Coarse-grained Zone Research Articles

Evading strength and ductility trade-off in an inverse harmonic structured metal

In this study, we demonstrated an inverse harmonic structured pure nickel fabricated through a powder metallurgy method, characterized by hard fine-grained zones three-dimensionally enclosed within a connected network of soft coarse-grained zones. This unique heterogeneous structured nickel exhibits excellent strength-ductility synergy, achieving a 100% increase in strength while maintaining ductility comparable to that of the coarse-grained one.

The effect of welding speed on the microstructure, mechanical properties, and fatigue failure of Q690D high-strength steel

This study analyzes the effect of welding speed on the microstructure and mechanical properties of CO2 gas-shielded welded joints of 4 mm thick Q690D high-strength steel. The results show that the weld zone is mainly composed of acicular ferrite. The coarse-grained zone and fine-grained zone contain lath bainite and lath martensite. As the welding speed increases, the degree of recrystallization in the weld zone initially increases and then decreases. At the optimal welding speed of 38 cm/min, the tensile strength of the welded joint reached 692.1 MPa, with an elongation of 9.63 %. The maximum microhardness of the welded joint was 303 HV, and the residual stress was a tensile stress of 167 MPa. After fitting the S-N curve, the ultimate fatigue strength of the welded joint was 210 MPa. Both fatigue and tensile fracture surfaces displayed second-phase precipitate particles, impacting the mechanical properties.

Improving strength-ductility synergy of twinning-induced plasticity steel by introducing a sandwich structure via submerged double-face friction stir processing

Twinning-induced plasticity steels typically exhibit remarkable plasticity and ultimate tensile strength, but the inferior yield strength has become a bottleneck limiting their application. In this work, a unique gradient sandwich structure that aims to break the bottleneck was successfully fabricated using a submerged double-face friction stir processing technique. The sandwich structure is composed of a coarse-grained zone in the core and two ultrafine-grained zones on the surfaces. An ultrahigh yield strength (1215 MPa) which is 2.2 times that of the as-received base material (543 MPa) is achieved for the sandwich structure. Additionally, significant elongation (19 %) and corrosion resistance are retained simultaneously. The coordinated deformation and effective strain transfer between the ultrafine-grained zones and coarse-grained zone facilitate the dispersion of stress and uneven deformation, which contributes to the excellent strength-ductility of the sandwich structure. This work provides a feasible method for fabricating high-performance steels.

EZNS: Elastic Zoned Namespace for Enhanced Performance Isolation and Device Utilization

Emerging Zoned Namespace (ZNS) SSDs, providing the coarse-grained zone abstraction, hold the potential to significantly enhance the cost efficiency of future storage infrastructure and mitigate performance unpredictability. However, existing ZNS SSDs have a static zoned interface, making them in-adaptable to workload runtime behavior, unscalable to underlying hardware capabilities, and interfering with co-located zones. Applications either under-provision the zone resources yielding unsatisfied throughput, create over-provisioned zones and incur costs, or experience unexpected I/O latencies. We propose eZNS, an elastic-ZNS interface that exposes an adaptive zone with predictable characteristics. eZNS comprises two major components: a zone arbiter that manages zone allocation and active resources on the control plane, and a hierarchical I/O scheduler with read congestion control and write admission control on the data plane. Together, eZNS enables the transparent use of a ZNS SSD and closes the gap between application requirements and zone interface properties. Our evaluations over RocksDB demonstrate that eZNS outperforms a static zoned interface by 17.7% and 80.3% in throughput and tail latency, respectively, at most.

Comparative study about the results of HAZ physical simulations on different high-strength steel grades

