Susceptibility of crystalline alloys to deformational amorphization during torsion under quasi-hydrostatic pressure

Features of the transition of Ni₅₀Ti₃₀Hf₂₀, Ti₅₀Ni₂₅, Zr₅₀Ni₁₈Ti₁₇Cu₁₅, and Fe₇₈B_8.5Si₉P_4.5 crystalline alloys with different susceptibilities to amorphization upon annealing and in the amorphous state during intense deformation in a Bridgman chamber are considered. The single- and two-phase crystalline states of the chosen alloys are obtained in different annealing modes. It is shown that the amorphizing rate of crystalline alloys differ substantially at the same degree of deformation; i.e., single-phase crystalline alloys based on titanium nickelide and iron amorphize well, while zirconium-based alloy amorphizes weakly in a manner similar to two-phase iron alloy. We believe that the LDA of crystalline alloys and their corresponding crystalline phases is determined by mechanical, thermodynamic, and concentration factors.     

Martensitic transformation in amorphous-crystalline Ti-Ni-Cu and Ti-Hf-Ni-Cu thin ribbons

Martensitic transformations and strain variation in amorphous-crystalline thin ribbons of Ti₅₀Ni₂₅Cu₂₅, Ti_40.7Hf_9.5Ni_44.8Cu₅ and Ti_40.7Hf_9.5Ni_39.8Cu₁₀ alloys were investigated. The amorphous-crystalline state in the samples was produced by an interruption of the isothermal crystallization in DSC apparatus. The volume fraction of the crystalline phase V_cryst in the samples was varied from 10 to 100%. Transformation temperatures in amorphous-crystalline thin ribbons of Ti₅₀Ni₂₅Cu₂₅ alloy depended only slightly on the crystalline volume fraction. On the contrary, the strong dependence of the martensitic transformation temperatures on the volume of crystalline phase was obtained in Ti_40.7Hf_9.5Ni_44.8Cu₅ and Ti_40.7Hf_9.5Ni_39.8Cu₁₀ alloys. Moreover, it was found that the intervals of the direct transformation are extremely narrow, whereas the intervals of the reverse transformation are unusually wide. As the crystalline volume fraction rises, both the direct transformation interval and the reverse one decrease, and the transformation temperatures increase. It was shown that increase of the Cu-content in Ti_40.7Hf_9.5Ni_{49.8 -x}Cu_x (x = 5, 10%) alloys results in decrease of the transformation temperatures, of the temperature interval of the reverse transformation on heating and of the hysteresis, while the temperature interval of the direct transformation on cooling widens     

Tendency of metallic crystals to amorphization in the process of severe (Megaplastic) deformation

The main features of the transition of crystalline Ni₅₀Ti₃₀Hf₂₀, Ti₅₀Ni₂₅Cu₂₅, Zr₅₀Ni₁₈Ti₁₇Cu₁₅, and Fe₇₈B_8.5Si₉P_4.5 alloys with various tendencies to amorphization into an amorphous state upon melt quenching and in the course of severe deformation in Bridgman anvils have been considered. The crystalline state of these alloys has been produced using various methods of annealing. In the iron-based alloy, single-phase and two-phase crystalline states have been studied. The nickel- and titanium-based alloys after annealing were in a single-phase crystalline state; the zirconium-based alloy, in a two-phase state. It is shown that at the same degree of deformation the rates of amorphization of crystalline alloys differ substantially; namely, the single-phase crystalline titanium- and iron-based alloys amorphize easily, whereas the Zr-based alloy amorphizes only poorly, just like the two-phase iron-based alloy. It can be assumed that the tendency to deformation-induced amorphization of crystalline alloys and the corresponding crystalline phases is mainly determined by three factors: mechanical, thermodynamic, and concentration-related.     

Formation and structure of nanocrystals in bulk Zr<Subscript>50</Subscript>Ti<Subscript>16</Subscript>Cu<Subscript>15</Subscript>Ni<Subscript>19</Subscript> metallic glass

The possible formation of a nanocrystalline structure in controlled crystallization of a bulk Zr₅₀Ti₁₆Cu₁₅Ni₁₉ amorphous alloy has been studied using differential scanning calorimetry, transmission and high-resolution electron microscopy, and x-ray diffraction. It was established that crystallization of the alloy at temperatures above the glass formation point occurs in two stages and brings about the formation of a nanocrystalline structure consisting of three phases. Local spectral x-ray analysis identified the composition and structure of the phases formed.     

