analytic modified embedded atom method (AMEAM) type many-body potentials have been constructed for ten hcp metals: Be, Co, Hf, Mg, Re, Ru, Sc, Ti, Y and Zr. The potentials are parametrized using analytic functions and fitted to the cohesive energy, unrelaxed vacancy formation energy, five independent second-order elastic constants and two equilibrium conditions. Hence, each of the constructed potentials represents a stable hexagonal close-packed lattice with a particular non-ideal c/a ratio. In order to treat the metals with negative Cauchy pressure, a modified term has been added to the total energy. For all the metals considered, the hcp lattice is shown to be energetically most stable when compared with the fcc and bcc structure and the hcp lattice with ideal c/a. The activation energy for vacancy diffusion in these metals has been calculated. They agree well with experimental data available and those calculated by other authors for both monovacancy and divacancy mechanisms and the most possible diffusion paths are predicted. Stacking fault and surface energy have also been calculated and their values are lower than typical experimental data. Finally, the self-interstitial atom (SIA) formation energy and volume have been evaluated for eight possible sites. This calculation suggests that the basal split or crowdion is the most stable configuration for metals with a rather large deviation from the ideal c/a value and the non-basal dumbbell (C or S) is the most stable configuration for metals with c/a near ideal. The relationship between SIA formation energy and melting temperature roughly obeys a linear relation for most metals except Ru and Re.

No abstract

Porous metals are of great interest as a potential engineering material in various industrial fields because of their unique properties such as their impact energy absorption capacity, their gas and liquid permeability, thermal conductivity, and electrical insulating properties.[1] However, their poor oxidation resistivity, poor corrosion resistance, and intolerance at elevated temperatures restrict their potential applications in much wider fields, such as high-temperature gas separation and as catalysis in rugged environments. It is well known that Ti-Al alloys are notable examples of intermetallic compounds containing a mixture of metallic and covalent bonds [2] that provide sound mechanical properties with outstanding corrosion resistance [3] and excellent oxidation resistance at elevated temperatures-particularly over 600°C. [4] In this Communication, we introduce a novel technique, based on the Kirkendall effect, to fabricate Ti-Al micrometer/nanometersized porous alloys with adjustable pore sizes ranging from several tens of micrometer to several tens of nanometers. In the late 1930s, Kirkendall et al. [5] discovered that the interface between copper and zinc in brass moved at an elevated temperature due to their different diffusion rates. This phenomenon was later referred to as the Kirkendall effect. It has been well documented that the Kirkendall effect leads to pore formation in materials. [6][7][8] In general, these pores impact negatively on the mechanical properties, so that significant efforts have been devoted to remove these pores for engineering applications. [9] However, in recent years, the Kirkendall effect has been used to fabricate special structures, such as hollow nanocrystals [7] and hierarchical porous iron oxide films. [8] In this regard, the Kirkendall effect can be used to produce porous alloys if there exists great diffusion-rate discrepancies between components in the alloy system. Two procedures for fabricating porous Ti-Al alloys via the Kirkendall effect are illustrated in Figure 1a. In the first procedure, commercial Ti and Al elemental powders were mixed and cold pressed into both disc and tubular compacts, followed by the solid sintering process (in which the Kirkendall effect occurs). Considerable expansion of these compacts has been observed and their examples are shown in Figure 1b and c. The final shape of a compact with the porous structure is important for practical applications. Through the precise control of the sintering process and the avoidance of the formation of the liquid phase, we have managed to preserve the sintered compacts in their original shapes (as shown in Fig. 1b and c) even though a great volume expansion occurred. Under an identical sintering condition (initially sintered at 600°C for 60 min and then sintered at 1300°C for 30 min), the porous materials of three structures of Ti-Al alloys (i.e., a 2 -Ti 3 Al, c-TiAl, and TiAl 3 ) could be fabricated when different ratios of Al and Ti powders were mixed. A typical example of the alloy with the nominal...

The previous model on surface free energy has been extended to calculate size dependent thermodynamic properties (i.e., melting temperature, melting enthalpy, melting entropy, evaporation temperature, Curie temperature, Debye temperature and specific heat capacity) of nanoparticles. According to the quantitative calculation of size effects on the calculated thermodynamic properties, it is found that most thermodynamic properties of nanoparticles vary linearly with 1/D as a first approximation. In other words, the size dependent thermodynamic properties P(n) have the form of P(n) = P(b)(1 -K/D), in which P(b) is the corresponding bulk value and K is the material constant. This may be regarded as a scaling law for most of the size dependent thermodynamic properties for different materials. The present predictions are consistent literature values.