Understanding The Fatigue Behavior Of Spray-deposited SiC_p/Al-Si Composites

The SiCp/Al-Si composite produced by spray deposition possess many merits such as low density, high specific strength, low thermal expansion coefficient and high wear resistance and so on, and thus exhibit incomparable advantages when used as brake rotors in vehicle. The structural components applied in automotive industry are often subjected to the cyclic loads during use, and the long-term transient internal stress may initiate fatigue cracks which often result in fatigue damage. The structure design and application of the as-sprayed composites are restricted by the lack of the research on their fatigue properties behavior. Therefore, a comprehensive study on the fatigue behavior of the as-sprayed SiCp/Al-Si composites is of great significance in the safety design for the braking components. In the present work, the SiCp/Al-Si composites ingots have been successfully prepared by spray deposition. The influence of the SiC particle size and the Si content on the microstructures, static tensile properties, low-cycle fatigue and fatigue crack propagation behavior of the as-sprayed have been systematically investigated and the effects in retarding the fatigue crack initiation have been especially analyzed.(1) The effect of particle size (4.5μm and20μm) on the microstructures and static tensile properties of the spray-deposited SiCp/Al-7Si composites was studied. The experimental dates demonstrate that the addition of the SiC particles is beneficial to the improvement on elastic modulus, but results in the reduction in the tensile strength and elongation. The SiC particles in both composites are partially aligned along the extrusion direction. Besides, the as-sprayed composite containing the small (4.5μm) SiC particles exhibits the smaller interparticle spacing as compared to the composite with the coarse (20μm) particles and thus the reinforcements probably share more portion of loading, contributing to higher elastic modulus and ultimate tensile strength. The interfacial debonding between the SiC particles and the matrix is the main fracture mechanism of the SiCp(4.5μm)/Al-7Si composite, while the cracking of the SiC particles is dominant in the SiCp(20μm)/Al-7Si composite.The tensile results of SiCp(4.5μm) particle reinforced Al-7Si, Al-13Si and Al-20Si alloy composite indicate that with the increase of the Si content, the volume fraction of the primary Si and the eutectic silicon phases increase. The as-sprayed composite with a higher Si content displays an increase in the elastic modulus, yield strength and ultimate tensile strength, but a decrease in elongation. The fracture reveals an increase in the degree of the brittleness and the cracking tendency of the primary Si particles with the Si content increasing.(2) The effect of particle size (4.5μm and20μm) on low-cycle fatigue properties of the spray-deposited SiCp/Al-7Si composites was inverstigated Both the composites with the different particle sizes and the matrix alloy exhibit the cyclically hardening effect and the cyclic hardening rates for the composites are greater than that for the matrix at the initial several hundred cycles for all the total strain amplitudes. The low-cycle fatigue endurance of the composite with the large particles is higher than that of the composite containing the small particles in the high strain regions, but is lower than that of the composite with small particles at low strains. A higher degree of cyclic hardening is observed for the composite with4.5μm SiC particles at the lower strain amplitudes (0.3%,0.35%, and0.4%) and this is probably due to the higher dislocation density and the lower softening rate. However, at the highest strain amplitude (0.5%), the degree of cyclic softening on the SiCp(4.5μm)/Al-7Si composite is reduced due to the higher softening rate induced by the fracture or the debonding of the SiC particles, and thus is beneficial to the linkage of the microscopic cracks in the region of the SiC particulate agglomeration.The low-cycle fatigue life of the SiCp(4.5μm)/Al-Si composites with the different Si content can be described by the Coffin-Manson relationship. The fatigue-ductility exponent (c) is-0.5284for the composite with7%Si,-0.4526for the composite with13%Si, and-0.5954for the composite with20%Si, respectively. The fatigue-ductility coefficient (ε’f) of composite with7%Si content (2.63) is higher than that the composite with13%Si (1.09) and20%Si (0.85), implying that the composite with a higher Si content exhibits a lower fatigue life. With the increase of the Si content, the degree of cyclic hardening is reduced. It can be ascribe to the increased fracture of the primary Si phase and the formation of micro-cracks originating from the nucleation of many voids at the interface between the Si phase and the matrix. A larger number cycle is beneficial to easier linking from one or more macroscopic cracks and thus leads to more serious softening, which causes stress relaxation and offsets part of the hardening effect derived from the increased dislocation density.(3) Fatigue crack behavior of the spray-deposited SiCp/Al-7Si composites with different particle size was investigated.The as-sprayed composite containing the small SiC particles (4.5μm)as the reinforcement exhibits a superior resistance to fatigue crack growth as compared to the composite with the SiC particles of20μm and the unreinforced alloy, with the threshold stress intensity factor range ΔKth of3.878MPa·√m for the former being slightly higher than that for the20μm SiC reinforcement composite (3.630MPa·(?)) and the unreinforced alloy (3.605MPa·(?)m). At the near threshold state and steady state, the composite reinforced with20μm SiC particulates exhibits the lower fatigue crack growth rates than the unreinforced alloy. However, a higher fatigue crack growth rate in the20μm SiC particulates composite is observed compared to the unreinforced alloy at the fast fracture state. Both crack deflection caused by the SiC particles and microcrack initiations in the SiC particles can bring about the ziggzag main crack propagation path, increase the surface energy during crack propagation and thus raise the crack propagation resistance. Especially, the microcrack initiation in the SiC particles is the main cause for the relatively higher closure effect in the SiCp (4.5μm)/Al-7Si composite. However, crack deflection caused by the SiC particles is the main cause for the higher threshold stress intensity factor of small fatigue cracks in the SiCp (20μm)/Al-7Si composite than the unreinforced alloy. With the threshold stress intensity factor increasing, a higher probablity is involved in the advancing crack propagation and thus the faster fatigue crack propagation is observed in the composties than the unreinforced alloy. The latter can be ascribed to the frequent separation between the SiC particles and the matrix and the continual rupture.The fatigue crack growth experiments of the SiCp(4.5μm)/Al-Si composites with the different Si content(7%,13%and20%) were conducted. The results indicate that the high Si content (20%) composite has the highest thereshold ΔKth at lower ΔK levels, which has been attributed to an increase in crack path deflections due to the crack avoiding the primary Si and eutectic silicon phases, and a decrease in the value of crack tip opening distance resulted from the higher elastic modulus and strength due to the increase of Si content, but shows worse fatigue resistance at high ΔK levels. This is attributed to an increased contribution of failure modes at the crak tip through the fracture of the primary Si phase and the formation of micro-cracks originating from the nucleation of many voids at the interface between the Si phase and the matrix at higher ΔK levels. With the threshold stress intensity factor increasing, the easier microcrack linking at the crack tip is the main reason for the gradually reduced fatigue crack resistance of the as-spray composites.(4) A lower fatigue crack growth rate is observed in the composite with the higher relative density. The curves of the fatigue crack propagation rate and the micro-hardness verus the fatigue cycle numbers exhibit the quasi-periodic up-and-down variations. It can be due to the variations of dislocation density caused by the alternate stress generation and relief. Fatigue hardening and softening occur alternately during the thermal fatigue. Crack propagatation through and around the Si particles are observed. Cracking of the SiC particles and debonding of the reinforcement/the matrix are proven to be the main mechanism when the crack tip interacts with the SiC particles.