Martensitic Thin Films Research Articles

Characterizations of Binary FeCr (AISI 430) Thin Films Deposited from a Single Magnetron Sputtering Under Easy Controllable Deposition Parameters

A series of 50-nm binary FeCr martensitic thin films were sputtered from a single source made of commercial AISI 430 ferritic stainless steel under the deposition rates gradually increased from 0.03 to 0.11 nm/s with 0.02 nm/s steps at stationary condition. And, under 0.09 nm/s deposition rate, a second series of the films were also deposited under the rotation speed of their substrates which was chosen at 0, 25, and 45 rpm. As far as we are concerned, this study is the first investigation of properties of the thin films produced from AISI 430. The atomic Fe content in the films increased from 79.3 to 98.6% while atomic Cr content decreased from 20.5 to 1.2% with the increase of deposition rate from 0.03 to 0.11 nm/s. According to compositional analysis, the Fe content increased while Cr content decreased with increasing deposition rate. The reason for this may be attributed to the relatively different bond energy/melting point of metals which have different contents sputtered from source material since this physical parameter is very significant for the sputtering process. And, the Fe content in the films decreased from 84.9 to 79.2 at. % while the Cr content increased from 14.9 to 20.6 at. % when the increase of rotation speed of substrate. The crystal structure of all films was observed to have a body-centered tetragonal phase and the intensity of (110) peak varied with the atomic Fe content. The surface observations of films performed by a scanning electron microscope exposed that the number of surface grains increased with the increase of deposition rate and decreased with the increase of rotation speed. According to surface roughness analysis done by an atomic force microscope, the roughness of the film surfaces increased as the deposition rate increased. And, the roughness of the film surfaces decreased as the rotation speed increased. This has been consistent with the grain size and roughness parameters. Thus, increasing deposition rate and decreasing rotation speed of the substrate caused an increase in grain size and roughness parameters. The magnetic measurements of the films achieved, by a vibrating sample magnetometer at room temperature, displayed that the saturation magnetization, Ms, values increased from 820.1 to 1700.4 emu/cm3, the remanence magnetization, Mr, values increased from 293 to 817 emu/cm3, and the coercivity, Hc, value also increased from 38 to 107 Oe with the increasing of deposition rate. It is also seen that the magnetic easy axis are in the film plane due to the shape anisotropy. With the increase rotation speed, the values of Ms and Mr increased and the Hc decreased. It was seen that variation of Fe content in the films influences the Ms values and the Hc values are consistent with the surface properties. It was concluded that the deposition rate and the rotation speed of the substrate play a considerable role on the structural and related magnetic properties of the sputtered FeCr thin films, and the properties of the films can be easily controlled by changing production parameters.

Ternary FeCrNi martensitic thin films sputtered on a flexible substrate from a single AISI 304 austenitic stainless steel source: Effect of deposition rate on structural and magnetic properties

Abstract The effect of deposition rates on the structural and magnetic properties of the ternary FeCrNi martensitic thin films produced via a source material made of the commercial AISI 304 austenitic stainless steel was investigated. The films with 50 nm thickness were deposited on a flexible amorphous polymer substrate by using dc magnetron sputtering. As increasing deposition rate, the Fe and Ni contents of the films increased while the Cr component decreased confirming the same materials in the source. And, all films examined by X-ray diffraction have a body centred tetragonal structure confirming the martensitic state, and the peak intensities changed depending on the film content which was varied by deposition rate. Also, the morphological analysis of the surfaces performed by a scanning electron microscope (SEM) displayed that the smoothest surface were obtained at high deposition rates. Further measurements by an atomic force microscopy disclosed an increase in the film surface smoothness and a decrease in the values of roughness parameters confirming the SEM images. As to magnetic analysis, the increase of total of Fe and Ni contents resulted in an increase in saturation magnetization, MS. The MS values of films, were detected to increase from 991.0 to 1283.1 emu/cm3 with decreasing coercivity from 131 to 74 Oe as the deposition rate increased from 0.04 to 0.12 nm/s. In this study, the increase of the Fe and Ni components and smoothness in surfaces may give rise to the ternary FeCrNi martensitic thin films gain the softer magnetic properties with increasing deposition rate. A probable explanation to this results may come from the hysteresis loops; the change in the deposition rate produces an increase in the MS and remanent magnetization, Mr, and decrease in the coercivities which led to almost unchanged shape of the loops since the Mr/MS ratio is between 6.3 and 6.8. The ternary FeCrNi thin films produced by a magnetron sputtering technique have a martensitic phase, which indicates the soft ferromagnetic state without any further treatment. To our knowledge, this has not been expressed in previous relevant studies. It is seen that the magnetic properties of the ternary FeCrNi martensitic thin films can be easily controlled by changing production parameters for potential electric and electronic devices on flexible substrates.

