Influence of the structure of the Ti-49.8 at % Ni alloy on its thermal expansion during the martensitic phase transformation

The temperature dependence of thermal expansion of the Ti-49.8 at % Ni alloy has been measured after rolling at temperatures of 470, 570, 670, 770, and 870 K. The maximum dilatation jump during the martensitic phase transition has been observed for the samples rolled at 570 and 670 K. A fragmented structure, in which the phase transformations are hampered, is formed during the low-temperature rolling. An increase in the rolling temperature to 770–870 K leads to the return processes and dynamic recrystallization of the material; as a result, the slope of dilatation curves changes and the range of phase transitions narrows.     

The kinetics of vacancy annihilation and short range order formation in Ni 11.4 at % Cr

In Ni-Cr alloys annealing after quenching from high temperatures leads to an increase of electrical resistivity which is attributed to a short range order state often denoted as K-state. Here the kinetics of the formation of the K-state in a Ni 10% Cr alloy is investigated by resistivity measurements for different quenching and annealing temperatures. The measurements are analyzed by an extended Schulze-Lücke method [22] which assumes that the rate of resistivity change is proportional to the vacancy concentration and that this rate as well as the rate of vacancy annihilation is described by a chemical rate equation.This analysis allowed a quantitative determination of the parameters determining the kinetics of short range order formation under different vacancy concentrations as well as the kinetics of annealing out of the quenched-in surplus vacancies. Among other quantities this treatment yielded the activation energies for vacancy formation H_F = 1.16 eV, for vacancy migration H_M = 1.56 eV and, as an independent cross check, for self diffusion H_D = 2.73 eV in good agreement with H_F + H_M = 2.72 eV.     

The effect of deformation on the superconducting transition temperature of a niobium-32% zirconium alloy

An increase of T_c by up to 0.5 with a 93 per cent area reduction of a Nb-32%Zr alloy is reported. Various possibilities for explaining this phenomenon are discussed.     

Mechanism for the separation of the Ni3Ti phase in the alloy Ni + 16.3% Cr + 5.8% Ti

A metallographic study has been made of the discontinuous separation of the -phase, and many cases of a branching of its laminae have been found. The effect of the quenching temperature on the discontinuous -phase separation has been studied. Measurements have been made of the microhardness of regions with continuous and discontinuous separation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, Vol. 12, No. 5, pp. 90–93, May, 1969.     

Accelerated formation of an ultrafine-grained structure in a two-phase Ti alloy during compression with decreasing temperatures

ABSTRACT

Ultrafine-grained (UFG) structure is beneficial for overcoming the strength-ductility trade-off and enhancing the superplasticity of two-phase Ti alloys. Recently, it has been demonstrated that compression with decreasing temperatures is effective for producing UFG two-phase Ti alloys initially with lamellar microstructures. However, the effect of lamellar thickness on the microstructural evolution during this process has not yet been fully elucidated. In this study, Ti-6Al-4V alloys with different lamellar thicknesses were compressed while the processing temperature was decreased from 800°C to 600°C. The thinner lamellar microstructure was preferable for preventing void/crack formation, while accelerating the continuous dynamic recrystallisation, thus providing a fully UFG structure at a relatively low strain of 1.4. In addition, the origin of different plastic flows in each sample was analysed in detail by analysing the microstructural evolution. These findings demonstrate that the processing method is effective for reducing the grain size of a two-phase Ti alloy without severe plastic deformation techniques, which require large strain (≥4). A reduction in the strain required to achieve the UFG structure would be beneficial because conventional metal-forming processes, i.e. rolling, extrusion, or forging, which are suitable for mass production, could be used.     

Critical point of martensitic transformation under stress in an Fe-31.2Pd (at.%) shape memory alloy

We have examined the transformation strain, Δε, in the [0 0 1] direction of an Fe-31.2Pd (at.%) shape memory alloy under compressive stress applied in the same direction. When the stress is absent, the alloy exhibits a cubic to tetragonal martensitic transformation at 230 K with |Δε| of 1.4%. As the stress increases, the transformation temperature increases linearly and |Δε| decreases linearly and vanishes at 40 MPa (280 K). This point is the critical point of this transformation at which the first-order nature disappears, and the critical exponent β is evaluated to be 0.47?±?0.04.     

