Binary Phase Diagrams for Mixed Valency Metals

2 Binary Phase Diagrams for Mixed Valency Metals 

Recall the conditions suggested by Hmne-Rothery that must be satisfied in order for two elements to combine in all proportions to form a substitutional solid solution  

If these conditions are not met, the two elements will have limited solid solubility or may form either compoimds or new phases. The ratio of Zn/Cu atomic radii is 1.04, so the first condition is satisfied. Cu crystallizes in the fee configuration whereas, Zn is hep. Also, Cu is univalent and Zn is divalent, so the last two conditions are not met. 

Note that a-brass, that forms up to 38 At% Zn, is fee. For higher concentration of Zn, the system undergoes a solid-phase transformation to /3-brass which is bcc up to 48 At%. At this point it becomes y (a complex cubic unit cell containing 52 atoms). The addition of still more Zn produces the e and rj phases which are hep with different eja ratios. Cu-Al, Cu-Sn, Ag-Zn, Ag-Al, Ag-Cd and other mixed valence systems show similar phase behavior. 

Cu-Zn phase diagram. The a phase is fee and extends to 36 At% Zn. The p phase is bee and extends to 48 At%. (From Massalski, T.B., Senior Editor, Handbook of Binary Alloy Phase Diagrams, Vols. 1-3, Ameriean Soeiety for Metals, 1990. Reprinted with permission of ASM International. All rights reserved.) 

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