Dislocation Density Research Articles

Achieving 1.7 GPa Considerable Ductility High-Strength Low-Alloy Steel Using Hot-Rolling and Tempering Processes.

To achieve a balanced combination of high strength and high plasticity in high-strength low-alloy (HSLA) steel through a hot-rolling process, post-heat treatment is essential. The effects of post-roll air cooling and oil quenching and subsequent tempering treatment on the microstructure and mechanical properties of HSLA steels were investigated, and the relevant strengthening and toughening mechanisms were analyzed. The microstructure after hot rolling consists of fine martensite and/or bainite with a high density of internal dislocations and lattice defects. Grain boundary strengthening and dislocation strengthening are the main strengthening mechanisms. After tempering, the specimens' microstructures are dominated by tempered martensite, with fine carbides precipitated inside. The oil-quenched and tempered specimens exhibit tempering performance, with a yield strength (YS) of 1410.5 MPa, an ultimate tensile strength (UTS) of 1758.6 MPa, and an elongation of 15.02%, which realizes the optimization of the comprehensive performance of HSLA steel.

Grain size dependence of twin nucleation in magnesium alloys

Twinning plays a prominent role in the plastic deformation of low-symmetry crystals such as magnesium alloys. However, the capture of twin nucleation has been challenging since the process is rapid and difficult to predict, making it impossible to observe directly. Because of this, the understanding of the fundamental twinning mechanism through experiments and simulation techniques incorporating twinning has progressed slowly. In this study, a quantitative analysis integrating an in-situ three-point bending experiment and crystal plasticity modeling was conducted on the twin nucleation in AZ31 magnesium alloys with different grain sizes. The threshold for triggering twin nucleation has been quantified. The prediction ability of three widely used criteria for twin nucleation, i.e., geometrically necessary dislocation (GND) density, von Mises stress, and stored energy density, is evaluated. The results suggest that only the stored energy density is able to capture the twin nucleation location accurately in all the considered grain sizes. The critical value exhibits an exponential reduction with the increase in grain size. Based on the quantitative analysis, the mechanism of the grain size effect on twin nucleation is proposed.

Very high cycle fatigue properties of ultrahigh strength steels in the presence of hydrogen

The very high cycle fatigue (VHCF) properties of 2000 MPa ultrahigh strength martensitic steels after hydrogen pre-charging were investigated by means of ultrasonic fatigue tests. Results showed that the VHCF lives of the experimental steels were drastically decreased by about two orders after pre-charged with a hydrogen concentration of CH = 1.23 wppm. The experimental steel tempered at a higher temperature of 300 °C with some epsilon-carbides and a lower density of dislocations showed higher VHCF lives in the presence of hydrogen. It could be attributed to the lower hydrogen diffusion coefficient since hydrogen might accelerate fatigue crack propagation in the granular bright facet (GBF) area.

Microstructure and mechanical properties of a low-alloyed Mg–Zn–Al–Ca alloy: Effect of extrusion speed

A low-alloyed Mg-1.2Zn-0.6Al-0.1Ca (wt.%) alloy was extruded at 200 °C with different ram speeds (0.5–4.0 mm/s), and the microstructure and mechanical properties were studied systematically. Heterostructures with fine dynamic recrystallized (DRXed) grains and coarse unDRXed grains were achieved at lower ram speeds of 0.5 mm/s and 1.0 mm/s, and fully-DRXed microstructure was attained at 4.0 mm/s. Increasing the extrusion speed resulted in an increase in DRXed grain size from 0.9 μm to 3.8 μm, and a transformation of the DRXed texture component from <10 to 10>−<11–20> to a new orientation that deviated by approximately 14°. The sample extruded at 0.5 mm/s presented an excellent tensile yield strength (TYS) of 369 MPa along with a 7.8% elongation, which was mainly due to the high hetero-deformation induced (HDI) strengthening provided by its heterostructures. Increasing ram speed resulted in an improved elongation despite a decreased TYS. The reasons for the decreased strength with increasing extrusion speed were mainly associated with grain growth, reduced dislocation density and weakened HDI strengthening. The reasons for the improved ductility with increasing extrusion speed were largely due to the increased DRXed grains fraction with soft orientations.

