Strain-hardening Rate Research Articles

On the Simultaneous Improving of Strength and Elongation in Dual Phase Steels via Cold Rolling

The ferrite-pearlite microstructure was cold-rolled to form dual phase (DP) steels, the percentage reduction of which varied. To do so, the steels were annealed in two steps and then the workpiece underwent water quenching. Accordingly, a decrease was observed in the average size of the ferrite grains, from above 15 µm to below 2 µm, subsequent to the thermomechanical processing. By an increase in the reduction percentage, the volume fraction of martensite grew. The balance between strength and elongation also improved nearly 3 times, equivalent to approximately 37,297 MPa% in DP in comparison to 11,501 MPa% in the ferrite-pearlite microstructure, even after 50% cold-rolling. Based on Hollomon and differential Crussard-Jaoul (DC–J) analyses, the DP steels under investigation deformed in two and three stages, respectively. The modified C–J (MC–J) analysis, however, revealed that the deformation process took place in four stages. The rate of strain hardening at the onset of the deformation process was rather high in all DP steels. The given rate increased once the size of the ferrite grains reduced; an increase in the volume fraction of martensite due to larger percentage of reduction also contributed to the higher rate of strain hardening. The observation of the fractured surfaces of the tensile specimens indicated ductile fracture of the studied DP steels.

Initial microstructure influence on Ti–Al–Mo–V alloy's superplastic deformation behavior and deformation mechanisms

Superplastic forming is an effective way to manufacture complex-shaped parts of titanium-based alloys. This paper studies the influence of the initial microstructure and its strain-induced evolution on superplastic deformation behavior and the formability of a titanium-based alloy. Two types of Ti–Al–V–Mo alloy samples having a different fraction of recrystallized grains before the start of the superplastic deformation were studied. The deformation behavior, including strain hardening and strain rate sensitivity of the flow stress, was analyzed in a temperature range of 775 °C–900 °C and a strain rate range of 10−5 to 10−2 s−1. Strain-induced changes of the microstructure within the bulk of the samples and on the surface of the pre-polished samples were studied during superplastic deformation with a constant strain rate. The dynamic recrystallization and dynamic grain growth in the volume of the samples and the multiple slip bands on the samples' surface were revealed after superplastic deformation. The grain structure evolution and slip bands localization depended on the samples’ initial microstructure. The results showed that the samples with an increased fraction of recrystallized grains exhibited better superplasticity and higher quality of the formed parts with a more uniform thickness distribution across the section than the samples with a lower initial recrystallized fraction.

Deformation Behavior and Structural Evolution of Stainless Cr–Mn–N Steel during Low-Temperature Tension

Abstract—The deformation behavior and structure of 16.5Cr–18.8Mn–0.53N–0.07C steel deformed by tension in the temperature range from –196 to + 20°С have been investigated. A stage with a constant strain-hardening rate has been shown to be present in the temperature range –65 < t ≤ 20°C, which at –65 < t < 0°C is periodically interrupted by the parabolic hardening stage. Multiple strain localization has been observed in the samples at test temperatures of –65 < t < 0°С. There is γ → e-transformation in steel at all deformation temperatures. Its contribution to the total ductility of steel increases as the test temperature decreases. Tension tests of steel at –196°C has resulted in γ → e → α' transformation at the strain localization stage.

Assessment of reverse gun taylor cylinder experimental configuration

Experimental efforts for Taylor-anvil impact tests have often been limited to near room temperature. The ‘Reverse Gun’ method proposed by Gust in 1982 allows for the Taylor impact specimen to be uniformly heated without temperature losses before impact. Through the use of finite element analysis, we explore two topics in this work. First, we examine whether the reverse gun experimental configuration is comparable to the traditional Taylor-anvil setup. Second, we assess the accuracy of several commonly employed flow strength models in terms of their ability to predict the reverse gun experimental results which involve dynamic loading conditions and complex thermo-mechanical coupling. The reverse gun simulations are performed for tantalum targets at initial temperatures in the range 295 K to 1295 K and velocities from 135 m/s to 242 m/s. We show that with suitable care in the modeling of the preheated reverse gun experiments one can make valuable assessments of flow strength models. Given the conditions explored, these observations probe the thermal softening, strain hardening, and strain rate sensitivity of the material.

