Hardening Rate Research Articles

Surface integrity analysis of Ti-5553 under different cooling strategies

Ti-5553 is one of the difficult-to-cut materials, which is prone to surface defects and plastic deformation during machining, resulting in its surface quality deterioration. In this paper, the experimental of machining Ti-5553 under LN2 cryogenic was carried to comparison with dry and high-pressure. The result indicated that Sq is the smallest under LN2 cryogenic. The depth of plastic deformation is the smallest during dry machining. The most influential factor for the surface hardening rate of the workpiece is cutting speed. The surface residual tensile stress gradually increases with the cutting speed and feed rate increase.

Structural aspects of deformation hardening with repeated drawing low carbon wire

While repeated drawing of low carbon wire, a decrease in the grain size of ferrite reduces the parameters of deformation hardening in the initial field of deformations with parabolic hardening and increases in the field of substructural hardening with large and extra‑large total compressions. The transition from large to super‑large total deformations goes along with the development of dynamic return processes that reduce the rate of deformation hardening. Preliminary thermal softening of cold‑formed wire increases the strength gain during subsequent drawing and can be used to increase the total compression of the wire rod before intermediate heat treatment.

Studies of High-Temperature Fatigue Behavior and Mechanism for Nickel-Based Superalloy Inconel 625

Strain-controlled continuous fatigue and creep–fatigue tests were carried out at 700 °C and 800 °C on Inconel 625 alloy. The effects of strain rate and tensile-hold time on cyclic stress response and fatigue life were investigated. Then, the microstructural analysis and the fractographic analysis of fatigue-fractured specimens were performed by scanning electron microscopy and transmission electron microscopy. The cyclic stress responses during high-temperature fatigue and the creep–fatigue–oxidation damage mechanism were discussed. The results showed that the strain rate and the tensile-hold time had little effect on the fatigue life at 700 °C, but there was a significant impact at 800 °C due to the creep–fatigue–oxidation interaction. The cyclic plastic deformation accelerated the precipitation of the γ″ phase, resulting in a continuous cyclic hardening and negative strain rate sensitivity. The fatigue failures at 700 °C under continuous fatigue conditions occurred with a transgranular fracture mode, while a transgranular-intergranular hybrid fracture manner was found at 800 °C. Furthermore, a frequency-modified total strain energy density model was proposed to consider the effects of creep and oxidation on fatigue life, and the predicted fatigue lives were located within the 1.5 times error band.

Role of thermally-stable deformation twins on the high-temperature mechanical response of an austenitic stainless steel

In the present study, a two-step thermo-mechanical processing consisting of cold work and heat treatment steps was performed to increase the operating temperature of 316 austenitic stainless steels. A hierarchical microstructure of thermally-stable, nano twin bands was achieved forming into bundles in elongated grains. The mechanical response of the samples with this microstructure was evaluated through uniaxial tension tests at temperatures ranging from 20 °C to 500 °C and compared with those from the fully annealed samples. The results demonstrate that such hierarchical microstructure leads to a significant increase in the elevated temperature yield strengths due to the presence of nano-twin boundaries and resulting decrease in dislocation mean free path and increase in dislocation storage capacity. In fact, the yield strength ratio of the twinned and annealed samples increases with increasing temperature up to 500 °C, indicating the effectiveness of pre-existing thermally-stable twin boundaries as the strengthening source at temperatures as high as 0.46 homologous temperature. The hierarchical microstructure also led to irregular serrations through dynamic strain aging in the stress-strain response at 500 °C, which is attributed to the bi-modal microstructural length-scales present in the structure affecting the diffusion distances during dynamic strain aging. This structure also increases the tensile strength, and without a total loss in ductility, even though the flow stress of the twinned samples surpasses the tensile strength of the annealed samples, especially at elevated temperatures. Total hardening rate is consistently higher in the twinned samples as compared to the annealed samples, indicating the positive role of nano-twin boundaries in the dislocation storage capacity at elevated temperatures. Overall, the present study clearly demonstrate the positive role of thermally stable nano-twins on the elevated temperature mechanical response of austenitic stainless steels.

