Study On Macroscopic And Microscopic Mechanical Behavior Of In-situ TiB₂ Particulate Reinforced Aluminum Matrix Composite

To satisfy the increasingly operational requirements for auto mobile engine, such as higher specific power output and lower fuel consumption, ceramic fiber/particle reinforced aluminum matrix composites（PRAMCs） are extensively used in pistons with their high specific strength and superior thermal stability. In situ TiB₂ PRAMCs become increasingly attractive because of its cleaner particle-matrix interface, finer particle size compared with the ex situ PRAMCs, which is a new type of piston material.In order to assess the high-temperature performance of aluminum-silicon alloy reinforced with titanium diboride particles as potential piston material, the tensile behaviors and low-cycle fatigue characteristics of in situ 4wt% TiB₂/Al-Si composite were investigated in the temperature range 25-350 °C. In addition, the micromechanical finite element method was employed to predict the macroscopic effective properties of the composite. The main research contents and conclusions are as follows:（1） The tensile results revealed that the composite exhibited higher modulus than the matrix alloy at all testing temperatures, but both the matrix alloy and the composite presented similar strength levels above 200 °C. The ductility of the composite was found to be lower than that of the unreinforced matrix alloy at 25 and 200 °C, but no obvious distinction was observed at 350 °C. At 25 and 200 °C, the fracture surfaces exhibited a mixture of cleavage facets, tear ridges and small dimples morphology due to the addition of TiB₂ particles, fracture was dominated by cracked silicon particles and separated TiB₂ particles, At 350 °C, the composite showed a dimpled appearance with the extent of plastic deformation of the matrix, indicating that void nucleation, growth and coalescence are the main fracture mechanism in the composite at high temperature.（2） The low-cycle fatigue test results showed that the composite exhibited cyclic stable at 200 °C and cyclic softening at 350 °C. The stable response was due to the balance between dislocaiton proliferation and its annihilation, and the softening could be attributed to the coarsening and decomposition of the strengthening precipitates and the decrement of dislocation density at high temperature. Increases in cyclic plasticity and fatigue life for the composite were observed as the temperature rose from 200 to 350 °C. Fractographic morphology studies indicated that fatigue cracks preferred to initiate at pores and inclusions near the specimen surface and propagate within the matrix and avoid TiB₂ particles. Under cyclic straining, more silicon particles and intermetallics fracture（200 °C） or debond（350 °C） and separated TiB₂ particles were found at both 200 °C and 350 °C.（3） The monotonic tensile behaviors and progressive damage process of the composite were simulated by using a 2D multi-particle random distribution representative volume element（RVE） generated by Random Sequential Adsorption（RSA） method. The results indicated that the simulated Young’s modulus and Strength of the composite were higher than the experimental results at 25 and 200°C, while the simulated results agreed with the corresponding experiments well at 350°C. The microscopic stress and strain analysis showed that a large degree of plastic deformation could be found in the matrix alloy, and the maximum stress appeared at the matrix-particle interface, which mean that matrix cracks would be inclined to divert at the interface as the tensile loading involved, leading to TiB₂ particle-matrix interface debonding, and the regions of clustered TiB₂ particles were also found to be the locations prone to damage in the composite at both room and high temperatures.（4） The uniaxial cyclic deformation of the composite was numerically simulated by using finite element code ABAQUS and employing the single-particulate cell model of the composite. In the simulation, a newly cyclic elastic-plastic constitutive model was proposed to describe the cyclic deformation behavior of the Al-Si matrix alloy. It was shown that the developed model simulated the uniaxial cyclic stable（200°C） and cyclic softening（350°C） deformation of the composite well. The microscopic stress and strain analysis indicated that the TiB₂ particles remained at the high stress level under fully-reversed loading and the maximum tensile stress and compressive stress appeared at the matrix-particle interface. Besides, the matrix alloy exhibited significantly plastic deformation and higher plastic zones could be observed near the TiB₂ particle-matrix interface, which clearly demonstrated that the damage evolution started earlier at interface under the cyclic loading.