Effect of annealing temperatures on microstructure and deformation behavior of Al0·1CrFeCoNi high-entropy alloy

Abstract Strength-ductility synergy of the Al0·1CrFeCoNi high-entropy alloy can be tuned largely by cold-rolling followed by annealing treatments. However, strain-hardening behaviors and the underlying deformation mechanisms of the low-to high-temperatures-annealed microstructures have not been well clarified. In this paper, the Al0·1CrFeCoNi high-entropy alloy was cold-rolled, followed by annealing at 673–1273 K to investigate the effect of annealing temperatures on the microstructure and deformation behavior. Annealing at 673, 873, 1,073, and 1273 K resulted in no recrystallization, partial recrystallization, full recrystallization, and grain-coarsening, which yielded a stretched-grained, heterostructured, fine-recrystallized, and coarse-grained microstructure, respectively. The heterostructured microstructure was comprised of stretched, partially recrystallized, and fine recrystallized grains. Uniaxial tension results show that the 873 K-annealed specimens possessed a good combination of strength (a yield strength of ~746 MPa) and ductility (a uniform elongation of ~28%), compared with the high-strength-yet-low-ductility cold-rolled and 673 K-annealed specimens, and the high-ductility-yet-low-strength 1073 and 1273 K-annealed specimens. The relationship between the yield strength and average grain sizes of the Al0·1CrFeCoNi high-entropy alloy follows the Hall–Petch equation with a friction stress of ~137 MPa, and a high Hall–Petch coefficient of ~664 MPa·μm1/2. Five and three different regimes existed in the strain-hardening rate versus true strain curves for the 873, and 1073 and 1273 K-annealed samples, respectively. The occurrence of the second and fourth regimes, and the second regime were detected essentially caused by nanotwinning. The correlation between microstructure, tension properties, deformation mechanism, and strain-hardening behavior is discussed.

Effects of grain size on body-centered-cubic martensitic transformation in metastable Fe46Co30Cr10Mn5Si7V2 high-entropy alloy

Systematic investigation of microstructure characterization and mechanical property was performed on metastable Fe46Co30Cr10Mn5Si7V2 high-entropy alloy (HEA) with varying the grain size. The grain coarsening led to the decrease in stability of face-centered-cubic (FCC) phase, and consequently generated the formation of laminate-morphology hexagonal-close-packed (HCP) and butterfly-morphology body-centered-cubic (BCC) martensite. During the tensile test, a transformation-induced plasticity (TRIP) from the FCC to BCC via the intermediate HCP occurred, and the accelerated transformation rate of BCC increased the strain-hardening rate. Our results suggest that microstructural evolutions related to thermally-induced martensitic transformation and strain-hardening mechanisms can be well interpreted by understanding the FCC to HCP to BCC martensitic transformation.

Constitutive Analysis of the Anisotropic Flow Behavior of Commercially Pure Titanium

Plastic anisotropy is an important issue for metals possessing a hexagonal close-packed structure. This study investigated the anisotropic deformation characteristics of commercially pure titanium with basal texture. A quasi-static uniaxial compression gave rise to clear differences in flow curves and strain-hardening rates depending on the loading direction. This study employed a constitutive approach to quantify the contribution of (i) dynamic Hall–Petch strengthening, (ii) dislocation pile-up, and (iii) texture hardening with respect to the total flow stress. Such an approach calculated a flow stress comparable to the measured value, providing logical validity. The microstructural and mechanical differences depending on the loading direction (i.e., anisotropy) were successfully interpreted based on this approach.

