The High-temperature Creep Behavior Of In-situ TiB₂ Particulate Reinforced Al-12Si Composite

Particle reinforced aluminum matrix composites are promising materials in aerospace,national defense and automobile industry,due to their favorable material characteristics,such as high specific strength,high elastic modulus,and elevated temperature properties,as well as simple and low-cost processing technique.Engineering materials often work at the high temperature condition.In this case,the slow plastic deformation has procceded with time.This is the creep deformation,which occurs and may lead to failures,even fracture.Therefore,the study of high temperature creep behavior for the composite is important.The in-situ method can refine the particles to the nano-scale in the aluminum matrix composites,which often yields better mechanical properties.The TiB₂/Al-12Si composite prepared by the Al-12Si matrix alloy,which is usually used at a high temperature range（523⁶23 K）,exhibits favorable room-temperature and elevated temperature mechanical properties.In this work,the in-situ TiB₂/Al-12Si composites were successful synthesized through the salt-metal reaction route.The chemical composition and the phase were analyzed by inductively coupled plasma optical emission spectroscopy（ICP-OES）and X-ray diffraction（XRD）.The microstructure and the fracture surface were observed by optical microscopy（OM）and scanning electron microscopy（SEM）.The tensile creep tests were carried out under the constant load in the range of 523⁶23 K on the creep machine.The high-temperature creep behavior of TiB₂/Al-12Si composites was studied in detail.The influences of the TiB₂ particle content and the heat treatment on the creep properties of composites were analyzed.The characteristics of creep for composites were also investigated.The research for the influence of reinforcement content on the creep behavior of composites shows that the composites exhibited a lower steady-state creep rate when compared to the unreinforced matrix alloy at high temperature.This shows the creep strength of Al-12Si alloy is enhanced under high temperature.However,the in-situ TiB₂/Al-12Si composites presented similar creep strength when the weight fraction of reinforcement increased from 4%to 9%.The calculation based on the stress exponent and apparent activation energy shows the high-temperature creep behavior for x TiB₂/Al-12Si composites（x=4 and 9 wt.%,T5）could be explained by the dislocation glide mechanism（523 K）and dislocation climb mechanism（573K and 623 K）.The TiB₂ particles bear the applied stress during the creep process and reduce the average stress on the matrix.Meanwhile,the reinforcements can work as dislocation obstacles.However,the study of the creep fracture surface reveals the severe TiB₂ clusters can accelerate the fracture process and have a detrimental influence on the creep resistance.The research for the influence of heat treatment on the creep behavior of composites shows that the similarities in creep behavior for T5 and T7composites,including the similar characteristics of creep curves and the stress exponent.Meanwhile,lower steady-state creep rate and longer creep lifetime was found in the 4 wt.%TiB₂/Al-12Si composite（T5 state）,as compared with that in the 4 wt.%TiB₂/Al-12Si composite（T7 state）.The result has been rationalized by using the load-partitioning model and relative to the evolution of the rigid phase.In general,the creep deformation of the4 wt.%TiB₂/Al-12Si composite at both states was controlled by the climb of dislocations in the aluminum alloy matrix.