Wire Axis Direction Research Articles

Cleavage Stress Producing Notch-Induced Anisotropic Fracture and Crack Path Deflection in Cold Drawn Pearlitic Steel

The fracture performance of axisymmetric notched samples taken from pearlitic steels with different levels of cold-drawing is studied. To this end, a real manufacture chain was stopped in the course of the process (on-site in the factory), and samples of all intermediate stages were extracted from the initial hot-rolled bar (not cold-drawn at all) to the final commercial product (prestressing steel wire). Thus, the drawing intensity or straining level (represented by the yield strength) is treated as the key variable to elucidate the consequences of manufacturing on the posterior fracture issues. On the basis of a materials science approach, the clearly anisotropic fracture behavior of heavily drawn steels (exhibiting deflection in the fracture surface) is rationalized on the basis of the markedly oriented pearlitic microstructure of the cold-drawn steel that influences the operative micromechanism of fracture. In addition, a finite element analysis of the stress distribution at the fracture instant allows the computation of the cleavage annular stress required to produce anisotropic fracture behavior and thus crack path deflection associated with mixed-mode cracking. Results show that such a stress is the variable governing initiation and propagation of anisotropic fracture by cleavage (a specially oriented and enlarged cleavage fracture) appearing along the wire axis direction in the case of sharply-notched samples of heavily drawn pearlitic steels.

On the cleavage hoop stress responsible for crack path deflection and notch-induced anisotropic fracture in cold drawn pearlitic steel: Revisiting John Ford’s Monument Valley

In this paper a study is presented of the fracture behaviour in the presence of notches of high-strength pearlitic steel with different cold drawing degree. To this end, axisymmetric notched samples with a circumferentially-shaped notch of very distinct geometries (sharp and blunt, deep and shallow) were taken from wires with different levels of drawing from the initial hot rolled bar (not cold drawn at all) to the final commercial product (prestressing steel wire: heavily cold drawn). Experimental results show that cold drawn steels exhibit a markedly anisotropic fracture behaviour in the form of fracture path deviation from the original transverse fracture path perpendicular to the wire axis or cold drawing direction. Present paper proposes a key variable governing the notch-induced anisotropic fracture process: the cleavage hoop stress as the relevant factor promoting this sort of fracture path deviation in the most heavily cold drawn pearlitic steel wires

In-situ high energy X-ray diffraction study of microscopic deformation behavior of martensite variant reorientation in NiTi wire

This study investigated the collective evolution of martensite variant structures, the development of crystallographic textures and the load partitioning among the variants during tensile deformation in a commercial polycrystalline NiTi wire by means of in-situ high-energy X-ray diffraction technique. The cold-worked and then crystallized NiTi wire was found to have three fiber textures along the wire axial direction in its thermally formed self-accommodating B19′ martensite, including (1¯20)/(120), (1¯02) and (102). These textural orientations are inheritance of the texture of the B2 phase and are associated with the <011> type II twins and (001) compound twins. Tensile deformation via martensite variant reorientation converted the (102) and (120) fiber textures into (1¯02). The texture evolutions and variant selections are driven by the difference in d-spacing values between the pairing variants in each twin structure in order to achieve maximum elongation deformation in the tensile loading direction. At the meantime, the lattice strains of the favored variants increased whereas those of the unfavored variants decreased (even to negative) abruptly at the occurrence of the Lüders deformation event when at the global stress remained constant over the stress plateau. These changes of the lattice strains are attributed to load transfer and repartitioning among the variants. Based on the lattice strain measurements, the load partitioning among variants is also estimated.