Electropulsing assisted aging with ultrafast hardening rate for AerMet100 steel

Carbide precipitation behavior is greatly improved for AerMet100 steel by high frequency electropulsing assisted aging (EAA). As a result, the EAA hardening efficiency of AerMet100 remarkably increased by ∼10 times compared with that of conventional furnace ageing. Fracture elongation exceeds 18% within 10 times of EAA peak aging time.

Experimental characterization and crystal plasticity modeling of dual-phase steels subjected to strain path reversals

This paper is concerned with monotonic and load reversal deformation of four dual-phase (DP) 590, 780, 980, 1180 and one martensitic (MS) 1700 steel sheets. While the monotonic data are presented for all steels, the load reversal data are provided for DP 590, DP 780, and DP 1180. Particularities pertaining to the reversal deformation including the decreasing hardening rate during forward tension, a linear and then a non-linear unloading, followed by the Bauschinger effect, and a shift in the hardening rate during continuous straining were quantified and discussed as a function of loading history. Moreover, parameters such as the reloading stress differential, reloading softening stress, ratcheting strain, and unloading deviation stress were determined and discussed as a function of martensite fraction. The data were interpreted and predicted using an elasto-plastic self-consistent (EPSC) crystal plasticity model incorporating anisotropic elasticity, a dislocation density based hardening law, and a slip system backstress law. The model parameters associated with the slip strengths of ferrite and martensite and backstress were established. This work demonstrated the ability of crystal plasticity modeling to account for the co-dependent nature of crystallographic slip in ferrite and martensite and the sources of hardening caused by history-dependent dislocation density evolution and backstress to predict not only monotonic but also hysteresis in plastic response during the forward-reversal cycles. The combination of comprehensive experimental data and modeling results allowed us to infer that the tradeoff between the magnitude of backstress per phase and the volume fraction of ferrite versus martensite per steel governs the unloading and subsequent yielding per steel, while the dissolution of dislocations facilitates capturing the hardening rates during load reversal deformation.

Dynamic compressive behavior of a single crystal nickel-base superalloy at ultra-high temperature: mechanism investigation with a modified electric synchronous SHPB technique

A novel experimental technique conducted at a modified automatic split Hopkinson pressure bar (SHPB) for measuring the dynamic compressive behavior at ultra-high temperature up to 1700 °C was developed. A bidirectional electric synchronous assembly system, two high-precision delay controllers and an automatic control mechanism were employed to automate each section in the whole experiment process with high precision and high reliability. The cold contact time (CCT) between specimen and loading bars can be accurately and stably controlled to be shorter than 5 ms and the effect of CCT on temperature distribution can be eliminated. Based on this automatic high-temperature SHPB technique, an experimental investigation into the responses of a new in-service single-crystal Nickel-base superalloy DD10 subjected to impact loading at high temperature was undertaken. The influence of strain rate and temperature on the flow behavior of DD10 was tested and analyzed over a wide range of strain rate (0.001/s-4000/s) and temperature (25°C–1200 °C). The high temperature has an evident effect on the flow stress, strain hardening and strain rate sensibility. An anomalous peak of stress in true stress–temperature curve can be found, and the peak temperature increases with strain rate increasing. Through the microstructural observation, the influence of strain rate and temperature on the compressive deformation and fracture mechanism was discussed. At the high temperature above 1000 °C, the shear deformation mechanism of both γ matrix phase and γ′ precipitate phase is the dominant deformation mechanism at high strain rate, instead of the rafting mechanism of γ′ precipitate phase of under quasi-static loading condition.

