Abstract
The relation between the nickel content and the hardening capability of Ni-rich NiTi compounds, and the effect of heat treatment on the mechanical properties of a NiTi compound containing 54.7 at%Ni have been investigated. NiTi compounds containing more than 52 at%Ni show a considerable increase in hardness by quenching, and the maximum hardness is obtained at about 54.5 at%Ni. According to the phase diagrams which are considered to be most reliable, the composition from which the hardening effect appears corresponds to the composition of the boundary between the NiTi phase and the NiTi+Ni3Ti phase at lower temperatures and the composition at which the maximum hardness is obtained coincides with the composition of the same boundary at the quenching temperature. This fact seems to suggest that the hardening capability in Ni-rich NiTi compounds is related to the existence of excess point defects which are introduced by the deviation from the stoichiometric composition at the quenching temperature. The NiTi compound containing 54.7 at% Ni is hardened by quenching from temperatures above 600°C, and the hardness increases with increasing temperature. When the NiTi is quenched into water from 1100°C, the Vickers hardness of about 700 is obtained. The hardness of the quench-hardened NiTi decreases with annealing time. However, no hardening effect due to the precipitation of Ni3Ti is observed. The tensile strength of the NiTi is 80 kg/mm2 in the annealed state and 100 kg/mm2 when annealed at a relatively high temperature after quenching. Although the Young’s modulus of the NiTi in the annealed state is anomalously low as compared with the values of the constituent metals, quenching from 1000°C gives a nearly 20% increase of the value.