Study on the plasticity improvement mechanism and grain refinement of AZ80 Mg alloy under cryogenic multi-directional forging

The introduction of twin lamellae and dislocation interaction through Multi-directional forging (MDF) at room temperature (RT) has been reported to significantly improve the mechanical properties of Mg alloys. However, the Mg alloys typically exhibited poor plasticity at RT, making it highly susceptible to cracking. This study further enhanced the deformation capacity of AZ80 Mg alloy by adding a pre-cryogenic step before MDF. Compared to direct MDF at RT, MDF at cryogenic temperature (CT, −196 °C) further increased the alloy's deformation capacity from 0.72 (4P) to 1.08 (6P), with the tensile yield strength (TYS) and ultimate tensile strength (UTS) increasing to ∼420 MPa and ∼512 MPa. The alloy subjected to cryogenic MDF exhibited denser twin lamellae and a higher proportion of {10–12}-{10–12} twin interactions. The more frequent twin interactions under CT inhibited excessive twin growth and further increased the dislocation density of the matrix, facilitating the nucleation of new twins. Additionally, with increasing deformation passes, the texture of the RT specimens became more single, while the CT specimen showed a trend toward texture dispersion. The study elucidated that poor forgeability of the alloy under MDF at RT was hindered by unfavorable texture and limited twin activation. However, this issue was mitigated under CT, where the activation of more atypical twins with low Schmid factors (SFs) significantly enhanced the deformation coordination ability. Furthermore, the activation of atypical twins and extensive twin interactions promoted texture dispersion in CT specimens, which facilitated the activation of various slips/twins and suppressed excessive twinning-induced hardening, thereby improving forgeability.

A framework toward fatigue life modeling of machining process verified in burnishing

In the present work a multiscale model was developed to predict fatigue lives of Inconel 718 after a machining process. The model captures the effects of surface integrity like residual stress, microstructure, hardness and roughness. Within this holistic predictive model, firstly burnishing process factors (viz static force, feed rate, spindle speed and pass number) are correlated to surface integrity aspects. Herein, residual stress is modeled using theory of incremental plasticity; also grain size and hardness were correlated to process factors through dislocation density evolution. In order to model the surface roughness, the Z-map approach considering the work hardening of the surface layers was utilized. The foresaid developed models of surface integrity aspects were utilized to predict the fatigue life using Navaro-Rios crack propagation together with Chen’s crack initiation models. Thus, a novel holistic model correlating machining parameters directly to fatigue life has been developed through this hybrid approach. The developed models of fatigue life and surface integrity aspects were then confirmed by series of burnishing experiments on Inconel 718. Later, the confirmed model was utilized to identify the parametric influence of burnishing process on variation of fatigue life where the results were justified using variation of surface integrity factors. Through this developed model, it was found that burnishing setting of 1000 N static force, 0.05 mm/rev feed rate, 300RPM spindle speed and 2 pass number results in improvement of HCF, MCF and LCF of 242 %, 184 % and 212 % where the obtained results from the developed framework were compatible well with experimental values.

Dislocation accumulation-induced strength-ductility synergy in TRIP-aided duplex stainless steel

In this study, we investigate the intrinsic mechanism of intensive and progressive transformation-induced plasticity (TRIP) effects and their different strength-ductility synergies using a resource-efficient 15Cr-2Ni duplex stainless steel. The progressive TRIP material exhibits a ductility that is more than twice that of the intensive TRIP material, as well as, a larger product of the ultimate tensile strength and ductility. This is attributed to the dislocation accumulation caused by different grain sizes of strain-induced martensite depending on the stability of the γ phase, which determines the strength and work hardening of steel. When the stability is low, the γ phase is sensitive to loaded stress and transformed into dispersed fine martensite immediately after yielding at a high rate. It induces a sigmoid-shaped dislocation accumulation to an approximately 10-fold increase in the dislocation density at a limited strain, resulting in intensive work hardening and a large ultimate tensile strength. As the stability is adequate, the γ phase is transformed into coarse martensite laths with a high critical load stress, which is initiated from a delayed strain at an extremely low rate and steadily accelerated as the strain increases. This process induces a gradually increased dislocation accumulation to a 2–3-fold increase in the dislocation density at large strains, resulting in progressive work hardening and an excellent ductility.

