Relaxation Of Internal Stresses Research Articles

REASONS AND REGULARITIES OF THE INFLUENCE OF MAGNETIC FIELDS ON THE MECHANICAL PROPERTIES AND STRUCTURE OF DEFORMABLE METALS

Background. The strength of metals greatly limits the possibility of obtaining products by plastic deformation. The electromagnetic nature of the processes of structure formation and plastic deformation provided the basis for the application of additional influence of the magnetic field. A fairly large volume of research material has been accumulated on the topic of additional influence of the magnetic field on ferro-, dia- and paramagnetic metals. The researches of recent years have an applied nature of studying the magnetoplasticity of technical alloys. Their generalization will make it possible to move from laboratory research to the development of equipment and technologies for combined pressure processing of metal products in a weak magnetic field. Objective. Generalization and analysis of the results of laboratory and theoretical studies of the additional application of the magnetic field in the processes of mechanical testing of metals and alloys.Methods. Literary review of materials of articles, monographs, dissertations. Results. Reasonable use of a magnetic field for plastic deformation of metals. The explanation of the mechanism of the influence of the magnetic field on the structural elements of metals based on the effect of magnetoplasticity has been made. The description of changes in the mechanical properties of metals and alloys under the additional influence of a magnetic field is given. Conclusions. The phenomenon of magnetoplasticity has been studied for a wide range of materials such as pure metals and their alloys, including industrial steels and alloys. Various types of positive effects of a magnetic field on the mechanical properties of metals have been established: a decrease in the yield strength and deformation resistance, an increase in strain, relaxation of internal stresses, and a decrease in dislocation density. There is also a reverse, negative effect of the influence of a magnetic field: increased rate of hardening, embrittlement, increased creep of metals. What will be the effect of a magnetic field on a specific metal cannot be guaranteed with high accuracy.

Exploring the temperature dependence of magnetic properties and magnetoimpedance effect in Co-rich microwires

We studied the temperature dependence of the magnetic properties and giant magnetoimpedance, GMI, effect in as-prepared and annealed Co69.2Fe3.6Ni1B12.5Si11Mo1.5C1.2 glass-coated microwires with nearly-zero magnetostriction. Substantial changes in the GMI ratio, ΔZ/Z, value and magnetic field, H, dependence, and in the hysteresis loops upon heating were observed. The modification in the hysteresis loop shape upon heating correlates with a change in the ΔZ/Z (H) dependencies. In as-prepared and most of the heat treated samples the hysteresis loop transformation from inclined to squared upon heating correlates with the change in ΔZ/Z(H) dependencies from double-peak to single-peak. However, the stress-annealed at 118 MPa samples present better thermal stability of the ΔZ/Z(H) dependencies and hysteresis loops. In all the studied samples an increase in the GMI ratio at 300 °C was observed. The origin of the observed temperature dependences is discussed in terms of the Hopkinson effect, temperature dependence and relaxation of internal stresses, induced magnetic anisotropy, and temperature dependence of the magnetostriction coefficient.

Optimization of Giant Magnetoimpedance Effect of Amorphous Microwires by Postprocessing

Magnetic microwires with amorphous structures can present a unique combination of excellent magnetic softness and giant magnetoimpedance (GMI) effects together with reduced dimensions and good mechanical properties. Such unique properties make them suitable for various technological applications. The high GMI effect, observed in as-prepared Co-rich microwires, can be further optimized by postprocessing. However, unexpected magnetic hardening and a transformation of the linear hysteresis loop into a rectangular loop with a coercivity on the order of 90 A/m were observed in several Co-rich microwires upon conventional annealing. Several routes to improve magnetic softness and GMI effect in Fe- and Co-rich magnetic microwires are provided. We observed that stress annealing could remarkably improve the magnetic softness and GMI ratio of Co-rich microwires. Thus, almost unhysteretic loops with a coercivity of 2 A/m and a magnetic anisotropy field of about 70 A/m are achieved in Co-rich microwires stress annealed at appropriate conditions. The observed change in hysteresis loops and the GMI effect is explained by stress-annealing-induced anisotropy, which is affected by the stresses applied during annealing and by the annealing temperature. While as-prepared Fe-rich amorphous microwires present a low GMI effect, appropriate postprocessing (annealing and stress annealing) allows for a remarkable GMI ratio improvement (an order of magnitude). The evaluated dependence of the maximum GMI ratio on frequency allows the identification of the optimal frequency band for the studied samples. The origin of stress-annealing-induced anisotropy and related changes in hysteresis loops and the GMI effect are discussed in terms of the relaxation of internal stresses, “back-stresses”, as well as structural anisotropy.

