Ultrafine-grained Interstitial-free Steel Research Articles

Mechanical response of dislocation interaction with grain boundary in ultrafine-grained interstitial-free steel

Interstitial-free (IF) steel with ultrafine-grained (UFG) size was used to clarify the underlying mechanisms of the ‘extra-hardening’ behavior from the Hall-Petch plot and the discontinuous yielding behavior from axial tensile tests. Using transmission electron microscopy (TEM) at different tensile strains, it was found that the TEM micrographs for the bulk sample of the UFG IF steel showed a decrease in dislocation density in the grain interior after experiencing a micro tensile strain of 1.0% before macro-yielding. Additionally, an increase in dislocation density was found in the grain interior, with a micro tensile strain of 15% in the necking region after macro-yielding. In-situ compression tests in a TEM for micropillars were performed to directly observe the motion of dislocations and corresponding interactions with grain boundaries in UFG IF steel. Before macro-yielding, pre-existing dislocations in the grain interior moved toward the grain boundary, wherein they were then absorbed. The dislocation density decreased significantly due to the annihilation, which was accompanied by an increase in load during pre-yielding. The decrease in dislocation density in the grain interior of the micropillar is consistent with the results obtained from the bulk sample. After macro-yielding, the burst-like dislocation emission emerged from the grain boundary, leading to a yield drop and a discontinuous yielding behavior. Furthermore, the higher yield stress and the yield drop in the UFG IF steel can be understood by the decrease in dislocation density according to a combination of the Orowan model as well as the Johnston-Gilman model.

The role of J3 in the Orientation Analysis of the Adiabatic Shear Band of Ultrafine‐Grained Interstitial‐Free Steel

Within the framework of classical plastic theory, the instability inclination angle of metal is 45 ° under axial compression loading. However, microscopic observations by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show the ultrafine‐grained (UFG) interstitial‐free (IF) steel inclination angle of adiabatic shear instability is less than 45 °. Herein, a constitutive model that accounts for the effect of the third stress invariant is proposed based on the Johnson–Cook model to investigate the effect of J3 on the adiabatic shear instability inclination angle of UFG IF steel. To give a clear and direct interpretation of this effect, the model is implemented in the commercial software ABAQUS/Explicit by user subroutine VUMAT. Then shear‐compression specimens with different groove angles are used to investigate the influence of J3 on the orientation of adiabatic shear band. Subsequently, dynamic tests of UFG IF steel shear‐compression specimen with different groove angles are conducted through Split‐Hopkinson Pressure Bar. The experimental results show that noncoaxial influence (the effect of J3 or Lode angle) deserves enough attention in the orientation analysis of adiabatic shear instability of UFG IF steel.

Dynamic Mechanical Responses of Ultrafine-Grained IF Steel Over a Wide Range of Temperatures

The ultrafine-grained interstitial free (UFG-IF) steel was fabricated using the equal channel angular pressing (ECAP) method. Transmission electronic microscope (TEM) observations showed that the grains were refined to sub-micrometer scale and the average grain size was several hundred nanometers (nm). Uniaxial compression tests were conducted over a wide range of strain rates (up to 6000/s) and temperatures (from −196[Formula: see text]C to 300[Formula: see text]C) to understand the dependence of mechanical behaviors on the strain rate and temperature. It is found that the UFG-IF steel showed a weak strain rate sensitivity and displayed a strong temperature sensitivity only within a very narrow temperature range. Microstructural observations on the specimens deformed dynamically at temperatures below −20[Formula: see text]C via scanning electronic microscopy (SEM) and TEM indicated the occurrence of adiabatic shear localization. In order to explain the reason why adiabatic shear localization formed at the low temperatures and high strain rates, the susceptibility factor to adiabatic shear band (ASB) was estimated. It is proposed that more heat generated during the adiabatic plastic deformation process due to the enhanced flow stress of the UFG-IF steel at lower temperatures may play a dominant role in the formation of ASB. Meanwhile, the experimental results also provide strong evidence that for the UFG-IF steel the dynamic recrystallization which results from the severe plastic deformation and high temperature rise only occurs at the later stage of ASB evolution. With regard to the formation of ASB in the UFG IF steel, the localized plastic flow at a lower stress level indicates that localized deformation happens firstly and the temperature rise in the localized shearing zone leads to the final plastic instability.

