Deformation behavior of Fe-based bulk metallic glass during nanoindentation

Fe-based bulk metallic glasses (BMGs) normally exhibit super high strength but significant brittleness at ambient temperature. Therefore, it is difficult to investigate the plastic deformation behavior and mechanism in these alloys through conventional tensile and compressive tests due to lack of distinct macroscopic plastic strain. In this work, the deformation behavior of Fe₅₂Cr₁₅Mo₉Er₃C₁₅B₆ BMG was investigated through instrumented nanoindentation and uniaxial compressive tests. The results show that serrated flow, the typical plastic deformation feature of BMGs, could not be found in as-cast and partially crystallized samples during nanoindentation. In addition, the deformation behavior and mechanical properties of the alloy are insensitive to the applied loading rate. The mechanism for the appearance of the peculiar deformation behavior in the Fe-based BMG is discussed in terms of the temporal and spatial characteristics of shear banding during nanoindentation. Supported by the National Natural Science Foundation of China (Grant Nos 50571109, 10572142 and 56771102) and the National Basic Research Program of China (973 Program)(Grant No 2007CB613900)     

Structure and peculiarities of nanodeformation in Ti–Zr–Ni quasi-crystals

Ti–Zr–Ni samples with a substantial predominance of icosahedral quasicrystalline phase were produced by the melt-spinning technique. Their structure and mechanical properties were studied by X-ray diffraction and nanoindentation methods. The quasicrystalline phase was found to have a primitive lattice with the quasicrystallinity parameter a _q = 0.5200–0.5210?nm. Quasicrystalline deformation behaviour under nanoindentation versus phase composition and structure is discussed in comparison with single crystal W–12?wt%?Ta. The estimated elastic modulus E of the quasicrystalline phase shows no correlation with the element composition. The nanohardness was shown to increase with increasing quasicrystalline-phase perfection. Load–displacement curves of Ti–Zr–Ni quasicrystals (QCs) show stepwise character with alternation of elastic and plastic sections. Such non-uniform plastic flow in QCs might be caused by the localization of plastic deformation in shear bands. The non-uniformity of the plastic deformation increases with the increasing quasicrystalline phase perfection.     

纳米多层膜“软"相结构参量对硬度的Hall-Petch表征

采用直流磁控溅射方法制备了不同调制比的Ni/Al纳米多层膜,利用X射线衍射技术和纳米压入连续刚度法分析了薄膜微结构及塑性变形的尺度依赖性.实验结果表明,尽管调制比有所不同,多层膜的硬度与“软"相的微结构特征参量随调制波长减小具有相似变化规律,说明多层膜的变形机制对“软"相的微结构约束存在敏感性.随着薄膜特征尺度的减小,为统一多层膜中晶界和膜界两种强化机制,提出一个与“软”相相关的表征参量r(r＝L_sub/d,L关键词： 纳米多层膜 塑性变形 调制波长 Hall-Petch关系     

Deformation Behavior and Structure of <Emphasis Type="Italic">i</Emphasis>-Al-Cu-Fe Quasicrystalline Alloy in Vicinity of Nanoindenter Indentation

The nanoindentation tests have been carried out for the quasicrystalline polygrain Al_62.4Cu_25.3Fe_12.3 alloy with the icosahedral structure i; the load P-displacement h diagrams have been used to estimate the contributions of plastic deformation (monotonic and intermittent), and the structures of the transverse microscopic sections have been studied in the vicinity of indentations by electron microscopy. It is shown that several systems of deformation bands are formed in the elasto-plastic zone in the vicinity of the indentations along the close-packed planes of the i lattice with the five-fold and two-fold symmetry axes; the bands often begin from cracks and manifest the signs of the dislocation structure. The traces of the phase transformation with the formation of the β-phase areas are observed only in a thin layer under an indenter. The effects of intermittent deformation are up to 50% of the total inelastic deformation and are related to the plastic behavior of the quasicrystal-activation and passage of deformation bands and also the formation of undersurface micro- and nanosized cracks.     

Nanoindentation of amorphous Ge-As-Se films

Changes in the nanohardness H and Young’s modulus E of Ge _x As _y Se_{100 ? x ? y} films have been studied as a function of the penetration depth of the Berkovich indenter. The values of H and E have been measured in the regime of harmonic modulation of a linearly increasing indenter load. It has been shown that the changes in E and H of the films under study during nanoindentation arise due to the peculiarities of their elastoplastic behavior, the formation of deformation zones near the nanocontact, and also size effects.     