Effect of deformation on structural-phase state of structural steel and on formation of its yield strength and degree of hardening

The work shows that the values of the yield strength of steel 20 after long-term operation and deformation, obtained experimentally and by calculation, differ significantly. This is primarily due to the fact that all coefficients require experimental verification. However, the data obtained can be used to determine the role of each contribution (in %) in relation to the total received. Relaxation of such high local internal stresses (due to the appearance and intensive growth of the elastic component) can (and does) occur only through the destruction of the sample, which is what is observed. Therefore, it is the emergence and increase of local internal stresses that leads to the destruction of the sample.

Digital Laser Direct Writing of Internal Stress in Shape Memory Polymer for Anticounterfeiting and 4D Printing.

Shape memory polymers (SMPs) are a type of smart shape-shifting material that can respond to various stimuli. Their shape recovery pathway is determined by the internal stress stored in the temporary shapes. Thus, manipulating the internal stress is key to the potential applications of SMPs. This is commonly achieved by the types of deformation forces applied during the programming stage. In contrast, we present here a digital laser direct writing method to selectively induce thermal relaxation of internal stress stored in the two-dimensional (2D) shape of a thermoplastic SMP. The internal stress field, while invisible under natural light, can be visualized under polarized light. Consequently, the digital stress pattern can be used for anticounterfeiting. In addition, further uniform heating induces the release of the programmed internal stress within the 2D film. This triggers its transformation into a three-dimensional (3D) shape, enabling 4D printing. The simplicity and versatility of our approach in manipulating internal stress and shape-shifting make it attractive for potential applications.

The Magnetostriction of Amorphous Magnetic Microwires: The Role of the Local Atomic Environment and Internal Stresses Relaxation

We studied the magnetostriction coefficients, λs, Curie temperature, Tc, and their dependence on annealing conditions in Fe47Ni27Si11B13C2 and Co67Fe3.9Ni1.5B11.5Si14.5Mo1.6 amorphous glass-coated microwires with rather different character of hysteresis loops. A positive λs ≈ 20 × 10−6 is observed in as-prepared Fe47Ni27Si11B13C2, while low and negative λs ≈ −0.3 × 10−6 is obtained for Co67Fe3.9Ni1.5B11.5Si14.5Mo1.6 microwire. Annealing affects the magnetostriction coefficients and Curie temperatures, Tc, of both Fe47Ni27Si11B13C2 and Co67Fe3.9Ni1.5B11.5Si14.5Mo1.6 glass-coated microwires in a similar way. Observed dependencies of hysteresis loops, λs and Tc on annealing conditions are discussed in terms of superposition of internal stresses relaxation and structural relaxation of studied microwires. We observed linear λs dependence on applied stress, σ, in both studied microwires. A decrease in the magnetostriction coefficient upon applied stress is observed for Co-rich microwires with low and negative magnetostriction coefficient. On the contrary, for Fe-Ni-rich microwires with a positive magnetostriction coefficient, an increase in the magnetostriction coefficient with applied stress is observed. The observed results are discussed considering the internal stresses relaxation and short range atomic rearrangements induced by annealing on hysteresis loops, magnetostriction coefficients and Curie temperatures of studied microwires.

Identification by inelastic X-ray scattering of bulk alteration of solid dynamics due to liquid wetting

We examine the influence at room temperature of the deposit of a water layer on the phonon dynamics of a solid. It is shown that the water wetting at the surface of an Alumina monocrystal has deep effects on acoustic phonons, propagating over several hundred µm distance and taking place on a relatively long time scale. The effect of the wetting at the boundary is two-fold: a hardening of both transverse and longitudinal acoustic phonons is observed as well as a relaxation of internal stresses. These acoustic phonon energy changes were observed by inelastic X-ray scattering up to 40 meV energy loss, allowing us to probe the solid at different depths from the surface.

