High Dislocation Density Research Articles

A customised novel hybrid post-treatment process achieved excellent mechanical properties in additively manufactured Haynes 230 alloy

ABSTRACT It is challenging to maintain decent plasticity while increasing strength in laser powder bed fusion (LPBF) manufactured Haynes 230 alloy. The fundamental issue of both inadequate affected depth and deteriorated plasticity exists for laser shock peening (LSP). To address this problem, the treatment method of heat treatment plus laser shock peening (HT-LSP) was proposed. After heat treatment, dispersive M23C6 carbides were precipitated, and dislocation was predominately decreased inside grains but increased at grain boundaries. The junction of three imperfect dislocations was identified at M23C6 after HT-LSP treatment, indicating the novel phenomenon of twin formation via the polar-axis mechanism of dislocation proliferation, given the inhibition of intergranular transfer of dislocations and adequate clean rooms without dislocation clusters inside grains. Grain rotation occurred after HT-LSP derived from dynamic recrystallisation and twin distortion. The hardness along depth direction of the sample demonstrated the increased effective affected depth of LSP from 900 to 1400 μm after heat treatment. The shear strength of the HT-LSP sample was increased to 662 MPa. The elongation of the sample reaching 13.4% was also higher than that with LSP. The mechanical performance improvement was mainly due to the gradient twin layer, carbides’ precipitation and high-density dislocation.

Tailoring charge transport in BaTiO3 crystals through dislocation engineering

AbstractDislocations in oxide ceramics significantly influence their physical properties by creating substantial local strain fields, new electronic states, and space‐charge layers. In this study, we investigated the effects of mechanically introduced dislocations on the electrical conductivity of BaTiO3 single crystals. High‐temperature plastic deformation was employed to introduce a high dislocation density with a {100}〈100〉 slip system. Impedance measurements revealed a significant anisotropy in the conductivity due to the presence of oriented dislocation structures. The crystals with dislocation lines aligned parallel to the measurement axis ([001] crystallographic direction) exhibited 16‐fold higher conductivity compared to those measured across the dislocations. Compared to the pristine crystals, this means an increase in conductivity when the measurements were carried out parallel to dislocation lines and a decrease in perpendicular measurements. Our study demonstrates that not only ferroelectric properties but also charge transport can be modified by dislocation introduction in BaTiO3.

Microstructure control and DRX characteristics of Ni–Co–W superalloys affected by changing deformation direction on [001] columnar grain

Controlling the evolution process of columnar grains is benefits to achieve microstructure regulation during subsequent hot processing in superalloys. In present research, it takes a Ni–Co–W superalloy as an example, aims to clarify the underlying connections between the compression direction and dynamic recrystallization (DRX) behaviors in microstructure evolution. The compression direction (CD) was parallel or perpendicular to [001] columnar has been defined as CD∥[001] and CD⊥[001], respectively. The columnar evolution and DRX characteristics under two sets of experimental during hot deformation were identified deeply. The results show that complete DRX is more easily to occur when CD⊥[001], but fine DRX grains are tendency to form when CD∥[001]. DRX nucleation within CD∥[001] and CD⊥[001] deformed microstructure under dislocation energy was discussed deeply. The critical size of nucleation is decreased while the nucleation density is increased in CD∥[001] with high dislocation density, which benefits to form numerous fine DRX grains along the original columnar boundaries. In addition, according to Taylor factors (TFs) criterion, TFs difference will always existed in CD∥[001], which promotes the necklace structure gradually replaced columnar structure and some of them developed into fine DRX bands. With increasing of trues strain, TFs difference gradually decreased in CD⊥[001], DRX nucleation was inhibited, thus the existed DRX grains further grow and finally coarse DRX grains were obtained. The findings clarified the flow behaviors and DRX characteristics of [001] columnar in two directions, and then proposed a microstructure control mechanism of superalloys with [001] columnar based on deformation vector and evolution decomposition.

Investigating the hot deformation behavior and microstructural evolution of Mo-14Re alloy at various strains and strain rates

This study examines the hot deformation behavior of Mo-14Re alloy at various true strains (15%, 35%, 65%) and strain rates (0.01 s−1, 10 s−1) at a temperature of 1400K. The findings indicate that dynamic recovery (DRV) and dynamic recrystallization (DRX) occur concomitantly as strain increases at a low strain rate of 0.01 s−1, with DRV being the predominant softening mechanism. At a strain of 65%, DRX emerges as the primary softening process. Conversely, under high strain rates of 10 s−1, DRX is inhibited, and the Mo-14Re alloy experiences work hardening due to an increase in dislocation density. Microscopic analysis shows that the high-density dislocations facilitate the continued nucleation and growth of recrystallized grains at low strain rates. At high strain rates, tangled dislocations hinder dislocation motion and recrystallization. Regarding texture evolution, stronger {100}//CD and weaker {111}//CD fiber texture is observed at low strain rates of 0.01 s−1, while stronger {111}//CD and weaker {100}//CD fiber texture forms at high strain rates of 10 s−1, with enhanced texture intensity. Mechanistic analysis confirms the activation of the {110}<111>, {112}<111>, and {123}<111> dislocation slip systems at elevated temperatures, with the {123}<111> system being the most dominant.

