First - Principles Study On The Fragility Of Iridium (Ir) Elements

Iridium has a very high melting point of2443℃, is not only the most resistant of all metals to corrosion, insoluble in all mineral acids including aqua regia and unattacked by other molten metals or by silicates at high temperatures, and is the only metal to maintain good mechanical properties in air at temperatures above1600℃. With excellent physical properties (high melting point, anti-oxidation, corrosion bresistance) and high temperature mechanical properties, iridium has impotant applications in the extreme environments, i.g. at elevated temperature. However, the mechanical properties of iridium, having face-centered cubic crystal structure, are extremely unusual:having the properties of body-centered cubic metals at low temperature, and having the properties of normal face-centered cubic metals at elevated temperature. This makes the plastic processing of iridium and its alloy become extremely difficult. Thus, the brittleness of iridium essentially restricts its applications and development.In this paper, with the help of first-principles calculations (using CASTEP package) and experimental analysis (SEM, HRTEM), we have analysied and discussed the embrittlement mechanism in iridium. The main works includes:calculated and analyzed the basic properties of the metal iridium (including the cohesive energy, equilibrium lattice constant, elastic properties, the chemical bonding); generalized stacking fault energy; the structure and properties of dislocations and fracture behaviors. The research results can be summarized as follows::1) Pure iridium has a high cohesive energy (about9.69eV/atom), thus the iridium atoms have a strong bonding force; the chemical bonding has obvious directionality, so the bonding is a mix-type bonds which mixed metal and covalent bond. The strong bongding is the reason for high melting point and high elastic modulus of iridium.2) Pure Iridium has high instable and intrinsic stacking fault energy, so the mechanism of plastic deformation in iridium is dislocation motion, rather than mechanical twinning. Due to a high instable stacking fault energy in the<112> direction, Shockley partial dislocation can not nucleate easily. Even Shockley partial dislocations have nucleated, the extended dislocation width is very narrow as high intrinsic stacking fault energy in<112> direction, and Shockley partial dislocation will become perfect dislocation easily because of constriction of dislocation during dislocation motion. Therefore, the mechanism of plastic deformation of pure iridium is the motion of<110> perfect dislocation.3) The high stacking fault energy is associated with directional chemical bond. The solute atoms can change the charge distribution of matrix atoms and improve anisotropy. Thereby, they can affect the stacking fault energy. In the selected solute atoms(C, Th, Al, Re, Zr, Nb), C (Th) can significantly improve (reduce) iridium stacking fault energy. High stacking fault energy will increase the resistance to dislocation motion; low stacking fault energy will reduce the resistance to dislocation motion, is beneficial to dislocation nucleation, and even deformation twinning can participate plastic deformation. Therefore, the carbon atom in iridium will deteriorate ductility, and thorium atom in iridium will be conducive to plastic deformation. This is consistent with phenomenons observed in the experiments.4) In the Peierls-Nabarro model framework, using the generalized stacking fault energy surface (y surface) to solve the inter-layer restoring force, replacing the sinusoidal approximation of inter-layer restoring force in classic Peierl-Nabarro model, can obtain more precise Peierls-Nabarro equation.Then taking the solution under sinusoidal approximation conditon as a particular solution, extrapolating the form of general solution of revised Peierls-Nabarro equation, can get general solution with undetermined coefficients. Finally, taking the general solution of into revised Peierls-Nabarro equation can slove undetermined coefficients, then gain the function expression of misfit distribution function fb(x). Using fb(x) can calculate the resistance to motion (Peierls-Nabarro force) of<110> perfect dislocation. The results indicate that iridium has a high Peierls-Nabarro force, which is much higher than aluminum. Therefore, the dislocations mobility is low, and the plastic deformation is difficult.5) The mechanism properties of iridium meet some experience criterions, such as G/B. The micro mechanism for the cleavage in irdium is as follows:the nucleation of microcracks may be achieved by dislocation reaction mechanism; the reason for microcracks extend in a brittle mode is that dislocation nucleation is difficult at crack tip, and tends to form a new surface.