Considering alloy materials it is said that metallic bonds cause increase in ductility. Why is so? And can the same be implemented in metal matrix composites.
The metallic bonding depends on the metals considered for the alloy-- the relative atomic radii, crystal structure, valency etc. An alloy can be substitutional or interstitial type of solid solution (new single phase) which collectively respond to the applied stress thus deeming the alloy ductile or brittle. Whereas in an MMC, the reinforcements do not form a single phase with the matrix, they remain as intergranular entities, and any chemical reactions of the reinforcement with the matrix is considered detrimental (e.g. Al reacting with SiC to form a brittle compound Al4C3 in Silicon free Aluminium alloys).
Nonmetallic bonds may have large amount of covalent bonds in addition to ionic contribution, which are uni-directional and their deformation or distortion requires bending and twisting rather then gliding during the plastic deformation by dislocations and disclinations in crystalline solids. For the edge dislocation climb process one requires nonconservative motion as far as the point defects (vacancies and interstitials) creation and annihilation are concerned. For ionic crystals due to charge neutrality requirement the mobility of point defects under the applied stress is rather very slag-gist, which is also the same for covalent bonding systems.
To say briefly, metallic bonds are more isotropic and hence more slip systems become almost equally easy to activate by external stresses. At the same time, the words (more) "ductile interface" are hardly proper. More ductile is bulk material and just this fact allows possible stress singularity at interfaces to be plasticallly accommodated.
Contrast to ionic and covalent bonds, delocalized electrons that surrounds the positive metal ions in metallic bond makes the metal more ductile and malleable (since the local bonds can be easily broken and formed). In metal matrix composites (MMC), the interfacial bonding can be mechanical or chemical. The chemical bond can be metallic (non-directional), ionic or covalent (directional) and the interface with metallic bond is more ductile. I have come across that the chemical bond can be through chemical reaction (brittle reaction product and is detrimental) or it can be electronic which is preferred as it gives better ductility.
Please see these references; may be useful:- (1) "Interfaces in Discontinuously Reinforced Metal-matrix Composites", Interfaces in Discontinuously Reinforced Metal-matrix Composites, Defense Science Journal, Vol 43, No 4, October 1993, pp. 397-418 (link in RG:-https://www.researchgate.net/publication/226043250_Interfaces_in_Discontinuously_Reinforced_Metal_Matrix_Composites). (2) "Metal Matrix Composites", (Chapter 6) Krishan K. Chawla, In Book 'Composite Materials', Springer, 2012, pp 197-248. (3) "Processing and fabrication of advanced materials", XVII: Volume 1, Editors. N. Bhatnagar and T.S. Srivatsan, 2009.
Article Interfaces in Discontinuously Reinforced Metal Matrix Composites
Zieman is right. Ductility is better considered as a property of the bulk. However, just for the sake of argument, and by definition where metallic bonding, which should be considered as delocalized covelent bond, prevails that particular locii should have more propensity to allow shifting of atoms, which would mean ductility for a bulk material. One additional point is important... Existance of pure bond is notthe most common case, to my best knowledge.
I agree with answers of Prof. Zisman and Prof. Kaya that ductility is certainly the property of the bulk. I have no experience on MMCs and (1) the paper "The ductility of metal matrix composites – Relation to local deformation behavior and damage evolution, Otmar Kolednik, Klaus Unterweger, Engineering Fracture Mechanics 75 (2008) 3663–3676" , and (2) "Metal and Ceramic Matrix Composites" edited by Brian Cantor, Fionn .P.E Dunne, Ian C Stone. IOP Publishing Ltd. 2004 may be useful to you Mr. Pranav.
Thanks for contributing all.
All your views are very useful to me, will get back to you soon after some study and results.
As previously discussed by other researchers, the metallic bonds makes metals ductile compared to ionic bonding. Ionic bonding such as in NaCl makes them brittle.
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