Shape memory effect in Ti-Ni-Cu alloy caused by Joule heating

The properties of melt-spun Ti₅₀Ni₂₅Cu₂₅ alloy have been investigated by multiple thermocycling (up to 8000 cycles) within the martensitic transformation temperature range under constant mechanical stress from 5 to 200MPa, using electric current heating. A nearly linear relation is established between the alloy strain and electric resistance, which is independent of the applied stress and the number of thermal cycles.     

Experimental valence-band study of Ti(NiCu) alloys with different compositions and crystal structures

The density of valence-band electronic states of Ti(NiCu) alloys with different crystal structures and elemental compositions has been studied by X-ray photoelectron spectroscopy. It has been established that the change in the crystal state initiated by a martensitic transformation or a transition from the amorphous state to the crystal state does not affect the valence-band electronic state density distribution of the Ti₅₀Ni₅₀ and Ti₅₀Ni₂₅Cu₂₅ alloys. It has been shown that a change in the elemental composition leads to a noticeable redistribution of the electronic density in alloys of the Ti₅₀Ni_{50 ? x} Cu _x system (x = 0, 10, 15, 25, 30, 38, 50 at. %). As the copper concentration in the Ti(NiCu) alloys increases, the contribution of the Ni d states in the vicinity of the Fermi level decreases, with the d band of nickel shifting toward higher binding energies, and that of copper, toward lower binding energies.     

On the nature of the “double” yield point in Ti<Subscript>50</Subscript>Ni<Subscript>25</Subscript>Cu<Subscript>25</Subscript> alloy upon high-pressure torsion

The anomalous phenomenon of the “double” yield point attributed with the “crystal ? amorphous state” phase transition during high-pressure torsion of Ti₅₀Ni₂₅Cu₂₅ alloy has been analyzed experimentally and theoretically. Satisfactory correlation between experimental data and theoretical hypothesis has been found.     

Action of a pulsed electron beam on Cu-Ni foils and accompanying phenomena

Influence of the action of a pulsed electron beam in the millisecond range on the composition of surface layers, structure, topography, and mechanical properties of Cu₅₀Ni₅₀ and Cu₈₀Ni₂₀ foils depending on the energy’s and pulse’s current density is studied. The formation of concentration inhomogeneities in ultrathin near-surface layers is found as well as variations in the structure, mechanical properties, and topography of the foils depending on the alloy composition and irradiation parameters.     

Effect of the chemical composition on amorphization of titanium-nickelide-based alloys with fast neutrons

The structural state of a Ti₅₀Ni₄₇Fe₃ single crystal irradiated with fast neutrons (F = 2.5 × 10²⁰ cm^?2) at a temperature of 340 K was studied using thermal neutron diffraction. The alloy of this chemical composition was chosen in searching for a radiation-resistant shape memory material. It is established that this alloy retains its crystalline state after irradiation, whereas the Ti₄₉Ni₅₁ crystal studied previously is completely amorphized after similar irradiation. A detailed analysis of the structural state of the irradiated ternary alloy allowed us to discover the main physical causes of its radiation resistance.     

Nanostructured shape memory alloys of the TiNi-TiCu system

We study the shape memory Ti₅₀Ni₂₅Cu₂₅ (at %) alloy fabricated by melt spinning. The techno-logical parameters were optimized to obtain the alloy in the amorphous state. The dynamic crystallization of the alloy by a single electric pulse with durations of 1 to 100 ms was used to form the nanostructural state. Transmission electron microscopy and differential scanning calorimetry study show that reducing the pulse duration to 2 ms resulted in considerable refinement of the alloy with the formation of nanosized martensitic plates (20–60 nm). It was established that nanostructurization of the alloy can lead to an increase in the value of the recovery strain when the shape memory effect is manifested.     

