Nickel-iron-based alloys control

High-purity metals and superalloys are required for the aeronautics, electronics, instruments, space, and defense industries the raw materials are at present imported. Primarily, these special metals include nickel-and cobalt-based superalloys, high-strength iron-based alloys, titanium-based alloys, controlled-expansion alloys, and magnetic materials. Keeping in view the importance of these metals and alloys and the expertise available in India for making them, the NCST has identified two projects for their development the setting up of a special metal and superalloys plant and the development of controlled-expansion alloys. 

The development of heat-resistant alloys was similarly lengthy and inefficient. In the 1970s the protective oxide layers on nickel-based alloys were much improved by alloying them with elements that resulted in a more stable and tenacious layers of alumina versus chromia. It is only now that the concept of alumina protective layers is being applied to more cost-effective and high-temperature iron-based heat-resistant alloys, effectively producing a new class of stainless steels based on the principles of selective oxidation and advanced microstructural control of precipitates for strengthening. ... 

Three types of metals are used exclusively stainless steels, nickel-chromium-iron alloys and, to a limited extent, cobalt-based alloys. These materials have provided many years of successfiil operation in similar control rod drive mechanisms. In the case of stainless steels, only austenitic and austenitic stainless steels are used. Where low or zero cobalt alloys are substituted for cobalt-based alloy pins, bars, or hard facing, Ihe substitute material is qualified by evaluation or test. The materials used for reactor internals are chosen to be compatible with the primary coolant chemistry and, as far as is possible, to be free from elements such as carbon or cobalt, which are prone to activation. Full details of the materials used, as well as the controls on fabrication, are provided in Section 4.5 of Reference 6.1. 

Cast irons can be cast into intricate shapes because of their excellent fluidity and relatively low melting points and can be alloyed for improvement of corrosion resistance and strength. With proper alloying, the corrosion resistance of cast irons can equal or exceed that of stainless steels and nickel-base alloys. The wide spectrum of properties of cast iron is controlled by three main factors the chemical composition of the iron, the rate of cooling of the casting in the mold, and the type of carbide or graphite formed. 

Silicon. Silicon is typically present only in minor amounts in most nickel-base alloys. In alloys containing significant amounts of iron, cobalt, molybdenum, tungsten, or other refractory elements, the level of silicon must be carefully controlled because it can stabilize carbides and harmful intermetallic phases. However, the use of silicon as a major alloying element has been found to greatly improve the... 

SMAs include copper-zinc-aluminum, copper-aluminum-nickel, and nickel-titanium (NiTi) alloys. NiTi alloys exhibit better mechanical properties than copper-based SMAs which were first developed in the early 1960s. SMAs have been widely used in retrofit and strengthening projects in the field of damping, active vibration control, and prestressing or posttensioning of structures with fibers and tendons, while recently, iron-based SMAs were developed that can reduce the cost. 

The arrangement of the melting and vacuum spray chambers is critical for guiding the liquid metal to eject into the vacuum chamber. Difficulties exist in precisely controlling the expulsion of the liquid metal into the vacuum chamber. Therefore, flaky droplets may be formed in vacuum atomization. Although vacuum atomization was developed mainly for the production of high-purity nickel and cobalt based superalloy powders, it is also applied to atomize the alloys of aluminum, copper and iron. 

Most LWR fuel rod cladding is made of Zircaloy (and its derivatives), which is an alloy of primarily zirconium and tin. Other alloying elements include niobium, iron, chromium, and nickel. Zircaloy was chosen because it has a very low cross section for thermal neutrons. Naturally occurring zirconium contains about l%-5% hafnium. The hafnium must be removed because it has a very high thermal neutron cross section and is often used in making control rods for reactors. The separation process used in the United States is a liquid-liquid extraction process. It is based on the difference in solubility of the metal thiocyanates in methyl isobutyl ketone. In Europe, a process known as extractive distillation is used to purify zirconium. This method employs a separation solvent that interacts differently with the zirconium and hafnium, causing their relative volatilities to change. This enables them to be separated by a normal distillation process. The separated zirconium is then alloyed with the required constituents. 

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