An investigation into the characterization of the hardening response of sheet metals using tensile and shear tests with surface strain measurement

The adoption of full-field digital image correlation (DIC) strain measurement in recent years has fundamentally transformed conventional analysis procedures for the mechanical characterization of sheet metals. The wealth of local strain data that is now available until fracture has enabled experimentalists to propose new analysis techniques for tensile and shear tests to obtain the hardening response to large strain levels. Although the tests show great potential, significant gaps remain surrounding their applicability. The choice of test geometry and suitability of the analysis across a broad range of sheet metals with diverse hardening and anisotropic behaviour remain open questions. Critically, the assumptions within the tensile and shear methodologies in applying surface strain measurements to large strains while omitting through-thickness gradients have not been verified. The objective of the present study is to perform a critical evaluation of constitutive characterization techniques for sheet metals under quasi-static, ambient conditions. A series of virtual experiments were developed to numerically replicate the testing and analysis procedures followed in lab-scale experiments to extract the hardening response. A simple shear and two standard tensile test geometries were evaluated across a broad range of plasticity characteristics including different hardening rates and anisotropic behaviour. For each case, the input hardening curve in the virtual experiment is treated as the exact solution and compared to the hardening response obtained from each analysis method. The so-called area reduction method (ARM) used for tensile specimens was found to be relatively insensitive to the specimen geometry, yield exponent, and plastic anisotropy but overestimated the flow stress. The performance of the ARM method hinges upon the aspect ratio of the cross-section to avoid shear band formation and requires empirical correlations to improve its predictions at large strains. Despite their simplicity, tensile analysis methods that use small extensometers or local DIC point data are shown to systematically overestimate the hardening behaviour. Simple shear tests can provide accurate estimations of the hardening response, but the choice of geometry is material dependant. The study concludes by considering a strongly anisotropic mild steel to evaluate the tensile and shear characterization strategies with the results of a hydraulic bulge analysis.

Dual heterogeneous structured medium-entropy alloys showing a superior strength-ductility synergy at cryogenic temperature

The tensile properties and the corresponding deformation mechanisms at both room and cryogenic temperatures for a CoCrNi-based medium entropy alloy with both a homogeneous structure and a dual heterogeneous structure have been studied and compared. The dual heterogeneous structure is found to be composed of both heterogeneous grain structure and coherent L12 nanoprecipitates. Both the homogeneous structure and the dual heterogeneous structure show a simultaneous improvement on yield strength and uniform elongation at cryogenic temperature as compared to those at room temperature, which can be attributed to higher hetero-deformation-induced hardening rate and higher density of geometrically necessary dislocations induced at cryogenic temperature. For the homogeneous structure, nano-spaced stacking faults along two slip systems with much smaller interspacing and higher density of Lomer-Cottrell locks should play the essential role in sustaining the strain hardening for better tensile ductility at cryogenic temperature. L12 nanoprecipitates are observed to be very effective on blocking dislocation slip and accumulation of dislocations around them. For the dual heterogeneous structure, more intense interactions between defects and L12 nanoprecipitates are observed after tensile deformation at cryogenic temperature, resulting in fragmentation and refinement of L12 nanoprecipitates for better tensile properties than at room temperature.

Explosion/impact response of pure molybdenum at ultra-high strain rate and large strains

In order to study the dynamic mechanical properties of molybdenum under high strain rates, experiments are conducted using Hopkinson dynamic compression testing of a small-sized sample and explosion impact dynamic X-ray study of a flat shaped-charge liner. Experimental data agrees well with theoretical calculations and numerical simulations. Results demonstrate that strain hardening, thermal softening, and strain rate strengthening effects during the impact response process of the molybdenum material vary significantly with different impact conditions in a temperature range of 297–1273 K and a strain rate range of 0.0001–48,000 s−1. Strain rate strengthening and thermal softening effects are particularly apparent in the molybdenum material under impact at medium and high strain rates. At extremely high strain rates and large deformations, the dynamic mechanical response provides evidence of thermal softening and strain hardening effects. Stresses that result in damage to the molybdenum material are primarily attributed to the latter.