The thermoelectric power of Ni and Ni−Cu alloy near the Curie temperature measured by an ac method

An ac technique for the measurement of the thermoelectric power is introduced. This technique has the advantage, that it can be applied parallel with the heat capacity measurement on the same sample. The principle of the method and some technical details are presented. The method was used to measure the thermoelectric power (and specific heat) of single crystal Ni and Ni-1.5 w% Cu alloy near their Curie temperatures. The experimental data are compared with the resistive anomaly and specific heat and it is concluded that the behavior of the Seebeck coefficient is very similar to the resistive anomaly around the critical temperature.This work was carried out partly at the Materials Research Laboratory, Physics Department, University of Illinois, and supported in part by the National Science Foundation under Grant GH-33634     

Helium bubble and blister formation for Ni and an amorphous Fe-Ni-Mo-B alloy during 5 keV implantation at 300 K

Results are presented for helium bubble formation in Ni and amorphous Fe₄₀Ni₃₈Mo₄B_l8 during 5 keV He-implantation at 300 K. The formation of helium blisters on the surface is also reported.     

Dilatation anisotropy upon the phase transition in the rolled Ti-49.8% Ni alloy

The temperature dependence of the thermal expansion of the Ti-49.8% Ni alloy rolled at 773 K has been measured in three mutually perpendicular directions with respect to the rolling direction. It has been found that, upon phase transition of the martensitic type, the sample is elongated stepwise in two directions, and it is contracted in the third direction. The observed effect is due to the crystallographic texture. The dilatation jump during the phase transformation is determined by the joint action of the change in lattice parameters and the process of twinning.     

Formation and structure of nanocrystals in an Al86Ni11Yb3 alloy

The formation and structure of the nanocrystalline phase in the Al₈₆Ni₁₁Yb₃ alloy are investigated using differential scanning calorimetry (DSC), transmission electron and high-resolution electron microscopy, and x-ray diffraction. The nanocrystalline phase is formed upon controlled crystallization of the amorphous alloy prepared by quenching of the melt on a rapidly moving substrate. It is revealed that the nanocrystalline alloy consists of aluminum nanocrystals (5–12 nm in size) randomly distributed in the amorphous matrix. The maximum fraction of the nanocrystalline phase does not exceed 25%. The nanocrystal size substantially increases at the initial stage of isothermal treatment (at 473 K) and then changes insignificantly. It is found that nanocrystals are usually free of defects. However, some nanocrystals have a more complex microstructure with twins and dislocations. The size distributions of nanocrystals are determined at several durations of isothermal treatment. It is demonstrated that the nucleation of nanocrystals predominantly occurs through the heterogeneous mechanism. The experimental distributions are compared with those obtained from a computer simulation. The activation energy of crystallization, the time-lag, and the coefficient of ytterbium diffusion in the alloy are estimated     

Effect of temperature on the dislocation structure of the ordered alloy Ni3(Fe,Cr)

Transmission electron microscopy is used to study the dislocation structure of ordered polycrystalline Ni₃(Fe, Cr) alloy after deformation at temperatures of 293, 473, 673, and 773°K. The flow stress σ is plotted as a function of the density of dislocations ρ. It is observed that there is a direct proportionality between σ and ρ^1/2, which indicates that the relation τ= αGbρ^1/2 is satisfied, where τ is the shear stress, G is the shear modulus, b is the Burgers vector, and α is a coefficient weakly dependent on the density of dislocations. The values of α are found for different deformation temperatures from the slope of the lines. It is found that α decreases with increasing deformation temperature. When the temperature is increased from 293 to 773°K the reduction in α is about 20% in agreement with estimates of the resistance to motion by superdislocations caused by nonconservative drag of dislocation jogs.     