Internal stress development within wood during drying: A master curve concept and its application on drying stress evaluation

The generation of internal stress (IS) in flat-sawn and quarter-sawn rubberwood boards during drying has been investigated using an online restoring force (RF) technique, which restrained a half-split rectangular wood specimen. Particular attention was given to the longest IS reversal regime. The IS development proceeds faster in the flat-sawn specimens than that in the quarter-sawn specimens while the maximum IS magnitudes in both specimens are rather similar. By normalizing the IS reversal period, a master IS profile, the derivative of the measured RF to the IS reversal time ratio versus the IS reversal time ratio, is proposed. This master curve, exhibiting some degree of independence from wood orientation and drying temperature, shows variations correlated with the free water content in the wet zone and the dry/wet zone fractions. The process of IS reversal, unaffected by temporary unrestraint, advances as the dry zone expands inwards and ends when the wet zone disappears. Assuming mechanical equilibrium between the dry and wet zones, the IS in both zones can be estimated from the RF data. The maximum tensile IS in the dry zone, indicating a risk of surface checks, evolves at slightly lower magnitudes at higher drying temperatures and is lower in the quarter-sawn specimens. The IS relaxation in the dry zone, still largely taking place in the absence of the applied RF, highlights the main contribution of the dry/wet zone fractions, continuing to proceed without restraining, to the IS development. These findings, emphasizing the significance of dry and wet zones, should pave the way for a better understanding of the IS development within wood during drying.

Experimental studies and modeling of strain rate- and temperature-dependent springback behavior of hot-deformed aluminum alloys

The unloading process of hot-deformed aluminum alloys exhibits obvious strain rate- and deformation temperature-dependent nonlinearity and the springback is generated by internal stress relaxation, which affects the forming accuracy of aluminum alloy components. The strain hardening and dynamic softening during deformation as well as static softening during unloading affect the springback of hot-deformed aluminum alloys. In this study, the homogeneous aluminum alloys samples were used to perform the single-pass isothermal hot compression and unloading tests with different deformation temperatures, loading strain rates and strains. The strain-, strain rate- and deformation temperature-dependent unloading and springback behavior of hot-deformed aluminum alloys were studied. The effects of strain hardening, dynamic and static softening on the springback behavior of alloys were revealed. The relationships between unloading stress, springback strain and instantaneous tangent modulus in unloading were analyzed. A polynomial-type equation was proposed according to the nonlinear unloading stress-strain curves. The microscopic mechanism of nonlinear unloading of hot-deformed aluminum alloys was also clarified based on the physical mechanism of internal stress relaxation and anelastic strain accumulation. A micro-mechanism-based model was developed to describe the springback and internal stress relaxation in unloading after hot deformation of aluminum alloys. The comparison of calculated unloading curves with experimental results indicates that the proposed polynomial-type equation and developed micro-mechanism-based model can describe accurately the nonlinear springback and stress decay in unloading of hot-deformed aluminum alloys, which provide a prerequisite for analyzing the final geometry and residual stress of precision hot-deformed alloy components.

Optimization of Magnetoimpedance Effect and Magnetic Properties of Fe-Rich Glass-Coated Microwires by Annealing.

As-prepared Fe-rich microwires with perfectly rectangular hysteresis loops present magnetization reversal through fast domain wall propagation, while the giant magnetoimpedance (GMI) effect in Fe-rich microwires is rather low. However, the lower cost of Fe-rich microwires makes them attractive for magnetic sensors applications. We studied the effect of conventional (furnace) annealing and Joule heating on magnetic-propertied domain wall (DW) dynamics and the GMI effect in two Fe microwires with different geometries. We observed that magnetic softness, GMI effect and domain wall (DW) dynamics can be substantially improved by appropriate annealing. Observed experimental results are discussed considering the counterbalance between the internal stresses relaxation and induced magnetic anisotropy associated with the presence of an Oersted magnetic field during Joule heating.