The effect of direct aging treatment on microstructure and mechanical properties of the GH4099 superalloys produced by selective laser melting

The effect of direct aging (DA) treatment on microstructure and mechanical properties of the GH4099 superalloys produced by selective laser melting (SLM) was investigated by the microhardness tests, room- (RT) and high-temperature (HT) tensile tests, field emission scanning electron microscopy (FE-SEM), and electron backscattered diffraction (EBSD) in this study. Both microhardness and RT tensile results show that DA treatment improves mechanical properties more than solution-aging (SA) treatment. By analyzing the effect of the microstructural features on the yield strength, we found that the dominant strengthening mechanisms responsible for higher yield strength of the DA specimens are the grain size hardening of fine γ matrix, high-density dislocation strengthening, Orowan dislocation bypass strengthening of smaller but dense M23C6-type carbides, and chemical strengthening of smaller but dense γ΄ nanoprecipitates. Furthermore, a transgranular fracture is a dominant fracture type for RT tensile specimens produced by DA treatment. Conversely, the failure type of SA tensile specimens is a typical intergranular fracture mechanism. The underlying mechanism for the above phenomenon is attributed to the effects of transgranular and intergranular M23C6-type carbides.

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.

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.

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.

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.

Laser clad CoCrFeNiTixNby hypoeutectic high-entropy alloy coating: Effects of Ti and Nb content on mechanical, wear and corrosion properties

High-entropy alloys (HEAs) exhibit significant potential for advanced wear and corrosion-resistant applications. This study investigates the influence of Ti and Nb doping on the microstructure, phase composition, and properties of CoCrFeNiTixNby HEAs synthesized using high-speed laser cladding (HLC). The results demonstrate that increasing Ti and Nb content transforms the coating structure from a face-centered cubic (FCC) phase to a hybrid structure comprising FCC, body-centered cubic (BCC), and Laves phases, leading to a linear enhancement in surface hardness. The incorporation of Ti and Nb not only promotes the preferred orientation of the FCC phase (111) crystal plane but also significantly enhances tensile strength, though this comes at the expense of reduced plasticity. Specifically, the CoCrFeNiTi0.6Nb0.15 coating attains an ideal balance between strength and ductility, with a tensile strength of 1641.8 MPa and a tensile strain of 15.9 %, achieved through the formation of a eutectic structure. This coating also exhibits superior wear resistance and outstanding corrosion resistance, which are attributed to the stability of the passivation film, reinforced by the (111) crystal plane and high-density dislocations. These findings provide both theoretical and empirical foundations for the design of high-performance HEAs, underscoring their potential in industrial applications requiring robust wear and corrosion resistance.

Altering tensile and crack initiation behavior in a metastable β titanium alloy via designing grain size and precipitates

Microstructure can play a vital role in defining mechanical properties of metallic materials. To elucidate this correlation in the case of Ti–15Mo–3Nb–3Al-0.2Si (TB8) alloy, herein, we designed various microstructures via heat treatment exploring the effects of grain size, precipitates and segregation on crack initiation behavior during tensile tests in metastable β-Ti alloy. After solution treatment at 830 °C, the TB8 alloy with equiaxed β grain displayed a good fracture elongation of 30.2 ± 0.63%. The adiabatic shearing band and β→α phase transformation were activated to increase the compatible deformation capability during tensile testing; however, the phase transformation caused the stress concentration in the boundary, resulting in crack initiation. For the samples prepared using solution and low aging at 440 °C, large grain, elements segregation at grain boundary and incomplete precipitates induced a slight reduction in ultimate tensile strength and elongation. After solution and aging at 520 °C, the short-rod or/and lamellar α phase precipitated in β grain effectively enhancing ultimate tensile strength (1398.71 ± 15.6 MPa). The increased boundaries provided the interface or precipitation strengthening effect, but high-density dislocations were also accumulated at the β/α interface, causing unstable deformation and crack initiation. These findings advance our understanding of the correlation between microstructure and crack initiation, and provide a basis for designing and customizing the mechanical properties of metastable β-Ti alloy.

