Alloy Structure Equivalency

Particles with the continuous ordered structure of PdCu3, with stacking faults, are obtained after a long annealing. Their are cap-shaped and mainly oriented (100) and (110) on MgO. For larger sizes (> 10-20 imi), long period superstructures with anti phase boundaries are obtained, with the same spacing as in the bulk alloy for equivalent concentrations. 

Poor Weldability a. Underbead cracking, high hardness in heat-affected zone. b. Sensitization of nonstabilized austenitic stainless steels. a. Any welded structure. b. Same a. Steel with high carbon equivalents (3), sufficiently high alloy contents. b. Nonstabilized austenitic steels are subject to sensitization. a. High carbon equivalents (3), alloy contents, segregations of carbon and alloys. b. Precipitation of chromium carbides in grain boundaries and depletion of Cr in adjacent areas. a. Use steels with acceptable carbon equivalents (3) preheat and postheat when necessary stress relieve the unit b. Use stabilized austenitic or ELC stainless steels. 

Steel may have some merit SSCC of weld repairs in well-head alloys was investigated by Watkins and Rosenberg who found that the repairs were susceptible to this problem because of the hard HAZs developed by welding. Post-weld heat treatment was an essential but not complete cure compared with unrepaired castings. In the case of hydrogen-assisted cracking of welded structural steels, composition is more important than mechanical properties and the carbon equivalent should be... 

Figure 3.31. cF16-MnCu2Al type structure (representative of the Heusler type alloys). The unit cell is shown in (a), an eighth of the cell is shown in (b). It degenerates into a cP2-CsCl type cell if the atoms at the cube comers (Mn and Al) are equivalent. If all the atoms were equivalent there would he a further degeneration into the cI2-W type. 

Notice that the structures presented in this paragraph are unary structures, that is one species only is present in all its atomic positions. In the prototypes listed (and in the chemically unary isostructural substances) this species is represented by a pure element. In a number of cases, however, more than one atomic species may be equally distributed in the various atomic positions. If each atomic site has the same probability of being occupied in a certain percentage by atoms X and Y and all the sites are compositionally equivalent, the unary prototype is still a valid structural reference. In this case, from a chemical point of view, the structure will correspond to a two-component phase. Notice that there can be many binary (or more complex) solid solution phases having for instance the Cu-type or the W-type structures. Such phases are formed in several metallic alloy systems either as terminal or intermediate phases. 

In discussing the topology of such structures, it is particularly appropriate then to focus on the structure of the four-connected nets of metal atoms - a procedure in tune with the philosophy of this paper. It is in fact an approach that has already been actively developed elsewhere (see e.g. Ref. ), so we do not pursue this topic further here, other than to include in Table 3 some examples of alumino-silicates in which the metal atom arrangement is topologically equivalent to corresponding network alloy types. 

The simple cubic structme, sometimes called the rock salt structure because it is the structme of rock salt (NaCl), is not a close-packed structure (see Figure 1.20). In fact, it contains about 48% void space and as a result, it is not a very dense structure. The large space in the center of the SC structme is called an interstitial site, which is a vacant position between atoms that can be occupied by a small impurity atom or alloying element. In this case, the interstitial site is surrounded by eight atoms. All eight atoms in SC me equivalent and me located at the intersection of eight adjacent unit cells, so that there me 8 x (1/8) = 1 total atoms in the SC unit cell. Notice that... 

Figure 18.7 Interfaces resulting from two types of continuous transformation, (a) Initial structure consisting of randomly mixed alloy, (b) After spinodal decomposition. Regions of B-rich and B-lean phases separated by diffuse interfaces formed as a result of long-range diffusion, (c) After an ordering transformation. Equivalent ordering variants (domains) separated by two antiphase boundaries (APBs). The APBs result from A and B atomic rearrangement onto different sublattices in each domain.

This class of smart materials is the mechanical equivalent of electrostrictive and magnetostrictive materials. Elastorestrictive materials exhibit high hysteresis between strain and stress (14,15). This hysteresis can be caused by motion of ferroelastic domain walls. This behavior is more complicated and complex near a martensitic phase transformation. At this transformation, both crystal structural changes induced by mechanical stress and by domain wall motion occur. Martensitic shape memory alloys have broad, diffuse phase transformations and coexisting high and low temperature phases. The domain wall movements disappear with fully transformation to the high temperature austentic (paraelastic) phase. 

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