Features of radiation damage of Ni-Ti alloy under exposure to heavy ions of gaseous elements

The consistent patterns of changes in structural and phase state, hardening and temperature ranges of martensitic transformations in Ni-Ti alloy with the shape memory effect after implantation of heavy ions 16O3+, 40Ar8+ and 84Kr15+ under comparable parameters have been experimentally studied. It is found that under the impact of 84Kr15+ ions, a two-layer surface structure with radiation-hardened second layer is formed, radiation-stimulated phase transformation B19'→B2 occurs in the near-surface layer and out-range area, and the martensitic transformation temperature increases toward higher values after implantation of 40Ar8+ and 84Kr15+ ions.


Introduction
Modification of materials by heavy ion implantation has become a promising technology. It is important to know which heavy ions are most effective in terms of creating radiation-resistant titanium nickelide alloys to be applied in nuclear power engineering or coatings for medical supplies.
Previously it was found [1][2][3] that modification by heavy krypton ions of low and high energy in the two-phase Ni-Ti alloy with the shape memory effect does not cause amorphization of Ti-Ni phases. Radiation-stimulated phase transformation B19'→B2 occurs, which is the major cause of deterioration of its physical-mechanical and functional properties, and temperatures ranges of martensitic transformation extend. In the case of implantation of high-energy krypton ions, a globular structure is formed on the Ni-Ti alloy surface and the degree of its homogeneity decreases as the fluence increases, radiation hardening is found to occur in the out-range area at low (~ 10 13 ion/m 2 ) fluences due to the reduced size of the structural fragments of Ni-Ti phases.
In [4,5], it is also shown that consistent high-energy krypton ion implantation and application of high-current electron beam increases the quality of the Ni-Ti alloy surface. In the out-range area, it causes formation of primary martensite phase B19' responsible for the shape memory effect, formation of nano-sized particles of NiTi R-phase, increase in the temperature interval of the martensitic transformation and further softening. In [6,7], it is found that the distinguishing feature of a purely thermal effect on the structure of Ni-Ti alloy formed by ion implantation is hardening caused by the ordering of the radiation defect structures (phases).
The paper presents the experimental results to show the effect of implantation of different heavy ions of gaseous elements on the structural and phase state, and physical and mechanical properties of Ni-Ti alloy with the shape memory effect.

Experimental methods and material
We studied Ni-Ti alloy with Ni of 53.46 wt.% and Ti of 46.54 wt.%, preferably consisting of NiTi with the B2 structure (austenite), NiTi with B19 structure (martensite) and a minor content of Ti, excess Ni in the form of solid solution and process particles similar in composition to Ti 2 Ni(C) [8]. Before implantation, a proven technology was used to prepare the surface of the samples: cutting out by spark cutting across the massive forged plate of semiindustrial Ni-Ti alloy, mechanical polishing with skins of different grits and buffing with GOI paste. The sample size was 15.000×~3900×3500 μm 3 .
The data of preliminary testing were used to choose Ni-Ti alloy with the titanium nikelid phase ratio B19'/B2 equal to ~ 0.7 which showed the highest values of martensitic transformation characteristic temperatures compared to those for higher values of the phase ratio. Testing of the samples before and after implantation was performed by X-ray diffraction analysis, scanning electron microscopy, measuring microhardness and the shape memory effect with D8 ADVANCE diffractometer, JSM-7500F (JEOL) microscope, microhardness tester PMT-3M and an apparatus for measuring the temperature hysteresis of the electrical resistance.