Body-centered-cubic martensite and the role on room-temperature tensile properties in Si-added SiVCrMnFeCo high-entropy alloys

We present a new class of metastable high-entropy alloys (HEAs), triggering deformation-induced martensitic transformation (DIMT) from face-centered-cubic (FCC) to body-centered-cubic (BCC), i.e., BCC-DIMT. Through the ab-initio calculation based on 1st order axial interaction model and combined with the Gibbs free energy calculation, the addition of Si is considered as a critical element which enables to reduce the intrinsic stacking fault energy (ISFE) in SixV(9-x)Cr10Mn5Fe46Co30 (x = 2, 4, and 7 at.%) alloy system. The ISFE decreases from -30.4 to -35.5 mJ/m2 as the Si content increases from 2 to 7 at.%, which well corresponds to the reduced phase stability of FCC against HCP. The BCC-DIMT occurs in all the alloys via intermediate HCP martensite, and the HCP martensite provides nucleation sites of BCC martensite. Therefore, the transformation rate enhances as the Si content increases in an earlier deformation range. However, the BCC-DIMT is also affected by the phase stability of FCC against BCC, and the stability is the highest at the Si content of 7 at.%. Thus, the 7Si alloy presents the moderate transformation rate in the later deformation range. Due to the well-controlled transformation rate and consequent strain-hardening rate, the 7Si alloy possesses the superior combination of strength and ductility beyond 1 GPa of tensile strength at room temperature. Our results suggest that the Si addition can be a favorable candidate in various metastable HEAs for the further property improvement.

Dynamic Tension Deformation of Rare-Earth Containing Mg-1.9Mn-0.3Ce Alloy Sheet along the Rolling Direction at Various Temperatures

The tensile properties of rare-earth containing Mg-1.9Mn-0.3Ce alloy sheet along the rolling direction were experimentally investigated within the strain rate and temperature ranges of 0.001–1300 s−1 and 213–488 K. The obtained stress-strain responses of the alloy sheet indicate that both yield strength and strain-hardening rate increase when the strain rate increases, whereas they decrease with increase of temperature. Microscopic examination results show that basal slip, prismatic slip, and {101¯2} tension twinning take place in the tensile plastic deformation, while the occurrence of twinning is not obviously affected by the rate and temperature. Tensile samples tend to fracture in a ductile mode with increasing strain rate and temperature.

A full-field crystal plasticity model including the effects of precipitates: Application to monotonic, load reversal, and low-cycle fatigue behavior of Inconel 718

This work advances a recently developed high-performance, full-field elasto-viscoplastic fast Fourier transform (MPI-ACC-EVPCUFFT) formulation to model large plastic deformation and low-cycle fatigue behavior of Inconel 718 (IN718). Specifically, the recently developed model of Eghtesad et al., 2020 [1] incorporating a strain rate, temperature, and strain-path sensitive hardening law based on the evolution of dislocation density and a slip system-level back-stress law influencing the resolved shear stress is advanced in several aspects. First, strengthening effects due to grain size and shape, solid solution, shearing of small precipitates, and Orowan looping around larger precipitates are incorporated into the initial slip resistance, which evolves with the hardening law including the latent hardening. Second, the resolved shear stress on the slip plane in the direction of slip is altered by accounting for the two orthogonal shear stress components and the three normal stress components, in addition to the slip system-level kinematic effects. The model is used to interpret the complex mechanical behavior and microstructural data for samples of alloy IN718 in additively manufactured (AM) forms before and after hot isostatic pressing (HIP) and in a wrought form. Voxel-based microstructural cells consistent with the characterization data are synthetically constructed to initialize the model setups for predicting simple compression, tension, load reversal, and low cycle fatigue behavior of the alloy. Variation in the microstructural features such as the distribution of grain size and shape, crystallographic texture, content of annealing twins, and precipitates among the samples facilitated reliable calibration and validation of the model parameters. Predicted anisotropy, tension/compression asymmetry, non-linear unloading upon the load reversal, the Bauschinger effect, reverse hardening, texture evolution as well as cyclic hardening/softening along with the mean stress relaxation during low-cycle fatigue are in good agreement with the corresponding experimental data for the alloy. Furthermore, the simulations based on the high-resolution microstructural cells facilitated the discussion of the mechanical fields induced by microstructure, and especially annealing twins.

