Microtexture and microstructure evolution during necking in a Zr-1% Nb alloy

The evolution of a microstructure and Schmid factors during tension under necking conditions are analyzed by electron backscatter diffraction. The oscillatory instability of the plastic flow in the deformation zone in the hardening-softening mode is found to be related to geometrical hardening (softening) during the reorientation of easy slip planes in grains with respect to the loading axis. A relation between the activities of various slip systems and the evolution of quantitative microstructure parameters is revealed.     

Plastic strain localization and necking in Zr-Nb alloy

Plastic strain macrolocalization at the parabolic stage of strain hardening in Zr-1 wt% Nb alloy is studied. The plastic flow is found to be unstable, which shows up as the oscillatory periodic space-time variation of the local strain distribution. The results are discussed in terms of a synergetic model of plastic flow evolution.     

Plastic behaviour of Fe-3% Si alloy single crystals

Fe-3% Si alloy single crystals of single slip and double slip orientation have been tested in tension at temperatures between 113 K and 473 K. The stress-strain curves exhibit a yield drop followed by inhomogeneous yielding (yield propagation stage), a parabolic and an approximately linear parts. The slip line observations carried out in various stages of deformation show that only at the beginning of the parabolic part of stress-strain curve the specimen is completely filled out with slip bands and the deformation becomes relatively homogeneous. With both single slip and double slip orientations deformation takes place predominantly in one slip system only. At higher temperatures ageing occurs during deformation.Na Slovance 2, Praha 8, Czechoslovakia.The authors would like to thank Dr. B. esták for suggestion of the problem and for his interest in discussion of the results. We are also grateful to Mr. J. Poucha for help with some experimental work and to Miss G. Výborná for help with specimen preparation. To Dr. S. Libovický and Dr. F. Kroupa we thank for helpful discussions.     

Plastic deformation macrolocalization during serrated creep of an aluminum-magnesium Al-6 wt % Mg alloy

The nonlinear dynamics of the space-time structure of macrolocalized deformation is studied by a set of high-speed in situ methods under the conditions of serrated creep in an aluminum-magnesium Al-6 wt % Mg alloy at room temperature. Macroscopic deformation jumps with an amplitude of several percent are detected in the creep curve of this alloy. It is found that a complex space-time structure of macrolocalized deformation bands moving in a correlated manner forms spontaneously in the material during the development of a deformation jump. The difference between the observed picture of deformation bands and the well-known Portevin-Le Chatelier classification of deformation bands is discussed.     

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.     

Influence of hydrogen on the properties of Zr-1%Nb alloy modified by a pulsed electron beam

The influence of hydrogen and a pulsed electron beam on the properties of Zr-1%Nb alloy is studied. It is shown that electron beam processing results in the formation of martensite with an intricate morphology in the subsurface layer of the alloy. It is found that the amount of hydrogen penetrating into the modified alloy is twice as low as in the as-prepared material. In addition, the modified alloy is more stable against the hydrogen action.     

Unstable plastic flow in the Al-3% Mg alloy in the process of continuous nanoindentation

Methods of dynamic nanoindentation were used to study unstable modes of plastic flow in micro-and submicrovolumes of the Al-3% Mg alloy. It was established that, depending on the rate of loading and dimensions of the deformed region, various regimes of unstable plastic deformation are realized. In the course of deformation, the irregular deformation curve (corresponding to a random process) reveals a quasi-periodic behavior with a characteristic amplitude of hardness oscillations.     

Study of rapid stress annealed nano-crystalline Fe74.5Cu1Nb3Si15.5B6 alloy

Rapid stress annealing induced changes in structural and magnetic properties in Fe_74.5Cu₁Nb₃Si_15.5B₆ are reported. Obtained results suggest changes in spin texture with preferred orientation along ribbon axis. Fraction of A site in the DO₃ lattice occupied by Si atoms, increases, with increase of applied stress during annealing. Volume fraction of the nanograins up to 60% (exhibiting quite similar mean grain diameter ～9 nm) is observed. Lattice parameter values suggest that Si content in the nanocrystalline phase is between 14% and 19% and increase of lattice parameter suggests the elongation of the unit cell. Studied stress annealed samples exhibit soft magnetic behavior (coercive field ranging between 4 and 8 Am^???1). Stress annealing reduces permeability whereas anisotropy field increases almost linearly exhibiting the induction of uniaxial and perpendicular to the ribbon axis anisotropy. Obtained stress-induced-anisotropy constant values range between 50 and 2,140 Jm^???3.     

