Ductile Dimple Research Articles

Microstructure and properties of ER120S-G and SS316L bimetallic structures manufactured via Arc-based direct energy deposition with woven trajectory

A bimetallic laminated steel structure (BLSS) was manufactured using Arc-based Direct Energy Deposition (Arc-DED), featuring a sandwich structure in the X-axis and a fish scale weaving structure in the Y-axis. The microstructural characteristics and mechanical behavior of this BLSS, composed of ER120S-G and SS316L, were comprehensively investigated. The microstructure transitioned from equiaxed and cellular in ER120S-G to columnar in the intermediate region, and then back to cellular in SS316L. Significant differences were observed in the mechanical properties of the sample between the X and Y directions. All fractures in both directions exhibited ductile characteristics, with numerous ductile dimples and evidence of plastic deformation. The BLSS achieved a maximum hardness of 461HV, surpassing that of single ER120S-G samples. Notably, the ultimate tensile strength (UTS) of the BLSS reached its peak at 1296 MPa when SS316L was centered in the Y direction, which was 34.7 % higher than that of ER120S-G samples. In BLSS samples, toughness was superior when SS316L was positioned at the center compared to when ER120S-G was centered.

Effect of an Ultrasonic Vibration on the Microstructure and Properties of Al Alloy/Steel Laser Welding-Brazing Joints

This study analyzes the influence of different ultrasonic amplitudes on the microstructure composition, microhardness, tensile strength, and corrosion resistance of Al alloy/steel laser welding-brazing joints assisted by ultrasonic vibration. The application of ultrasonic vibration did not change the microstructure composition of the joints but refined them. The joints were all composed of θ-Fe(Al, Si)3 and τ5-Al7.2Fe1.8Si formed at the interface reaction zone, as well as an α-Al solid solution and Al-Si eutectic phase generated in the weld seam zone. Meanwhile, the thickness of the IMCs at the interface decreased with an increase in the ultrasonic amplitude. When the ultrasonic amplitude was 8 μm, the IMCs thickness was a minimum of 1.62 μm. In this condition, the reduction of the IMCs thickness and the refined grain of joints made the microhardness and tensile strength reach the maximum. The fracture of joints with ultrasonic amplitudes of 0 and 4.8 μm began at the weld seam and extended to the interface reaction zone at the steel side, while the fracture of joints was located in the heat-affected zone (HAZ) of the Al alloy side when the ultrasonic amplitude was 8.0 and 11.2 μm. The fracture mode of the former presented a typical mixed fracture with cleavage steps and tearing edges, and that of the latter showed ductile fracture with uniform and fine ductile dimples. The corrosion resistance of the joints was improved by adding ultrasonic vibration. When the ultrasonic amplitude was 8 μm, its corrosion resistance was optimum; it was ascribed to a dense oxide film formed on the surface of the metal under the action of ultrasonic vibration.

Study on the mechanical properties of X80 pipeline steel under pre-charged high-pressure gaseous hydrogen

Hydrogen embrittlement (HE) is a significant challenge to the safe operation of hydrogen-blended natural gas pipelines. The mechanical properties of X80 steel were studied after H2 pre-charging followed by mechanical properties tests and fracture morphologies observation. Results indicated that the hydrogen saturation time of X80 steel was roughly 48 h. H2 pre-charging induced a decline in strength, plasticity, and fatigue properties, while hydrogen-assisted fatigue crack growth occurred during the early stage of fatigue crack growth rate tests. Additionally, the fracture morphologies transited from primary microvoid coalescence to a mixed mode characterized by ductile dimples and quasi-cleavage planes with increasing hydrogen. The HE susceptibility increased with increasing hydrogen partial pressure and decreasing loading frequency, but there existed a critical value (6 × 10−7 s−1) for the strain rate effect on the HE susceptibility. Under H2 pre-charging conditions, the predominant HE mechanism was the hydrogen-enhanced local plasticity (HELP) mediated hydrogen-enhanced decohesion (HEDE) mechanism.