A Finite Similitude Approach to Scaled Impact Mechanics

The response characteristics of large-scale structures subjected to impact loading can in principle be determined by scaled experiments. Unfortunately, scaling suffers from scale effects and for impact mechanics, the non-scalability of strain rate and strain hardening can diminish the effectiveness of scaled trials. To resolve this difficulty, a new scaling method has recently appeared in the open literature called finite similitude. The theory is founded on the metaphysical concept of space scaling, where the idea is that by expanding or contracting space, changes in the governing mechanics can be assessed.In this paper the finite-similitude theory is further developed, where it is demonstrated how the constraints imposed by dimensional analysis can be broken. A new form of similarity is introduced but at the cost of requiring two scaled experiments at distinct scales. It is shown however, how the theory is able to combine the information from the two scaled trials to predict outcomes that can be markedly superior to what can be achieved with experiments at a single scale. All scale dependencies are accounted by the theory and consequently the new formulation attempts to capture scale effects, so provides a more realistic approach to scaled experimentation.Unlike dimensional analysis, the new first-order finite similitude theory can simultaneously target two independent physical properties of common dimension (e.g. initial-yield stress and linear strain hardening). The advantage offered by this feature is demonstrated analytically and numerically in the paper with a focus on axisymmetrical tube buckling and energy absorption. The analytical model serves to expound the theory and the numerical highlights its capabilities and the kinds of accuracy achievable with the new approach.

Prediction of slamming pressure considering fluid-structure interaction. Part I: numerical simulations

ABSTRACT Marine structures are subjected to slamming loads characterised by high hydrodynamic pressure within short time durations. Such loads can cause local and global damages to structures. This paper, which is Part I in a series, reports a numerical investigation of slamming loads acting on flat stiffened plates and their dynamic response. The nonlinear explicit finite element code LS-Dyna with the Multi-Material Arbitrary Lagrangian-Eulerian (MMALE) solver was adopted to simulate the slamming impact on marine structures. The numerical method was validated by the relevant experimental data from the open literature in a comparison of slamming pressure and deflection data, as well as existing formulations. The Eulerian formulation was applied to describe the fluid flow, and a Lagrange formulation was employed to model flat stiffened plates. A penalty coupling algorithm was utilised to realise the fluid-structure interaction (FSI) between the plate and the fluid. Bilinear strain hardening with no strain-rate hardening and effect of the heat-affected zone (HAZ) were considered for the numerical simulation of the aluminum model, while nonlinear strain hardening and strain-rate hardening were adopted to verify the steel models. Effects of key influenced parameters such as water impact velocity, material behaviour, and air cushion on the slamming response were addressed accordingly. Additionally, a simulation of water hitting structure was proposed to investigate the slamming load characteristics acting on the bottom decks of offshore structures. As the next paper in this continuing series, Part II will present empirically derived formulations for prediction of slamming loads based on extensive parametric studies of actual scantlings of offshore structures, using the simulation methodology of water hitting proposed herein in Part I.

Research on dynamic compression properties and deformation mechanism of Ti6321 titanium alloy

This research reports detailed investigations on the dynamic compression properties and the deformation mechanism in Ti6321 titanium alloy subjected to high strain rate loading by using the split Hopkinson pressure bar (SHPB). Microstructures of deformed samples were analyzed by scanning electron microscopy (SEM) with an electron backscatter diffraction (EBSD) detector and transmission electron microscopy (TEM). The experimental results demonstrate that with the increase of strain rate, the strength of Ti6321 titanium alloy increases significantly, which indicates the phenomenon of strain rate hardening. Due to twinning, the initial plastic deformation mechanism is by twin shear, the twins' interface is straight, continuous, and symmetrically distributed. The majority of twins are found at low strain stage (the strain range of 0.03 ∼ 0.06), which justifies the early initiation and contribution of twins in plastic deformation of the material. It also indicates that during plastic deformation twinning dominates the dynamic deformation mechanism. As the strain (e) increases further (when the strain is greater than 0.06), the number of twins also increases up to ɛ = 0.20. After that, the dynamic deformation mechanism started shifting from twinning deformation to dislocation slip.