On the dislocation mechanisms of dynamic strain ageing in fatigued plain carbon steels

Abstract The fatigue life of plain ferritic carbon steels fatigued in stress-controlled (plastic-strain-controlled) tests exhibits a maximum (minimum) at a temperature of approx. 300 °C as a result of dynamic strain ageing (DSA). In the regime of DSA, an enhanced cyclic hardening related to an increased dislocation density is observed, and the dislocation arrangement changes from a dislocation cell structure (at intermediate stress amplitudes) at room temperature to a dense edge di-/multipolar bundle/wall/vein structure in the temperature range of DSA and back to a dislocation cell/ subgrain structure at higher temperatures. Related effects occur also in α-iron containing some carbon when fatigued at room temperature at a very low strain rate. This behaviour is considered to be typical of body-centred cubic materials. In the present study, the dislocation mechanisms responsible for this behaviour are discussed in detail, and a dislocation glide model is developed which can explain both the enhanced dislocation density and the change of the dislocation arrangement in the regime of DSA. The model is based on the different mobilities of mobile edge and less mobile screw dislocations in body-centred cubic metals and on the strong tendency towards multiple slip leading to a dislocation cell structure at lower temperatures. In the regime of DSA, it is proposed that the edge dislocations are hindered more strongly in their glide motion by the solute carbon atoms than the screw dislocations so that screws and edges are then more or less equally mobile. Taking into account that screw dislocations of opposite sign annihilate mutually by cross slip, this explains the observed dislocation arrangement of dense edge di-/multipolar bundles/ walls/veins of high dislocation density which, in turn, leads to an enhanced cyclic hardening rate. The dislocation sub-grain structure observed at higher temperatures reflects the increasing tendency for the transformation from dislocation cell to subgrain structures, as the temperature is increased.

Complex Additives for Concrete Based on Substances of Organic and Mineral Nature

The production of concretes with a hard hardening rate is not feasible without the use of modifying additives. The development of the chemical industry has led to the emergence of a large number of additives with different properties. Some increase the frost and corrosion resistance, the strength of building materials, while others affect the setting of concrete and the water-cement ratio. Of greatest interest is the joint work of the additives in the composition of the cement stone. The paper analyzes and determines the effect of various combinations of chemical additives (sodium carbonate Na2CO3, sodium chloride NaCl and sodium nitrate NaNO3) on the structure formation and properties of cement stone as part of a complex additive using the simplex-lattice experiment planning method. In the course of the research, to obtain cement stone, Portland cement M500 D0 has been used with the addition of Na2CO3, NaCl and NaNO3 in the established proportions. As a result of tests, it has been found that the following proportions of the components in the composition of the complex additive are optimal: NaNO3 – 12–38 %; NaCl – 34–54 %; Na2CO3 – 26–42 %. With the introduction of this complex into the composition of the concrete mixture, positive dynamics have been observed in neutralizing the corrosion effect on reinforcement in concrete, as well as an accelerated set of concrete hardness. The results obtained can be used at specialized enterprises of the chemical industry, as well as in further research in the development of modifying additives.

A phase field model for high-speed impact based on the updated Lagrangian formulation

High-speed impact problems usually undergo a form of highly localized plastic deformation under high strain rate with intense shock waves. In this paper, we present a coupled phase field model to simulate fracture in these situations. Our model considers the dynamic finite deformation with both strain hardening and strain rate hardening by the Johnson–Cook plasticity model. In particular, the formulation is posed in an updated Lagrangian framework and the constitutive update is realized with a return-mapping algorithm. Considering the tension–compression asymmetry, a new history variable is developed to enforce the irreversibility condition of crack propagation. The effectiveness of the proposed method is validated with a numerical example of 45 steel impact experiment along with two numerical simulations of split Hopkinson bar experiments on concrete and aluminum alloy specimens, where strong shock waves are generated. Besides, the effect of a few parameters is studied, namely, the regularization length parameter, the energy release rate, the initial impact velocity, and parameters of the Johnson–Cook model. These show the correct trend in terms of the time and extent of crack propagation.