Laser contacting of hairpin windings in electrical vehicle motors using ring-mode laser beams based on angularly-uniform Lissajous trajectories.

The contacting process of hairpin windings is deemed the most important and challenging process in manufacturing electric vehicle motors. This paper proposes a ring-mode laser contacting method for copper hairpin windings based on Lissajous-shaped cyclic scanning with a uniform angular speed. Two key variables, trajectory cycle n and angular multiplier k, which represent the laser energy input and distribution respectively, were investigated via single-factor experiments. Morphology, microstructure, mechanical and electrical properties were analyzed consequentially to show the influence on the contact quality. The results show that, as n increases, contact appearance is criticized for being under-contacted, well-contacted, or over-contacted. Molten pool behaviors affected by n variations determine morphology formation. Larger k facilitates molten pool evolution and improves laser contacting efficiency. Fusion zone (FZ) has directional columnar grains, while heat-affected zone (HAZ) has overgrown equiaxed grains coarser than those of base metal (BM). The microstructure of FZ becomes coarser if n increases, or finer if k increases. The highest tensile force of 712.5 N was reached when k = 4, n = 6. Ductile dimple fractures in FZ indicate the contacts' excellent strength and toughness. FZ and HAZ show lower microhardness than BM due to thermal softening. FZ is slightly harder than HAZ due to its densely-interlaced microstructure. Poor contacts cause reduction in their electrical conductivity and increase in their resistance.

Microstructural analysis and mechanical characterization of dissimilar welds between nickel-based and steel-based superalloys post-cryogenic treatment using hotwire GTAW

This study explores the dissimilar welding of nickel superalloy 59 with Fe-Ni superalloy 904 L using the Gas Tungsten Arc Welding with Hot Wire technique (HW-GTAW). The HW-GTAW process produced defect-free weldments with complete penetration, as confirmed by macroscopic analysis. Microstructural analysis using FESEM and EDS revealed fine equiaxed and columnar dendrites in the fusion zone and identified a secondary phase enriched with chromium. XRD analysis supported the presence of predominant austenitic phases and indicated the formation of Cr23C6-type carbide. The grain size of the weld joint, determined using the Gaussian method and Scherer formula, was 44.53 nm, slightly smaller than base metals. Microhardness tests revealed values ranging from 226 ± 5 HV 10 to 243 ± 5 HV 10, with the highest values observed in the deep cryogenic treatment (DCT). The Mo in the ERNiCrMo-13 filler and the effects of DCT, which promote grain refinement, are responsible for this increase in hardness. Residual stress measurements revealed a maximum tensile stress of 315 MPa along the longitudinal axis and a maximum compressive stress of 355 MPa along the transverse axis. DCT enhanced the weldment’s tensile strength by up to 12% compared to as-welded samples, with a 6% increase compared to shallow cryogenic-treated (SCT) samples. Charpy impact testing indicated a value of 88 J in the fusion zone, which is 1.2% less than alloy 59 and 9% higher than 904 L. Tensile fracture analysis revealed ductile dimples, while impact fractography showed a mix of ductile dimples and brittle, glassy cleavage facets.

Optimized control of laser processing parameter on microstructural evolution and mechanical behavior in CoCrNi medium entropy alloy

In this study, the processing parameters of a laser powder bed fusion (LPBF)-fabricated CoCrNi alloy were systematically investigated for their impact on mechanical properties, microstructure, and fracture characteristics when the relative density exceeds 99%. The maximum relative density of 99.89% was attained at a laser power of 225 W, and scan speed of 1000 mm/s, accompanied by a hardness value of 276.83 HV. However, it is important to note that the highest relative density did not correspond to the maximum σUTS. Tensile testing revealed that increasing the laser power at the same scan speeds led to a decrease in elongation. The LPBF-fabricated CoCrNi alloy exhibited enhanced defect tolerance due to its cellular microstructure, which confined ductile dimple sizes to the nanoscale and impeded crack propagation through pinning effects. At scan speeds of 1000 mm/s and energy densities below 83 J/mm³, an increased presence of unmelted powder and pores was observed. Besides, it was confirmed that the <110> crystallographic texture displayed the strongest orientation within the sample. Theoretical calculations indicated that over 50% of the yield strength is attributed to the dislocation structure. These findings contribute to the process optimization of LPBF, providing a valuable reference for adjusting process parameters to enhance material mechanical properties, particularly when targeting relative densities above 99%.

