Reaction kinetics studies on Ti-Al-Cr-Nb-B intermetallics

Abstract

In reaction synthesis (RS) route of powder metallurgy, heat of reaction between elemental powders is utilised to obtain intermetallic compounds. Prior to reaction synthesis of quaternary alloy Ti48Al2Cr2Nb (with 10 at. % B), reaction kinetics studies were carried out. Powder mixture was prepared and Differential Scanning Calorimetry (DSC) experiments were carried out at 3 different heating rates. Activation energy and Avrami Index 'n' are obtained by Johnson-Mehl-Avrami (JMA) equation. Formation of Al 3 Ti was confirmed through X-ray diffraction (XRD). Reaction mechanisms are identified in accordance with 'n' values. In the temperature range of 1060-1150K, it proceeds as instantaneous nucleation and three dimensional growth with an activation energy of 176 kJ mole -1 . Theoretical backgroundNotation: D :the reacted fraction, t : time, T : the absolute temperature, R :the gas constant = 8.3143 J/mol/K n :Avrami Index parameter depending on the nucleation mechanism and number of growth dimensions E : the activation energy, A : the pre-exponential factor, E : the heating rateThe reaction rate of a solid-state transformation obeying JMA kinetic model can be written in the following form [6];The above equation is differentiated and equated to zero (d 2 D/dt 2 = 0) for the maximum reaction rate at T m temperature to obtain final equation as follows;ln E/T m 2 = -E/RT m + C1where C1 is a constant.T m is calculated from the DSC plots for all the different heating rates with the assumption that the peak of respective DSC plot for a particular heating rate (E) is the maximum reaction rate point. The plot for ln E/T m 2 versus 1/T m is linear and the slope of this curve is used to calculate the activation energy E.Integration of equation 1 and after taking double natural logarithm and using Doyle's approximation [7]; ln[ln (1/1-D)] = C2 -1.05nE/RT( 3) where C2 is a constant

Development of TiB2 reinforced in-situ Ti aluminide matrix composite through reaction synthesis

“…Endothermic peak is due to melting of Al. After melting of pure Al exothermic reaction starts for formation of intermetallic phases of Ti aluminide and Ti diboride almost simultaneously [12,13]. Change in free energy for TiB 2 formation is marginally less as compared to aluminide [14] so, boride formation and aluminide formation shall be at very short interval.…”

Study on Generation of Fine Grained Titanium Aluminide Through Ball Milling of Ti, Al and Ni-P Coated Graphite Powder

“…The curve shown in the Fig. 4a, indicates formation of Ti aluminide which is an exothermic reaction and it takes place at around 800 0 C [20,21]. It means that simple powder mixture undergoes phase transformation after 800 0 C whereas, in ball milling transformation starts after 20 hrs.…”