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 pre-strain on thermal aging of austenitic stainless steel weld metal

Austenitic stainless steel welds (ASSWs) suffer from severe pre-strain during assembly process, which threatens the long-term operation safety of pipelines in nuclear power plant. In this work, the 316L weld metals (WMs) with 0 % and 8 % pre-strain were thermally aged at 400 °C for up to 39000 h to investigate the pre-strain effect on thermal aging of ASSWs. The results showed that the pre-strain caused work hardening and further promoted the hardening of thermally aged ferrite. After thermal aging for 39000 h, the nano-hardness increment of ferrite with 8 % pre-strain was about 1.6 GPa more than that without pre-strain. By microstructure characterization, it is found that the high dislocation density induced by pre-strain promoted spinodal decomposition and G-phase precipitation. The spinodal decomposition morphology and corresponding element concentration fluctuations were more obvious in the WM with 8 % pre-strain. Moreover, the size and density of G-phase along dislocations were larger in the ferrite with 8 % pre-strain than those without pre-strain.

Cu[formula omitted]Se/MXene (Ti[formula omitted]C[formula omitted]T[formula omitted]) composite achieved ultra-low thermal conductivity and enhanced thermoelectric performance

Due to the significant mismatch in phonon density of states between carbon (C) and Cu2Se, the thermal conductivity of Cu2Se can be notably enhanced by incorporating low-dimensional carbon-based materials. This study propose an innovative approach where two-dimensional MXene material Ti3C2Tx is introduced into Cu2Se, creating numerous Cu2Se/MXene heterogeneous interfaces within the matrix. These interfaces induce high-density dislocations, which enhance multi-scale phonon scattering efficiency. Consequently, they significantly reduce lattice thermal conductivity across the entire temperature range tested. Additionally, the heterogeneous interfaces facilitate energy filtration, selectively filtering out low-energy carriers and thereby optimizing the high intrinsic carrier concentration of Cu2Se to a certain extent. Finally, the total thermal conductivity of the Cu2Se/0.4 wt% Ti3C2Tx sample at 795 K is markedly lower at 0.27 Wm−1K−1 compared to the average level of Cu2Se materials. As a result, the ZTmax reaches 2.11, reflecting a 109% enhancement compared to pristine Cu2Se.

INFLUENCE OF CRYOGENIC COOLING AFTER DRAWING ON CHANGES IN PROPERTIES OF STEEL WIRE

One of the promising and little-studied methods for obtaining an ultrafine-grained structure and enhanced mechanical properties is the so-called cryogenic deformation - deformation at temperatures below 120K. It is assumed that low deformation temperatures suppress recovery processes, thus contributing to the accumulation of an extremely high dislocation density and increase internal stresses, as well as activate deformation twinning, which together will accelerate grain refinement. In this regard, in this work, we studied the drawing of steel wire under cryogenic cooling in liquid nitrogen. The results of the laboratory experiment show that the application of cryogenic deformation treatment after wire drawing improves mechanical properties compared to conventional wire drawing. In this case, after two cycles of deformation, the relative contraction after stretching decreases by 8 %, the ultimate strength increases by 40 %, and the conditional yield strength by 26 %. The results show that the deformation conditions during cryogenicdrawing are an additional factor for the realization of structural resources of steel wire physical and mechanical properties optimizing.

Effect of heat input on the microstructure and hydrogen embrittlement susceptibility of the coarse-grain heat-affected zone of CrMo steels generated using a Gleeble 3500 welding simulator

The changes in microstructure and hydrogen embrittlement (HE) susceptibility of the simulated coarse-grain heat-affected zone (CGHAZ) of CrMo steels under three distinct heat input conditions were investigated. The cooling time from 800 °C to 500 °C, represented by t8/5, increased from 100 s to 200 and 400 s.. The sample with t8/5 = 100 s has a microstructure consisting of martensite and a small amount of lower bainite. When the t8/5 was increased to 200 s and 400 s, the HAZ consisted of 65.43 % martensite and 34.57 % lower bainite and 4.53 % martensite with 95.47 % bainite, respectively. The HE susceptibility was assessed by slow strain rate tensile (SSRT) testing of hydrogen-charged samples, and the HE susceptibility index (IHE) declined from 87.89 % to 61.37 % when t8/5 increased from 100 to 400 s. The high dislocation density and hydrogen concentration in the sample with t8/5 = 100 s led to intergranular fracture and incresaed HE susceptibility. The martensite lath and cementite/ferrite interface are the sites of crack initiation in the sample with t8/5 = 200 s. In the sample with a cooling time of 400 s, the martensite/ferrite interfaces are the main crack initiation sites under hydrogen charging.

