Dear Shahin,

All the above answers are correct and cover several factors. Before I add on, it is worth reiterating that the statement of the question need not always be true - many impurities deteriorate the strength of metals and some second phases do improve the strengths of composites, so there really is no generalisation.

One fundamental difference between strengthening metals and composites is this:

(1) Metals are strengthened by obstructing movement of dislocations, which is essentially in the plastic regime.

(2) Composites are strengthened by adding a stronger second phase, which takes most of the load. This is usually in the elastic regime.

So while strengthening of metals is because of various complex dislocation interactions, strengthening of composites is because of a simple rule-of-mixtures formulation, where the stronger phases simply raises the 'average' strength of the composite. Now remember that in a composite, the matrix has to 'transfer' the load to the second phase- if this is not done effectively, the second phase can become a stress riser (as correctly stated by I. Dinaharan).

In a metal, the second phase (say dispersoid or precipitate) is not really 'bearing' much more load than the matrix, so this possibility is reduced. Certainly, the degree of strengthening can reduce if the size and spacing of dispersoids/precipitates changes, but stress-concentration would usually not take place. The exceptions are cases when a precipitate/impurity can change local properties such as wettability (eg. hot cracking in welds) or boundary cohesion (eg. sensitisation in stainless steel), which then leads to failure.

Around a dislocation, there is a strain field which is a pair of compression and tension. And the strain decays as apart from the dislocation centre following Saint Venant's principle. 

So, I think this decaying length is the factor that tells the difference between the impurity atoms and element of composites. If the size of the composite elements (fibres or particles) were smaller than this length, they work like impurity atoms. 

Does this make sense?        

There are no general results for the introduction of impurities or additives. In alloys they tend to reduce the melting point, with semi-metals impurities affect the electrical behavior of them. Composites are not exception, the additives may improve mechanical properties, like hardness and tensile strength, of polymer composites . The effects are functions of the particle size and additive contents.  

Dear Shihan

In general composite materials have less coherent microstructures, less continuum microstructures, in a comparison to the metallic alloy microstructures.   In other words, the physical properties, i.e., yield strength, ultimate strength, of the metallic alloys , with solid solution phases & impurities, would comply with continuum mechanics more than the composite materials.  Consequently, the experimental (obtained) physical properties of metallic alloys would be closer to their theoretical values than the experimental properties of the composite materials to their theoretical values.

It is worth noting that continuum mechanics of materials is based on the following constrained assumptions:

1-The materials are continuous, coherent, with no point defects ,i.e., interstitial & substitution atoms,   line defects, i.e., dislocation, and planar defects, i.e., stacking faults

2-The physical properties of materials are in a steady state condition, the properties don't change with respect to aging time of the materials.

3-The materials comply with the constitutional laws of mass & energy.

So, from the above constrained assumptions, one would see that the metallic alloys are more compliance with continuum mechanics than the composite materials.     

In solid solutions it is atomic level strengthening resulting from resistance to dislocation motion. In composites the constituents are mutually not soluble and strength improvement is mainly due to interface strengthening. It is like difference between age hardening and dispersion strengthening.

There is a exchange of electrons between alloying elements. One elements dissolve into other element upto solubility limit in alloys. Some alloy systems generate eutectic or eutectoid compounds. The lattice of the one element is distorted by addition of another element due to size difference. All the above are totally absent in metal matrix composites. If the MMC is produced by liquid metallurgy processing such as stir casting the large difference of coefficient of thermal expansion between matrix material and reinforcement create large amount of strain fields around the reinforcement particles. The interfacial bonding is a crucial factor which helps to transfer the load effectively to the reinforcement. A good distribution of reinforcement particles may operate Orowan strengthening. Hence, the behavior of alloys and MMCs are different. MMCs exhibit superior properties compared to alloys i.e. solid solutions.

Dear Shahin,

All the above answers are correct and cover several factors. Before I add on, it is worth reiterating that the statement of the question need not always be true - many impurities deteriorate the strength of metals and some second phases do improve the strengths of composites, so there really is no generalisation.

One fundamental difference between strengthening metals and composites is this:

(1) Metals are strengthened by obstructing movement of dislocations, which is essentially in the plastic regime.

(2) Composites are strengthened by adding a stronger second phase, which takes most of the load. This is usually in the elastic regime.

So while strengthening of metals is because of various complex dislocation interactions, strengthening of composites is because of a simple rule-of-mixtures formulation, where the stronger phases simply raises the 'average' strength of the composite. Now remember that in a composite, the matrix has to 'transfer' the load to the second phase- if this is not done effectively, the second phase can become a stress riser (as correctly stated by I. Dinaharan).

In a metal, the second phase (say dispersoid or precipitate) is not really 'bearing' much more load than the matrix, so this possibility is reduced. Certainly, the degree of strengthening can reduce if the size and spacing of dispersoids/precipitates changes, but stress-concentration would usually not take place. The exceptions are cases when a precipitate/impurity can change local properties such as wettability (eg. hot cracking in welds) or boundary cohesion (eg. sensitisation in stainless steel), which then leads to failure.

Many thanks for all contributions and answers, I am so pleased to see such quality answers.

Also, Prof. Liang Tian from Michgan Uni offered his following works to enlighten and to be shared with interested people:

Tian, Liang, et al. "The microstructure-strength relationship in a deformation processed Al–Ca composite." Materials Science and Engineering: A 570 (2013): 106-113.

Tian, Liang, Alan Russell, and Iver Anderson. "A dislocation-based, strain–gradient–plasticity strengthening model for deformation processed metal–metal composites." Journal of materials science 49.7 (2014): 2787-2794.