Papers by Author: Karimbi Koosappa Mahesh

Abstract: The demand of emerging joining techniques for shape memory alloys (SMA) has become of great importance, as their functional properties, namely shape memory effect (SME) and superelasticity (SE) present unique solutions for state-of-the-art applications. Literature shows that significant efforts have been conducted on laser welding of these alloys, although very limited results concerning mechanical properties are repeatedly achieved. A better understanding of the mechanical behaviour of these welded joints may be got through a detailed analysis of the structural characteristics of the material from the base metal to the weld bead. Such studies have been carried out on a series of Ni-rich Ni-Ti SMA laser welded plates using synchrotron radiation.

Abstract: A systematic study on the phase transformation of Ni-Ti shape memory and superelastic alloys subjected to Severe Plastic Deformation (SPD) – High Pressure Torsion (HPT) technique has been carried out. Ni-Ti alloys of three compositions were chosen for the study. Specimens of these alloys in as-received (AR) condition and after HPT have been subjected to Differential Scanning Calorimetry (DSC) and X-ray Diffraction (XRD) analyses. In this study, while comparing the results of DSC thermograms and XRD spectra for the same sample conditions, some differences were observed. In the case of NiTi-H alloy after HPT, there appeared one stage phase transformation with DSC both while heating and cooling suggesting Martensite↔Austenite transformation but, with respect to XRD spectra while cooling, at the intermediate temperature of 55°C, the R-phase peaks corresponding to (1 1 2)R and (3 0 0)R planes appeared. In the thermogram obtained for the NiTi-B alloy subjected to HPT, it is observed that, while cooling, the Austenite to R-phase transformation is merged with R-phase to Martensite transformation. The results of the XRD obtained at -180°C show the presence of R-phase along with M-phase. The DSC curve of the NiTi-S alloy subjected to HPT corresponds to one stage phase transformation both while heating and cooling but, the diffractogram of the sample obtained at -180°C corresponds to the presence of both R-phase and M-phase.

Abstract: In shape memory alloys (SMA), the texture can be an interesting factor influencing the anisotropic physical and mechanical characteristics during the phase transformations. It is well known that the texture significantly influences the stress-strain curve and shape memory strain of NiTi SMA. The aim of the present experiment was to analyze the textural modifications in the Ti-rich Ni-Ti SMA after annealing at moderate (500°C for 30 min) and subsequent low level of cold work reduction (10% thickness reduction). The textural results were obtained by X-Ray Diffraction (XRD) during thermal cycling in three points: (i) at room temperature (B19’ phase, after cold work), (ii) at 180°C (B2 phase), and (iii) at room temperature (B19’ phase, after cooling from 180°C). The phase transformations were characterized by Differential Scanning Calorimetry (DSC) and XRD.

Abstract: The Shape Memory Effect on Ni-Ti thin films is strongly dependent on several factors: (i) chemical composition of the matrix, (ii) presence of precipitates and (iii) preferential orientation. Ni-Ti alloys derive their unique nonlinear and anisotropic mechanical behavior from stress-induced martensitic transformations, where the resulting strains are affected by crystallographic orientation. The influence of the texture on the transformation characteristics of Ni-Ti thin films is discussed on the basis of models and experimental results of the literature. A brief review of the texture build-up on thin films obtained by different fabrication techniques (sputtering, melt spinning, diffusion treatment of ultra-fine laminates, …) is presented. Details about in situ techniques allowing the identification of the preferential orientation during the fabrication process are presented. The processing parameters that more strongly influence the preferential orientation of the Ni-Ti thin films are identified. The mechanisms for the different microstructures are summarized and a special emphasis is put on the type of preferential orientation and its evolution along the processing time.

