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76 5.Alloys and Compounds 5.2.1 A particular phase diagram in which only two elements (such as Isomorphous copper and tin)or two compounds(such as Mgo and NiO)are in- volved is called a binary phase diagram.If complete solute solubil- Phase ity between the constituents of a compound or an alloy is encoun- Diagram tered,the term isomorphous binary phase diagram is utilized.Figure 5.3 depicts the Cu-Ni isomorphous binary phase diagram as an ex- ample.Referring to Figure 5.3,we note that at temperatures above 1455C (i.e.,the melting point of nickel),any proportion of a Cu-Ni mixture is liquid.On the other hand,at temperatures below 1085C (the melting point of copper),alloys of all concentrations should be solid.This solid solution of nickel in copper is designated as a-phase. In the cigar-shaped region between the liquid and the a-solid-so- lution areas,both liquid and solid copper-nickel coexist(like cof- fee and solid sugar may coexist in a cup).This area is appropriately termed a two-phase region.The upper boundary of the two-phase region is called the liquidus line,whereas the lower boundary is re- ferred to as the solidus line.Within the two-phase region,the free- dom to change the involved parameters is quite limited.The degree of freedom,F,can be calculated from the Gibbs phase rule: F=C-P+1, (5.1) where C is the number of components and P is the number of phases.!In the present case,there are two components(Cu and Ni)and two phases (a and liquid),which leaves only one degree T [c] Liquid 1455 1210 c+L 1085 FIGURE 5.3.Copper-nickel isomorphous bi- nary phase diagram.The composition Solid here is given in mass percent(formerly Q called weight percent)in contrast to atomic percent.This section uses exclu- sively mass percent.For simplicity,the Cu 23 34 Ni latter is generally designated as % Mass Ni In(5.1)the pressure is assumed to be constant as it is considered in virtually all of the cases in this chapter.If the pressure is an additional variable,then the Gibbs phase rule has to be modified to read F=C-P+2. (5.1a)A particular phase diagram in which only two elements (such as copper and tin) or two compounds (such as MgO and NiO) are involved is called a binary phase diagram. If complete solute solubility between the constituents of a compound or an alloy is encountered, the term isomorphous binary phase diagram is utilized. Figure 5.3 depicts the Cu–Ni isomorphous binary phase diagram as an example. Referring to Figure 5.3, we note that at temperatures above 1455°C (i.e., the melting point of nickel), any proportion of a Cu–Ni mixture is liquid. On the other hand, at temperatures below 1085°C (the melting point of copper), alloys of all concentrations should be solid. This solid solution of nickel in copper is designated as -phase. In the cigar-shaped region between the liquid and the -solid-solution areas, both liquid and solid copper–nickel coexist (like coffee and solid sugar may coexist in a cup). This area is appropriately termed a two-phase region. The upper boundary of the two-phase region is called the liquidus line, whereas the lower boundary is referred to as the solidus line. Within the two-phase region, the freedom to change the involved parameters is quite limited. The degree of freedom, F, can be calculated from the Gibbs phase rule: F C P 1, (5.1) where C is the number of components and P is the number of phases.1 In the present case, there are two components (Cu and Ni) and two phases ( and liquid), which leaves only one degree 5.2.1 Isomorphous Phase Diagram 76 5 • Alloys and Compounds Mass % Ni T [C] + L Cu 23 34 Ni 1085 1210 1455 Liquid Solid FIGURE 5.3. Copper–nickel isomorphous binary phase diagram. The composition here is given in mass percent (formerly called weight percent) in contrast to atomic percent. This section uses exclusively mass percent. For simplicity, the latter is generally designated as %. 1In (5.1) the pressure is assumed to be constant as it is considered in virtually all of the cases in this chapter. If the pressure is an additional variable, then the Gibbs phase rule has to be modified to read F C P 2. (5.1a)