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Given two materials A and B such that a has higher young modulus as well as plastic limit than B , which of the two is more ductile?

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closed as off-topic by user191954, Jon Custer, John Rennie, Aaron Stevens, Chris Oct 19 '18 at 23:41

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Materials which can undergo plastic deformation without undergoing failure are called ductile. Youngs modulus determines the elasticity of the material and the yield point is the stress at which the material undergoes considerable deformation (usually 0.2%). These factors have nothing to do with ductility.

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  • $\begingroup$ Poissons ratio is generally valid only in the elastic regime. Ductility is plastic deformation. $\endgroup$ – Jon Custer Oct 18 '18 at 14:41
  • $\begingroup$ I know that. This is an observation made. Brittle materials have a low poisson's ratio and undergo failure easily without considerable deformation $\endgroup$ – Mechanic7 Oct 18 '18 at 14:42
  • $\begingroup$ And rubber undergoes lots of elastic deformation and isnt necessarily plastic... $\endgroup$ – Jon Custer Oct 18 '18 at 14:44
  • $\begingroup$ I was answeing in the context. Youngs modulus or yield point dont have any effect on ductility. $\endgroup$ – Mechanic7 Oct 18 '18 at 14:49
  • $\begingroup$ That is correct, so why bring in another parameter that also does not apply? $\endgroup$ – Jon Custer Oct 18 '18 at 14:51
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I'll list some of the factors here.

Ductility has to do with the nondirectionality of the interatomic bonds in the material. If those bonds have preferred directions and bond angles, then adjacent atoms cannot slip and slide past one another easily, which is key to furnishing ductility. This means that in an ideally ductile material, the nature of the bonds between its atoms must be maximally metallic in nature and exhibit no covalency.

Ductility is limited by the exhaustion of dislocation mobility mechanisms; if we try to further deform the material beyond that point the material begins to develop microcracks which then lead to failure. The most common causes of dislocation exhaustion are 1) pinning of dislocations at point defects in the crystalline structure of the material and 2) pileup of dislocations at grain boundaries.

Ductility can then be enhanced by increasing the purity of the material so as to eliminate the presence of foreign atoms as substitutions or interstitials within the crystal lattice, by furnishing a mechanism of dislocation climb that is active at the service temperature of the material so as to unlock the dislocations from point defects, and by allowing dislocation travel across grain boundaries.

These microstructural effects are very important but unfortunately difficult to deliberately design into a metallic alloy. The reason for the outstanding ductility of, for example, pure silver and gold is that their bonding is entirely metallic and dislocations in them can move across grain boundaries.

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