Investigation of radiation-induced mixed interstitial configurations in a dilute nickel 1 at.% silicon alloy

Abstract

X-ray energy spectra induced by 1 MeV protons and the energy spectra of the backscattered protons have been examined in the <100> and <110> directions of Ni crystals containing 1 at.% Si. The channeling method was used to investigate interactions between the radiation-induced defects and the silicon atoms. From the measured minimum yields and from the shape of the angular scans the fraction of silicon atoms is determined, which are displaced into the <100> and <110> channels due to proton or helium irradiations and subsequent annealing treatments. In undamaged crystals about 98 % of the silicon atoms are on normal lattice sites. After irradiation a dose dependent fraction of the silicon atoms is displaced 0.05 nm away from the substitutional position indicating the formation of a mixed dumbbell consisting of one selfinterstitial atom and one silicon atom. However, the experimental data can also be interpreted by the assumption of a NiSi₂ complex, in which the silicon atoms are displaced 0.08 nm from the lattice site. Subsequent annealing from 50 K to 160 K does not change the configuration and the concentration of the silicon complexes. At room temperature the silicon atoms in the complex are positioned 0.04 nm from the lattice position. The silicon complexes were totally am ihilated at 400 K.     

Temperature dependence of amorphous and interface phases in the Fe80Nb7Cu1B12 nanocrystalline alloy

Temperature measurements (77–625 K) of Fe₈₀Nb₇Cu₁B₁₂ nanocrystalline alloy prepared from amorphous precursor annealed for 1 h at 470^°C and 620^°C are presented. Structural and magnetic behaviours of the crystalline phase, the amorphous residual matrix, and the interface zone between crystalline grains and the amorphous phase are studied by distributions of hyperfine magnetic fields. Magnetic regions are developing in the retained amorphous phase with rising temperature of annealing. They can be suppressed, however, at high enough measuring temperatures turning the amorphous matrix into paramagnetic state. As a consequence, the respective spectral components do not interfere so much and the role of interface zone can be studied. This revised version was published online in July 2006 with corrections to the Cover Date.     

Kinetics of localized plastic flow during deformation and fracture of a D1 alloy

The stress-strain curve of a polycrystalline duralumine (D1) is studied to find three basic deformation stages: linear hardening, parabolic hardening (n = 1/2), and prefracture (n < 1/2). The results obtained show special features of macrolocalization of the plastic flow of the alloy under review. The distribution patterns of localized plastic flow domains develop according to deformation stages. The prefracture stage is characterized by self-correlated motion of the domains to the point of subsequent fracture. It follows from an analysis of the plastic flow localization kinetics that both hardening and softening domains coexist in the specimen in the prefracture stage. The domains move with a constant velocity inherent to each of them and linearly dependent on the position of their nucleation point. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 68–73, November, 2007.     

Polarization rotation via a monoclinic phase in the piezoelectric 92% PbZn(1/3)Nb(2/3)O3-8% PbTiO3

The origin of ultrahigh piezoelectricity in the relaxor ferroelectric PbZn(1/3)Nb(2/3)O3-PbTiO3 was studied with an electric field applied along the [001] direction. The zero-field rhombohedral R phase starts to follow the direct polarization path to tetragonal symmetry via an intermediate monoclinic M phase, but then jumps irreversibly to an alternate path involving a different type of monoclinic distortion. Details of the structure and domain configuration of this novel phase are described. This result suggests that there is a nearby R-M phase boundary as found in the Pb(Ti,Zr)O3 system.     

Influence of structurization of amorphous Fe73.5Si13.5B9Nb3Cu1 alloy on its spectral properties

The optical properties of Fe_73.5Si_13.5B₉Nb₃Cu₁ ferromagnetic alloy in the amorphous, nanocrystalline, and crystalline states have been investigated by optical ellipsometry in the spectral range of 0.22–18 μm. It is established that the crystallization of alloy during heat treatment leads to a significant change in its optical constants, as well as the frequency dependences of dielectric functions calculated based on these constants. The structural ordering is accompanied by a considerable increase in the intensity of the fundamental absorption band and an occurrence of its blue shift. Some characteristics of conduction electrons are determined, the numerical values of which also depend on the degree of atomic order.     