Elastic–plastic deformation of Pb(Zn1/3Nb2/3)O3–(6–7)% PbTiO3 single crystals during nanoindentation

The elastic–plastic deformation behavior of (001)- and (011)-oriented single crystal solid solutions of Pb(Zn_1/3Nb_2/3)O₃–(6–7)% PbTiO₃ (PZN–PT) have been studied using a nanoindentation technique. A procedure is presented here to isolate the elastic, elastic–plastic and plastic contributions to the deformation using the unloading data, and a parameter, referred to as relaxation, is defined to characterize the elastic–plastic deformation during nanoindentations. This relaxation parameter increases with the maximum indentation load due to the higher indentation stress induced, and it also causes less recovery of the material upon indentation unloading compared to predicted pure elastic recovery. For a (001) surface, the relaxation value remains virtually unchanged within the range of the maximum indentation load of 10–50 mN, possibly due to a complete localized depoling of the non-180° domain switching. It is also found that the unpoled surface is more prone to stress-induced depolarization compared to the poled surfaces. Furthermore, by applying the continuous stiffness measurement (CSM) technique, the effects of multiple loading/unloading are studied for both (001)- and (011)-oriented PZN–PTs using the maximum indentation loads of 20 and 50 mN. With more loading/unloading cycles at higher CSM frequencies, stress-induced depolarization becomes prevalent and the contribution of the domain reorientation towards elastic recovery is significantly reduced. As a consequence, the relaxation value is increased, indicating more elastic–plastic deformation. This CSM effect is especially pronounced for poled (011) surfaces.     

Al–Pd–Mn icosahedral quasicrystal: deformation mechanisms in the brittle domain

The extreme brittleness of Al–Pd–Mn quasi-crystalline alloys over a wide range of temperatures drastically restricts investigation of their plastic deformation mechanisms over a small high-temperature regime. Recently, plastic deformation of Al–Pd–Mn quasicrystal has been achieved in the brittle domain (20?≤?T?≤?690°C) using specific deformation devices, which combined a uniaxial compression deformation or a shear deformation with a hydrostatic pressure confinement (0.35–5?GPa). Results of these experimental techniques, which provide various deformation conditions giving rise to a range of Al–Pd–Mn plastic features in the brittle domain, are discussed. On this basis, we propose that low and intermediate temperature plastic properties of Al–Pd–Mn are controlled by non-planar dislocation core extensions specific to the non-periodic structure.     

Lamellar structure and nanomechanical properties of quasicrystalline Al-Cu-Fe alloys

The kinetics of structural phase transformations in quasicrystal-forming Al-Cu-Fe alloys with compositions in the region of stability of the icosahedral (i) phase has been investigated. It has been shown that, depending on the development of metastable transformations i → pentagonal phases P1 and P2, a homogeneous lamellar structure (i + P1 + P2) or a polygrain i-phase is formed in the alloys. The P-h diagrams obtained upon nanoindentation, atomic force microscopy, and scanning electron microscopy of indentations have demonstrated signs of elasto-plastic deformation of the alloys with lamellar and polygrain icosahedral structures. It has been found that, in contrast to the polygrain icosahedral alloys with a normal size effect of nanoindentation, the alloys with a lamellar structure are characterized by a nonmonotonic dependence of the hardness (H) on the maximum load (P _max) and exhibit the effect of strain hardening in the range of loads 50 mN ≤ P _max < 500 mN. The strain hardening is considered as the result of resistance exerted by boundaries of the lamellar structure to the development of plastic deformation.     

Nanoindentation stress–strain curves as a method for thin-film complete mechanical characterization: application to nanometric CrN/Cr multilayer coatings

The mechanical behavior of CrN/Cr multilayer coatings deposited by rf magnetron sputtering has been investigated by nanoindentation measurements performed with indenters of different geometries. Nanoindentation stress–strain curves generated from these measurements allow us to characterize the complete mechanical behavior of these coatings in the elastic, elastoplastic, plastic and fracture deformation regimes. In particular, indentation measurements carried out with a 100-m-radius spherical indenter allowed us to study the elastic deformation regime and estimate the yield stress parameter through the initial indentation yielding point. The elastoplastic deformation regime has been studied using a 5-m-radius spherical indenter and the stationary yielding regime (fully plastic regime) has been investigated with a pyramidal indenter of Berkovich geometry. The use of a pyramidal cube-corner indenter allowed us to study coating fracture characteristics. Nanometric CrN/Cr multilayer structures as well as single CrN and Cr coatings have been characterized. The study has shown that multilayered coatings with period thicknesses less than 46 nm present values of yield stress, Youngs modulus, hardness and toughness higher than those for single-layer CrN and Cr coatings. PACS 62.20.Dc; 62.20.Qp; 68.60.Bs     

