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Engineering Materials 2E VOLUME2 Episode 4

Tham khảo tài liệu 'engineering materials 2e volume2 episode 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 66 Engineering Materials 2 from the solid-liquid interface for the same sort of reason that your hands lose heat much more rapidly if you wear gloves than if you wear mittens . And the faster solidification that we get as a consequence pays for the high boundary energy. Large driving forces produce fine dendrites - which explains why one can hardly see the dendrites in an iced lollipop grown in a freezer -10 C but they are obvious on a freezing pond -1 C . To summarise the shapes of the grains and phases produced during transformations reflect a balance between the need to minimise the total boundary energy and the need to maximise the speed of transformation. Close to equilibrium when the driving force for the transformation is small the grains and phases are primarily shaped by the boundary energies. Far from equilibrium when the driving force for the transformation is large the structure depends strongly on the mechanism of the transformation. Further even the scale of the structure depends on the driving force - the larger the driving force the finer the structure. Further reading D. A. Porter and K. E. Easterling Phase Transformations in Metals and Alloys 2nd edition Chapman and Hall 1992. M. F. Ashby and D. R. H. Jones Engineering Materials I 2nd edition Butterworth-Heinemann 1996. G. A. Chadwick Metallography of Phase Transformations Butterworth 1972. P. G. Shewmon Diffusion in Solids 2nd edition TMS Publishers 1989. Problems 6.1 The solidification speed of salol is about 2.3 mm min-1 at 10 C. Using eqn. 6.15 estimate the energy barrier q that must be crossed by molecules moving from liquid sites to solid sites. The melting point of salol is 43 C and its latent heat of fusion is 3.2 X 10-20 J molecule-1. Assume that the molecular diameter is about 1 nm. Answer 6.61 X 10-20 J equivalent to 39.8 kJ mol-1. 6.2 Glass ceramics are a new class of high-technology crystalline ceramic. They are made by taking complex amorphous glasses like SiO2-Al2O3-Li2O and making