Effect of Sn and Al additions on the microstructure and mechanical properties of amorphous Ti–Cu–Zr–Ni alloys

Amorphous Ti–Cu–Zr–Ni alloys with minor addition of Sn and Al were prepared by melt spinning technique.The effects of Sn and Al additions on the microstructures and mechanical properties of glassy ribbons were investigated.The amorphous state of ribbons was confirmed by x-ray diffraction and transmission electron microscopy,where those ribbons with Sn addition exhibited a fully amorphous state.The characteristic temperature indicates that Ti_(45)Cu_(35)Zr_(10)Ni_5Sn_5 alloy has a stronger glass-forming ability,as proven by differential scanning calorimetry.Ti_(45)Cu_(35)Zr_(10)Ni_5Al_5 alloy showed a better hardness of 9.23 GPa and elastic modulus of 127.15 GPa and good wear resistance.Ti_(45)Cu_(35)Zr_(10)Ni_5Sn_5 alloy displayed a pop-in event related to discrete plasticity according to nanoindentation.When the temperature is below 560 K,Ti_(45)Cu_(35)Zr_(10)Ni_5Sn_5 alloy mainly exhibits elasticity.When the temperature rises between 717 K and 743 K,it shows a significant increase in elasticity but decrease in viscoelasticity after the ribbon experiences the main relaxation at 717 K.When the temperature is above 743 K,the ribbon shows viscoplasticity.     

Growth kinetics and precipitation phenomena in undercooled Nd–Fe–B melts with Ti and C additions

The microstructure and the solidification kinetics of stoichiometric Nd–Fe–B alloy with Ti and C additions were investigated using the electromagnetic levitation technique. In situ temperature–time characteristics were carried out. A strong reduction of the growth velocity of the Nd₂Fe₁₄B phase was observed in the Nd–Fe–B–Ti–C alloy compared to the addition-less Nd–Fe–B alloy. The undercoolability of the melt depends on the alloy composition. Moreover, at high TiC contents, the maximum undercooling level is strongly reduced turning to low cooling rates. The TiC solution and its formation were studied in overheated and undercooled samples, respectively after subsequent quenching. The cooling rate prior to solidification influences drastically the morphology of the TiC precipitates which affects strongly the nucleation of the properitectic γ-Fe phase in the undercooled stage.     

High glass formability for Cu–Hf–Ti alloys with small additions of Y and Si

The effects of small substitutions of Si and Y on the glass-forming ability of a Cu₅₅Hf₂₅Ti₂₀ glassy alloy are reported and discussed. Fully glassy rods with diameters up to 7 and 6.5 mm were produced for Cu_54.5Hf₂₅Ti₂₀Si_0.5 and Cu_{55? x} Hf₂₅Ti₂₀Y_0.3 alloys, respectively. The addition of Si enlarged ΔT_x (= T_x ? T _g, where T _g and T_x are crystallisation and glass transition temperatures, respectively) considerably, from 25 to 53 K for the Cu₅₄Hf₂₅Ti₂₀Si₁ alloy. However, the results showed that the parameters obtained from thermal analysis, such as T _rg , ΔT_x and γ[= T_x /(T _g + T _l)] are not reliably correlated with the glass-forming ability (GFA), at least for these bulk glass-forming alloys. The scavenging effects of the Y and Si, in particular the possibility of Y reducing the oxides, could be responsible for enhancing the GFA. It is proposed that the effectiveness of small additions of Si in enhancing the GFA may be the result of the possible formation of HfSiO₄ having a very large negative enthalpy of formation and, as a strong network former, it would form glassy particles which would be ineffective as nucleating agents.     

High-strength and ductile glassy-crystal Ni–Cu–Zr–Ti composite exhibiting stress-induced martensitic transformation

We present a Ni-based crystal-glassy composite material having superior strength paired with a considerable ductility of 15%. The formation of a metastable crystalline phase in a glassy matrix during solidification has been proven capable of promoting a strain-induced martensitic transformation leading to enhanced plasticity under compression at room temperature. Underlying mechanisms of plastic deformation are discussed in terms of the interplay between dislocation slip in the crystalline phase and shear deformation in the glassy matrix. We suppose that the strain-induced martensitic inclusions serve as strong barriers for shear band propagation, promoting shear band branching and multiple shear band formation, thus extending the ductility and preventing a premature brittle fracture. The acoustic emission technique has been employed to clarify the kinetics of transformation and stages of plastic deformation.     

