The Room-and Elevated-temperature Mechanical Behaviors Of Al-Cu Matrix Composites Reinforced With Different-sized TiC_p

Due to the advantages of high specific strength,high specific modulus,good wear resistance,low density and low fabrication cost,particles reinforced aluminum matrix composites have broad application prospects in automobile and aerospace industries.The particle size has a significant effect on the properties of the aluminum matrix composites.Generally speaking,adding micron-sized particles into the metal matrix can improve the strength and wear resistance while sacrificing the ductility.The nano-sized particles have a more significant strengthening effect than the micron-sized ones.Besides,the incorporation of nano-sized particles into the metal matrix can simultaneously improve the strength and ductility.However,the high nano-sized particle content results in particle agglomeration,which limits the strength improvement of the nano-sized particles reinforced aluminum matrix composites.Introducing the mixture of micron-sized and nano-sized particles into the matrix seems promising to give play to their respective advantages and thus further improving the mechanical properties of the composites.The particles can be divided into ex situ particles and in situ particles according to the sources of the particles.Compared with the ex situ particles,the in situ particles have the advantages of clean surfaces without contaminations,strong interfacial bonding with the matrix and relatively uniform distribution in the matrix.However,now there is limited researches on the mechanical properties and strengthening effects of different-sized?micron-sized,nano-sized and micron+nano bimodal-sized?in situ particles reinforced cast aluminum matrix composites.Especially the researches on the influence laws and mechanisms of the micron+nano bimodal-sized in situ particles on the elevated-temperature tensile properties and creep behaviors of cast aluminum alloys are much few.Therefore,studying the influence laws and mechanisms of the different-sized?micron-sized,nano-sized and micron+nano bimodal-sized?in situ particles on the room-and elevated-temperature mechanical behaviors of the cast aluminum alloys will provide technical and theoretical basis for further improving the room-and elevated-temperature mechanical properties of the particle reinforced cast aluminum matrix composites and expanding their application ranges.In this thesis,the monomodal-sized?micron or nano?and bimodal-sized?micron+nano?TiCp/Al-Cu composites were successfully casted by using the micron-sized TiCp-Al and nano-sized TiC_p-Al master alloys which were fabricated by the self-propagating high-temperature synthesis reactions of Al-Ti-C/CNTs systems.The influence laws and mechanisms of the different-sized?micron-sized,nano-sized and micron+nano bimodal-sized?in situ TiCp on the room-and elevated-temperature tensile properties and creep behaviors of the cast Al-Cu alloys were investigated.The room-and high-temperature strengthening mechanisms and the mechanisms of creep resistance improvement were revealed.The detailed results are as follows:1)It was revealed that the addition of micron-sized,nano-sized or micron+nano bimodal-sized TiCp could refine the?-Al grains and??precipitates in the Al-Cu alloys.TiCp could serve as effective heterogeneous nucleation sites for?-Al during solidification and refine the?-Al grains.During solution treatment in the T6 heat treatment,the refined grains lead to the shorter diffusion distance of Cu atoms,which came from Al2Cu distributed in grain boundaries.Cu atoms were distributed more homogeneously within grain interiors.A larger number of finer and more uniformly distributed??precipitates were precipitated during aging treatment.The refining effects of different-sized Ti Cp on the?-Al grains and??precipitates in descending order were as follows:bimodal-sized TiC_p?nano-sized TiC_p?micron-sized TiC_p.2)It was revealed that the incorporation of micron+nano bimodal-sized,nano-sized and micron-sized TiC_p could improve the room-and elevated-temperature?453 K and 493 K?strength and ductility of the Al-Cu alloys.The improvement effects of different-sized TiCp on the room-and elevated-temperature strength and ductility of the Al-Cu alloys in descending order were as follows:bimodal-sized TiC_p?nano-sized TiC_p?micron-sized TiC_p.The M-1.0+N-0.3 TiC_p/Al-Cu composite exhibited relatively high strength and the highest fracture strain.At room temperature,the yield strength,tensile strength and fracture strain of the M-1.0+N-0.3 TiCp/Al-Cu composite were 363 MPa,541 MPa and 17.5%,which were 56 MPa,69 MPa and 10.0%higher than those of