Mechanical response of NiTi alloys with different initial phase transition temperatures at high strain rates

In order to obtain the physical and mechanical properties of NiTi alloys with different initial phase transition temperatures under high strain rates, the responses of NiTi alloys with different initial phase transition temperatures were systematically studied under quasi-static compression and tension at strain rate 10⁻³ s⁻¹, quasi-isentropic compression at strain rate 10⁵ s⁻¹, and shock compression at strain rate 10⁷ s⁻¹. Dog-bone specimens and cylindrical rod specimens were used in the quasi-static tension and compression experiments, respectively. A series of quasi-isentropic compression and planar shock wave compression experiments were performed by using the pulsed power generator CQ-4, which can deliver pulsed currents with peak values of 3–4 MA and a rise time of 470–600 ns to short circuit loads. Velocities were measured by a photonic Doppler velocimetry (PDV) system with accuracies of 1%. The quasi-static loading stress-strain curves showed twice modulus changes for both the initial martensitic and initial austenitic NiTi alloys. The modulus changes were caused by crystal reorientation and plastic deformation of the martensitic NiTi alloy. In experiments of the initial austenitic phase, the modulus changes were caused by martensitic phase transition and plastic deformation after phase change. The Lagrangian sound speed increased continuously with the particle velocity for the initial martensitic NiTi alloy under quasi-isentropic loading. However, there are discontinuities in the sound speed curves for the initial austenite phase. The sound speed decreases intermittently from the transverse wave speed to the longitudinal wave speed and then increases linearly with the particle velocity. In shock experiments of initial martensitic NiTi alloy, a double-wave structure appeared at the free surface velocity of about 34 and 100 m/s for the initial sample temperature of 302 and 402 K, respectively. The martensite-austenite phase transition occurred during sample heating of the initial martensitic NiTi alloy. The inflection points on the velocity curve were caused by plastic yielding of martensitic and austenitic phases separately. For the initial austenite NiTi alloy, an obvious elastic-plastic transformation of austenite NiTi alloy was observed at a free surface velocity of approximately 260 m/s. The elastic limit of austenitic NiTi alloy increased from about 2 GPa to about 4 GPa with the increase of strain rate from about 10⁵ s⁻¹ to 10⁷ s⁻¹. The elastic limit decreased to 1.7 GPa at a strain rate of 10⁷ s⁻¹ with the initial sample temperature of 402 K. The results show that the elastic limit of NiTi alloy is greatly affected by temperature and strain rate.