Abstract
In order to clarify the cause of quench-hardening in Ni-rich NiTi, the relation between composition and electrical resistivity, and the change in the electrical resistivity at a constant rate of heating for NiTi have been measured.
On the Ni-rich side of the stoichiometry, the electrical resistivity increases with increasing nickel content for water-quenched specimens and remains constant at non-stoichiometric composition for slowly cooled specimens. The measurements of electrical resistivity at a constant rate of heating for water-quenched Ni-rich NiTi show a significant decrease in the electrical resistivity around 300 and 700°C. Above 700°C the electrical resistivity changes in the same manner as slowly cooled specimens. Activation energies were 1.1 eV for the low temperature reaction and 3.7 eV for the high temperature reaction. The electrical resistivity for the specimens of stoichiometric composition and those containing 55 at%Ni after being ordered just below the critical temperature increases with increasing temperature. No significant change in the electrical resistivity due to the order-disorder transformation or the eutectoid reaction was observed.
The analysis of the results of the electrical resistivity measurement indicates that the ordered NiTi phase is stable but has a restricted homogeneity range at low temperatures, the precipitation of Ni3Ti in the water-quenched state is hardly conceivable and the existence of the order-disorder transformation and the eutectoid reaction is improbable. Therefore, it can be concluded that the quench-hardening in non-stoichiometric NiTi is due to the existence of structure defects.