With continuous improvements, structural steels are available in even higher strength grades above 1000 MPa yield strength. As the great majority of these steels are used in welded structures, their weldability needs to be taken into account. Several factors can cause difficulties during welding of these steels, but in this paper the softening behavior and the toughness characteristics of the heat-affected zone (HAZ) are examined. As the critical parts of the HAZ in a real welded joint are relatively small, their investigating ability is limited. However, the physical simulation provides a way of evaluating specimens made from a given material to produce the specified HAZ areas in a suitable size range for subsequent testing. In this research work, three different strength categories of high-strength structural steels (with yield strength of 960 MPa, 1100 MPa, and 1300 MPa) are investigated by physical simulation. In the case of different technological variants of gas metal arc welding (GMAW) process, the effect of the cooling time t8/5 is investigated in different HAZ subzones considered to be critical. The thermal cycles were determined according to the Rykalin 3D model. The investigated cooling times were t8/5 = 5 s, 15 s, and 30 s. The properties of the selected coarse-grained, intercritical and intercritically reheated coarse-grained zones are analyzed by laser scanning microscope, scanning electron microscope, hardness test, and instrumented Charpy V-notch impact toughness test. Furthermore, additional investigation like JMatPro calculations, electron backscatter diffraction measurements, and prior austenite grain size calculation were carried out. As a result of the tests, the investigated heat-affected subzones indicated higher sensitivity to the welding heat input compared to conventional structural steels. Overall, the results of the tests show that the application of shorter t8/5 cooling time can be beneficial for the investigated high-strength steel grades, since significant toughness reduction and the risk of softening occur in the whole cooling time range.

Evaluation of Simulated Heat Affected Zone of Molybdenum- and Niobium Alloyed Ultra-High-Strength Steels

Gleeble 3800-thermomechanical simulator was used to simulate the heat affected zone of quenched and tempered 0.16 wt.% C steels with variation of molybdenum-and niobium contents. The purpose of the study was to evaluate the effect of alloying content on the properties of the coarse-grained zone of HAZ region (CGHAZ) and partially re-austenitised inter-critical zone (ICHAZ) with two different t8/5 times (5 s and 15 s). Results showed that Mo and Nb decreased the amount of softening in the HAZ-region, especially with longer t8/5 -time (15 s). 0Mo steel had mixed microstructure of bainite and martensite in the CGHAZ region with t8/5 time of 15 s, which led to higher degree of softening compared to other steels. Shorter t8/5 time of 5 s produced martensitic microstructure in CGHAZ region in all cases leading to higher hardness values. Impact energy values at-40 °C were at least 34 J/cm2 in all cases. Highest hardness values in the ICHAZ-region were achieved in the case of 0.5Mo steel. Also, at-40 °C impact energy values of ICHAZ were at least 34 J/cm2 in all cases, however Mo-free steel achieved clearly higher impact energies in ICHAZ region, which is result from softer microstructure with relatively low hardness compared to other steels. Overall, it can be concluded that longer t8/5 time can be used, which corresponds to higher heat input in welding, if Mo and/or Nb alloying is used.

CNT-induced heterogeneous matrix grain structure in CNTs/Al composites

Ductility collapse is a stubborn issue for developing high-strength Al matrix composites reinforced with carbon nanotubes (CNTs). In order to solve this problem, here we manipulated the fabrication process and crafted a heterostructured CNTs/Al composite consisting of fine-grained zone (FGZ) and quasi-continuously networked coarse-grained zone (CGZ). Such heterostructured CNTs/Al composite with CGZ in a quasi-continuously networked fashion has never been reported before. Microstructural analysis showed that the majority of CNTs resides in FGZ, which stimulated dynamic recrystallization of Al during hot deformation and resulted in the formation of FGZ. Tensile tests showed that the heterostructured composite possesses a tensile property of 549 MPa of strength and 11.2% of ductility, which is superior to those in literatures. Further in-situ tensile EBSD and post-tensile TEM analyses revealed that during tensile deformation, the stiff FGZ undertook most of the stress, whereas the CGZ facilitated hetero-deformation induced hardening, which helped to resist the high flow stress of FGZ and assisted the ductility retention. This study may provide some guidance for the development of strong yet ductile CNTs/Al composites.

Investigation of the Abnormally Large Grains of a Super304H Heat-Resistant Steel Tube over Long-Term, High-Temperature Service

Abstract Abnormally large grains (ALGs) were detected in the outer walls of Super304H superheater steel tubes of an ultra-supercritical unit in long-term service. In this study, the microstructure and mechanical properties of steel tube samples were investigated to reveal the formation mechanism of the ALGs. The results showed that the room-temperature and high-temperature strength and plasticity of the coarse-grain zone were much lower than those of the fine-grain zone. Inhomogeneous residual strain distribution in the outer wall led to high distortion energy differences in the as-supplied steel tube. The low-distortion austenite grains annex the neighboring high-distortion grains via the strain-induced grain boundary migration mechanism. Moreover, the two-dimensional step mechanism of the faceted grain boundaries also causes the ALGs to form.