Evolution process of T1 precipitate in Al–Cu–Li–TiC/TiB2 alloy during aging treatment

In this study, particle-reinforced aluminium matrix composites (PRAMCs) of an Al–Cu–Li alloy were prepared using nano-sized TiB₂+TiC particles. The relationship between TiB₂+TiC nanoparticles and T1 precipitates during the ageing process, as well as the influence of TiC + TiB₂ particles on the growth process of T1 precipitates during the ageing process, were investigated. The grain sizes of the A0 and A1 samples were found to be 249.89 μm and 86.42 μm, respectively. The dislocation density is greater in the deformed A1 sample. The coefficient of thermal expansion (CTE) effect generated by TiC and TiB₂ particles stimulates the precipitation process of T1 precipitates. The A1 alloy reached the peak ageing state in a shorter time and yielded a greater number of T1 precipitates. The precipitation process of T1 precipitate is: SSSS→T1P→T1. The transition from T1P (face-centered cubic, FCC) to T1 (hexagonal close packing, HCP) entails a modification in the order of packing. The recently formed T1P precipitate is attached to the established T1 precipitate and extends along the c-axis through the shared copper-rich layer that has formed on both sides of the mature T1 precipitate. The mechanical properties of sample A1 are optimal at T = 22h, with a yield strength, tensile strength, and elongation of 520 MPa, 553 MPa, and 8.2%, respectively. In comparison to the A0 sample, the yield strength and tensile strength exhibited an increase of 5.05% and 5.13%, respectively.

Achieving high mechanical and corrosion properties of AA2050 Al-Li alloy: The creep aging under plastic loading

The influences of elastic/plastic loading (100–220 MPa) on the creep behavior, mechanical properties, and corrosion behavior of creep-aged AA2050 alloys were investigated. The results show that the creep rate increased from 0.35 % to 0.61 % with the increase of stress from 100 MPa to 220 MPa. The creep rate was increased rapidly under plastic loading (220 MPa) due to the increased dislocation density. Meanwhile, the plastic loading shortened the peak-aged time of creep-aged alloys and achieved outstanding strength (UTS=534 MPa, YS=496 MPa, peak aged), which increased by 33 MPa and 32 MPa compared with elastic loading, respectively. The strength enhancement was attributed to the increase in dislocation density, weak oriented precipitation effect, and dense precipitation of T1 phases. Additionally, compared with elastic loading, GBPs under plastic loading coarsened and distributed discretely, their elements content distributed evenly, and the Cu content increased. Therefore, the intergranular corrosion (IGC) depth and stress corrosion cracking (SCC) susceptibility index (ISSRT) decreased from 174 μm, and 8.7 % to 121 μm, and 5.9 %, respectively. These findings pave a way in breaking curvature limit of creep aging technology.

Effect of interlayer ultrasonic impact on the microstructure, mechanical and corrosion properties of wire arc additive manufacturing AZ31 Mg alloy thin wall

AZ31 Mg alloy thin walls are prepared using wire arc additive manufacturing (WAAM) and interlayer ultrasonic impact (UI) techniques with cold metal transition (CMT) serving as the heat source. The microstructure, mechanical and corrosion properties of thin walls prepared by WAAM and WAAM + UI are studied. Results showed that the recrystallization area fraction along traveling direction (TD) increased by 68.6% after UI treatment, and many fine equiaxed crystals were formed, resulting in grain refinement, anisotropy reduction, mechanical and corrosion properties improvement. The average grain size along TD decreased from 66.6 ± 3.5 μm to 32.7 ± 1.6 μm. Through UI treatment, the ultimate tensile strength (UTS) and elongation (EL) along TD increased from 205 MPa to 230 MPa and 13.5%–17%, respectively. The anisotropic percentage of UTS and EL were decreased from 10.8% to 4.5%, and 42.1%–9.7%, respectively. Electrochemical experimental results showed that the average corrosion rate along TD decreased from 1.93 mm year−1 to 1.53 mm year−1. Grain refinement, dislocation density variation and texture strength reduction were the main reasons for these results.