Microstructure and shape memory behaviour of annealed Ti51.5Ni(48.5- x )Cu x ( x = 6.5–20.9) thin films

Ti-rich Ti–Ni–Cu amorphous films (Ti_51.9Ni_41.6Cu_6.5, Ti_51.6Ni_36.8Cu_11.6, Ti_51.5Ni_33.1Cu_15.4 and Ti_51.7Ni_27.4Cu_20.9), formed by sputtering, were annealed at 773, 873 and 973?K for 1?h and their structures and shape memory behaviours investigated. All the films annealed at 773?K for 1?h exhibited Guinier–Preston (GP) zones, but these precipitates were absent after annealing at 873?K or higher. Instead of GP zones, coherent plate precipitates of a Ti₂Cu phase were formed in films annealed at 873?K for 1?h, when the Cu content was between 11.6 to 20.9 at.%. The strain–temperature curves under constant stresses of Ti_51.6Ni_36.8Cu_11.6 and Ti_51.5Ni_33.1Cu_15.4 films showed a two-step deformation associated with the B2???B19???B19′ transformation, whereas Ti_51.9Ni_41.6Cu_6.5 and Ti_51.7Ni_27.4Cu_20.9 films showed a single-step deformation associated with the B2?B19′ and B2?B19 transformations, respectively. The two kinds of plate precipitates, GP zones and a Ti₂Cu phase were found to be effective to increase the critical stress for plastic strain.     

Crystallization behaviour in a new multicomponent Ti16.6Zr16.6Hf16.6Ni20Cu20Al10 metallic glass developed by the equiatomic substitution technique

A new amorphous Ti_16.6Zr_16.6Hf_16.6Ni₂₀Cu₂₀A1₁₀ alloy has been developed using the novel equiatomic substitution technique. Melt spinning Ti_16.6Zr_16.6Hf_16.6Ni₂₀Cu₂₀A1₁₀ forms an amorphous phase with a large supercooled liquid region, ΔT=70°C. After isothermal annealing within the supercooled liquid region for 3 h at 470°C, the amorphous alloy crystallizes to form a fine-scale distribution of 2–5 nm nanocrystals, and the supercooled liquid region increases to ΔT=108°C. Atomic-scale compositional analysis of this partially crystalline material using a three-dimensional atom probe (3DAP) is unable to detect any compositional difference between the nanocrystals and the remaining amorphous phase. After annealing for 1 hr at 620°C, the amorphous alloy crystallizes to form 20–50nm equiaxed grains of a hexagonal-type C14 Laves phase with lattice parameters a = 5.2Å and c = 9.0 Å. 3DAP analysis shows that this Laves phase has a composition very close to that of the initial amorphous phase, suggesting that the alloy crystallizes via a polymorphic rather than a primary crystallization mechanism, despite the complexity of the alloy composition.     

Microstructure of annealed Ti48.5Ni(51.5– x )Cu x (x = 6.2–33.5) thin films

(Ni, Cu)-rich Ti–Ni–Cu amorphous films with a Cu content of 6.2–33.5 at. % formed by sputtering were annealed at 773, 873 and 973 K for 1 h and their microstructures investigated. Two types of precipitate were observed in the annealed films: a Ti(NiCu)₂ phase for the Ti_48.5Ni₄₀Cu_11.5, Ti_48.6Ni_35.9Cu_15.5, Ti_48.3Ni_28.4Cu_23.3 and Ti_48.3Ni_23.9Cu_27.8 films, plus a TiCu phase for the Ti_48.5Ni₁₈Cu_33.5 films. These precipitates were found to have coherency with the B2 matrix in the films annealed at 773 K and were densely distributed within the grains. However, in the films annealed at 873 K, their size increased 10-fold and their density decreased. Annealing at 973 K promoted grain-boundary precipitation and, accordingly, the density of the precipitates in the grain interiors decreased. On the other hand, the annealed Ti_48.9Ni_44.9Cu_6.2 films showed no precipitates in their grain interiors, but the number of grain-boundary precipitates increased with increasing annealing temperature. It was also found that grain size decreased with increasing Cu content and was significantly decreased for the Ti_48.5Ni₁₈Cu_33.5 films.     

Effect of alloying on the structure and phase transformations in Ni-Mn alloys alloyed by titanium

The crystal structure of NiMn alloy alloyed by titanium in a wide range of temperatures and compositions has been investigated using resistivity measurements, transmission electron microscopy, electron diffraction, and X ray diffraction. It is found that alloying by titanium not only decreases the martensitic transformation temperature but also changes the martensite crystal structure. The martensitic transformation temperatures are determined and the diagram of martensitic transformations for Ni₅₀Mn_{50 ? x} Ti _x alloys is constructed.     

