Characterization of Transformation Temperatures with the Bend and Free Recovery Technique: Parameters and Effects

This study was conducted to investigate factors such as the deformation strain, straining temperature, and the number of testing cycles on the measurement of transformation temperatures using the bend and free recovery (BFR) technique. Ti-56.0%Ni wire with approximately 40% cold work and a 2 mm diameter was heat treated in an air furnace for 10 min at 490 °C to obtain an A _f of approximately 21 °C. Wire specimens were deformed with one of two mandrels to apply an outer fiber strain of 2.4 or 5.8%. Deformation was performed at one of four straining temperatures: 0, ?30, ?50, or ?65 °C. Specimens were tested ten times to investigate the effect of repeated testing. The resulting BFR curves were analyzed to determine the trends in the R-phase start ( $ R_{{\text{s}}}^{\prime} $ ) and austenite finish (A _f) temperatures. For specimens strained at both 2.4 and 5.8% there was no detectable change in A _f resulting from changes in the deformation temperature. Increasing the deformation strain from 2.4 to 5.8% tended to increase the measured A _f by approximately 1 °C independent of deformation temperature. Repeat testing seemed to result in a slight increase in A _f but the intrinsic scatter of the BFR data made it impossible to conclusively identify a trend.     

Martensitic transformation and mechanical properties of Ti-rich Ti-Ni-Cu melt-spun ribbon

Martensitic transformation, mechanical and thermomechanical properties of a Ti-rich Ti₅₂Ni₂₃Cu₂₅ melt spun ribbon annealed at a temperature below the crystallization temperature were studied by XRD, DSC and DMA. After annealing the initially amorphous ribbon at 400 °C for 10 h, the ribbon is fully crystallized and exhibits one-stage B2?B19 phase transformation with the temperature hysteresis of 14 °C. The annealed ribbon is composed of B2, B19 and B11-TiCu phase with (001) preferential orientation. On the stress―strain curves, the rearrangement of the martensite variants and stress-induced martensitic transformation are observed below the M_f temperature and above the A_f temperature, respectively. The annealed ribbon exhibits up to 1.6% superelastic shape recovery with small stress hysteresis of 25 MPa. No flat stress-plateau is associated with the superelasticity. The annealed ribbon shows a well-defined shape memory effect during thermal cycling from ?60 to 100 °C. The transformation strain and recovery strain increase with increasing the applied external stress. Under the external stress above 150 MPa, the shape recovery strain is not sensitive to it and keeps stable at about 1.74%.     

Compound forming technology of outside 3D integral fin of copper tubes

Using rolling-ploughing-extrusion compound processing methods, a 3D integral-fin structure on outside surface of red copper tube with diameter of 16.0 mm and wall thickness of 1.5 mm was obtained. When both rolling depth and ploughing-extrusion (P-E) depth were 0.2 mm, rotating speed was 50 r/min, feed speed was 0.16 mm/r, 3D fin structures with height of 0.25 mm were gotten. Two different fin structures were obtained in grooves formed with rolling-ploughing-extrusion compound forming technology and observed by scanning electron microscope(SEM). One is the compound structure with V-shaped groove and U-shaped groove, and the other is the single structure with V-shaped grooves. Two kinds of groove structures obtained by rolling processing and ploughing extrusion processing are restricted together by groove interval and rolling depth, and pitch and P-E depth, respectively. Based on the analysis of interaction of rolling and P-E processing, it is found from the result that the outside 3D integral-fin can be achieved by rolling-ploughing-extrusion compound processing when single V-shaped groove structures are formed by both rolling and P-E processing.     

Effect of heat treatment on the structural transformations and properties of high-nitrogen chromium steels

Conclusions  

An Examination of the Mechanical Behavior of Shape Memory Nitinol Wires at Various Temperatures

Shape memory Nitinol alloys are commonly used in a myriad of strain recovery applications. Understanding the behavior of the material at different temperatures influences the choices of the application designer when selecting different grades of Nitinol. This study focuses on six Nitinol ingot austenitic start (A _s) temperatures (Ingot A _s = 95.0, 84.0, 62.3, 41.6, 29.3, and 13.4 °C). Each sample was drawn to wire diameters ranging from 0.076 to 1 mm, two sizes per ingot A _s, with varying heat treatments to achieve various active austenite transformation temperatures ranges (TTR). TTR behavior was measured utilizing a linear variable displacement transducer (LVDT) bend and free recovery (BFR) method, per ASTM F2082. Mechanical characterization was determined by a series of uniaxial tensile tests performed at 5, 10, 15, 20, 30, 50, 75, and 100 °C above respective active A _f transformation temperatures. This study will detail the methods utilized in determining the results and potential impact on current industry test modalities and standards.     