Change in electrical resistivity of an Al-4% Cu alloy during plastic deformation

The paper reports the results of measuring the change in electrical resistivity of an Al-4% Cu alloy containing Guinier-Preston zones during plastic extension at various temperatures and strain rates. The deformation temperature has a marked influence on the nature of the observed effect. The results obtained are discussed on the assumption that there are two different mechanisms by which plastic deformation affects the resistivity of the alloy: a diffusionless mechanism (subdivision of the zones as a result of their being cut by moving dislocations) and a diffusion mechanism (supplementary decomposition and a redistribution of the zones with respect to the volume of the alloy and the zone size).     

Change in the crystallographic structure of grain boundaries in an order-disorder phase transition in Ni3Fe alloy

Electron diffraction microscopy and metallography are used to investigate the crystallographic structure of grain boundaries in Ni₃Fe alloy with short-range and long-range atomic order. It is found that the fraction of special boundaries in alloys with short-range and long-range order is 0.3–0.4. Heat treatment, which leads to ordering, causes a reorientation of some of the grains with boundaries of a general type, boundary migration, and also faceting of some of the boundaries of general type.Tomsk Engineering-Construction Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 3–10, July, 1992.     

Sequence of phase formation during solid-state synthesis in Al/Ni films (Al: Ni = 60: 40 at %)

The structure formed during solid-state synthesis in thin bilayer Al/Ni films with the ratio Al: Ni = 60: 40 (at %) has been investigated. The films were obtained by thermal evaporation in vacuum with a residual pressure of 10^?5–10^?6 Torr. Solid-state synthesis was performed by diffusion reaction. The sequence of phase formation upon vacuum annealing of bilayer Al/Ni films has been established: Al + Ni → Al₃Ni + Ni (T _ann = 180°C) → Al₃Ni₂ (T _ann = 220°C).     

In situ phase changes in a Fe-40 at.% Al ordered alloy during ion implantation: T.E.M. study

The radiation damage and disordering phenomena produced at room temperature during Fe⁺, Bi⁺⁺ and Xe⁺ ion implantations in a Fe-40 at.% A1 ordered alloy were studied by in situ transmission electron microscopy. Progressive, but not total disordering was achieved in the thinnest areas of the specimens during high fluence implantations (> 10¹⁵ ions/cm²). Simultaneously, the athermal formation of a new phase occurs in the case of Fe⁺ and Bi⁺⁺ implantations. The diffusion-less mechanism suggests a martensitic like transformation.

During post-implantation annealing, reordering and long-range order domain growth occurred in the temperature range up to 570 K. Above 670 K another phase appeared, which was stable up to 1070 K. The results are discussed and compared with those obtained on melt-quenched or neutron irradiated specimens of the same alloy.     

Investigating aspects of the formation of structure in FePd alloy upon ordering

The Mössbauer effect is used to study changes in the structure of Fe₅₀Pd₅₀ alloy in the course of annealing for ordering at T = 450°C from different initial states: cast and quenched from 950°C, then subjected to severe plastic deformation by shear under pressure, and that obtained by fast quenching from the melt. Differences in the kinetics of phase transformations are observed depending on the initial state of the material.     

Stress‐induced Curie temperature increase in the Fe64Ni36 invar alloy

Structural and magnetic changes on invar Fe₆₄Ni₃₆ alloy (T_C = 500 K) produced by mechanical milling followed by heating up to 1073 K, were investigated by neutron diffraction, magnetization measurements, X‐ray diffraction under high pressures and X‐ray absorption at both Fe and Ni K‐edges. We argue that the strain induced in the Fe₆₄Ni₃₆ material after this treatment mainly affects the Fe sites due to the magnetovolume coupling, the most notorious feature being the increase of the Curie temperature (ΔT_C = 70 K). (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Change in the critical exponent of magnetization in maghemite in the temperature range of the structural phase transition

The magnetization of maghemite and magnetite thin films is measured as a function of the temperature and orientation of the film. It is established that the temperature dependence of the spontaneous magnetization deviates from the Bloch law at low temperatures and is adequately described by the linear function M/M _s = 2.2(1-T/T _C) below the Curie temperature T _C. The linear temperature dependence of the magnetization below the Curie temperature is explained by the change in the spin of iron ions in tetrahedral positions due to local deformations of the crystal lattice.