上海交通大学：《材料与文明》课程教学资源（参考资料）Understanding Mater_Chapter 5 - Alloys and Compounds

5.2·Phase Diagrams 79 and interdendritic segregation.In general terms,the center of the dendrites is rich in that element which melts at high temperatures (in the present case,Ni),whereas the regions between the dendrites contain less of this element.The resulting mechanical properties have been found to be inferior to those of a homogeneous alloy. Further,the nonequilibrium alloy may have a lower melting point than that in the equilibrium state,as demonstrated by Tie Line 4 in Figure 5.5.This phenomenon is called hot shortness and may cause partial melting (between the dendrites)when heating the al- loy slightly below the equilibrium solidus line. It is possible to eliminate the inhomogeneities in solid solu- tions.One method is to heat the alloy for many hours below the solidus line.This process is called homogenization heat treat- ment.Other procedures involve rolling the segregated solid at Liquid Solid Secondary dendrites (a) (b) FIGURE 5.6.Microstructure of an alloy revealing dendrites:(a) schematic,(b)photomicrograph of a nickel-based superalloy

and interdendritic segregation. In general terms, the center of the dendrites is rich in that element which melts at high temperatures (in the present case, Ni), whereas the regions between the dendrites contain less of this element. The resulting mechanical properties have been found to be inferior to those of a homogeneous alloy. Further, the nonequilibrium alloy may have a lower melting point than that in the equilibrium state, as demonstrated by Tie Line 4 in Figure 5.5. This phenomenon is called hot shortness and may cause partial melting (between the dendrites) when heating the al￾loy slightly below the equilibrium solidus line. It is possible to eliminate the inhomogeneities in solid solu￾tions. One method is to heat the alloy for many hours below the solidus line. This process is called homogenization heat treat￾ment. Other procedures involve rolling the segregated solid at 5.2 • Phase Diagrams 79 (a) Solid Secondary dendrites Liquid FIGURE 5.6. Microstructure of an alloy revealing dendrites: (a) schematic, (b) photomicrograph of a nickel-based superalloy. (b)

80 5.Alloys and Compounds high temperatures (called hot working)or alternately rapid so- lidification,which entails a quick quench of the liquid alloy to temperatures below the solidus line. Only very few binary phase diagrams are isomorphous as just described.Indeed,unlimited solid solubility is,according to Hume-Rothery,only possible if the atomic radii of the con- stituents do not vary more than 15%,if the components have the same crystal structure and the same valence,and if the atoms have about the same electronegativity.Other restrictions may ap- ply as well.Actually,most phase diagrams instead consist of one or more of the following six basic types known as eutectic,eu- tectoid,peritectic,peritectoid,monotectic,and monotectoid.They are distinguished by involving reactions between three individ- ual phases.This will be explained on the following pages. Complete solute solubility as discussed in this section is not restricted to selected metals only.Indeed,isomorphous phase di- agrams also can be found for a few ceramic compounds,such as for NiO-MgO,or for FeO-MgO,as well as for the orthosilicates Mg2SiO4-Fe2SiO4,in which the Mg2+and the Fe2+ions replace one another completely in the silicate structure. 5.2.2 Eutectic Some elements dissolve only to a small extent in another element. Phase In other words,a solubility limit may be reached at a certain solute concentration.This can be compared to a mixture of sugar and cof- Diagram fee:One spoonful of sugar may be dissolved readily in coffee whereas,by adding more,some of the sugar eventually remains undissolved at the bottom of the cup.Moreover,hot coffee dissolves more sugar than cold coffee;that is,the solubility limit (called solvus line in a phase diagram)is often temperature-dependent. Let us inspect,for example,the copper-silver phase diagram which is depicted in Figure 5.7.When adding small amounts of copper to silver,a solid solution,called a-phase,is encountered as described before.However,the solubility of copper into silver is restricted.The highest amount of Cu that can be dissolved in Ag is only 8.8%.This occurs at 780C.At any other temperature, the solubility of Cu in Ag is less.For example,the solubility of Cu in Ag at400°℃is only1.2%. A similar behavior is observed when adding silver to copper. The solubility limit at 780C is reached,in this case,for 8%Ag in Cu.Moreover,the solubility at 200C and lower temperatures is essentially nil.This second substitutional solid solution is ar- bitrarily called the B-phase. In the region between the two solvus lines,a mixture of two solid phases exists.This two-phase area is called the a+B region. To restate the facts for clarity:The a-phase is a substitutional solid solution of Cu in Ag comparable to a complete solution of