Influence of Different Weld Heat Input on Microstructure and Properties of New Martensitic Heat-resistant Steel Joints by Automatic Welding

In this article, the influence of different automatic weld heat input about the microstructure and properties of the joint of new martensitic heat resistant steel of P92 is studied. Based on this, the key factors to obtain excellent quality of P92 joint are proposed. The results show that the weld is tempered martensite microstructure, the grain of Heat-affected zone (HAZ) is finer with the weld heat input lower. The width of the slats and the crystalline size of the coarse-grained zone (CGHAZ) in the weld metal microstructure is gradually larger with the heat input higher. Meanwhile, as the welding heat input increase, the hardness of weld bead in the welded joint of P92 steel has a gradual downward trend, the impact toughness has a significant decrease, and the impact fracture morphology changes from ductile fracture to brittle fracture. Therefore, when the automatic welding technology is used to repair P92, so as to obtain better performance of welded joints, appropriate gas protection flow and welding parameters should be ensured according to welding thickness, and heat input should be strictly controlled.

Insights into laser powder bed fused Scalmalloy®: investigating the correlation between micromechanical and macroscale properties

Additively manufactured Scalmalloy® using laser powder bed fusion (LPBF) has been showing increasing potential for industrial adoption. Its intended application in space, aerospace, and automotive industries requires extensive testing and characterization to produce parts with repeatable properties and safe operations. In the present paper, a thorough investigation of the association between small-scale (micro/nano-scale) and macro-scale (e.g., tensile) properties and microstructural features has been conducted. The prime novelty is the correlation of the small-scale hardness with the tensile properties, which is studied in both as-built and heat-treated conditions (325 °C/4 h) along parallel and perpendicular build directions of the samples. Advanced microstructural characterization including scanning and transmission electron microscopies (SEM/TEM), as well as electron backscatter diffraction (EBSD), was carried out on each condition to correlate microstructure and mechanical properties. While significant improvement in strength was found from as-built to heat-treated conditions, due to the precipitation of secondary Al3Sc and Al3(Sc, Zr), grain size or texture did not change considerably. The grain size analysis revealed a fine-grained zone (FGZ: mostly between 0.5 and 1 μm) next to the coarse-grained zone (CGZ: mostly between 2 and 15 μm). All samples show texture-free FGZ and textured CGZ with a predominance of <001> crystal directions parallel to the building direction.

Heterogeneous microstructure improves the strength and ductility of a flash-butt welded IN718 alloy joint

Higher strength and ductility are obtained in a flash-butt welded IN718 alloy joint due to an improved heterogeneous microstructure distribution by adjusting the upsetting deformation. With the increase in upsetting deformation, the microstructure distribution of the characteristic zones is effectively adjusted which contains a complete recrystallization zone (RxZ), a thermo-mechanical affected zone (TMAZ) and a coarse grain zone (CGZ). Finer grains and more residual stress are obtained in the heat affected zone (HAZ), which increases the strength and hardness. The improvement of the strength in the HAZ delays the concentration rate of local strain at the weld seam (WS) and enhances the coordination between the HAZ and the matrix. Therefore, the additional ductility is obtained by the coordination between the HAZ and the matrix, which results in a welded sample of UD0.86 achieving the same high strength and ductility as that of the base material.

Effects of Built Direction and Deformation Temperature on the Grain Refinement of 3D Printed AlSi10Mg Alloy Processed by Equal Channel Angular Pressing (ECAP).

In this work, we used an AlSi10Mg alloy produced by selective laser melting (SLM) to study the effects of build direction and deformation temperature on the grain refinement process. Two different build orientations of 0° and 90° and deformation temperatures of 150 °C and 200 °C were selected to study this effect. Light microscopy, electron backscatter diffraction and transmission electron microscopy were used to investigate the microtexture and microstructural evolution of the laser powder bed fusion (LPBF) billets. Grain boundary maps showed that the proportion of low-angle grain boundaries (LAGBs) dominated in every analysed sample. It was also found that different thermal histories caused by the change in build direction resulted in microstructures with different grain sizes. In addition, EBSD maps revealed heterogeneous microstructures comprising equiaxed fine-grained zones with ≈0.6 μm grain size and coarse-grained zones with ≈10 μm grain size. From the detailed microstructural observations, it was found that the formation of a heterogeneous microstructure is closely related to the increased fraction of melt pool borders. The results presented in this article confirm that the build direction has a significant influence on the microstructure evolution during the ECAP process.