Estimation of residual stresses from elastic recovery of nanoindentation

In this study, an empirical model based on finite element simulations is presented for residual stress determination from the elastic recovery of nanoindentation. Finite element simulations show that the ratio of elastic recovery of nanoindentation to the maximum penetration depth, h _e /h _max, has a linear relationship to the ratio of residual stress to yield stress, σ _r/σ_y , and increases for compressive residual stress, whereas it decreases for tensile stress. Nanoindentation tests performed on a bent fused quartz beam in this study, and on diamond-like carbon (DLC) and Au coatings in the literature, also confirm the existence of residual stress effects on the elastic recovery of nanoindentation. The empirical model has been used to derive the plastic properties and to estimate the residual stress of the mechanically polished fused quartz beam.     

Transition temperature to crystal phase of Al86Mn14 quasicrystal ultrafine particles determined by direct observation and characterization of surface oxide layer

Al–Mn quasicrystal ultrafine particles can be produced by the advanced gas evaporation method (AGEM), which is a method of preparing ultrafine alloy particles by coalescence growth among the particles near the evaporation sources. We investigated the phase transition temperature from a quasicrystal to a stable crystal, by examining successive electron diffraction patterns of an ultrafine particle in an in situ experiment using a transmission electron microscope. In spite of the report that the Al₈₆Mn₁₄ quasicrystal transforms into the crystal phase at around 400–670 °C on thin film specimens, the quasicrystal ultrafine particle transformed at 800 °C, i.e., the quasicrystal ultrafine particle is more stable. Since the cross-sectional view of the surface oxide layer of the quasicrystal ultrafine particles can be easily observed, the surface oxides of η-Al₂O₃ and MnO were characterized as a result of the oxidation of residual atoms on the surface of the produced alloy particles including the quasicrystals. The conditions required for Al–Mn quasicrystal ultrafine particle formation by the AGEM can be estimated under the cooling rate of 10⁵ K/s.     

Temperature dependence of crack propagation in a two-dimensional model quasicrystal

The propagation of cracks in two-dimensional decagonal model quasicrystals is studied under mode I loading by means of molecular dynamics simulations. In particular, we investigate the dependence on temperature, applied load and underlying structure. The samples are endowed with an atomically sharp crack and strained by linear scaling of the displacement field. Three different regimes of propagation and discernible with increasing temperature. For low temperatures the crack velocity increases monotonically with increasing applied load. We observe that the crack follows the path of dislocations nucleated at its tip. For temperatures above 0.3?T _m, where T _m is the melting temperature, the crack does not remain atomically sharp but becomes blunt spontaneously. In the temperature range between 0.7?T _m and 0.8?T _m the quasicrystal fails by nucleation, growth and coalescence of microvoids. This gradual dislocation-free crack extension is caused by plastic deformation which is mediated by localized rearrangements comparable with the so-called shear transformation zones. These are also observed in amorphous solids. Thus, at low temperatures the crack propagates along crystallographic planes just as in periodic crystals, whereas at high temperatures a glass-like behaviour is dominant.     

Atomic dynamics of a <Emphasis Type="Italic">d</Emphasis>-AlNiFe decagonal quasicrystal

The atomic dynamics of an Al_71.3Ni₂₄Fe_4.7 decagonal quasicrystal has been investigated using the isotopic contrast method for inelastic neutron scattering. The partial vibrational spectra of the Ni, Fe, and Al atoms and the spectrum of the thermal vibrations of the alloy have been reconstructed directly from the experimental data without any model assumptions. The cutoff energies and the positions of the main features of the spectra have been determined. It has been revealed that the average binding energy of the nickel atoms in the quasicrystal under investigation is lower than that of the iron atoms and the vibrational spectrum of the aluminum atoms is noticeably harder than the spectrum of the pure metal. The results obtained for the d-AlNiFe decagonal quasicrystal have been compared with the previously published data for an i-AlCuFe icosahedral quasicrystal.     

Study of the magnetic properties of GdZn compound using PAC spectroscopy with 140Ce and 111Cd as probe nuclei

Nanocrystalline Al₆₄Cu₂₃Fe₁₃ icosahedral quasicrystal has been obtained by milling of solid quasicrystal precursors prepared by arc-melt. The local structure around Fe atoms was studied by Mössbauer spectroscopy using a quadrupole splitting distribution method. Mössbauer results of annealed and milled samples show the existence of a broadened distribution of Fe sites which is associated to intrinsic disorder. The structural characterization was determined using x-ray diffraction. The average grain-size of the nanostructured quasicrystal, obtained from the line broadening of the X-ray diffraction peaks, was estimated to be of the order of 10 nm for a sample milled by 5 h.     

