Solidification Of Alloys

1)      Solid solution alloys exhibit different freezing.

2)      Differential freezing promotes growth in a manner other than by the advance of a smooth interface.

3)      Solid solution alloys base upon solid solubility which is the irreversible mixing of two solids into a single phase. Phase may be defined as a physically distinct region of matter having characteristic atomic structure and properties which change continuously with temperature, composition or other thermodynamic variables. The various phases of the system, in principle, can be mechanically separated.

4)      A solid solution binary alloy system is one in which two metals are completely soluble in both solid and liquid state.

5)      A solid solution is the result of metals dissolving in each others crystal lattice.

6)      Copper- nickel and gold- silver are two examples of solid solution alloys.

7)       Following fig. shows the cooling curve of a binary solid solution alloys.

a)      From A to B, the alloy is in liquid state.

b)      Solidification starts at point B and completes at C.

c)      Unlike pure metals, solidification occurs throughout the temperature range (i.e. from Tb to Tc)

d)     Latent heat of fusion is liberated gradually from B to C and it tends to increase the time required for the solidification.

Phase diagram:

1)      If two metals of binary solid solution system are mixed in different proportions and a cooling curve is constructed for each composition, the resulting diagram will be one shown in fig. below which is phase diagram for the alloy system.

1)      A phase diagram shows two different and distinct phases, one is liquid metal solution and other is solid solution.

2)      Within these two phases i.e. liquidus and solidus, the two phases- liquid and solid exit together. These two phages existing together can be mechanically separated by decantation of the liquid phase.

3)      Liquidus is that line (a) above which the alloy is in liquid state and (b) where solidification starts.

4)      Solidus is that line (a) below which the alloy is in solid state and (b) where solidification completes.

5)      If in a phase diagram, for each change of phase, adequate time is allowed for the change to complete so that phage change takes place under equilibrium conditions, the phase diagram will be also known as equilibrium diagram.

6)      Alloy solidification occurring under equilibrium conditions is known as equilibrium solidification.

7)      Equilibrium conditions are not generally attained during the solidification of the castings because the diffusion involved may be extremely sluggish due to fast cooling rate of the castings. Thus most frequently castings solidify under non- equilibrium conditions and the solidification process is known as non-equilibrium solidification.

8)      Non- equilibrium solidification results in porous, columnar (or dendrite), cored material which is usually of very inhomogeneous composition.

9)      Non-equilibrium solidification involves fast cooling rate and does not permit complete diffusion in the solid state and thus result in coring and segregation.