Sensing Shape Recovery Using Conductivity Noise in Thin Films of NiTi Shape Memory Alloys

Low frequency fluctuations in the electrical resistivity, or noise, have been used as a sensitive tool to probe into the temperature driven martensite transition in dc magnetron sputtered thin films of nickel titanium shape-memory alloys. Even in the equilibrium or static case, the noise magnitude was more than nine orders of magnitude larger than conventional metallic thin films and had a characteristic dependence on temperature. We observe that the noise while the temperature is being ramped is far larger as compared to the equilibrium noise indicating the sensitivity of electrical resistivity to the nucleation and propagation of domains during the shape recovery. Further, the higher order statistics suggests the existence of long range correlations during the transition. This new characterization is based on the kinetics of disorder in the system and separate from existing techniques and can be integrated to many device applications of shape memory alloys for in-situ shape recovery sensing.

Direct physical evidence for the back-transformation of stress-induced martensite in the vicinity of cracks in pseudoelastic NiTi shape memory alloys

Crack loading and crack extension in pseudoelastic binary NiTi shape memory alloy (SMA) miniature compact tension (CT) specimens with 50.7 at.% Ni (austenitic, pseudoelastic) was investigated using infrared (IR) thermography during in situ loading and unloading. IR thermographic measurements allow for the observation of heat effects associated with the stress-induced transformation of martensite from B2 to BIT during loading and the reverse transformation during unloading. The results are compared with optical images and discussed in terms of the crack growth mechanisms in pseudoelastic NiTi SMAs. Direct experimental evidence is presented which shows that crack growth occurs into a stress-induced martensitic microstructure, which immediately retransforms to austenite in the wake of the crack.

The effect of ageing on microstructure and nanoindentation behaviour of dc magnetron sputter deposited nickel rich NiTi films

Nickel rich NiTi films were sputter deposited on p-doped Si left angle bracket1 0 0right-pointing angle bracket substrates maintained at 300 °C. The films were subsequently solution treated at 700 °C for 30 min followed by ageing at 400 and 500 °C for 5 h. The microstructure of the films was examined by TEM and these studies revealed that the NiTi films were mostly amorphous in the as-deposited condition. The subsequent solution treatment and ageing resulted in crystallization of the films with the film aged at 400 °C exhibiting nanocrystalline grains and three phases viz. B2 (austenite), R and Ni3Ti2 whereas the film aged at 500 °C shows micron sized grains and two phases viz. R and Ni3Ti2. Nanoindentation studies revealed that the nature of the load versus indentation depth response for the films aged at 400 and 500 °C was different. For the same load, the indenter penetrated to a much greater depth for the film aged at 400 °C as compared to the film aged at 500 °C. Also the ratio of the residual indentation depth (hf) to maximum indentation depth (hmax) is lower for the film aged at 400 °C as compared to the film aged at 500 °C. This was attributed to the occurrence of stress induced martensitic transformation of the B2 phase present in the film aged at 400 °C during indentation loading which results in a transformation strain in addition to the normal elastic and plastic strains and its subsequent recovery on unloading. The hardness and elastic modulus measured using the Oliver and Pharr analysis was also found to be lower for the film aged at 400 °C as compared to the film aged at 500 °C which was also primarily attributed to the same effect.

Effect of Substrate and Annealing Temperatures on Mechanical Properties of Ti-rich NiTi Films

The effect of substrate and annealing temperatures on mechanical properties of Ti-rich NiTi films deposited on Si (100) substrates by DC magnetron sputtering was studied by nanoindentation. NiTi films were deposited at two substrate temperatures viz. 300 and 400 degrees C. NiTi films deposited at 300 degrees C were annealed for 4 h at four different temperatures, i.e. 300, 400, 500 and 600 degrees C whereas films deposited at 400 degrees C were annealed for 4 h at three different temperatures, i.e. 400, 500 and 600 degrees C. The elastic modulus and hardness of the films were found to be the same in the as-deposited as well as annealed conditions for both substrate temperatures. For a given substrate temperature, the hardness and elastic modulus were found to remain unchanged as long as the films were amorphous. However, both elastic modulus and hardness showed an increase with increasing annealing temperature as the films become crystalline. The results were explained on the basis of the change in microstructure of the film with change in annealing temperature.

