Solid solutions have the solute atoms and matrix atoms distrubuted randomly. The lattice type is that of the matrix material. Intermetallics have ordered arrangements of the different atom types. The lattice type is different from that of the matrix material(s) - a bigger unit cell with more atoms per lattice point. A classic example is CuZn: at high T, the lattice type is BCC. with Cu and Zn randomly on the lattice sites. At low T it is simple cubic, with one Cu and one Zn per lattice point: one atom type at the lattice point, the other type at 1/2,1/2,1/2 away from it. There is a second-order transition between the two forms at a fixed temperature.

Solid solutions have the solute atoms and matrix atoms distrubuted randomly. The lattice type is that of the matrix material. Intermetallics have ordered arrangements of the different atom types. The lattice type is different from that of the matrix material(s) - a bigger unit cell with more atoms per lattice point. A classic example is CuZn: at high T, the lattice type is BCC. with Cu and Zn randomly on the lattice sites. At low T it is simple cubic, with one Cu and one Zn per lattice point: one atom type at the lattice point, the other type at 1/2,1/2,1/2 away from it. There is a second-order transition between the two forms at a fixed temperature.

Dear Prof. Steve Roberts,

Thank you so much for your response.

It was very good.

Is it correct?

"In the intermetallics, atomic bonds is metallic-covalent, while in solid solution alloys, atomic bonds is metallic only."

With best regards

Yours sencerley

Saeed

Probably an over-simplistic distinction: but you'd need really to speak with a "quantum-mechanics of bonding" person... which I am not!

Dear Prof. Steve Roberts,

Thank you so much for your attention.

However, your response was very useful for me.

With bests

Steve's answer is quite complete, but you should reallize there are cases in which the intermetallic is not ordered. As an example, the beta phase in Cu-Zn disorders at higher temperature and becomes "conventional" BCC (just as any solid solution of metals with the BCC structure). The best distinction between an intermetallic and a simple substitutional solid solution is to require the following conditions to be satisfied by the candidate compound:

1)It has to be formed by at least two different elements (preferably metals, but there are exceptions)

2)It must have a different crystal structure from the one observed in the pure components.

Examples:

CuAu and Cu3Au order at low temperatures from a FCC continuous solid solution at high temperatures, these are intermetallics

The Beta phase in Cu-Zn and Cu-Sn (and many other Cu-bearing systems) form a disordered BCC phase at high temperature, the phase is an intermetallic because the second alloying element has never the BCC structure (there is no beta phase in Cu-Cr, check it out)

Any other compound with more complex crystal structure (like the sigma phase in Fe-Cr, for example) is an intermetallic

Also, your hypothesis about the role of covalent bonding in intermetallic stabilization is not completely true. Usually there is a great deal of covalent bonding in pure metals, in particular transition metals. There are particular cases which shows that. For instance, the alpha phase in Mn shows a complex cubic structure (labelled A13) which is also observed in many intermetallic phases (we believe Mn atoms act as two different "elements" in this structure, being therefore a "intermetallic").

You see that intermetallic research is a fascination field, which shows why researchers keep returning to this theme over and over again since the first works in the beginning of the XXth century, which coined the term.

ِDear Dr. Cláudio Geraldo Schön,

Thank you so much for you complete response.

Your response is very useful.

Yours sincerley

Saeed

Claudio - thanks for these additions, all of which I support.

A solid solution is formed when two metals are completely soluble in liquid state and also completely soluble in solid state. In other words, when homogeneous mixtures of two or more kinds of atoms (of metals) occur in the solid state, they are known as solid solutions.

Intermetallic compounds are generally formed when one metal (for example magnesium) has chemical properties which are strongly metallic and the other metal (for example antimony, tin or bismuth) has chemical properties which are only weakly metallic. Examples of intermetallic compounds are Mg2Sn, Mg2Pb, Mg3Sb2 and Mg3Bi2 . These intermetallic compounds have higher melting point than either of the parent metal. This higher melting point indicates the high strength of the chemical bond in intermetallic compounds.

http://www.newagepublishers.com/samplechapter/000627.pdf

A solid solution:  solid-state solution of one or more solutes in a solvent.

Intermetallic: solid phases containing two or more metallic elements, with optionally one or more non-metallic element. stronger bonding than solid solution

An intermetallic compound is a chemical compound just like H2O or CH4 or any other compound. It exists as its own thing, different from its constituents. Water is nothing like hydrogen or oxygen. In a phase diagram, an intermetallic compound will exist as an essentially straight vertical line (it could be at one of the edges of the diagram). There can be solid solutions where one constituent is an intermetallic compound and the other constituent is an element. There could be a eutectic point between them. 

If you go to this page http://www1.asminternational.org/asmenterprise/apd/help/intro.aspx

and scroll down to the "Intermediate Phases" you will see what I am talking about.

Bob Esser

Hi, Sorndurai, the high temperature beta phase is simple BCC (A2), space group Im(-3)m, Person symbol cB2, atoms are located in 

1(a) 0,0,0 (Cu,Zn)

1(b) 1/2,1/2/1/2 (Cu,Zn)

As for lattice parameter, I am sure there is some publication which reports it, but in case you cannot find it, I suggest you use the same lattice parameter of the low temperature phase (ordered B2) as a first approximation.

Hope this helps.

Claudio 

Hi Sornadurai, you understood wrong Steve' s answer, the B2 structure, ordered, is stable at room temperature, the high temperature phase is the A2 structure, the transformation is second order and it is depicted as a critical temperature line in the phase diagram.

Dear Prof. Cláudio Geraldo Schön 

Thank you so much for your good response.

What is difference between the second-order and first-order transition?

With Bests

Saeed

Saed, your question would deserve a topic for itself. There are many definitions of what is the order of a phase transition. The first (and most famous, you find it in many textbooks) is due to Ehrenfest (I believe) and is loved by physicists. It relates to which order of derivative (of the free energy with respect to any state function) is non-continuous: a first-order transition has a first derivative that is discontinuous, a second-order transition, has a second derivative which is discontinuous and so on. This definition, although appealing, is useful only for theoreticians. Another useful definition would be to look at the way thermochemical properties vary during a phase transition. A first-order transition has a step discontinuity (for example, there is a definite difference of composition between the equilibrium phases, or their specific volumes are quite different), while in a second-order transition both phases present the same values of thermochemical properties at the phase transition point (composition, density and so on, are the same for both phases). In other words, the second-order transition is continuous. Examples of first-order transitions are the ones we are used to, solidification, the alpha -> gamma transition in iron and so on. Second-order transitions are not so famous, but you find then everywhere, for example, the ferromagnetic -> paramagnetic transition of iron is second-order. Hope this helps.  

Dear Prof. Cláudio Geraldo Schön

Thank you so much for your useful response.

With Best Regards

Saeed

Solid solutions of metals formed in the wider area of concentration of the components, while the lattice type does not change. Lattice parameters smoothly change when content components (e.g. system Cu-Au). Intermetallic compounds are formed when a specific ratio of component in a fairly narrow area concentrations. So this new chemical compounds are formed with a different type of lattice, or other parameters of the lattice (e.g. system Sm-Co).

The discussion could also include random solid solution versus ordered solid solutions (textbook Shackelford) and stoichiometric intermetallics versus non-stoichiometric IMCs (intermediate solid solutions). See http://www.slideshare.net/N.Prakasan/intermetallics?next_slideshow=1 

Dear Saeed Hasani,

Could you give me some reference related to intermetallic? I am interested in this topic.