Effect of thermocycling on the temperatures of phase transformations,structure, and properties of the equiatomic alloy Ti<Subscript>50.0</Subscript>Ni<Subscript>50.0</Subscript>

This article is devoted to studying the influence of thermocycling in the range of temperatures of the thermoelastic martensitic transformation B2–B19' on the microstructure, the temperatures of the martensitic transformations, and the mechanical properties of the equiatomic alloy Ti₅₀Ni₅₀ in the coarse-grained (CG) and ultrafine-grained (UFG) states, the latter obtained by equal-channel angular pressing (ECAP). One hundred cycles of thermocycling and the related increase in the dislocation density in the CG alloy led to a decrease in the temperatures of martensitic transformations. In the UFG alloy, the temperatures of the forward transformation (M_s, M_f) decrease by 2–3 K, and the temperatures of the reverse transformation (A_s, A_f) increase by 6 K. The ultimate strength remains almost unaltered upon the thermocycling, but the yield stress increases substantially from 430 to 550 MPa and from 935 to 1120 MPa for the CG and UFG states, respectively.     

Differential Scanning Calorimetric (DSC) Analysis of Rotary Nickel-Titanium (NiTi) Endodontic File (RNEF)

To determine the variation of A _f along the axial length of rotary nickel-titanium endodontic files (RNEF). Three commercial brands of 4% taper RNEF: GTX (#20, 25?mm, Dentsply Tulsa Dental Specialties, Tulsa, OK, USA), K3 (#25, 25?mm) and TF (Twisted File #25, 27?mm) (Sybron Kerr, Orange, CA, USA) were cut into segments at 4?mm increment from the working tip. Regional specimens were measured for differential heat-flow over thermal cycling, generally with continuous heating or cooling (5?°C/min) and 5?min hold at set temperatures (start, finish temperatures): GTX: ?55, 90?°C; K3: ?55, 45?°C; TF: ?55, 60?°C; using differential scanning calorimeter. This experiment demonstrated regional differences in A _f along the axial length of GTX and K3 files. Similar variation was not obvious in the TF samples. A contributory effect of regional difference in strain-hardening due to grinding and machining during manufacturing is proposed.     

Ferromagnetic behaviour in the LaKMnMoO6 double perovskite

The structural and magnetic properties of a LaKMnMoO₆ double perovskite sample have been investigated. The powder sample has been prepared using a conventional solid state reaction at 1100 °C. Structural studies at room temperature show that our sample crystallizes in an orthorhombic structure with the P222 space group. Measurements of the magnetization versus temperature on the LaKMnMoO₆ sample show a magnetic transition from paramagnetic to ferromagnetic state with decreasing temperature. The Curie temperature T_C is found to be 180 K. The critical exponent γ, defined by M_sp=M_sp (0) (1−T/T_C)^γ and deduced from field dependent magnetization measurements at several temperatures below T_C, is found to be 0.33. The moment per Mn ion, deduced from the Curie constant, C, is found to be 5.68 μ_B which is slightly smaller than the Mn²⁺ moment equal to 5.90 μ_B.     

Modification of the titanium nickelide surface using frictional treatment and subsequent heating in air

