Proceedings of SerbiaTrib '13

(a)(b)Figure 4. Showing (a) variation of tensile strength forthe monolithic Al-Mg-Si alloy, single reinforced andhybrid reinforced Al-Mg-Si/RHA-SiC composites; and(b) variation of yield strength for the monolithic Al-Mg-Si alloy, single reinforced and hybrid reinforced Al-Mg-Si/RHA-SiC composites.It has been well reported that particle reinforcedAMCs achieve improved strength due to loadtransfer from the matrix to the particles (directstrengthening) and creation of more dislocationswhich serve as constraints to plastic deformation bythermal mismatch between the particles and theAluminium matrix arising from their differences incoefficient of thermal expansion (indirectstrengthening) [27-28]. Thus even in a scenariowhere the particles are not sufficiently strong toinduce strengthening via the ‘direct route’ of loadtransfer from matrix to particles, the indirectstrengthening it could offer is adequate to inducesome strength improvements well and above that ofthe monolithic alloy. In the present case underinvestigation, the reduction in strength observedwith increase in the RHA content of the compositesis as a result of the decrease of the directstrengthening capacity of RHA which containspredominantly silica. Silica is noted to be a softerceramic with elastic modulus of 60-70 GPa, whichis within the range of Aluminium unlike SiC whichhas an elastic modulus of 400GPa. Thus theefficiency of load transfer from the Al matrix to theparticles (load carrying capacity) of the hybridparticulates will be dependent on the amount of SiCthan RHA. However, it should be noted that samplesB5, C5, and D5 which contain only RHA, show aprogressive increase in tensile strength and yieldstrength with the increased weight percent of RHAsupporting our hypothesis that the indirectstrengthening mechanism (which entails dislocationgeneration results in higher dislocation densities withincreased weight percent of the particles) can resultin modest improvement in strength with increase inthe weight percent of the reinforcing particles.The variation of the specific strength of thecomposites produced with weight ratio of RHA/SiCis presented in Figure 5. It is observed that thespecific strengths of the composites generallyincreased with increase in the weight percent of thereinforcing phase (that is RHA-SiC weightpercent). Also the specific strength values decreaseswith increase in the RHA content in the hybridreinforcement. However, the % decrease in specificstrength of the composites is generally lower incomparison with that of the ultimate tensile strengthanalyzed earlier. For the 5 wt% compositions, it isobserved that 3.1, 6.8, 8.75, and 11.9 % reductionin specific strength is obtained. For the 7.5 wt %compositions (grades) 3.93, 6.2, 10 and 13.9 %reductions were obtained. In the case of the 10 wt%grade, 2.6, 5.3, 6.54, and 11.9 % reductions wereobtained. The results show that the composites withcomposition of 1:3 weight ratio RHA:SiC (25%RHA: 75% SiC) can offer comparable specificstrength values at reduced cost of production of thecomposite since its difference is less than 4 % for thethree weight percents of reinforcement worked on.Figure 5. Variation of specific strength for themonolithic Al-Mg-Si alloy, single reinforced and hybridreinforced Al-Mg-Si/RHA-SiC composites.The results of the variation of strain to fractureof the composites with weight percentreinforcement and weight ratio RHA/SiC ispresented in Figure 6. It is observed that there is ageneral decrease in ductility of the composites withincrease in the weight percent of reinforcing phase13 th International Conference on Tribology – Serbiatrib’13 165