Microstructure and Tribological Behavior of Cr-Mn-N Steel with Age-Hardened Near-Surface Layer including CrN and Fe2N Particles Intended for Use in Orthopedic Implants

This paper presents the results of a study of 17%Cr-19%Mn-0.53%N high-nitrogen austenitic stainless steel with a 25 µm thick dispersion-hardened near-surface layer intended for orthopedic applications. It was modified using a mechanical–thermal treatment (MTT) that included both friction processing and subsequent electron beam processing. The friction processing enabled the formation of a microstructure with a high dislocation density and strain twins, and it also initiated strain aging in the near-surface layer. At this stage, the hardening was achieved via the formation of CrN particles coherent to the matrix with the face-centered cubic (FCC) lattice and via the relaxation of internal stresses. After electron beam processing, the volume fraction of the nanodispersed phases increased. In the near-surface layer, a highly dispersed microstructure with a grain size of 3 μm, reinforced with CrN and Fe2N nanoparticles, was observed using transmission electron microscopy. The MTT increased the microhardness of the surface layer, and this contributed to the enhancement in both the H/E and H3/E2 ratios. This indicated an improvement in the crack resistance of the steel under frictional loads. The MTT also enhanced both the yield point (up to 580 MPa) and the wear resistance (by 50% to 100%, depending on the applied load) compared with those of the same steel after it had undergone quenching. In addition, the wear resistance was many times greater than that of the Ti-6Al-4V alloy typically used for manufacturing orthopedic implants. After the MTT, the properties of the near-surface layer of the steel indicated its suitability for biomedical applications.

Nonlinear unloading-reloading behavior and macro/micro models during multi-pass cyclic hot compression of aluminum alloys

Unloading-reloading behavior is of crucial importance in multi-pass forming of metal materials, the nonlinear of which creates challenges for the analysis of springback and residual stress of the precision manufacture components. Especially for the hot bulk-formed material at elevated temperature, the thermal-mechanical coupling effect makes the unloading-reloading process more complicated and thus more difficult to model and control, which remains to be solved. In this paper, the unloading-reloading behavior dependent on strain rate and deformation temperature and its model of hot compressive deformed aluminum alloys at elevated temperature were studied from the macroscopic and microscopic perspectives. The effect of deformation temperature, strain rate and strain on the unloading-reloading behavior of hot compressive deformed aluminum alloys were revealed, and corresponding micro-mechanism was elucidated and then discussed. Macroscopically, a strain-, strain rate- and temperature-dependent nonlinear macroscopic model describing unloading–reloading process was established, which can be used to describe the unloading stress relaxation and predict springback during multi-pass hot bulk forming. Microscopically, it is suggested that the relaxations of back stress and forest dislocation hardening stress collectively affect the unloading stress relaxation, due to the decrease of mobile and immobile (forest) dislocation density in unloading, while the dislocation-related behavior before yield, i.e. the dislocations’ bowing-out under pinning, affects the reloading behavior. Based on the micro-mechanism, a physically-based micro-model for describing unloading–reloading was constructed which further is used to clarify the physical mechanism of internal stress relaxation and inelastic strain recovery under multi-pass hot compression deformation. The comparison of the calculation results of the models with those of experiments indicated that the established unloading–reloading macroscopic and microscopic models are able to predict the stress–strain response upon the unloading–reloading loop of multi-pass cyclic hot compression deformation.

Crack-Free Structural Color Materials Prepared without Disrupting the Particle Arrangement by Controlling the Internal Stress Relaxation and Interactions of the Melanin Particles.

In fabricating structural color materials with assembled colloidal particles, there is a trade-off between the internal stresses acting on the particles and the interactions between the particles during solvent volatilization. It is crucial to fabricate crack-free materials that maintain the periodic arrangements of the particles by understanding the mechanism for crack initiation. Here, we focused on the composition and additives of melanin particle dispersions to obtain crack-free structural color materials without disturbing the particle arrangements. The use of a water/ethanol mixture as a dispersant effectively reduced the internal stresses of the particles during solvent evaporation. Furthermore, the addition of low-molecular-weight, low-volatility ionic liquids ensured that the arrangement and interactions of the particles were maintained after solvent volatilization. Optimization of the composition and additives of the dispersion made it possible to achieve crack-free melanin-based structural color materials while maintaining vivid, angular-dependent color tones.

