PSI - Issue 69

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

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ratio of maximum normal stress to maximum shear stress under the applied load. The larger the value α is, the softer is the stress state and the more prone is material to plastic deformation and ductile fracture. This also results in higher stress values under torsion as compared to compression.

Fig. 2. Torsion flow curves of NiTi obtained at 300-600 °C.

The peak strains e max and steady-state starting strains e st with corresponding values of peak stresses σ max and steady state starting stresses σ st for different deformation temperatures are shown in Table 1.

Table 1. Peak stress, peak strain, stready-state stress and stready-state starting strain after torsion deformation at 300-600 ºC.

Deformation temperature (ºC)

σ max ( MPa )

e max 0.21 0.19 0.14 0.09 0.07

σ st ( MPa )

e st

300 350 400 500 600

1174 1088

-

-

835 620 431 335

0.58 0.51 0.37 0.27

860 547 380

Both σ max and σ st decrease with increase of deformation temperature. The highest value of peak stress σ max is observed at 300 °C ( σ max = 1174 MPa) and this value is more than three times higher than at 600 °C ( σ max = 380 MPa). The peak strain e max decreases from 0.19 to 0.07 with increase of deformation temperature from 350 to 600 °C, so the kinetics of softening processes activation is higher at high temperatures. After reaching e max , softening processes begin to prevail over hardening processes, and the stress falls to the steady-state value σ st at e st . The σ st and e st also decrease with increase of deformation temperature. The steady state stress is maximum at 350 °C ( σ st = 835 MPa) and falls with increase of deformation temperature ( σ st = 335 MPa). The difference between two stresses σ max - σ st decreases from 245 to 45 MPa as the deformation temperature increases from 350 to 600 °C (Table 1). That occurs due to decrease of strain-hardening coefficient and increase of diffusional dynamic softening processes rate. The torsional deformation at 350-600 °C and strain rate of 0.1 s -1 allows achievement of much higher strains ( e > 3) as compared to compressive deformation (e < 1.2) under the same conditions. The stresses required for torsion deformation are higher as compared to compression. Deformation by torsion at 350-600 °C leads to the formation of steady state flow stress stage. The polygonized structure can be formed in this temperature range and true strain range