Hot Deformation Behavior And Strengthening Mechanism Of Dual-scale SiC_p/A356 Composites

Silicon carbide particle（SiC_p）reinforced Al matrix composites have the advantages of high specific strength,stiffness and modulus,as well as good thermal conductivity,electrical conductivity,wear resistance and dimensional stability.It has broad application prospects in high-end equipment manufacturing fields such as aerospace and automotive.The volume fraction and size of SiC_p reinforcement and the composition of the matrix alloy play a decisive role in the physical and mechanical properties of the composites.The rapid development of high and new technology has put forward higher requirements for the properties of composites.The strengthening effect of a single scale reinforcement on the properties of composites is limited,which will restrict the optimization and improvement of the comprehensive properties.The micron and nano dual-scale reinforcements are adopted to make full use of the respective size advantages and coupling effects of the dual-scale reinforcements.The composites with excellent comprehensive properties can be prepared to meet the comprehensive requirements for the mechanical properties,wear resistance and thermal expansion.For this reason,the single and dual-scale SiC_p/A356 composites with different volume fractions were prepared by powder metallurgy combined with hot extrusion in this paper.Transmission electron microscopy（TEM）,scanning electron microscopy（SEM）,X-ray diffractometer（XRD）,optical microscopy（OM）,in-situ TEM tensile testing machine,electron tensile testing machine,friction and wear testing machine and thermal simulation testing machine were used.The microstructure evolution and interface structure of dual-scale composites during the fabrication and hot working were studied in detail.The influence of the dual-scale SiC_p on the microstructure,mechanical properties and hot deformation behavior of the composites was discussed,and the corresponding mechanism was clarified.The main research results are as follows:（1）The micron,nano and dual-scale SiC_p/A356 composites with different volume fractions were prepared.The effect of dual-scale SiC_p on the microstructure and mechanical properties of the composites was analyzed.The content of SiC_p in the dual-scale SiC_p/A356 composites was optimized.It is found that with the increase of the volume fraction of dual-scale SiC_p,the uniformity of SiC_p distribution first increases and then decreases.When the volume fraction of dual-scale SiC_p is 25%（2 vol.%-80nm+23 vol.%-10μm）,the mechanical properties of the composites are the highest.Compared with A356 alloy and single-scale SiC_p/A356 composites,the dual-scale SiC_p/A356 composites have more remarkable enhancement effect.（2）The microstructure evolution of dual-scale SiC_p/A356 composites during heat treatment was investigated.It is found that during the process of solution heat treatment at 510℃～550℃for10 mim～10 h,with the increase of solution temperature and time,the second phase in the matrix is fully dissolved.The shape of Si particles in the composites becomes more rounded,and the size is mostly 2～5μm.The hardness of the composite reaches the maximum value after solution at 530℃for 2 h.The precipitate sequence of the dual-scale composites during aging is as follows:spherical GP zones with a size of several nanometers→needle-likeβ’’phase with a length of ten nanometers→rod-likeβ’phase with a length of several hundred nanometers.The hardness of the composite reaches the maximum value after aging at 170℃for 4 h.And the main precipitates are needle-shapedβ’’phase that are coherent with the matrix.In addition,the hardness,strong plasticity,expansion coefficient and wear resistance of composite reinforced with SiC_p of different scales after solution treatment at 530℃for 2 h and then aging at 170℃for 4 h were studied in detail.Compared with A356 alloy and single-scale SiC_p/A356 composites,it is found that dual-scale SiC_p/A356 composites have more excellent comprehensive properties.（3）The hot deformation behavior of dual-scale SiC_p/A356 composites with SiC_p of15 vol.%（2 vol.%-80 nm+13 vol.%-10μm）and 25 vol.%（2 vol.%-80n m+23 vol.%-10μm）under the conditions of deformation temperature of 460～520℃and strain rate of 0.01～5 s^-1 was investigated.Results show that the true stress-strain curves of the dual-scale composites are characterized by dynamic recrystallization（DRX）,and the peak stress increases with the decrease of deformation temperature or the increase of strain rate.The constitutive equations and processing maps of dual-scale composites were established,respectively.It is found that the 25 vol.%dual-scale composites have higher flow stress and the hot deformation activation energy.Besides,the rheological instability zone is widened,and the thermal processing safety interval is narrowed.The hot working process criteria of dual-scale SiC_p/A356 composites are proposed.（4）The DRX behavior of 25 vol.%dual-scale SiC_p/A356 composites was analyzed.Based on the true stress-true strain curve data,the critical strain model and kinetic model of DRX of the composites were established.Combined with the grain size analysis after hot deformation,the DRX grain size model was established.The main softening mechanism during hot deformation is found to be DRX mechanism,including grain boundary bowing mechanism,particle stimulated nucleation（PSN）mechanism and subgrain rotation induced nucleation mechanism.In the PSN mechanism,both micron and nano SiC_p can stimulate the DRX nucleation of Al matrix.（5）The microstructure of the dual-scale SiC/Al interface in the composite was characterized and evaluated.The interface of micron SiC/Al in the dual-scale SiC_p/A356composites prepared in this paper is well bonded.The interface type is mainly clean interface,accompanied by a few reactive interfaces and a very few amorphous layer interfaces.There is no definite crystal orientation relationship at the clean interface.The semi-coherent interface is formed between the micron SiC/Al.The interface type of nano SiC/Al is semi-coherent clean interface.The interface matching degree of nano SiC/Al is better than that of micron SiC/Al.The fracture behavior of SiC/Al interface in the composite is evaluated by in situ TEM tensile testing.It is found that the bonding strength of SiC/Al clean interface is higher than the matrix.When there is a Mg Al₂O₄interface layer with thickness of about 100 nm,the microcracks preferentially form and propagate at the Mg Al₂O₄/Al interface,indicating that the bonding strength of the Mg Al₂O₄/SiC interface is greater than that of Mg Al₂O₄/Al interface.（6）The strengthening mechanism of SiC_p/A356 composites with different state was revealed.The strengthening mechanism of the hot extruded dual-scale SiC_p/A356composite is mainly Orowan strengthening.Meanwhile,there are also thermal mismatch strengthening,load transfer strengthening and fine grain strengthening.In addition to the above strengthening mechanism,the contribution of precipitation strengthening to the strength of aged composites is also large.The tensile tests show that the dislocation density around micron and nano SiC_p in the composite gradually increases with the tensile deformation,the strength of the composites can be improved by load transfer strengthening of micron SiC_p and Orowan strengthening of nano SiC_p.