Decrease In Dislocation Density Research Articles

Effect of Tempering Temperature on the Microstructure, Deformation and Fracture Properties of an Ultrahigh Strength Medium‐Mn Steel Processed by Quenching and Tempering

Herein, a medium‐Mn steel containing 4 wt% Mn is designed to achieve ultrahigh tensile strength and good ductility by the quenching and tempering process. The effects of tempering temperature on the microstructure, tensile behavior, and fracture properties under different stress states are explored. It is observed that tempering temperature influences the fracture mechanisms of the ultrahigh‐strength steels. Both ductile fracture strain and cleavage fracture strength of the investigated medium‐Mn steel are affected by the tempering temperature. The impacts of tempering temperature on volume fraction, carbon content of retained austenite (RA), and tensile properties are quantitatively analyzed. Detailed microstructure analysis reveals that both volume fraction and carbon content of the RA are increased in the tempered steels, accompanied by improved tensile properties than in quenched conditions. The superior tensile properties are achieved after tempering at 250 °C, with the ultrahigh yield strength of 1590 MPa, ultrahigh tensile strength of 1963 MPa, and total elongation of 12%. The corresponding relationship between microstructure and mechanical properties is investigated, with a focus on the change of residual stress, the occurrence of carbon redistribution/carbide precipitation, the decrease of dislocation density of martensite, and the presence of transformation‐induced plasticity effect during deformation.

Elucidating the influence of temperature and strain rate on superplasticity and microstructure evolution of near-α TNW700 titanium alloy

The superplastic deformation and microstructure evolution of a new near-α TNW700 titanium alloy with short-term service temperature up to 700 °C were systematically investigated in the temperature range of 900–975 °C and strain rate range of 0.0005–0.01s−1. The results illustrate that TNW700 alloy has excellent superplasticity with elongation above 300% and m value above 0.41 at 900–950 °C, and the maximum elongation of 523% is obtained at 950°C-0.001s−1 due to the appropriate grain size and phase ratio. In addition, TNW700 alloy exhibits high temperature softening and strain rate hardening behaviors. The increase of β phase volume fraction induced by dynamic phase transformation is a crucial cause for flow softening, and this phenomenon is accelerated with increasing temperature and decreasing strain rate. The refinement of primary α grains and the inconspicuous growth of β grains at lower temperatures are favorable for grain boundary sliding and superplasticity enhance, whereas the overgrowth of β grains at 975 °C results in superplasticity absence. The decrease of dislocation density in primary α grains caused by strain partitioning, dynamic recovery (DRV) or dynamic recrystallization (DRX) is another important factor for high temperature softening; the increase of dislocation density and velocity, and the decrease of annihilation time contribute to strain rate hardening. The appearance of random texture indicates the activation of grain boundary sliding (GBS) and grain rotation. GBS is the overarching mechanism of superplastic deformation of TNW700 alloy, accompanied by grain rotation, dislocation movement, dynamic phase transformation and grain refinement or coarsening.

Effect of Cu Alloying on Mechanical Properties of Medium-C Steel after Long-Time Tempering at 500 °C

This research studies the influence of the copper alloying of medium-carbon steel on mechanical properties after quenching and tempering at 500 °C. The microstructure was characterised using SEM, EBSD, TEM, and XRD analysis. The mechanical properties were comprehensively investigated using hardness measurements, tensile and Charpy impact tests and solid solution, grain boundary, dislocation, and precipitation strengthening contributions were estimated. Higher yield strength for Cu-alloyed steel was confirmed at about 35-73 MPa. The precipitation strengthening contribution from Cu precipitates in the range of 11-49 MPa was calculated. The interaction between Cu precipitates and dislocations retards the decrease in dislocation density. Similar values of effective grain size of martensite crystals were measured for Cu-alloyed and Cu-free steel as well. Copper alloyed steel exhibited significantly deteriorated impact toughness, total plastic elongation, and reduction of area. The size of Cu precipitates ranged from 8.3 nm after tempering at 500 °C for 6 h to 13.9 nm after tempering for 48 h.