Hierarchical microstructure evolution in cold drawn pearlitic steels: In the conceptual frame of Fray Luis de León, Miguel de Cervantes, Victor Vasarely, Maurits Cornelis Escher & Johann Sebastian Bach

Cold drawn pearlitic steels posses a hierarchical microstructure consisting of pearlitic colonies (first microstructural level) and pearlite (ferrite/Fe & cementite/Fe3C) lamellae (second microstructural level) that evolves during the manufacturing process by cold drawing towards a preferential orientation aligned in the drawing (wire axis) direction, so that these materials acquire microstructural anisotropy that influences their posterior fracture behaviour. The paper establishes an analogy with the literature of Spanish writers Fray Luis de León and Miguel de Cervantes (through the alternate distribution of ferrite/cementite lamellae), as well as with the painters Maurits Cornelis Escher and Victor Vasarely (through the multi-level organization of their paintings) and the composer Johann Sebastian Bach (through the hierarchical structure of his music).

Identification of a new microstructural unit in cold drawn pearlitic steel: The pearlitic pseudocolony

This paper provides a definition and identification of a new (non-conventional) microstructural unit in cold drawn pearlitic steel: the pearlitic pseudocolony, a special pearlitic colony in which the lamellae are not oriented along the wire axis or cold drawing direction (contrarily to what happens in the normal conventional colonies), thereby producing a clearly anomalous (very high) local interlamellar spacing, making the pseudocolonies weakest units or potential fracture initiation loci and promoting crack deflection associated with anisotropic fracture behavior and mixed-mode propagation

HELP versus HEDE in progressively cold-drawn pearlitic steels: Between Donatello and Michelangelo

Abstract This paper reviews previous research by the author in the field of hydrogen effects on progressively cold-drawn pearlitic steels in terms of hydrogen degradation (HD), hydrogen embrittlement (HE) or, at the micro-level, hydrogen-assisted micro-damage (HAMD), thus affecting their microstructural integrity and compromising the (macro-)structural integrity of civil engineering structures such as prestressed concrete bridges. In particular, the two classical hydrogen-assisted deterioration micro-mechanisms are discussed, namely, hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE), together with their role in the embrittlement process as a function of the degree of cold-drawing undergone by each steel (associated with a different level of microstructural orientation of pearlitic colonies and lamellae). It is seen that hydrogen effects in pearlitic microstructure (either oriented or not) are produced at the finest micro-level by plastic tearing in the form, in general, of hydrogen damage topography (HDT) with different appearances depending of the cold drawing degree, evolving from the so-called tearing topography surface (TTS) in hot-rolled (not cold-drawn at all) or slightly cold-drawn pearlitic steels to a sort of enlarged and oriented TTS (EOTTS) in heavily drawn steels (the pronounced enlargement and marked orientation being along the wire axis or cold drawing direction). Whereas the pure TTS mode (null or low degree of cold drawing) resembles the Michelangelo stone sculpture texture (MSST), the EOTTS mode does the same in relation to the Donatello wooden sculpture texture (DWST).

Structural and magnetic anisotropy of directionally-crystallized ferromagnetic microwires

Amorphous ferromagnetic microwires have drawn attention primarily due to their excellent soft magnetic properties as the elements of sensors. In this work, semi-hard magnetic microwires of composition Fe4.3Co67.7Si11B14Cr3 were obtained after a process of directional crystallization from amorphous state. The XRD analysis of crushed and whole wires identified the formation of the face-centered Co-modification as the main phase of directionally crystallized core alloy, which explains a large increase in anisotropy and coercivity. The application of a magnetic field during crystallization may orient the easy anisotropy axis of crystallites along the wire. This is confirmed by the investigation of crystallite orientation with respect to the wire axis and X-rays direction in diffractometer. The obtained results revealed that the formation of crystallites in amorphous Co-rich microwires occurs with the predominant orientation of the crystallographic direction [111] along the wire axis and the direction of a magnetic field during the directional crystallization process.