The effect of a combined treatment including severe plastic deformation under the conditions of dry sliding friction and heating in air to temperatures of 300?C480°C (holding for 1 h) on the structure and wear resistance of the surface layer of the Ti_49.4Ni_50.6 alloy has been investigated. It has been shown that this frictional treatment results in an amorphous-nanocrystalline structure in the surface layer (of thickness to 10 ??m) of the Ti_49.4Ni_50.6 alloy. Heating to 300°C brings about the complete crystallization of the amorphous phase; as a result, the structure of the deformed surface layer of the alloy becomes single-phase, consisting of nanocrystals of the B2 phase. At 400°C, in this deformed surface layer there arises a nanocrystalline oxide (TiO₂) phase whose amount reaches tens of volume percent. The sizes of crystals of the B2 phase and oxide TiO₂ are in the range of 1?C50 nm. The arising two-phase (B2 + TiO₂) nanocrystalline structure is located just below the oxide TiO₂ film, which is less than 1 ??m thick. With an increase in the heating temperature to 480°C, the deformed surface layer under consideration retains the nanocrystalline two-phase (B2 + TiO₂) structure, but an increase in the amount of the oxide phase and a decrease in the microhardness of this structure are observed. In some cases (heating at temperatures of 430 and 450°C), the presence of the two-phase (B2 + TiO₂) nanocrystalline surface layer leads to a noticeable (to ??25%) enhancement in the adhesive wear resistance of the Ti_49.4Ni_50.6 alloy upon sliding friction in pair with steel 40Kh13.     

The crystal structure of trans,trans-1,3,5,7-octatetraene as a model for fully-ordered trans-polyacetylene

The crystal structure of trans,trans-1,3,5,7-octatetraene has been determined using x-ray diffraction techniques. This crystal structure, the first for an unsubstituted polyene, is used to predict the molecular and crystallographic structures of trans-polyacetylene. The structure of the octatetraene crystal is monoclinic, P2₁/c, and has two centrosymmetric molecules in a unit cell with dimensions $\text{a = 10.228(8)}\text{A}\text{?}\text{, b = 4.120(1)}\text{A}\text{?}\text{, c = 8.313(3)}\text{A}\text{?}\text{,}$ and β = 96.53(4)°. The carbon skeleton is planar to within the experimental error and the carbon-carbon bond lengths starting from the chain end are 1.336(4), 1.451(3), 1.327(4), and 1.451(5) Å. The average CCC angle is 125.0(3)°. From the intermolecular packing mode of octatetraene, the predicted structure of trans-polyacetylene is monoclinic, P2₁/n, with $\text{a = 7.38}\text{A}\text{?}\text{, b = 4.12}\text{A}\text{?}\text{, c = 2.47}\text{A}\text{?}\text{(}\text{chain axis}\text{), β = 97.8°}$, and two chains per unit cell (2C₂H₂ units). These axial lengths deviate at most by 1% from the average of three experimental determinations using x-ray diffraction and suggest that alternate proposed cells are less likely. Likewise, the predicted setting angle of the polymer chain (52.7° relative to the (010) of the present cell) is in good agreement with the results of crystal energy calculations (51.0°) and x-ray diffraction analyses (55°). Using the octatetraene data, the predicted single-bond length, double-bond length and CCC angle in trans-polyacetylene are 1.451(5) Å, 1.327(4) Å, and 125.3(4)°. These results are in reasonable agreement with bond lengths derived from nutation n.m.r. studies on trans-polyacetylene ($\text{1.44 ± 0.02}\text{a}\text{?}\text{and}\text{1.36 ± 0.02}\text{A}\text{?}$). The dimerization parameter (μ₀) derived from the octatetraene results is 0.035(1) Å, which can be compared with values previously estimated from the analysis of the limited available electron and x-ray diffraction intensities for poorly ordered trans-polyacetylene ($\text{0.012 ± 0.001}\text{A}\text{?}\text{and}\text{0.03}\text{A}\text{?}$, respectively.     

Effects of Crimping on Mechanical Performance of Nitinol Stent Designed for Femoral Artery: Finite Element Analysis

Nitinol stents are used to minimize improper dynamic behavior, low twistability, and inadequate radial mechanical strength of femoral artery stents. In this study, finite element method is used to investigate the effect of crimping and Austenite finish temperature (A _f) of Nitinol on mechanical performance of Z-shaped open-cell femoral stent under crimping conditions. Results show that low A _f Nitinol has better mechanical and clinical performance due to small chronic outward force, large radial resistive force, and appropriate superelastic behavior.     