Influence of microstructure evolution and element segregation on high temperature tensile behaviors in GH4706 alloy

This work systematically investigated the influence of microstructure evolution and element segregation on tensile properties and fracture behaviors of GH4706 superalloy under different temperatures (25 °C, 400 °C, 500 °C, 600 °C and 650 °C). The advanced methods of TEM, AES and SIMS were conducted to characterize the variation of microstructure and element distribution. Experimental results showed that the yield and tensile strength of the test alloy decreased while the corresponding elongation increased as the increment of test temperature. The dominated fracture mode was intergranular brittle fracture at the temperature of 25 °C, gradually transforming to ductile dimple fracture as the temperature increased. This phenomenon was determined to be closely related to the increment of average grain size and size of γ'/γ″ coprecipitates as temperature rose. It was worth noticing that the elongation performed abnormally low (16.3%) at the temperature of 500 °C, exhibiting intermediate temperature brittleness (ITB) phenomenon. The SIMS analysis showed that the segregation of S towards grain boundary was the most obvious at the temperature of 500 °C, which was considered as the main factor leading to ITB. In addition, the coarsening of carbides at grain boundaries and local strain non-uniformity caused by dislocations could also resulted in ITB. The main reason for maintaining good strength and plasticity at the temperature of 650 °C was contributed to the softening of γ' phases by repeated cutting of dislocations and deformation twins, reducing the hindrance to the movement of dislocations. Moreover, composite strengthening effect of P and B could improve bonding strength, stabilize grain boundaries, and effectively suppress the initiation of intergranular cracks.

Low‐Temperature Annealing of Asymmetrically Cold‐Rolled Electrolytic Tough‐Pitch Copper: Anneal Hardening and Bimodal Microstructure

In this research, 90% asymmetric unidirectional rolling and post‐annealing are applied to electrolytic tough‐pitch (ETP) copper. The effect of low‐temperature (200 °C) annealing on the microstructure and texture evolution, mechanical, and electrical properties are investigated. Static recovery (SRV) is the main softening mechanism at the annealing time range of 1–5 min. In contrast, complete static recrystallization (SRX) is observed after annealing for 10 min. Bimodal microstructure, consisting of coarse and fine grains, is evolved for annealing times more than 10 min. With increasing the post‐annealing duration, the intensity of deformation and shear textures is continuously decreased; however, the recrystallization (Cube) orientation becomes stronger up to 11.2 × R. After 1, 2, and 5 min of post‐annealing, the hardness, yield strength, tensile strength, ductility, and toughness of copper are higher than those of the 90% rolled sample due to annealing hardening. After 10 min of annealing, the hardness, yield strength, and tensile strength suddenly decrease and the ductility and toughness significantly enhance owing to the complete recrystallization. With increasing the post‐annealing time, the fraction of flat surfaces is decreased and the fraction of ductile dimples is increased. With the post‐annealing time increasing from 1 to 120 min, the electrical conductivity gradually increases from 84.9%IACS to 96.7%IACS.

Development of a method for computer processing of fractographic images to assess the cohesion of inclusions to the matrix in the weld metal after its operational degradation and hydrogenation

