Slightly Ni-rich NiTiHf and NiTiZr alloys are considered serious candidates for HTSMAs, since they can exhibit good mechanical and functional properties above 200 ºC when they are reinforced with nano-precipitate H-phase. However, the microstruc-tures and functionalities of any HTSMA might be modified by long exposure of the ma-terial to high working temperatures.
In the present work, three polycrystalline materials (Ni50.3Ti29.7Hf20, Ni50.1Ti24.9Hf25 and Ni50.3Ti29.7Zr20) have been studied after three different initial thermal treatments: solu-tion heat treatment, ageing for 3h at 550ºC, and furnace cooling from 700ºC to 100ºC in 48 h. Each initial treatment produces distinct microstructures: precipitate free, high den-sity of H-phase nanoparticles of few tens of nm in size, and lower density of H-phase particles of hundreds of nm, respectively.
The thermal stability and microstructural evolution after long term ageing at tempera-tures near their martensitic transformation have been studied by DSC and TEM for each composition and initial thermal treatment. The results demonstrate that the presence of H-phase precipitates enhances the thermal stability in all three compositions, especially for the furnace cooling treatment. Furthermore, NiTiHf alloys show better stability than NiTiZr under austenite aging, martensite aging, and thermal cycling, with the Ni50.3Ti29.7Hf20 composition exhibiting the most stable behavior upon aging in austenite.
Compression superelastic tests show that the best dimensional stability and superelastic behaviour is obtained after ageing for 3 hours at 550ºC for all three compositions, alt-hough the superelastic window is only 30-40ºC. Furnace cooled alloys do not show su-perelastic behavior but do exhibit high toughness, as they can withstand stresses up to 2 GPa and reach strain levels above 12% without macroscopic crack generation. Finally, the solution heat treated NiTiZr alloy shows superelasticity above 35 ºC even though no martensitic transformation peaks can be observed by DSC.