DEVELOPMENT OF TiAl BASED INTERMETALLICS BY MECHANICAL ALLOYING AND SINTERING

Abstract

This study is a part of larger effort towards the fundamental understanding of structure-property relationship of mechanically alloyed titanium aluminide based nanocomposites containing Cr, Nb and Ni-P coated graphite and carbon powders as second phase reinforcement. The aluminides are synthesized using mechanical alloying and two routes of sintering i.e. pressure less and pressurized sintering. The applications of these aluminides as thermo structural part need strength and in turn densification. Mechanical alloying and the formation of aluminides during sintering hinder the densification process. The formation of aluminides consists of self propagating high temperature reaction, which causes in the bulging of the compacts. The nanograined feature of the compacts though gave higher hardness and strength but the densification were not found desired, which is due to the lack of compressibility of workhardened mechanically alloyed powders. Morphological and phase analysis of the mechanically alloyed partiecles was studied using TEM, FESEM, and XRD. Archimedes principle was used to study the density of sintered samples and the percentage densification after pressure less and pressurized sintering. The pressurized sintering was carried out in GLEEBLETM thermo mechanical simulator by the help of plungers. The hardness of the compacts was calculated using Vickers hardness tester. The thermal behavior of the as received powder blends and mechanically alloyed powders were determined using DTA. As the mechanically alloyed powders having lesser densification, in order to determine the route for better densification, reaction synthesis route is used for the formation of Titanium Aluminides. In reaction synthesis route Titanium and Aluminium powders were mixed in a 50:50 at% composition. To see the effect of particle size on the densification and activation energy, Ti and Al powders were milled separately, to attain particle sizes of 100μ, 25-27μ and 7-9μ. After optimization by milling separately, Ti powders were milled for 1h, 5h and 10h respectively to attain above particle sizes. Al powders were milled for 1h,4h and 7h ton attain above particle sizes respectively. To determine effect of Al particle sizes keeping the Ti particle size constant (44μ), Various particle size of Al were mixed with Ti, to attain Ti-Al (50:50 at%). Similarly to determine the effect of Ti particle size Al powder size were made constant and various particle sizes Ti were mixed with it to prepare powder blends. To determine the cumulative effect of ii particle sizes, approximately equal particle sizes of Ti and Al were mixed with each other to prepare powder blends. The thermal analysis behavior of the powder blends were calculated using DTA and effect of heating rates were determined using three different heating rates. The reaction sintering temperatures (exothermic temperatures) are used to calculate the activation energy of the above mixtures. FESEM analysis was carried out to determine the microstructural evolution. Densification after reaction sintering was detrmined using Archimedes principle. The above powder mixtures with various Ti particle size, various Al Particle size and cumulative particle size were also subjected to dilatometric calculations. In order to calculate the bulging/expansion during reaction synthesis the dilation behavior is calculated. Dilatometric analyses was calculated at five different soaking temperatures i.e 6000C,6500C,7000C,7500C and 8000C to explain solid state sintering, transient liquid phase sintering and formation with completion of Titanium Aluminides. Using ANN(artificial neural network) and Fuzzy modeling (ANFIS), a model is prepared, where composition, soaking temperature and relative percentage of expansion was given as the input and the average shrinkage rate is the output and the resultis calibrated and validated with 50% of the results.