Exploring the Influence Mechanisms of Tempering Temperature and N‐alloying on Mechanical Properties of AISI M42 High‐Speed Steel

The variations in microstructure and mechanical properties of N‐free and high‐nitrogen (0.18 wt%) M42 HSS with tempering temperatures are investigated. The microstructure of steels after austenitizing primarily consists of martensite, retained austenite (RA), undissolved M2C, and M6C carbides. With increasing tempering temperature (450–600 °C), the RA content and martensite lattice strain of steels gradually decrease. At 540 and 600 °C, RA is almost or fully transformed into martensite, and massive nanosized M2C secondary carbides are precipitated. The M2C secondary carbides at 600 °C obviously coarse compared to those at 540 °C. Further, the addition of N increases RA content and martensite lattice strain. With increasing tempering temperature, the bulk hardness and compressive strength of steels first increase and then decrease, whereas their impact toughness shows an opposite trend. This is ascribed to the combined effect of RA decomposition, precipitation and coarsening of secondary precipitates, and decrease in dislocation density and martensite lattice strain. Compared to N‐free steel, 0.18N steel tempered at peak‐hardening temperature (540 °C) and above has higher bulk hardness and compressive strength because of the increased solid solution, dislocation, and precipitation strengthening, but lower impact toughness owing to higher matrix hardness and martensite lattice strain.

Effect of Co/P micro-alloying on stress relaxation behaviour of Cu-15Ni-8Sn alloy

To improve the stress relaxation resistance of the Cu-5Ni-S8n alloy, the effects of Co or P addition on its stress relaxation resistance were investigated by scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction), X-ray diffraction, and high-temperature creep durability tester. The results revealed that the relaxation rate increased dramatically as the temperature rose. The thermal stress relaxation resistance of Cu-15Ni-8Sn alloy was significantly improved with Co addition; After 100 h of stress relaxation tests at 200°C or 250°C, the residual stress value of the Cu-15Ni-8Sn alloy increased by 23 MPa or 22 MPa with Co addition, which was associated with the consumption of vacancies by Co atoms and the pinning effect of Co atoms on dislocation. However, at 250°C, the stress relaxation resistance of the Cu-15Ni-8Sn-0.2P alloy was lower than that of the Cu-15Ni-8Sn alloy. This difference was due to the inclusion of P, which promoted the recrystallization reaction. Article highlights Recrystallization and the decrease of dislocation density are the major reasons for the stress relaxation of the samples. The stress relaxation resistance of Cu-15Ni-8Sn alloy was significantly improved with Co addition. The addition of the P element is not conducive to the improvement of thermal stress relaxation resistance.

Effect of annealing process on microstructure and properties of 980MPa grade hot-dip galvanized dual-phase steels

A hot-dip galvanizing simulator studied the effects of soaking temperature and rapid cooling end temperature on the microstructure and mechanical properties of 980MPa grade hot galvanizing dual-phase steel. The results show that at the end of a rapid cooling temperature of 290°C, with the increase of soaking temperature, the tensile strength of the test steel increases, the yield strength decreases at first and then increases, and the elongation decreases as a whole. The microstructure is composed of ferrite and martensite. At the soaking temperature of 800°C, the strength decreases, and the elongation increases with the increase of the end temperature of rapid cooling. When the end temperature of rapid cooling is lower than 320°C, the martensite tempering is not apparent, and there are many high-density dislocations in the ferrite near the neighbor. When the cooling end temperature is 350°C, some martensites are tempered. The twins in martensite are widened, carbides are unevenly precipitated along the twin planes, and the dislocation density decreases. When the soaking temperature is between 800°C and 820°C, the end temperature of rapid cooling is between 260°C and 290°C. Annealing and zinc plating are beneficial to obtain dual-phase steel with excellent mechanical properties.

