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Comparative study on the high-temperature oxidation resistance of porous and solid TiNi-based alloys
(2021)
The present work aims to characterize the surface features of solid and porous (sintered and SHS) TiNi-based alloys subjected to oxidation at 1000 °C in static air in the context of their resistance to high-temperature atmospheric attack. Clear differences between the intact and oxidated surfaces indicate the complexity of a chemicothermal diffusion process evolving therein. Microscopic and XRD studies showed that the dominant superficial constituent in all oxidated samples is titanium dioxide in the rutile modification. The phase and structural properties of the surface layers suggest that porous sintered and solid alloys are most susceptible to high-temperature corrosion due to bare reactive surfaces, which negatively affects their overall biocompatibility. Surface morphology analysis revealed microporous and loose superficial layers having a thickness of 8–10 and 50–60 μm, respectively in the solid and sintered alloy. Also, these alloys showed a high content of leaching NiO and free Ni within the surface layer. Conversely, a thin (0.5–0.6 μm), dense, and multifarious layer of oxycarbonitrides Ti4Ni2(O,N,C) concealing the porous SHS-TiNi matrix inhibits the negative effect of high-temperature oxidation.
The study aimed to characterize the structural-phase composition of the porous SHS TiNi surface explored by the GIXRD method. The surface layers at a depth of up to 100 nm mainly consist of amorphous nanocrystalline intermetallic oxycarbonitrides Ti₄Ni₂(O,N,C) with nonmetallic inclusions of different structural variants and routes of origin. Fine-porous alloys were synthesized at ignition temperatures of 450-480 °C. A distinct feature of the surface of crystalline phases therein was shown to be a low degree of crystallinity (up to 40%) and presence of multifarious glass and cermet phases evident as NiSi₂, NaAlSiO (SO₄), SiO₂, MgSi₂, and CaCO₃. Conversely, large-pore alloys ignited at temperatures of 280-330 °C have a higher degree of crystallinity (up to 70%). An individually selected GIXRD technique and precision structural phase analysis are capable to determine a set of other superficial nonmetallic and cermet phases reported as CaTiO₃, Si (P2O₇), CaSiO₃, MgAl₂O₄, TiNiAl, as well as the Ti₃SiC₂MAX phase.