Bimodal Grain Research Articles

Deformation Behavior of Harmonic Structure Designed CoCrFeMnNi HE Alloys

This paper presents the deformation behavior of CoCrFeMnNi high-entropy alloy (HEA) produced from plastically undeformed and deformed powders. The investigation commenced with the preparation of ball-milled HEA powder at 90ks and 180ks, with a ball-to-powder ratio of 2:1, to induce severe plastic deformation on the outer layer of the HEA powder. Subsequently, the plastically undeformed and deformed HEA powders were successfully consolidated in a spark plasma sintering (SPS) furnace. The SEM observations revealed that the sintered plastically undeformed HEA powder exhibited conventional homogeneous microstructures (Homo), while sintered deformed HEA powders exhibited a regulated bimodal grain distribution known as harmonic structure (HS). The mechanical properties of the materials were characterized by performing tensile tests at room temperature (RT) and a low temperature of 193 K. The HS HEA exhibited an increase in proof strength of 16% to 40%, while the ultimate tensile strength (UTS) increased by 8% to 20% compared to the Homo HEA. Moreover, the strain hardening rate (SHR) curve indicates that both Homo and HS HEA exhibited oscillatory behavior at low temperature. However, Homo HEA exhibited more pronounced oscillation than HS HEA. The current results indicate that oscillatory behavior in the SHR curve is indicative of twin deformation. At low temperatures, the high rate of twinning deformation in Homo HEA was attributed to its higher ductility, while a good combination of high strength and high ductility produced by HS material was found to be influenced by HS design in addition to twinning deformation.

Corrosion studies of Al-Mg-Sc-Zr Alloy with Low Sc/Zr ratio fabricated by laser powder bed fusion for marine environments

This study investigates the corrosion behavior of a laser powder bed fusion (LPBF) Al-Mg-Sc-Zr alloy with a Sc/Zr ratio of less than 1 in 3.5wt% NaCl solution. X-ray diffraction (XRD) analysis revealed the presence of an Al matrix with face-centered cubic (FCC) structure and the secondary phase Al3(Sc,Zr) with L12 crystal structure. Microstructural analysis indicated a bimodal grain size distribution with fine equiaxed and columnar grains influenced by thermal gradients and secondary phase Al3(Sc,Zr). Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests in 3.5wt% NaCl solution demonstrated that the alloy exhibits a less negative corrosion potential (Ecorr) and lower corrosion current density (icorr) compared to other Al-Mg-Sc-Zr alloys with higher Sc/Zr ratios, indicating superior corrosion resistance. The enhanced performance is attributed to the fine grain structure and the formation of a stable protective oxide layer facilitated by the higher Zr content.

Microstructure and dynamic deformation mechanism of laser powder bed fusion of 316L-containing Ti–6Al–4V with dual-phase heterostructure

In this paper, 316L-containing Ti–6Al–4V alloy was fabricated by laser powder bed fusion (LPBF) through mixing Ti–6Al–4V and 4.5 wt% 316L powders, and the dynamic compressive mechanical properties and deformation mechanism at different strain rates were studied. The results show that the volume fraction of β phase sharply increases from 1.8% (in TC4) to 61.4% (in HST), and micro- and submicron-sized grains coexist in the HST alloy, producing heterostructure in the form of bimodal grain size distribution. The presence of the heterostructure led to superior heterogeneous deformation induced stress, thus facilitating the additional strain hardening and deformation coordinating ability, thus made the HST alloy achieved ultimate compressive stress of ∼1750 MPa and fracture strain of ∼17%, showing excellent comprehensive mechanical properties. Under quasi-static load, the deformation mode of HST alloy is mainly attributed to progressive deformation-induced martensitic transformation. However, at high strain rates, the impact loading inhibited the activation of progressive DIMT, and thus the deformation mode changed to combination of dislocation slipping and deformation twinning.