Microstructure evolution and deformation behaviors of pulse electro-assisted deformation in M50 bearing steel

M50 bearing steel, as a typical high-carbon and high-alloy steel, exhibits low cold working capacity with high susceptibility to produce voids during the ring rolling process. In this paper, the deformation behavior and microstructure evolution during the pulse electro-assisted tensile of M50 bearing steel was studied. The results show that the maximum flow stress of the pulse electro-assisted deformation (PEAD) specimen decreases by 24.4% compared with that of the traditional deformation (TD) specimen. However, the section shrinkage rate of the PEAD specimen is increased by 32.2%, which is even higher than that of the hot deformation (HD) specimens. This indicates the positive role of pulse current on the increase of plasticity. Besides, a constitutive model of the PEAD process was established by fitting the experimental data. With the assistance of pulse current, the deformation resistance and work hardening are reduced, so the number of voids is significantly reduced due to the easier metal flow. Moreover, the energy barrier of recrystallized grain nucleation is decreased, while the nucleation rate is increased under pulse current. As a result, the occurrence of recrystallization leads to the reduction of average grain size from 4.51 μm to 1.62 μm. According to the high-resolution transmission electron microscopy (HRTEM) and electron backscatter diffraction (EBSD) characterizations, the dislocation density decreases by 24% under the applied current, which should be ascribed to the increase of atomic flux and lattice diffusion coefficient. This work provides a new idea for solving the plastic deformation problem of M50 bearing steel.

Effect of Recrystallization Behavior of AZ31 Magnesium Alloy on Damping Capacity

For a wide industrial application of magnesium alloys, a method for imparting high damping properties while maintaining mechanical properties is required. Controlling the crystallographic texture seems to be useful, because dislocations are known to have a significant influence on the damping characteristics of magnesium alloys. In addition, textures are affected by the microstructure and texture variation when the deformation or annealing is applied. However, there were less reports about their effect on damping capacity. Therefore, the effect of twinning and annealing, which can affect the recrystallization, were investigated in this study. An AZ31 alloy was hot rolled at 673 K with a reduction ratio of 10% and 50%, and then annealed at 673 K and 723 K for 0.5, 1, 2, and 3 h, respectively. SEM-EBSD was used to examine the microstructure and texture. In addition, each specimen's hardness and internal friction were contemporarily measured. As a result, hot rolling produced tensile twins and their fraction increased with internal friction when the reduction ratio increased. Due to annealing, a discontinuous type of static recrystallization occurred within the twinning grains, and was highly activated along with the increasing annealing temperature and the fraction of twinning. In the samples annealed at 723 K, the internal friction continuously increased over the annealing time, whereas in the samples annealed at 673 K, the decrease in dislocation density was delayed while the internal friction showed a relatively low value.

Microstructure and Mechanical Properties Evaluation of Microalloyed Low‐Carbon Reduced Activation Ferritic/Martensitic Steel

A novel method for reducing the amount of M23C6 carbides, increasing the amount of MX carbonitrides, and decreasing the coarsening rate of M23C6 carbides is put forward to improve the high‐temperature properties of reduced activation ferritic/martensitic (RAFM) steel. Scanning electron microscopy, transmission electron microscopy, X‐ray diffraction, tensile tests, and impact tests are used to systematically investigate the microstructure evolution and mechanical properties of RAFM (RAFM‐0.1C) steel, low‐carbon RAFM (RAFM‐0.04C) steel, and minor boron and nitrogen microalloyed low‐carbon RAFM (RAFM‐CBN) steel. Some δ ferrite develops when C decreases from 0.1 to 0.04 wt% and subsequently disappears after 0.01 wt% N addition. The amount and size of M23C6 carbides and dislocation density decrease with the decrease of C (RAFM‐0.04C), while the amount of MX carbonitrides is 2‐3 times, and dislocation density is ≈2 times higher than that of RAFM‐0.04C steel after 0.01 wt% N addition (RAFM‐CBN). Compared with RAFM‐0.1C steel, the yield strength at room temperature and 550 °C slightly decreases for RAFM‐0.04C steel and considerably increases for RAFM‐CBN steel. The contributions of microalloy on strengthening mechanisms of RAFM steel at room temperature are also systematically revealed by combining the experimental and theoretical data.