Structural integrity of progressively cold-drawn pearlitic steels: From Raffaello Sanzio to Vincent van Gogh

Abstract: This papers deals with fracture behavior and structural integrity of progressively cold-drawn pearlitic steels on the basis of their microstructural evolution during manufacturing by multi-step cold drawing. It is seen that the manufacture technique by progressive cold drawing in several steps produces a microstructural evolution in the form of progressive slenderizing and orientation (in the wire axis or cold drawing direction) of the pearlitic colonies (first microstructural level), as well as increasing orientation and densification of the ferrite/cementite lamellae (second microstructural level) linked with a decrease of pearlite (ferrite/cementite) interlamellar spacing. Thus the microstructure of the cold-drawn pearlitic steel wires becomes progressively oriented as the cold-drawing degree increases and this microstructural fact influences their macroscopic behavior by inducing anisotropic fracture and crack path deflection. Therefore, this paper offers a micro- and macro-approach to the fracture and structural integrity of cold-drawn pearlitic steels, introducing the new concept of microstructural integrity .

Stress Corrosion Cracking of Progressively Cold-Drawn Pearlitic Steels: From Tintoretto to Picasso

Progressive cold drawing in eutectoid steels produces a preferential orientation of the pearlitic microstructure in the wire axis or drawing direction. This affects the posterior behaviour of the steels under conditions of stress corrosion cracking (SCC). The experimental results show that cold drawing induces strength anisotropy in the steel, and thus the resistance to SCC is a directional property that depends on the angle in relation to the drawing direction. Therefore, an initial transverse crack changes its propagation direction to approach that of the wire axis, thus producing mixed mode propagation, the deflection angle being an increasing function of the cold drawing degree. This experimental result may be explained by micro-mechanical considerations on the basis of the lamellar microstructure of the steels. A relationship is established between the microstructural angles and the deflection angles of the macroscopic SCC crack, thus providing a materials science type relationship between the microstructure and the macroscopic crack paths with regard to hydrogen assisted cracking (HAC) associated with the cathodic regime and in the matter of localised anodic dissolution (LAD) linked to the anodic regime. Whereas in hot rolled (not cold drawn at all) and slightly drawn steels the phenomenon of SCC develops in mode I, i.e., the SCC crack is a straight line linked with the classical linear perspective painting by Tintoretto, in the case of heavily drawn steels, the SCC deflected crack is a polygonal line associated with the multi-perspective cubist painting by Picasso, these conclusions being valid for both HAC and LAD.

Hydrogen Effects on Progressively Cold-Drawn Pearlitic Steels: Between Donatello and Michelangelo

This paper reviews previous research by the author in the field of hydrogen effects on progressively cold-drawn pearlitic steels in terms of hydrogen degradation (HD), hydrogen embrittlement (HE) or, at the micro-level, hydrogen-assisted micro-damage (HAMD), thus affecting their microstructural integrity and compromising the (macro-)structural integrity of civil engineering structures such as prestressed concrete bridges. It is seen that hydrogen effects in pearlitic microstructure (either oriented or not) are produced at the finest micro-level by plastic tearing in the form, in general, of hydrogen damage topography (HDT) with different appearances depending of the cold drawing degree, evolving from the so-called tearing topography surface (TTS) in hot-rolled (not cold-drawn at all) or slightly cold-drawn pearlitic steels to a sort of enlarged and oriented TTS (EOTTS) in heavily drawn steels (the pronounced enlargement and marked orientation being along the wire axis or cold drawing direction). Whereas the pure TTS mode (null or low degree of cold drawing) resembles the Michelangello stone sculpture texture (MSST), the EOTTS mode does the same in relation to the Donatello wooden sculpture texture (DWST).

On the exact solutions of nonlinear Schrödinger equation with spin current

An integrable nonlinear Schrödinger (NLS) equation driven by spin polarized current governing the magnetization dynamics of a ferromagnetic nanowire is considered. The exact soliton solution of the NLS equation propagating along the direction of wire axis which is also the current direction along which nonuniform magnetization occurs is obtained through the application of exponential function method. The solution of the system admits a class of solitons such as kink and periodic solitons in the nanowire along the direction of the electric current.