Effect of Mg content on the microstructure, texture, and mechanical performance of hypoeutectic extruded Zn-Mg alloys

The effect of Mg addition on the microstructure, texture, and mechanical performance of two hypoeutectic Zn-Mg alloys with 0.68 and 1.89 wt% Mg content was studied and compared to pure Zn after extrusion. The SEM/EDX, EBSD, and XRD investigations were carried out to study the effect of alloying on (1) phases’ composition, morphology, and size and (2) crystallography of the grains and the dominating slip systems. Through the mentioned investigations, hypothesized strengthening mechanisms could be estimated to identify the contribution of solid solution, grain boundary, secondary phase, dislocation, and texture rations of strengthening to improve the overall calculated yield strength. Uniaxial tensile testing was performed to evaluate the ultimate tensile strength (UTS), the yield tensile strength (YTS0.2 %), and the elongation to failure (Ef) of the pure and alloyed Zn and to compare the yield tensile strength with the computed ones. The combined impacts of the bimodal grains, the solubility of Mg, favorable morphology and distribution of secondary phase, and weakened bimodal basal texture with simultaneous twin-non basal slip mode deformation mechanisms resulted in an extraordinary strength-ductility synergy of the as extruded Zn-0.68Mg alloy (∼326 MPa and ∼16 %). The current research provides a new pathway for designing high-performance Zn-based alloys as an alternative load-bearing material for orthopedic implant applications.

Bimodal grain structure, phase transformation, and mechanical properties of a Mg-Gd-Y-Zn-Zr alloy during hot extrusion

In this study, we investigated the evolution of grain structures and second phases of a Mg-9.5Gd-4Y–2Zn-0.3Zr alloy during two-stage deformation that took place in the metal during hot extrusion, as well as their effects on its mechanical properties. We found that the alloy exhibited a bimodal grain structure consisting of equiaxed dynamic recrystallized (DRXed) grains and elongated unDRXed grains. As the extrusion process progressed, DRX occurred on a large scale, resulting in an increase in the proportion of DRXed grains from 17.8% to 93.2%, and a reduction in average grain size from 80.3 μm to 9.41 μm. The grain orientation also underwent a transition from a single orientation of an extrusion direction (ED) parallel to <0001> (i.e. ED//<0001>) to mixed orientations of ED//<01–10> and ED//<0001>. The primary mechanism of dynamic recrystallization (DRX) during extrusion involved particle-stimulated nucleation (PSN) accompanied by both continuous and discontinuous DRX. The Mg-9.5Gd-4Y–2Zn-0.3Zr alloy mainly contained intergranular 18R-long period stacking order (LPSO) phases and intragranular 14H-LPSO. As the extrusion process progressed, 18R-LPSO changed from blocky to lamellar to accommodate deformation, and β-Mg5(Gd,Y) phase particles dynamically precipitated along DRXed grain boundaries. The 14H-LPSO phase was found to be primarily distributed within coarse unDRXed grains. As deformation via extrusion progressed, the quantity of 14H-LPSO gradually decreased. Ultimately, with the gradual dissolution of Mg5(Gd,Y), 14H-LPSO reprecipitated within the recrystallized grain boundaries. As the extrusion process progressed, both the yield strength (YS) and elongation (EL) increased from 132.6 MPa to 287.6 MPa and from 10.8% to 15.5%, respectively. This was mainly attributed to the significant reduction in grain size and second phase size.