Mössbauer studies of magnetic texture in nanocrystalline Fe73.5Cu1Nb3Si13.5B9 alloy

Mössbauer measurements have been carried out using non-polarized absorbers in order to investigate the magnetic texture in nanocrystalline Fe_73.5Cu₁Nb₃S_13.5B₉ prepared from the amorphous state by crystallization. The results indicate a significant variation of magnetic domains in ribbons with the annealing temperature. Upon crystallization the component of the magnetization vector oriented parallel to the long axis of ribbons increases significantly and becomes dominant. The non-field annealing-induced magnetic texture probably arises from the shape anisotropy and intensely influences the initial magnetization process.     

非晶Fe73.5Cu1Nb3Si13.5B9合金激光纳米化的超精细结构研究

在CO2激光功率为50-300W、扫描速度为20mm/s、激光散光斑为20mm照射条件下,诱导非晶Fe73.5Cu1Nb3Si13.5B9带中发生结构重组,产生定量纳米α-Fe(Si)晶相形成双相组织结构材料. 利用穆斯堡尔谱研究了非晶Fe73.5Cu1Nb3Si13.5B9合金激光纳米化的超精细结构. 实验结果表明,激光诱导非晶Fe73.5Cu1Nb3Si13.5B9纳米化后,其超精细磁场的分布随着激光功率变化由单峰向双峰变化,在高功率辐照时, 出现了双峰分布,并且峰位向高场移动. 高激光功率辐照非晶Fe73.5Cu1Nb3Si13.5B9合金纳米晶化相有四种超精细结构,即2个超精细磁场较小的初晶相和2个超精细磁场较大的纳米晶化相. 其中超精细磁场较大(17-25MA/m)的α-Fe(Si)相为DO3结构.     

A TEM study of α-phase stability in Zr-2.5 Nb pressure tubes following neutron irradiation (A TEM study of α-phase stability)

The α-phase in Zr-2.5 Nb pressure tubes has been analysed after irradiation with neutrons for temperatures between 523 and 573 K. The α-phase contains Nb in supersaturated solid solution (about 0.5−1 wt% Nb) in the as-fabricated condition. Irradiation results in the formation of small discrete precipitates having diameters of about 5 nm. Energy Dispersive X-ray (EDX) analysis using a scanning transmission electron microscope (STEM) was complicated by the presence of a self-generated X-rays in the irradiated material and by the fact that the precipitates were small relative to the foil thickness (typically 100–200 nm). Techniques developed to measure segregation of impurities at interfaces were employed to show that the precipitates were Nb-rich, their formation being consistent with a decrease in Nb concentration in the matrix.     

Magnetic and structural behaviours of nanocrystalline Fe 70.8 Nb 3.7 Cu 1 Al 2.7 Mn 0.7 Si 13.5 B 7.6 alloy

The crystallization behaviour of Fe 70.8 Nb 3.7 Cu 1 Al 2.7 Mn 0.7 Si 13.5 B 7.6 alloy prepared in the form of amorphous ribbons by melt-spinning technique was studied using differential scanning calorimetry and the temperature variation in resistivity. An X-ray diffaction and transmission electron microscopy study showed the formation of f-Fe(Si, Al) and/or Fe 3 (Si, Al) nanoparticles after the first stage of crystallization. The activation energy for this nanophase formation was 68 kcal mol m1 . The brittleness of the alloy increased with the formation of nanoparticles after heat treatment. Superior soft magnetic properties were achieved when the material was heat treated at 790 K for 15 min. The particle size at the optimum heat treatment condition for superior soft magnetic properties was found to be 6.0 -0.5 nm which was less compared than for the Fe-Nb-Cu-Si-B system. The observed coercivity value at the optimum heat treatment condition was found to be 0.32 A m m1 (approximately 4 mOe). The presence of Al in the alloy reduced the particle size and the magnetic anisotropy energy of the system, which resulted in superior soft magnetic properties of the heat-treated materials.