Composition modulated alloys

Gai Aljf As, Hgi Cd Te) alloys, and quaternary Ai B CyDi y (e.g., Ini jfGaj j) l j,) alloys. The spectral features in Figure 1, e.g., Fq and Ej vary with alloy composition. Modulation Spectroscopy thus can be employed conveniently for this purpose even at 300 K. 

Tench and White (12) have shown that the room-temperature tensile strength of CMA (composition-modulated alloy) Ni-Cu exhibits values around three times that of nickel itself. The hardness of the same CMA has been demonstrated by Gimunovich et al. (13) to be many times greater. This is so as long as the thickness of the CMA layers is less than lOOnm. Stress due to lattice mismatch may be the prime cause of this. 

Electrodeposition of composition-modulated films was first performed by Brenner in 1939 (1) by employing two separate baths for the two components and a periodic immersion of the deposit in the two baths. This is too cumbersome a method to be adopted in practice. Deposition from a single bath with the presence of salts of the two components of the multilayer is what is desired, but there was a serious problem with the deposition of two metals from one bath. Specifically, whereas a layer of the more noble member can be deposited by choosing the potential to be between the reduction potentials of the two metals, one can expect that when the potential is set to a value appropriate for reduction of the less noble member, both will be deposited, resulting in an alloy layer rather than a pure metal. Thus, to nobody s surprise, even as recently as 1983, Cohen et al. (2), were able to deposit only a layered structure of alloys rather than pure metals. In addition they cast doubt on the possibility that a modulation cycle (the thickness of the basic layers, the periodic repetition of which... 

Recent results were able to demonstrate that the coatings produced by electrodeposition display the same coherence and layer thickness uniformity as those of composition-modulated alloys produced by vacuum evaporation or sputter deposition. 

The choice of materials for metallic systems is still expanding and at present various examples of combinations with different atomic radii are being prepared. Here multilayered techniques also show possibilities for new material syntheses. In contrast to materials prepared by chemical procedures, supedattices are made far from equilibrium. The various possibilities for layering the artificial supedattice materials are given in Figure 16c—f. Most of the stacked layers (c, d, f) have more or less sharply defined boundaries and some have a noncrystalline structure in the individual layers (c) or one of the layers is noncrystalline (d). In such situations the structural information is not transmitted between adjacent layers and therefore, stricdy speaking, no supedattice is formed. In the case of an unsharp boundary (e), compositionally modulated alloy-layered structures have been made. The amplitude of composition modulation in the center of a layer can be in the range of 0 to 100%. Supedattices can also be formed with sharp boundaries (<5% of the thinnest layer) between the two components. 

Tench and White have shown (12) that the room-temperature tensile strength of CM A (composition-modulated alloy) Ni-Cu exhibits values around three times that... 

In 1986 Yahalom and Zadok (4) pointed to methods to produce composition-modulated alloys by electrodeposition, initially for the copper-nickel couple. They obtained modulation to thicknesses down to 8 A. The principle of the method is as follows. 

They proposed hence to apply this technique for improving the electrodeposition of composition modulated alloys (CMA) [101]. They showed that combining potential and flow modulations it is possible to control the symmetry of concentration distribution of the element in the alloy which is electrodeposited under mass transport conditions [102],... 

Different authors have shown that compositionally modulated 3D metallic alloys can be electrochemically deposited using cyclic polarization conditions and multicomponent electrolytes [6.108, 6.109, 6.113, 6.114]. Alloy formation takes place alternately by diffusion and charge transfer control of the deposition reactions of different metal components. Similar conditions have been used for the deposition of ultrathin metal films and heterostructures (cf. Section 6.4). 

As far as the method of formation of thin Pd layers, electroless plating has given excellent results without showing any problem of hydrogen embrittlement (see ECN module experimental data) PVD sputtering represents a valid alternative for Pd alloy film, in order to overcome the difficulty to control the film alloy composition. 

Yahalom J, Zadok O (1987) Formation of composition-modulated alloys by electrodepositiotL J Mater Sci 22 499 503... 

Tench DM, White JD (1990) Considerations in electrodeposition of compositionally modulated alloys. J Electrochem Soc 137 3061-3066... 

The interfaces may be regarded as compositionally modulated disordered R-T alloys because X-ray diffraction shows an amorphous structure in the interface region. In the next subsection a brief overview of the interactions and structures of R and R-T alloys is given, on the basis of which the magnetic structure of interfaces and the origin of PMA may be well understood. 

Figure 23.18. Pourbaix diagram of Co-water system [84]. (Reprinted by permission of ECS—The Electrochemical Society, from Balirololoom ME, Gabe DR, Wilcox GD. Development of a bath for electrodeposition of zinc-cobalt compositionally modulated alloy multilayered coatings.)...

The principles described have been used to solve an interesting case of galvanic corrosion observed in compositionally modulated metal multilayers (CMMs) and alloys. The electrodeposition of copper and nickel alloys or metal multilayers has received attention due to their practical importance for corrosion protection (Jensen et al, 1998 Fei arrd Wilcox, 2006) as well as rrtagnetic (Foecke and Lashmore, 1992 Lenczowski et al, 1995 Miyazaki et al, 1999 Bakor i et ai, 2002 Spray and Nowak, 2006) and mechanical (Tsalakos arrd Jarrkowski, 1986) applications. Details of the rrses and fabrication are elaborated elsewhere (Roy, 2009). 

The Phase Diagram module permits the generation of phase diagrams of three basic types (1) with two potential axes (e.g. T vs P(02) for constant alloy composition, or P(S2) vs P(02) for constant temperature and composition of the alloy components), (2) with one potential axis and one ratio of extensive properties (e.g. T vs X (mole fraction), or P(02) vs X with all other variables constant), and (3) with two ratios of extensive properties on the axes (e.g. Xj (mole fraction of component i) vs Xj (mole fraction of component j) with all other variables constant). 

We have recently demonstrated that it is possible to electrodeposit nanoscale ceramic superlattices based on the TlPbO system (5). The idea of electrochemically depositing nanomodulated superlattices is not new, but it has not been applied previously to the deposition of nonmetallic materials. Several research groups have shown that compositionally modulated metallic alloys can be electrochemically deposited from a single plating bath by cycling either the potential or current (6-9). The interest in nanomodulated metallic systems stems from their enhanced mechanical and magnetic properties (7,10),... 

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