Preparing gradient microstructure to achieve enhanced performance in GH4586 superalloy: Experiments and models

In this present study, a combined method of controlling strain deformation (CSD) and gradient-temperature heat treatment (GTHT) was adopted to prepare gradient microstructure in GH4586 superalloy. Consequently, it brought enhanced tensile strength from 1313.1±18.7 MPa at intermediate transition (IT) region to 1506.3±15.7 MPa at high-strain and low-temperature (HS-LT) region, presenting an improvement of 11.6%. And it also enhanced fracture toughness from 77.7±0.3 MPa⋅m1/2 at IT region to 86.7±3.7 MPa⋅m1/2 at low-strain and high-temperature (LS-HT) region, presenting an improvement of 14.7%. The formation of gradient microstructure along the longitudinal direction of CSD+GTHT-treated sample was thoroughly investigated from perspectives of precipitates distribution, activated slip systems and dislocations evolution. During CSD, the size and volume fraction of γ′ precipitates decreased under the effect of strain-induced dissolution behavior. The activation of (1‾ 11)[101] and (11‾ 1)[011] slip systems dominated dislocation movement at each region and there retained increasing dislocation density due to more extensively activated slip systems at HS region. During GTHT, the high-density dislocations promoted recrystallization while the concentratedly distributed γ′ precipitates obstructed movement of recrystallized front, leading to formation of heterogeneous structure with high-density grain boundaries and dislocations at HS-LT region. Finally, a model reflecting the effect of grain boundary strengthening, precipitation strengthening and dislocation strengthening on mechanical property of gradient microstructure was established and successfully applied to property prediction of GH4586 superalloy.

Microstructure and Mechanical Properties of Low Stacking-Fault Energy Cu-Based Alloy Wires

Innovations in electronic devices and their capabilities have driven the demand for improved conductive materials relevant to device fabrication. To gain insights on developing solid solution-type Cu alloy thin wires with a superior balance of strength and conductivity, this study investigated variations in the microstructures and properties of pure Cu wires and Cu–5 at. pct Zn, Cu–5 at. pct Al, and Cu–5 at. pct In alloy wires during intense drawing and analyzed the effects of stacking-fault energy (SFE) of Cu alloys on their microstructural evolution. During the initial drawing stages, lower SFE Cu–5 at. pct Al and Cu–5 at. pct In alloys yielded more high-density deformation twins than pure Cu and Cu–5 at. pct Zn. Deformation twins promoted grain refinement during drawing. Effective grain refinement and dislocation accumulation during drawing in low-SFE Cu alloys substantially strengthened them without adversely impacting electrical conductivity. During intense drawing in the Cu–5 at. pct In alloy wires, ultrafine fibrous grains (diameter ~ 80 nm) and a high-dislocation density yielded excellent tensile strength and conductivity. These results indicate that adjusting the solute element content in Cu matrix to reduce SFE and optimizing deformation strain via wire drawing significantly improve alloy wire performance.

Superior strength-ductility synergy in additively manufactured CoCrFeNi high-entropy alloys with multi-scale hierarchical microstructure

Additive manufacturing offers a feasible route for fabricating net-shape metallic materials with advanced physicochemical and mechanical properties, which is unattainable through conventional routes. This study focuses on the processing parameters, microstructural, and mechanical properties of the CoCrFeNi high-entropy alloys (HEAs) fabricated via laser powder bed fusion (LPBF). A near-dense LPBF CoCrFeNi HEA was manufactured by fine-tuning the parameters. Compared with conventionally processed CoCrFeNi HEAs, the as-built samples exhibit excellent strength-ductility synergy with 200 % higher yield strength and unsacrificed ductility. Microstructural analyses of the LPBF samples demonstrate a hierarchical structure including melt pools, columnar grains, dislocation cells, and elemental segregations. This excellent strength is attributed to the high-density dislocation cellular structure and Cr2O3 nanoparticles that impede the dislocation motion. The excellent ductility correlates to the steady work-hardening regulated by the interactions between cellular structure, dislocations, deformation twins, and nanoprecipitation. This work provides promising outlook towards fabricating high-performance alloys with tailored hierarchical microstructure.

Investigation of Rolling Contact Fatigue Damage Behavior of a ER9 Wheel Tread in Relation to Surface Microstructural Evolution

In the present study, we systematically analyze the relationship between the surface microstructural evolution and rolling contact fatigue (RCF) damage behavior of the ER9 wheel tread. In the unfatigued zone (Zone A), where the shear stress was low, a thin plastic deformation layer with low hardness formed. The ferrite grains exhibited obvious plastic flow; however, these grains were not refined, and the cementite showed no significant fragmentation or dissolution. Additionally, the dislocation density remained low on the wheel surface in Zone A. Conversely, in the fatigue zone (Zone B), where the shear stress was high, a thicker plastic deformation layer with increased hardness developed. The ferrite grains in this zone were notably refined, and a substantial amount of lamellar cementite fragmented and dissolved, leading to a high dislocation density on the wheel surface. The severe plastic deformation in Zone B facilitated the formation of fatigue wear cracks on the wheel, which initiated the RCF crack in Zone B. Furthermore, the interface between pearlite and proeutectoid ferrite grains in the wheel accelerated the RCF crack propagation, ultimately leading to RCF failure.