PSI - Issue 69

Carlo Alberto Biffi et al. / Procedia Structural Integrity 69 (2025) 61 – 68

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the alloys appears to be more dependent by the chemical composition, as shown in Figure 5. In fact, NiTa shows a thermal hysteresis of about 40-70°C, minimized after TT2, as occurred also for NiTaB-A2. Different behaviour is associated to the NiTaB-A1, which shows higher thermal hysteresis in the range 100-140°C.

Fig. 4. Thermal cycling of NiTaB-A1 after TT2 till the 20th cycle.

Table 1. Characteristic temperature and heats exchanged during the MT of the alloys.

Alloys

Conditions

Ms [°C]

Mf [°C]

HA è M [J/g]

As [°C]

Af [°C]

HM è A [J/g]

as cast

258 262 273 274 255 259 268 257 264

240 243 238 255 234 243 218 235 253

1.5 1.3 1.1 0.4 1.6 1.6 0.9 1.2 1.2

311 303 288 317 326 349 299 305 292

320 319 320 340 393 366 310 321 332

2.0 1.9 1.3 0.8 2.2 1.8 1.2 1.6 1.6

NiTa

TT1 TT2

as cast

NiTaB-A1

TT1 TT2

as cast

NiTaB-A2

TT1 TT2

3.3. Mechanical behavior Figure 6a shows the deformation in compression at room temperature, prior to the shape recovery. The three alloys show a non linear strain hardening at the beginning of the test. It can be seen that NiTa binary alloy presents lower stress under compression (about 150 MPa) by 1,5% of strain while higher stress is reached for the B addition, above 300 MPa. The strain recovery is improved with the B addition in both the ternary alloys with respect to the binary one. In fact, the recovered strain has been increased from 0.5% of NiTa to 1.0% of NiTaB-A2. The B addition increased the strength of the alloys at room temperature (below Mf) but not the ductility, even if the test was concluded by 1.5 % in strain. The behavior under compression of the NiTa alloy is more brittle, as confirmed by the SEM pictures of Figure 6b, in which the presence of some fractures were observed on the sample surface after the compression test. No fractures were seen on the ternary alloys (see Figure 6c); the explanation of this fact could be due to the effect of the B addition, probably as grain refiner. Table 2 summarizes the characteristic stress and strain values of the alloys, measured from the mechanical test of Figure 6.