Experimental results and discussion
SEM studies at low magnifications revealed that in the case of 16 O 3+ ion implantation, traces of sputtering, i.e. tuberosity, pits, remnants of Ti 2 Ni(C) process particles are observed on the Ni-Ti alloy surface. As the ion mass increases 2.5 times as much (ions 40 Ar 8+ ), the surface becomes smoother with little roughness. Part of the particles protrudes over the surface which indicates that ion etching occurs alongside with sputtering. In case of heavier 84 Kr 15+ ions, etching completely dominates.
Higher magnification of SEM-images indicates formation of bright particles of a round shape, bubbles of about 1 nm, and tracks of 8 to 25 nm or conglomerates made up of several tracks with a dome, and circular bubble chains after implantation of 16   studied the samples of the two-phase Ni-Ti alloy with higher (~ 0.7) ratio of these phases than that of the samples studied previously.
In figure 2 the data of X-ray diffraction analysis obtained for a divergent beam mode shows that the phase composition nearby the out-range area (the depth of the analysis is comparable with the calculated ion range), as well as the structure of the implanted surface depend on the ion mass. In the case of 16 O 3+ ion implantation (figure 2, curve 2), partial radiation-stimulated phase transformation B19'→B2 can be observed. The martensite content ~3.5 times decreases compared to unimplanted Ni-Ti alloy (figure 2, curve 1). However, 40 Ar 8+ ion implantation (figure 2, curve 3) and 84 Kr 15+ ion implantation (figure 2, curve 4) are characterized by complete radiation-stimulated phase transformation B19'→B2 followed by dissolving Ti phase and Ni 3 Ti 3 O compound. As a result of this implantation, Ni-Ti alloy becomes single phase with the B2 structure (austenite).

Figure 2.
Parts of diffraction patterns for Ni-Ti alloy before (curve 1) and after 16 O 3+ ion implantation (curve 2), 40 Ar 8+ ion implantation (curve 3) and 84 Kr 15+ ion implantation (curve 4). Note that as the ion mass increases, the intensity of the primary X-ray line (110) of the B2 structure grows. The parameter of the body-centered cubic structure compared to unimplanted Ni-Ti alloy reduces, and this indicates approaching to the stoichiometric composition of titanium nickelide.
Similar patterns of change in the phase composition of the surface layer after 16 O 3+ , 40 Ar 8+ and 84 Kr 15+ ion implantation were obtained by shooting in grazing (2°) beam (analysis depth ~ 1 µm). In addition, firstly, in contrast to the out-range area, we detected slight splitting of the X-ray reflection line (110) of the B2 structure. This, as reported in [5,9,10], confirms the formation of nanoparticles of the R-phase during 16 O 3+ ion implantation. Secondly, the content of Ni 3 Ti 3 O compound does not virtually change in the out-range area and considerably reduces in the surface layer. This implies that the strain-hardened layer, formed on the Ni-Ti surface due to the technology used for its preparation, is partially sputtered during 16 O 3+ ion implantation, its base being the Ni 3 Ti 3 O compound.
Earlier, the data obtained in SEM-studies of the Ni-Ti alloy surface etched prior to implantation [5] were used to attribute the surface layer hardening to titanium oxide formation during sample preparation. To make it more exact we conducted additional research in the geometry of the grazing beam at an angle of 1° to the etched surface of Ni-Ti alloy by X-ray structural analysis. The surface was found to get enriched with titanium due to release of nickel during chemical etching. This confirms the data obtained in X-ray analysis, especially as the technology of sample preparation was based on mechanical methods only.
Data on microhardness of Ni-Ti alloy measured after 16 O 3+ , 40 Ar 8+ and 84 Kr 15+ ion implantation indicates that, in contrast to the unimplanted alloy, distinct indentation appears even under very low (0.098 N) load. This is another evidence for the sputtering of the strain-hardened layer in interaction of heavy ions with the polished surface.
The data obtained in microhardness measurement indicates that the double-layer structure of the near-surface layer is formed with different degrees of hardening regardless of the ion mass. Radiation  figure 3. In comparison to the corresponding R (T)-curves, measured before implantation, the martensitic transformation (MT) temperature range shifts and widens, the shape and area of the electrical resistance hysteresis loop changes. All the loops are straight, and the degree of variation depends on the ion mass, as in the case of the structure. It was found that under the impact of 16 O 3+ ions the temperature range shifts towards lower MT temperatures and 3.6 times increases mainly due to the greater degree of decrease in the temperature of М end (figure 3 a). Martensitic transformation is preceded by pre-martensitic phase transformation B2→R, which indicates positive values of dρ/dT coefficients [11]. Changing of the shape and area of the electrical resistance hysteresis loop is related to the pre-martensitic transformation and extending temperature range reverse to MT due to a significant increase in temperature when it ends A end .
As the mass of inert gases increases, the MT temperature range shifts, on the contrary, towards higher temperatures, and its widening is due to the greater degree of the increase in the temperature when MT starts M st . In the case of 40 Ar 8+ ion implantation (figure 3 b), firstly, direct MT is of distinctly stepped character compared to 16 O 3+ ion implantation (figure 3 a) and 84 Kr 15+ ion implantation (figure 3 c).
Secondly, in the R(T)-curve of the inverse martensite transformation we can find the electric resistance scale maximum exceeding by 0.033 μОhm•m compared to the temperature of the start M st of the direct MP, and then a sharp decline as the temperature keeps increasing. A similar effect has been previously observed after heat treatment of nickel-titanium alloys doped with the third component [12].
Note that for both ions of inert gases the pre-martensitic transformation area is more extended and more pronounced. It contains the region where electrical resistance increases as the temperature falls, and the "plateau" with ρ = const in the R-curve of direct MT (figure 3 b and c). According to [12], two processes are responsible for this "plateau". The first one is related to crystal lattice distortions in transition of the R-phase, which increases electrical resistance, and the second one caused by the decrease in the amplitude of thermal vibrations (phonon component of the electric resistance) contributes to electrical resistance reduction. The process of phase transformation B2→R in heavy ion implantation to a relatively low fluence determined by measuring the temperature dependence of the resistivity is consistent with its greater sensitivity as compared to the results of X-ray analysis.

