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Strengthening Mechanisms Metallurgy for the Non-Metallurgist.

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Presentation on theme: "Strengthening Mechanisms Metallurgy for the Non-Metallurgist."— Presentation transcript:

1 Strengthening Mechanisms Metallurgy for the Non-Metallurgist

2 Learning Objectives After completing this lesson, students will be able to: o Describe the effects of alloying elements on the strength of metals o Define age (precipitation) hardening o Define cold working and annealing o Explain the optimal strengthening mechanisms for several groups of alloys

3 Introduction Introduction : Strengthening Mechanisms Examine alloying, grain size, dislocation content(strain hardening or cold working), age or precipitation hardening Annealing Diffusion Precipitation heat treatments Delay coverage of martensitic transformations(steels)

4 Schematic representation of two possible diffusion mechanisms. (a) Exchange mechanism. (b) Ring mechanism

5 Diffusion temperature dependence Exponential…. ….because the number of vacancies increases exponentially with temperature. Interstitial diffusion is rapid because the number of vacancies is always high.

6 Schematic depiction of diffusion by vacancy movement

7 The effect of cold work on the tensile properties of yellow brass (66Cu- 34Zn)

8 Microstructure of alloy 260 (cartridge brass, 70%), reduced 50% by cold rolling from 6.071–3.048 mm (0.239–0.120 in). Nominal tensile strength, 593 MPa (86,000 psi). Original magnification, 75×

9 Microstructure of alloy 260 (cartridge brass, 70%). Same process as in Fig. 4 shown after recrystallization anneal at 427 °C (800 °F). Grain size, mm. Original magnification: 75×

10 The effect of annealing temperature on the mechanical properties and grain size of yellow brass (65Cu-35Zn) previously cold reduced 60%

11 Recrystallization: Replacement of worked grains with entirely new grains New grains are dislocation free Hi MP metals have high rex’t temperaures Increased cold work decreases rex’t temp Increased annealing time decreases rex’t temp Finer grain size: lower rex’t temp Alloying increases rex’t temp

12 Microstructure of alloy 260 (cartridge brass, 70%). Same process as in Fig. 4 shown after recrystallization and grain growth. Grain size, mm. Original magnification: 75×

13 Effect of annealing time and temperature on the grain size of yellow brass (65Cu-35Zn) that was heavily cold worked before annealing

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15 Substitutional Solid Solutions: Hume-Rothery Rules For extended solid solubility….. Size of solute within 15% of solvent Same crystal structure Solute same or higher valency Similar electronegativity

16 Lattice distortion caused by interstitial and substitutional solute additions

17 Effects of alloying elements on yield strength of copper

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19 A portion of the phase diagram for the hypothetical alloy system A-B

20 Stages of Precipitation Equilibrium solution at elevated T Quenching for maximum nucleation, Low T Initial growth of coherent precipitates, Med T Development of maximum strength with t Loss of coherency, overaging, Med T, too much time May chose slight overage for toughness, stress corrosion resistance

21 Relation between aging curve and microstructure

22 A portion of the aluminum-copper phase diagram. Note that the solubility of copper in aluminum decreases as the temperature increases. Additionally note that the maximum amount of copper that can be added to aluminum and still be able to solution anneal the alloy is 5.65%.

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25 Some of the other suffix designations and their meanings are: T1: Cooled from an elevated temperature shaping process and naturally aged to a substantially stable condition T2: Annealed (cast products only) T3: Solution treatment then cold worked T5: Artificial aging only. This applies to alloys that have not been solution treated, but have been rapidly cooled after an elevated-temperature fabricating process such as casting or extrusion T7: Solution heat treated and then stabilized T8: Solution heat treated, cold worked, and then artificially aged T9: Solution heat treated, artificially aged, and then cold worked T10: Cooled from an elevated-temperature shaping process, cold worked, and then artificially aged

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29 The copper-zinc phase diagram

30 Typical microstructure of Cu-40Zn (known as Muntz metal) furnace cooled from 825 °C (1515 °F). Coarse α plates (light) have precipitated at the grain boundaries and within the original β grains. Original magnification: 50×

31 Precipitation hardening of Cu-40Zn brass (see copper-zinc diagram, Fig. 14). Samples were heated 30 min at 800 °C (1470 °F), quenched to retain the supersaturated β′ phase, and then aged 30 min at various temperatures to precipitate fine dispersion of α phase in the β′.

32 Effect of vanadium on the α-to-β transition in a Ti6Al alloy

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34 Microstructures in Ti-6Al-4V alloy after the following heat treatments: (a) Forged at 955 °C (1750 °F) just below β transus. Elongated α (white) in a matrix of β in which time α has precipitated on cooling. (b) Forged at 1040 °C (1900 °F) in all-β phase, slow cooled to form large plates of α (white), then annealed at 705 °C (1300 °F). Some residual β remains between the α plates (dark). (c) Forging annealed at 955 °C (1750 °F), air cooled, annealed at 705 °C (1300 °F), and slowly cooled. Equiaxed α grains in matrix of β in which some coarse α plates have precipitated

35 Summary: Strengthening Mechanisms(RT) Same alloy maybe ductile, low strength or less ductile and high strength Fine grains very effective Solid solution good at RT, high T Working increases dislocation content, strength As strength increases, ductility decreases Precipitation very effective, controlled by nucleation and growth, refer to phase diagrams Annealing: stress relief(recovery), recrystallization, grain growth, solution


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