Microstructure Characteristics and Mechanical Properties of Flash Butt Welded 590 MPa V-N Microalloyed Heavy-Duty Truck Wheel Steel

This study reports the welded joint of a novel 590 MPa V-N microalloyed wheel steel on microstructure and mechanical properties after flash butt welding. The welding parameters were flash current 48°/582.0 A, upsetting current 44°/516.6 A, and workpiece clearance 1.5 mm. The evolution of microstructure in the welded joint occurred as follows: welding seam (ferrite side plate + acicular ferrite +martensite)→coarse-grained zone (acicular ferrite + granular bainite)→fine-grained zone (fine-grained ferrite + M/A island)→base metal (equiaxed ferrite + pearlite). The standard impact energy value of welding seam, coarse grain zone, fine grain zone, and base metal at −40 °C was 116, 128, 144, and 88 J, respectively. The mechanical property of the joint was excellent. The microstructure, the number of grain boundaries, and the dislocation density directly affected the strength and hardness of the joint. The increase of large angle grain boundaries and the decrease of effective grain size were beneficial to the improvement of toughness. The hot-rolled 590 MPa V-N microalloyed wheel steel had superior weldability.

Microstructure heterogeneity and mechanical properties of Mg-Gd-Y-Zr alloy fabricated by force-controlled additive friction stir deposition

Multi-layer single-pass deposits of high-performance Mg-8Gd-3Y-0.5Zr alloy were successfully prepared using force-controlled additive friction stir deposition (AFSD) with a high deposition rate. The microstructure and mechanical properties heterogeneity along the build direction and transverse direction of the final deposits were investigated. The grain size of the coarse grain zone (CGZ) formed on advancing side (AS) is roughly 11–17 μm, two to three times larger than that of the other regions. Because of the evident grain refinement during deposition, the tensile properties in the center and retreating side (RS) regions of as-deposited final deposits are superior to those of the extruded Mg-8Gd-3Y-0.5Zr alloy.

Selective corrosion behavior of X80 steel heat affected zone by SRB in coastal saline soil

The corrosion behavior of heat affected zone of X80 pipeline steel was investigated in coastal saline soil with or without sulfate-reducing bacteria (SRB). In the sterile soil environment, the diameter, depth, and density of corrosion pits detected in the coarse grain zone (CGZ) were larger than those in the fine-grained zone (FGZ) and fusion zone (FZ). In the soil environment inoculated with SRB, the microbes enhance the corrosion of FGZ, CGZ, and FZ. The corrosion susceptibility of CGZ coupons is stronger in the presence of SRB than those of FGZ and FZ coupons.

Achieving high strength and high ductility in submicron-MgO/AZ31 composites with an innovative honeycomb-like structure

Honeycomb-like AZ31 matrix composites with 1, 2, and 4 wt% submicron-MgO were prepared using low-energy milling and extrusion. The composites consist of fine-grained and coarse-grained zones. The fine-grained zones enriched with submicron MgO form a honeycomb-like structure, and the coarse grains without MgO are filled into unit cells. Both the strengths and elongations of the newly developed MgO/AZ31 composites are enhanced compared with AZ31. Specifically, the tensile strength of composites containing 1 wt% MgO is improved to 344.1 MPa, and at the same time, its elongation is increased up to 21.5%. The excellent strength-ductility combination is due to the introduction of submicron-MgO reinforcements with unique distribution. Such achievements could be beneficial to the structural design of composites.

Effects of solid-state interfacial reaction on the mechanical properties of carbon nanotubes reinforced aluminum matrix composites with heterogeneous structure

To overcome strength-ductility trade-off in carbon nanotubes (CNTs) reinforced Al matrix composites, we developed a novel and controllable strategy via a combination of different length of CNTs and fine/coarse-grained matrices. Microstructure analysis showed that Al4C3 nanorods about 100 nm were distributed in the fine grain zone (FGZ), while CNTs of micron length were distributed in the coarse grain zone (CGZ). Different sizes of reinforcements play different strengthening roles, and the bimodal grains can effectively coordinate deformation, thereby achieving a good combination of strength and ductility. Heat treatment does not affect the microstructure of the FGZ, but significantly affect the CNT/Al interface bonding in the CGZ. Heat treatment at an appropriate temperature can further improve the strength of the composite by improving the interface bonding. When the heat treatment temperature is too high, the coarsening of Al4C3 in CGZ will deteriorate the mechanical properties of the composite.