Microstructure and mechanical behavior of a shape memory Ni–Ti bi-layer thin film

Abstract Two different single-layers and a bi-layer Ni–Ti thin films with chemical compositions of Ni45Ti50Cu5, Ni50.8Ti49.2 and Ni50.8Ti49.2/Ni45Ti50Cu5 (numbers indicate at.%) determined by energy dispersive X-ray spectroscopy were deposited on Si (111) substrates using DC magnetron sputtering. The structures, surface morphology and transformation temperatures of annealed thin films at 500 °C for 15 min and 1 h were studied using grazing incidence X-ray diffraction, transmission electron microscopy (TEM), atomic force microscopy and differential scanning calorimetry (DSC), respectively. Nanoindentation was used to characterize the mechanical properties. The DSC and X-ray diffraction results indicated the austenitic structure of the Ni50.8Ti49.2 and martensitic structure of the Ni45Ti50Cu5 thin films while the bi-layer was composed of austenitic and martensitic thin films. TEM study revealed that copper encourages crystallization in the bi-layer such that crystal structure containing nano-precipitates in the Ni45Ti50Cu5 layer was detected after 15 min annealing while the Ni50.8Ti49.2 layer crystallized after 60 min at 500 °C. Furthermore, after annealing at 500 °C for 15 min, a precipitate free zone and thin layer amorphous were observed closely to the interface in the top layer. The bi-layer was completely crystallized at 500 °C for 1 h and the orientation of the Ni-rich precipitates indicated a stress gradient in the bi-layer. The bi-layer thin film showed different transformation temperatures and mechanical behavior from the single-layers. The developed bi-layer has different phase transformation temperatures, the higher temperatures of shape memory effect and lower temperature of pseudo-elastic behavior compared to the single-layers. Also, the bi-layer thin film exhibited a combined pseudo-elastic behavior and shape memory effect with a reduced hysteresis at the same time similar to the austenitic and martensitic thin films, respectively.

Functionally Graded Shape-Memory and Pseudoelastic Response in Ni-Rich/Ti-Rich and Vice Versa NiTi Multilayer Thin Films Deposited on Si(111)

In this study, functionally graded NiTi multilayer thin films were created by radio frequency magnetrun sputtering and subsequent annealing. The chemical compositions of the multilayer thin films which determined by energy-dispersive x-ray spectroscopy are about Ni51Ti/Ni49Ti (numbers indicate at.%). The structures, surface morphology, and transformation temperatures of annealed thin films at 500°C for 1 h were studied using grazing incidence x-ray diffraction, atomic force microscopy, and differential scanning calorimetry (DSC), respectively. Moreover, a coupled nanoindentation/atomic force microscopy technique is employed to characterize both the superelastic and shape memory effects. Specifically, an analysis of recoverable energy through the stress-induced phase transformation and recoverable strain through de-twinning and the subsequent thermally induced phase transformation were performed. The DSC and x-ray diffraction results indicated the multilayer was composed of austenitic and martensitic thin films. The continuous stiffness measurement technique showed that the stiffness and elastic modulus of the multilayer change gradually through the thickness of the multilayer thin films. Also, the thin films exhibited a combined pseudoelastic behavior and shape-memory effect which produces a two-way shape-memory effect in both thin films.

MFM Domain Imaging of Textured Ni-Mn-Ga/MgO(100) Thin Films

Magnetic, micromagnetic, and texture properties of the martensitic thin films prepared by magnetron sputtering of Ni49.5Mn28Ga22.5 target on MgO(100) wafer are studied as a function of film thickness. The films exhibit 220-fiber texture. Moreover, in contrast to thick films, a considerable in-plane texture component is giving rise to the pronounced in-plane magnetic anisotropy found in the thin film. The texture features together with a large magnetocrystalline anisotropy lead to the particular domain structure and different magnetic anisotropy components of films depending on the film thickness and substrate nature.

Multivariant model of martensitic microstructure in thin films

A novel multivariant model of martensite is developed through competing energetics to describe the coarsening, refinement and selection of the microstructure. In contrast with the conventional phase-field methods, a new set of field variables motivated by the hierarchical structure of multirank laminates is employed to represent each variant. As a result, the energy-well structure can be expressed explicitly in an elegant and unified fashion. The framework is applied to the investigation of pattern formation in martensitic thin films with trigonal symmetry. Various intriguing patterns are predicted and are found to be in good agreement with those observed in experiments. In addition, film orientations and patterns necessary to achieve large actuation strains are suggested for dome-shaped and tunnel-shaped microactuators. It is found that the resulting morphologies evolve with coherent interfaces under various loading conditions. This suggests that compatible walls provide a low-energy path during evolution, and the understanding of them leads to novel strategies of large strain actuation.