Electron backscatter diffraction and performance of a severely deformed steel

This study investigated electron backscatter diffraction and performance of ultrafine-grained interstitial-free (IF) steel fabricated by an asymmetric rolling process (ASR). For ASR operations, plastic strain was induced with a thickness reduction of 30% per pass and a roll speed ratio of 1:4 for the lower and upper rolls, respectively. As the number of operations increased, the deformed microstructure developed gradually from a band-like structure, which was observed after a single pass, to almost equiaxed ultrafine grains (~0.7 μm) divided by high-angle boundaries after 4-pass ASR. The mechanical behaviors analyzed by microhardness and tension tests showed that the mechanical strength of the IF steel samples improved with increasing number of ASR operations, which was associated with the structural refinement and textural evolution in the ASR-deformed sample.

Bulk Ultrafine-Grained Interstitial-Free Steel Processed by Equal-Channel Angular Pressing Followed by Flash Annealing

Interstitial-free steel workpieces are deformed by equal-channel angular pressing (ECAP) for equivalent strain εvm = 3 and εvm = 21 followed by flash annealing. Microstructures are analyzed by optical microscopy, scanning electron microscopy and transmission electron microscopy. Mechanical properties are evaluated by hardness testing. Yield strength of materials is calculated from hardness values. Flash annealing (at 675 °C) of ECAPed samples for εvm = 3 and εvm = 21 results in abnormal subgrain growth and abnormal grain growth, respectively. Flash annealing at 700 °C of ECAPed (at εvm = 3) IF steel converts abnormally grown subgrains to grains which serve as nuclei for recrystallization and that result in bimodal grain size distribution. Bimodal grain size distribution is also produced when ECAPed IF steel for εvm = 21 is flash annealed at 675 °C due to abnormal grain growth or secondary recrystallization. Flash annealing of IF steel samples ECAPed for low εvm, in the temperature range 600-675 °C, decreases the hardness continuously with increase in the annealing temperature but it increases at high εvm. The former is due to annihilation of defects but the later is caused by ordering of nonequilibrium boundaries. The hardening and strengthening behaviors are similar.

Microstructure and Mechanical Properties of Ultrafine-Grained Interstitial-Free Steel Processed by ECAP

Equal-channel angular pressing (ECAP) of interstitial-free (IF) steel at equivalent strain, evm = 12 has been employed to develop ultrafine-grained (UFG) microstructure with high fraction of low angle grain boundaries, that enhances strength significantly with reduced tensile ductility. ECAPed IF steel has been deformed further by cold rolling/cryorolling at −50 °C to >90 % reduction in area. It is observed that the UFG structure gets refined with an improvement in high angle grain boundary fraction and heavily stressed non-equilibrium grain boundaries in cryorolled state resulting in significant strengthening. However, the decrease in grain size to an ultrafine level with the increased lattice strain lowers the work hardening ability of the material that limits its ductility. Hence, the rolled samples are flash annealed at 675 °C in order to recover the ductility of the material by achieving partially recrystallized structures. Consequently, the increased subgrain size as well as the grain size, the reduced residual lattice strain, lower hardness and strength with marginal recovery of ductility is maintained in order to attain the yield strength 2–3 times compared to that of as-received coarse-grained IF steel.