Substrate-induced stress and transformation characteristics of a deposited Ti–Ni–Cu thin film

This article presents the investigation results on the transformation characteristics of a sputter-deposited Ti–Ni–Cu shape memory alloy thin film and its relation to the substrate-induced stress. Experimental results show that, with the substrate attachment, the transformation interval increases while the transformation hysteresis decreases in comparison with those of the same thin film in the free-standing condition. By assuming a stress distribution through the film thickness, a layer-by-layer transformation sequence in the substrate-attached film is proposed and the transformation interval and hysteresis are analysed. The analysis results show qualitative agreement with the experimental observations, suggesting that the approach taken is plausible. This approach may also be used to examine the transformation characteristics of other thin films having thermally induced phase transformations.     

Interdiffusion and solid solution strengthening in Ni–Co–Pt and Ni–Co–Fe ternary systems

Diffusion couple experiments are conducted in Co–Ni–Pt system at 1200?°C and in Co–Ni–Fe system at 1150?°C, by coupling binary alloys with the third element. Uphill diffusion is observed for both Co and Ni in Pt rich corner of the Co–Ni–Pt system, whereas in the Co–Ni–Fe system, it is observed for Co. Main and cross interdiffusion coefficients are calculated at the composition of intersection of two independent diffusion profiles. In both the systems, the main interdiffusion coefficients are positive over the whole composition range and the cross interdiffusion coefficients show both positive and negative values at different regions. Hardness measured by performing the nanoindentations on diffusion couples of both the systems shows the higher values at intermediate compositions.     

Thermomechanical response of an Ni–Ti–Cr shape-memory alloy at low and high strain rates

The thermomechanical response of an Ni–Ti–Cr shape-memory alloy is investigated at various initial temperatures, over a wide range of strain rates, using an Instron hydraulic testing machine and one of the modified split-Hopkinson-bar systems at the Center of Excellence for Advanced Materials, University of California, San Diego. The transition stress for the stress-induced martensite formation is observed to be quite sensitive to the initial deformation temperature, but the yield stress of the resulting martensite is not. The linear transition stress–temperature relation with a slope of 8.5?MPa?K^?1, obtained in a quasistatic loading regime, seems to remain valid for strain rates up to 500–700?s^?1. The transition stress and the yield stress of the stress-induced martensite show strain-rate sensitivity, increasing monotonically with increasing strain rate. There exists a certain critical strain rate at which the transition stress equals the yield stress of the material. This critical strain rate determines the material's deformation behaviour; the material deforms by the formation of stress-induced martensites and their subsequent yielding, when the strain rate is less than this critical value, and through dislocation-induced plastic slip of the parent austenite, when the strain rate exceeds the critical value. It appears that the critical strain rate increases slightly with decreasing initial temperature.     

Incommensurate–commensurate transition and nanoscale domain-like structure in iron doped Ti–Ni shape memory alloys

Precursor phenomena of displacive transformation have been studied by optical and transmission electron microscope observation and X-ray diffraction of Ti–(50???x)Ni–xFe (x?=?2,?4,?6,?8 in at.%) alloys. We found that a Ti–44Ni–6Fe alloy exhibits a second-order-like incommensurate–commensurate transition without latent heat and discontinuity in lattice parameters. In other words, diffuse scatterings appear in an electron diffraction pattern at an incommensurate position on cooling; they move gradually towards 1/3? 110? as the temperature decreases and lock into the commensurate position at 180?K. The commensurate phase is not expanded along one of the ? 111? directions, unlike the R-phase formed by a first-order transformation in Ti–48Ni–2Fe and Ti–46Ni–4Fe alloys. In addition, the commensurate phase shows a nanoscale domain-like structure, which is inherited from the incommensurate state of the parent phase. Thus, the anomalies in physical properties observed in the incommensurate state are most likely the precursor phenomena of the commensurate phase in the Ti–44Ni–6Fe alloy. In the case of a Ti–42Ni–8Fe alloy, the incommensurate state remains even at 19?K.     

Thermal and structural characterization of Cu–Al–Mn–X (Ti,Ni) shape memory alloys

