Microstructure And Creep Behavior Of Al-Cu-Mg-Ag Composites Reinforced With In-situ TiB₂

Due to the advantages of low density,high specific strength,good wear resistance and elevated creep resistance,particles reinforced aluminum matrix composites have become new structural materials which are widely used in the field of aerospace.Among the commonly used ceramic particle reinforced phases,TiB₂particles show the advantages of high melting point,thermal stability,high interface bonding strength with Al matrix,good lattice matching withα-Al and no reaction with Al matrix.In recent years,with the increasing requirements for the flight speed and service time of aerospace equipment,as a common structural material,Al-Cu-Mg-Ag alloy is prone to friction damage,creep fracture and other problems.Therefore,it is particularly important to further improve the room temperature and elevated temperature mechanical properties of Al-Cu-Mg-Ag alloy and its composite.In this work,3 and 6 wt.%TiB₂/Al-Cu-Mg-Ag composites were fabricated by salt-reaction method.The microstructure of Al-Cu-Mg-Ag alloy and its composites in different states were observed by optical microscopy（OM）,X-ray diffraction analysis（XRD）,scanning electron microscope（SEM）and transmission electron microscopy（TEM）.The mechanical properties of Al-Cu-Mg-Ag alloy and its composites at room and elevated temperature were tested by tensile test,hardness test and elevated temperature creep tests.The results show that:（1）The addition of In-situ TiB₂ particles has a great influence on the microstructure and precipitation process of Al-Cu-Mg-Ag alloy.The grain size of as-cast Al-Cu-Mg-Ag alloy is about 130μm.After the addition of TiB₂ particles,the grain sizes of as-cast 3 and 6 wt.%TiB₂ composites were refined to 98μm and 86μm,respectively.The extruded microstructure of Al-Cu-Mg-Ag alloy and its composites are all intermittent fibrous,and the extruded grain width of composites is significantly smaller than that of the Al-Cu-Mg-Ag alloy.The addition of In-situ TiB₂ particles shorten the peak aging（T6）time of the composites from 10h to 6h.TEM observation shows thatΩprecipitates are mainly precipitated in Al-Cu-Mg-Ag alloy;θ′precipitates are mainly and uniformly precipitated in 3 wt.%TiB₂ composite,and someΩprecipitates are retained;6 wt.%TiB₂ composites are mainly composed ofθ′precipitates.（2）The addition of In-situ TiB₂ particles significantly improves the room temperature mechanical properties of Al-Cu-Mg-Ag alloy.After T6 heat treatment,the tensile strengths of 3 and 6 wt.%TiB₂ composites are 472.7MPa and 537MPa,which are 10.5%and 25.6%higher than that of the Al-Cu-Mg-Ag alloy,respectively.The elongation of 3 wt.%TiB₂ composite decreased slightly,while the elongation of6 wt.%TiB₂ composites decreased from 9.71%of the Al-Cu-Mg-Ag alloy to 4.69%.（3）After the creep temperature at 180-220℃under applied stresses of150-275MPa,3 wt.%TiB₂ composite shows the lowest steady creep rate,smallest creep strain and longest creep life than those of the Al-Cu-Mg-Ag alloy and 6 wt.%TiB₂ composite under the same creep conditions.The steady creep rates of 3 wt.%TiB₂ composite are 45-320%lower than those of the Al-Cu-Mg-Ag alloy,and the threshold stresses are increased by 13-19MPa,which means that 3 wt.%TiB₂composite exhibites the best creep resistance.Before and after the creep test,θ′precipitates with high number density and small size together with the In-situ TiB₂particles effectively hinder the movement of dislocations during creep.The strengthening ofθ′precipitates and TiB₂ particles increases the threshold stresses of 3wt.%TiB₂ composite and improves its creep resistance.The true stress exponent n of the matrix alloy and composites is confirmed to be 5,indicating that the dislocation climb mechanism is the dominant creep mechanism with the existence of threshold stress.In summary,in-situ TiB₂ particles can strengthen the Al-Cu-Mg-Ag alloy.Among them,3 wt.%TiB₂ composite shows excellent elevated temperature creep properties,and 6 wt.%TiB₂ composite shows excellent room temperature mechanical properties.The results obtained in this work provide a new technical idea and experimental basis for the development of ceramic particle reinforced Al-Cu-Mg-Ag matrix composites.