AH_D_lecture11 - Materials Science & Metallurgy Pt IB Phase...

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Materials Science & Metallurgy Pt IB Phase Transformations: Course A Lecture 11 _____________________________________________________________________________________________________________________ Lecture 11: Solid-state diffusionless transformations and twins 11.1 Twinning: no phase change. No composition change or phase change occurs and involves: either shear transformation during deformation or diffusion during recrystallisation. mirror image of original structure across twin plane. Twinned regions have same crystallographic structure but material is in a different orientation hence possible to see twins macroscopically in polished etched samples. 11.1.1 Deformation (mechanical) twins Shape change occurs by shear and not by dislocation movement, i.e no slip occurs. Each atom moves only very small distance but overall shear (deformation) can be large and seen macroscopically. Occurs if: (a) sample has low crystal symmetry few active slip systems for general plasticity; hexagonal metals (ideally only 3 slip systems); tetragonal Sn also. But note that such materials may still deform by slip (plastically flow) at higher temperatures when non-close packed systems are themally activated. (b) high strain rate - need to dissipate mechanical energy very quickly; - twins grow at speed of sound (much faster than dislocation movement); - occurs in explosively formed material e.g. bcc Fe. (c) dislocations are heavily pinned and cannot move (martensite formation in high C steel). Appearance: 2005 - 2006 Page 63 Rob Wallach
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Materials Science & Metallurgy Pt IB Phase Transformations: Course A Lecture 11 _____________________________________________________________________________________________________________________ 11.1.2.Annealing (growth) twins: f.c.c. metals only Form during recrystallisation as new grains with lower strain energy grow by diffusion from sub-grains. No deformation involved in their formation.
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This note was uploaded on 12/05/2011 for the course MSE 4100 taught by Professor Hennig during the Fall '11 term at Cornell University (Engineering School).

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AH_D_lecture11 - Materials Science & Metallurgy Pt IB Phase...

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