Assignment deadline Monday
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Transcript of Assignment deadline Monday
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Assignment deadline Monday
• Explain in a one page essay, how an iron alloy can be designed to emulate the nickel based superalloy, containing ordered precipitates which are coherent with the matrix.
• For information on nickel based alloy see:• http://www.msm.cam.ac.uk/phase-trans/
2003/Superalloys/superalloys.html
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Interstices in Cubic-I, Cubic-F and HCP Structures
Crystallography H. K. D. H. Bhadeshia
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C 62 pm Ni 126 pm Fe 124 pm Cr 130 pm
(Andrew Fairbank)
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silicon in iron
carbon in iron
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Insterstitial solid solution strengthening
(Ghosh & Olson)
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Cubic-I
Close-packed direction?
(Andrew Fairbank)
Ferrite
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radius of iron atom in ferrite
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octahedral interstice in ferritepoint group symmetry?
Carbon does not fit. Therefore, placing in octahedral site reduces the tetragonality of the site.
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Largest atom that can fit in octahedral site
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tertrahedral interstice in ferrite
Vector joining centres of iron and carbon?
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Structure projection of 4 unit cells of ferrite
What it the vector joining iron and carbon atom neighbours?
x
y
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structure projection
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3 octahedral sites per Fe6 tetrahedral sites per Fe
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austenitecubic-F
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octahedral interstice in austenitepoint group symmetry?
Carbon does not fit. It causes uniform expansion.
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Largest atom that can fit in octahedral site
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tertrahedral interstice in austenite
Vector joining centres of iron and carbon?
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Insterstitial solid solution strengthening
(Ghosh & Olson)
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ferrite
austenite
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Ferrite
• Carbon in “smaller” anisotropic octahedral interstices
• Resulting strain is anisotropic• Strong interaction with deviatoric and
dilatational strain fields of dislocations• Intense strengthening• 3 octahedral and 6 tetrahedral holes per iron
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Austenite
• carbon in larger isotropic octahedral interstices
• therefore, behaves like substitutional solute with weak interactions with dislocations
• mild strengthening• 1 octahedral and 2 tetrahedral holes per iron
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(a)
BAIN STRAIN
(c) Body-centered
tetragonal austenite
(d) Body-centered
cubic martensite
a
a
a1
2
3 b3
b1 b2
(b)
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tetragonal martensite?
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