Magnetic properties of a monocrystalline quasicrystal in the Al-Pd-Mn system

Magnetic susceptibility of a monocrystalline icosahedral Al_70.2Pd_21.3Mn_8.5 quasicrystal was measured over the temperature range from 4 to 1100 K. The susceptibility was found to include the temperature-independent diamagnetic contribution, the temperature-dependent Curie’s contribution, and the contribution from the Pauli paramagnetism of an electron system with energy gap. An analysis of the low-temperature susceptibility revealed the presence of about 0.008% of ions with magnetic moment 4µ_B in the quasicrystal at 4 K. It is assumed that the ions with uncompensated magnetic moments appear near the structural vacancies in the quasicrystal lattice. The energy gap between the valence and conduction bands is estimated at Δ=0.64 eV, and the effective mass of charge carriers is equal to approximately 70 electron masses.     

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.     

Bridging room-temperature and high-temperature plasticity in decagonal Al–Ni–Co quasicrystals by micro-thermomechanical testing

Ever since quasicrystals were first discovered, they have been found to possess many unusual and useful properties. A long-standing problem, however, significantly impedes their practical usage: steady-state plastic deformation has only been found at high temperatures or under confining hydrostatic pressures. At low and intermediate temperatures, they are very brittle, suffer from low ductility and formability and, consequently, their deformation mechanisms are still not clear. Here, we systematically study the deformation behaviour of decagonal Al–Ni–Co quasicrystals using a micro-thermomechanical technique over a range of temperatures (25–500 °C), strain rates and sample sizes accompanying microstructural analysis. We demonstrate three temperature regimes for the quasicrystal plasticity: at room temperature, cracking controls deformation; at 100–300 °C, dislocation activities control the plastic deformation exhibiting serrated flows and a constant flow stress; at 400–500 °C, diffusion enhances the plasticity showing homogenous deformation. The micrometer-sized quasicrystals exhibit both high strengths of ~2.5–3.5 GPa and enhanced ductility of over 15% strains between 100 and 500 °C. This study improves understanding of quasicrystal plasticity in their low- and intermediate-temperature regimes, which was poorly understood before, and sheds light on their applications as small-sized structural materials.     

Accumulation of dislocations and thermal strengthening of alloys having an L12 superstructure

This paper discusses the dislocation-accumulation mechanism in alloys having an L1₂ superstructure, which is associated with the formation of Kira-Wilsdorf barriers and the retardation of superdislocations during plastic deformation. A model of the dislocation-accumulation kinetics during plastic deformation is constructed, on the basis of which a mathematical model is formulated for the thermal and deformation strengthening of single crystals of alloys having the L1₂ superstructure. The results of numerical calculations based on the model are compared with the experimentally observed regularities of the deformation and thermal strengthening of single crystals of Ni₃Ge. Fiz. Tverd. Tela (St. Petersburg) 41, 454–460 (March 1999)     

Microstructure of a composite with a quasicrystalline phase fraction obtained by directional solidification of Al61Cu27Fe12 alloy

The microstructure of a composite containing a quasicrystal phase, i.e. so-called crystal–quasicrystal (CQ) composite, was studied. The CQ composite was obtained by the Bridgman method via solidification of Al₆₁Cu₂₇Fe₁₂ alloy (numbers indicate at%). The process was conducted at a pull out rate of v = 0.07 mm/min. The average temperature gradient in the heating zone was 43 K/cm. The composite matrix consisted of cubic β phase Al(Fe, Cu), with reinforcement of λ-phase rod-shaped fibres surrounded by a quasicrystal icosahedral ψ phase, which also existed in the fibre core. The fibres were rhomboidal in cross-section. The composite was studied using X-ray and electron diffraction, light-optical and scanning electron microscopy (SEM), X-ray topography and Laue diffraction.     

Serrated deformation of a Pd<Subscript>40</Subscript>Cu<Subscript>30</Subscript>Ni<Subscript>10</Subscript>P<Subscript>20</Subscript> bulk amorphous alloy during nanoindentation

Depth-sensing (indentation) testing is used to study the characteristics of a serrated plastic flow in a Pd₄₀Cu₃₀Ni₁₀P₂₀ bulk amorphous alloy, and the boundaries between the regions of serrated and homogeneous plastic deformation are determined.