Gradient changes in structural condition of the B2 phase of NiTi surface layers after electron-beam treatments

Structural conditions of the B2 phase of the Ti[49.5]Ni50.5] alloy surface layers before and after electron-beam treatments (pulse duration [tau]=150 [mu]s, number of pulses n=5, beam energy density E<=20 J/cm{2}) were studied by X-ray diffraction analysis. Analysis of the X-ray patternsdemonstrates that surface layers modified by electron beam treatment contain phase with B2{surf} structure. It is revealed that the lattice parameter of the B2{surf} phase in the surface (modified) layer is also higher than the lattice parameter of the B2 phase in the underlying layer ( a[B2]=3.0159+=0.0005 ). The values of lattice parameter of phase B2{surf} amounted a{surf}[B2]=3.0316+=0.0005 Å and a{surf}[B2]=3.0252+=0.0005 Å, for the specimens after electron-beam treatment at E[1]=15 J/cm{2} and E[2] =20 J/cm{2}, respectively. Inflated lattice parameters a{surf}[B2] are associated with changes in the chemical composition and the presence of residual stresses in the surface region of the samples after electron-beam treatments

Phase composition in NiTi near-surface layers after electron beam treatment and its variation depending on beam energy density

In the work, we study the mechanisms of structural phase state formation in NiTi surface layers after low-energy pulsed electron beam irradiation depending on the electron beam energy density. It is revealed that after electron beam treatment of the NiTi specimens at energy densities E[1]=15 J/cm{2}, E[2]=20 J/cm{2}, and E[3]=30 J/cm{2}, a series of effects is observed: the absence of the Ti[2]Ni phase and the presence of new peaks correspond to the B19 martensite phase with monoclinic structure. Estimation of the relative volume content of the B2 and B19 phases from the total intensity of their peaks shows that the percentage of the martensite phase increases from ~5 vol.% in the NiTi specimen irradiated at E[1]=15 J/cm{2} to ~80 vol.% in the NiTi specimen irradiated at E[3]=30 J/cm{2}. It is found that in the NiTi specimens irradiated at E<=20 J/cm{2}, the layer that contains a martensite phase resides not on the surface but at some depth from it

Phase composition in NiTi near-surface layers after electron beam treatment and its variation depending on beam energy density

In the work, we study the mechanisms of structural phase state formation in NiTi surface layers after low-energy pulsed electron beam irradiation depending on the electron beam energy density. It is revealed that after electron beam treatment of the NiTi specimens at energy densities E[1]=15 J/cm{2}, E[2]=20 J/cm{2}, and E[3]=30 J/cm{2}, a series of effects is observed: the absence of the Ti[2]Ni phase and the presence of new peaks correspond to the B19 martensite phase with monoclinic structure. Estimation of the relative volume content of the B2 and B19 phases from the total intensity of their peaks shows that the percentage of the martensite phase increases from ~5 vol.% in the NiTi specimen irradiated at E[1]=15 J/cm{2} to ~80 vol.% in the NiTi specimen irradiated at E[3]=30 J/cm{2}. It is found that in the NiTi specimens irradiated at E<=20 J/cm{2}, the layer that contains a martensite phase resides not on the surface but at some depth from it

Influence of surface modification of nitinol with silicon using plasma-immersion ion implantation on the alloy corrosion resistance in artificial physiological solutions

Cyclic voltammetry and potentiostatic polarization have been applied to study electrochemical behavior and to determine corrosion resistance of nitinol, which surface was modified with silicon using plasma-immersion ion implantation, in 0.9% NaCl solution and in artificial blood plasma. It was found out that continuous, and also homogeneous in composition, thin Si-containing layers are resistant to corrosion damage at high positive potentials in artificial physiological solutions due to formation of stable passive films. Breakdown potential Eb of Si-modified NiTi depends on the character of silicon and Ni distribution at the alloy surface, Eb values amounted to 0.9–1.5 V (Ag/AgCl/KCl sat.) for the alloy samples with continuous Si-containing surface layers and with decreased Ni surface concentration

The Effect of Hydrogen on Martensite Transformations and the State of Hydrogen Atoms in Binary TiNi-Based Alloy with Different Grain Sizes

The analysis presented here shows that in B2-phase of Ti49.1Ni50.9 (at%) alloy, hydrogenation with further aging at room temperature decreases the temperatures of martensite transformations and then causes their suppression, due to hydrogen diffusion from the surface layer of specimens deep into its bulk. When hydrogen is charged, it first suppresses the transformations B2↔B19′ and R↔B19′ in the surface layer, and when its distribution over the volume becomes uniform, such transformations are suppressed throughout the material. The kinetics of hydrogen redistribution is determined by the hydrogen diffusion coefficient DH, which depends on the grain size. In nanocrystalline Ti49.1Ni50.9 (at%) specimens, DH is three times greater than its value in coarse-grained ones, which is likely due to the larger free volume and larger contribution of hydrogen diffusion along grain boundaries in the nanocrystalline material. According to thermal desorption spectroscopy, two states of hydrogen atoms with low and high activation energies of desorption exist in freshly hydrogenated Ti49.1Ni50.9 (at%) alloy irrespective of the grain size. On aging at room temperature, the low-energy states disappear entirely. Estimates by the Kissinger method are presented for the binding energy of hydrogen in the two states, and the nature of these states in binary hydrogenated TiNi-based alloys is discussed