high temperatures (called hot working) or alternately rapid so￾lidification, which entails a quick quench of the liquid alloy to temperatures below the solidus line. Only very few binary phase diagrams are isomorphous as just described. Indeed, unlimited solid solubility is, according to Hume–Rothery, only possible if the atomic radii of the con￾stituents do not vary more than 15%, if the components have the same crystal structure and the same valence, and if the atoms have about the same electronegativity. Other restrictions may ap￾ply as well. Actually, most phase diagrams instead consist of one or more of the following six basic types known as eutectic, eu￾tectoid, peritectic, peritectoid, monotectic, and monotectoid. They are distinguished by involving reactions between three individ￾ual phases. This will be explained on the following pages. Complete solute solubility as discussed in this section is not restricted to selected metals only. Indeed, isomorphous phase di￾agrams also can be found for a few ceramic compounds, such as for NiO–MgO, or for FeO–MgO, as well as for the orthosilicates Mg2SiO4–Fe2SiO4, in which the Mg2 and the Fe2 ions replace one another completely in the silicate structure. Some elements dissolve only to a small extent in another element. In other words, a solubility limit may be reached at a certain solute concentration. This can be compared to a mixture of sugar and cof￾fee: One spoonful of sugar may be dissolved readily in coffee whereas, by adding more, some of the sugar eventually remains undissolved at the bottom of the cup. Moreover, hot coffee dissolves more sugar than cold coffee; that is, the solubility limit (called solvus line in a phase diagram) is often temperature-dependent. Let us inspect, for example, the copper–silver phase diagram which is depicted in Figure 5.7. When adding small amounts of copper to silver, a solid solution, called -phase, is encountered as described before. However, the solubility of copper into silver is restricted. The highest amount of Cu that can be dissolved in Ag is only 8.8%. This occurs at 780°C. At any other temperature, the solubility of Cu in Ag is less. For example, the solubility of Cu in Ag at 400°C is only 1.2%. A similar behavior is observed when adding silver to copper. The solubility limit at 780°C is reached, in this case, for 8% Ag in Cu. Moreover, the solubility at 200°C and lower temperatures is essentially nil. This second substitutional solid solution is ar￾bitrarily called the -phase. In the region between the two solvus lines, a mixture of two solid phases exists. This two-phase area is called the   region. To restate the facts for clarity: The -phase is a substitutional solid solution of Cu in Ag comparable to a complete solution of 5.2.2 Eutectic Phase Diagram 80 5 • Alloys and Compounds

5.2·Phase Diagrams 81 T rc] 1085 L 1000 962 x+L 28.1% L+B 800 B 8.8% 780C 92% 600 snAlOS a+β 400 200 FIGURE 5.7.Binary copper-silver phase Ag 20 40 60 80 Cu diagram containing a eutectic transfor- Mass Cu mation. sugar in coffee.Consequently,only one phase (sweet coffee)is present.(The analogue is true for the B-phase.)In the a +B re- gion,two phases are present,comparable to a mixture of blue and red marbles.The implications of this mixture of two phases to the strength of materials will be discussed later. We consider now a silver alloy containing 28.1%copper called the eutectic composition (from Greek eutektos,"easy melting"). We notice that this alloy solidifies at a lower temperature(called the eutectic temperature)than either of its constituents.(This phe- nomenon is exploited for many technical applications,such as for solder made of lead and tin or for glass-making.)Upon slow cooling from above to below the eutectic temperature,two solid phases (the a-and the B-phases)form simultaneously from the liquid phase according to the three-phase reaction equation: L28.1%Cua8.8%Cu+B92%Cu. (5.2) This implies that,for this specific condition,three phases (one liq- uid and two solid)are in equilibrium.The phase rule,F=C-P+ 1 [Eq.(5.1)]teaches us that,for the present case,no degree of free- dom is left.In other words,the composition as well as the temper- ature of the transformation are fixed as specified above.The eutec- tic point is said to be an invariant point.The alloy therefore remains at the eutectic temperature for some time until the energy differ- ence between solid and liquid (called the latent heat of fusion, AHf)has escaped to the environment.This results in a cooling curve which displays a thermal arrest (or plateau)quite similar to that of pure metals where,likewise,no degree of freedom remains dur- ing the coexistence of solid and liquid.A schematic cooling curve for a eutectic alloy is depicted in Figure 5.8

