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Phase transformations in nickel-rich nickel-titanium alloys : influence of strain-rate, temperature, thermomechanical treatment and nickel composition on the shape memory and superelastic characteristics

  • Author(s): Adharapurapu, Raghavendra R.
  • et al.
Abstract

Nearly four decades of academic research and industrial interest on Nitinol has largely focused on the superelastic applications in the bio-medical sector and in the development of 'smart' shape-memory based sensors involving the low-strain rate (̃0.001/s) response of NiTi. It is only within the last decade that there has been a growing interest in the exploitation of Nitinol towards high-strain rate applications such as seismic damping, blast-mitigation or energy-absorbing applications. However, a systematic study of the influence of high-strain rate and temperature on the shape memory characteristics of NiTi is severely lacking. The current research program reports the findings on: (1) The phase-transformation mechanisms in Ni-rich Ni-Ti alloys. These include (a) diffusionless multiple-stage martensitic transformations and (b) diffusion-based phase transformations that govern the precipitation reactions in Ni-rich alloys and the overall time-temperature-transformation (TTT) curves. (2) The systematic study of the high-strain rate response of Ni-rich NiTi alloys as a function of temperature (between -196⁰C and 400⁰C) and thermomechanical treatment, viz., fully annealed, work-hardened and precipitation hardened conditions. Two Ni-rich Nitinol alloys, a commercial 50.8- NiTi (at.%) and a new 55-NiTi (at.%), were selected for the study, since the Ni composition determines the precipitation processes and, critically, the transformation temperatures in NiTi alloys. It was observed that the presence of dislocations (through work- hardening) and the presence of Ni-rich precipitates (through age-hardening) contribute to a more complex two- stage or multiple-stage transformations and also improve the overall strength of the NiTi alloy. Based on the microstructural changes, such as recovery, recrystallization and precipitation formation in 50.8-NiTi alloys, the current work uniquely provides a unified and general understanding of the various multiple-stage transformations reported in the literature, specifically providing the transition between two main transformation sequence mechanisms rationalized on the basis of partial differential scanning calorimetry (DSC) studies. Additionally, the work also identified unusual multiple- stage transformations in 55-NiTi. Aging in Ni-rich 55NiTi elicited precipitation reactions with the formation of Ti₃Ni₄, Ti₂Ni₃ and TiNi₃ in sequence. A time-temperature- transformation diagram for 55NiTi was constructed, as well as the upper temperature limit of formation for several precipitates has been estimated for Ni-rich NiTi alloys system as a function of Ni concentration between 50.6-56 at.%. Superelasticity and shape memory characteristics in Ni-rich 55NiTi, thought to be unfeasible, have been successfully demonstrated with recoverable strains up to ̃4-6%

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