Aniruddha Biswas

A study of self-propagating high-temperature synthesis of NiAl in thermal explosion mode

Abstract Processing of NiAl in the thermal explosion mode of Self-propagating High-temperature Synthesis (SHS) was investigated at different heating rates varying between 5 and 60 °C/min. Three different Ni particle sizes were used and experiments were conducted in vacuum and argon atmospheres to arrive at an optimized processing window. On-line videography of the process provided the visual evidence of the different stages of thermal explosion. Evolution of the process was studied by on-line thermometry, optical and scanning electron microscopy, XRD and mercury intrusion porosimetry. The results showed that the combustion behaviour was independent of the atmosphere but strongly dependent on the rate of heating and hence on the extent of pre-combustion reaction. Process modelling was carried out to describe the effect of the rate of heating both on the pre-combustion reaction and the combustion in the case of thermal explosion. This model could successfully rationalize the experimental results.

Neutron diffraction evidence for kinetic arrest of first order magneto-structural phase transitions in some functional magnetic materials

Neutron diffraction measurements, performed in the presence of an external magnetic field, have been used to show structural evidence for the kinetic arrest of the first order phase transition from (i) the high temperature austenite phase to the low temperature martensite phase in the magnetic shape memory alloy Ni37Co11Mn42.5Sn9.5, (ii) the higher temperature ferromagnetic phase to the lower temperature antiferromagnetic phase in the half-doped charge ordered compound La0.5Ca0.5MnO3 and (iii) the formation of glass-like arrested states in both compounds. The cooling and heating under unequal fields protocol has been used to establish phase coexistence of metastable and equilibrium states, and also to demonstrate the devitrification of the arrested metastable states in the neutron diffraction patterns. We also explore the field–temperature dependent kinetic arrest line TK(H), through the transformation of the arrested phase to the equilibrium phase. This transformation has been observed isothermally in reducing H, as also on warming in constant H. TK is seen to increase as H increases in both cases, consistent with the low-T equilibrium phase having lower magnetization.

Calorimetric Studies of Dissolution Kinetics of Ni2(Cr,Mo) Phase in Ni-Cr-Mo Alloys Using Non-Isothermal Approach

The kinetics of dissolution of ordered phase with Pt2Mo structure has been studied in two nickel chromium alloys – one without molybdenum and another with molybdenum - using differential scanning calorimetry. The activation energy for dissolution, determined using three nonisothermal approaches was found to be ~ 418 kJ /mole for both the alloys. This value agreed very well with the activation energy for coarsening of g″ precipitates in Ni-Cr-Mo matrix and is close to activation energy for mobility of chromium and molybdenum in complex nickel alloy matrix.