K-absorption spectrum of aging Al-Zn alloys

The K-absorption spectra of aging Al-Zn alloys experience a systematic shift of the main edge during natural aging toward the short wavelength side. This is apparently due to filling of the 4S–4P shells of the zinc atoms by electrons at the expense of the aluminum atoms. Additions of Mg and Sn lead to a more intense redistribution of the outer electrons. The early zone stage of aging of Al alloys exhibits an electronic character.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 7, pp. 18–22, July, 1971.In conclusion the authors wish to thank Prof. A. E. Bryukhanov and docent V. I. Kotlyat for constant interst shown in the study and valuable comments made while discussing the results.     

Quantitative analysis of Al-Zn alloys in the early stage of decomposition

The method of low-angle x-ray scattering is used to perform a detailed analysis of the early stage of decomposition of Al-Zn alloys with 4.41 and 9.41 at. % Zn quenched at different homogenization temperatures. The parameters that characterize the post-quench state of the alloys are determined. It is found that a reduction in the quenching temperature is accompanied by a reduction in the size of the Guinier-Preston (GP) zones and their content of the dissolved element. Here, the density of the zones also decreases. An attempt is made to establish a connection between the parameters of the GP zones and the regions containing closely-spaced lamina formed in the alloys at the homogenization temperature.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 29–34, August, 1989.We thank A. N. Bekrenev and L. A. Naumov for their advice on conducting the experiment.     

On the configuration of Guinier-Preston zones in Al-Ag and Al-Zn alloys

An experimental investigation of the short-range ordering in the spatial distribution of isotropic G.-P. zones in supersaturated solid solutions of Ag or Zn in Al is presented. The intensity distribution of the small-angle diffuse scattering can be satisfactorily explained by inter-zone X-ray interferences when using the data on metastable miscibility gaps determined by Gerold for Al-Ag and by Hillert for Al-Zn. The investigation was carried out for a wide interval of annealing temperatures on Al-Ag (6 at. % Ag) and on Al-Zn (11 at. % Zn) samples.The authors wish to express their gratitude to their colleagues of the Institute of Solid State Physics: J. Laek, C.Sc, for valuable discussions and for chemical analyses, V. íma for alloy preparations, and Z. iký for his assistance in the measurements and computations.     

Electronic topological transitions and phase stability in the fcc Al-Zn alloys

We report on a detailed investigation of the phase equilibria and the Fermi surface in the Al-Zn system. Our calculation are based on the density functional theory and we use the linear muffin-tin orbital method and the Green's function technique. The calculated free energies of alloy formation exhibit the existence of a miscibility gap between the alloys containing approximately 10 and 55 at.% of Zn, in agreement with the phase diagram of the Al-Zn system. Seven electronic topological transitions (ETT) were found in Al-Zn system within the stability range of the fcc solid solution. A relation between these ETT and the phase stability of the fcc Al-Zn solid solutions is established. We show that extremum points on the concentration dependencies of the thermodynamic properties of Al-Zn alloys can be explained by band-filling effects. Received 6 February 2002 Published online 19 November 2002     

Solvus line of the metastable rhombohedral phase in the Al-Zn system for Al-rich alloys

The X-ray diffraction on single crystals and on coarse-grained foils of Al-Zn alloys was used to determine the solvus temperatures of the metastable rhombohedral α′ _R -phase. The dissolution of α′ _R -phase was studied simultaneously by application of DTA technique. Results obtained from different heat-treatments of the investigated alloys were critically interpreted in terms of the nucleation theory. The dissolution of large α′ _R -particles was necessary to obtain the correct α′ _R -solvus line unaffected by the reversion of the solid solution below the solvus temperatures.     

Plastic deformation instability in copper alloys

A systematic investigation was made of the temperature and velocity conditions for the appearance of stepwise deformation in copper alloys with a high alloying content (the Portevin-Le Chatelier effect) and the deformation mechanics of crystalline materials was analyzed from the viewpoint of an emsemble of dislocations. A mathematical model of the Portevin-Le Chatelier effect was constructed, based on general ideas concerning the behavior of dislocations and their interaction with alloying elements, and from the viewpoint of the theory of oscillations, taking into account the rigidity of the test-piece-machine system. Mechanical tests were performed on copperalloy test pieces in the temperature range 20–400°C. A characteristic feature found for the oscillation modes was that the peak-to-peak value Δσ was independent of ɛ and that there was a plateau with a weak dependence σ(ɛ) at the upper σ level. The dependences of the oscillation period on the temperature and the given plastic deformation velocity were in good agreement with experiment. State Scientific-Research and Design Institute of Alloys and Working of Nonferrous Metals. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 14–20, February, 1993.     

