Heterogeneous Lamella Structure Research Articles

Effect of texture on deformation behavior of heterogeneous Mg-13Gd alloy with strength–ductility synergy

Heterostructure metals as a new class of materials can effectively break the traditional strength–ductility trade-off dilemma. In this study, the extruded sheet with the small extrusion ratio (ER) of 3.9 (ER3.9) presented a heterogeneous lamella structure (HLS) and texture, where the fine dynamical recrystallized (DRXed) grains formed a random texture and coarse un-DRXed grains exhibited a strong basal texture. The ER3.9 sample presented an excellent combination of strength and ductility. The texture strengthening in coarse grains and hetero-deformation induced (HDI) strengthening contributed to the enhanced strength of the ER3.9 sample besides grain refinement. The improving ductility mainly stems from the weakened texture in fine grains. Interestingly, in coarse grains, the strong basal texture, the occurrence of cross slip, low stacking fault energy (SFE), and dislocation pinned by precipitates weaken the HDI hardening effect. While the traditional dislocation hardening mainly generated by fine grains dominates overall strain hardening. Meanwhile, the activation of non-basal slips, especially pyramidal 〈c + a〉 slip, and the generation of cross slips in fine grains benefit for coordinating plastic deformation. The ability for coordinate plastic deformation in fine grains is higher than that of coarse grains, which was confirmed by the digital image correlation technology. This work will promote the development of the heterogeneous theory in textured Mg alloys.

Enhanced interfacial strength and ductility of stainless steel/carbon steel laminated composite by heterogenous lamella structure

Interfacial performance of stainless steel/carbon steel (SS/CS) laminated composite is crucial for further manufacturing and application. Here, we reported a new strategy for fabricating the SS/CS laminated composites that composed of heterogeneous lamella structure at the cladding interface zone, by employing liquid–solid bonding and subsequent hot rolling. Different rolling temperatures were applied for comparison of the formation of heterogeneous lamella structure and their influences on the mechanical properties of the cladding interface, and the mechanisms of enhanced interfacial shear strength and ductility by the heterogenous lamella structure were discussed. A metallurgical bonding interface with intensive elements diffusion was obtained by liquid–solid bonding method, and coarse equiaxed grains and a wider decarburized layer formed at the cladding interface zone. Under rolling temperature of 900 °C, the mechanical incompatibilities of cladding interface zone and lower temperature resulted in heterodeformation and inefficient driving force of recrystallization, resulting in a heterogenous lamella structure consisted of hardened austenite region, coarse-grained ferrite region and fine-grained pearlite and ferrite region at the cladding interface zone of SS/CS laminated composite, which obviously improved the interfacial shear strength and ductility of the SS/CS laminated composites due to back stress strengthening and hardening.

A new strategy for fabrication of unique heterostructured titanium laminates and visually tracking their synchronous evolution of strain partitions versus microstructure

• A new strategy for producing Ti laminates with gradient structure (GS) and heterogeneous lamella structure (HLS) is proposed. • Strain partitions versus microstructure evolution is visualized synchronously using ultrahigh-resolution SEM-DIC combined with EBSD. • GS/HLS Ti exhibited a superior combination of strength and ductility, which is attributed to HDI stress strengthening and HDI hardening. Heterostructured (HS) material with extraordinary mechanical properties has been regarded as one of the most promising structural materials. Here, we reported a new strategy for preparing heterostructured pure titanium laminates that possess a good combination of strength and ductility by combining gradient structure (GS) and heterogeneous lamella structure (HLS). The deformation characteristic versus microstructure evolution of GS/HLS titanium laminates, namely the strain partitions between different-sized grains (480–25 μm) was visualized using a scanning electron microscope (SEM) equipped with electron backscattered diffraction (EBSD) mode combined with the digital image correlation (SEM-DIC) with an ultrahigh spatial resolution for the first time. As a result, the hetero-deformation of unique GS/HLS structure by the characteristic of strain partitions could be accurately captured. While the hetero-deformation could result in the hetero-deformation induced (HDI) stress strengthening and HDI hardening, which were regarded as the key reason that the resulting GS/HLS Ti laminates showed a superior combination of strength and ductility. This could promote a more in-depth understanding of the strengthening-toughening mechanism of heterostructured material.