Origins and optimization mechanisms of periodic microstructures in Al-Cu alloys fabricated by wire arc additive manufacturing combined with Interlayer Friction Stir Processing

Inter-layer Friction Stir Processing on Arc Additive Manufactured Components offers significant potential for performance enhancement, yet the homogenization of microstructures has become a challenge for industrial application. To address this, 2319 Al-Cu alloy components were fabricated through the Wire Arc Additive Manufacturing + Interlayer Friction Stir Processing (WAAM-IFSP)1 technique, and a systematic study was conducted on the effects of deformation degree, deformation temperature, and strain rate changes on the microstructural evolution, revealing three distinct periodic microstructures. An increase in deformation degree resulted in a uniform periodic microstructure, attributed to the more uniform internal deformation caused by the increased deformation degree. At a deformation degree of 24.1 %, higher strength and ductility were achieved, along with a notable bimodal grain structure (BGS). Further research indicated that this structure would distribute more widely within the matrix with increasing strain rate, aiding in further performance breakthroughs, ultimately reaching an ultimate tensile strength (UTS) of 404 MPa and an elongation (EL) of 32 %. However, imperfections in the process led to inhomogeneity in the structure. Focusing on the dynamic recrystallization (DRX) mechanisms during stirring can improve this phenomenon. The research revealed that the fine grains in the BGS were controlled by a synergy of dominant discontinuous dynamic recrystallization (DDRX) and auxiliary continuous dynamic recrystallization (CDRX) nucleation mechanisms. The findings are expected to provide a theoretical basis for the optimization of WAAM-IFSP processes and microstructural and performance regulation, which is crucial for enhancing the industrial application prospects of aluminum alloys.

300 MPa grade high-strength ductile biodegradable Zn-2Cu-xMg (x = 0.08, 0.15, 0.5, 1) alloys: The role of Mg in bimodal grain formation

300 MPa grade biodegradable Zn-2Cu-xMg (0.08, 0.15, 0.5, and 1 wt.%) alloys with different bimodal grain structures were obtained by casting and hot extrusion. The effects of the Mg element on the microstructure, mechanical properties, and dynamic recrystallization (DRX) behavior of the as-extruded Zn-2Cu-xMg alloys were investigated. The obtained results showed that CuZn4 butterfly particles and eutectic net structure (η-Zn + Mg2Zn11) are formed in the as-cast Zn-2Cu-xMg alloys. The as-extruded Zn-2Cu-0.08Mg and Zn-2Cu-0.15Mg alloys exhibited finer DRXed and coarser unDRXed grains with an average grain size of 8.5–8.8 μm, while Zn-2Cu-0.5Mg and Zn-2Cu-1Mg alloys were almost composed of completed DRXed grains with an average grain size of 4.2–6.5 μm. Nanoprecipitates ε-CuZn4 were uniformly precipitated in both DRXed regions and unDRXed regions. Continuous DRX (CDRX) and twinning-induced DRX (TDRX) were the main mechanisms at a low Mg content; Discontinuous DRX (DDRX) and particle-stimulated nucleation (PSN) were strengthened with the addition of Mg. The improved yield strengths in Zn-2Cu-xMg originate from grain boundary strengthening, Orowan strengthening, and hetero-deformation-induced (HDI) strengthening. The fracture elongations are mainly affected by the synergistic effect of bimodal grains, non-basal < c + a > dislocations, and the secondary phases.

Comparative Study on Mechanical Properties and Microstructure Evolution of Mg-3Zn-1Mn/Sn Alloy through Ca-La-Ce Addition.

This study systematically investigates the influence of the composite addition of Ce, La, and Ca elements on the microstructure evolution and mechanical properties of Mg-3Zn-1Mn/Sn (wt.%) alloys. It indicates that the strength of Mg-Zn-Mn series alloys is superior to that of Mg-Zn-Sn series alloys, due to the stronger restriction of nanosized Mn particles on the recrystallization process and grain growth compared with Mg2Sn phases. The addition of the Ca-La-Ce elements significantly enhances the strength of the Mg-3Zn-1Sn alloy (YS increased by approximately 92.5%, UTS increased by approximately 29.2%, and EL decreased by nearly 52.2%), while for the Mg-3Zn-1Mn alloy, a balanced effect on both the strength and performance can be achieved. This difference mainly lies in the more pronounced refined effect on the grain size and the formation of a bimodal grain structure with strip-like un-DRXed grains and surrounding fine DRXed grains for the Mg-3Zn-1Sn alloy. In contrast, the addition of the Ca-La-Ce elements has a less obvious hindrance on the recrystallization process in the Mg-Zn-Mn series alloy, while significantly weakening the extrusion texture while refining the grains. Through in-depth characterization and experimental analysis, it is found that Sn and Ca can promote the formation of brittle and fine secondary phases. A nanoscale Sn phase (Mg2Sn phase) is more likely to accumulate at the grain boundaries, and the size of the nanoscale Ca2Mg6Zn3 in Mg-Zn-Mn series alloys is finer and more dispersed than that in Mg-Zn-Sn series alloys, thus strongly hindering recrystallization and refining the recrystallized structure of the alloy.

