High Temperature Specific Heat Capacity Measurement of Ni2+XMn1-XGa

The investigation of Ni2MnGa ferromagnetic shape memory alloys relies on accurate and complete phase diagrams for the various phase transitions these materials exhibit. However, very little work has been performed to investigate the high temperature order/disorder phase transitions. This project tries to fill some of the gap in the phase diagram through a systemic composition based investigation of Ni2-XMn1+XGa (X=-0.2, 0.0, 0.2, 0.8 and 1.0) using high temperature specific heat measurements. The work has produced a phase diagram in the composition range measured and demonstrated that the order/disorder transition temperatures and Curie temperatures follow the predicted trend as seen in similar studies of different alloy compositions. This work has also characterised the melt temperature of the five compositions measured. As part of the investigation of the high temperature phase transitions, a ‘High Temperature Adiabatic Calorimeter” was developed, which was commissioned using four reference samples (copper, Stainless steel, graphite and molybdenum). The developed system used a linearly drifting baseline to improve the speed, accuracy and stability of the system. This modification to the standard adiabatic calorimeter measurement scheme required extensive development of the analysis techniques and development of new approaches (e.g. heating phase analysis and finite element modelling analysis). The system was demonstrated to operate well between room temperature and 1350 K, producing accurate specific heat data that compared well with the standard data for the commissioning samples. Due to time constraints this system was not used for the Ni2MnGa investigation.