Abstract: Ni-Ti Shape Memory Alloy thin films are suitable materials for microelectromechanical devices. During the deposition of Ni-Ti thin films on Si substrates, there exist interfacial diffusion and chemical interactions at the interface due to the high temperature processing necessary to crystallize the film. For the present study, Ni-Ti thin films were prepared by magnetron cosputtering from Ni-Ti and Ti targets in a specially designed chamber mounted on the 6-circle goniometer of the ROssendorf BeamLine (ROBL-CRG) at ESRF, Grenoble (France). The objective of this study has been to investigate the interfacial structure resulting from depositions (at a temperature of ≈ 470°C) on different substrates: naturally oxidized Si(100), Si(111) and poly-Si substrates. A detailed High-Resolution TEM analysis of the interfacial structure has been performed. When Ni-Ti is deposited on Si(100) substrate, a considerable diffusion of Ni into the substrate takes place, resulting in the growth of semi-octaeder A-NiSi2 silicide. In the case of Ni-Ti deposited on Si(111), there appears an uniform thickness plate, due to the alignment between substrate orientation and the [111]-growth front. For Ni-Ti deposited on poly-Si, the diffusion is inhomogeneous. Preferential diffusion is found along the columnar grains of poly-Si, which are favourably aligned for Ni diffusion. These results show that for the Ni-Ti/Si system, the morphology of the diffusion interface is strongly dependent on the type of substrates.

Abstract: Ni-Ti SMA are smart materials undergoing first order martensitic transformations driven by temperature and/or stress. In the form of film they are very attractive candidates for microelectro- mechanical system (MEMS) applications. Future directions include the production of functionally graded films by changing deliberately the ratio Ti/Ni across their thickness. However, for the successful development of this type of films, it is important to characterize, model and control the variations in composition, crystalline structure and transformation temperatures. Our approach is in-situ XRD study of the actual growth of the films of varying composition along the thickness carried out using a deposition chamber installed at a synchrotron radiation beamline. These studies were complemented with ex-situ analysis techniques. The results achieved on a Ni-Ti film co-sputtered from Ni-Ti and Ti targets on a TiN buffer layer are presented in this paper. The deposition started by using optimised parameters for a near equiatomic composition. After 1 h (≈330 nm thick film), the Ti power was increased from 20 to 25 W, leading to the precipitation of Ti2Ni. The evolution of the lattice parameter values of the B2 phase, calculated from the corresponding XRD data, is clearly linked with the increase of the Ti power. The depth profile of the atomic concentrations determined by Auger Electron Spectroscopy (AES) is in agreement with the in situ XRD results. The temperature dependence of the electrical resistivity was used to monitor phase transformations, Scanning Electron Microscopy (SEM) has shown the presence of twinned martensite on the film’s surface at room temperature.

Abstract: Shape memory effect (SME) in Nickel-Titanium (Ni-Ti) alloys is ascribed to the thermoelastic reversible martensite phase transformation. Phase transformation is established to be affected by the pre- thermal and mechanical history of the alloy. The present work deals with the effect of mechanical working, known as ‘marforming’ and ‘ausforming’, on the phase transformation characteristics and mechanical behaviour of Ti-rich Ni-Ti alloy. Tensile study and measurement of the hardness data were carried out at room temperature. Mere heat treatment or heat treatment at 773 K after the marforming shows similar characteristics, whereas, the as-received and the ausformed samples exhibit different behaviours. Hardness numbers of the heat treated samples are found to be smaller than those of the as-received and mechanically worked samples.

Abstract: In the present study, interrupted mechanical cycles are performed to observe ‘micromemory effect’ for the Ni-rich (Ni51.0at%-Ti) Ni-Ti Shape Memory Alloy (SMA) with different strain rates. In addition, the tensile test is coupled with the four-probe electrical resistivity (ER) measurements. Coupled measurements of stress-strain and electrical resistivity for mechanical cycling (with SIM) have not yet been reported in the literature. The behaviour of the alloy during the interrupted cycles is discussed on the basis of the ER profile

Abstract: The aim of the present experiment was to analyse the structural evolution during annealing of Nickel-Titanium (Ni-Ti) SMA subjected to different thermomechanical treatments. As structural evolutions are accompanied by the changes in preferential orientations, pole figures were employed to study the in-situ conditions.