Alloys strengthening

Once the precipitates grow beyond a critical size they lose coherency and then, in order for deformation to continue, dislocations must avoid the particles by a process known as Orowan bowing(23). This mechanism appHes also to alloys strengthened by inert dispersoids. In this case a dislocation bends between adjacent particles until the loop becomes unstable, at which point it is released for further plastic deformation, leaving a portion behind, looped around the particles. The smaller the interparticle spacing, the greater the strengthening. 

Zirconium is used as a containment material for the uranium oxide fuel pellets in nuclear power reactors (see Nuclearreactors). Zirconium is particularly usehil for this appHcation because of its ready availabiUty, good ductiUty, resistance to radiation damage, low thermal-neutron absorption cross section 18 x 10 ° ra (0.18 bams), and excellent corrosion resistance in pressurized hot water up to 350°C. Zirconium is used as an alloy strengthening agent in aluminum and magnesium, and as the burning component in flash bulbs. It is employed as a corrosion-resistant metal in the chemical process industry, and as pressure-vessel material of constmction in the ASME Boiler and Pressure Vessel Codes. 

Many materials of practical interest are multi-phase solids with heterogeneous surfaces that can be either regular (orientated eutectics, unidirectional composites, etc) or random ( hard alloys processed by liquid phase sintering, or alloys strengthened by precipitation, etc). 

Table 2 summarizes the best data obtained with binary and complex alloys strengthened with borides, and borides and silicides [9],... 

The objectives of this Workshop, to present and discuss new results in the development and processing of metallic materials with high structural efficiency and to establish new interactions and networks between the participants, were successfully met. A number of new developments were presented and discussed at this Workshop, including recent information on titanium alloys modified with relatively small amounts of B and Si to provide dramatic improvements in strength and stiffness, and new A1 alloys strengthened with quasicrystalline precipitates for good elevated temperature properties. In addition, a current extensive update on severe plastic deformation and on nanocrystalline metals for structural applications was provided. 

Relationship of Composition and Heat Treatment to Environment-Sensitive Cracking of Aluminum Alloys. Those aluminum alloys strengthened by cold working only, particularly the 1000-series alloys, do not develop susceptibility to SCC. The so-called high-strength alloys are strengthened by thermal/mechanical treatments, which result in solid-state precipitation of one or more intermetallic phases that restrict dislocation motion and, hence, increase strength. Their susceptibility to SCC varies extensively with alloy composition and the thermal/mechanical treatment. While susceptibility tends to increase with... 

Miy] Miyafiiji, M., Copper Alloy Strengthened by Iron Phosphides (in Japanese), Kobe Res. 

Nickel-based alloys, which form the bulk of alloys produced, are basically nickel-chrome alloys with a face-centered cubic solid-solution matrix containing carbides and the coherent intermetallic precipitate y-NijlAfTi). This latter precipitate provides most of the alloy strengthening and results in useful operating temperatures up to 90% of the start of melting. Further additions of aluminum, titanium, niobium, and tantalum are made to combine with nickel in the y phase, and additions of molybdenum, tungsten, and chromium strengthen the solid solution matrix. 

This is a nonmagnetic nickel-chromium-based alloy strengthened with refractory metals. The chemical composition is given in Table 15.24. It possesses excellent corrosion properties, strength, ductility, and toughness, and has outstanding structural stability. 

In summary, we have discussed the three mechanisms that may be used to strengthen and harden single-phase metal alloys strengthening by grain size reduction, solid-solution strengthening, and strain hardening. Of course, they may be used in conjunction with one another for example, a solid-solution strengthened alloy may also be strain hardened. 

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