V. Sampath

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

Elementary Transformation and Deformation Processes and the Cyclic Stability of NiTi and NiTiCu Shape Memory Spring Actuators

Abstract: The present work addresses functional fatigue of binary NiTi and ternary NiTiCu (with 5, 75, and 10 at pct Cu) shape memory (SM) spring actuators We study how the alloy composition and processing affect the actuator stability during thermomechanical cycling Spring lengths and temperatures were monitored and it was found that functional fatigue results in an accumulation of irreversible strain (in austenite and martensite) and in increasing martensite start temperatures We present phenomenological equations that quantify both phenomena We show that cyclic actuator stability can be improved by using precycling, subjecting the material to cold work, and adding copper Adding copper is more attractive than cold work, because it improves cyclic stability without sacrificing the exploitable actuator stroke Copper reduces the width of the thermal hysteresis and improves geometrical and thermal actuator stability, because it results in a better crystallographic compatibility between the parent and the product phase There is a good correlation between the width of the thermal hysteresis and the intensity of irrecoverable deformation associated with thermomechanical cycling We interpret this finding on the basis of a scenario in which dislocations are created during the phase transformations that remain in the microstructure during subsequent cycling These dislocations facilitate the formation of martensite (increasing martensite start (M S ) temperatures) and account for the accumulation of irreversible strain in martensite and austenite

Influence of aluminum and manganese concentration on the shape memory characteristics of Cu–Al–Mn shape memory alloys

Abstract: Four sets of Cu–Al–Mn shape memory alloys, in the range of 10–15 wt.% of aluminum and 0–10 wt.% of manganese, were prepared by ingot metallurgy. The Al content was kept constant, whereas the Mn content was varied in two sets. The Mn content was kept constant in the other two sets, whereas the Al content was varied. The transformation temperatures, shape memory and superelastic behavior of these SMAs are highly sensitive to variations in aluminum and manganese content. The influence of variation in aluminum and manganese on these properties has been studied by differential scanning calorimetry, bend and tensile tests. With an increase in the aluminum and manganese concentrations of the alloy, the martensite morphology is modified and the transformation temperatures decrease. A linear regression analysis correlates the changes in aluminum and manganese concentration to the variations in the transformation temperatures. There is no appreciable variation in the shape memory effect with variation in the concentrations of aluminum and manganese. On the other hand, an increase in the aluminum content decreases the superelasticity of the alloys, while at the same time an increase in the manganese content increases it.

Effect of composition and ageing on damping characteristics of Cu–Al–Mn shape memory alloys

Abstract: Cu–Al–Mn shape memory alloys in the range of 10–15 wt.% of aluminum and 0–10 wt.% of manganese, exhibiting β-phase at high temperatures and manifesting shape memory effect upon quenching to lower temperatures, were prepared through ingot metallurgy. The damping behavior of the alloys was studied using a dynamic mechanical analyzer. The damping capacity of the alloys increases with an increase in the aluminum content when the Cu/Mn ratio or amount of manganese is maintained constant. The damping capacity of the alloys decreases with an increase in the manganese content when the Cu/Al ratio or the amount of aluminum is maintained constant. The alloys exhibit an internal friction peak in the transition zone and that all of them exhibit higher damping capacity in the martensitic condition compared with that in the austenitic condition. While ageing of the alloys at 300 °C increases the transformation temperatures of the alloys, there is a reduction in the damping capacity of the alloys due to the formation of precipitates. The alloys aged at 500 °C do not exhibit the austenite to martensite transformation due to the formation of precipitates rich in aluminum and therefore exhibit very poor damping capacity.

Influence of quaternary alloying additions on transformation temperatures and shape memory properties of Cu–Al–Mn shape memory alloy

Abstract: The transformation temperatures, shape memory effect and superelasticity of Cu–Al–Mn shape memory alloys are highly sensitive to variations in composition. A ternary shape memory alloy Cu–12.5 wt.% Al–5 wt.% Mn was chosen for the present study and different amounts (1, 2 and 3 wt.%) of quaternary elements Zn, Si, Fe, Pb, Ni, Mg, Cr and Ti were added to the ternary (base) alloy. The influence of quaternary additions on shape memory characteristics of these alloys was studied by differential scanning calorimetry, bend and tensile test. Zn and Ni as quaternary additions were found to increase the transformation temperatures, whereas Fe, Cr, Ti, Si and Mg decrease them. These additions were found to increase the extent of strain recovery by shape memory effect, whereas decrease the superelasticity of the alloys.