Structural characterization of ultrafine-grained interstitial-free steel prepared by severe plastic deformation

Interstitial free steel with ultrafine-grained (UFG) structure was prepared by high-pressure torsion (HPT). The development of the microstructure as a function of the number of HPT turns was studied at the centre, half-radius and periphery of the HPT-processed disks by X-ray line profile analysis (XLPA), positron annihilation spectroscopy (PAS) and electron microscopy. The dislocation densities and the dislocation cell sizes determined by XLPA were found to be in good agreement with those obtained by PAS. The evolution of the dislocation density, the dislocation cell and grain sizes, the vacancy cluster size, as well as the high-angle grain boundary (HAGB) fraction was determined as a function of the equivalent strain. It was found that first the dislocation density saturated, then the dislocation cell size reached its minimum value and finally the grain size got saturated. For very high strains after the saturation of grain size the HAGB fraction further increased. The PAS investigations revealed that vacancies introduced by severe plastic deformation agglomerated into small clusters consisting of 9–14 vacancies. The evolution of the yield strength calculated from the microhardness as a function of strain was explained by the development of the defect structure.

Effect of dislocation and grain boundary on deformation mechanism in ultrafine-grained interstitial-free steel

Ultrafine-grained interstitial-free steel fabricated by the accumulative roll-bonding method was subjected to tensile tests and analyses of AFM, TEM and XRD to identify the effects of interaction between dislocations and grain boundaries (GB) on the deformation mechanism. The AFM analyses indicated that the main deformation mechanism of this material changed from dislocation motion to grain boundary sliding (GBS) with decreasing strain rate. TEM observations and XRD analysis revealed showed that dislocations piled up at GB and the dislocation density decreased with increasing strain. Those suggest the dislocations are absorbed into GB during deformation, activating slip-induced GBS.

Formability of Ultrafine-Grained Interstitial-Free Steels

The stretch formability of ultrafine-grained (UFG) interstitial-free steel (IF-steel) produced by equal-channel angular extrusion/pressing (ECAE/P) via various strain paths was investigated with a miniaturized Erichsen test. A coarse-grained (CG) sample demonstrated high formability with an Erichsen index (EI) of 4.5 mm. Grain refinement by ECAE decreased the formability, but increased the required punch load (FEI) depending on the applied strain paths. The EI values were 0.35, 2.90, and 3.91 mm for 8A-, 8Bc-, and 8C-processed samples, respectively. Decrease in the biaxial stretch formability was attributed to the limited strain-hardening capacity of the UFG microstructure. Also, the grain morphology of the UFG microstructure was found to be very influential on stretch formability. Heavily elongated grain morphology in the 8A-processed microstructure resulted in the lowest formability due to the increased cracking tendency through elongated grain boundaries. However, the UFG microstructures with equiaxed grains obtained after 8C and 8Bc ECAE resulted in better formability compared to 8A. The UFG microstructure reduced the roughness (orange peel effect) of the free surface of the biaxial stretched samples by decreasing the non-uniform grain flow leading to the so-called orange peel effect. It should be noted that the strength and ductility values gained from uniaxial tensile tests are not comparable directly to the EI and FEI values determined from the Erichsen tests. Finally, it is important to emphasize that the UFG microstructure produced by a suitable strain path leading to equiaxed grains below 1 μm could be highly deformed even under multiaxial stress conditions.

Effect of strain rate on deformation mechanism for ultrafine-grained interstitial-free steel

Ultrafine-grained (UFG) interstitial-free steels with grain sizes of 0.39, 0.42, and 0.51μm were used to ascertain effects of the strain rate (ε̇) on the primary deformation mechanism at room temperature. Tensile tests were performed to obtain the strain-rate sensitivity exponent of 0.2% proof stress. The value was evaluated as 0.02 at high strain rates but as −0.01 at low strain rates. The transition was observed at ε̇ of 10−3s−1 for each sample. Although the negative m value might result from strain aging, the influence of grain boundary sliding (GBS) increased remarkably at a low strain rate because it reached 76% of plastic strain and became about six times as much as that at a high strain rate. Therefore, it is claimed that the dominant deformation mechanism was changed by the strain rate from dislocation motion to GBS with decreasing strain rate across ε̇≈10−3s−1.