EBSD Analysis of Hot Deformation Behavior of Oxide-Doped Molybdenum Alloys

Hot compression tests of Mo–1.5 wt% Al2O3/ZrO2 molybdenum alloys were carried out using the Gleeble–1500 simulator at 0.01 s−1−5 s−1 strain rates and 1000–1500 °C deformation temperatures. The microstructural changes of the alloy at 1000–1500 °C were studied. The changes in the hot deformation process for the Mo−1.5 wt% Al2O3/ZrO2 molybdenum alloys were analyzed by means of EBSD. The ZrO2 particles had a greater effect on improving the thermal deformation resistance of molybdenum alloys than did the Al2O3 particles. The activation energy of the molybdenum alloy doped with ZrO2 (403.917 kJ/mol) was lower than that of the molybdenum alloy doped with Al2O3 (440.314 kJ/mol). Due to the occurrence of recrystallization, the intensity of {100} the texture first increased and, then, dropped down with increase in the temperature, while the change law of {111} texture was the opposite. Above 1200 °C, the higher deformation temperature made the texture more random by lowering the texture intensity. The dislocation density was sacrificed to promote recrystallization. When dynamic recrystallization occurred, the sub–grain boundaries absorbed dislocations and transformed them into high–angle grain boundaries, resulting in a decrease in dislocation density and an increase in high–angle grain boundaries at high temperatures and low strain rates. At 0.01 s−1 strain rate, the average grain size of Mo–1.5 wt% ZrO2 alloy increased from 2.38 μm to 4.67 μm, and the proportion of large angle grain boundaries increased from 59.8% to 86.6%.

Effect of electric current on mechanical properties and microstructure of Ti/Al laminated composite during electrically assisted tension

Electrically assisted (EA) forming technology is often applied to difficult to form materials for its convenience and efficiency. As one of the common metal clad plates, explosive welded Ti/Al laminated composite (LC) is hard to form at room temperature (RT). EA forming provides a new idea for Ti/Al LC forming. Explosive welded Ti/Al LC was used as the experimental material to conduct EA tensile experiments with different parameters in this paper. The mechanical properties and microstructure evolution of Ti/Al LC during EA tension were investigated. The flow stress of Ti/Al LC during EA tension was reduced, but the elongation of Ti/Al LC was less than that tension at RT. The reason that induced the fracture in advance was that the local Joule heat temperature was too high. The Ti/Al LC microstructure characterization results showed that dislocation density of matrix diminished, the recrystallization in matrix and atomic diffusion in interface were facilitated. Besides, the high frequency current promoted the transition of Al matrix from ductile fracture to brittle fracture. The primary reason for the decrease of Ti/Al LC flow stress is the decrease of dislocation density in the matrix, and the change of microstructure is also due to dislocation evolution.

Characterization of Microstructural Evolution in Heat-Affected Zone of Cu-Bearing Ultra-High-Strength Steel with Lamellar Microstructure.

The advanced lamellar microstructure significantly improves the toughness of Cu-bearing ultra-high strength steel by delamination toughening (yield strength: 1370 MPa, impact toughness at -40 °C: 60 J). The lamellar microstructure affects the microstructure evolution of heat-affected zone (HAZ), resulting in separate distributions of lath martensite and granular bainite in the complete austenitizing zone and the formation of cluster fresh martensite in the partial austenitizing zone. The grain refinement and decrease in dislocation density are predominant features, especially for the complete austenitizing zone, where the grain is refined to 4.33 μm, and dislocation density is decreased by 27%. With the degree of austenitizing increase, the dissolution of Cu-rich precipitates (CRPs) aggravates during welding. A small fraction of CRPs in the complete austenitizing zone implies the onset of reprecipitation of CRPs. The reason for softening in HAZ is attributed to a combined effect of granular bainite forming, dislocation density decreasing, and CRPs dissolving. After PWTH, large numbered reprecipitation of coherent CRPs occurs, contributing to the hardness recovery of HAZ. Meanwhile, due to the high density of dislocation of lamellar microstructure inherited by partial austenitizing zone, coarsening of coherent CRPs is easy to occur, and various incoherent structures are observed.

Effect of strain rate on the microstructure evolution and tensile behavior of medium manganese steel

• The TRIP effect is predominant at lower strain rates. • An increase in the strain rate facilitates mechanical stability of austenite. • Excellent tensile properties are resulted at lower strain rates. In the present study, the effect of strain rate on the microstructure evolution and tensile behavior of intercritically annealed medium manganese steel is investigated. Tensile tests were performed on the annealed samples at strain rates of 1.33×10 -4 , 0.665×10 -3 and 1.33×10 -3 s -1 . The amount of austenite to martensite transformation and dislocation density decrease with the increase in strain rates. The sample at the strain rate of 1.33×10 -4 s -1 exhibits superior tensile properties due to the synergistic influences of the transformation-induced plasticity (TRIP) effect and high dislocation density.

