Phase diagrams lead-tin

Figure 4.11 The ceDtral region of the lead-tin phase diagram...

Figure 8.4 (a) The binary lead-tin phase diagram Ce is the eutectic composition, (b) The microstmcture of an alloy of... 

FigMre 9.9 The lead-tin phase diagram. For a 40 wt% Sn-60 wt% Pb alloy at 150°C (point B), phase compositions and relative amounts are computed in Example Problems 9.2 and 9.3. 

Depending on composition, several different types of microstructm-es are possible for the slow cooling of alloys belonging to binary eutectic systems. These possibilities will be considered in terms of the lead-tin phase diagram. Figure 9.8. 

In the past, the vast majority of solders have been lead-tin alloys. These materials are reliable and inexpensive and have relatively low melting temperatures. The most common lead-tin solder has a composition of 63 wt% Sn-37 wt% Pb. According to the lead-tin phase diagram, Rgure 9.8, this composition is near the eutectic and has a melting temperature of about 183°C, the lowest temperature possible with the existence of a liquid phase (at equilibrium) for the lead-tin system. This alloy is often called a eutectic lead-tin solder. 

Figure 9.18 The lead-tin phase diagram used in computations for relative amounts of primary a and eutectic microconstituents for an alloy of composition C4.

In a eutectic reaction, as found in some alloy systems, a hquid phase transforms isothermally into two different solid phases upon cooUng (i.e., L + /3). Such a reaction is noted on the copper-silver and lead-tin phase diagrams (Figures 9.7 and 9.8, respectively). 

FE From the lead-tin phase diagram (Figme 9.8), O which of the following phases/phase combinations is present for an alloy of composition 46 wt% Sn-54 wt% Pb that is at equihbrimn at 44°C ... 

And now for a real phase diagram. We have chosen the lead-tin diagram (Fig. 3.1) as our example because it is pretty straightforward and we already know a bit about it. Indeed, if you have soldered electronic components together or used soldered pipe fittings in your hot-water layout, you will already have had some direct experience of this system. 

Fig. 3.1. The phase diagram for the lead-tin alloy system. There ore three phases L - a liquid solution of lead and tin (Pb) - a solid solution of tin in lead and (Sn) - o solid solution of lead in tin. The diagram is divided up into six fields - three of them are single-phase, and three ore two-phose.

Fig 3 3 Diagrams showing how you can find the equilibrium constitution of any lead-tin alloy at 200°C. Once you have had a little practice you will be able to write down constitutions directly from the phase diagram without bothering about diagrams like (b) or ( ). 

The other place where the constitution is not fully defined is where there is a horizontal line on the phase diagram. The lead-tin diagram has one line like this - it runs across the diagram at 183°C and connects (Sn) of 2.5 wt% lead, L of 38.1% lead and (Pb) of 81% lead. Just above 183°C an alloy of tin -i- 38.1% lead is single-phase liquid (Fig. 3.5). Just below 183°C it is two-phase, (Sn) -i- (Pb). At 183°C we have a three-phase mixture of L -I- (Sn) -I- (Pb) but we can t of course say from the phase diagram what the relative weights of the three phases are. 

Fig. 3.6. (a) The copper-nickel diagram is a good deal simpler than the lead-tin one, largely because copper and nickel are completely soluble in one another in the solid state. (b) The copper-zinc diagram is much more involved than the lead-tin one, largely because there are extra (intermediate) phases in between the end (terminal] phases. However, it is still an assembly of single-phase and two-phase fields. 

Figure A 1.1 shows a phase diagram for the lead-tin system (the range of alloys obtained by mixing lead and tin, which includes soft solders). The horizontal axis is composition Xpg (at%) below and Wpg (wt%) above. The vertical axis is temperature...

The Pb-Sn system has a eutectic. Look at the Pb-Sn phase diagram (Fig. AT. 26). Above i27°C., liquid lead and liquid tin are completely miscible, that is, the one dissolves in the other completely. On cooling, solid first starts to appear when the lines (or boundaries) which limit the bottom of the liquid field are reached. 

Not all alloys in the lead-tin system show a eutectic pure lead, for example, does not. Examine the Pb-Sn phase diagram and list the composition range for which a eutectic reaction is possible. 

Figure 9. (opposite) Liquidus and separation surfaces of the copper-tin-lead system after Ref, 13. Tne appropriate portions of the phase equilibrium diagrams for Cu-Sn and Cu-Pb appear on the sides of the triangle, ( ), Intersection of the liquidus contours with the liquidus of the binary phase diagrams. One should really picture this as a solid triangular prism viewed from the top the sides of the prism would show the binary phase equilibrium diagrams. 

The Binary System Lead-Tin. The phase diagram for the lead-tin system of alloys is shown as Figure 24-5. This system rather closely resembles the system arsenic-lead, except for the fact that there is an appreciable solubility of tin in crystalline lead and a small solubility... 

Tin additions to lead-calcium alloys change dramatically the method of precipitation and age-hardening from discontinuous precipitation of PbsCa to a mixed discontinuous and continuous precipitation of PbsCa and (PbSn)3Ca and, finally, to a continuous precipitation of SnsCa. Such precipitation reactions have been described for alloys that contain low tin contents [45,48,68-70]. The reactions are not influenced or modifled by impurities in the lead alloys [71,72]. A ternary phase diagram has been proposed [41], which sets the areas of stability of PbsCa, SusCa, and mixed (PbSn)3Ca precipitates, and this is shown in Fig. 2.5. The phase diagram has been conflrmed [73]. 

The sodium lead system is important, since NaPb compounds are applied to chemical processes. The intermetallic compound NaPb is fabricated in large amounts as an intermediate for the production of tetra ethyl lead. The Na—Pb phase diagram indicates the formation of several compounds, as shown in Fig. 14. The solubility of lead in liquid sodium is considerably high, the saturated solution contains 3 at- % Pb at 250 °C. The sodium-tin system shows a similar solution and compound formation behaviour. 

The vast majority of binary phase diagrams are more complex than the example described above. Typical of many is the diagram of the lead-tin (Pb-Sn) system (Figure 4.9). 

Figure 4.9 The lead-tin (Pb-Sn) phase diagram at atmospheric pressure...

Not all systems have parent structures that show solid solution formation. Solid solution formation is generally absent if the crystal stmctures and compositions of the parent phases are quite different from each other. In general, the phase diagrams of metallic systems, drawn schematically in Figure 4.13(a), are similar in form to the lead-tin diagram. 

The phase diagram of the Be0-Y203 system is shown in Figure 4.25. Explain why this figure differs from that of the lead-tin system, Figure 4.9, and comment on the composition axis chosen. [Note answer is not provided at the end of this book.]... 

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