Impact Toughness of Ultrafine-Grained Interstitial-Free Steel

Impact toughness of an ultrafine-grained (UFG) interstitial-free (IF) steel produced by equal-channel angular extrusion/pressing (ECAE/P) at room temperature was investigated using Charpy impact tests. The UFG IF steel shows an improved combination of strength and impact toughness compared with the corresponding coarse-grained (CG) one. The CG IF steel samples underwent a transition in fracture toughness values with decreasing temperature because of a sudden change in fracture mode from microvoid coalescence (ductile) to cleavage (brittle) fracture. Grain refinement down to the submicron (≈320 nm) levels increased the impact energies in the upper shelf and lower shelf regions, and it considerably decreased the ductile-to-brittle transition temperature (DBTT) from 233 K (−40 °C) for the CG steel to approximately 183 K (−90 °C) for the UFG steel. Also, the sudden drop in DBTT with a small transition range for the CG sample changed to a more gradual decrease in energy for the UFG sample. The improvement in toughness after UFG formation was attributed to the combined effects of grain refinement and delamination and/or separation in the heavily deformed microstructure. Although an obvious change from the ductile fracture by dimples to the brittle fracture by cleavage was recognized at 233 K (−40 °C) for the CG steel, no fully brittle fracture occurred even at 103 K (−170 °C) in the UFG steel.

Microstructure and mechanical properties after annealing of equal-channel angular pressed interstitial-free steel

The evolution of microstructure, microtexture and mechanical properties during isothermal annealing of an ultrafine-grained interstitial-free steel after eight passes of route B C room temperature equal-channel angular pressing (ECAP) was studied. The microstructure and microtexture were characterized by electron back-scattering diffraction, and mechanical properties were assessed by shear punch and uniaxial tensile testing. Homogeneous coarsening via continuous recrystallization of the ECAP microstructure is accompanied by minor changes in the ∼63% high-angle boundary population and a sharpening of the original ECAP texture. This is followed by abnormal growth during the final stages of softening due to local growth advantages. Linear correlations between shear and tensile data were established for yield, ultimate strength and total elongation. After yield, the changes in uniaxial tensile behaviour from geometrical softening after ECAP to load drop, Lüders banding and continuous yielding after annealing is attributable to a coarsening of the microstructure.

Formability of ultrafine-grained interstitial-free steel fabricated by accumulative roll-bonding and subsequent annealing

The miniaturized Erichsen test was successfully developed for evaluating the stretch formability of the ARB and annealed IF steel sheets. The Erichsen value tends to decrease as the average grain size becomes small. However, it should be emphasized that even in the specimen with ultrafine grain, a high Erichsen value is obtained. This result indicates that the ultrafine grained IF steel sheet could be highly deformed under multiaxial stress.

Fatigue Damage Evolution in Ultrafine‐Grained Interstitial‐Free Steel

AbstractThe current work presents the crack propagation behavior in ultrafine‐grained (UFG) interstitial‐free (IF) steel, and in particular, focuses on the damage evolution in UFG IF steel under cyclic loading. The current results indicate that equal‐channel angular pressing (ECAP) has a major influence on the cyclic deformation response of the UFG IF steel, such that the failure and the crack path depend on the inclination plane during ECAP. Furthermore, the UFG IF steel demonstrates significant notch sensitivity in comparison to its coarse‐grained counterpart. This is attributed to the ultrafine grains with a large volume fraction of high‐angle grain boundaries, where glide of dislocations is hindered and the resulting internal stresses increase the stress concentration further in the presence of a pre‐existing notch.

On the coupled temperature–strain rate sensitivity of ultrafine-grained interstitial-free steel

The coupled temperature–strain rate sensitivity of ultrafine-grained interstitial-free steel was investigated. The current findings indicate that the grain boundary misorientation angle distribution dictates the observed increased strain rate sensitivity and strain hardening rate at elevated temperatures.