Hot Deformation Behavior of PM Ni with Harmonic Microstructure

The hot deformation behavior of a harmonic-structured pure nickel has been studied and compared with the hot deformability of a homogeneously structured nickel. Both materials were produced via the powder metallurgy route through the Spark Plasma Sintering (SPS) of mechanical milled and un-milled powders. Hot deformation was evaluated through compression tests at three different temperatures (400 °C, 800 °C, and 1300 °C), covering a wide range in the homologous temperature spectrum for Ni (from 0.39 to 0.91), and at three different strain rates (0.001, 0.01, and 0.1 s−1). The evaluation of the stress–strain curves showed a higher hot compression resistance for the harmonic-structured nickel, together with higher strain hardening and strain rate sensitivity, thanks to the peculiar microstructural features of this material. Through the metallographic analysis of the specimens after hot compression, different mechanisms were identified as responsible for the deformation behavior in relation to the temperature of testing. While at 400 °C dynamic recrystallization has slightly started, at 800 °C it is widely diffused, and at 1300 °C it is replaced by grain growth and diffusion creep phenomena.

ФИЗИЧЕСКОЕ УРАВНЕНИЕ УПЛОТНЯЕМОЙ УПРУГОВЯЗКОЙ СРЕДЫ

Experienced information about the rheological properties of materials used in the manufacture of abrasive mixtures for the subsequent technology of manufacturing wheel blanks by rolling, pressing and punching made it possible to classify it as a heterogeneous compacted elastoviscous, characterized by deformation and rate hardening, a significant proportion of elastic deformation, elastic aftereffect and relaxation phenomena. This, in turn, made it possible to approach the derivation of the defining physical equation. The conclusion that defines the physical equation in the entirety of the parameters included in it is a difficult task. At the same time, the creation of an exact theory of physical equations is one of the main problems. There are various approaches to the construction of defining physical equations. Some of them claim to be a deep study of the essence of phenomena, lead to rather complex constitutive physical equations, and are difficult to use in applied problems. The article proposes a variant of constructing the defining physical equation, which is based on plastometric curves, their approximation by polynomials, a step-by-step procedure for processing experimental data, the choice of a mechanical analogue and the derivation of a physical equation in a differential form for a small time interval, the determination of the constants of a physical equation by the least squares method, and as a result, obtaining the dependence of the modulus of elasticity and the coefficient of viscosity on the deformation conditions. The presence of a defining physical equation makes it possible to proceed to the solution of applied problems.

Strain hardening behavior of friction welded beta titanium alloy Titan 1023 used for aeronautical applications

The experimental true stress – true strain data has been evaluated for the solution treated and aged beta titanium alloy Titan 1023 in comparison with the solid state welded samples. 14 mm diameter rods extracted out of Titan 1023 rolled bars were welded employing rotary friction welding with optimized parameters. Strain hardening parameters for samples in solution treated, aged condition and welded samples were determined using Holloman equation with the data obtained by carrying out tensile testing at ambient temperatures employing a strain rate of 3x10−4S−1. Strain hardening rate in terms of Crussard-Jaoul (C-J) plots of the alloy in all pre and post treated conditions were elucidated. Work hardening capacity of the alloy in STA and welded conditions were determined and presented. The effect of resultant microstructures on the strain hardening behavior is discussed in the current study.

3D numerical investigation of the detonation wave propagation influence on the triangular plate deformation using finite rate chemistry model of LS-DYNA CESE method

In this article, an immersed boundary method which couples a Lagrangian structure solver with an Eulerian fluid solver with a chemistry model capable of detonation computation for H2O2 is developed to achieve the fluid-structure interaction, the deformation, and the damage of a thin plate subjected to a gaseous detonation loading. The Johnson-Cook material model was used for the plate which incorporates strain hardening, temperature softening, and strain rate effects. The detonation wave is modeled using LS-DYNA finite rate chemistry model incorporated in conservative element solution element solver. Other objectives of this numerical simulation such as estimation of deflection and stress state failure mechanism of material at high strain rate were derived based on a strain-rate dependent failure criterion. The simulated deformation pattern and the effect of pre-detonation pressure are compared with experimental results and a good agreement was acquired. Further additional simulations revealed that the pre-detonation pressure, ignition point location, and longitudinal capacity of the cylinder have great influences on the deformation and sensed pressure of the plate.