Dislocation Density Measurements from x-ray Diffraction of Austenite and Ferrite Phases in Superduplex UNS S39274

Dislocation Density Measurements from x-ray Diffraction of Austenite and Ferrite Phases in Superduplex UNS S39274

The influence of nano structuring and combined doping (Ag and Sb) on the thermoelectric properties of PbTe thin films

Nanocrystalline PbTe and Ag & Sb co-doped PbTe thin films are prepared using an integrated physical-chemical approach by evaporating chemically synthesized PbTe and Ag & Sb co-doped PbTe nanopowders on glass substrates. The X-Ray Diffractogram (XRD) reveals that all samples are NaCl-type structure and the structural parameters such as crystallite size D, dislocation density δ, strain ε, lattice constant, volume of the cell and number of crystallites are calculated and reported the results. The crystallite size of the films is around 24 nm. The absolute value of the Seebeck coefficient of nanocrystalline PbTe and Ag and Sb co-doped PbTe thin films is 1.069 mV/K and 0.569 mV/K respectively and the positive values indicate their p-type conductivity of the film. The values of Seebeck coefficient and power factor are higher than that of bulk which implies an enhancement in the thermoelectric properties with a reduction in the particle size and co doping.

Study on the evolution of multistage and multiscale Ti-bearing precipitation and microstructure in ultrahigh-strength titanium microalloyed weathering steels

In this study, 900 MPa ultrahigh-strength weathering steels were successfully developed through thermomechanical controlled processing (TMCP). Advanced microstructure characterization, combined with precipitation thermodynamics and kinetics models, elucidated the evolution of Ti-bearing precipitation and microstructure. The results showed that coiling temperature (CT) significantly impacts phase fractions, grain boundary density, and misorientation angles, while both CT and finishing rolling temperature (FRT) influence grain sizes in acicular ferrite (AF) and granular bainite. The lower coiling temperature resulted in a higher dislocation density of the test steel, which provided more nucleation sites for AF and TiC, favoring a higher number of TiC particles and a higher proportion of AF. Above 1050 °C, the addition of nitrogen changed the shape of the precipitation kinetics curve, reduced the nucleation energy barrier of TiC, decreased the critical nucleation size, and improved the nucleation rate. Meanwhile, the addition of nitrogen accelerated the precipitation transformation of TiC, which promoted the formation of Ti(C, N). Furthermore, increasing the deformation stored energy (DSE) further accelerated the precipitation of Ti(C, N) and significantly increased the nucleation rate. The formation mechanism of large-size Ti(C, N) and the transformation mechanism of Ti(C, N) to TiC are revealed by precipitation thermodynamics and kinetics. The completion time of TiC precipitation on dislocations is shorter than that on grain boundaries, which results in the TiC on dislocations being prone to coarsening during prolonged coiling. These findings provide crucial insights for optimizing the industrial production of ultrahigh-strength titanium microalloyed weathering steels.

Investigation of white etching area at non-carbides origin in M50 bearing steel under rolling contact fatigue

White etching areas (WEAs) were originated in regions free of carbides or non-metallic inclusions under rolling contact fatigue (RCF) in M50 bearing steel. Two types of WEAs with largely sheared and compact morphologies were observed. They were investigated using Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The results show that WEAs involve martensite-to-austenite phase transformation and amorphization, in contrast to the butterfly-winged WEAs consisting of only ferrite nanocrystallines. It is suggested that the nanocrystallines are the result of recrystallization. The high micro shear strain of 0.9–1.1 drives phase transformation. The solid-state amorphization occurs due to accumulation of high dislocation density. The dislocation density in the compact WEA is much higher than that in the sheared WEA.

