Research On Micromechanics Behavior Of In-situ Particle Reinforced Aluminum Matrix Composites

Particle reinforced aluminum matrix composites have attracted extensive attention and intensive research due to their higher specific strength,specific modulus,fatigue performance,damping and wear resistance than corresponding matrix metals,and have been applied in many fields.The mechanical properties of these materials depend on the properties and microstructural characteristics of their constituent phases,such as the strength,size,shape and distribution of particles,properties of particle-matrix interface,dislocation density,void volume fraction and anisotropy of Al matrix.This paper mainly focuses on the mechanical behavior of in-situ TiB₂ particle reinforced 2024 Al matrix composite(referred to as TiB₂/2024 composite hereafter)prepared by mixed salt reaction method.The microstructural feature and fracture mechanism of TiB₂/2024 composite were studied by means of experiments.Analysis of scanning electron microscope(SEM)and transmission electron microscope(TEM)were carried out to give a straight knowledge on TiB₂ particle of their size,shape,distribution and the dislocations around them.The particles are smooth and without sharp corners with their sizes ranging from30 to 500nm.The particle distribution has the following characteristics:the sub-micron particles segregate at grain boundaries of the matrix to form the particle segregation zone,while the nano-particles disperse inside the matrix grain.In addition,a large number of dislocations near the dispersed nano-particles were found in TEM test,which preliminarily indicated that the dispersed nano-particles are taking indirect strengthening effect on the matrix.In-situ SEM tensile test of TiB₂/2024 composite was carried out.The observed phenomena of the initiation,accumulation and coalescence of micro-cracks in particle segregation zone have confirmed that the fracture of TiB₂/2024 composites under quasi-static loading belongs to ductile-brittle mixed fracture.In order to clarify the particles strengthening effect,indentation experiments on TiB₂/2024 composite and 2024 alloy were carried out.By using Oliver-Pharr method,the mechanical properties change of the matrix relative to 2024 alloy is revealed from the perspective of indentation modulus and hardness.The analysis results show that compared with 2024 alloy,the indentation modulus of the matrix has not been notably changed,but the indentation hardness is increased by 33.7%,which indicates that the flow stress of the matrix has been strengthened,also confirms that the dispersed TiB₂nano-particles have considerable indirect strengthening effect on matrix.It is also shown that the improvement of the elastic modulus of TiB₂/2024 composite is mainly due to the high elastic modulus of TiB₂ particles.In order to determine the plastic properties of the matrix strengthened by dispersed nanoparticles,indentation test was performed at matrix micro-regions with different indentation force levels.Dimension analysis of the variables involved in indentation process was carried out.Elastic-plastic finite element model was established and used to implement the simulation of indentation process for 46"virtual"materials.Based on the results of the indentation test and corresponding simulation,the yield strength and strain hardening parameters of the matrix were obtained by an inverse analysis.To investigate and revisit the damage and fracture behavior of TiB₂/2024composite at mesoscale,a 2D unit cell finite element models with hexagonal particle segregation structure was established based on the microstructural features of particle distribution.The maximum principal stress criterion was used to estimate the brittle fracture of the particles and Rice-Tracey failure criterion is used to characterize the damage and fracture of the matrix.The formulae of the radial return algorithm was deduced and applied in elastoplastic iteration.The damage and fracture behavior of TiB₂/2024 composites in the unit cell model was simulated by FE analysis.The analysis results show that the mean values of the maximum principal stress in TiB₂ particles and damage variables of the matrix are not significantly altered by particle segregation levels.However,the more severe the particle segregation degree is,the greater are the variation degree of the maximum principal stress in particles and damage in the matrix,and the easier to initiate micro-cracks in TiB₂/2024 composite.With the combination of Mori-Tanaka homogenization method,GTN(Gurson-Tvergaard-Needleman)porous ductile failure model for the matrix and Weibull probability fracture model for the particles,a multi-scale analysis method characterizing the ductile-brittle fracture behavior of TiB₂/2024 composite is proposed.Cracked particle volume fraction is taken as the nucleation volume fraction of microvoids to consider the effect of particle crack on the ductile failure process of matrix region.The uniaxial tensile response and fracture of notched round bars with different notch radii were reasonably predicted.The method should provide theory and evaluation tool for the design of the structures using the particle reinforced Al matrix composite materials.