Ion implantation in Ni-Ti shape memory alloy thin films
The present thesis focuses on the effect of 5 MeV Ni ion irradiation on the microstructure and the thermomechanical behavior in prestrained (ε∼4%) martensitic Ti-rich NiTi thin films. At this ion energy, damage is limited to a depth of approximately 2 µm (the ion projected range predicted the Transport and Range of Ions in Matter (TRIM) code). When applied to a pre-deformed 6µm thick film, this technique may be used to excite reversible out-of-plane bending which can perform useful work as an actuator in microdevices. Conceptually, the frustration of the martensitic transformation by ion beam damage creates a differential latent strain between the beam-damaged and undamaged layer upon reverse transformation. Furthermore, the beam-damaged layer acts as an intrinsic bias spring that allows a reversible bending motion during thermal cycling. Thus, the actuator and bias spring can be realized in a single thin film element, using a single and simple planar processing. Due to the potential complexity of the thermomechanical response of the bimorph, it is obvious that detailed information about the effects of high-energy heavy-ion irradiation on microstructure and transformation characteristics are needed if the performance of these ion-biased bimorphs is to be optimized. Thus, the present thesis work has been undertaken to develop a better understanding of the influence of ion irradiation parameters on thermally-induced cyclic deflection, with a strong emphasis on the development of the ion damaged microstructure. Cross-sectional transmission electron microscopy (TEM) investigations and X-ray diffraction (XRD) studies of the irradiated microstructure showed that both NiTi and Ti2Ni precipitates were readily amorphized with 5 MeV Ni ions at doses above 5×1013 ions/cm2, especially at lower irradiation temperatures, i.e. <130 K. It was concluded from the observed microstructural evolution with dose that amorphization occurs by a damage accumulation mechanism, where the damage builds-up to quantities that are high enough to drive the transformation to the amorphous phase. From a Gibbons's modeling, it is postulated that the accumulated, overlapped damage from 3 cascades is necessary to induce an amorphous zone. The average cascade volume calculated from this analysis has a similar size as a cascade produced by 30 keV Ni ions. The energy deposited in crystal by 3 such cascades, is on the order of 0.3 eV/atom, which under a thermal spike model, is sufficient to melt NiTi and exceeds the energy needed to crystallize an irradiation-induced amorphous NiTi film (ΔHca=0.011 eV/atom). The amorphization processes can be rationalized in terms the subcascade damage, since 30 keV PKAs deposit signifigantly higher energy into crystal, than 5 MeV ions. One of the most striking results of this thesis work was the observed variation in amorphization with depth. The surface regions of the film were significantly amorphized at relatively low fluences, which was not expected since damage produced by nuclear stopping events is low and defect annihilation is high in surface regions. The enhanced amorphization at low ion penetration depths was evident in specimens irradiated to a fluence of 1×1014 ion/cm2, where the maximum amorphous fraction was observed at a depth 0.7 µm shallower than the maximum atomic displacements from nuclear events predicted by TRIM simulations. Since this shift and surface amorphization were attributed to electronic stopping effects, which are more influential at low ion penetration depths, swift ion irradiation (high energy ions that were not nuclear-stopped within the films) experiments had been conducted to evaluate the effects of electronic stopping on the damage production. It was shown in 40 MeV Ne irradiation experiments, for the first time, that significant damage was produced in metallic targets (NiTi) using extremely low electronic stopping powers (4 keV/nm). Since these stopping powers are near those of 5 MeV Ni ions (3 keV), electronic stopping should also affect the damage processes from irradiation with these ions. A model based on the enhanced cascade damage (nuclear stopping) induced by the synergetic lattice softening resulting from electronic stopping effects, was used to explain the increased amorphization in surface regions and to account for the shift the peak amorphization to shallower depths. Since the deformation of the martensitic films prior to irradiation is important prerequisite for the actuator design, the effects of deformed martensite on the irradiation damage processes were studied by TEM and XRD. Indeed, the prior martensite deformation had a considerable effect on the irradiation-induced amorphization processes, where the amorphous fraction was observed to increase with increasing amount of martensite pre-strain. It is believed that under irradiation, the martensite is transformed coherently to austenite, which should therefore result in the subsequent recovery of the prior martensite deformation. However, since the sample was constrained on the irradiation heat sink, the martensite pre-strain was not recovered, causing large stresses to develop. These recovery stresses increase the crystal's strain energy, thereby decreasing the free energy change needed for amorphization. In this sense, the stresses are thought to promote amorphization in pre-deformed martensitic samples and explain the observed trends in the experimental data. The curling motion of the irradiated films was expressed in terms of pseudo-bending strain (ε=film thickness/2*ρ) calculated from the film curvature (ρ) and was monitored as a function of temperature, and the obtained hysteric pseudo-strain- temperature curves were evaluated as function of ion fluence and thermal cycles. The films irradiated to low fluences of 1×1013 ion/cm2 bent in a direction opposite (about the irradiated face) to the expected curling (away from the irradiated face). This was related to the partial, irradiation-induced transformation of the martensite into austenite, causing a variation in both microstructure and transformation temperatures within the beam-damaged layer. In samples irradiated to high doses >1014 ions/cm2, significant bending movements were achieved. However, a loss in the film's reversible bending motion was observed during initial cycling, especially at high temperatures, that was attributed to the relaxation of the high elastic stresses in beam-damage layer due to the structural relaxations within the amorphous material. For use as actuator material, it is recommended that the NiTi thin films be irradiated above a fluence of 1×1015 ions/cm2 for two reasons, (1) the beam-damaged layer has a more homogeneous microstructure, and thus mechanical properties should be more uniform properties, (2) the bi-morph is more thermally stable and has lower cyclic fatigue.