Influence of Substrate Temperature and Deposition Rate on Structural and Mechanical Properties of Shape Memory NiTi Films

NiTi thin films deposited by DC magnetron sputtering of an alloy (Ni/Ti:45/55) target at different deposition rates and substrate temperatures were analyzed for their structure and mechanical properties. The crystalline structure, phase-transformation and mechanical response were characterized by X-ray diffraction (XRD), Differential Scanning Calorimetry (DSC) and Nano-indentation techniques, respectively. The films were deposited on silicon substrates maintained at temperatures in the range 300 to 500 degrees C and post-annealed at 600 degrees C for four hours to ensure film crystallinity. Films deposited at 300 degrees C and annealed for 600 degrees C have exhibited crystalline behavior with Austenite phase as the prominent phase. Deposition onto substrates held at higher deposition temperatures (400 and 500 degrees C) resulted in the co-existence of Austenite phase along with Martensite phase. The increase in deposition rates corresponding to increase in cathode current from 250 to 350 mA has also resulted in the appearance of Martensite phase as well as improvement in crystallinity. XRD analysis revealed that the crystalline film structure is strongly influenced by process parameters such as substrate temperature and deposition rate. DSC results indicate that the film deposited at 300 degrees C had its crystallization temperature at 445 degrees C in the first thermal cycle, which is further confirmed by stress temperature response. In the second thermal cycle the Austenite and Martensite transitions were observed at 75 and 60 degrees C respectively. However, the films deposited at 500 degrees C had the Austenite and Martensite transitions at 73 and 58 degrees C, respectively. Elastic modulus and hardness values increased from 93 to 145 GPa and 7.2 to 12.6 GPa, respectively, with increase in deposition rates. These results are explained on the basis of change in film composition and crystallization. (C) 2010 Published by Elsevier Ltd

Nanoindentation studies in NiTi films deposited on Si wafer

Nanoindentation tests were carried out at different locations in a Ti rich NiTi film deposited on a 3 `' silicon wafer by dc magnetron sputtering. The purpose of doing nanoindentation at different locations was to check the uniformity of the sample with respect to its mechanical behaviour and shape memory effect. The results showed that elastic modulus and hardness measured by nanoindentation was similar at different locations in the 3 `' wafer. Nanoindcntation coupled with depth profiling of residual indents using AFM also showed that the extent of shape memory recovery obtained by heating the film above its martensite to austcnite phase transformation temperature was also similar at different locations in the 3 `' wafer. However, the measured recovery ratio was lower than that predicted from theoretical calculations for indents made using Berkovich indenter. The results showed that the deposition process resulted in a NiTi film with uniform composition, mechanical properties and shape memory behaviour.

Stress Evolution With Temperature in Titanium-Rich NiTi Shape Memory Alloy Films

The effect of deposition temperature on residual stress evolution with temperature in Ti-rich NiTi films deposited on silicon substrates was studied. Ti-rich NiTi films were deposited on 3? Si (100) substrates by DC magnetron sputtering at three deposition temperatures (300, 350 and 400 degrees C) with subsequent annealing in vacuum at their respective deposition temperatures for 4 h. The initial value of residual stress was found to be the highest for the film deposited and annealed at 400 degrees C and the lowest for the film deposited and annealed at 300 degrees C. All the three films were found to be amorphous in the as-deposited and annealed conditions. The nature of the stress response with temperature on heating in the first cycle (room temperature to 450 degrees C) was similar for all three films although the spike in tensile stress, which occurs at similar to 330 degrees C, was significantly higher in the film deposited and annealed at 300 degrees C. All the films were also found to undergo partial crystallisation on heating up to 450 degrees C and this resulted in decrease in the stress values around 5560 degrees C in the cooling cycle. The stress response with temperature in the second thermal cycle (room temperature to 450 degrees C and back), which is reflective of the intrinsic film behaviour, was found to be similar in all cases and the elastic modulus determined from the stress response was also more or less identical. The three deposition temperatures were also not found to have a significant effect on the transformation characteristics of these films such as transformation start and finish temperatures, recovery stress and hysteresis.

Interrelation of grain boundary microstructure and texture in a hot rolled Ni-rich NiTi alloy

This investigation deals with the evolution of grain boundary microstructure and crystallographic texture during hot rolling of a Ni-rich NiTi alloy. Electron backscattered diffraction studies revealed the occurrence of several coincidence site lattice (CSL) boundaries. The origin of these boundaries has been identified. The crystallographic texture of the deformed sample consists mainly of (1 1 1)parallel to normal direction fiber. The texture components on the fiber exhibit some correlation with the type of CSL boundary. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Effect of thermal and thermo-mechanical cycling on the microstructure of Ni-rich NiTi shape memory alloys

Microstructural changes of Ni-rich NiTi shape memory alloy during thermal and thermo-mechanical cycling have been investigated using Electron Back Scattered Diffraction. A strong dependence of the orientation of the prior austenite grain on the misorientation development has been observed during thermal cycling and thermo-mechanical cycling. This effect is more pronounced at the grain boundaries compared to grain interior. At a larger applied strain, the volume fraction of stabilized martensite phase increases with increase in the number of cycling. Deformation within the martensite leads to stabilization of martensitic phase even at temperatures slightly above the austenite finish temperature. Modulus variation with respect to temperature has been explained on the basis of martensitic transformation.

