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EQUILIBRIUM DIAGRAMS HAVING INTERMEDIATE PHASES OR COMPOUNDS The magnesium–lead phase diagram. [Adapted from Phase Diagrams of Binary Magnesium Alloys,

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Presentation on theme: "EQUILIBRIUM DIAGRAMS HAVING INTERMEDIATE PHASES OR COMPOUNDS The magnesium–lead phase diagram. [Adapted from Phase Diagrams of Binary Magnesium Alloys,"— Presentation transcript:

1 EQUILIBRIUM DIAGRAMS HAVING INTERMEDIATE PHASES OR COMPOUNDS The magnesium–lead phase diagram. [Adapted from Phase Diagrams of Binary Magnesium Alloys, A. A. Nayeb-Hashemi and J. B. Clark (Editors), 1988 Reprinted by permission of ASM International, Materials Park, OH.]

2 PERITECTIC REACTIONS Liquid + Solid 1 Solid 2 cooling Heating Therefore,this is an example of an incongruent-melting intermediate alloy

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4 Alloy 1, 90A-10B, remains liquid until the liquidus line is reached at T 1. Solidification now takes place by forming crystals of the pure metal A. As the temperature falls, the liquid is decreasing in amount, and its composition is moving down along the liquid us line. Let us examine the conditions that exist just above the peritectic temperature Tp

5 The liquid contains 60A, while A m B n contains 70A. The liquid is not rich enough in A to form the compound by itself. The liquid must therefore react with just the right amount of solid A, in this case 8 percent, to bring its composition to that of the compound A m B n. The following reaction must have taken place at the peritectic temperature

6 The reaction takes place all around the surface of each grain of solid A where the liquid touches it. When the correct composition is reached,the layer solidifies into A m B n material surrounding every grain of A. Further reaction is slow since it must wait for the diffusion of atoms through the peritectic wall of A m B n in order to continue (see Fig. 6·40). When diffusion is completed, all the liquid will have been consumed, and since only 8 percent of pure A was required for the reaction, there will be 67 percent of A left. The final microstructure will show grains of primary A surrounded by the compound A m B n

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8 Since there was 87.5 percent liquid before the reaction and 50 percent liquid after the reaction, it is apparent that 37.5 percent of the liquid reacted with 12.5 percent of solid A to give 50 percent of the compound A m B n at the peritectic temperature

9 As cooling continues, the liquid now separates crystals of A m B n. The liquid becomes richer in B, and its composition gradually moves down and to the right along the liquidus line until it reaches point E, the eutectic temperature. At this temperature, there is only 5/ 50 x 100 or 10 percent liquid left. Since the liquid has reached the eutectic point, it now solidifies into the eutectic mixture of A m B n + B. This alloy, at room temperature, will consist of 90 percent primary or proeutectic A m B n surrounded by 10 percent of the eutectic (A m B n + B) mixture. Figure 6-42 shows the cooling curve and the changes in microstructure at various points in the slow cooling of this alloy.

10 β α pt Ag

11 Monotectic reaction Two liquids partly soluble in the liquid state

12 Two metal insoluble in the liquid and solid states

13 Transformation in the solid state Allotropy

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15 Order – disorder transformation

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18 The eutectoid reaction

19 ϒ

20 The peritectoid reaction

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22 Examples

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24 Eutectoid reaction Peritectic reaction A region of the copper–zinc phase diagram that has been enlarged to show eutectoid and peritectic invariant points, labeled E (560C, 74 wt% Zn) and P (598C, 78.6 wt% Zn), respectively. [Adapted from Binary Alloy Phase Diagrams, 2nd edition, Vol. 2, T. B. Massalski (Editor-in-Chief), Reprinted by permission of ASM International, Materials Park, OH.]

25 A portion of the nickel–titanium phase diagram on which is shown a congruent melting point for the - phase solid solution at 1310C and 44.9 wt% Ti. [Adapted from Phase Diagrams of Binary Nickel Alloys, P. Nash (Editor), Reprinted by permission of ASM International, Materials Park, OH.]

26 Figure 11-5 A hypothetical phase diagram

27 Considerations for effective dispersion strengthening: (a) The precipitate phase should be hard and discontinuous, while the matrix should be continuous and soft, (b) the dispersed phase particles should be small and numerous, (c) the dispersed phase particles should be round rather than needle-like, and (d) larger amounts of the dispersed phase increase strengthening.

28 Phase Rule Josiah Willard Gibbs (1839–1903) was a brilliant American physicist and mathematician who conducted some of the most important pioneering work related to thermodynamic equilibrium Gibbs developed the phase rule in 1875–1876. It describes the relationship between the number of components and the number of phases for a given system and the conditions that may be allowed to change (e.g., temperature, pressure, etc.). It has the general form: C is the number of chemically independent components, usually elements or compounds, in the system; F is the number of degrees of freedom, or the number of variables (such as temperature, pressure, or composition), that are allowed to change independently without changing the number of phases in equilibrium; and P is the number of phases present (please do not confuse P with pressure).

29 at point A at point B at point X Unary system

30 Binary system

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32 Eutectic, peritectic, eutectoid, peritectoid, monotectic

33 Ternary phase diagram

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36 Example

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38 Freeze near 360 o C, with primary β forming first. Near 275 o C, ϒ will also begin to form. Finally, at 160 o C,α forms and the last liquid freezes. The final microstructure contains α, β, and ϒ.


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