the Al-Cu matrix alloy?307 MPa,472 MPa and 7.5%?,respectively.At 493 K,the yield strength,tensile strength and fracture strain of the M-1.0+N-0.3 TiC_p/Al-Cu composite were 261 MPa,314 MPa,17.0%,which were 27 MPa,31 MPa,5.1%and 34 MPa,35 MPa,6.4%higher than those of the N-0.3 TiC_p/Al-Cu composite and M-1.0 TiC_p/Al-Cu composite,respectively;and were 51 MPa,73 MPa,10.8%higher than those of the Al-Cu matrix alloy?210 MPa,241 MPa,6.2%?.3)It was revealed that the main strengthening mechanisms of the micron-sized TiC_p/Al-Cu composites at room temperature were grain refinement strengthening,precipitation strengthening and load transfer strengthening of micron-sized TiC_p.The main strengthening mechanisms of the nano-sized TiC_p/Al-Cu composites were grain refinement strengthening,precipitation strengthening and Orowan strengthening of nano-sized TiCp.The main strengthening mechanisms of the bimodal-sized TiCp/Al-Cu composites were grain refinement strengthening,precipitation strengthening,Orowan strengthening of nano-sized TiC_p and load transfer strengthening of micron-sized TiCp.4)It was revealed that after the high-temperature tensile tests,the average diameters of the coarsened??precipitates in the composites and Al-Cu matrix alloy in ascending order were as follows:bimodal-sized TiC_p?nano-sized TiC_p?micron-sized TiC_p?Al-Cu matrix alloy.The main strengthening mechanisms of the micron-sized,nano-sized and micron+nano bimodal-sized TiC_p/Al-Cu composites at high temperatures were as follows:??precipitate strengthening,load transfer strengthening and grain boundary pinning strengthening of micron-sized TiCp in the micron-sized TiCp/Al-Cu composites;the strengthening effect of nano-sized TiCp and??precipitate strengthening in the nano-sized TiCp/Al-Cu composites;the strengthening effect of nano-sized TiCp,??precipitate strengthening,load transfer strengthening and grain boundary pinning strengthening of micron-sized TiCp in the bimodal-sized TiC_p/Al-Cu composites.5)It was revealed that the yield stress,tensile stress and fracture strain of both the Al-Cu matrix alloy and composites increased with increasing strain rate from 1×10^-4 s^-1 to 1×10^-2 s^-1 at 453 K and 493 K.Under the same temperature and strain rate,the strength and ductility of the composites were higher than those of the Al-Cu matrix alloy.At lower strain rate,the specimens of the Al-Cu matrix alloy and composites stayed at high temperature for longer time,thus the??precipitates coarsened more significantly.Under the same temperature and strain rate,the sizes of??precipitates in the composites were smaller than those in the Al-Cu matrix alloy.6)It was revealed that at 453-493 K and under the applied stresses of 120-200 MPa,the creep threshold stresses of the Al-Cu matrix alloy and composites in descending order were as follows:M-1.0+N-0.3 TiC_p/Al-Cu?N-0.3 TiC_p/Al-Cu?M-1.0 TiC_p/Al-Cu?Al-Cu matrix alloy.The M-1.0+N-0.3 TiCp/Al-Cu composite exhibited the lowest steady creep rates,which were almost 1/38-1/10 of the Al-Cu matrix alloy,1/6-1/4 of the M-1.0 TiC_p/Al-Cu composite and 1/4-1/3 of the N-0.3 TiC_p/Al-Cu composite.Therefore,compared with the monomodal-sized TiCp/Al-Cu composites,the bimodal-sized TiCp/Al-Cu composite exhibited better creep resistance.7)It was revealed that the creep mechanism of both the Al-Cu matrix alloy and different-sized TiCp/Al-Cu composites was dislocation climb mechanism.The mechanisms of improved creep resistance of the nano-sized TiC_p/Al-Cu composites compared with the Al-Cu matrix alloy were TiCp strengthening and??precipitate strengthening.The improved creep resistance of the bimodal-sized TiCp/Al-Cu composites compared with the monomodal-sized TiCp/Al-Cu composites were attributed to the precipitation strengthening effect of the finer??precipitates,strengthening effect of nano-sized TiCp and the grain boundary pinning strengthening and load transfer strengthening of micron-sized TiCp.In the present thesis,by comparing the room-and elevated-temperature mechanical behaviors of the different-sized?micron-sized,nano-sized and micron+nano bimodal-sized?TiCp/Al-Cu composites,it was revealed that the bimodal-sized?micron+nano?particles had a more significant strengthening effect than the monomodal-sized?micron or nano?particles.The room-and elevated-temperature strength and ductility as well as the creep resistance of the composites could be further improved by reinforcing with bimodal-sized particles.This thesis will provide experimental basis,technical and theoretical reference for researching and developing the particles reinforced cast aluminum matrix composites with superior room-and elevated-temperature mechanical properties.