The X-ray diffraction and electron microscopic investigation of stable and metastable equilibria in Al-rich Al-Zn alloys

The solvus line for the metastable cubic α′-phase and for the stable α- and α′-solubility limit for Al-Zn alloys with zinc content from 15 to 50 wt.% were determined. X-ray diffraction on single crystals was used to study the structures of phases developed during the applied heat-treatments. The dissolution of α′-phase at temperatures close below and above the determined solvus temperatures was observed by the transmission electron microscopy. The shift of the known (α+α′)-phase boundary to lower temperatures of about 8 °C was found. Similarly, small changes were obtained in the shape of the curve giving the solid solubility limit of Zn in Al.     

Structural changes in TiC-TiNi alloys on deformation

The deformation of a composition material with a binding phase of shearing-unstable alloy, namely, titanium nickelide, is considered. X-ray structural analysis and electron—positron annihilation methods are used to investigate the structural changes in the composite material as a function of the degree of deformation. It is shown that the deformation produces the structural phase transition B2 B19, which is confirmed by x-ray structural investigations and electron positron annihilation.Institute of Strength Physics and Materials Science, Siberian Branch, Russian Academy of Sciences. Translated from Izvestiya Uchebnykh Zavednii, Fizika, No. 4, pp. 100–103, April, 1994.     

Nucleation of deformation twins in nanocrystalline fcc alloys

An earlier dislocation model for predicting the grain size effect on deformation twinning in nanocrystalline (nc) face-centred-cubic (fcc) metals has been found valid for pure metals but problematic for alloys. The problem arises from the assumption that the stacking-fault energy (γ_SF) is twice the coherent twin-boundary energy (γ_fcc), which is approximately correct for pure fcc metals, but not for alloys. Here we developed a modified dislocation model to explain the deformation twinning nucleation in fcc alloy systems, where γ_SF ≠ 2γ_twin. This model can explain the differences in the formations of deformation twins in pure metals and alloys, which is significant in low stacking-fault energy alloys. We also describe the procedure to calculate the optimum grain size for twinning in alloy systems and present a method to estimate γ_twin.     

Modern aspects of plastic deformation in metals

After a historical survey of crystal plasticity the rôle of dislocations in plastic deformation of metals is outlined. The theory of plastic deformation in metal single crystals before the impact of transmission electron microscopy is described. Recent experimental results on glide and workhardening in single crystals of b.c.c. metals are reviewed and explained by a simple dissociation model of the screw-dislocation cores. A relative success of this explanation together with calculations of atomic structure of dislocation cores support the conclusion that different structures of screw dislocation cores are responsible for both differences and similarities between plastic behaviour of f.c.c. and b.c.c. metals. Further developments in the field of metal crystal plasticity by modern experimental techniques (transmission electron microscopy of deformed crystals in the stress-applied state, magnetic studies of dislocations in ferromagnetic crystals) and by atomic calculations of defect configurations are discussed.Na Slovance 2, Praha 8, Czechoslovakia.Invited paper presented on a plenary session of the First European Conference on the Physics of Condensed Matter organized by the Board of the Condensed Matter Division of E.P.S., in Florence on 14–17 September 1971 (Chairman: Prof. S. F. Edwards; Schuster Laboratory, University of Manchester).     

The influence of tensile stress on grain and cell boundary precipitation in supersaturated Al-Zn alloys during creep

The results are presented of the optical microscopic and X-ray diffraction study of the stress-induced nucleation and growth of (Zn) precipitates at grain and cell boundaries (GB's and CB's) during uniaxial creep at 200 °C of supersaturated AlZn20 and AlZn30 alloys. The rate of precipitation is increased mainly owing to the modifying effect of tensile stress on diffusion processes in alloy samples during their anneal. The diffusion of Zn atoms toward GB's and CB's from adjacent regions of grains is accompanied during creep by diffusive flux of Zn along boundaries parallel or nearly parallel to the tensile creep axis toward boundaries with near to normal orientation to that axis. Enhanced precipitation of results then preferentially at the latter and is supressed at the former boundaries where even the dissolution of preexisting has been found during a later application of tensile stress. The stress-induced precipitation of at GB's gradually ceases with prolonged creep exposures due to the lengthening of duffusion paths of Zn atoms from grain interior to GB's.Dissolution of lamellae by their regress toward GB's and CB's is assisted with the stress-induced diffusion of Zn along epitaxial / lamellar interfaces. Copious precipitation of at the parts of GB's and/or CB's with near to normal orientation to the creep axis is then observed on account of Zn from dissolved lamellae. Creep strain also leads to the fragmentation of lamellae and thus also to breaking down of the paths for diffusion of Zn along / interfaces. Spheroidization of fragmented parts of lamellae is then observed. Spheroids of remain embedded within the former lamellar regions.Large creep strains and high strain rates observed on fine-grained alloy samples may be associated with an enhanced viscous GB sliding due to the stress-dependent flow of Zn along GB's and/or CB's.     