Conclusion
In the experimental study, we found the features of the impact of implantation of Ni-Ti alloy with 16 O 3+ , 40 Ar 8+ and 84 Kr 15+ heavy ions on its structure, phase composition, and physical and mechanical properties under relatively similar parameters A/Z, E ion /a.e.m., Ф and J beam . The direct relationship between increase in the ion mass and radiation damage of Ni-Ti alloy with the shape memory effect is not found, but it is manifested in the degree of the processes, namely, sputtering, track formation, phase transformations and changes in the temperature ranges of martensitic transformations.
The preliminary testing of Ni-Ti alloy revealed the following features of radiation damage when exposed to 16 O 3+ , 40 Ar 8+ and 84 Kr 15+ ions: -the process of sputtering prevails in the case of lighter 16 O 3+ ions, and the process of ion etching dominates under the effect of 84 Kr 15+ heavy ions. The strain-hardened layer based on Ni 3 Ti 3 O compound is dispersed regardless of the ion mass; -for ions with M≤40, the formation of ~ 1 nm-sized bubbles can be observed, whereas, in the case of 84 Kr 15+ ions, we observe tracks with a dome surrounded by bubble chains; -in implantation, partial ( 16 O 3+ ) and complete (M≥40) radiation-induced phase transformation B19'→B2 occurs both in the near-surface layer and nearby the out-range area. The phase composition of titanium nikelid consists mainly of the B2 structure (austenite); -regardless of the ion mass, the near-surface layer is a double layer structure. As the mass of the ion increases, the degree of the second layer hardening decreases from 71 to 39%. In the out-range area, on the contrary, it grows from 7 to 16%; -the martensitic transformation temperature range with pre-martensitic phase transformation B2→R is found to shift towards lower and higher temperatures for 16 O 3+ ions and M≥40, respectively.
The results obtained suggest new possibilities for modification by different heavy ions of gaseous elements to make titanium nickelid alloys for medical use which are radiation-resistant to ionizing radiation and retain the ability of the shape memory effect.