Search results for "Shape-memory alloy"
showing 10 items of 24 documents
Wavelets Image Analysis for Friction Stir Processed TiNi Functional Behavior Characterization
2015
Abstract A key topic regarding Ti Ni Shape Memory materials concerns the possibility to attain welded junctions that preserves the shape memory properties of material. Other research topic for SMAs regards the retention of the shape memory effect cyclic stability; in fact, good shape memory properties frequently decrease during SME cycling of material. A method able to improve the cyclic stability of TiNi shape memory effect is the grain refinement. Considering these above mentioned research topics, a solid state welding process, as the Friction Stir Welding, is thus attractive for SMA joining and it exhibits potentials for achieving welded joints affected by microstructural changes that pr…
Recent Progress in FSMA Microactuator Developments
2009
The giant magneto-strain effect is particularly attractive for actuator applications in micro- and nanometer dimensions as it enables contact-less control of large deformations, which can hardly be achieved by other actuation principles in small space. Two different approaches are being pursued to develop ferromagnetic shape memory (FSMA) microactuators based on the magnetically induced reorientation of martensite variants: (1) the fabrication of free-standing epitaxial Ni-Mn-Ga thin film actuators in a bottom-up manner by magnetron sputtering, substrate release and integration technologies and (2) the top-down approach of thickness reduction of bulk Ni-Mn-Ga single crystals to foil specime…
Temperature-induced martensite in magnetic shape memory Fe2MnGa observed by photoemission electron microscopy
2012
The magnetic domain structure in single crystals of a Heusler shape memory compound near the composition Fe2MnGa was observed during phase transition by photoelectron emission microscopy at Beamline 11.0.1.1 of the Advanced Light Source. The behavior is comparable with recent observations of an adaptive martensite phase in prototype Ni2MnGa, although the pinning in the recent work is an epitaxial interface and in this work the effective pinning plane is a boundary between martensitic variants that transform in a self-accommodating way from the single crystal austenite phase present at high temperatures. Temperature dependent observations of the twinning structure give information as to the …
Shape memory NiTi thin films deposited at low temperature
1999
Abstract NiTi shape memory alloy (SMA) thin films have the potential to become high performance actuators for micro-electromechanical systems. Low temperature crystallized NiTi films would ensure a good compatibility with microelectronic processes and polymers. To avoid the drawbacks induced by annealing, we have tried to obtain low temperature crystallized RF sputtered NiTi films by optimising deposition parameters. We have found that NiTi films containing an excess of Ti (∼52%) were crystallized when deposited on Si(100) substrates heated up to only 473 K. NiTi/Si(n) Schottky diodes I–V characteristics showed a temperature dependence indicating structural transition in the NiTi electrode.…
Gradient-enhanced model and its micromorphic regularization for simulation of Lüders-like bands in shape memory alloys
2018
Abstract Shape memory alloys, notably NiTi, often exhibit softening pseudoelastic response that results in formation and propagation of Luders-like bands upon loading, for instance, in uniaxial tension. A common approach to modelling softening and strain localization is to resort to gradient-enhanced formulations that are capable of restoring well-posedness of the boundary-value problem. This approach is also followed in the present paper by introducing a gradient-enhancement into a simple one-dimensional model of pseudoelasticity. In order to facilitate computational treatment, a micromorphic-type regularization of the gradient-enhanced model is subsequently performed. The formulation empl…
Electronic and Magnetic Properties of Li<sub>1.5</sub>Mn<sub>0.5</sub>As Alloys in the Cu<sub>2</sub>Sb Structure
2013
We investigated two formula-units of Li1.5Mn0.5As alloys, such as Li3MnAs2, in the Cu2Sb crystal structure using an ab-initio algorithm. By interchanging Mn with each Li located at different positions of the Li4As2unit cell, four separate alloys are formed. At the optimized lattice constant, two of these alloys are predicted to be ferromagnetic metals and the other two are half metals. The possibility of half metallicity in the first two is also explored. Both the modified Slater-Pauling-Kübler rule and the ionic model can characterize the magnetic moments of the half metals.
Numerical Analysis of Piezoelectric Active Repair in the Presence of Frictional Contact Conditions
2013
The increasing development of smart materials, such as piezoelectric and shape memory alloys, has opened new opportunities for improving repair techniques. Particularly, active repairs, based on the converse piezoelectric effect, can increase the life of a structure by reducing the crack opening. A deep characterization of the electromechanical behavior of delaminated composite structures, actively repaired by piezoelectric patches, can be achieved by considering the adhesive layer between the host structure and the repair and by taking into account the frictional contact between the crack surfaces. In this paper, Boundary Element (BE) analyses performed on delaminated composite structures …
Image Enhancement Algorithm for Optical Microstructural Characterization of Shape Memory TiNi Friction Stir Processed
2017
Abstract A key topic regarding TiNi alloys concerns the possibility to attain junctions that preserve the shape memory properties of material. Experimental tests, previously performed on TiNi sheet friction stir processed, have highlighted the need to develop an appropriate image analysis method to quantify the various phases percentages present in the characteristics zones of Friction Stir Welding process. A proper Image Processing procedure has been performed in order to quantify the amount of the martensitic phase and to detect its morphology modification along to the processed region. Particularly each micrographic image, firstly, has been denoised using the 2D Wavelet transform techniq…
Electronic structure of the austenitic and martensitic state of magnetocaloric Ni-Mn-In Heusler alloy films
2013
Changes of the electronic and magnetic structure near the martensitic phase transition of Ni-Mn-In Heusler alloys doped with Co are investigated by experiment and theory. The nonstoichiometric Ni${}_{48}$Co${}_{5}$Mn${}_{35}$In${}_{12}$ epitaxial film undergoes a transition from a weakly magnetic martensitic phase below ${T}_{m}=350$ K to a ferromagnetic austenitic phase above ${T}_{m}$. Element-specific magnetic moments and the unoccupied density of states function is investigated using x-ray magnetic circular dichroism. We find an antiparallel alignment of Mn and Ni/Co magnetic moments in both phases. The electronic structure is calculated using the SPR-KKR Green's function approach consi…
Strain-Controlled Giant Magnetoresistance in Spin Valves Grown on Shape Memory Alloys
2019
We report a strain-mediated giant magnetoresistance (GMR) in spin valves (SPVs) grown on shape memory alloys (SMAs). The SPVs with a stacking structure of Al2O3/Co90Fe10/Cu/Co90Fe10/IrMn/Pt were de...