In this study, the Cu–Al–Mn–X (X = Ni, Ti) shape memory alloys at the range of 10–12 at.% of aluminum and 4–5 at.% manganese were produced by arc melting. We have investigated the effects of the alloying elements on the transformation temperatures, and the structural and the magnetic properties of the quaternary Cu–Al–Mn–X (X = Ni, Ti) shape memory alloys. The evolution of the transformation temperatures was studied by differential scanning calorimetry with different heating and cooling rates. The characteristic transformation temperatures and the thermodynamic parameters were highly sensitive to variations in the aluminum and manganese content, and it was observed that the nickel addition into the Cu–Al–Mn system decreased the transformation temperature although Ti addition caused an increase in the transformation temperatures. The effect of the nickel and the titanium on the thermodynamic parameters such as enthalpy and entropy values was investigated. The structural changes of the samples were studied by X-ray diffraction measurements and by optical microscope observations at room temperature. It is evaluated that the element Ni has been completely soluble in the matrix, and the main phase of the Cu–Al–Mn–Ni sample is martensite, and due to the low solubility of the Ti, the Cu–Al–Mn–Ti sample has precipitates, and a martensite phase at room temperature. The magnetic properties of the Cu–Al–Mn, Cu–Al–Mn–Ni and Cu–Al–Mn–Ti samples were investigated, and the effect of the nickel and the titanium on the magnetic properties was studied.     

Improvement of microstructure and magnetic properties of Nd–Fe–B alloys by Nb and Co additions

In order to establish the role of niobium on the hydrogenation, disproportionation, desorption and recombination (HDDR) behavior of near-stoichiometric alloys, two alloys: NdI3Fe8OB7 and Nd13Fe78Nb1Co1B7 (at%) were investigated before, during and after the HDDR process. The microstructure of the as-cast Nb-free alloy before employing the HDDR process was found to consist of three phases, the matrix Nd₂Fe₁₄B (φ) phase, Nd-rich phase and a significant amount of free iron; whereas, the microstructure of the Nb-containing alloy consisted of only the first two phases.     

Structure and thermoelastic martensitic transformations in ternary Ni–Ti–Hf alloys with a high-temperature shape memory effect

The effect of alloying by 12–20 at % Hf on the structure, the phase composition, and the thermoelastic martensitic transformations in ternary alloys of the quasi-binary NiTi–NiHf section is studied by transmission electron microscopy, scanning electron microscopy, electron diffraction, and X-ray diffraction. The electrical resistivity is measured at various temperatures to determine the critical transformation temperatures. The data on phase composition are used to plot a full diagram for the high-temperature thermoelastic B2 ? B19’ martensitic transformations, which occur in the temperature range 320–600 K when the hafnium content increases from 12 to 20 at %. The lattice parameters of the B2 and B19’ phases are measured, and the microstructure of the B19’ martensite is analyzed.     

Phase transformation studies in unirradiated and proton beam irradiated Ni–Ti alloy between 25 and 100°C

The stress-induced phase transformation characteristics of unirradiated and proton beam irradiated NiTi alloy were investigated at different tests temperatures. The wire-shaped NiTi specimens were irradiated by 2?MeV proton beam for 30?min at room temperature to a flux of 10¹⁹ protons/m² s. Engineering stress–strain (S-S) curves of both unirradiated and irradiated specimens were obtained using a materials testing machine at 25, 50, 75 and 100°C. The results indicate a single-stage phase transformation from austenite to martensite (B2–B19′) in unirraidated specimens at all the test temperatures. In contrast, in the case of the irradiated specimens, a two-stage austenite–rhombohedral–martensite (B2–R–B19′) phase transformation is observed at 25 and 50°C. The B2–R–B19′ phase transformation, however, is found to change into B2–B19′ transformation at 75 and 100°C. The stress required to initiate the B19′ phase transformation (σ_MS) and the plateau range are found to be lower in irradiated specimens compared with those of the unirradiated specimens. The results obtained are discussed on the basis of the formation of Ni₄Ti₃ precipitates in irradiated specimens and their consequences on the phase transformations.     

High temperature strain glass in Ti–Au and Ti–Pt based shape memory alloys

Strain glass is a frozen short-range strain ordered state found in shape memory alloys recently, which exhibits novel properties around the ideal glass transition temperature T₀. However, the T₀ of current strain glass systems is still very low, limiting their potential applications and experimental studies. In this paper, we reported two new strain glass systems with relatively high T₀. In Ti₅₀Au_50-xCr_x alloys, the strain glass appears at x = 25, and exhibits a T₀ of 251 K, while in Ti₅₀Pt_50-yFey alloys, the strain glass takes place at y = 30, and shows a T₀ of 272 K. Both of them are comparable with the highest T₀ value reported so far. Moreover, the phase diagrams of main strain glass systems in Ti-based alloys were summarized. It is found that the influence of the martensitic transformation temperature of the host alloy on the T₀ of the strain glass is limited. This work may help to design new strain glass systems with higher T₀ above ambient temperature.     

