Preparation Of Nanosized Ti₃AlC₂/Cu Composite Powders And The Fabrication Of The Bulk Composites

To solve the process problems in fabrication Ceramic Particle Reinforced Metal Matrix Composites(CPRMMCs)such as controlling of particle size of the ceramic particles,weak interfacial strength between the reinforcement and matrix and large amount of microstructural defects as well as difficulty in adding the cermic particles into the metal matrix etc.aiming to improve the properties and reliabilities of MMCs,various ceramic particles were selected as reinforcement for metal matrix composites by using variety of processing methods including using Ti3AlC2 or TiCx deintercalated from Ti3AlC2 as reinforcements for Cu and Ni based composites.To obtain easy handled ultra-fine ceramic reinforcements(submicron and nano)with higher surface energy and solve the problems in enhancement of the properties and reliabilities of CPRMMCs,this thesis focused on preparation of nanosized Cu-Ti3AlC2 powders using Cu as a milling media by mechanical alloying,and using them as reinforcements for Cu and Ni based composites.The main achievements were as following:1.It was very difficult to milling down the Ti3AlC2 powders up to submicron level by directly high energy ball milling which often led to the decompositon of Ti3AlC2 and self burning.Using Cu as milling media duing milling process,it can solve the problems of decompositon and self-burning of Ti3AlC2 powders.After milling under certain conditions,the grain size of both Ti3AlC2 and Cu was reduced to less than 100 nm.The main milling mechanisms were due to the chipping effect from the sharp Ti3AlC2 powders to the Cu powders and the hindering effect from Cu to the chipping of Ti3AlC2 powders as well as the chemical saturation effects of the bonds of Ti3AlC2.2.The crystal size of milled powders determined by Rietveld refinments and TEM observations demonstrated that the grain size of Ti3AlC2 was decreased into 18-22.6 nm range after milling 2h under a vacuum environment and a further decreasing into 12-13 nm range by prolonging the milling time up to 10h.At the same time,the grain size of Cu was reduced into 35?46 nm range after milling 2h and further reduced to 8-12 nm after milling up to 10h,respectively.Due to the high surface energy,the Ti3AlC2 and Cu composite powder became aggregated after the milling process.However,the big aggregated composite powder did not affect the densification process to fabricate the bulk materials.3.Using mechanical alloyed Ti3AlC2/CU composite powder as starting materials,it could fabricate high desne Cu composites with high volume ceramic content using hot pressing techniques at 950 ? which was lower than the melting point of Cu.The density of as prepared composites was improved with the prolonging milling time.The main densification mechanisms were due to the reduction of melting point of Cu resulting in a liquid sintering process and good wettability between Cu and Ti3 AIC2 as well as shorter diffusion distances resulting from the mechanical alloying.4.The mechanical and physical properties of as prepared composites using milled Ti3AlC2/Cu powders as starting materials was enhanced with the prolonged milling time.The compressive strength,hardness and electrical conductivity of the composites made from 60 Ti3AlC2/Cu composite powders with a milling time of 6h can reach to 1267 MPa,4.9 GPa,5.19%IACS,respectively.5.TiCX reinforced Ni based composites were prepared by pressureless sintering at 1150? for 30 min by using milled Ti3AlC2/Cu powder mixtures and Ni as starting materials.The TiCx deintercalated from Ti3AlC2 was very homogenously distributed in Ni based matrix with an average particle size of 300 nm.The compressive strength and three-point bending strength of TiCx reinforced Ni based compoisites prepared by using 20Ti3AlC2/13.3Cu/66.7Ni as starting materials can reach 1123 ±15 MPa and 482 ±20 MPa,respectively.The main innovations of this thesis were as following:1.By applying mechanical alloying and Cu as milling media,it solved the problems in preparing submicron/nanosize Ti3AlC2 particles,stabilizing those ultra-fine and highly actived Ti3AlC2 particles in air and avoiding them self-burning.2.By using milled Ti3AlC2/Cu as starting materials or precursors,the Ti3AlC/Cu composites with high strength and conductivity could be prepared at lower tempreature as 950 ?.It could significantly improve the distribution of Ti3AlC2 or TiC,in the metal matrice.3.By applying milled Ti3AlC2/Cu and Ni as precursors,the particle size of in situ formed TiCx reinforcement was reduced and their distributions were significantly improved as well as the density of the as prepared composites.4.The method used to prepare Cu composites coupled with mechanical alloying and hot pressing can also applied to prepare other MAX phase-metal composites.