Structure of martensite in sputter-deposited (Ni,Cu)-rich Ti–Ni–Cu thin films containing Ti(Ni,Cu)2 precipitates

The martensite structure in sputter-deposited thin films of Ti_48.6Ni_35.9Cu_15.5 was studied. The Ti(Ni,Cu)₂ phase precipitates during the annealing process. Fine Ti(Ni,Cu)₂ precipitates can be deformed by the shear deformation of martensitic transformation, but they obstruct the movement of the twin boundaries to some extent. Coarse Ti(Ni,Cu)₂ precipitates seriously impede the growth of martensite plates and lead to a rectangular-cell-like structure of martensite in the film annealed at 873 K. The resistance of Ti(Ni,Cu)₂ precipitates to the growth of the martensite plates enhances with the coarsening of Ti(Ni,Cu)₂ precipitates, which is one of the reasons for the decrease in the maximum recoverable strain with increasing annealing temperature. B19′ martensite with (0 0 1) compound twinning is frequently observed near coarse Ti(Ni,Cu)₂ precipitates and grain boundaries in films annealed at 873 and 973 K. The local stress concentration should be responsible for the presence of B19′ martensite.     

On the Ti2Ni precipitates and Guinier–Preston zones in Ti-rich Ti–Ni thin films

After heat treatment, there may exist different types of precipitates in Ti-rich thin films, i.e. spherical Ti₂Ni precipitates and plate-like Guinier–Preston (GP) zones. While the Ti₂Ni precipitates impede the shear deformation of martensitic transformation, the GP zones do not stop both the shear deformation of martensitic transformation and the twinning shear of (001) deformation twin in the martensite phase. An elastic deformation model of the GP zone during martensitic transformation and subsequent deformation in the martensite was built up. The model can explain the GP zones-related shape memory properties self-consistently. These results supply microstructural explanation for the previous results that Ti-rich Ti–Ni thin films with GP zones show a large transformation strain despite the presence of the GP zones, while thin films with Ti₂Ni precipitates show a relatively small transformation strain.     

Microstructural features and orientation correlations of non-modulated martensite in Ni–Mn–Ga epitaxial thin films

Epitaxially grown thin films with nominal composition Ni₅₀Mn₃₀Ga₂₀ and thickness 1.5 μm were prepared on MgO(1 0 0) substrate with a Cr buffer layer by DC magnetron sputtering. The surface layer microstructures of the as-deposited thin films consist of non-modulated (NM) martensite plates with tetragonal structure at ambient temperature, which can be classified into the low and high relative contrast zones of clustered plates (i.e. plate colonies) with parallel or near-parallel inter-plate interface traces in secondary electron images. Orientation analyses by electron backscatter diffraction revealed that individual NM plates are composed of alternately distributed thicker and thinner lamellar variants with (1 1 2)_Tetr compound twin relationship and coherent interlamellar interfaces. In each plate colony, there are four distinct plates in terms of the crystallographic orientation of the thicker lamellar variants and therefore, in total, eight orientation variants. For the low relative contrast zones, both thicker and thinner lamellar variants in adjacent plates are distributed symmetrically across their inter-plate interfaces (along the substrate edges). At the atomic level, there are no unbalanced interfacial misfits and height misfits, resulting in long and straight inter-plate interfaces with homogeneous contrast. However, in the high relative contrast zones, the thicker and thinner lamellar variants in adjacent plates are oriented asymmetrically across their inter-plate interfaces (at ～45° to the substrate edges). Significant atomic misfits appear in the vicinity of the inter-plate interfaces and in the film normal direction. The former result in bending of the inter-plate interfaces, and the latter give rise to the high relative contrast between adjacent plates.     

Structure and phase formation in Cu–Mn alloy thin films deposited at room temperature

Transmission electron microscopy characterization of Cu–Mn alloy thin films deposited by DC magnetron sputtering is applied to reveal the formation of phases throughout the composition range. Pure Cu and Mn films exhibit face-centred cubic (fcc) Cu and α-Mn phases, respectively. At room temperature the low Mn content films have fcc structure (γ-phase). Mn can substitute Cu in the fcc Cu lattice up to ～35 at.% Mn. The lattice parameter of fcc Cu–Mn alloy films follows a linear relationship of a₀ = a_Cu + 0.322c (in Å), where a_Cu = 3.615 Å is the lattice parameter of Cu and c is the Mn atomic concentration. At high Mn content, above 50 at.% Mn, a homogeneous one-phase structure is observed, possessing the short-range order of α-Mn. The incorporation of Cu into Mn suggests that this structure changes from crystalline α-Mn to disordered structure as the Cu content increases. A narrow two-phase region exists between 35 and 45 at.% Mn. A grain size minimum of 2–3 nm was observed in the 35–65 at.% Mn region.     