Simultaneous measurement of mechanical and electrical contact resistances during nanoindentation of NiTi shape memory alloys

The nanoindentation technique can be employed in shape memory alloys (SMAs) to discern the transformation temperatures as well as to characterize their mechanical behavior. In this paper, we use it with simultaneous measurements of the mechanical and the electrical contact resistances (ECR) at room temperature to probe two SMAs: austenite (RTA) and martensite (RTM). Two different types of indenter tips - Berkovich and spherical - are employed to examine the SMAs' indentation responses as a function of the representative strain, epsilon(R). In Berkovich indentation, because of the sharp nature of the tip, and in consequence the high levels of strain imposed, discerning the two SMAs on the basis of the indentation response alone is difficult. In the case of the spherical tip, epsilon(R) is systematically varied and its effect on the depth recovery ratio, eta(d), is examined. Results indicate that RTA has higher eta(d) than RTM, but the difference decreases with increasing epsilon(R) such that eta(d) values for both the alloys would be similar in the fully plastic regime. The experimental trends in eta(d) vs. epsilon(R) for both the alloys could be described well with a eta(d) proportional to (epsilon(R))(-1) type equation, which is developed on the basis of a phenomenological model. This fit, in turn, directs us to the maximum epsilon(R), below which plasticity underneath the indenter would not mask the differences in the two SMAs. It was demonstrated that the ECR measurements complement the mechanical measurements in demarcating the reverse transformation from martensite to austenite during unloading of RTA, wherein a marked increase in the voltage was noted. A correlation between recovery due to reverse transformation during unloading and increase in voltage (and hence the electrical resistance) was found. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Nanoscale compositional analysis of NiTi shape memory alloy films deposited by DC magnetron sputtering

The formation of surface oxide layer as well as compositional changes along the thickness for NiTi shape memory alloy thin films deposited by direct current magnetron sputtering at substrate temperature of 300 degrees C in the as-deposited condition as well as in the postannealed (at 600 degrees C) condition have been thoroughly studied by using secondary ion mass spectroscopy, x-ray photoelectron spectroscopy, and scanning transmission electron microscopy-energy dispersive x-ray spectroscopy techniques. Formation of titanium oxide (predominantly titanium dioxide) layer was observed in both as-deposited and postannealed NiTi films, although the oxide layer was much thinner (8 nm) in as-deposited condition. The depletion of Ti and enrichment of Ni below the oxide layer in postannealed films also resulted in the formation of a graded microstructure consisting of titanium oxide, Ni3Ti, and B2 NiTi. A uniform composition of B2 NiTi was obtained in the postannealed film only below a depth of 200-250 nm from the surface. Postannealed film also exhibited formation of a ternary silicide (NixTiySi) at the film-substrate interface, whereas no silicide was seen in the as-deposited film. The formation of silicide also caused a depletion of Ni in the film in a region similar to 250-300 nm just above the film substrate interface. (C) 2013 American Vacuum Society.

Effect of chemical treatment on surface characteristics of sputter deposited Ti-rich NiTi shape memory alloy thin-films

NiTi thin-films were deposited by DC magnetron sputtering from single alloy target (Ni/Ti: 45/55 aL.%). The rate of deposition and thickness of sputter deposited films were maintained to similar to 35 nm min(-1) and 4 mu m respectively. A set of sputter deposited NiTi films were selected for specific chemical treatment with the solution comprising of de-ionized water, HF and HNO3 respectively. The influence of chemical treatment on surface characteristics of NiTi films before and after chemical treatment was investigated for their structure, micro-structure and composition using different analytical techniques. Prior to chemical treatment, the composition of NiTi films using energy dispersive X-ray dispersive spectroscopy (EDS), were found to be 51.8 atomic percent of Ti and 48.2 atomic percent of Ni. The structure and morphology of these films were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD investigations, demonstrated the presence of dominant Austenite (110) phase along with Martensite phase, for untreated NiTi films whereas some additional diffraction peaks viz. (100), (101), and (200) corresponding to Rutile and Anatase phase of Titanium dioxide (TiO2) along with parent Austenite (110) phase were observed for chemically treated NiTi films. FTIR studies, it can be concluded that chemically treated films have higher tendency to form metal oxide/hydroxide than the untreated NiTi films. XPS investigations, demonstrated the presence of Ni-free surface and formation of a protective metal oxide (TiO2) layer on the surface of the films, in both the cases. The extent of the formation of surface oxide layer onto the surface of NiTi films has enhanced after chemical treatment. (C) 2014 Elsevier B.V. All rights reserved.

Microstructure dependent elastic modulus variation in NiTi shape memory alloy

Modulus variation of NiTi shape memory alloy has been investigated at microstructural level through nano dynamical mechanical analysis and compared with bulk experimental measurements. The differences between the modulus values at the macro and micro level as well as within the micro level are discussed and the corresponding variations have been explained based on the crystal structure, orientation and misorientation. The experimental results confirm a higher modulus value for the martensite phase that is in agreement with the theoretical predictions. (C) 2015 Elsevier B. V. All rights reserved.