sugar in coffee. Consequently, only one phase (sweet coffee) is present. (The analogue is true for the -phase.) In the   re￾gion, two phases are present, comparable to a mixture of blue and red marbles. The implications of this mixture of two phases to the strength of materials will be discussed later. We consider now a silver alloy containing 28.1% copper called the eutectic composition (from Greek eutektos, “easy melting”). We notice that this alloy solidifies at a lower temperature (called the eutectic temperature) than either of its constituents. (This phe￾nomenon is exploited for many technical applications, such as for solder made of lead and tin or for glass-making.) Upon slow cooling from above to below the eutectic temperature, two solid phases (the - and the -phases) form simultaneously from the liquid phase according to the three-phase reaction equation: L28.1% Cu
8.8% Cu 92% Cu. (5.2) This implies that, for this specific condition, three phases (one liq￾uid and two solid) are in equilibrium. The phase rule, F
C  P 1 [Eq. (5.1)] teaches us that, for the present case, no degree of free￾dom is left. In other words, the composition as well as the temper￾ature of the transformation are fixed as specified above. The eutec￾tic point is said to be an invariant point. The alloy therefore remains at the eutectic temperature for some time until the energy differ￾ence between solid and liquid (called the latent heat of fusion, Hf) has escaped to the environment. This results in a cooling curve which displays a thermal arrest (or plateau) quite similar to that of pure metals where, likewise, no degree of freedom remains dur￾ing the coexistence of solid and liquid. A schematic cooling curve for a eutectic alloy is depicted in Figure 5.8. 5.2 • Phase Diagrams 81 1000 800 600 400 200 Ag 20 40 60 80 Cu Mass % Cu T [C] 962 L 28.1 % 8.8 % 780C 92 %  +   Solvus L +   1085  + L FIGURE 5.7. Binary copper–silver phase diagram containing a eutectic transfor￾mation

82 5.Alloys and Compounds FIGURE 5.8.Schematic representa- tion of a cooling curve for a eu- tectic alloy (or for a pure metal). The curve is experimentally ob- amiejadwL tained by inserting a thermome- ter (or a thermocouple)into the liquid alloy and reading the tem- perature in periodic time inter- vals as the alloy cools. Time The microstructure,observed by inspecting a eutectic alloy in an optical microscope,reveals a characteristic platelike or lamel- lar appearance;see Figure 5.9.Thin a and B layers (several mi- crometers in thickness)alternate.They are called the eutectic mi- croconstituent.(A microconstituent is a phase or a mixture of phases having characteristic features under the microscope.)This configuration allows easy interdiffusion of the silver and the cop- per atoms during solidification or during further cooling. Alloys which contain less solute than the eutectic composition are called hypoeutectic (from Greek,"below").Let us assume a Ag-20%Cu alloy which is slowly cooled from the liquid state.Upon crossing the liquidus line,initially two phases(a and liquid)are pre- sent,similar as in an isomorphous alloy.Thus,the same consider- ations apply,such as a successive change in composition during B B (a) (b) FiGURE 5.9.(a)Schematic representation of a lamellar or platelike mi- crostructure as typically observed in eutectic alloys.(b)Photomicro- graph of a eutectic alloy,180x(CuAl2-Al).Reprinted with permission from Metals Handbook,8th Edition,Vol.8(1973),ASM International, Materials Park,OH,Figure 3104,p.156

FIGURE 5.9. (a) Schematic representation of a lamellar or platelike mi￾crostructure as typically observed in eutectic alloys. (b) Photomicro￾graph of a eutectic alloy, 180 (CuAl2–Al). Reprinted with permission from Metals Handbook, 8th Edition, Vol. 8 (1973), ASM International, Materials Park, OH, Figure 3104, p. 156. The microstructure, observed by inspecting a eutectic alloy in an optical microscope, reveals a characteristic platelike or lamel￾lar appearance; see Figure 5.9. Thin  and  layers (several mi￾crometers in thickness) alternate. They are called the eutectic mi￾croconstituent. (A microconstituent is a phase or a mixture of phases having characteristic features under the microscope.) This configuration allows easy interdiffusion of the silver and the cop￾per atoms during solidification or during further cooling. Alloys which contain less solute than the eutectic composition are called hypoeutectic (from Greek, “below”). Let us assume a Ag–20% Cu alloy which is slowly cooled from the liquid state. Upon crossing the liquidus line, initially two phases ( and liquid) are pre￾sent, similar as in an isomorphous alloy. Thus, the same consider￾ations apply, such as a successive change in composition during 82 5 • Alloys and Compounds Temperature Time FIGURE 5.8. Schematic representa￾tion of a cooling curve for a eu￾tectic alloy (or for a pure metal). The curve is experimentally ob￾tained by inserting a thermome￾ter (or a thermocouple) into the liquid alloy and reading the tem￾perature in periodic time inter￾vals as the alloy cools.       (a) (b)

