1. Solidification  MADE BY-Yash Jadhav - 140110119026 - Mihir Jariwala - 140110119027 - Jay Rajawat - 140110119028 - Jeet Shingala - 140110119029 - Harsh Jiyani - 140110119030  SUBJECT - Material Science and Metallurgy  Guided by - Prof. Ela Jha

2. Contents Solidification of metal Nucleation and Growth Macrostructures Effect of structure on mechanical properties

3. Solidification of a pure metal. TEMPERATURE TIME

4. Cooling Curve A cooling curve is a graphical plot of the changes in temperature with time for a material over the entire temperature range through which it cools.

5. Cooling Curve for Pure Metals Under equilibrium conditions, all metals exhibit a definite melting or freezing point. If a cooling curve is plotted for a pure metal, It will show a horizontal line at the melting or freezing temperature.

6. Cooling Curve of Alloys In this method, alloys with different compositions are melted and then the temperature of the mixture is measured at certain time intervals while cooling back to room temperature. A cooling curve for each mixture is constructed and the initial and final phase change temperatures are determined.

7. Cooling Curve Then these temperatures are used for the construction of the phase diagrams

8. Mechanism of Solidification of Metals The solidification of metals occur by nucleation and growth transformation. In nucleation and growth transformation, the nuclei of the solid phase are formed and then they grow.

9. Solidification of Metals 1. During solidification, the liquid changes in to solid during cooling. 2. The energy of liquid is less than that of the solid above the melting point. Hence liquid is stable above the melting point. 3. Below the melting point, the energy of liquid becomes more than that of the solid. 4. Hence below the melting point, the solid becomes more stable than the liquid. 5. Therefore at the melting point, liquid gets converted in to solid during cooling. 6. This transformation of liquid into solid below melting point is known as solidification.

10. Click to edit the outline text format  Second Outline Level Third Outline Level  Fourth Outline Level Fifth Outline Level Sixth Outline Level Seventh Outline Level Cooling curve for a pure metal showing possible undercooling. The transformation temperature, as shown on the equilibrium diagram, represents the point at which the free energy of the solid phase is equal to that of the liquid phase. Thus, we may consider the transition, as given in a phase diagram, to occur when the free energy change, ΔGV, is infinitesimally small and negative, i.e. when a small but positive driving force exists due to undercooling.

11. Click to edit the outline text format  Second Outline Level Third Outline Level  Fourth Outline Level Fifth Outline Level Sixth Outline Level Seventh Outline Level Nucleation and Growth of Crystals At the solidification temperature, atoms from the liquid, such as molten metal, begin to bond together and start to form crystals. The moment a crystal begins to grow is know as nucleus and the point where it occurs is the nucleation point. When a metal begins to solidify, multiple crystals begin to grow in the liquid. The final sizes of the individual crystals depend on the number of nucleation points. The crystals increase in size by the progressive addition of atoms and grow until they impinge upon adjacent growing crystal. a)Nucleation of crystals, b) crystal growth, c) irregular grains form as crystals grow together, d) grain boundaries as seen in a microscope.

16. Nucleation and Growth Transformation Nucleation – The physical process by which a new phase is produced in a material. In the case of solidification, this refers to the formation of tiny stable solid particles in the liquid. Growth - The physical process by which a new phase increases in size. In the case of solidification, this refers to the formation of a stable solid particle as the liquid freezes.

17. crystal growth and grain formation nuclei → crystals → grains polycrystalline – solidified metal containing many crystals grains – crystals in solidified metal grain boundaries – the surfaces between the grains two major types of grain structures: (1) equiaxed grains – crystals grow about equally in all directions, commonly found adjacent to a cold mold wall (2) columnar grains – long, thin, coarse grains, created when metal solidifies rather slow in the presence of a steep temperature gradient. columnar grains grow perpendicular to the mold surface

18. Nucleation and Growth Transformation in solid solution wt% Ni 20 1200 1300 304050 1100 L (liquid)  (solid) L +  L +  T(°C) A 35 C0C0 L: 35wt%Ni 46 35 43 32  : 43 wt% Ni L: 32 wt% Ni B  : 46 wt% Ni L: 35 wt% Ni C E L: 24 wt% Ni  : 36 wt% Ni 24 36 D

19. Dendrites

20. In metals, the crystals that form in the liquid during freezing generally follow a pattern consisting of a main branch with many appendages. A crystal with this morphology slightly resembles a pine tree and is called a dendrite, which means branching. The formation of dendrites occurs because crystals grow in defined planes due to the crystal lattice they create. The figure to the right shows how a cubic crystal can grow in a melt in three dimensions, which correspond to the six faces of the cube. For clarity of illustration, the adding of unit cells with continued solidification from the six faces is shown simply as lines. Secondary dendrite arms branch off the primary arm, and tertiary arms off the secondary arms and etcetera.

22. Dendrites During freezing of a polycrystalline material, many dendritic crystals form and grow until they eventually become large enough to impinge upon each other. Eventually, the interdendriticspaces between the dendrite arms crystallize to yield a more regular crystal. The original dendritic pattern may not be apparent when examining the microstructure of a material. However, dendrites can often be seen in solidification voids that sometimes occur in castings or welds, as shown in the next slide..

23. SOLIDIFICATION OF AN INGOT

25. Effect of Structure on Mechanical Properties  The size of the grains,or average grain diameter,in a polycrystalline metal influences the mechanical properties.  Atomic disorder within a grain boundary region will result in a discontinuity of slip planes from one grain into other.  A fine grain material one that has small grain is harder and stronger than one that is coarse grain.

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