Internal structures and shape memory properties of sputter-deposited thin films of a Ti–Ni–Cu alloy

Low-temperature heat treatments of the sputter-deposited amorphous films, which were previously proved to be a new method to produce very good shape memory properties for Ti-rich Ti–Ni alloys, have been applied to a ternary Ti–43.0Ni–6.2Cu alloy (at.%). The basically same nanometric structures as in the binary alloy are formed, i.e. the nanometric structures consist of extremely thin plate precipitates of bct structure, which are formed on {100} planes of the parent B2 structure and have the c-axis normal to the habit planes. High-shape recovery stresses of about 500 MPa with recoverable shape strains of 5% are obtained without accompanying any permanent strains. A shape recovery stress of more than 870 MPa is attained if it is allowed to involve about 1% permanent strain. Although these bct precipitates have large tetragonalities, they are perfectly coherent with the parent bcc lattice. The maximum shape recovery stress is nearly twice that of the Ti-rich Ti–Ni binary alloy having a similar nanometric structure. It is suggested that this remarkable increase in recovery stress may be attributed to the change in Burgers vector of dislocations caused by partial disordering in Ti–Ni–Cu alloys. It is emphasized that the shape recovery stress in this ternary alloy is four times that of the Ti₂Ni containing samples and 10 times that of a bulk Ti–45Ni–5Cu alloy.     

Crystallization process and shape memory properties of Ti–Ni–Zr thin films

The crystallization process of as-deposited Ti–Ni–(10.8–29.5)Zr amorphous thin films was investigated. The Ti–Ni–Zr as-deposited films with a low Zr content exhibited a single exothermic peak due to the crystallization of (Ti,Zr)Ni with a B2 structure. In contrast, a two-step crystallization process was observed in the Ti–Ni–Zr thin films with a high Zr content. Shape memory behavior of Ti–Ni–Zr thin films heat treated at 873–1073 K was investigated by thermal cycling tests under various stresses. The martensitic transformation start temperature increased with increasing Zr content until reaching the maximum value, then decreased with further increasing Zr content. The inverse dependence of transformation temperature on Zr content in the thin films with a high Zr content is due to the formation of a NiZr phase during the crystallization heat treatment. The formation of the NiZr phase increased the critical stress for slip but decreased the recovery strain.     

Tensile properties and fractographs of Ti–2.5Al–1.5Mn foils at different temperatures

The tensile properties and fractographs of Ti–2.5Al–1.5Mn foils at different temperatures were investigated. It is observed that material properties closely correlate with the thickness(T) to grain size(d) ratio and deformation temperature. Tensile analysis shows that local deformation is the main deformation feature in foils forming at room temperature, which may lead to premature fracture. The causes of inhomogeneous deformation behavior are the limited number of deformable grains contained in deformation zone and the weak transferability of hardening among different grains. Fracture analysis reveals that the size of dimples can represent the ductility of foils at room temperature. With the further increase of deformation temperature, the main plastic deformation mode of foils is transformed from intragranular dislocations and twin crystal to grain-boundary gliding and rolling. In conclusion, foil forming at elevated temperature can increase the hardening transferability and the number of deformable grains in deformation zone, which is an effective method to improve the formability and reduce the scatter of material properties.     

Shape memory behavior and internal structure of Ti–Ni–Cu shape memory alloy thin films and their application for microactuators

Internal structure and shape memory behavior of Ti–38.3Ni–9.3Cu (at.%) thin films heat-treated at 873 K, 923 K, 973 K and 1023 K were investigated by TEM observation and thermal cycling tests under various constant stresses. The thin film heat-treated at 873 K contained two types of precipitates, i.e., fine platelets and Ti₂Ni particles. The density of the platelets decreased with increasing heat-treatment temperature and annihilated completely when the heat-treatment temperature reached 973 K. The Ti₂Ni precipitates increased in volume fraction with increasing heat-treatment temperature from 873 K to 923 K, then their volume fraction was almost kept constant above 923 K. The recoverable strain decreased and the M_s increased with increasing heat-treatment temperature from 873 K to 923 K. Both the recoverable strain and the M_s became almost constant when the heat-treatment temperature was above 923 K. A diaphragm-type microactuator utilizing a Ti–38.0Ni–10.0Cu (at.%) thin film was fabricated. The diaphragm was square with the width of 500 μm. The actuation properties were investigated under conditions of both quasi-static and dynamic actuation. The M_s and the transformation temperature hysteresis of the microactuator were determined to be 352 K and 6 K, respectively. The microactuator operated at a working frequency of 100 Hz.     