Effect of temperature on magnetic properties and magnetoimpedance effect in Fe-rich microwires

Herein, the temperature dependence of hysteresis loops and the Giant magnetoimpedance, GMI, effect of amorphous as-prepared and stress-annealed FeSiBC microwires have been thoroughly analyzed. We observed a remarkable improvement of GMI ratio and modification of hysteresis loops from rectangular to inclined with increase in temperature for as-prepared FeSiBC microwires. Observed changes have almost completely reversible character. On the other hand, rather different behavior is observed in stress-annealed FeSiBC microwires: a substantial GMI ratio improvement is observed in stress-annealed FeSiBC microwires. However, a decrease in GMI effect is observed with temperature increasing. Observed experimental results are discussed considering relaxation of internal stresses upon heating, Hopkinson effect and modification of the thermal expansion coefficients upon heating.

In-situ synchrotron high energy X-ray diffraction study on the internal strain evolution of D019-α2 phase during high-temperature compression and subsequent annealing in a TiAl alloy

The residual stress in the D019-α2 phase is known to be significantly higher than that in the L10-γ phase in TiAl alloys after deformation due to the poor plasticity and strong mechanical anisotropy of the α2 phase. However, the internal stress accumulation and relaxation in the α2 phase during high-temperature deformation and annealing are scarcely investigated. In this study, for the first time, the internal strain evolution and load partitioning between the α2 and γ phases at high temperatures are characterized by in-situ synchrotron high energy X-ray diffraction (HEXRD) technique. The plastic deformation is at least initiated at a stress of roughly 200 MPa in the γ phase and 775 MPa in the α2 phase. The intergranular strains in the α2 phase are generated by the onset of dislocation glide in the γ phase, and accentuated with the accumulated dislocations and the ensuing twinning activity. After unloading, great intergranular strains are preserved in the α2 phase constrained by the heavily plastically deformed γ phase. During subsequent heating from 400 to 1000 °C, the internal strains in the α2 phase are almost fully relaxed by substantial dislocation annihilation and rearrangement in the γ phase. During annealing at 800 °C, the internal strain relaxation is rapid in the initial 10 min, whereas considerably retarded subsequently. The extent of relaxation after holding at 800 °C for 1 h is equivalent to that of heating in an effective temperature range of 680–880 °C for 10 min. The in-situ lattice strain measurements with various thermal relaxation schemes provide guidance for the stress relief annealing of TiAl components.

Structural Changes in Ceramic Carbonized Hydroxyapatite as a Result of Long-Term Storage at Room Temperature

Carbonated hydroxyapatite (CHA) is the basic mineral component of animal and human bone. Therefore, it is widely used in medicine to repair bone defects. In orthopedic surgeries, ceramic implants are usually used as a biologically active defect filler. In the lattice of CHA carbonate ions can occupy two non-equivalent positions - A and B. A position corresponds to the position of OH- anions in the lattice of hydroxyapatite (HA), and B - PO43-. It is well known that substitution of B-positions with carbonate groups leads to significant distortions of HA lattice, which causes microstresses and crystalline defects in it. Therefore, CHA ceramics as a result of sintering is characterized by significant internal stresses whose relaxation at room temperature can lead to a change in both its phase composition and biological activity. By methods of chemical and X-ray structural analysis, infrared spectroscopy and electron scanning microscopy the ageing process of pressed CHA at room temperature, sintered in an atmosphere of dry carbon dioxide at temperatures 800÷1200 °C was studied. The phase composition and structure of freshly prepared and aged for two years ceramic samples were compared. It is shown that relaxation of internal stresses arising during sintering of presses causes plastic deformation of crystallites accompanied by redistribution of carbonate ions from B to A-position. As a result, displacement of OH- ions from channel (A) positions and decomposition of B-type CHA on CaO and A-type CHA becomes energetically advantageous.