Heterogeneous lamella design to tune the mechanical behaviour of a new cost-effective compositionally complicated alloy

A heterogeneous lamella (HL) design strategy was applied to manipulate mechanical properties of a new cost-effective Fe35Ni35Cr25Mo5 compositionally complicated alloy (CCA). The HL structure was produced by single-step heat treatment (800 °C for 1 h) after cold rolling. This HL structure consists of alternative lamellae regions of coarse-grained FCC matrix (5‒20 μm), and regions containing ultra-fine grains or subgrains (200‒500 nm) together with nanoprecipitates (20‒500 nm) and annealing twins. As compared with other cost-effective CCAs, the 800 °C annealed sample with HL structure demonstrated a comparable tensile property, with yield strength over 1.0 GPa and total elongation of ~13%. Formation of the annealing twins and nanoprecipitates decorated HL structure was a result of the concurrent partial recrystallization and precipitation of σ phase at the shear bands with a high density of lattice defects (e.g. high-density dislocation walls and deformation twins). The latter restricted the growth of recrystallized grains, leading to the formation of ultrafine subgrains within the HL structure. The high yield strength resulted from the multistage hetero-deformation induced (HDI) strengthening and precipitation strengthening associated with heterogeneous lamella structures containing nanoprecipitates. The ductility was originated from the coexistence of multiple deformation mechanisms, which started with dislocation slip and formation of stacking faults at the initial stage, followed by nano-twinning at the higher strain level. This HL design strategy, comprising composition and thermomechanical process designs, and the resultant microstructure tuning, open a broader window for the development of cost-effective CCAs with enhanced performance.

Achieving high hetero-deformation induced (HDI) strengthening and hardening in brass by dual heterostructures

• A dual heterostructured brass with superior combination of strength and ductility has been fabricated. • High hetero-deformation induced (HDI) strengthening and hardening is achieved by the design of the microstructure. • Extra interfaces generated by the dual heterostructure become the reason for the high HDI strengthening and hardening. Heterostructured materials have a superior combination of strength and ductility, due to their ability to produce hetero-deform induced (HDI) strengthening and hardening. Therefore, achieving high HDI strengthening and hardening is the primary goal for designing heterostructures. Here we report a dual heterostructure in brass that consists of the heterogeneous lamella and gradient structure, fabricated by rolling, partial annealing, and rotationally accelerated shot peening (RASP). The dual heterostructures are able to generate extra interfaces of heterogeneity compared to the single heterostructures, which could lead to higher HDI strengthening and hardening, and a more superior combination of strength and ductility. This finding presents a new pathway to designing heterostructures in metallic materials.

Optimization in strength-ductility of heterogeneous Mg-13Gd alloy via small extrusion ratio combined with pre-aging

In this work, the extruded Mg-13Gd alloy sheet was fabricated by hot extrusion with small extrusion ratio of 3.9 (ER3.9) combined with the aging prior to extrusion (APE) treatment. The APE-ER3.9 and ER-3.9 samples show apparent heterogeneous lamella structure (HLS) and heterogeneous texture (HT), where fine dynamically recrystallized (DRXed) grains form a random texture and coarse un-DRXed grains exhibit a strong basal texture. Besides, amounts of pre-existing nano-precipitates are introduced in the APE-ER3.9 sample. The APE-ER3.9 sample exhibits the optimal combination of strength and ductility, which is attributed to the synergy of HT between coarse and fine grains, the elevated heterogeneous deformation-induced (HDI) strengthening and hardening, the pre-existing precipitates and the dynamic generation of nanoscale chain-like precipitates during deformation. The coarse grains mainly rely on the activation of basal and prismatic <a> slips to maintain its work hardening. In fine grains, the basal <a> slip dominates the whole deformation process, while the combined effects of activated pyramidal <c + a> slip and cross slip play a significant role in accommodating the late deformation. This work provides a guide for designing Mg-RE alloys with excellent strength and ductility.

Excellent cryogenic strength-ductility synergy in duplex stainless steel with heterogeneous lamella structure

Ferrite and austenite duplex stainless steel with heterogeneous lamella structure was fabricated by severe cold-rolling and short-time annealing. The processed steel exhibited superior cryogenic mechanical properties with a tensile strength of ∼1650 MPa and an ultimate tensile strain of ∼30%. The excellent combination of cryogenic strength and ductility was attributed to fine grain size, nanotwins, back-stress hardening, and deformation-induced martensitic transformation.