5.2·Phase Diagrams 83 (a) (b) FiGURE 5.10.(a)Schematic representation of a microstructure of a hy- poeutectic alloy revealing primary a particles in a lamellar mixture of a and B microconstituents.(b)Microstructure of 50/50 Pb-Sn as slowly so- lidified.Dark dendritic grains of lead-rich solid solution in a matrix of lamellar eutectic consisting of tin-rich solid solution (white)and lead- rich solid solution (dark)400X,etched in 1 part acetic acid,1 part HNO3,and 8 parts glycerol.Reprinted with permission from Metals Handbook,8th Ed.Vol 7,page 302,Figure 2508,ASM International, Materials Park,OH(1972). cooling,dendritic growth,and the lever rule.When the eutectic tem- perature(780C)has been reached,the remaining liquid transforms eutectically into a-and B-phases.Thus,the microstructure,as ob- served in an optical microscope,should reveal the initially formed a-solid-solution (called primary a,or proeutectic constituent)inter- spersed with lamellar eutectic.Indeed,the micrographs depicted in Figure 5.10 contain gray,oval-shaped a areas as well as alternating black (@)and white(B)plates in between.A schematic cooling curve for a Ag-20%Cu alloy which reflects all of the features just dis- cussed is shown in Figure 5.11(a).For comparison,the cooling curve for an isomorphous alloy is depicted in Figure 5.11(b). Silver alloys containing less than 8.8%Cu solidify similar to an isomorphous solid solution.In other words,they do not con- tain any eutectic lamellas.However,when cooled below the solvus line,the B-phase precipitates and a mixture of a-and B- phases is formed,as described previously in Section 5.2.1. Hypereutectic alloys(from Greek,"above")containing,in the present example,between 28.1 and 92%Cu in silver,behave quite analogous to the hypoeutectic alloys involving a mixture of pri- mary B-phase (appearing dark in a photomicrograph),plus plate- shaped eutectic microconstituents

cooling, dendritic growth, and the lever rule. When the eutectic tem￾perature (780°C) has been reached, the remaining liquid transforms eutectically into - and -phases. Thus, the microstructure, as ob￾served in an optical microscope, should reveal the initially formed -solid-solution (called primary , or proeutectic constituent) inter￾spersed with lamellar eutectic. Indeed, the micrographs depicted in Figure 5.10 contain gray, oval-shaped  areas as well as alternating black () and white () plates in between. A schematic cooling curve for a Ag–20% Cu alloy which reflects all of the features just dis￾cussed is shown in Figure 5.11(a). For comparison, the cooling curve for an isomorphous alloy is depicted in Figure 5.11(b). Silver alloys containing less than 8.8% Cu solidify similar to an isomorphous solid solution. In other words, they do not con￾tain any eutectic lamellas. However, when cooled below the solvus line, the -phase precipitates and a mixture of - and - phases is formed, as described previously in Section 5.2.1. Hypereutectic alloys (from Greek, “above”) containing, in the present example, between 28.1 and 92% Cu in silver, behave quite analogous to the hypoeutectic alloys involving a mixture of pri￾mary -phase (appearing dark in a photomicrograph), plus plate￾shaped eutectic microconstituents. 5.2 • Phase Diagrams 83      FIGURE 5.10. (a) Schematic representation of a microstructure of a hy￾poeutectic alloy revealing primary  particles in a lamellar mixture of  and  microconstituents. (b) Microstructure of 50/50 Pb-Sn as slowly so￾lidified. Dark dendritic grains of lead-rich solid solution in a matrix of lamellar eutectic consisting of tin-rich solid solution (white) and lead￾rich solid solution (dark) 400 , etched in 1 part acetic acid, 1 part HNO3, and 8 parts glycerol. Reprinted with permission from Metals Handbook, 8th Ed. Vol 7, page 302, Figure 2508, ASM International, Materials Park, OH (1972). (a) (b)