On the Temperature Sensitivity of Dwell Fatigue of a Near Alpha Titanium Alloy: Role of Strain Hardening and Strain Rate Sensitivity

On the Temperature Sensitivity of Dwell Fatigue of a Near Alpha Titanium Alloy: Role of Strain Hardening and Strain Rate Sensitivity

In-situ microstructural investigations of the TRIP-to-TWIP evolution in Ti-Mo-Zr alloys as a function of Zr concentration

Aiming at overcoming the strength-ductility trade-off in structural Ti-alloys, a new family of TRIP/TWIP Ti-alloys was developed in the past decade (TWIP: twinning-induced plasticity; TRIP: transformation-induced plasticity). Herein, we study the tunable nature of deformation mechanisms with various TWIP and TRIP contributions by fine adjustment of the Zr content on ternary Ti-12Mo-xZr (x = 3, 6, 10) alloys. The microstructure and deformation mechanisms of the Ti-Mo-Zr alloys are explored by using in-situ electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The results show that a transition of the dominant deformation mode occurred, going from TRIP to TWIP major mechanism with increasing Zr content. In the Ti-12Mo-3Zr alloy, the stress-induced martensitic transformation (SIM) is the major deformation mode which accommodates the plastic flow. Regarding the Ti-12Mo-6Zr alloy, the combined deformation twinning (DT) and SIM modes both contribute to the overall plasticity with enhanced strain-hardening rate and subsequent large uniform ductility. Further increase of the Zr content in Ti-12Mo-10Zr alloy leads to an improved yield stress involving single DT mode as a dominant deformation mechanism throughout the plastic regime. In the present work, a set of comprehensive in-situ and ex-situ microstructural investigations clarify the evolution of deformation microstructures during tensile loading and unloading processes.

Effects of boron content on the microstructure and mechanical properties of twin-roll strip casting borated steel sheets

High borated steel sheets containing 0.25 wt% to 4.0 wt% boron including hypoeutectic (0.28 wt% B and 2.11 wt% B), eutectic (2.43 wt% B) and hypereutectic (4.01 wt% B) compositions were tried to be fabricated by a novel strip casting technology, and the effects of boron content on sub-rapid solidification behavior and subsequent microstructural evolution together with mechanical properties was studied. It was found that the morphology of borides depended greatly on the boron content. When increasing boron, the morphological change of borides was network-like → grainy → cluster-like → plate-like. Various orientation relationships between different morphological borides and γ-Fe in as-cast steels were also identified. After subsequent hot-rolling and solution-treating, ultra-fine (<10 μm) borides were obtained in hypoeutectic and eutectic steels. Benefiting from that, excellent mechanical properties was achieved, which was much better than that of steels prepared by traditional ingot casting. Particularly, when the boron content approached eutectic composition, of which the effect on the microstructure and mechanical properties was more sensitive. The steels containing 2.11 wt% and 2.43 wt% boron exhibited different morphological borides with a total elongation of 14.1% and 8.0%, respectively. Moreover, a thin hypereutectic borated steel sheet was also prepared, but the borides were very large and the total elongation was very low. The correlation between microstructure and strength was clarified based on the Hall-Petch behavior and intermetallic strengthening effect. Smaller γ-grains and higher volume fraction of borides were responsible for higher strength. The plastic behavior of steels was influenced by strain-hardening rate that was determined by the volume fraction of γ-Fe matrix. A better sustainable strain-hardening capability was beneficial to extend the stage of uniform deformation and enhance the ductility. In addition, the mechanical properties of steels were also decided by the fracture mechanism that was dominated by the size of borides.