Improved Mechanical Property and Corrosion Resistance of SAF2507 Duplex Stainless Steel with Heterogeneous Lamella Structure

Severe cold rolling and subsequent annealing are implemented on ferrite and austenite duplex stainless steel (DSS). The microstructure and mechanical properties of the processed sample are investigated systematically. The results show that the heterogeneous lamella (HL) structure can be generated in the cold‐rolled DSS by an appropriate annealing strategy. After annealing at 900 °C for 2 min, the HL‐structured sample with an alternative distribution of micrograined lamellae in ferrite and ultrafine‐grained lamellae in austenite obtains a unique combination of high strength and good ductility; the corresponding yield strength and uniform tensile elongation are ≈730 MPa and ≈23%, respectively. In addition, the corrosion behavior of the DSS in different processing states is investigated by the electrochemical method. The HL‐structured DSS possesses a superior corrosion resistance compared with other counterparts, which is attributed to the formation of a more stable passive layer on the surface. The precipitation of the sigma phase after long‐term annealing leads to deterioration of corrosion performance and mechanical properties.

Fabrication of high-strength metallic materials with heterostructure prepared via flow control extrusion

This paper proposes a novel approach called flow control extrusion (FCE) to obtain functional properties in metallic materials. The microstructure of pure copper subject to this technology combine ultrafine-grained structure in the surface layer, heterogeneous lamella structure inward, and gradually transits to bimodal grain structure in the center. Compared with the sample subject to conventional extrusion (CE), FCE extruded pure copper have higher yield strength (443 MPa) and work hardening rate. The high strength mainly depends on the significant refinement of microstructure. The high work hardening rate attributes to the back-stress-induced strengthening and dislocation hardening developed from the heterostructure of grain size and crystal orientation.

Mechanical properties and electrical conductivity of cold rolled Al-7.5wt%Y alloy with heterogeneous lamella structure and stacking faults

Strength, ductility, and electrical conductivity are always mutually exclusive to each other in conductor materials. Although numerous strategies were proposed, aiming at breaking the dilemmas between strength and ductility, or strength and conductivity, a better solution to solve them all is still a thirst for Al alloys. In this paper, we produced an Al-7.5Y alloy with heterogeneous lamella structure and stacking faults (SF) via cold rolling. The results indicate that the as-rolled Al-Y specimen shows a simultaneous increase in strength and electrical conductivity compared to that of the as-cast counterpart. Interestingly, the as-rolled Al-Y specimen has nearly the same uniform elongation as the pure Al counterpart. The deformation behavior, strengthening mechanism as well as the conductive mechanism of the as-rolled Al-Y alloy were revealed by electron microscopy and in-situ X-ray diffraction. Data AvailabilityThe authors confirm that the data supporting the findings of this study are available within the article.

A novel fracture behavior of the 304L stainless steel with heterogeneous lamella structure

The tensile fracture behavior of a 304L stainless steel with heterogeneous lamella structure (HLS) is studied by observation of its fracture appearance and analysis of its microstructure evolution. The HLS steel has two disparate fracture appearances of ductile clusters and brittle planes while the conventional steel with homogenous structure (HS) has a uniform fracture only consisting of ductile clusters. It is proposed that the fracture initiation is originated from boundaries between ultra-fine grains (UFGs) and recrystallized micro-grains (Micro-RGs) in the HLS steel while that is from some dislocation migration barriers in the HS steel.

Enhanced strength and ductility AZ91 alloy with heterogeneous lamella structure prepared by pre-aging and low-temperature extrusion

In this study, AZ91 alloy with heterogeneous structure comprised coarse, fine grained layers and the heterogeneous precipitates was obtained via pre-aging and low-temperature extrusion. The aging prior to extrusion (APE) sample with heterogeneous lamella structure presented an optimum combination of ultimate tensile strength (405.3 MPa), yield strength (216.5 MPa) and elongation (18.5%), which are much superior than extrusion (E) and extrusion prior to aging (EPA) samples with homogeneous structure. The observed excellent mechanical properties stemmed from the synergy effect of more stable precipitations at low temperature extrusion, grain refinement and texture. Meanwhile, heterogeneous deformation-induced (HDI) strengthening and work hardening also play a significant role on improving mechanical properties. The macroscopic and microscopic strain distributions demonstrate that APE sample is conducive to strain transfer and alleviate local stress concentration during collaborative deformation of coarse and fine grained layers, resulting in the improvement of strength and ductility. The research of this work will enrich the preparation methods of heterogeneous materials with intense precipitation strengthening potential.

