PSI - Issue 69

Alberto Coda et al. / Procedia Structural Integrity 69 (2025) 26–34

31

800

as cast 1200°C for 1h 1200°C for 4h 1350°C for 1h 1350°C for 4h

700

600

500

400

300

Microhardness [HV]

200

100

0

1

2

3

4

5

Alloy

Figure 5 Microhardness results of the 5 investigated alloys as cast and after heat treatment.

3.3 Thermal Transformation Thermal analysis demonstrated that the alloys are very stable during cycling, as shown in Figure 6a for alloy 1. Under heating a small shift in the transformation temperatures of the direct MT peak can be observed while any variation can be seen under cooling during the reverse MT. Already after the second thermal cycle, the temperature shift is neglectable. Compared to the binary alloy, the transformation temperatures decreased after Cu and Co substitution (for Ni and Ta, respectively) as shown in Figure 6b.

a)

b)

DSC [mW]

1 th cycle 2 nd cycle 3 rd cycle

Temperature [°C]

Figure 6 DSC Results: a) First three thermal cycles of Alloy 1 in the zone of interest; b) Thermal transformation comparison of Alloy 1, 3 and 5.

An overview of the effect of alloying and heat treatments is shown in Figure 7. As can be seen, the third element addition decreased the transformation temperatures without affecting the thermal stability while the heat treatments have a very low impact. Moreover, it must be underlined that even if the shift of temperatures to lower values is relevant, especially for Cu addition, in all cases the alloys have still a thermal transformation in the HT region, above 150°C. Just to give some references, NiTiHf and NiTiZr have a thermal transformation typically in the range 80 300°C depending on Hf and Zr content and on Ni/Ti ratio. The high temperature thermal transformation and related stability could be an asset in applications, where the temperatures often could reach elevated values for extended