Synthesis of γ-LiV2O5/VO2 mixture by thermal lithiation of vanadium （＋4, ＋5） oxides

γ-LiV2O5/VO2 composites were synthesized through thermal lithiation reaction of mixed valence （＋4, ＋5） vanadium oxides by lithium bromide. The phase evolution, morphology and discharge behavior at 500℃ were investigated by thermal gravimeter/differential thermal analysis （TG/DTA）, X-ray diffraction（XRD）, scanning electron microscopy（SEM） and specific surface analysis（BET）. The mixed vanadium oxides are obtained from the pyrolytic decomposition of ammonium metavanadate, with V6O13 as main phase. Results show that the lithiation reaction begins at about 258℃, with γ-LiV2O5 and VO2（B） as the product. VO2（B） can transit to VO2（R） in the range of 400-500℃, following by grain growth and crystalline development with the increase of temperature and roasting time. The ratio of γ-LiV2O5 to VO2 can be modified by the additive content of lithium bromide. A lattice shearing model about the nucleation and growth of LixV2O5 and VO2（B） inside mixed valence （＋4, ＋5） vanadium oxides （e.g. V6O13, V3O7） is speculated, which is relative to oxygen-/vacancy-diffusion and structural evolution inspired by lithium-insertion. The open-circuit voltage of 2.6 V is observed in the single cell of Li-B/LiCl-KCl/（γ-LiV2O5/VO2） at 500℃, and the specific capacities of 146 and 167 mA.h/g （cut-offvoltage 1.4 V） are measured for the positive material at 100 mA/cm^2 and 200 mA/cm^2, respectively.     

Excited states absorptions in oligomers of PPV

Recent experimental studies based on pump-probe spectroscopy of phenyl-based polymers reveal quite different relaxation dynamics of low and high energy even parity states reached by photoinduced absorption (PA). One class of even parity states (mA_g) experiences ultrafast internal conversion to the lowest singlet state (1B_u), whereas the other class (kA_g) in violation of the Vavilov–Kash's rule undergoes a different pathway. We therefore investigate theoretically the nature of higher excited states, both even as well as odd parity states, for various sizes of oligomers of poly(para-phenylene vinylene) (PPV). We study Third Harmonic Generation (THG) of oligomers of PPV within a rigid band correlated Pariser–Parr–Pople (P–P–P) model Hamiltonian, using the π-electron basis and the powerful multireference single and double configuration interaction (MRSDCI) method. We also use single–double–triple–quadruple configuration interaction (SDTQCI) in limited cases. We reconfirm the nature of (mA_g) states, which are responsible for the low energy PA band. We also show that in THG three kinds of one-photon states 1B_u, jB_u and nB_u (where j < n) appear whereas two kinds of even parity states 2A_g and mA_g appears. While mA_g absorptions appear just after the nB_u, the 2A_g absorption peak appears immediately after the jB_u absorptions. The later absorption features were not seen in case of linear chain materials like polydiaccetylene (PDA).     

Phase transformation and microstructural features of NiAl/Ni<Subscript>3</Subscript>Al two-phase alloys containing ternary elements

Various ternary elements were added to observe the effects on the microstructural features of β+γ′ two-phase alloys. The microstructural features of β+γ′ two-phase (Ni₆₆Al₃₄)_100-χX_χ(X=Ti, Si, Nb) depended on the A_s (austenite start) temperature of β-martensite with alloying elements. For A_s>250°C, a lamellar microstructure was found to form by the following phase transformation: Martensite→Ni₅Al₃→β+γ’. For A_s<250°C, two-type microstructures, mesh and Widmanstätten, were formed depending on the ternary element. When Ti or Nb was added as a ternary element, the β→Ni₅Al₃ transformation occurred very quickly. Conversely, this transformation proceeded very slowly in the case of Si addition, and the resultant microstructure assumed somewhat different features. Consequently, it could be suggested that the microstructures of NiAl/Ni₃Al two-phase alloys are determined by not only the A_s temperature but also by the β→Ni₅Al₃ transformation.     

Half-metallic full-Heusler compound Ti2NiAl: A first-principles study

Full-Heusler compound Ti₂NiAl with Hg₂CuTi-type structure is demonstrated to be a new half-meltal ferromagnet by first-principles calculations. The compound has a complete (100%) spin polarization around the Fermi level in the total density of state. The band structure calculations show that the majority spin is strongly metallic, while the minority spin shows an insulating behavior. The compound has a total magnetic moment of −3.0 μ_B per formula on first-principles calculations which complies well with the Slater-Pauling (SP) rule. Though having different atomic surroundings, the profiles of atom-projected density of states of Ti(A) and Ti(B) are similar. The half-metallic character is retained when the lattice constant ranging from −12.8% to +4.9%.     