Study of Texture Analysis on Asymmetric Cryorolled and Annealed CoCrNi Medium Entropy Alloy

CoCrNi equiatomic medium entropy alloy sheets were prepared by asymmetric rolling, cryorolling, and asymmetric cryorolling. The asymmetric cryorolled samples exhibited a noteworthy ultra-fine-grain heterogeneous lamella structure. The microstructure and corresponding hardness obtained by different rolling processes and subsequent annealing are compared. It can be seen from the results that the cryogenic deformation temperature had a stronger effect on the mechanical properties of the medium entropy alloys (MEA), compared with the shear strain caused by the asymmetric cryorolling. The effect of annealing temperature on texture components and volume fractions of the specially rolled samples was also analyzed. The result revealed that the recrystallized MEA exhibited similar texture components and the corresponding volume fraction, which indicated that the rolling process had limited influence on the formation of annealing texture. The recrystallized texture after annealing retained the deformation texture and twin related orientations appeared. Asymmetric rolled MEA showed strong random composition than symmetric rolled MEA regardless of rolling temperature. The recrystallized textures of the species obtained by the three rolling processes did not exhibit a significant dependence on the annealing temperature.

Heterogeneous Lamella Design to Tune the Mechanical Behaviour of a New Cost-Effective Compositionally Complicated Alloy

A novel heterogeneous lamella (HL) design strategy was proposed to improve mechanical properties of a new cost-effective Fe 35 Ni 35 Cr 25 Mo 5 compositionally complicated alloy (CCA). A HL structure was produced by single-step heat treatment (800 °C for 1h) after cold rolling. This HL structure consists of alternative lamellae regions of coarse-grained FCC matrix (5∼20 μm), and regions containing ultra-fine grains or subgrains (200∼500 nm) together with nanoprecipitates (20-500 nm) and annealing twins. The 800 °C annealed sample with HL structure demonstrated a superior tensile property, with yield strength over 1.0 GPa and ductility of ~13%. Formation of the annealing twins and nanoprecipitates decorated HL structure was a result of the concurrent partial recrystallization and precipitation of σ phase at the shear bands with a high density of lattice defects (e.g. high-density dislocation walls and deformation twins). The latter restricted the growth of recrystallized grains, leading to the formation of ultrafine subgrains within the HL structure. The high yield strength resulted from the multistage hetero-deformation induced (HDI) strengthening and precipitation strengthening associated with heterogeneous lamella structures containing nanoprecipitates. The ductility was originated from the coexistence of multiple deformation mechanisms, which started with dislocation slip and formation of stacking faults at the initial stage, followed by nano-twinning at the higher strain level. This HL design strategy, comprising of composition and thermomechanical process designs, and the resultant microstructure tuning, open a broader window for development of cost-effective CCAs with enhanced performance.

Aged metastable high-entropy alloys with heterogeneous lamella structure for superior strength-ductility synergy

High-entropy alloys containing multi-principal-element systems significantly expand the potential alloy design space, and offer the possibility of overcoming the strength-ductility trade-off in metallurgical research. However, the gain in ultra-high strength through traditional grain refinement and precipitation-strengthening mechanisms inevitably leads to a drastic loss of ductility. Here, we report on the design and fabrication of heterogeneous-lamella structured, aged bulk high-entropy alloy, which attains gigapascal tensile strength while retaining excellent ductility (UTS ~1.4 GPa, elongation ~30%; UTS ~1.7 GPa, elongation ~10%). Our work shows that the improved strength-ductility synergy arises due to various complementary strengthening mechanisms, including solid-solution, interfaces, precipitation and martensitic transformation, which influence the hardening and deformation processes at different strain levels. In particular, the hetero-deformation that is associated with the formation of microbands as well as the stress-induced martensite promotes additional hardening and hence high ductility. The strategy described here, that is leveraging the concept of heterogeneous microstructure design, provides a practical and novel method for fabricating high-performance structural materials.

