| In this paper,in-situ(TiC,TiBZ)reinforced aluminum-based composites were produced by in-situ formation method based on Al-Ti-C and Al-Ti-B system.The effect of synergistic effects of TiC and TiB2 on the microstructure of piston-aluminum alloys was studied.The tensile properties at different temperatures and creep properties of T6-state different composites were tested,and the creep mechanism was analyzed.Because the thermal stability of the alloy has a great influence on the service life of the material,the composites were processed by T6+350?/100 h.The tensile and creep properties of the composites after thermal exposure were tested and the microstructure was observed.The main findings are as follows:The ?-Al dendrite of T6-state TiC/Al and TiB2/Al composites becomes shorter,the secondary dendrite spacing decreases and the grain size is refined.When TiB2 and TiC particals were added in the composites together,the grain refinement of ?-Al is remarkable.After thermal exposure at T6+350?/100h,eutectic silicon becomes bulky and short rod-like at partial zones,primary silicon edges are passivated,and CuNiMnFe phase grows up and coarsens obviously.Some of the partial strip-like CuNiMnFe phases have been fractured,and the Mg2Si phases,like black Chinese character,have been precipitated.With the increase of TiB2 content from 0 wt.%to 0.15 wt.%,the tensile strength of TiB2/Al at room temperature increases firstly and then decreases.When TiB2 reached to 0.1 wt.%,the tensile strength reaches to the highest value(358.02 MPa),which increased by 5%;With the increase of TiC content from 0%to 0.06 wt.%,the tensile strength of TiC/Al at room temperature increases at first and then decreases.The tensile strength at room temperature increases to 341.6 MPa when TiC content reached to 0.03 wt.%.According to the mechanical properties at room temperature,the optimum content of TiB2 particles is 0.1 wt.%and TiC particles is 0.03 wt.%,respectively.When the optimum content of TiC and TiB2 are 0.03 wt.%and 0.1 wt.%respectively,the room temperature elongation is increased by 3%.The mechanical properties at room temperature are the same as those at high temperature.After T6+350?/100h heat exposure,the tensile properties decreases at room temperature and high temperature,and the rule is the same as that of T6 state.The tensile fracture mechanism of TIB2/Al and TiC/Al composites is cleavage fracture at room temperature.The room temperature fracture mechanism of(TiB2+TiC)/Al composite is mixed fracture of cleavage and partial dimples.The fracture mechanism at high temperature of three kinds of composite is cleavage and mixed fracture of some dimples.The fracture forms are brittle fracture.After T6+350?/100h heat exposure,the fracture mechanism at room temperature is cleavage and mixed fracture of some dimples,and the fracture form is brittle fracture.The fracture mechanism at high temperature is the mixed fracture mechanism of cleavage and micropore aggregation.The creep rates of 0.03 wt.%TiC/Al,0.1 wt.%TIB2/AI and(0.1 wt.%TiB2+0.03 wt.%TiC)/Al composites increases with the increase of stress at 350?.The creep resistance of 0.03 wt.%TiC/Al and 0.1 wt.%TiB2/AI composites is better than(0.1 wt.%TiB2+0.03 wt.%TiC)/Al composite.When TiB2 and TiC cooperate with each other,grain boundary specific surface area increases with ?-Al grain refinement,grain boundary sliding obviously and creep resistance decreased.Grain boundary sliding plays an important role in creep deformation mechanism of(0.1 wt.%TiB2+0.03 wt.%TiC)/Al composites.After T6+350?/100h thermal exposure,the creep rate increases by one order of magnitude than that of T6 state,and the creep resistance decreases greatly.Among them,the creep properties of 0.03 wt.%TiC/Al are better than those of 0.1 wt.%TIB2/Al and(0.1 wt.%TiB2+0.03 wt.%TiC)/Al composites.The creep fracture life and creep resistance of 0.03 wt.%TiC/Al composites under stress 32 MPa are increased by 7 hours compared with those of matrix alloys.The nominal stress indices of the three materials are 8.0,7.5 and 6.8 respectively.The creep mechanism is mainly dislocation climbing mechanism,accompanied with grain boundary sliding mechanism.The creep fracture mechanism of 0.1 wt.%TiB2/Al,0.03 wt.%TiC/Al and(0.1 wt.%TiB2+0.03 wt.%TiC)/Al composites is microporous aggregation fracture.After T6+350?/100h thermal exposure,the nominal stress indices of the three composites are 10.3,10.4 and 11,respectively.The creep mechanism is mainly dislocation climbing,accompanied by grain boundary sliding,and the creep fracture mechanism is microporous aggregation fracture. |
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