Solidification of metals
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This document discusses the properties of liquid and solid phases of metals. It notes that while solids and gases can be easily classified, liquids have many unknown properties. X-ray diffraction has shown that liquids have short-lived ordered atomic structures similar to solids. There is only a small difference in bonding between solids and liquids. The document also examines how the free energy of solids, liquids, and gases varies with temperature and pressure, and discusses the melting point, boiling point, and triple point of phase changes.
Solidification of metals
- 2. LIQUID PHASE Solids and gases are on two different extremes of classification and can be classified easily while many properties of liquid are unknown. E.g. the solids are considered as the most ordered state and gas as the most disordered state but liquid has an intermediate entropy which is not completely studied like other properties of liquid.
- 3. CONT. … There is a small difference in bonding of solid and liquid as the change of liquid from solid gives a density change of 2-6% Further the latent heat of fusion during melting is 1/25 to 1/40 as compare to its latent heat of vaporization. All of these thing lead to a conclusion that solid and liquid have the same bonding.
- 4. CONT. … X-ray diffraction also confirmed this by showing that liquid has a short lived group of atoms having ordered structure like solids and having same coordination num. like solids. Due to many defects in liquids the ordered structure is not long lived. Due to high mobility of liquid atoms the diffusion in liquid is high just below its melting point as compare to solids just above the M.P.
- 5. CONT. … The high diffusion is also due to defects in liquids. The liquid phase is an ever changing phase with changing structure and properties with time. A difference in solid and liquid is the diffusion in solid is by vacancy mechanism and will not change the structure but in liquids the diffusion due to defects will continuously change its structure.
- 6. CONT. …. The liquid phases of most metals are similar in their properties as compare to metals in solid states having different properties. The properties of liquid metal depends upon the structural defects than the nature of bonding which is contrary to solids. The solid phase has the least entropy and internal energy contrary to liquid while liquid has an intermediate between the two.
- 7. CONT. …. The free energy of gas falls more rapidly, next of liquids and in the last of solids but this varies with pressure. From the figure if we consider the 3 points aa , bb & cc and draw the curves of solid , liquid and gas showing the relation between free energy and temperature at constant pressure.
- 9. CONSIDER THE ISOBAR AA…
- 10. ISOBAR AA… This shows that below Tm the solid phase has lowest free energy and at Tm the internal energy of liquid start to decrease which become more stable until Tb and after that gas is stable. By figure the Tm is point on which liquid and solid curve meets and Tb is point on which liquid and gas meet.
- 12. ISOBAR BB… It passes through the triple point on which all the 3 phases coexist and below this point the only stable phase is solid and after this the only stable phase is the gaseous phase.
- 14. ISOBAR CC… This lies below the triple point and the curve also shows that the free energy of liquid is greater than the solid and gas thus it will not exist. A point exist where the solid is transformed to gas this is the temperature of sublimation.
Editor's Notes
- #4: Ives