Latent strain in titanium–nickel thin films modified by irradiation of the plastically-deformed martensite phase with 5 MeV Ni2+

Lattice damage brought on by heavy ion irradiation is able to alter the displacive transformation characteristics of near equiatomic titanium–nickel. Irradiation of sputtered TiNi thin films can modify thermomechanical response to a depth of more than a micron, and may thus be used to create a perfectly bonded heterophase that deploys materials of sharply differing latent thermal strain on opposite sides of a thin sheet. If the alloy film is first martensitized, and then deformed in tension prior to partial-depth exposure to ion beam damage at temperatures well below A_s, a novel active-passive bilayer results that expresses pronounced bending displacements on subsequent heating. In the present paper, describing experiments on stretched 6-μm thick sputtered Ti_50.2Ni_49.7 films irradiated with 5 MeV Ni²⁺, we show that ion-induced latent bending can be cyclically reversed in temperature-displacement space, and that appreciable mechanical work can be extracted. Marked effects are observed at doses as low as 5×10¹³ Ni²⁺ cm⁻². The approach, in which nominally planar processing is used, derives mechanical robustness from a naturally diffuse interface between the beam-damaged stratum and the adjacent unmodified shape-memory layer.     

Density change upon crystallization of amorphous Zr–Cu–Al thin films

Systematic deflection measurements with micro-cantilevers and a combinatorial-deposition method have been used to investigate the density change upon crystallization of amorphous Zr–Cu–Al thin films. It is found that, in general, the density change decreases from Cu-/Al-rich compositions to Zr-rich compositions, and the previously reported good glass-forming compositions are found to correspond to global minima of the density change inside respective local eutectic systems. Furthermore, we propose that the general trend of density change as a function of alloy compositions discovered in this work may have implications for the macroscopic plasticity of metallic glasses.     

Microstructure and nanomechanical properties of Mn-rich Ni–Mn–Ga thin films

We report the effect of post-annealing on the crystalline phase, grain growth, magnetic and mechanical properties of Ni–Mn–Ga thin films deposited at room temperature followed by post-annealing at different temperatures. The phase and microstructural analysis reveal that amorphous to crystalline transformation occurs in as-deposited films after post-annealing above 873 K. The transformation of disordered phase into nanocrystalline phase by the influence of annealing has been confirmed by transmission electron microscopy. The crystalline films exhibit soft magnetic behavior with the Curie temperature of 314 K, while the amorphous films exhibit the Pauli-paramagnetic behavior even down to 4 K. The mechanical properties like hardness and elastic modulus of the films also show a strong dependence on the annealing temperature with crystalline film exhibiting maximum values of 6 GPa and 103 GPa, respectively. The Ni–Mn–Ga film annealed at 873 K exhibits enhanced nanomechanical properties and room temperature ferromagnetism which make this a potential candidate for use in MEMS devices.     

Chemical ordering and texture in Ni–25 at% Al thin films

This paper describes a novel study of the microstructural development in sputter-deposited thin films from a target of the intermetallic compound Ni₃Al. Films were deposited on oxidized Si substrates at different temperatures, namely 45°C, 200°C, and 400°C. These films have been characterized by X-ray diffraction and transmission electron microscopy. In contrast to the behavior of sputter-deposited elemental metals and disordered alloys, the films deposited at the two higher temperatures consisted of refined microstructures and exhibited a low degree of texturing. This anomalous behavior has been explained by the role of exothermic heating accompanying chemical ordering.     

