Study On High Temperature Strength And Deformation Mechenism Of Ti₂ AlN/TiAl Composites

In the paper, Ti-47Al alloy, 20% and 50% Vol. Ti2AlN/TiAl composite were succesfully fabricated by powder metallurgy technology. Uniform microstructure was gained by heat treatment later. True stress-ture strain curve and high temperature strength of thermo-compressive deformation of the three materials at different temperature and different strain rate were obtained by Gleeble^-1500D. The effect of different process parameters on the high temperature strength was discussed. Also the strength enhancement of the composites was discussed. Metallomicroscope (MS), scanning electron microscope (SEM), Transmission electron microscopy (TEM) were used to observe the microstructure before and after thermo-compressive deformation. Activation energy of defomation at defferent temperature was calculated. Deformation behavior and mechanism was discussed by activation energy and defomation microstrcture.The results showed that at the temperature ranging from 800 to 1100℃and strain rates ranging from 0.001 to 0.1s^-1, the ture stress-ture strain curve of thermo-compressive deformation of the three materials had three stages: elastic deformation stage, work harding stage and strain softing stage. The peak stress found on the ture stress-ture strain curve was defined as the compressive strength. It decreaced with the increasing of temperature and increased with the increasing of strain rate. The optimum enhancement effect of the composite was obtained at low temperature and low strain rate or at high temperature and high strain rate. Compared with the Ti-47Al, the compressive strength of the composite got the maximum reinforce percentage (50.24%), which is 618.4MPa.The microstructure analysis showed that the inclination of the lamellar and the deformation direction minished and the lamellar kinked during the thermo-compressive process. At low temperature, the kink of the lamellar and the slippage of the boundary was the main way of the deformation of Ti-47Al alloy. At high temperature area, the slippage of the dislocation in the grain and slippage of the grain boundary should be the deformation mechanism of Ti-47Al alloy. The deformation machenism of the Ti2AlN/TiAl composite was the slippage of the dislocation in the grain. The Ti 2AlN phase blocked the movement of the dislocation. Activation energy of the Ti-47Al alloy is 398.6KJ/mol at the temperature ranging from 800 to 1100℃and strain rates ranging from 0.001 to 0.1s^-1. In the temperature range 800¹100℃, activation energy of the 20%Vol. Ti2AlN/ TiAl composite are 566.8KJ/mol (0.001⁰.01s^-1) and 334.1KJ/mol (0.01⁰.1s^-1), activation energy of the 50%Vol. Ti2AlN/TiAl composite is 547.8KJ/mol (0.001⁰.01s^-1 ) and 352.0KJ/mol (0.01⁰.1s ^-1). TEM showed that during the deformation process of the three mateials the dislocation mainly distributed in theα2 phase of the lamellar. The dislocation density in theγphase was low. The deformation mode of the lamellar depended on the deformation mode ofα2 phase. Dynamic recovery and recrystallization took place during the deformation. They induced the strain softing of the material.