Cold-workable refractory complex concentrated alloys with tunable microstructure and good room-temperature tensile behavior

A novel non-equiatomic NbTaTi-based refractory complex concentrated alloy (RCCA) capable of being cold-rolled up to a reduction in thickness of over 90% directly from the as-cast state is studied. Instead of conventional heat-treatment at temperatures greater than 0.8Tm for long durations, the excellent cold-workability in this RCCA allows for tunable microstructural features, including fine-grained, coarse-grained, and heterogeneous lamella (HL) structures, through proper heat-treatment at relatively lower temperatures for shorter duration. By tailoring microstructures, balanced tensile properties are achieved in this RCCA, through back-stress strengthening of HL structures. This study demonstrates an energy-saving and efficient way to fabricate high-performance RCCAs.

Achieving High Hetero-Deformation Induced (HDI) Strengthening and Hardening in Brass by Dual Heterostructures

Heterostructured materials have a superior combination of strength and ductility, due to their ability to produce hetero-deform induced (HDI) strengthening and hardening. Therefore, achieving high HDI strengthening and hardening is a goal for designing heterostructures. Here we report a dual heterostructure in brass that includes heterogeneous lamella and gradient structures, fabricated by rolling, partial annealing and rotationally accelerated shot peening (RASP). Extraordinary HDI strengthening and hardening were achieved by the dual heterostructures, which led to a more superior combination of strength and ductility, than the single heterostructures. This finding presents a new pathway to designing heterostructures in metallic materials.

Enhancing strength and ductility synergy through heterogeneous structure design in nanoscale Al2O3 particulate reinforced Al composites

Nanoparticle reinforced aluminum matrix composites fabricated by a high energy ball milling method showed significantly improved strength, however, their ductility was relative lower. This study proposed for the first time a heterogeneous structure strategy toward good strength and ductility synergy for Al matrix composites. Nanoscale Al2O3 particulate reinforced pure Al composites were fabricated as model materials. Three ball milling procedures were conducted to construct uniform ultrafine grain, random bi-modal grain, and heterogeneous lamella grain structures. It was shown that all composites exhibited significantly improved tensile strength. The heterogeneous lamella structure provided the composites with increased tensile strength and ductility compared to the uniform or the random bi-modal grain structure. Compared to ball milled pure Al, the heterogeneous structure composite had 160% and 121% increases in yield stress and ultimate tensile stress, respectively. Meanwhile, its elongation was as high as 8%. The results were rationalized through back stress theory. Geometrically necessary dislocations were believed to be the main reason for the good ductility without strength sacrifice.

Non-equiatomic FeNiCoAl-based high entropy alloys with multiscale heterogeneous lamella structure for strength and ductility

A new method based on conventional thermomechanical processing is proposed for design of non-equiatomic FeNiCoAlTaB (NCATB) high entropy alloy (HEA) with multiscale heterogeneous lamella (HL) structures to achieve outstanding mechanical properties. Two kinds of microstructures are produced in this study: 1) specimens annealed at 1050 °C exhibit structure composed of fine grains and ultrafine grains, which exhibits a combination of high strength (UTS ~1050 MPa) and good ductility (elongation ~23%); and 2) specimens annealed at 1200 °C which exhibit structure composed of fine grains and coarse grains, which demonstrates excellent synergy of good strength (UTS ~890 MPa) and exceptional ductility (elongation ~43%). It has been shown that shear bands formed in the cold rolling step are responsible for the fine-grain area in both specimens. Additionally, precipitation of NiAl (B2) particles also affects the formation of HL structures due to Zener pinning effect. The back-stress strengthening mechanism, responsible for high strength and ductility, is verified through combined “loading-unloading-reloading” (LUR) cyclic tensile tests and electron backscatter diffraction (EBSD) based geometrically necessary dislocation (GND) density analysis.

Deformation mechanisms of 304L stainless steel with heterogeneous lamella structure

Deformation behavior of 304L stainless steel with heterogeneous lamella structure mainly contains three stages: yield drop stage (~0.2–2%), involving accumulation of stacking faults/geometrically necessary dislocations and initiation of strain-induced martensitic transformation (SIMT); high-speed work-hardening stage (~2–15%), related to dislocation slipping and SIMT; steady work-hardening stage (~15–32%), dominated by SIMT.