Effect of Annealing on the Magnetic Properties of Co2MnSi-Based Heusler Alloy Glass-Coated Microwires

In the current study, we concentrated on the influence of annealing on the magnetic behavior of Co2MnSi-based Heusler microwires. We set the annealing temperature at 1023 K for 2 h, as the sample did not show any significant changes in the magnetic properties at lower temperatures, while annealing at temperatures above 1023 K damages the glass coating. Strong in-plane magnetocrystalline anisotropy parallel to the microwire axis was evident in the magnetic behavior at room temperature for as-prepared and annealed samples. The coercivity of the annealed sample was four times higher than that of the as-prepared sample across a wide range of measuring temperatures. Both annealed and as-prepared samples exhibit quite stable coercivity behavior with temperature, which may have interesting applications. The an nealed sample did not exhibit magnetic saturation for M-H loops measured below 50 K. Sharp irreversible magnetic behavior has been detected for annealed samples at a blocking temperature of 220 K; at the same time, the blocking temperature for the as-prepared sample was 150 K. The strong internal mechanical stress induced during the fabrication of Co2MnSi microwires in addition to the internal stress relaxation caused by the annealing induced the onset of magnetic phases resulting in unusual and irreversible magnetic behavior.

Temperature influence on magnetic properties and magnetoimpedance effect of Fe-rich glass-coated microwires

Giant magnetoimpedance, GMI, effect and magnetic properties upon temperature influence of as-prepared and stress-annealed amorphous Fe75B9Si12C4 glass-coated microwires produced by the Taylor-Ulitovsky technique are analyzed. Remarkable change in the hysteresis loops and GMI effect is observed for both samples upon heating. Tuning of the stress-annealing conditions allows one to vary the temperature dependence. Furthermore, it is observed almost complete reversibility of the changes induced by the temperature. Observed dependences are explained by the heating effect on the internal stresses relaxation, by the modification of the thermal expansion coefficients of the metallic nucleus and the glass coating, and by the Hopkinson effect.

Microstructural evolution during post heat treatment of the Ti–6Al–4V alloy manufactured by laser powder bed fusion

The purpose of the present study is to investigate the effect of post heat treatment as well as the manufacturing strategy on the microstructure evolution during martensite decomposition of Ti–6Al–4V manufactured by laser powder bed fusion (LPBF). The microstructural evolution was tracked using in situ high-energy synchrotron X-ray diffraction and differential thermal analysis. The phase fraction, the mean lattice parameters, d-spacing and the Full Width at Half Maximum (FWHM) variations were determined by Rietveld refinement from XRD patterns during continuous heating up to the single β phase domain with different heating rates and manufacturing strategies. The complementarity of characterization tools clearly evidences a shift of the α/α’ → β transformation kinetics toward higher temperatures as the heating rate increases. The variations in transformation kinetics are discussed with regard to the predicted values at thermodynamic equilibrium. In addition, the combined analysis of d-spacing and FWHM shows the deviations from linearity at intermediate and high temperatures that are related to the heating and manufacturing strategy. These variations were analyzed in regard of internal stress relaxation or changes of chemical composition. Moreover, the anisotropic elastic distortions in martensite α’ caused by the manufacturing strategy were examined.

Manipulation of magnetic and structure properties of Ni2FeSi glass-coated microwires by annealing

In the current work, we demonstrate that the magnetic and structural properties of Ni2FeSi glass-coated microwires are strongly dependent on annealing conditions. A noticeable change in the magnetic and structural properties has also been compared to as- prepared samples. Annealing conditions increase the crystalline phase content from 34% to 72% for annealing duration 1 h and 6 h, respectively. While as- prepared samples show square hysteresis loops for a wide range of measuring temperatures, annealed samples show inclined hysteresis loops shape. Additionally, a marked increase in coercive and magnetic anisotropy fields has been observed for annealed samples as compared to as- prepared ones at and below room temperature. Meanwhile, highly reduced magnetic remanence was measured in the as-prepared samples. Temperature dependence of the coercivity shows anomalous behavior for as-prepared and annealed samples as well. An increase in the magnetization of the metallic nuclei of Ni2FeSi glass-coated observed in annealed microwire has been attributed to the internal stresses relaxation caused by annealing. The anomalous magnetic behavior at low temperature is due to the coexistence of ordered and disordered phases induced by annealing conditions. The difference in magnetization behavior, remanence, and coercivity values for Ni2FeSi glass-coated microwires samples indicates the influence of internal stresses created by the presence of the glass coating. These findings reveal the sensitivity of Heusler-based glass-coated microwires to temperature and annealing conditions, which will open the door to the design of spintronic devices and sensing applications.