Effect of strain ratio and pre-strain on the low-cycle fatigue behavior of 4130X steel at different strain amplitudes

This study investigates the impacts of strain ratio and pre-strain on the low-cycle fatigue properties and microscopic damage mechanisms of 4130X steel across varying strain amplitudes. Under symmetric loading, initial cycles at low strain amplitudes demonstrate clear peak/valley stress hardening followed by cyclic softening in later stages of fatigue. In contrast, the hardening behavior vanishes at higher strain amplitudes. Increasing strain ratio and pre-strain cause the initial hardening behavior at low strain amplitudes to gradually disappear. Additionally, at lower strain amplitudes, fatigue life decreases as strain ratio and pre-strain rise, attributed to substantial tensile mean stress. Microscopic examination reveals that symmetric cyclic loading at low strain amplitudes releases stress concentrations caused by quenched and tempered processing and reduces both dislocation density and localized plastic strain. Conversely, at higher strain ratios and pre-strains, increased tensile mean stresses not only intensify dislocation multiplication, resulting in high dislocation density, but also activate multi-slip system, leading to dislocation cross-slip. Moreover, at a high strain amplitude of 0.45 %, pre-strain aids in martensite recovery and promotes dislocation annihilation during recovery, thus inducing cyclic softening. Finally, the fatigue lives under varied loading conditions are compared with the fatigue design curve prescribed by ASME VIII-II code.

A cryogenic incremental sheet forming process for improving the formability of AA6061 to reveal the dual enhancement effect and microstructure evolution mechanism

Based on the dual enhancement effect of hardening and plasticity in aluminum alloys at cryogenic temperatures, the cryogenic incremental sheet forming process is introduced in this paper for manufacturing complex components. Experimental results indicate that incremental sheet forming at cryogenic environments results in a remarkable enhancement of formability. The ultimate forming height of specimen at 113K presents an increase of 32.4% compared with the specimen at 295K. Meanwhile, it is confirmed that cryogenic conditions increase the work-hardening ability and reduce the microcrack generation on the specimen surface. Moreover, the mechanism of dual enhancement effect on 6061 alloy during cryogenic incremental forming was studied. At 295 K, the dislocation distribution was localized due to significant cross-slip in the specimens. It was also observed that dislocation entanglement occurred at grain boundaries, which tends to cause stress concentration and therefore reduced formability. In contrast, at 113 K, the decrease in stacking fault energy leads to suppression of cross-slip, and the uniform slip leads to a significant increase in dislocation density. As a result, the cryogenic temperature exhibits enhanced work-hardening ability and formability. The rolling texture evolves mainly along the α and β orientation lines during the forming process, the 295K specimen generates a large number of Goss textures in the final forming region due to the uneven deformation which makes it difficult for the texture to evolve further. In contrast, at 113 K the texture fully evolves and intersects in the Brass texture along the two orientation lines due to the enhanced formability.

The effect of coordinated deformation on the microstructure and corrosion performance of 316L stainless steel/20# carbon steel composite pipes during hot compression

Through hot compression experiments under different deformation conditions (0.1 s−1 at 900 °C/1050 °C/1200 °C), the effect of coordinated deformation on microstructure evolution was studied, and the influence of different initial microstructures on corrosion performance was subsequently investigated. As a result, strain preferentially concentrated in 20#, resulting in dynamic recrystallization (DRX). Subsequently, grain refinement and DRX occurred in 316L. Additionally, the farther from the interface, the weaker the coordinated deformation effect. Compared to only 316L, the 316L in composite pipes has higher density of dislocations and low-angle grain boundaries. Corrosion resistance is related to the microstructure after coordinated deformation, decreasing with the increase in dislocation density, low-angle grain boundaries, and recrystallized grain size. At 1200 °C, the effect of coordinated deformation on 316L is significantly reduced. The passivation film resistance is 2.14 × 106 Ω·cm2, similar to that of only 316L. Therefore, appropriately increasing the hot rolling temperature and the thickness of 316L in the billet can reduce the effect of coordinated deformation on the DRX of 316L, thereby maintaining the pitting corrosion resistance of 316L.