In this paper, the method for automated processing of digital fractographic SEM images based on the modified local Otsu approach was developed. The effectiveness of the developed method was demonstrated. The method is used to identify inclusions or their traces at the bottom of dimples in weld metal fractures with subsequent assessment of their quantity. It is assumed that a decrease in the cohesion of inclusions to the matrix is one of the signs of damage to the weld metal at the microstructural level. Consequently, the more inclusions were found at the bottom of ductile fracture dimples, the stronger their connection with the matrix and, accordingly, the less damage in the weld metal. The effect of operational degradation of the weld metal on a circumferential weld on thick pipes of thermal power plant steam pipelines is analyzed. For this purpose, the features of cohesion of inclusions with the matrix in the weld metal in the initial state and after operation on the main steam pipeline for 23 years were compared. The influence of hydrogen (absorbed by the weld metal during long-term operation, as well as during its additional electrolytic charging) on the cohesion of inclusions in its structure with the surrounding matrix is also considered. A decrease in cohesion between these two components of the weld metal structure under the influence of all analized factors is shown. Thus, a method for quantitative assessment of fractographic features caused by the decohesion of inclusions from the matrix developed on the basis of analysis and computer processing of scanning electron microscope images, can be used to assess the operational damage of the weld metal at the microscopic level.

The Variation Patterns of the Martensitic Hierarchical Microstructure and Mechanical Properties of 35Si2MnCr2Ni3MoV Steel at Different Austenitizing Temperatures.

This study investigates the influence of varying austenitizing temperatures on the microstructure and mechanical properties of 35Si2MnCr2Ni3MoV steel, utilizing Charpy impact testing and microscopic analysis techniques such as scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The findings reveal that optimal combination of strength and toughness is achieved at an austenitizing temperature of 980 °C, resulting in an impact toughness of 67.2 J and a tensile strength of 2032 MPa. The prior austenite grain size initially decreases slightly with increasing temperature, then enlarges significantly beyond 1100 °C. The martensite blocks' and packets' structures exhibit a similar trend. The proportion of high-angle grain boundaries, determined by the density of the packets, peaks at 980 °C, providing maximal resistance to crack propagation. The amount of retained austenite increases noticeably after 980 °C; beyond 1200 °C, the coarsening of packets and a decrease in density reduce the likelihood of trapping retained austenite. Across different austenitizing temperatures, the steel demonstrates superior crack initiation resistance compared to crack propagation resistance, with the fracture mode transitioning from ductile dimple fracture to quasi-cleavage fracture as the austenitizing temperature increases.

Microstructure Features and Mechanical Properties of Modified Low-Activation Austenitic Steel in the Temperature Range of 20 to 750 °C

A new low-activation austenitic steel with a modified composition and high austenite stability is proposed. The features of its microstructure after solution treatment (ST) and cold rolling (CR) are studied. The mechanical properties and features of the fracture behavior of this steel under tensile tests in the temperature range of 20–750 °C are discussed. After ST, an austenitic structure with stacking faults and dispersed carbide particles of the MC and M23C6 types is observed in the steel. After CR, the grains are refined, and the average grain size decreases from 41.4 µm (after ST) to 33.9 µm. High-density microtwin packets form in the material, and the dislocation density increases relative to that after ST. As the test temperature increases from 20 to 750 °C, the yield strength of the steel decreases by approximately two times, from ≈300 to 150 MPa (for ST) and from ≈700 to 370 MPa (for CR). In the studied temperature range, the steel demonstrates up to 2.6 times higher values of elongation to failure, ≈40–80% (for ST) and ≈13–27% (for CR), compared to steels of similar compositions and lower manganese content. Mechanical twinning contributes to the high steel ductility up to 300 °C. Signs of discontinuous flow in the tensile curves after ST in the temperature range of 500–600 °C and a decrease in the elongation to failure in the close temperature range indicate dynamic strain aging (DSA). Steel fracture after tension at all test temperatures mainly occurs via a ductile dimple transcrystalline mechanism with elements of ductile intercrystalline fracture. It is shown that cracks nucleate on clusters of dispersed second-phase particles. The mechanisms of plastic deformation, fracture, and strengthening of the proposed modified low-activation austenitic steel are discussed.