Synchrotron high energy X-ray diffraction study of microstructure evolution of severely cold drawn NiTi wire during annealing

Microstructure evolution of a cold-drawn NiTi shape memory alloy wire was investigated by means of in-situ synchrotron high-energy X-ray diffraction during continuous heating. The cold-drawn wire contained amorphous regions and nano-crystalline domains in its microstructure. Pair distribution function analysis revealed that the amorphous regions underwent structural relaxation via atomic rearrangement when heated above 100 °C. The nano-crystalline domains were found to exhibit a strong cold work induced lattice strain anisotropy along 〈111〉, which coincides with the crystallographic fiber orientation of the domains along the wire axial direction. The lattice strain anisotropy systematically decreased upon heating above 200 °C, implying a structural recovery. Crystallization of the amorphous phase led to a broadening of the angular distribution of 〈111〉 preferential orientations of grains along the axial direction as relative to the original 〈111〉 axial fiber texture of the nanocrystalline domains produced by the severe cold wire drawing deformation.

Impurity-limited resistance and phase interference of localized impurities under quasi-one dimensional nano-structures

The impurity-limited resistance and the effect of the phase interference among localized multiple impurities in the quasi-one dimensional (quasi-1D) nanowire structures are systematically investigated under the framework of the scattering theory. We derive theoretical expressions of the impurity-limited resistance in the nanowire under the linear response regime from the Landauer formula and from the Boltzmann transport equation (BTE) with the relaxation time approximation. We show that the formula from the BTE exactly coincides with that from the Landauer approach with the weak-scattering limit when the energy spectrum of the in-coming electrons from the reservoirs is narrow and, thus, point out a possibility that the distinction of the impurity-limited resistances derived from the Landauer formula and that of the BTE could be made clear. The derived formulas are applied to the quasi-1D nanowires doped with multiple localized impurities with short-range scattering potential and the validity of various approximations on the resistance are discussed. It is shown that impurity scattering becomes so strong under the nanowire structures that the weak-scattering limit breaks down in most cases. Thus, both phase interference and phase randomization simultaneously play a crucial role in determining the impurity-limited resistance even under the fully coherent framework. When the impurity separation along the wire axis direction is small, the constructive phase interference dominates and the resistance is much greater than the average resistance. As the separation becomes larger, however, it approaches the series resistance of the single-impurity resistance due to the phase randomization. Furthermore, under the uniform configuration of impurities, the space-average resistance of multiple impurities at room temperature is very close to the series resistance of the single-impurity resistance, and thus, each impurity could be regarded as an independent scattering center. The physical origin of this “self-averaging” under the fully coherent environments is attributed to the broadness of the energy spectrum of the in-coming electrons from the reservoirs.

A nano lamella NbTi–NiTi composite with high strength

A hypereutectic Nb60Ti24Ni16 (at%) alloy was prepared by vacuum induction melting, and a nano lamellae NbTi–NiTi composite was obtained by hot-forging and wire-drawing of the ingot. Microscopic analysis showed that NbTi and NiTi nano lamellae distributed alternatively in the composite, and aligned along the wire axial direction, with a high volume fraction (~70%) of NbTi nano lamellae. In situ synchrotron X-ray diffraction analysis revealed that stress induced martensitic transformation occurred upon loading, which would effectively weaken the stress concentration at the interface and avoid the introduction of defects into the nano reinforced phase. Then the embedded NbTi nano lamellae exhibited a high elastic strain up to 2.72%, 1.5 times as high as that of the Nb nanowires embedded in a conventional plastic matrix, and the corresponding stress carried by NbTi was evaluated as 2.53GPa. The high volume fraction of NbTi nano lamellae improved the translation of high strength from the nano reinforced phase into bulk properties of the composite, with a platform stress of ~1.7GPa and a fracture strength of ~1.9GPa.