Processing and property assessment of NiTi and NiTiCu shape memory actuator springs

Abstract: Among the multifarious engineering applications of NiTi shape memory alloys (SMAs), their use in actuator applications stands out. In actuator applications, where the one-way effect (1WE) of NiTi SMAs is exploited, SM components are often applied as helical coil springs. Ingots are generally used as starting materials for the production of springs. But before SM actuator springs can be manufactured, the processing of appropriate wires from NiTi ingots poses a challenge because cold and hot working of NiTi SMAs strongly affect microstructure, and it is well known that the functional properties of NiTi SMAs are strongly dependent on their microstructure. The objective of the present paper is therefore to produce binary Ni50Ti50 and ternary Ni40Ti50Cu10 SMA actuator springs, starting from ingots produced by vacuum induction melting. From these ingots springs are produced using swaging, rolling, wire drawing and a shape-constraining procedure in combination with appropriate heat treatments. The evolution of microstructure during processing is characterized and the mechanical properties of the wires prior to spring-making are documented. The mechanical and functional characteristics of the wires are investigated in the stress-strain-temperature space. Finally, functional fatigue testing of actuator springs is briefly described and preliminary results for NiTi and NiTiCu actuator springs are reported. Untersuchungen zur Herstellung und zu den Eigenschaften von NiTi- und NiTiCu-Aktorfedern NiTi-Formgedachtnislegierungen (FGL) zeichnen sich durch eine hohe Attraktivitat fur verschiedene Aktorik-Anwendungen aus. Dabei werden FGL haufig in Form von zylindrischen Federn verwendet, wobei der Einwegeffekt genutzt wird. Die Herstellung von solchen Aktor-Federn ist jedoch keinesfalls trivial. Sowohl die Herstellung von geeignetem Drahtmaterial als auch die Formgebungsbehandlung stellen in gewisser Weise eine Herausforderung dar. Die funktionellen Eigenschaften von NiTi-FGL hangen sehr stark von mikrostrukturellen Randbedingungen ab, und erforderliche thermomechanische Behandlungen sind mit weit reichenden mikrostrukturellen Veranderungen verbunden. Das Ziel dieser Arbeit war, eine Prozesskette zur Herstellung von Ni50Ti50 und Ni40Ti50Cu10-Aktorfedern ausgehend vom Gussmaterial zu erarbeiten und metallkundlich zu charakterisieren. Aus den Gussblocken wurden durch Rundkneten, Walzen, Drahtziehen und speziellen Formgebungsbehandlungen Aktorfedern hergestellt. Dabei wurden die mikrostrukturellen und funktionellen Anderungen der beiden FGL charakterisiert. Zum Schluss wurden die mechanischen und funktionellen Eigenschaften der verschiedenen Draht-Materialien und der fertigen Federn untersucht. Dabei wurde ein besonderes Augenmerk auf die funktionelle Ermudung der Aktorfedern gelegt.

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 .

On the effect of alloy composition on martensite start temperatures and latent heats in Ni–Ti-based shape memory alloys

Abstract: In the present work we explain the concentration dependence of the martensite start temperature ( M S ) in Ni–Ti-based shape memory alloys (SMAs). We briefly review the present level of understanding and show that there is a need for further work. We then investigate the strong dependence of M S on alloy composition in binary Ni–Ti, ternary Ni–Ti–X (X = Cr, Cu, Hf, Pd, V, Zr) and quaternary Ni–Ti–Cu–Y (Y = Co, Pd) SMAs. For binary Ni–Ti, we combine differential scanning calorimetry experiments with insight gained through the application of the density functional theory (DFT) to show that heats of transformation Δ H decrease as Ni concentrations increase from 50.0 to 51.2 at.%. This causes a shift in the Gibbs free energy curves of austenite G A ( T ) and martensite G M ( T ), which in turn results in a lower M S temperature. Our DFT results suggest that the strong decrease of Δ H is caused by a stabilization of the B2 phase by structural relaxations around Ni antisite atoms, together with a gradual destabilization of B19′. The martensite start temperatures and the latent heats of transformation for binary, ternary and quaternary Ni–Ti-based SMAs are closely related. We observe smaller latent heats when the geometrical differences between the crystal structures of austenite and martensite decrease.

Optimization of pulsed TIG welding process parameters on mechanical properties of AA 5456 Aluminum alloy weldments

Abstract: The present work pertains to the improvement of mechanical properties of AA 5456 Aluminum alloy welds through pulsed tungsten inert gas (TIG) welding process. Taguchi method was employed to optimize the pulsed TIG welding process parameters of AA 5456 Aluminum alloy welds for increasing the mechanical properties. Regression models were developed. Analysis of variance was employed to check the adequacy of the developed models. The effect of planishing on mechanical properties was also studied and observed that there was improvement in mechanical properties. Microstructures of all the welds were studied and correlated with the mechanical properties.

Recent advances in ageing of 7xxx series aluminum alloys: A physical metallurgy perspective

Abstract: Al-Zn-Mg-Cu alloys (7xxx series Al alloys) are extensively used for their superior mechanical and corrosion performance. These properties are microstructure-sensitive and highly dependent on the formation, growth and coarsening of precipitates. To date, a wide variety of ageing procedures have been developed to tailor the evolved microstructures so as to yield a good combination of mechanical capacity and corrosion resistance of 7xxx series Al alloys. Among these methods, isothermal ageing, multi-stage ageing, non-isothermal ageing, retrogression and re-ageing (RRA), and stress ageing (i.e. creep ageing) are the most significant. In the present review, all of these approaches are comprehensively introduced and their potential effects on the microstructure and properties of Al-Zn-Mg-Cu alloys are fully reviewed. Also, recent advances and future prospect in this field are addressed.