Selection, Phase Diagrams & Microstructure-Property Relationships Essay Example

Selection, Phase Diagrams & Microstructure-Property Relationships 5

Selection, Phase Diagrams & Microstructure-Property Relationships

Q. 1. The cracking and leaking occurred due to a process called dezincification. This is a form of corrosion that occurs when zinc is leached out of the alloy leaving a weakened porous copper fitting. Dezincification occurs particularly when the temperature is increased. This process leads to copper being re-deposited in the region that is cracked, obscuring the damage. Since, the re-deposited copper is spongy, brittle, and weak, it leads to failing and leaking. Therefore, dezincification appears to be a possible explanation.

Q.2 Ceramic and concrete are hard materials that cannot be reshaped without breakage, as opposed to mild steel which can be bent. These materials can crack when hit by items, hence are not suitable in car panel construction.

Q.3 (a). Rapidly cool to 1000oc and hold for 1 second, then rapidly cool to 600oc and hold for 1 minute, then rapidly cool to 400oc then hold for 5 minutes. Quench to room temperature.

(b). rapidly heat to 1400oc then cool 100 seconds.

(c). rapidly heat to 1400oc then cool slowly for 1000 seconds.

(d). rapidly cool to 1000oc then hold for 5 seconds.

Q.4. Aging is the third step in precipitation hardening where the supersaturated solid solution is heated below the solvus temperature to produce a finely dispersed precipitate. Atoms diffuse only short distances at this aging temperature.

In the age hardening of Al-Cu alloys, four structures of the precipitates can be recognized: These are GP-1 zones, the GP-2 Zones, the ‘θ’ Phase, and then the (CuAl2) phase. GP-1 zones grow rapidly up to a certain size, and then growth practically stops. At room temperature this size is 3-7 nm thick. At 70-130°C the diameter is of the order of 10-15 nm. Hardening and a decrease in ductility accompany the formation and growth of the GP-1 zones. GP-2 zones then follows after. These zones are larger than the GP-1 zones and their number is correspondingly smaller since the amount of solvent in the zones does not change. The size range for this phase is 10-100 nm diameter, and 1-4 nm thick. With continued aging, the ‘θ’ phase will begin to form causing recrystallization, softening, and a decrease in strength, a process called over-aging. The ‘θ’ phase has a tetragonal structure with a different lattice parameter from the matrix; no coherency strains exist, but each particle is surrounded by a ring of
dislocations. The size of the θ’ phase depends on time and temperature; size ranges from 10 to 600 nm in diameter with a thickness of 10-15 nm.

Finally, the (CuAl2) phase occurs which has the same structure and composition as the θ phase found by solidification.

Over-aging heat treatment is be used to enhance the stress corrosion resistance without unacceptable strength penalties in Al-Zn-Mg alloys containing copper, a process called intergranular corrosion.

Q.5. (a). (i). Liquid phase present. L

(ii). CL= 25wt% Cu- 75wt% Ag

(iii). Relative amount present = 100%

(b). (i). Two phases present; α + L

(ii). Cα= 9 wt% Ag — 91 wt% Cu

CL = 44 wt% Ag – 56 wt% Cu

(iii). Wα = Selection, Phase Diagrams & Microstructure-Property Relationships = Selection, Phase Diagrams & Microstructure-Property Relationships 1 = 0.472 = 47.2%

WL = Selection, Phase Diagrams & Microstructure-Property Relationships 2 = Selection, Phase Diagrams & Microstructure-Property Relationships 3 = 0.527 = 52.7%

(c).
(i). Two phases present; α + β

(ii). Cα= 7 wt% Ag — 93 wt% Cu

Cβ = 92 wt% Ag – 8 wt% Cu

(iii). Wα = Selection, Phase Diagrams & Microstructure-Property Relationships 4 = Selection, Phase Diagrams & Microstructure-Property Relationships 5 = 0.212 = 21.2%

Wβ = Selection, Phase Diagrams & Microstructure-Property Relationships 6 = Selection, Phase Diagrams & Microstructure-Property Relationships 7 = 0.788 = 78.8%

Bibliography

Alper A.M. (1970) Phase diagrams; materials science and technology, New York: Academic

Bullens D. K. (1948) Steel and its heat treatment, (5th ed.). New York: J. Wiley. 33-40

Grossmann, M. A., & Bain, E. C. (1964) Principles of heat treatment, (5th ed.). Metals Park,

Ohio: American Society for Metals. 10-22

Kakani S.L. and Kakani A. (2004). Material science, New Delhi: New Age International. 90-107

Porter D. A. and Easterling K. E. (1981) Phase transformations in metals and alloys. New York:

Van Nostrand Reinhold. 20-31

Skalny J. and Mindess S. (1989) Materials science of concrete. Westerville, OH: American

Ceramic Society. 55-73