Hydrogen embrittlement behavior in the nugget zone of friction stir welded X100 pipeline steel

Hydrogen-induced damage is an inevitable challenge in pipeline safety applications, especially, the fusion welded joints owing to microstructure heterogeneity caused by welding process. In this work, X100 pipeline steel was subjected to friction stir welding (FSW) at rotation rates of 300–600 rpm under water cooling, and the relationship among the microstructure, hydrogen diffusivity, and hydrogen embrittlement (HE) behavior of the nugget zone (NZ) were studied. The NZ at 600 rpm had the highest effective hydrogen diffusion coefficient (Deff) of 2.1 × 10−10 m2/s because of the highest dislocation density and lowest ratio of effective grain boundary. The Deff decreased with decreasing rotation rate due to the decrease of dislocation density and the increase of ratio of effective grain boundary, and the lowest Deff of 1.32 × 10−10 m2/s was obtained at 300 rpm. After hydrogen charging, the tensile strength of all specimens decreased slightly, while the elongation decreased significantly. As the rotation rate decreased, the elongation loss was obviously inhibited, and ultimately a lowest elongation loss of 31.8% was obtained at 300 rpm. The abovementioned excellent mechanical properties were attributed to the fine ferrite/martensite structure, low Deff, and strong {111}//ND texture dramatically inhibiting hydrogen-induced cracking initiation and propagation.

Effects of electropulsing frequency on mechanical properties, corrosion behavior and microstructure of a creep-aged 7050 aluminum alloy

In the experiment, the effects of electropulsing frequency (150 Hz, 300 Hz, 450 Hz, 600 Hz) on the creep age formability, mechanical properties and corrosion behavior of 7050 aluminum alloy were studied. The results show that the creep age formability of the alloy is improved as the electropulsing frequency increases from 150 Hz to 600 Hz. The tensile strength and yield strength of the 600 Hz sample are 627 MPa and 590 MPa, which correspondingly increased by 5.2% and 6.1% compared with the 150 Hz sample. In addition, the maximum corrosion depth of the 600 Hz sample is 25.9 μm while that of the 150 Hz sample is 61.9 μm. The application of higher frequency electropulsing promotes the dislocation movement, so the dislocation density decreases and the creep strain is larger. The electropulsing accelerates the precipitation by enhancing the movement of vacancy and solute atoms and the depletion of vacancy inhibits the growth of precipitates. Hence, the higher frequency electropulsing leads to finer and denser intragranular precipitates, which contributes to the optimized mechanical properties. The electropulsing frequency of 600 Hz results in the discontinuous distribution of grain boundary precipitates and the reduction in the width of PFZ, which are beneficial to improve the corrosion resistance of the alloy.

Orientation-induced anisotropy of plasticity and damage behavior in monocrystalline tantalum under shock compression

In this work, the role of crystallographic orientation on dynamic mechanical response in single-crystalline tantalum subjected to impact loading is investigated using non-equilibrium molecular dynamics simulations. Interestingly, the correlation of spall strength with void distribution is characterized statistically. Moreover, it is discovered that the entanglement of dislocation lines plays an important role in void nucleation, and then high dislocation density is beneficial to the increase in potential void nucleation sites. Oppositely, the movement of dislocation lines is deteriorated by voids, and then the dislocation density within the spall region dramatically drops after void nucleation in each orientation. There is a strong sensitivity of dislocation line evolution to void growth and coalescence in the later stage of cavitation. For [100], fewer potential void nucleation sites are found to occur in narrower spall zone, and then the existing voids are easier to be aggregated. Consequently, dislocation lines cannot be absorbed by surrounding voids. In contrast, the void distribution area is wider enough to facilitate to annihilate more dislocation lines through voids in [110]. Particularly, the decrease of dislocation density is inclined to the fact that the space for dislocation line survival is preoccupied by void nucleation sites along the [111] direction.