Precipitation and fracture behaviour of Fe–Mn–Ni–Al alloys

The effects of Al addition on the precipitation and fracture behaviour of Fe–Mn–Ni alloys were investigated. With the increasing of Al concentration, the matrix and grain boundary precipitates changed from L1₀ θ-MnNi to B2 Ni₂MnAl phase, which is coherent and in cube-to-cube orientation relationship with the α′-matrix. Due to the suppression of the θ-MnNi precipitates at prior austenite grain boundaries (PAGBs), the fracture mode changed from intergranular to transgranular cleavage fracture. Further addition of Al resulted in the discontinuous growth of Ni₂MnAl precipitates in the alloy containing 4.2?wt.% Al and fracture occurred by void growth and coalescence, i.e. by ductile dimple rupture. The transition of the fracture behaviour of the Fe–Mn–Ni–Al alloys is discussed in relation to the conversion of the precipitates and their discontinuous precipitation behaviour at PAGBs.     

Synthesis and performance of Zn–Ni–P thin films

The electroplating of Zn–Ni–P thin film alloys from a sulfate bath containing phosphoric and phosphorous acid was investigated. The bath composition and the deposition parameters were optimized through Hull cell experiments, and the optimum experimental conditions were determined(p H = 2, temperature = 298–313 K, zinc sulfate concentration =30 g·L-1, EDTA concentration = 15 g·L-1, and current density = 1.0–2.0 A·dm-2). The SEM analysis of the coating deposited from the optimum bath revealed fine-grained deposits of the alloy in the presence of EDTA. Optical microscopy analysis indicated an electrodeposited thin film with uniform thickness and good adhesion to the steel substrate. The good adherence of the coatings was also demonstrated by the scratch tests that were performed, with a maximum determined value of 25 N for the critical load. Corrosion resistance tests revealed good protection of the steel substrate by the obtained Zn–Ni–P coatings, with values up to 85.89% for samples with Ni contents higher than 76%. The surface analysis of the thin film samples before and after corrosion was performed by X-ray photoelectron spectroscopy(XPS).     

Microstructure investigations and magnetic after-effect in amorphous and nanocrystalline Fe–Zr–Ti–B–Cu alloy

The microstructure evolution and low field magnetic properties i.e. initial magnetic susceptibility, stabilization field and magnetic after-effect as disaccommodation of the amorphous and nanocrystalline Fe₈₀Zr₄Ti₃B₁₂Cu₁ alloy were investigated. The heat treatment of the as-quenched Fe₈₀Zr₄Ti₃B₁₂Cu₁ alloy at 773 K for 1 h leads to its nanocrystallization. It was stated that initial magnetic susceptibility increases and intensity of disaccommodation decreases with increasing of annealing temperature. The magnetic after-effect of the investigated nanocrystalline samples is connected with relaxation processes that occur in the amorphous matrix.     

Ag implantation-induced modification of Ni–Ti shape memory alloy thin films

Nanocrystalline thin films of Ni–Ti shape memory alloy are deposited on an Si substrate by the DC-magnetron co-sputtering technique and 120?keV Ag ions are implanted at different fluences. The thickness and composition of the pristine films are determined by Rutherford Backscattering Spectrometry (RBS). X-Ray diffraction (XRD), atomic force microscopy (AFM) and four-point probe resistivity methods have been used to study the structural, morphological and electrical transport properties. XRD analysis has revealed the existence of martensitic and austenite phases in the pristine film and also evidenced the structural changes in Ag-implanted Ni–Ti films at different fluences. AFM studies have revealed that surface roughness and grain size of Ni–Ti films have decreased with an increase in ion fluence. The modifications in the mechanical behaviour of implanted Ni–Ti films w.r.t pristine film is determined by using a Nano-indentation tester at room temperature. Higher hardness and the ratio of higher hardness (H) to elastic modulus (E_r) are observed for the film implanted at an optimized fluence of 9?×?10¹⁵ ions/cm². This improvement in mechanical behaviour could be understood in terms of grain refinement and dislocation induced by the Ag ion implantation in the Ni–Ti thin films.     

Design and Fabrication of Ni/Ti Multilayer for Neutron Supermirror

In the applications of neutron guides and focusing devices, by using the Ni/Ti multilayer supermirrors （SM）, the neutron flux is significantly enhanced, because the critical reflective angle of supermirrors increases m times compared to the one of natural bulk Ni. We design and fabricate the Ni/Ti multilayer supermirrors by considering the effect of the interfacial imperfection, such as interface roughness and diffusion, and by using the direct current magnetron sputtering technology. The reflective performances of these supermirrors are measured on a V14 neutron beam line at the Berlin Neutron Scattering Centre （BENSC）, Germany. The measurement data suggest that the critical angles of the supermirrors are 1.5 and 2.2 times that of bulk Ni, respectively.