Reversible cyclic deformation mechanism of gold nanowires by twinning–detwinning transition evidenced from in situ TEM

Mechanical response of metal nanowires has recently attracted a lot of interest due to their ultra-high strengths and unique deformation behaviours. Atomistic simulations have predicted that face-centered cubic metal nanowires deform in different modes depending on the orientation between wire axis and loading direction. Here we report, by combination of in situ transmission electron microscopy and molecular dynamic simulation, the conditions under which particular deformation mechanisms take place during the uniaxial loading of [110]-oriented Au nanowires. Furthermore, by performing cyclic uniaxial loading, we show reversible plastic deformation by twinning and consecutive detwinning in tension and compression, respectively. Molecular dynamics simulations rationalize the observed behaviours in terms of the orientation-dependent resolved shear stress on the leading and trailing partial dislocations, their potential nucleation sites and energy barriers. This reversible twinning-detwinning process accommodates large strains that can be beneficially utilized in applications requiring high ductility in addition to ultra-high strength.

First-principles study of (InAs)1/(GaSb)1 superlattice nanowires

As the active areas and the connection part, semiconductor nanowires have ideal shapes which are beneficial to restricting the electron motion and atomic periodicity to one one-dimensional structure. The effective selection of material components in nanowires can enhance the advantages of low-dimensional structures by analyzing the features of bulk materials. Furthermore, the nanowire properties can also be tailored by controlling the internal or intrinsic characteristics such as diameters, crystallographic growth direction, structural phase, surface crystallographic plane or saturation degree, and by applying external influences such as electric, magnetic, thermal and force fields. The bulk InAs and GaSb have approximate lattice constants, thereby resulting in small lattice mismatch for InAs/GaSb heterostructures that can finally be grown into excellent infrared optoelectronic materials. Moreover, the bulk InAs has the lowest electron effective mass in binary III-V compound semiconductors, leading to high transport features for electrons distributing most in InAs layers of InAs/GaSb superlattices. In the present work, the zinc-blend (InAs)1/(GaSb)1 superlattice nanowires (subscript denotes the number of molecular or double-atomic layers) with [001] and [111] crystallographic wire-axes have been studied by first-principles calculations for their structural, electronic and mechanical properties together with the rule of different nanowire diameters (from ～0.5 to ～2.0 nm). We also analyze the stress effects from external forces and examine the electronic band-edge changes with strain in wire-axis direction to determine the deformation potentials.

Magnetic behavior in arrays of Ni79Fe21 and Ni79Fe21/Cu nanowires

Ordered hexagonal arrays of NiFe homogeneous nanowires and NiFe/Cu multilayer nanowires were fabricated by electrodeposition into alumina membranes with pore diameter about 70nm and 35nm and 105nm interpore distance, with a NiFe nanodisc thickness of 60 and 70nm, and a Cu thickness of 50 and 10nm. The hysteresis loops were measured with the applied magnetic field parallel and perpendicular to the wire axis using a VSM. The homogeneous NiFe nanowire array exhibits a nearly isotropic magnetic behavior. In the multilayer nanowire arrays of 70nm diameter, the preferential direction of magnetization is perpendicular to the wire axis and the measured coercive field decreases as the tCu/tNiFe increases. The magnetization processes in multilayer nanowire arrays of 70nm diameter have been investigated by means of micromagnetic simulations of isolated, 2 and up to 9, wires. The magnetization distributions for the two 21(NiFe60/Cu50) and 21(NiFe70/Cu10) multilayer nanowire arrays of 70nm diameter, with the applied magnetic field in the wire axis direction, indicate that magnetization occurs by vortex nucleation at the ends in all the wire nanodiscs. For the samples of the same composition mentioned above, but of 35nm diameter, the reversal magnetization starts by nucleation of a domain wall. A random orientation of the magnetization in the different nanodiscs for the sample of 21(NiFe60/Cu50) can be observed.