Research on warm laser shock sheet micro-forging

This paper introduces warm laser shock sheet micro-forging, which combines the advantages of sheet bulk metal net forming, laser shock dynamic micro forming and warm forming. The influence of AZ31B magnesium alloy on laser energy and forming temperature was investigated by experiment and simulation. The mechanism that warm temperature has better properties than room temperature has been revealed from the aspects of microstructure, stress wave transmission and material flow. The results show that the forming depth of workpiece increases with the increase of laser energy. Forming depth increases with increasing temperature at the same laser energy, but serious ablation occurs on the surface of workpiece at 250 °C, which affects the forming quality of workpiece. EBSD results show that with the increase of forming temperature, the number of twins decreases, the dislocation density decreases and the forming depth increases. The grain refinement after laser shock forming is obvious, and the increase of grain dislocation density promotes the improvement of microhardness, which indicates that laser shock can improve the mechanical properties of materials. The numerical simulation results show that the elastic wave propagates faster than the plastic wave. The stress on the workpiece at 150 °C is less than that at room temperature, but the equivalent plastic strain is higher than that at room temperature. After plastic deformation, the material in the workpiece gradually flows to the cavity. At 150 °C, the material flow inside the workpiece is faster, the filling effect is better and the forming uniformity is better.

Achieving high strength and large elongation in a strip casting microalloyed steel by ageing treatment

The microstructure and mechanical properties of a Nb–V microalloyed steel produced by strip casting in as-coiled and aged conditions were investigated using scanning electron microscopy, electron backscatter diffraction, X-ray diffraction, transmission electron microscopy, atom probe tomography and tensile tests. Tensile test results show that, after ageing at 650 °C for 5 min, accompanying an increase of yield strength from 698 ± 3.6 MPa to 758 ± 7.9 MPa, the elongation increases from 7.1 ± 0.4% to 12.6 ± 0.4% simultaneously. Microstructural analysis shows that both the as-coiled and aged specimens possess a microstructure predominantly comprising a mixture of acicular ferrite and bainitic ferrite. Hardness tests for specimens aged at different temperatures show very similar peak hardness values but higher ageing temperature requires less time for peak ageing. Ageing treatment leads to a decrease of dislocation density, which is responsible for the enhancement of elongation for aged specimens, while Nb–N-rich and Nb–V–N-rich clusters develop successively during ageing, which contributes to the increase of yielding strength after ageing. The strengthening contributions from solid solution, dislocations, grain boundaries and clusters were calculated, and the results indicated that dislocations and grain boundaries are the dominant strengthening mechanisms for the current strip casting steels.

Residual stress reduction in wire arc additively manufactured parts using in-situ electric pulses

Components fabricated in metal additive manufacturing, including wire arc additive manufacturing, undergo complex thermal cycles, resulting in residual stresses and thermal distortions. The present work investigates the effect of applying in-situ electric pulses to the component after the deposition of every layer to reduce residual stresses. The experimental results revealed that electropulsing resulted in dislocation rearrangement/annihilation, thereby decreasing dislocation density. A significant reduction in the fraction of low angle grain boundaries was observed for electropulse-treated samples, indicating a decrease in residual stress. Further, X-ray diffraction results also confirm a reduction in residual stress (24.0–29.4% reduction compared to untreated samples). The method can effectively be used to address specific regions selectively in addition to in-situ reduction of residual stresses in deposited components. Abbreviations: EBSD: electron backscattered diffraction; EPT: electropulsing treatment; EWF: electron wind force; GND: geometrically necessary dislocations; KAM: Kernel average misorientation; LAGBs: low angle grain boundaries; WAAM: wire arc additive manufacturing; XRD: X-ray diffraction

On the sulfide stress cracking resistance of a 125 ksi grade high-strength low-alloyed steel for oil country tubular goods

Excellent sulfide stress cracking (SSC) resistance was achieved in a 125 ksi grade high-strength low-alloyed steel with increased C content and Cu alloying. Results showed that increased C content allowed tempering at higher temperature, obtaining low dislocation density without decreasing strength. Moreover, Cu alloying refined martensite blocks and inhibited dislocation annihilation, but hardly influenced the precipitation of Cr-rich carbides. The decrease in dislocation density was the main reason for improving SSC resistance, and sufficient precipitation of Cr-rich carbides came in second. Despite Cu alloying failing to improve the SSC resistance, it increased the yield strength without sacrificing SSC resistance.