Combining indentation and diffusion couple techniques for combinatorial discovery of high temperature shape memory alloys

We demonstrate the possibility of accelerated identification of potential compositions for high-temperature shape memory alloys (SMAs) through a combinatorial material synthesis and analysis approach, wherein we employ the combination of diffusion couple and indentation techniques. The former was utilized to generate smooth and compositionally graded inter-diffusion zones (IDZs) in the Ni–Ti–Pd ternary alloy system of varying IDZ thickness, depending on the annealing time at high temperature. The IDZs thus produced were then impressed with an indenter with a spherical tip so as to inscribe a predetermined indentation strain. Subsequent annealing of the indented samples at various elevated temperatures, T_a, ranging between 150 and 550 °C allows for partial to full relaxation of the strain imposed due to the shape memory effect. If T_a is above the austenite finish temperature, A_f, the relaxation will be complete. By measuring the depth recovery, which serves as a proxy for the shape recovery characteristic of the SMA, a three-dimensional map in the recovery–temperature–composition space is constructed. A comparison of the published A_f data for different compositions with the T_a data shows good agreement when the depth recovery is between 70% and 80%, indicating that the methodology proposed in this paper can be utilized for the identification of promising compositions. Advantages and further possibilities of this methodology are discussed.     

Investigation of Fe content in Cu–Al–Ni Shape Memory Alloys

Polycrystalline Cu–Al–Ni–Fe-based shape memory alloys with different chemical composition were produced in an arc-melting furnace under an argon atmosphere. Homogenized and aged specimens were prepared for multiple analyses. The temperatures of reversible martensitic transformations, namely A_s, A_f, M_s, M_f, A_max and ΔH enthalpy values were determined by a DSC device. The phase transition analysis from the room temperature to 850°C was undertaken by DTA. To characterize the lattice structure, an XRD analysis was conducted, the results of which were confirmed by microstructure images obtained from optical microscope observations.     

Microstructure and high-temperature shape-memory effect in Ni54Mn25Ga21 alloy

1 Introduction Shape memory alloys (SMAs) have developed rapidly in the past few decades as new functional materials, with commercial applications in pipe couplings, medical implants, electrical connectors and various actuators etc[1?3]. But the highest …     

Thermophysical properties of rare-earth stannates: Effect of pyrochlore structure

Compounds with an A₂B₂O₇-type pyrochlore structure have been identified as a class of materials with low thermal conductivity. To ascertain the effect of the pyrochlore structure, the thermophysical properties of rare-earth stannates which constitute the longest isostructural series with this unique crystal structure were measured and the thermal conduction behavior was investigated by analyzing the variation in the inverse phonon mean free path with temperature in terms of the phonon-scattering theory. The results show that the contraction of rare-earth ions and the accompanying deviation from the ideal pyrochlore structure result in a simultaneous increase in the Young’s modulus and the thermal expansion coefficient. Scattering due to the strain field fluctuation caused by the systemic displacement of 48f oxygen ions rather than scattering from the so-called 8a site “oxygen vacancies” and thermal defects predominates in reducing the phonon mean free path and consequently the thermal conductivity at relative low temperature. At a certain critical temperature, the thermal conduction behavior of the stannates undergoes a transition to a minimum phonon mean free path mechanism, which is responsible for the remarkable variation in temperature dependence of thermal conductivity across the compositions.     

Theoretical analysts of order–disorder transformation for binary alloys with stoichiometrical composition based on EAM potential

The order–disorder transformation for binary ordered intermetallics with stoichiometrical composition was analyzed using embedded atom method (EAM), The order–disorder behavior of AB ordered phases such as NiAl always exhibit a continuous transformation type. For A_xB_y (x≠y) type ordered phase such as A₃B, the order–disorder transformation may be in three different types: model I—first-order transformation at T_c, only; model II—transferring from continuous transformation to first-order transformation at T_c; model III—continuous transformation only. The order-disorder behaviors, of A₃B and B₃A compounds might be different despite same kind of ordered structure. the theoretical analysis has been applied to NiAl and Ni₃Al. The order–disorder behavior of Ni₃Al consists with the model I of first-order transformation at T_c. The theoretical prediction is consistent with the experimental results from references. In addition, the present calculation predicts the possibility of finding a thermodynamic, stable ordered structure with middle value of long range order (LRO) at 0 K. This is a kind of new ordered structure called “semi-ordered” structure.