| In this research, TiN/Ti composites was prepared by powder metallurgy method, Titanium powder was first ball-milled with N2, aiming to form the composite powder,and then through hot-press sintering to fabricate TiN/Ti composites. according to the different ball-milling time, components and properties of cmposites were studied. The ball-milling time in this research were 0.5h, 1h, 1.5h, and 2h respectively, the atmosphere was N2, the pressure was 4atm, the hot-press sintering parameters were 1100℃/20MPa/2h. components and microstructures were analysed by XRD, EMS, SEM and TEM. Mechanical properities were tested and the reinforcing mechanism was studied. Except that, The kinetics of TiN synthesized with Ti and N2 was also studied, aiming to provide a theoretical basis for the preparation of Titanium metal matrix composites.The ball-milling process had an obvious refining effect on Tatinium powder, with increasing of ball-milling time, the size of partical was smaller and smaller, when ball-milling for 0.5h, the prtical size droped to 26.87 nm from 44μm dramaticly, and when ball-milling for 2h, it droped to 16.33 nm, at the same time, powder began to agglomerate. After ball-milling, N2 absorb on the surface of Ti particle to form TiNx composite powders. Because of the great lattice distortion and the weak of interatomic action force, it didn’t form the stable TiN after ball-milling. And when sintered at high temperature, it recombine to an ordered structure and N diffusion futher into Ti particle to form a non-stoichiometric ratio TiNx. the components after sintering were βTi+αTi+TiNx.The microstructure of composites after sintering showed that the density is good, no obvious holes or crack were found, with ball-milling time increasing, crystal size decreased obviously. The matrix contains two-phase region βTi and αTi, a lot of martensite exist in βTi. The size of TiNx was very small, from 20 nm to 200 nm, it distribute uniformly in matrix and made a good dispersion strengthening effect.Mechanical properties test showed that the hardness increased with ball-milling time increasing, compared with matrix Titanium, with ball-miling time increasing, it increased 31.13%, 58.08%, 93.6%, and 101.71% respectively. The maximum compressive yield strength increased first and then decreased with ball-milling time increasing, compared with matrix Titanium, it increased 17.27%, 30.92%, 35.93%, and 18.11% respectively, when ball-milled for 1.5h, it reached to a maximum value of 2440 Mpa. The bending strength decreased with ball-milling time increasing. On the one hand, the product TiN have high hardness and high strength, it is fragile, moderate TiN is good for improving the properties of composites, but,when it is excessive, it will have a bad influence on the properties on the contrary, except that, with increasing TiN, the melting point of cmposites will also increase, which will lead to a decrease of the density of composites, so, the bending strength decreased with ball-milling time increasing.Through analysis of its microstructure and appearance of fracture, the reinforce mechanisum could be described as follows: the formed TiN improved its bearing capacity; ball-miling process decreased the particle size; the solution N plays a good solution strengthening role.The kinetics of TiN synthesized showed that Ti could react with N2 at 800-1100℃, with isothermal time increasing, the amount of product TiN increasd, at 1100℃, within 4h, Ti could transform to TiN totally. The whole process includes two stages, during the first 5s, the controlling mechanism was nucleation and growth, which was represented by the following kinetic equation: [-ln(1-a)]4=kt. During the folowing time,the controlling mechanism was three-dimensional diffusion which was represented by the following kinetic equation: [1-(1-a)1/3]2=kt. The activation energies obtained for two stages were 214.84 KJ/mol and 124.01 KJ/mol respectively. |
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