EFFECT OF SOLUTION TREATMENT ON THE MICROSTRUCTURE AND MECHANICAL BEHAVIOR OF THE NICKEL-BASED ALLOY N06625

We have studied the effects of different solution temperatures and holding times on the microstructure and room-temperature mechanical properties of the alloy using methods such as electron backscatter diffraction, hardness testing and tensile testing. The results showed that the average grain size increased significantly as the solution temperature increased, and many annealing twins appeared in the grains. The fraction of twin boundaries reached its maximum value at 1080 °C. The hardness, yield strength and tensile strength of the alloy decreased with an increase in the solution temperature, but abnormal yielding occurred at 1160 °C. The elongation after fracture and the area reduction increased with an increase in the solution temperature. The fracture morphology of the alloy was irregular, conical, accompanied by a large number of ductile dimples, and the material exhibited typical ductile fracture. At the same time, as the solution temperature increased, the number of dimples on the fracture surface continued to increase, the depth of the dimples gradually increased, and the distribution became more uniform.

High-Temperature Mechanical Behavior of an As-Extruded Al-5Zn-2Mg-0.3Cu (in wt.%) Alloy

To ensure that Al alloys are being served as the high-temperature structural components for applications in aerospace and transportation, it is necessary to investigate their high-temperature mechanical behavior and failure mechanism. In this paper, the mechanical behavior of the as-extruded Al-5Zn-2Mg-0.3Cu (in wt.%) alloy was studied and compared under different high-temperature tensile-testing conditions. It was found that the yield strength and the ultimate tensile strength of the alloy gradually decreased with the increase in temperature, but its elongation ratio showed a slightly increasing trend. Failure analysis demonstrated that there were a lot of ductile dimples on the fracture surfaces and that obvious necking occurred for the samples being tensile-tested at different temperatures. Surface observation revealed that the initiation of micro-cracks was mainly attributed to the self-cracking of the brittle phase particles. Moreover, when the testing temperature was between 450 °C and 550 °C, micro-cracks could also occur at the interface between phase particles and the Al matrix.

Comparison of a copper processed by asymmetric unidirectional rolling (AUR) and cross rolling (ACR): Strain distribution, microstructure, texture, and mechanical properties

In this work, the effect of strain path on the strain distribution, microstructural evolution, crystallographic texture, and mechanical behavior of electrolytic tough-pitch copper was investigated by simulation and experimental procedures. The asymmetric unidirectional and cross rolling processes with a thickness reduction of 90% were performed at room temperature. The asymmetric cross-rolling (ACR) gives much better through-thickness uniformity of shear strain distribution compared to the asymmetric unidirectional-rolling (AUR). The average equivalent strain of AUR and ACR after 90% rolling reduction was 3.99 ± 1.22 and 3.66 ± 0.26, respectively. Both AUR and ACR processes improved the particle distribution in the matrix. However, the particle distribution of the AUR sample was slightly more uniform compared to the ACR sample. The shape of primary grains in the AUR copper was straight while that in the ACR sample was zigzag. The average dislocation density of the AUR sheet was larger than that of the ACR sample (2.41 × 1016 m−2 vs. 2.31 × 1016 m−2). The major recrystallization mechanism in the AUR and ACR samples was discontinuous and continuous dynamic recrystallization, respectively. The main deformation textures (Copper, S, and Brass components, and α and β fibers) were weak in both samples due to the domination of recrystallization and shear textures in the cold-rolled copper. The recrystallization texture components in the upper region were stronger than in the lower region. The uniformity of the microhardness profile of the ACR sample was more than that of the AUR sample. The AUR sample had a larger strength and a smaller ductility compared to the ACR sample. The results showed that the strain path of asymmetric rolling was effective on the strain-hardening behavior of copper during the tensile test. The fractograph of the ACR sheet exhibited ductile dimples. However, the fractograph of the AUR sample revealed many ductile shear dimples and flat surfaces.