Strain rate dependent shear localization and deformation mechanisms in the CrMnFeCoNi high-entropy alloy with various microstructures

CrMnFeCoNi high-entropy alloys (HEA) with various microstructures have been produced using cold rolling followed by critical annealing at various temperatures. Shear deformation behaviors of this HEA with various microstructures at a wide range of strain rates (2 × 10-3-5 × 104 s-1) have been characterized using hat-shaped specimens. Strain hardening exponent and strain rate sensitivity have been obtained for various microstructures. No shear localization was observed up to shear strain of 8 for all microstructures under lower strain rates (2 × 10-3-1 × 101 s-1), while stress drop and shear localization were found to occur at various critical shear strains for various microstructures under dynamic shear loading (5 × 104 s-1). A new formula, considering competition of strain hardening and strain rate hardening against thermal softening, was proposed to estimate the critical shear strains under dynamic shear loading and the predicted results were found to be in a fairly good agreement with the experimental data. Based on micro-hardness testing, the strain hardening due to the microstructure evolution was found be much stronger under dynamic shear loading than that under quasi-static loading at the same interrupted shear strain, which can be attributed to the more efficient grain refinement and the triggered hierarchical deformation nanotwins under dynamic shear deformation.

The Effect of Material Behaviour on the Surface Roughness during Machining of H13 Tool Steel

The aim of the present study is to know the effect of material properties on surface roughness during dry turning of H13 tool steel. Machining was performed using Tungaloy made carbide insert. Chip reduction coefficient (CRC) and surface roughness values (Ra) were experimentally determined. Twenty seven experiments were conducted following 33 factorial design. Subsequently, chip samples were examined under scanning electron microscope. Surface roughness values were found to be influenced by strain hardening and strain rate hardening of the material depending upon the values of speed, feed and depth of cut (d.o.c). Surface quality improves because of strain rate hardening at higher cutting speed.

Effect of tempering conditions on adiabatic shear banding during dynamic compression and ballistic impact tests of ultra-high-strength armor steel

In this study, roles of strain-hardening rate on susceptibility of adiabatic shear band (ASB) formation and subsequent cracking were investigated in two ultra-high-strength armor steel plates heat-treated differently. The quenched and tempered steel contained ~2% of retained austenite in the tempered martensitic matrix, while the quenched and austempered steel contained ~4% retained austenite in the bainitic matrix partly with the tempered martensite. The actual ballistic impact test results revealed the lower sensitivity of ASB formation in the austempered steel than in the tempered steel, which corresponded well to the higher critical strain for ASB formation in the dynamic compressive test using a laboratory-scale split Hopkinson pressure bar (SHPB). The austempered steel caused the higher internal stress among various constituents, and all the retained austenite transformed into martensite during the deformation, thereby leading to transformation-induced plasticity (TRIP) effect. The higher strain-hardening rate induced by these higher internal stress and TRIP effect increased resistance to ASB formation, which was confirmed by a calculation of ASB susceptibility. Thus, the austempered steel was much less susceptible to the ASB formation during the ultra-high-speed deformation. Consequently, the increased resistance to ASB formation retarded the initiation and propagation of ASBs and cracks.

Effect of silicon addition on the microstructures, mechanical properties and helium irradiation resistance of NiCoCr-based medium-entropy alloys

A series of NiCoCrSix (x = 0, 0.1, 0.2, 0.3) medium-entropy alloys were synthesized to investigate the effect of Si addition on the microstructures, mechanical properties and helium irradiation resistance of the NiCoCr-based alloys. The microstructures and the phase evolution of these alloys were examined using electron microscopy as well as the existing models. The results show that the alloys exhibit a single face-centered-cubic structure when the Si content is in the range of 0–6.25 at.%, whereas phase decomposition occurs as the Si content further increases (∼9 at.%). Comparing with NiCoCr, NiCoCrSi0.2 exhibited improved strength without sacrificing the ductility. The yield strength and ultimate tensile strength increased from 438 to 599 MPa and 921 to 1032 MPa, respectively, whilst the ductility kept as high as ∼50%. Our analysis indicates that the increased strength is mainly ascribed to the grain boundary strengthening. The large ductility is primarily due to the low stacking fault energy and shear modulus of the NiCoCrSi0.2 alloy, which makes mechanical twinning in small grains accessible and provides a steady strain-hardening rate in a wide strain regime. Besides, the enhanced compositional complexity and local lattice distortion induced by the introduction of Si can improve the helium irradiation resistance of NiCoCr, resulting in reduced average bubble size.