V. Sampath

Bio: V. Sampath is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topics: Shape-memory alloy & Microstructure. The author has an hindex of 13, co-authored 45 publications receiving 705 citations. Previous affiliations of V. Sampath include Ruhr University Bochum.

Influence of Aging Temperature on Functional Fatigue Behavior of a Ti 50 Ni 45 Cu 5 Shape Memory Alloy

Abstract: Smart actuators, using materials with a memory, are an attractive alternative to conventional actuators due to their unique properties, such as high energy density, low power-to-weight ratio, simplicity of design, and miniaturization of size. However, the continuous cyclic operation of such devices, within their transformation temperature range, leads to the degradation of their functional properties. In this paper, the degradation of functional properties, such as recovery strain, permanent strain, and critical transition temperatures, of an Ni45Ti50Cu5 (at.%) shape memory alloy, aged at four different temperatures, ranging from 450 to 600 °C, was experimentally investigated under constant stress. The results reveal that all alloys underwent a single-step transition from B2 → B19’ at all aging temperatures. The aging temperature has a significant impact on recovery strain and permanent strain. The permanent strain accumulation after every cycle is minimized as the temperature of aging is raised to 550 °C due to the strengthening of the matrix by precipitate particles. Above this temperature, it starts to increase due to the coarsening of the precipitate particles. Aging treatment also helps to achieve faster cyclic stability during thermomechanical cycling.

Modelling of Tensile Behaviour of NiTinol SMA Wire by Finite Element Analysis

Abstract: Shape Memory Alloys (SMA) are used in diverse engineering and medical applications capitalizing on their sensing and actuation capabilities. This is attributed to the high magnitude of recovery strain and recovery stress that they are capable of generating. But shape memory alloys exhibit complex behaviour in the presence of electrical and magnetic fields as well as mechanical stress. Their behaviour in these domains is to be rightly and comprehensively understood if these are to be used in these applications. Over the years finite element analysis has proved to be a powerful tool for studying the behaviour of SMAs under different conditions and has therefore been applied in many important applications. In this paper an attempt has therefore been made to understand the behaviour of an equiatomic NiTi alloy wire under mechanical stress by finite element analysis using commercially available software MSC Marc.Simulation of an SMA wire with 0.5 mm diameter and 351 mm length was carried out in uniaxial tension by finite element analysis. The simulation involves three stages: geometric modelling using the software MSC.Patran. Finite element modelling also uses the same software, and finally arriving at the solution using the software MSC.Marc.The finte element model is created by meshing the geometric model using Element type Tetmesh4. Boundary conditions are then applied using MSC Mentat software followed by arriving at the solution using Auricchio model. The stress-strain behaviour of the equiatomic NiTinol wire under simple uniaxial tensile loading at different temperatures was simulated.

Manifestations of the effects of a “weak” sublattice in an YIG film with the replacement of yttrium by Gd ions

Abstract: Ferrite-garnet films of submicron thickness were investigated by ferromagnetic resonance method and by the autodyne oscillator method. Compensation of the magnetization of such a film is detected on the curve of its temperature dependence, which is associated with the inclusion of gadolinium ions from the gadolinium-gallium garnet substrate in its composition. It was found that the magnetization of the sample in perpendicular to the surface of the film magnetic field showed an abnormal change in the magnetization and also rotation of the magnetic axis. This behaviour of a ferromagnetic material in the presence of a magnetic field can be explained by the expressions for the linear magnetostrictive effect, and the piezomagnetic effect arising from the para-processes operating within a “weak” rare-earth magnetic sublattice which is located inside a unidirectional exchange field of “strong” iron sublattice.

Observation of Transformation Strain Arrest During Partial Thermomechanical Cycling of an NiTi Shape Memory Alloy

Abstract: In the present study, an arrest in transformation strain was observed during the partial thermomechanical cycling (thermal cycling under constant stress) of an equiatomic NiTi shape memory alloy. Sheet specimens of the alloy were thermomechanically cycled between the transformation temperatures (from below Mf to below Af) under constant stress, ranging from 50 to 150 MPa. The upper temperature for cycling was sequentially arrested at different temperatures in the subsequent cycles till T>Af. After every partial cycling, the arrest in transformation strain was observed in the immediate subsequent cycle. Unlike the temperature memory effect (due to thermal arrest during partial thermal cycling), the strain arrest effect remembers the magnitude of the previous cycle strain. It requires slight overheating than the previous cycle arrest temperature in order to continue to undergo further transformation strain. As a result, there is a deviation in the local strain path from the global strain path. These results suggest that the shape memory alloy can remember a temporary intermediate shape, between the transformation temperatures, other than the global shape, which is commonly called the one-way shape memory effect. The effect of applied stress levels on the strain arrest phenomenon was also studied. A methodology to determine the strain arrest characteristics is developed and presented in detail in this paper.

Simulation of the Electric Current Flow in Amorphous-Crystalline Ti2NiCu Alloy with Shape Memory Effect

Abstract: A new technique for the production of nanograined alloys from rapidly quenched amorphous ribbons by serial electric pulses has been proposed recently [1]. The present work involves a theoretical study of electric current flow in a nonhomogeneous Ti2NiCu alloy consisting of an amorphous matrix with a crystalline phase of spherical morphology embedded in it. The electric current density distribution was calculated in the vicinity of a spherical nucleus, which has an electrical resistance that is only 0.4 times that of the amorphous matrix. The calculation of Joule heat density was done in the nucleus and in the amorphous volume surrounding it. It was shown that during the current pulse the Joule heat evolution in nucleus exceeds one in equatorial region in matrix, but less than near the poles. The dependence of relative resistivity of nonhomogeneous amorphous-crystalline alloy on volume fraction of spherical crystalline nuclei was calculated

Identification of Quaternary Shape Memory Alloys with Near‐Zero Thermal Hysteresis and Unprecedented Functional Stability

Abstract: Improving the functional stability of shape memory alloys (SMAs), which undergo a reversible martensitic transformation, is critical for their applications and remains a central research theme driving advances in shape memory technology. By using a thin-film composition-spread technique and high-throughput characterization methods, the lattice parameters of quaternary Ti-Ni-Cu-Pd SMAs and the thermal hysteresis are tailored. Novel alloys with near-zero thermal hysteresis, as predicted by the geometric nonlinear theory of martensite, are identified. The thin-film results are successfully transferred to bulk materials and near-zero thermal hysteresis is observed for the phase transformation in bulk alloys using the temperature-dependent alternating current potential drop method. A universal behavior of hysteresis versus the middle eigenvalue of the transformation stretch matrix is observed for different alloy systems. Furthermore, significantly improved functional stability, investigated by thermal cycling using differential scanning calorimetry, is found for the quaternary bulk alloy Ti 50.2 Ni 34.4 Cu 12.3 Pd 3.1 .