It was found that iridium was more effective insoften and more » ductilize tungsten than rhenium at room temperature, and the minimum hardness and maximum elongation occurred at iridium content about 0.4 wt.%. The yield strength and fracture behavior of iridium and iridium-tungsten alloys were found to be very sensitive to trace impurities with concentrations in the ppM range. The Ir-2% alloy has the more » best toughness because of its high strength and good ductility at 1370/sup 0/C. In connection with this increase, the fracture mode changes from grain-boundary separation to transgranular fracture at temperatures around 800/sup 0/C and finally to ductile rupture with close to 100% reduction in area above 1000/sup 0/C. The ductilities of the alloys increase steadily with test temperature. Alloying with tungsten increases the yield strength and slightly lowers the ductility. Tensile properties were determined as a function of test temperature up to 1400/sup 0/C. Also, alloying with 2% tungsten raises the recrystallization temperature of iridium by 400/sup 0/C and retards the grain growth at high temperatures. No internal oxidation or grain-boundary penetration was observed in this alloy. The Ir-2% W has good oxidation resistance below 1000/sup 0/C. The solubility of oxygen and carbon in the alloys appears to be quite low, at a level of 5 ppM for oxygen and 10 ppM for carbon up to 1300/sup 0/C. The iridium-tungsten alloys are readily fabricable and are compatible with graphite, low-pressure oxygen, and simulated heat source environments up to at least 1300/sup 0/C.
Iridium – Properties Element Iridium Atomic Number 77 Symbol Ir Element Category Transition Metal Phase at STP Solid Atomic Mass 192.217 Density at STP 22.65 Electron Configuration 4f14 5d7 6s2 Possible Oxidation States +3,4 Electron Affinity 151 Electronegativity 2.2 1st Ionization Energy 9.1 Year of Discovery 1803 Discoverer Tennant, Smithson Thermal properties Melting Point 2410 Boiling Point 4130 Thermal Conductivity 150 Specific Heat 0.13 Heat of Fusion 26.The physical and mechanical properties of iridium and iridium-tungsten alloys containing up to 4% tungsten were evaluated as potential cladding materials in space isotopic heat sources. The melting point is the temperature at which the disruptive vibrations of the particles of the solid overcome the attractive forces operating within the solid. At some point, the amplitude of vibration becomes so large that the atoms start to invade the space of their nearest neighbors and disturb them, and the melting process initiates. As a solid is heated, its particles vibrate more rapidly as the solid absorbs kinetic energy. The motion of individual atoms, ions, or molecules in a solid is restricted to vibrational motion about a fixed point. The atoms in a solid are tightly bound to each other, either in a regular geometric lattice (crystalline solids, which include metals and ordinary ice) or irregularly (an amorphous solid such as common window glass), and are typically low in energy. Solids are similar to liquids in that both are condensed states, with particles that are far closer together than those of a gas. The first theory explaining the mechanism of melting in bulk was proposed by Lindemann, who used the vibration of atoms in the crystal to explain the melting transition. When considered as the temperature of the reverse change from liquid to solid, it is called the freezing point or crystallization point. The melting point of a substance depends on pressure and is usually specified at standard pressure. Adding heat will convert the solid into a liquid with no temperature change. In thermodynamics, the melting point defines a condition where the solid and liquid can exist in equilibrium. When considered as the temperature of the reverse change from vapor to liquid, it is called the condensation point. The pressure at which vaporization (boiling) starts to occur for a given temperature is called the saturation pressure. The temperature at which vaporization (boiling) starts to occur for a given pressure is called the saturation temperature or boiling point. In thermodynamics, saturationdefines a condition in which a mixture of vapor and liquid can exist together at a given temperature and pressure. Note that these points are associated with the standard atmospheric pressure. Iridium – Melting Point and Boiling Point