Composition-dependent ground state of martensite in Ni–Mn–Ga alloys

For Ni–Mn–Ga alloys, giant magnetic-field-induced strains may be achieved in a modulated martensitic state, offering attractive chances for academic and practical exploration. However, the metastability of modulated martensite imposes a severe constraint on the capacity of these alloys as promising materials for sensors and actuators. In the present work, we conduct both experimental examinations and ab initio calculations to seek potential remedies of this critical problem through composition tuning. Results show that, for Group II alloys having modulated martensite at reasonable temperatures, the increase in Ni addition results in an enhanced tendency to the formation of non-modulated (NM) martensite, whereas the proper Mn addition leads to the stabilization of seven-layered modulated (7M) martensite, which serves as the structural ground state of martensite. By correlating the microstructural evolutions with the two-stage phase transformation (i.e. austenite → 7M martensite → NM martensite), it is demonstrated that the 7M martensite possesses lower energy barriers in terms of the lattice distortion of parent austenite and the interfacial energy of martensitic variants, which plays a vital role in bridging the austenite to NM martensite transformation. This result is expected to provide useful information for the design of these new functional materials.     

Phase transformations in Al–Cu thin films: precipitation and copper redistribution

The (micro-)structural changes occurring in thin Al–Cu films of about 500 nm thickness and containing up to 1 at% Cu have been investigated. In particular, the copper distribution and the precipitation behaviour have been studied in situ during thermal cycling between 323 and 773 K. After slow cooling, large second-phase particles are observed which are mostly located at grain boundary triple points; the shape and size depend on the copper concentration. The average distance between these particles is about 16 μm, which is larger than the average grain size of approximately 1 μm. No second-phase particles have been observed within the aluminium grains. After cooling, a relatively large amount of copper (about 0.2 at%) as compared with bulk Al–Cu (0.001 at%) is not contained in second-phase particles. Most likely, this copper has segregated at grain boundaries, interfaces and dislocations. The temperature at which Al₂Cu starts to form in the thin films is well below the copper solvus for bulk material.     

Control of crystallinity in sputtered Cr–Ti–C films

The influence of Ti content on crystallinity and bonding of Cr–Ti–C thin films deposited by magnetron sputtering have been studied by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and Raman spectroscopy. Our results show that binary Cr–C films without Ti exhibit an amorphous structure with two non-crystalline components; amorphous CrC_x and amorphous C (a-C). The addition of 10–20 at.% Ti leads to the crystallization of the amorphous CrC_x and the formation of a metastable cubic (Cr_1?xTi_x)Cy phase. The observation was explained based on the tendency of the 3d transition metals to form crystalline carbide films. The mechanical properties of the films determined by nanoindentation and microindentation were found to be strongly dependent on the film composition in terms of hardness, elasticity modulus, hardness/elasticity ratio and crack development.     

Crystallization behavior of amorphous Zr70Cu20Ni10 alloy annealed at 380℃

Crystallization behavior of amorphous Zr70Cu20Ni10 alloy isothermally annealed at 3800℃ was first investigated by employing the differential scanning calorimetry (DSC) and transmission electron microscopy(TEM). It has been found that an exothermic peak appears in the DSC trace when the annealing time is about 17-18min,in dicating a certain phase transformation occurs in the matrix of amorphous Zr70 Cu20 Ni10 alloy.Meanwhile,isothermal annealing experiments for amorphous Zr70Cu20Ni10 alloy ranging from 370℃ to 400℃with a temperature interval of 10℃ were also carried out,from which no exothermic reaction can be observed except for the case of 380℃.This behavior indicates that the phase transformation during isothermal annealing of amorphous Zr70Cu20Ni10 alloy is strongly temperature-and time-dependent.Further investigations are required to reveal the nature of such phenome non.     

Hydrogen embrittlement behavior induced by dynamic martensite transformation of Ni–Ti superelastic alloy

The hydrogen embrittlement behavior induced by the martensite transformation of Ni–Ti superelastic alloy subjected to a dynamic cyclic tensile test with hydrogen cathodic charging has been investigated by hydrogen thermal desorption analysis. The critical stress for the martensite transformation steeply decreases with increasing number of deformation cycles, whereas the critical stress for the reverse transformation only slightly changes. The dynamic stress-induced martensite transformation markedly enhances hydrogen absorption, compared with that of the martensite phase itself. The hydrogen concentration at the surface layer of the specimen is evaluated to be above 3500 mass ppm; nevertheless, no fracture associated with the stress-induced martensite transformation occurs. In addition, no hardening is observed at the surface layer of the specimen despite the formation of the hydride and hydrogen enrichment. The hydrogen thermally desorbed at a low temperature markedly increases, indicating that the hydrogen states are changed by the dynamic martensite transformation. Note that interactions between hydrogen and the phase transformation are probably irreversible, although the phase transformation is reversible. The present study shows, for the first time, that the hydrogen embrittlement behavior of the alloy strongly depends on the dynamic change of the hydrogen states accompanied by the martensite transformation.