PSI - Issue 69

Victor Komarov et al. / Procedia Structural Integrity 69 (2025) 76–79

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of the most effective methods for the formation of an ultrafine-grained structure (UFG) in NiTi SMA which is characterized by the best combination of functional properties [6-10]. Development of SPD methods is concluded in a searching for thermomechanical procedures which allow producing bulk samples with UFG structure [11]. Severe torsion deformation (STD) is a promising technique that allows for high plastic strain accumulation without significant dimensional changes. Potential of the STD in improving the strength and ductility of various materials has been shown [12-15], but it has not been explored extensively for bulk NiTi SMA. In this study, the STD at 300-600 °C is applied to a NiTi SMA to achieve significant strain accumulation. This deformation temperature range is chosen because it is a range of the formation of dynamically polygonized dislocation substructure [16,17] that must provide the highest number of turns and facilitate the formation of the UFG structure. The goal of this work is to evaluate the possibility of application the severe torsional deformation to bulk NiTi samples at relatively low temperatures (in the range of the dynamic polygonization) to accumulate high strains and study torsion deformation behavior of NiTi SMA. 2. Materials and methods Near-equiatomic NiTi 6 mm-diam. rods with nickel content about 50.1 at. % were used. Torsional deformation was carried out using the multidirectional test system "BÄHR MDS-830”. To induce torsional deformation, cylindrical specimens with a reduced gauge section for strain localization were prepared (Fig. 1). The 130 mm-long specimens were cut from the rods. The central part of the specimens of 20 mm length and 4.5 mm diameter was lathe turned until achieving the dimensions shown in Fig. 1. The samples were inductively heated, and the temperature was monitored using a thermocouple welded to a sample. Samples were cooled by directing compressed air flow to the deformed region through two tubes. Torsional deformation was carried out in the temperature range of 300-600°C at a strain rate of 0.1 s -1 in air atmosphere. Deformation was applied until failure or to 10 turns, which is equivalent to the true accumulated strain of e = 3.

Fig. 1. Schematic representation of the sample for STD.

3. Results and discussion The torsion flow curves obtained at different temperatures (300-600 °C) at strain rate of 0.1 s -1 are presented in Fig. 2. All flow curves have initial peaks at true strain of e < 0.3 that are more pronounced for low deformation temperatures (300-400 °C) and have less sharp configuration for high deformation temperatures (500-600 °C). The increase of stress values upon an extremum occurs due to the intensive work hardening. When sufficiently high strains are attained, a steady state flow stress stage is established because the balance between work hardening rate and dynamic softening (recovery, polygonization and recrystallization) rate is reached. Torsion at 300 °C is characterized by the most intensive work hardening and brittle fracture at true strain of e = 0.23 and maximum true stress σ max of 1200 MPa. The raising of deformation temperature up to 350-400 °C leads to the formation of steady state flow stress stage after work hardening, and ductile fracture takes place at much higher true strains. No fracture occurs after 30 turns at 500 °C [18]. The brittle failure of the specimens deformed by torsion at 300 °C is predictable because no steady state flow stress was reached during compression at this temperature. No fracture was observed after compression at 300 °C up to e = 0.9 because of a softer stress state [16,17]. Indeed, the softness coefficient of the stress state α is 0.8 for torsion and 2 for uniaxial compression [19]. This coefficient is a