Ductility selected metals

Molybdenum (Mo) is a silvery-white, hard, fairly ductile, refractory metal suitable for alloys that are required to exhibit a combination of high strength and rigidity at temperatures as high as l,650°C. Selected properties of molybdenum are listed in Table 2, in comparison with the respective properties of tungsten and... 

Cladding may be less expensive than selective electro deposition when coatings greater than 1 p.m of a noble metal are required, but may be more expensive than electro deposition for thinner coatings. Selective techniques are most easily used for sheet metal substrates that are to be machine stamped and formed into contacts. Clad noble metals are considerably more ductile (and less hard) than comparable electro deposits and, therefore, are better suited to forming operations. Contacts that are made into separate parts from rod by screw machining are usually coated on all exposed surfaces by barrel electroplating. 

Intergranular Corrosion Selective corrosion in the grain boundaries of a metal or alloy without appreciable attack on the grains or crystals themselves is called intergranular corrosion. When severe, this attack causes a loss of strength and ductility out of proportion to the amount of metal actually destroyed by corrosion. 

The selection of materials for high-temperature applications is discussed by Day (1979). At low temperatures, less than 10°C, metals that are normally ductile can fail in a brittle manner. Serious disasters have occurred through the failure of welded carbon steel vessels at low temperatures. The phenomenon of brittle failure is associated with the crystalline structure of metals. Metals with a body-centred-cubic (bcc) lattice are more liable to brittle failure than those with a face-centred-cubic (fee) or hexagonal lattice. For low-temperature equipment, such as cryogenic plant and liquefied-gas storages, austenitic stainless steel (fee) or aluminium alloys (hex) should be specified see Wigley (1978). 

NOTE Good design practice should be followed in the selection of fabrication methods, welding procedures, and materials for vendor-furnished steel pressureretalning parts that may be subject to temperatures below the ductile-brittle transition temperature. The published design-allowable stresses for metallic materials in internationally recognised standards such as the ASME Code and ANSI standards are based on minimum tensile properties. Some standards do not differentiate between rimmed, semi-killed, fully killed hot-rolled and normalised material, nor do they take into account whether materials were produced under fine- or course-grain practices. The vendor should exercise caution in the selection of materials intended for services between 0 °C (-20 °F) and 40 °C (100 °F). 

Another factor to be considered is the time required to fabricate additional liners if the initial supply is depleted. Recently, General Atomics claimed it was able to fabricate 20-mil thick liners of the required diameter for the reactor. General Atomics plans to float a precious-metal liner in a cylindrical Hastelloy pressure vessel and use cooled elastomeric O-rings that have performed satisfactorily on other SCWO systems to form the SCWO reactor. The annular space between the liner and the vessel wall will be monitored for leaks to indicate when change-out of the liner is required. No decision has been made yet on whether to use platinum or Pt-20%Ir. Because they have markedly different mechanical properties, these two liner materials may require significantly different fabrication methods. Platinum is relatively weak and very ductile Pt-20%Ir is less ductile but 10 times stronger. Final selection of the liner material for use at the NECDF was scheduled for early 2000. Fabrication... 

Care has to be taken in selecting materials for the die and punches. Metals are of little use above 1000 °C because they become ductile, and the die bulges under pressure so that the compact can only be extracted by destroying the die. However, zinc sulphide (an infrared-transparent material) has been hot pressed at 700 °C in stainless steel moulds. Special alloys, mostly based on molybdenum, can be used up to 1000 °C at pressures of about 80 MPa (5 ton in-2). Alumina, silicon carbide and silicon nitride can be used up to about 1400 °C at similar pressures and are widely applied in the production of transparent electro-optical ceramics based on lead lanthanum zirconate as discussed in Section 8.2.1. 

Properties. Most of the alloys developed to date were intended for service as fuel cladding and other structural components in liquid-metal-cooled fast-breeder reactors. Alloy selection was based primarily on the following criteria corrosion resistance in liquid metals, including lithium, sodium, and NaK, and a mixture of sodium and potassium strength ductility, including fabricabiUty and neutron considerations, including low absorption of fast neutrons as well as irradiation embrittlement and dimensional-variation effects. Alloys of greatest interest include V 80, Cr 15, Ti 5... 

Erosion Similar to abrasion cutting in ductile metal fracture (of brittle material) very small chips or particles (e.g., impellers, propellers, fans) Reduce fluid velocity to eliminate turbulence select harder alloy (high chromium) hard coatings such as cement lined pipe, rubber lining... 

To achieve the desired pressure containment at elevated temperatures, most pressure vessels are made of metal or metal alloys. Suitable materials must be ductile that is, the material must expand (strain) with applied pressure (stress). It is important to select a material of construction that is in its proportional range such that the strain is linear, or proportional, to the stress. In this region, the proportionality is called Young s modulus. At sufficiently high stress (the proportional limit), the strain will be more than that predicted by the linear ratio. Up to this point, the material will return to its original dimensions when the stress is removed. Above the proportional limit, permanent deformation will occur and the material will not completely return to its original dimension when the stress is removed. The yield point of many materials is defined as the stress at which a permanent deformation of 0.2% is measured. 

The present contribution focuses on the effect of various elements added by ion implantation on the isothermal and cyclic oxidation behaviour of the 7-Ti A1 based inter-metallic alloys Ti-48Al-2Cr and Ti-48Al-2Cr-2Nb at 800°C in air.These particular materials were selected since the ternary chromium addition improves the mechanical properties especially room temperature ductility [9-11] and the quaternary niobium addition improves the oxidation resistance [8, 14, 15]. Comparison will be made between materials modified by ion implantation and alloys in which elements were added by alloying techniques. 

Dealloying corrosion involves the preferential dissolution of one or more components of a metallic alloy. This selective oxidation eliminates the more reactive components from the alloy and alters the residual material so that it is less dense and frequently has lost much of its strength, hardness, and macroscopic ductility. During the deaUoying process, the rearrangement of the residual atoms of the less reactive components can result in the development of a filamentary layer called a nanoporous structure as shown in Figiu e 4.1. 

Two Nl-based superalloys (Inconel 600 and 625) were studied for braze alloys In ceramic-metal joints for high temperature applications. These alloys were selected based on their high-temperature performance and ductility. 

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