PSI - Issue 69

Roman Karelin et al. / Procedia Structural Integrity 69 (2025) 35–40

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The results of the optical microscopy reveals that the average size of the structural elements does not change depending on the sample number and equals 40±7 μm. This fact is explained by the deformation at high temperatures and, as a consequence, the processes of dynamic recrystallization. It should also be noted that there is a large amount of the Ti 2 Ni phase in the structure that was formed during the melting process and does not dissolve even when heated to high temperatures (up to 1000 °C). This may be explained by the technological scheme of melting process that leads to the increased oxygen content and possible liquation in the ingot directly after melting. 3.3. Mechanical and functional properties of obtained NiTi tubes The mechanical properties of NiTi seamless tubes depending on the screw rolling pass number defined by mechanical tensile tests and hardness measurement are shown Table 2.

Table 2. Mechanical properties of NiTi SMA seamless tubes depending on the screw rolling pass number.

δ (%)

HV

σ cr (MPa)

σ y (MPa)

σ в (MPa)

D σ (MPa)

Treatment

1 2 3 4

590 615 668 580

755 815 995 850

165 200 327 270

1012 1095 1178 1086

24 23 32 22

355±14 330±8 391±12 310±13

Some differences in hardness values are observed depending on the sample number. The maximum hardness value of 391 HV units was obtained for sample 3, and the minimum for sample 4. This may be due to different cooling rates after rolling and the precipitation or dissolution of Ti 3 Ni 4 phase particles [7, 16]. Based on the tensile test results yield strength and tensile strength values increase insignificantly compared to RT (tensile strength 1040 MPa, yield strength 801 MPa), except for sample 3, which is characterized by a noticeable increase in strength characteristics (tensile strength 1178 MPa, yield strength 995 MPa). Just as in the case of hardness, this may be due to different cooling rates after rolling and the precipitation of Ti 3 Ni 4 phase particles. It should also be noted that the phase yield strength increases significantly after rolling for all the samples under consideration, which leads to a decrease in the D σ parameter and may negatively affect the magnitude of recoverable strain. The characteristic temperatures of martensitic transformations in NiTi SMA seamless tubes after screw rolling are presented in Table 3. The calorimetric curves of the studied alloy after various stages of screw rolling are characterized by the superposition of peaks of forward B2 B2 → R and R → B19' B19' transformation and reverse → R and R → indistinguishable. -phase transformation are temperatures devoted to the R transformation. Therefore,

Table 3. Characteristic temperatures of martensitic transformations NiTi seamless tubes after screw rolling.

A p (°C)

A f (°C)

M s (°C)

M p (°C)

M f (°C)

A s (°C)

Treatment

RT

40 40

0 0

-30 -40 -50 -63 -48

-18 -18 -51 -53 -64

11

35 35 30 20 28

1 2 3 4

8

9

-28 -56 -32

-27 -36 -30

-13

5

Screw rolling leads to a decrease in the intervals of forward and reverse transformations compared to RT. With respect to forward transformation, all three characteristic temperatures of transformations shift, however, for samples 1 and 3, the temperature of the starting of forward transformation remains above 0. With respect to the reverse transformation, the temperatures of the starting and peak shift noticeably toward negative temperatures, while the temperature of the end of transformation remains in the range of 20-35 °C.