Crystal structure and physical properties of magnetic shape memory alloys Ni50 − x Cu x Mn29Ga21

The phase composition, crystal structure, and physical properties (magnetization, electrical resistivity, thermoelectric power, relative elongation, and thermal expansion coefficient) of the stoichiometric alloy Ni₅₀Mn₂₅Ga₂₅ and nonstoichiometric alloys Ni_{50 ? x} Cu _x Mn₂₉Ga₂₁ (x = 0, 1, 2) with the thermoelastic martensitic transformation have been investigated. The influence of the chemical composition on the transformations and physical properties of the alloys has been determined.     

Change in the phase composition of Ti-Ni binary alloys in a nonuniform magnetic field

The effect of long-term exposure of titanium nickelide in a dc nonuniform magnetic field on the phase transformation in it is studied. The magnetic field is shown to change the phase composition of an equiatomic Ti-Ni alloy and to substantially change its magnetic properties. This effect can be related to the appearance of magnetic Ti₂Ni and Ni₃Ti phases, which result from local deformation of the crystal lattice of TiNi.     

Effects of nickel on the crystallization of Zr70Cu20Ni10 amorphous alloy

Differential scanning calorimetry (DSC) and x-ray diffraction (XRD) are employed to investigate the effects of nickel on the crystallization of the amorphous Zr₇₀Cu₂₀Ni₁₀ alloy. We have found that the crystallization process of the amorphous Zr₇₀Cu₂₀Ni₁₀ alloy is strongly influenced by the addition of nickel. Addition of 10 at% Ni to the Zr₇₀Cu₃₀ amorphous alloy makes the crystallization process proceed from a single-stage mode to a double-stage mode. The activation energy for crystallization of the amorphous Zr₇₀Cu₂₀Ni₁₀ alloy is calculated to be about 388kJ·mol^-1 on the basis of the Kissinger equation. The effects of nickel on the crystallization of the amorphous Zr₇₀Cu₂₀Ni₁₀ alloy are discussed in terms of the genetics of metals.     

Research on Zr50Al15−xNi10Cu25Yx amorphous alloys prepared by mechanical alloying with commercial pure element powders

Amorphous Zr₅₀Al_15−xNi₁₀Cu₂₅Y_x alloy powders were fabricated by mechanical alloying at low vacuum with commercial pure element powders. The effects on glass forming ability of Al partial substituted by Y in Zr₅₀Al₁₅Ni₁₀Cu₂₅ and thermal stability of Si₃N₄ powders addition were investigated. The as-milled powders were characterized by X-ray diffraction, scanning electron microscopy and differential scanning calorimeter. The results show that partial substitution of Al can improve the glass forming ability of Zr₅₀Al₁₅Ni₁₀Cu₂₅ alloy. Minor Si₃N₄ additions raise the crystallization activation energy of the amorphous phase and thus improve its thermal stability.     

Formation of nanostructured states in ternary TiNiFe-based shape memory alloys during megaplastic deformation and subsequent heat treatment

The ternary metastable TiNiFe alloys that exhibit a low-temperature shape memory effect and are subjected to plastic deformation by rolling or high-pressure torsion followed by heat treatment are studied by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and electrical resistivity measurements. It is found that moderate plastic deformation of a Ti₅₀Ni₄₉Fe₁ alloy at room temperature initiates the thermoelastic B2 ? B19’ martensitic transformation and the formation of a developed banded dislocation and twin substructure in the B19’ martensite. This deformation of a Ti₅₀Ni₄₇Fe₃ alloy forms a similar dislocation substructure but in B2 austenite. Megaplastic deformation by high-pressure torsion causes amorphization in the Ti₅₀Ni₄₉Fe₁ alloy and nanofragmentation in the Ti₅₀Ni₄₇Fe₃ alloy. The evolution of the nanostructure and the martensitic transformations in TiNiFe-based ternary alloys is studied during plastic deformation and subsequent annealing at various temperatures.     

On segregation manifestations of the long-range effect during the ion implantation of rolled copper-nickel foils

Segregation manifestations of the long-range effect during the irradiation of rolled nonequilibrium Cu₈₀Ni₂₀ alloy foils with B⁺ ions are studied. The composition separation of Cu and Ni in the surface layers of the unirradiated side of the foil is revealed. This separation leads to the formation of the nonmonotonic concentration profiles of Ni and Cu. A phenomenological model for the formation of the chemical composition of the surface layers on the unirradiated side of the foil is proposed. It is based on the assumption of the simultaneous effects of the generation of elastic waves and changes in local mechanical stresses.