Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Strengthening mechanisms of materials wikipedia , lookup
Condensed matter physics wikipedia , lookup
State of matter wikipedia , lookup
Glass-to-metal seal wikipedia , lookup
Bottom-blown oxygen converter wikipedia , lookup
Work hardening wikipedia , lookup
De re metallica wikipedia , lookup
السنة االولى/ قسم الهندسة الميكانيكية المحاضرة الثالثة عشر/هندسة المعادن م اسراء فيصل غازي. م The Superalloys The superalloys have superlative combinations of properties. Which must withstand exposure to severely oxidizing environments and high temperatures for reasonable time periods. Mechanical integrity under these conditions is critical. Density is an important consideration because centrifugal stresses are diminished in rotating members when the density is reduced. These materials are classified according to the predominant metal in the alloy, which may be cobalt, nickel, or iron. Other alloying elements include the refractory metals (Nb, Mo,W, Ta), chromium, and titanium. In addition to turbine applications, aircraft, these alloys are utilized in nuclear reactors and petrochemical equipment. Nickel-Chromium High-temperature Alloys The main feature of the nickel-chromium alloys is Their ability to resist oxidation at elevated temperatures. If further suitable alloy additions are made the strength is increased under conditions of stress at high temperatures. Alloys rich in nickel and chromium have a high specific resistance to electricity, which makes them admirable materials for the manufacture of resistance wires, and (because of their low rate of oxidation at high temperatures) heater elements of many kinds capable of working at temperatures up to bright red heat. Further groups of high-temperature, nickel-chromium-base alloys are those containing iron and, sometimes, molybdenum and tungsten. Of these the Inconel series has been long established. Inconel 600 contains 76Ni; 16Cr; and 7Fe Is resistant to many acids and alkalis as well as oxidising atmospheres at high temperatures. Since it retains reasonable strength at high temperatures it is used for furnace equipment including retorts, muffles, heat-treatment trays, supports and nitriding boxes The Refractory Metals Metals that have extremely high melting temperatures are classified as the refractory metals. Included in this group are niobium (Nb), molybdenum (Mo), tungsten (W), and tantalum (Ta). Melting temperatures range between (2468 C) for niobium and (3410 C), the highest melting temperature of any metal, for tungsten. Interatomic bonding in these metals is extremely strong, which accounts for the melting temperatures, In addition, large elastic moduli and high strengths and hardness's, at ambient as well as elevated temperatures. The applications of these metals are varied. 1. For example, tantalum and molybdenum are alloyed with stainless steel to improve its corrosion resistance. 2. Molybdenum alloys are utilized for extrusion dies and structural parts in space vehicles; incandescent light filaments, x-ray tubes, and welding electrodes employ tungsten alloys. 3. Tantalum is immune to chemical attack by virtually all environments at temperatures below and is frequently used in applications requiring such a corrosion-resistant material. The Noble Metals The noble or precious metals are a group of eight elements that have some physical characteristics in common (noble metals are silver, gold, platinum, palladium, rhodium, ruthenium, iridium, and osmium). They are expensive (precious) and are superior or notable (noble) in properties Characteristically soft, ductile, and oxidation resistant. The noble metals are silver, gold, platinum, palladium, rhodium, ruthenium, iridium, and osmium; the first three are most common and are used extensively in jewelry. Silver and gold may be strengthened by solid-solution alloying with copper; sterling silver is a silver–copper alloy containing approximately 7.5 wt% Cu. Platinum is used for chemical laboratory equipment, as a catalyst (especially in the manufacture of gasoline), and in thermocouples to measure elevated temperatures Shape-Memory Alloy A shape-memory alloy (SMA, smart metal, smart alloy) is an alloy that "remembers" its original shape and that when deformed returns to its predeformed shape when heated. This material is a lightweight, The two main types of shape-memory alloys are copper-aluminium-nickel, and nickel-titanium (NiTi) alloys. Many metals have several different crystal structures at the same composition, but most metals do not show this shape-memory effect. The special property that allows shape-memory alloys to revert to their original shape after heating is that their crystal transformation is fully reversible. In most crystal transformations, the atoms in the structure will travel through the metal by diffusion, changing the composition locally, even though the metal as a whole is made of the same atoms. A reversible transformation does not involve this diffusion of atoms, instead all the atoms shift at the same time to form a new structure, much in the way a parallelogram can be made out of a square by pushing on two opposing sides.