Effects of cold plastic deformation on microstructure and magnetic susceptibility of Au-Pt alloys

Formation of the alloys of paramagnetic gold (Au) and diamagnetic platinum (Pt) holds promise for developing materials with ultra-low magnetic susceptibility. This study investigates the effects of cold plastic deformation on the mechanical properties and magnetic susceptibility of Au-29Pt alloys by microstructure characterization. After cold rolling, the Vickers hardness of the Au-Pt alloy increases from 138.35 HV to 218.28 HV at 80 % deformation. Meanwhile, the volume magnetic susceptibility exhibits a change from −19.10 ppm to −28.60 ppm with minor difference between the results measured from different orientations. The crystal orientation of the Au-29Pt alloys changes but no new phases or no textures are formed. With the increase of the deformation level from 0 % to 60 %, EBSD measurements demonstrates a ∼40-fold increment in average geometric dislocation density. The results indicate the formation of new defects, which are identified as edge dislocations and stacking faults via HRTEM. Consequently, the larger diamagnetic susceptibility at a higher deformation level is mainly correlated with the formation of defects. The results of this study could provide useful guidelines for the design of ultra-low magnetic susceptibility alloys.

Effect of the Laser Cladding Parameters on Microstructure and Elevated Temperature Wear of FeCrNiTiZr Coatings.

In order to prepare coating with good friction and wear resistance at elevated temperature on the surface of hot-working tool steel, by using a CO2 laser, FeCrNiTiZr high-entropy alloy coating with different laser scanning speeds (360, 480 and 600 mm/min, respectively) was successfully fabricated by using laser cladding technology on the surface of H13 steel in this paper. Phase constitutions, microhardness, microstructure, and wear characteristics of FeCrNiTiZr coatings under different laser scanning speeds were analyzed. It was determined that 480 mm/min was the optimal laser scanning speed. The results showed that the coating at the scanning speed of 480 mm/min consists of a BCC phase with significant lattice distortion and high dislocation density; the crystal structure is cellular crystal and dendrite crystal. The coating demonstrates the highest microhardness (842 HV0.2), which is 4.2 times that of the substrate (200 HV0.2). Its average friction coefficients at room temperature and 823 K are approximately one-seventh and one-third of the substrate's, respectively, and its wear volume is reduced by about 98% and 81% under these conditions. Compared to the substrate, the coating underwent slight abrasive wear, adhesive wear, and oxidative wear at both room temperature and 823 K. In contrast, the substrate underwent severe abrasive wear, adhesive wear, oxidative wear, and even fatigue wear.

Revealing the fatigue strengthening and damage mechanisms of surface-nanolaminated gradient structure

Extending the fatigue life of metals is a critical concern for maintaining material and component integrity in engineering systems. The integration of gradient structures within materials represents a highly promising approach to enhance the fatigue properties in metallic materials, while a detailed mechanistic understanding of the fatigue damage evolution of such structures is yet to be developed. Here, we report that the surface-nanolaminated gradient structure comprised of nanolaminates and hierarchical twins imparts remarkable resistance to both low-cycle and high-cycle fatigue. A dislocation-based strain gradient crystal plasticity model is developed to investigate the strengthening and damage mechanisms of our gradient structure. The size dependence of the initial dislocation density, its evolution and back stress hardening are taken into account and verified by the experimental data. The simulation results reveal that the strain delocalization and back stress hardening induced by the structure gradient significantly mitigate the fatigue damage accumulation. Additionally, in contrast to conventional gradient structures, the mechanical stability of the present structure enables these strengthening mechanisms to persist until crack initiation. These effects, combined with the sequential toughening mechanisms activated in the surface-nanolaminated gradient structure, ensure a marked life extension under low-cycle fatigue (by a factor of four), outperforming conventional gradient and other microstructural design strategies. Finally, a multiscale anti-fatigue design principal for damage homogenization is given based on the prior quantitative analysis.