Mechanisms for creep rupture of 12Cr/alloy 4715 dissimilar weldments

The mechanism for creep rupture of 12%Cr /alloy 4715 welded joints was investigated by employing traditional uniaxial tensile creep in this study. The creep tests were maintained at 580 °C with creep loading stresses gradually decreasing from 300 to 240 MPa, and the time to failure was tested. The rupture location shifts from base metal (BM) to BM + fusion line (FL) then to FL + fine grain zone + coarse grain zone (CGZ), and finally to FL + CGZ as holding time increases. Meanwhile, a softening phenomenon is also observed in the heat-affected zone (HAZ) in crept specimens for a more extended holding time, where voids, carbides, and dislocation density are further discussed. The carbides are identified as Cr and Mo-rich M23C6 type, which forms mainly at grain boundaries during creep testing. In HAZ, M23C6 gradually increases and coarsens, providing nucleation sites for creep voids. Finally, the Larson–Miller parameter method is employed to provide a theoretical reference for predicting use cases that satisfy the industry service condition.

Effect of γ' phase and microtwins on the microstructural evolution and mechanical properties of a novel Ni–Co base superalloy

A new Ni–Co base precipitation-hardened superalloy designed using the low stacking fault energy (SFE) was used primarily in the hot components of aeroengines designed with low SFE exposed to high temperatures. Different microstructures of the new alloys were obtained by controlling the solution treatment temperature. The corresponding mechanical properties were tested at 25 °C and 750 °C. The primary γ' precipitates dissolved with increasing solution temperature, causing grain growth and an increasing microtwin fraction. Fine-grained, transient, and coarse-grained zones were formed in sequence because of the interaction between the γ′ phase and microtwins. The tensile strength decreased as the grain grew when tested at room temperature, and the strengthening mechanism was dislocation strengthening. However, when the tensile experiment temperature was increased to 750 °C, the tensile properties increased from the fine-grained to the transient zone and then decreased to the coarse-grained zone. Moreover, the fracture mechanism changed from ductile fracture to a mixed ductile and brittle fracture. The size of the secondary γ′ precipitates increased by 32 nm because the dissolved primary γ′ precipitated when the solution temperature was increased from 1090 °C to 1120 °C, which improved the tensile properties. The electron channeling contrast imaging and transmission electron microscopy results confirmed that microtwins were formed during the tensile process at 750 °C. The microtwins hindered the dislocation movement. Therefore, the transient zone of the low-SFE alloy demonstrated the best mechanical properties, which guided the optimization direction of the alloy design and service.

Microstructures and Properties Investigation on DP980 Dual-Phase Steel CMT + P Welded Joints.

The increasing demand for the lightweight production of advanced high-strength steel puts forward higher requirements for the quality of welded joint forming. The common CMT welding process has certain limitations and is difficult to meet the needs of lightweight manufacturing. In this study, the CMT + P welding technology was used to weld the DP980 dual-phase steel plate with 1.2 mm thickness. The ER120S-G welding wire was used as the filling material to conduct a 70° angle lap welding experiment. The effects of wire feeding speed (3 m/min~5 m/min) and welding speed (350 mm/min~600 mm/min) on the forming, microstructures, and mechanical properties of DP980 dual-phase steel welded joints were analyzed. The results show that the CMT + P welding process can produce lap weldments with good formability and properties. The welded joints can be divided into the weld zone, the HAZ, and the base metal zone, where the HAZ can be divided into the coarse-grained zone and the softened zone. The role of the elements Ni and Si is to promote the production of martensite and to increase the ferrite strength in welded joints. As the wire feeding speed increases, the grain size of the coarse grain zone in the HAZ increases from 31.90 μm to 50.93 μm; while the welding speed increases, the grain size of the coarse grain zone decreases from 45.48 μm to 35.73 μm. The average microhardness of the weld zone is 420 HV. In contrast, the average microhardness of the softening zone in HAZ is reduced to 250 HV. When the wire feeding speed is 4 m/min and the welding speed is 550 mm/min, the tensile properties of the weldment are optimal, its tensile strength can reach 973 MPa, and the tensile fracture is ductile fracture.