A directional framework of similarity laws for geometrically distorted structures subjected to impact loads

• The directional framework of similarity laws for structural impact is proposed, especially for geometric distortion . • Spatial directions and structural geometric characteristics are explicitly expressed in dimensionless numbers . • Correction methods of the velocity, the density and the geometry are used to address various distortion problems at scaling. • Various dimensionless response equations can be clearly, concisely and perfectly expressed. • Refined scaling analyses under the proposed framework are given numerically. A directional framework of similarity laws, termed oriented-density-length-velocity (ODLV) system, is suggested for the geometrically distorted structures subjected to impact loads. The distinct feature of the framework is that the newly proposed oriented dimensions, dimensionless numbers and scaling factors for all basic physical quantities are explicitly expressed by three characteristic lengths of spatial directions, which overcomes the inherent defects only with one scalar length in the traditional dimensional analysis. Meanwhile, similarity laws of the directional stresses, strains and displacements are expressed by different power law relationships of the ratios of undistorted characteristic lengths to distorted characteristic lengths. Therefore, the ability of similarity theory to express structural geometric characteristics are effectively developed. Based on the newly proposed framework, the non-scalabilities of geometric and material distortion (including strain hardening and strain rate effects) and the gravity effects could be compensated by correction methods of velocity, density and geometry. The analytical models of beams subjected to impact mass and impulsive velocity are verified. The results show that the proposed framework has excellent performance for expressing various dimensionless response equations and geometrically distorted scaling. A numerical model of circular plate subjected to dynamic pressure pulse is further carried out to verify the geometrically distorted scaling of the directional components of displacement, strain and stress. The refined analysis results show that, structural dimensionless responses in different directions can behave good consistency between the scaled model and the prototype in both the spatial and the temporal fields, with the correction of the directional physical quantities using different powers of the ratios of characteristic lengths.

Texture induced planar anisotropy of dwell fatigue response in titanium: Insights from experiments and crystal plasticity simulations

This study elucidates the mechanism of texture induced planar anisotropy in ambient temperature (∼25 °C) dwell fatigue response of an annealed sheet of commercially pure titanium (cp-Ti). Dwell fatigue resistance on loading along rolling direction (RD) was observed to be markedly superior when compared to the loading along transverse direction (TD). Texture caused significant anisotropy in the strain hardening and strain rate sensitivity (SRS) which altered the dwell-fatigue response between RD and TD. RD direction had higher strain hardening and lower SRS both of which synergistically led to higher dwell-fatigue life compared to the TD direction. A combination of electron back scattered diffraction (EBSD) and Crystal Plasticity Fast Fourier Transform (CPFFT) simulations confirmed higher activity of prismatic slip in RD which led to higher strain hardening compared to TD. An analytical model was applied to deconvolute the contributions of strain hardening and SRS on dwell-fatigue life. The effect of strain hardening was observed to be higher when compared to that of SRS. The results of this work indicate that crystallographic texture is a remarkable tool to improve the dwell fatigue life of titanium.

Tension-compression asymmetry and shear strength of titanium alloys

The relation between the tension-compression asymmetry (TCA) and shear strength in a wide variety of Ti alloys was examined. For this purpose, uniaxial tensile and compression, and double shear strength tests were performed on 13 Ti alloys, which have different microstructural phases (α, α+β, and β) and underwent different thermo-mechanical histories. Results show that the tensile-to-compressive yield strength and shear-to-tensile strength ratios of the alloys vary widely (between 0.79 to 1.24 and 0.52 to 0.86, respectively). A linear dependence between shear yield strength and tensile or compressive yield strength (whichever is lower) of all the alloys is found. While the ultimate shear and tensile strengths also show a linear dependence, data obtained on the fully β alloys are distinct. Finite element analyses employing the Drucker-Prager (DP) yield condition (to account for pressure-dependence on yield), Ludwick's strain hardening model, and Rice-Tracey failure criterion were performed to gain insights into the experimental trends. Results show that TCA decreases the shear yield stress while the shear strength is sensitive to the rate of hardening. A good correlation between the experimental and simulated shear-to-tensile yield strength ratios was observed. Based on a large, simulated dataset (generated using different combinations of material parameters), an empirical relation between shear strength, TCA, and tensile strengths was identified.