Influence of Overheating on High-Cycle Fatigue Characteristics of the Base Metal and Weld Metal of Low-Carbon Steel Welded Joints

The results of a high-cycle fatigue testing of our samples were obtained, and a comparative assessment of the properties of the base metal and weld metal of 09G2S-type steel (13Mn6 according to the DIN17145-80 standard) before and after overheating (1200 °C, 3.7 h) was performed. The welded joints between the sheets of 09G2S steel were obtained through automatic argon arc welding. The fatigue tests were carried out under repeated tensile loading. The “maximum cycle stress—number of cycles to failure” fatigue curves of the samples were plotted. The fracture surfaces of the samples were studied, and the fatigue failure mechanisms were analyzed. It was shown that, during testing, all samples demonstrated cyclic hardening behavior. The samples of the base metal as delivered had the highest endurance limit, and the smallest endurance limit was found in the samples of the base metal and weld metal after overheating, the endurance limits of which were similar. The fracture mechanism of all samples was quasi-brittle with the presence of very thin fatigue micro-grooves. The final rupture of all samples had a ductile dimple type.

Evolution of microstructure and strength of a high entropy alloy undergoing the strain-induced martensitic transformation

In a recent work, we have reported outstanding strength and work hardening exhibited by a metastable high entropy alloy (HEA), Fe42Mn28Co10Cr15Si5 (in at. %), undergoing the strain-induced martensitic transformation from metastable gamma austenite (γ) to stable epsilon martensite (ε). However, the alloy exhibited poor ductility, which was attributed to the presence of the brittle sigma (σ) phase in its microstructure. The present work reports the evolution of microstructure, strength, and ductility of a similar HEA, Fe38.5Mn20Co20Cr15Si5Cu1.5 (in at. %), designed to suppress the formation of σ phase. A cast and then rolled plate of the alloy was processed into four conditions by annealing for 10 and 30 min at 1100 °C and by friction stir processing (FSP) at tool rotation rates of 150 and 400 revolutions per minute (RPM) to facilitate detailed examinations of variable initial grain structures. Neutron diffraction and electron microscopy were employed to characterize the microstructure and texture evolution. The initial materials had variable grain size but nearly 100% γ structure. Diffusionless strain induced γ→ε phase transformation took place under compression with higher rate initially and slower rate at the later stages of deformation, independent on the initial grain size. The transformation facilitated part of plastic strain accommodation and rapid strain hardening owing to a transformation-induced dynamic Hall-Petch-type barrier effect, increase in dislocation density, and texture. The peak strength of nearly 2 GPa was achieved under compression using the structure created by double pass FSP (150 RPM followed by 150 RPM). Remarkably, the tensile elongation exhibited by the alloy was nearly 20% with fracture surfaces featuring a combination of ductile dimples and cleavage.

In situ synchrotron x-ray imaging and diffraction study of additively manufactured AlSi10Mg alloy under uniaxial tension

In situ synchrotron x-ray imaging and diffraction are used to investigate deformation dynamics in two kinds of additively manufactured AlSi10Mg with different pore configurations. The pre-existing pore configurations are characterized with synchrotron x-ray computed tomography. The specimens are subjected to tensile loading along the build direction and the longitudinal direction. Bulk stress–strain curves (macroscale), strain fields (mesoscale) and x-ray diffraction patterns (microscale) are obtained simultaneously. Scanning electron microscopy characterization is performed as a complement. For the small-pored AlSi10Mg, preferred orientations of pores lead to anisotropic ductility, and only ductile dimples are observed. For the large-pored AlSi10Mg, ductility loss is significant due to its higher porosity, and no remarkable anisotropy is found; both ductile and brittle fracture features are identified. For both types of AlSi10Mg, strain localization occurs around the pores. At the lattice scale, the Al matrix deforms plastically but the Si phase is still strained elastically, and thus, significant stress concentration occurs in the Si phase for the small-pored AlSi10Mg, while stress is more evenly partitioned between the Al matrix and the Si phase for the large-pored AlSi10Mg.

