Effects Of Heat Treatment On The Microstructures And Mechanical Properties Of SiC_p/6061Al Composite Fabricated By Powder Thixoforming And Research On The Corresponding Strengthening And Toughening Mechanisms

A novel technology for fabricating particulate reinforced aluminum matrix composites,powder thixoforming?PTF?,was utilized in this work to prepare SiC particle?SiC_p?reinforced 6061 aluminum matrix?PTF-SiC_p/6061Al?composite.Firstly,the relationship between microstructures and tensile properties of the as-fabricated PTF-SiC_p/6061Al composite was investigated,and by comparing it with those of 6061 alloy fabricated by PTF method?PTF-6061?and 6061 alloy by permanent mold cast?PMC?technique?PMC-6061?,the features of PTF method and merits of composite were clarified.Subsequently,the effects of heat treatment on the microstructures and tensile properties of the composite were studied,and the results indicated that solution treatment could simultaneously enhance the composite's tensile strengths and elongation,while aging treatment induced a distinct effect of aging hardening to the composite.Calculations based on building models revealed the strengthening mechanisms and toughening mechanisms of the composite during solution treatment as well as the strengthening mechanisms during aging treatment.The results show that:1.Compared with the traditional PMC technique,PTF method could significantly enhance the density of microstructure and reduce the sizes of grains and eutectic structures.The microstructure of the as-fabricated composites consisted of SiC_p,near-spheroidal primary?-Al phase?with a mean size of 16.51?m?and intergranular secondarily solidified structures.While the microstructure of the PMC alloy was composed of coarse equiaxed grains?90.01?m?and intergranular net-like eutectic structures.Owing to the strengthening effect of SiC_p on?-Al phase,the ultimate tensile strength and yield strength of the composite attained values of 230MPa and128MPa,obviously higher than those of PTF-6061 alloy and PMC-6061 alloy.2.When the composite was subjected to solutionization at 560?for 6h,the eutectic structures were basically dissolved;Due to the impeding of SiC/Al interface to heat transfer and the pinning effect of SiC_p on grain boundary,the coarsening rate of composite's microstructure was comparatively slower,and its overall solution progress was slower than that of PTF-6061 alloy;The ultimate tensile strength,yield strength and elongation of the composite reached peak values of 276 MPa,214 MPa and 7%,respectively,which indicated enhancements of 20.0%,67.2%and 169.2%when compared with its as-fabricated state.3.During solution treatment,the strength contribution from solid solution strengthening was the largest,while those from grain refinement strengthening,CTE mismatch strengthening and geometrically necessary dislocation strengthening were moderate and nearly the same.The strength contribution from Orowan strengthening was minor owing to the micro size of SiC_p and large inter-particle spacing.By combining various strengthening mechanisms and the failure situation of SiC _p,a modified shear lag model was proposed in this work which well predicted the yield strength of the composite in the process of solution treatment.The toughening mechanisms of the composite during solution treatment are:?1?the size of SiC_p was relatively smaller?its mean size was 6.94?m with 90%of them being lower than10?m?,which could effectively reduce the probability of premature failure at certain particles having relatively larger size;?2?the average aspect ratio of SiC_p was 2.43,which balanced well between tensile strength and elongation;?3?pre-oxidization of SiC_p led to the formation of MgAl₂O₄ phase at the SiC_p/Al interface,providing a good interface bonding strength;?4?application of PTF method greatly improved the SiC_p distribution uniformity among the matrix,degrading the stress concentration at the particle clusters.The aforementioned four factors laid foundation for the subsequent solution treatment,after being solutionized at 560?for 6h,a?5?ductile matrix and?6?much higher SiC_p/Al interface bonding strength were achieved,consequently leading to a significantly improved elongation of the composite.4.During aging treatment at 175?,the decrease in solute concentration and increase in dislocation density led to a phenomenon that,the mean size of precipitates in the composite was larger than that of primary?-Al phase in the PTF-6061 alloy throughout the whole aging process.In the peak-aged and over-aged state,the volume fraction of precipitate in the composite was lower than that of PTF-6061 alloy,owing to dislocation assisted strain release in the matrix and consuming of Mg element by interfacial reactions.The hardness,ultimate tensile strength and yield strength of the composite first increased and then decreased,and reached peak values of 119.5HV,309MPa and 241MPa at 10h,representing improvements of 83.0%,12.0%and 12.6%,respectively,when compared with the as-fabricated state.While the elongation of the composite was continuously reduced throughout the whole aging stage.The aging progress of the composite was faster than that of the matrix alloy due to the accelerated precipitation by dislocations,while the composite showed a relatively lower aging hardening rate than the matrix alloy under the same condition,resulting from the larger mean size and lower volume fraction of precipitates in the composite.5.During aging treatment,the yield strength contribution from solid solution strengthening was continuously reduced and that from precipitate strengthening first increased and then decreased,reaching maximum value at the peak-aged state,while those from grain refinement strengthening,CTE mismatch strengthening and geometrically necessary dislocation strengthening remained nearly the same.In general,the former two strengthening mechanisms contributed relatively larger yield strength to the composite,whereas the latter three ones contributed less yield strength and almost equally.The modified shear lag model developed in this work,which fully considered various strengthening mechanisms and the failure fraction of SiC_p,could be used to accurately predict the yield strength of particulate reinforced aluminum matrix composites during aging treatment.