Structural phase states in nickel-titanium surface layers doped with silicon by plasma immersion ion implantation

The paper reports on a study of NiTi-based alloys used for manufacturing self-expanding intravascular stents to elucidate how the technological modes of plasma immersion ion implantation with silicon influence the chemical an

Plasma immersion ion implantation for surface treatment of complex branched structures

The paper presents experimental results demonstrating the capabilities of plasma immersion ion implantation of silicon (Si) for surface treatment of complex branched structures such are self-expanding intravascular nickel-titanium (NiTi) stents. Using NiTi stents of diameter 4 and 8 mm, it is shown that plasma immersion ion implantation can provide rather homogeneous doping of their outer and inner surfaces with Si atoms. Also presented are research data on the processes that determine the thickness, composition, and structure of surface layers subjected to this type of treatment

Structural Defects in TiNi-Based Alloys after Warm ECAP

The microstructure, martensitic transformations and crystal structure defects in the Ti50Ni47.3Fe2.7 (at%) alloy after equal-channel angular pressing (ECAP, angle 90°, route BC, 1–3 passes at T = 723 K) have been investigated. A homogeneous submicrocrystalline (SMC) structure (grains/subgrains about 300 nm) is observed after 3 ECAP passes. Crystal structure defects in the Ti49.4Ni50.6 (at%) alloy (8 ECAP passes, angle 120°, BC route, T = 723 K, grains/subgrains about 300 nm) and Ti50Ni47.3Fe2.7 (at%) alloy with SMC B2 structures after ECAP were studied by positron lifetime spectroscopy at the room temperature. The single component with the positron lifetime t1 = 132 ps and t1 = 140 ps were observed for positron lifetime spectra (PLS) obtained from ternary and binary, correspondingly, annealed alloys with coarse-grained structures. This t1 values correspond to the lifetime of delocalized positrons in defect-free B2 phase. The two component PLS were found for all samples exposed by ECAP. The component with t2 = 160 ps (annihilation of positrons trapped by dislocations) is observed for all samples after 1–8 ECAP passes. The component with t3 = 305 ps (annihilation of positrons trapped by vacancy nanoclusters) was detected only after the first ECAP pass. The component with t3 = 200 ps (annihilation of positrons trapped by vacancies in the Ti sublattice of B2 structure) is observed for all samples after 3–8 ECAP passes

Crystal Structure Defects in Titanium Nickelide after Abc Pressing at Lowered Temperature

The experimental results regarding the effect of warm (573 K) abc pressing with an increase in the specified true strain, e, up to 9.55, on the microstructure and crystal structure defects (dislocations, vacancies) of the Ti49.8Ni50.2 (at %) alloy are presented. It is shown that all samples (regardless of e) have a two-level microstructure. The grains-subgrains of the submicrocrystalline scale level are in the volumes of large grains. The average sizes of both large grains and subgrain grains decrease with increasing e to 9.55 (from 27 to 12 µm and from 0.36 to 0.13 µm, respectively). All samples had a two-phase state (rhombohedral R and monoclinic B19′ martensitic phases) at 295 K. The full-profile analysis of X-ray reflections of the B2 phase obtained at 393 K shows that the dislocation density increases from 1014 m−2 to 1015 m−2 after pressing with e = 1.84 and reaches 2·1015 m−2 when e increases to 9.55. It has been established by positron annihilation lifetime spectroscopy that dislocations are the main type of defects in initial samples and the only type of defects in samples after abc pressing. The lifetime of positrons trapped by dislocations is 166 ps, and the intensity of this component increases from 83% in the initial samples to 99.4% after pressing with e = 9.55. The initial samples contain a component with a positron lifetime of 192 ps (intensity 16.4%), which corresponds to the presence of monovacancies in the nickel sublattice of the B2 phase (concentration ≈10−5). This component is absent in the positron lifetime spectra in the samples after pressing. The results of the analysis of the Doppler broadening spectroscopy correlate with the data obtained by the positron annihilation lifetime spectroscopy

Simulation of benzylpenicillin molecule distribution in slit-shaped Si nanopores

A molecular dynamics study of the behavior of benzylpenicillin molecules in slit-shaped nanopores was carried out. A model silicon material with a pore size from 10 to 50 nm was chosen as a nanoporous structure. The interaction between benzylpenicillin molecules was described by a pair potential, built on the basis of modelling the molecule behavior by all-atom force fields. It was shown that an adsorbed layer of benzylpenicillin molecules is formed near the pore walls. With a decrease in the pore size, the maximum density of molecules in the adsorbed layer decreases, while the fraction of adsorbed molecules in the whole pore increases