Preparation And Properties Of Cu Coated Ti₃AlC₂ And Its Reinforced Ni Matrix Composites

Ni based materials have the advantages of excellent thermal stability,high melting point（1455 ℃）,good conductive and thermal conductivity and mechanical properties etc.Ti₃AlC₂ is a kind of ternary layered carbide ceramics,with excellent properties of metal and ceramics.It is a kind of very ideal reinforcement of composite materials.Therefore,it is very meaningful to study the Ti3AIC2 reinforced Ni matrix composites by combining both of their advantages.This thesis aiming at the difficulties in preparing Ni based composite materials,such as weak interface bonding,poor wettability and large particle size,adopting the mechanical alloying method to make Cu coating Ti₃AlC₂ to improve wettability,and Ti₃AlC₂ is used as a precursor to separate TiC_x through A-site dissociation.The preparation process of Cu-coated Ti3AIC2 reinforced Ni matrix composites has been intensively studied.The phase composition and microstructure of the composites were varying with the different sintering processes,the relationship of which was analyzed by means of XRD,SEM and EDS.In addition,the mechanical properties of the composites were tested by the universal testing machine,then the reaction mechanism of the composites and relationship between microstructure and the mechanical properties were analyzed preliminarily.The experimental results were showed as following:（1）The ultrafine crystal Cu coated Ti3AIC2 and its reinforced Ni matrix composites with solid interface bonding and excellent comprehensive properties can be prepared successfully as adding Cu by mechanical alloying（MA）technology and the two-step hot press sintering the Ti3AIC2 and Ni powders.（2）The best mechanical alloying process is speed at 400 rpm,ball ratio 20:1,and vacuum ball milling time in10 h,which was determined by analysing the different ball milling parameters on the effect of Cu coated Ti₃AlC₂,including the ball milling time,ball milling speed,ball ratio and raw material ratio.In order to obtain the finer raw material,the Ti3AIC2 powder and Cu powder were selected for mechanical alloying process mentioned above.（3）The microstructure of Cu coated Ti3AIC2 and its reinforced Ni matrix composites is mainly composed of matrix,TiC_x reinforcement and few defects.With the increase of sintering temperature,the TiC_x grows gradually,and the defects decrease gradually in contrast,and with the increase of the volume fractions of Ti₃AlC₂,the distribution of TiC_x is more uniform.The distribution of Cu elements is uniformly dispersed in the matrix,which is due to the increase of liquid fluidity when the temperature is higher than the melting point of Cu,so the soluability of Cu into the Ni matrix increased.（4）The reaction mechanism of the composite system is that Ti₃AlC₂ can transform into TiC_x and Al through A-site dissociation approach,then Cu reacts with Ni to form the matrix phase of Ni（Cu）alloy;at the meantime,A1 and Ti would further react with Ni to form Ni3（Al,Ti）without other impurity phase.（5）The overall relative density of materials prepared in this paper is above 96%.Mechanical alloying led to the refinement of matrix grains,which is beneficial to improving the hardness and strength of the material.The Vickers hardness of the composites is up to 5.82 GPa,and the hardness and standard deviation of the material decrease with the increase of the load applied.（6）The Cu coated Ti₃AlC₂ and its reinforced Ni matrix composites of volume ratio 40:20:40 prepared at 1250℃ exhibit the most excellent comprehensive mechanical properties,which is attributed to the most excellent microstructure of the composites prepared under the sintering conditions.（7）The outstanding effect of adding Cu with mechanical alloying is to produce submicron TiC_x particles,thus improving the wettability to strengthen the interface between reinforcement and the matrix,according to the principle of fine grain strengthening,the composites have more excellent mechanical properties as a result.When the sintering temperature and volume fraction are at the same,the compressive strength,bending strength and fracture toughness of composites prepared by Cu high energy ball milling are 1519 MPa,957 MPa and 28.13 MPa·m 1/2 respectively,which are higher than 937.50 MPa,523.91 MPa and 13.52 MPa m^1/2 of composites prepared by pure roller ball milling through contrast experiment.