PPT Hardening (1).pptx this is the type of Heat treatment

precipitaion hardening tell us about the strengthening of materials . the materials grain boundry effect

1. Precipitation Hardening The strength and hardness of some metal alloys may be improved by the formation of extremely small, uniformly dispersed particles ( precipitates ) of a second phase within the original phase matrix. Other alloys that can be precipitation hardened or age hardened : Copper-beryllium (Cu-Be) Copper-tin (Cu-Sn) Magnesium-aluminum (Mg-Al) Aluminum-copper (Al-Cu) High-strength aluminum alloys

2. c11f40 Criteria: Maximum solubility of 1 component in the other ( M ); Solubility limit that rapidly decreases with decrease in temperature ( M→N ). Process: Solution Heat Treatment – first heat treatment where all solute atoms are dissolved to form a single-phase solid solution. Heat to T and dissolve B phase. Rapidly quench to T 1 Nonequilibrium state ( a phase solid solution supersaturated with B atoms; alloy is soft, weak -no ppts ). Phase Diagram for Precipitation Hardened Alloy

3. c11f43 The supersaturated a solid solution is usually heated to an intermediate temperature T 2 within the a+b region ( diffusion rates increase). The b precipitates ( PPT ) begin to form as finely dispersed particles. This process is referred to as aging . After aging at T 2 , the alloy is cooled to room temperature. Strength and hardness of the alloy depend on the ppt temperature (T 2 ) and the aging time at this temperature. Precipitation Heat Treatment – the 2 nd stage

4. 10 20 30 40 50 wt% Cu L  + L a a+q q  + L 300 400 500 600 700 (Al) T (°C) composition range available for precipitation hardening CuAl 2 A Precipitation Hardening • Particles impede dislocation motion. • Ex: Al-Cu system • Procedure: -- Pt B : quench to room temp. (retain a solid solution) -- Pt C : reheat to nucleate small q particles within a phase. Temp. Time -- Pt A : solution heat treat (get a solid solution) Pt A (solution heat treat) B Pt B C Pt C (precipitate ) At room temperature the stable state of an aluminum-copper alloy is an aluminum-rich solid solution (α) and an intermetallic phase with a tetragonal crystal structure having nominal composition CuAl 2 (θ).

5. c11f43 Precipitation Heat Treatment – the 2 nd stage PPT behavior is represented in the diagram: With increasing time, the hardness increases, reaching a maximum ( peak ), then decreasing in strength. The reduction in strength and hardness after long periods is overaging (continued particle growth). Small solute-enriched regions in a solid solution where the lattice is identical or somewhat perturbed from that of the solid solution are called Guinier -Preston zones.

6. • 2014 Al Alloy: • TS peak with precipitation time. • Increasing T accelerates process. Influence of Precipitation Heat Treatment on Tensile Strength (TS), %EL precipitation heat treat time tensile strength (MPa) 200 300 400 100 1min 1h 1day 1mo 1yr 204°C non-equil. solid solution many small precipitates “aged” fewer large precipitates “overaged” 149°C • %EL reaches minimum with precipitation time. % EL (2 in sample) 10 20 30 1min 1h 1day 1mo 1yr 204°C 149 °C precipitation heat treat time

7. c11f45 Effects of Temperature Characteristics of a 2014 aluminum alloy (0.9 wt% Si, 4.4 wt% Cu, 0.8 wt% Mn , 0.5 wt% Mg) at 4 different aging temperatures.

8. 8 Step 1: Solution Treatment Step 2: Quench Step 3: Age Guinier-Preston (GP) zones - Tiny clusters of atoms that precipitate from the matrix in the early stages of the age-hardening process. Section 11.5 Microstructural Evolution in Age or Precipitation Hardening

9. 9 ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure 11.10 The aluminum-rich end of the aluminum-copper phase diagram showing the three steps in the age-hardening heat treatment and the microstructures that are produced.

10. 10 Compare the composition of the a solid solution in the Al-4% Cu alloy at room temperature when the alloy cools under equilibrium conditions with that when the alloy is quenched. Example 11.2 Composition of Al-4% Cu Alloy Phases ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure 11.5 The aluminum-copper phase diagram and the microstructures that may develop curing cooling of an Al-4% Cu alloy.

11. 11 Example 11.2 SOLUTION From Figure 11.5, a tie line can be drawn at room temperature. The composition of the α determined from the tie line is about 0.02% Cu. However, the composition of the α after quenching is still 4% Cu. Since α contains more than the equilibrium copper content, the α is supersaturated with copper.

12. 12 The magnesium-aluminum phase diagram is shown in Figure 11.11. Suppose a Mg-8% Al alloy is responsive to an age-hardening heat treatment. Design a heat treatment for the alloy. Example 11.3 Design of an Age-Hardening Treatment ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure 11.11 Portion of the aluminum-magnesium phase diagram .

13. 13 Example 11.3 SOLUTION Step 1: Solution-treat at a temperature between the solvus and the eutectic to avoid hot shortness. Thus, heat between 340 o C and 451 o C. Step 2: Quench to room temperature fast enough to prevent the precipitate phase β from forming. Step 3: Age at a temperature below the solvus, that is, below 340 o C, to form a fine dispersion of β phase.

14. 14 Section 11.6 Effects of Aging Temperature and Time Figure 11.13 The effect of aging temperature and time on the yield strength of an Al-4% Cu alloy. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license.

15. 15 The operator of a furnace left for his hour lunch break without removing the Al-4% Cu alloy from the furnace used for the aging treatment. Compare the effect on the yield strength of the extra hour of aging for the aging temperatures of 190 o C and 260 o C. Example 11.4 Effect of Aging Heat Treatment Time on the Strength of Aluminum Alloys ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure 11.13 The effect of aging temperature and time on the yield strength of an Al-4% Cu alloy.

16. 16 Example 11.4 SOLUTION At 190 o C, the peak strength of 400 MPa (60,000 psi) occurs at 2 h (Figure 11.13). After 3 h, the strength is essentially the same. At 260 o C, the peak strength of 340 MPa (50,000 psi) occurs at 0.06 h. However, after 1 h, the strength decreases to 250 MPa (40,000 psi). Thus, the higher aging temperature gives lower peak strength and makes the strength more sensitive to aging time.

17. 17 The alloy system must display decreasing solid solubility with decreasing temperature. The matrix should be relatively soft and ductile, and the precipitate should be hard and brittle. The alloy must be quenchable. A coherent precipitate must form. Section 11.7 Requirements for Age Hardening

18. Aluminum rivets Alloys that experience significant precipitation hardening at room temp and after short periods must be quenched to and stored under refrigerated conditions. Several aluminum alloys that are used for rivets exhibit this behavior. They are driven while still soft, then allowed to age harden at the normal room temperature.