On the fatigue crack growth–microstructure relationship in ultrafine-grained interstitial-free steel

The crack growth behavior in an ultrafine-grained (UFG) interstitial-free (IF) steel processed by equal channel angular pressing (ECAP) was investigated utilizing miniaturized compact-tension specimens with different microstructural characteristics. The current results demonstrate that both the ECAP processing route and the direction of crack growth with respect to the extrusion direction dictate the crack growth behavior in UFG IF steel. Specifically, the highest crack growth rates and the lowest threshold values were observed for the lowest grain size. Moreover, an unusual deviation from the expected direction of crack expansion was observed, where the deviation depended on the processing route and direction of crack growth. This deviation is attributed to the presence of elongated structures in the microstructure, which were mainly detectable in the UFG IF steel following a small number of pressings, and to a smaller extent in the optimized microstructures. Specifically, these elongated structures formed parallel to the material’s plastic flow during ECAP processing and moved the crack away from the expected direction of growth due to the high stress concentration zones they created along with the process-induced damages.

An Assessment of Bulk and Local Stored Energy Measurements in Ultrafine Grained Interstitial Free Steel

The evolution of stored energy and associated thermal behaviour was investigated for an ultrafine grained Ti-IF steel severely deformed by Equal Channel Angular Pressing (ECAP) followed by cold rolling at ambient and liquid nitrogen temperatures. Bulk stored energy measurements by Differential Scanning Calorimetry (DSC) returned 350-600 whereas local stored energy estimates from microhardness, Electron Back-Scattering Diffraction (EBSD) and X-ray line profile analysis resulted in 5-140 . Higher bulk stored energy values correspond to the enthalpy release from all sources of strain in the material volume as well as Ti precipitation during annealing while the lower local stored energy range alludes only to dislocation content or internal stresses. An apparent activation energy of 500-550 suggests sluggish recrystallisation due to excess of Ti in solid solution.

Monitoring the fatigue-induced damage evolution in ultrafine-grained interstitial-free steel utilizing digital image correlation

The digital image correlation (DIC) technique was successfully utilized to detect fatigue-induced damage and monitor its evolution in ultrafine-grained interstitial-free steels of three different microstructures in the low-cycle fatigue regime. Specifically, visualization of strain localization with DIC allows for detecting the crack initiation sites after only a few cycles into the deformation. Furthermore, optical microscopy, atomic force microscopy and electron backscatter diffraction analyses revealed a direct correlation between elongated grains and the crack initiation sites. The results of a crystal plasticity model demonstrated that higher overall stresses are prevalent in the microstructures with elongated grains, which is attributed to strain localizations, and corresponding stress concentrations responsible for crack initiation. Overall, the current findings show that DIC is a novel and promising non-destructive technique for determining the crack initiation sites at the very early stages of cyclic deformation.

Cyclic stability of ultrafine-grained interstitial-free steel at elevated temperatures

The fatigue performance of an ultrafine-grained interstitial-free steel was investigated at elevated temperatures up to a homologous temperature of 0.39. The significantly altered cyclic stability under combined thermal-mechanical loading is attributed to localized grain growth.

The Effect of Texture on the Fatigue Properties of Ultrafine-Grained Interstitial-Free Steel

The present paper reports on the effect of texture on the cyclic stress-strain response (CSSR) and the fatigue life of ultrafine-grained (UFG) interstitial-free (IF) steel. Tests in the lowcycle fatigue (LCF) regime were conducted on material that was processed by the equal channel angular extrusion (ECAE) technique along the so called “efficient” route 8E. This route has been shown to result in a homogeneous microstructure with a high fraction of high angle grain boundaries (HAGBs), which are beneficial for a stable CSSR. In addition, the evolution of the microstructure was characterized by means of electron optical techniques, including electron backscattered diffraction, and by X-Ray diffractometry. It was found that the initial texture of specimens cut from the ECAE billet along different orientations with respect to the extrusion direction (ED) has a substantial effect on the CSSR of the UFG IF steel. Furthermore, microscopy results indicated the notable influence of the last ECAE processing step on the evolution of damage in the material.