Analysis of cracking characteristics and mechanism of high-strength steel after chemical oxidation

The tooth surface of the 20CrNi2Mo worm underwent cracking after chemical oxidation. The appearance inspection results showed that there were multiple cracks on the tooth surface of the worm slice, and the direction of the cracks was perpendicular to the machining pattern. There were many microcracks on both sides of the main crack. The profile morphology shows that the crack depth reaches 1100 μm. The crack extends perpendicular to the tooth surface to a certain depth and then propagates in two transverse directions. After opening the crack on the tooth surface, observation shows that the overall section is black, and the artificially opened area is silver-white. At the interface between the artificially opened area and the initial fracture surface, the fracture surface changes from intergranular morphology to ductile dimple morphology. The energy spectrum shows that the oxygen content in the intergranular cracking area is much higher than that in the ductile dimple area, indicating that the worm has undergone intergranular corrosion cracking. After the worm tooth surface is ground, there will be certain tensile stress on the surface. During the chemical oxidation process, the worm will be exposed to many corrosive media such as OH−, and NO2 −. Under the combined action of tensile stress and corrosive ions, the tooth surface will undergo stress corrosion cracking. To effectively avoid cracking on the worm gear surface, three measures have been proposed to reduce the risk of cracking: (1) improving machining parameters to reduce grinding feed, (2) increasing destressing before chemical oxidation to reduce residual tensile stress, (3) improving chemical oxidation process to reduce solution concentration, treatment time, and treatment temperature.

Effect of Artificial Aging Treatment on the Mechanical Properties and Regulation of Precipitated Phase Particles of High-Pressure Die-Cast Thin-Wall AlSi10MnMg Longitudinal Carrier.

This study investigates the artificial aging treatment process for AlSi10MnMg longitudinal carriers with optimal strength and ductility. Experimental results illustrate that the peak strength is observed under single-stage aging at 180 °C × 3 h, with a tensile strength of 332.5 MPa, Brinell hardness of 133.0 HB, and elongation of 5.56%. As aging time increases, tensile strength and hardness initially increase and then decrease, while elongation displays an inverse pattern. The amount of secondary phase particles at grain boundaries increases with aging temperature and holding time, but stabilizes as aging progresses; the secondary phase particles begin to grow, eventually weakening the alloy's strengthening effect. The fracture surface exhibits mixed fracture characteristics, including ductile dimples and brittle cleavage steps. Range analysis indicates that the influence of distinct parameters on mechanical properties post-double-stage aging is as follows: first-stage aging time, first-stage aging temperature, followed again by second-stage aging time, and second-stage aging temperature. For peak strength, the optimal double-stage aging process includes a first-stage aging temperature of 100 °C × 3 h and a second-stage aging temperature of 180 °C × 3 h.

Development of creep resistant high yttria 18Cr ferritic ODS steel through hot powder forging route

The present work reports the creep behaviour of a high yttria Fe-18Cr-2W-0.57Ti-1Y2O3 (all compositions in wt.%) ferritic oxide dispersion strengthened (ODS) steel produced by powder forging. The creep tests were carried out in a temperature range of 873 K to1023K and a stress range of 150 MPa to 425 MPa. Creep data is analysed using the power law creep equation. A large value of stress exponents (n ∼ 10 at 873 K, ∼ 18 at 923 K and 8.5 at 973 K) and a high activation energy (Q ∼581 kJ/mol) are obtained. These high values are attributed to threshold stress (σth) generated due to attractive interaction between dislocation and oxide particles. Threshold stress for different creep conditions was determined. True stress exponent values of 4 to 5 were obtained after accounting for the threshold stress. Electron back scattered diffraction (EBSD) revealed an equiaxed strain free grain structure in the powder forged ODS steel. Transmission electron microscopy (TEM) of creep tested samples indicates that microstructure remains stable after creep with no significant coarsening of nano size oxide particles. Dislocation pinning by the oxide particles was observed in creep tested samples. The observed values of true stress exponent suggest dislocation climb over the nano oxide particles to be the creep mechanism operating in the ODS steel. Samples sections close to creep fracture surfaces were investigated by EBSD. Void formation at triple junctions followed by crack propagation normal to the loading direction was observed. All the samples exhibited mixed mode (cleavage + ductile dimple) failure. Conventional ODS steel compositions contain up to 0.35% yttria. A significant improvement in creep life as well as an increase in activation energy for creep was observed because of high (1%) yttria content used in the ODS steel studied.