Grain boundary deformation in Cd-Zn and Zn-Al alloys

Activation energies for creep have been measured on fine-grained and coarse-grained specimens of pure cadmium, zinc, and Cd-Zn and Zn-Al alloys in the temperature range (0·4–0·8)T _m. It is found that in the case of fine-grained specimens the activation energies for creep are equal to the activation energies for grain boundary diffusion in cadmium and zinc, and in the case of coarse-grained specimens — to that of volume self-diffusion.     

Cyclic deformation of aluminium and its alloys

The formation of slip bands is the main mechanism of cyclic deformation in pure Al. Their density, orientation and heights in polycrystalline Al were investigated during cycling. Types, sizes and densities of precipitates are responsible for the mode of cyclic deformation in AlCu4 pure alloy. In technical Al alloys intermetallic phases have detrimental effects on deformation homogeneity and largely govern the fatigue mechanism of the material and especially microcrack initiation.     

NiTi形状记忆合金形变机制的应变率相关性研究

利用高速拉伸实验机在宽的应变率范围内(0.001–1200 s^-1), 研究了NiTi形状记忆合金的宏观力学性能随应变率的变化规律, 并借助透射电子显微镜深入研究了微观结构在不同应变率下的演变机制. 研究发现: NiTi合金马氏体(B19’相)孪晶的解孪晶应力随应变率的升高而近乎线性增大, 表明NiTi 合金解孪晶应力具有正向应变率相关性. 在拉伸应变率为10 s^-1的样品微观结构中发现了大量的解孪晶区域, 而当应变率进一步增大到100 s^-1和 1200 s^-1时, 在样品中没有发现解孪晶区域的存在, 样品微观组织以孪晶形式存在. 该结果表明, NiTi合金的马氏体解孪晶速率应在 10–100 s^-1范围内. 在高应变率下(≥qslant10 s^-1)均发现了热引发奥氏体相(B2)的存在, 表明随应变率的增加, 拉伸过程由等温过程逐渐变为绝热过程. 此外, 在1200 s^-1 的样品差示扫描热量曲线中还发现了一个小肩峰, 表明相变过程由一步相变变为两步相变. 关键词： NiTi形状记忆合金 高速拉伸 应变率相关性 透射电子显微镜     

Precipitation imaging of deformation structure of age-hardened alloys

Ageing of supersaturated solid solution of Al 20 wt % Zn alloy after cyclic deformation results in a preferential precipitation of Zn in regions of higher dislocation density. Characteristic inhomogeneities of the deformation structure revealed by this precipitation image correspond well to the surface relief observed by optical and scanning electron microscopy. The posibility of using the method of precipitation imaging to study spatial arrangement of high and low dislocation density regions in deformed age-hardened alloys is discussed.     

Forced chemical mixing in alloys driven by plastic deformation

Molecular dynamics simulations of forced atomic mixing in crystalline binary alloys during plastic deformation at 100 K are performed. Nearly complete atomic mixing is observed in systems that have a large positive heat mixing and in systems with a large lattice mismatch. Only systems that contained a hard precipitate in a soft matrix do not mix. The amount of mixing is quantified by defining a mean square relative displacement of pairs of atoms, sigma(2)(R,t), that were initially separated by a distance R. Analysis of sigma(2)(R,t) and visual inspection of the displacement fields reveal that forced mixing results from dislocation glide, and that it resembles the forced mixing of a substance advected by a turbulent flow. Consideration of sigma(2)(R,t) also provides a rationalization of compositional self-organization during plastic deformation at higher temperatures.     

Size effects under martensitic deformation of shape-memory alloys

The effect of the grain size and film thickness on the parameters of martensitic transitions and deformation behavior of shape-memory alloys is studied in terms of the theory of smeared martensitic transitions. Comparison of theoretical results with experimental data suggests that the transformation kinetics related to the growth mechanism and shrinkage of martensite lamellas when transformation steps (dislocations) move along their boundaries is most sensitive to the size factor.