Article on web

http://www.eng-tips.com/faqs.cfm?fid=1441

The physical basis of material
properties like Young's modulus can be understood by examining
materials on the atomic scale. There are two main things that influence
the value of the modulus:

1.)     The atomic microstructure
2.)     The interatomic bonds.

 So, what is the reason of change of elastic modulus with plastic deformation? Any References??