I have fabricated Aluminium metal matrix composites with different fraction of reinforcement (0, 4, 6 and 8). During tensile testing what i have observed that % elongation decreases (compared to 0%i.e. base alloy) with the addition of reinforcement up to 6% reinforcement. But at 8% reinforcement % elongation is more compared to 6% reinforcement. What can be the possible reasons for this increase in % elongation?
Dear Pardeep Saini ;
The maximum stress that a material can withstand while being stretched or pulled before breaking (UTS) and yield strength of the metal matrix increase with an increase in the weight fraction of the reinforcement particles, whereas elongation, decreases with the increase in reinforcement. The increase in strength of the reinforced composites is due to the increase in dislocation density. Moreover, adding the reinforcements cause an acceleration in the precipitation kinetics of the composites, as detailed in the following reference;
Article Investigations on mechanical properties of aluminum hybrid composites
Dear Sir
Thankyou for your timely response. I agree with your point that strength increases due to increase in dislocation density. But my question still remains the same that % elongation decreases (compared to 0%i.e. base alloy) with the addition of reinforcement up to 6% reinforcement. But at 8% reinforcement % elongation is more compared to 6% reinforcement. My query is about increase in % elongation of specimen (i.e. 8 % reinforcement). why this specimen have more % elongation compared to 6% specimen.
Dear Pardeep Saini ;
this may be caused by the defects in reinforced particles due to ball milling, lead to decrease and agglomerate of the particles also Increase reinforced particles agglomeration forming large porosity in the composite, thus resulting in a decrease in the mechanical properties after a certain value. or maybe it caused by insufficient binding due to lack of wetting at the interface between the particles and base metal, causing failure of directed load transfer due to the random arrangement of the added particles. fid more information in fig 6 in the following reference;
Article A short review on mechanical properties of graphene reinforc...
and;
K. Chu, X. Wang, Y. Li, D. Huang, Z. Geng, X. Zhao, et al. Mater Des, 140 (2018), pp. 85-94
regards
The percentage elongation of the composite materials is decreased due to the inclusion of reinforcement particles. But the strength of the material is increased. The addition of hard oxide/ceramic particles in the soft matrix, makes the composite brittle and the hardness is increased, Thus the elongation (ductility) is decreased while adding reinforcement in the matrix.
If the composite was fabricated perfectly (uniform distribution at every part of the composite with excellent interfacial bonding), the elongation will keep on decreasing with an increase in volume content irrespective of the type of the reinforcement. The tensile strength will also keep on increasing until a threshold value (depends upon the type of the matrix and the reinforcement) of volume fraction is reached and drop afterwards. Reinforcement particles usually strengthens the composite to increase the tensile strength. The strengthening effect reduces the plastic flow of material during the tensile test and causes the elongation to drop. This phenomenon takes place in every material and is not limited to metal matrix composites. As the strength of the material goes up, there will be a corresponding drop in elongation.
It is a general phenomenon that in tensile testing, the percentage elongation decreases after addition of ceramic particulates in MMCs while an increase in the tensile strength could be observed. If 8 wt% reinforcement reported increase in the elongation value, it is unconventional. You may repeat the experiments to come to a conclusion.
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