Direct simulation of turbulent heat transfer in swept flow over a wire in a channel

We investigate heat transfer characteristics of a turbulent swept flow in a channel with a wire placed over one of its walls using direct numerical simulation. This geometry is a model of the flow through the wire-wrapped fuel pins, the heat exchanger, typical of many civil nuclear reactor designs. The swept flow configuration generates a recirculation bubble with net mean axial flow. A constant inward heat flux from the walls of the channel is applied. A key aspect of this flow is the presence of a high temperature region at the contact line between the wire and the channel wall, due to thermal confinement (stagnation). We analyze the variation of the temperature in the recirculation bubble at Reynolds number based on the bulk velocity along the wire-axis direction and the channel half height of 5400. Four cases are simulated with different flowrates transverse to the wire-axis direction. This configuration is topologically similar to backward-facing steps or slots with swept flow, except that the dominant flow is along the obstacle axis in the present study and the crossflow is smaller than the axial flow, i.e., the sweep angle is large. The temperature field is simulated at three different Prandtl numbers: 10 −2, 10 −1 and 1. The lower value of Prandtl number is characteristic of experimental high-temperature reactors that use a molten salt as coolant while the high value is typical of gas (or water vapor) heat exchangers. In addition, mean temperature, turbulence statistics, instantaneous wall temperature distribution and Nusselt number variation are investigated. The peak Nusselt number occurs close to the reattachment location, on the lee side of the wire, and is about 50–60% higher compared to the case without crossflow. The high temperature region follows the growth of the recirculation bubble which increases by about 65% from the lowest to highest amount of crossflow. Particular attention is devoted to the temperature distribution on the walls of the channel and the surface of the wire. The behavior of the heat-flux across the mean dividing streamline of the recirculation bubble is investigated to quantify the local heat transfer rates occurring in this region.

A Monte-Carlo study of the phonon transport in nanowire-embedded composites

This work aims at investigating the thermal conductivities of nanowire-embedded composites in use of a Monte-Carlo simulator developed based on the assumptions of grey media, bulk phonon dispersion relations, and bulk intrinsic scattering rates. Those along and those perpendicular to the wire axis direction are both targeted. The simulation results show that when the interface surfaces are totally diffuse, the cross-wire thermal conductivities of either the silicon-wire-germanium-host or germanium-wire-silicon-host nanocomposites decrease with increasing wire volume fraction due to the increasing, dominant, interface scattering; when the interface surfaces are perfectly smooth, a competition exists between the decreasing intrinsic and increasing interface scatterings as the wire volume fraction increases and a minimum thermal conductivity associated with the silicon-wire-germanium-host composite is existent. When heat flows mainly along the wire axis direction in either the wire-host composites or the double-layered nanowires, the interface and boundary effects dominate the effective thermal conductivities of the two components, which together with the cross-sectional-area ratio determine the major heat pathway. A minimum effective thermal conductivity can thus also be observed when these two factors are well-matched in strength and when the effective thermal conductivity of the nanowires is larger than that of the host/shell component.

Direct simulation of turbulent swept flow over a wire in a channel

Turbulent swept flow over a cylindrical wire placed on a wall of a channel is investigated using direct numerical simulations. This geometry is a model of the flow through the wire-wrapped fuel pins, the heat exchanger, typical of many nuclear reactor designs. Mean flow along and across the wire axis is imposed, leading to the formation of separated flow regions. The Reynolds number based on the bulk velocity along the wire axis direction and the channel half height is 5400 and four cases are simulated with different flowrates across the wire. This configuration is topologically similar to backward-facing steps or slots with swept flow, except that the dominant flow is along the obstacle axis in the present study and the crossflow is smaller than the axial flow, i.e. the sweep angle is large. Mean velocities, turbulence statistics, wall shear stress and instantaneous flow structures are investigated. Particular attention is devoted to the statistics of the shear stress on the walls of the channel and the wire in the recirculation zone. The flow around the mean reattachment region, at the termination of the recirculating bubble, does not exhibit the typical decay of the mean shear stress observed in classical backward-facing step flows owing to the presence of a strong axial flow. The evolution of the mean wall shear stress angle after reattachment indicates that the flow recovers towards equilibrium at a rather slow rate, which decreases with sweep angle. Finally, the database is analysed to estimate resolution requirements, in particular around the recirculation zones, for large-eddy simulations. This has implications in more complete geometrical models of a wire-wrapped assembly, involving hundreds of fuel pins, where only turbulence modelling can be afforded computationally.