A justification of the theory of martensitic thin films in the absence of an interfacial energy

In the framework of Nonlinear Elasticity, the asymptotic behavior of the free energy of a martensitic material is studied as the height of the sample tends to zero. The effective thin film energy is identified in terms of parametrized probability measures, which allows for a justification of the kinematic compatibility conditions proposed in Bhattacharya and James' theory of thin films of martensitic materials, in the absence of higher order interfacial energy contributions to the three-dimensional bulk energy.

Pattern formation in martensitic thin films

Pattern formation in martensitic materials refers to the accommodation problem of how to mix martensitic variants coherently to minimize the strain energy. A framework motivated by energy-minimizing multirank laminated patterns is proposed to study this problem in martensitic films. It is found that the interfaces between the variants of martensite can be quite different in thin films than in bulk materials, and they typically have a simpler structure. Various intriguing and fascinating self-accommodation patterns are predicted for martensitic thin films with different orientations. The results are in good agreement with the Bhattacharya-James thin-film theory [K. Bhattacharya and R. D. James, J. Mech. Phys. Solids 47, 531 (1999)] as well as with experimental observations.

A finite element model for martensitic thin films

A finite element approximation of the thin film limit for a sharp interface bulk energy for martensitic crystals is given. The energy density models the softening of the elastic modulus controlling the low-energy path from the cubic to the tetragonal lattice, the loss of stability of the tetragonal phase at high temperatures and the loss of stability of the cubic phase at low temperatures, and the effect of compositional fluctuation on the transformation temperature. The finite element approximation is then used to simulate the hysteresis of a martensitic thin film obtained after applying a biaxial loading cycle to the film below the transformation temperature. Keywords: finite element, phase transformation, martensite, austenite, thin film Mathematics Subject Classification (1991): 65C30, 65Z05, 74K35, 74N10, 74N15, 74S05

Magnetic domains in Ni–Mn–Ga martensitic thin films

A series of martensitic Ni52Mn24Ga24 thin films deposited on alumina ceramic substrates has been preparedby using RF (radio-frequency) magnetron sputtering. The film thickness,d, variesfrom 0.1 to 5.0 µm. Magnetic domain patterns have been imaged by the MFM (magnetic forcemicroscopy) technique. A maze domain structure is found for all studiedfilms. MFM shows a large out-of-plane magnetization component and arather uniform domain width for each film thickness. The domain width,δ, depends on the film thickness as in the whole studied range of film thickness. This dependence is the expected one formagnetic anisotropy and magnetostatic contributions in a perpendicular magnetic domainconfiguration. The proportionality coefficient is also consistent with the values ofsaturation magnetization and magnetic anisotropy determined in the samples.

The energy density of martensitic thin films via dimension reduction

A variational limit defined on the space of bi-dimensional gradient Young measures is obtained from three-dimensional elasticity via dimension reduction. The obtained limit problem uniquely determines the energy density of the thin film. Our result might be used to compute the microstructure in membranes made of phase transforming material.

Nickel ion irradiation of plasitically deformed martensitic titanium nickel thin films

At present, little is known about the response of the martensitic phase in TiNi alloys to heavy ion irradiation. However, previous studies [2,3] of ion and electron irradiation show that these alloys are highly susceptible to disorder and amorphization at damage levels below 1 dpa. This substantially affects their transformation characteristics and their shape memory effect. The present study focuses on the effect of 5 MeV Ni ion irradiation of plastically strained (epsilon-4%) martensitic TiNi thin films, which is used as a processing technique for a novel out of plane bending actuator. Conceptually, the frustration of the martensitic transformation due to ion beam damage in a 2mum surface layer of a 6mum thick film will create a sharp differential latent strain on reverse transformation. This latent strain causes a two-way bending motion during cycling heating and cooling. This processing technique can be used to do useful mechanical work on both beating and cooling. To better understand the behavior of these ion irradiated thin films, TEM observations and motion experiments were conducted. Results are presented and discussed as they relate to the ion induced microstructure and its influence on the martensitic transformation.

On the Numerical Modeling of Deformations of Pressurized Martensitic Thin Films

We propose, analyze, and compare several numerical methods for the computation of the deformation of a pressurized martensitic thin film. Numerical results have been obtained for the hysteresis of the deformation as the film transforms reversibly from austenite to martensite.