Autogenic Formation of Bimodal Grain Size Distributions in Rivers and Its Contribution to Gravel‐Sand Transitions

AbstractRiverbeds often fine downstream, with a gravel‐bedded reach, a relatively abrupt gravel‐sand transition (GST), and a sand‐bedded reach. Underlying this behavior, bed grain size distributions are often bimodal, with a relative paucity (gap) around the range 1–5 mm. There is no general morphodynamic model capable of producing the grain size gap and gravel‐sand transition autogenically from a unimodal sediment supply. Here we use a one‐dimensional morphodynamic model including size‐specific bedload and suspended load transport, to show that bimodality readily evolves autogenically even under unimodal sediment feed. A GST forms when we include a floodplain width that abruptly increases at some point. Upstream of the transition, non‐gap gravel ceases to move and gap sediment is preferentially transported. At the transition, non‐gap sand rapidly deposits from suspension, enhancing gap sediment mobility and diluting its presence on the bed.

Spinning-additive hybrid manufacturing of Al-Zn-Mg-Cu alloys: Microstructure evaluation and mechanical properties

This study employs wire-arc direct energy deposition (wire-arc DED) to produce high-strength aluminum alloys on the spinning matrix. Spinning-additive hybrid manufacturing specimens were produced. The evolution mechanism of the interfacial microstructure and fracture behavior of the hybrid manufacturing (HM) specimens was analyzed. The results indicate that the microstructure morphology can be divided into three regions: the spinning zone (SZ), columnar grain (CG) zone of additive manufacturing (AM), and equiaxed grain (EG) zone of AM. The SZ exhibits a bimodal grain structure, while the AM-CG zone consists of epitaxial growth from the SZ, and the AM-EG zone comprises equiaxed grains. Due to the influence of the hot spinning process, a large amount of eutectic structure and η phase coexist in the spinning matrix, resulted in a small amount of undissolved η phase remaining in the spinning matrix after heat treatment. A small amount of η' phase was generated in the AM zone at as-deposited state, due to the effect of in situ thermal cycling of the subsequent additive manufacturing process. Nearly all of η phase which dissolves into the α-Al matrix after heat treatment. The tensile strength of the hybrid manufactured samples increased from 256 MPa in the as-deposited state to 528 MPa after heat treatment. The fine-grained region of the spinning matrix experiences deformation and crack propagation initially, attributed to the softer properties of the fine-grained region within the bimodal grain structure of the SZ matrix, ultimately resulting in fracture at the SZ in all hybrid manufactured specimens.

Hierarchical modification of bimodal grain structure in Al/Ti laminated composites for extraordinary strength-ductility synergy

Al/Ti laminates with altering Al grain sizes was fabricated via hot press sintering. Fine Al powders results in low sintering density and obvious cracks at Al/Ti interface. Large Al powders greatly increased the grain size, grain aspect ratio, LAGBs fraction, and recrystallization fraction of the Al layers. The texture heterogeneity is also significant, with rolling texture in Ti layer and random texture in Al layer. Ti5Si3 phase precipitated at Al/Ti interface, and it gradually partitioned Ti atoms from TiAl3 and hindered the formation of TiAl3. Moreover, numerous stacking faults, dislocation loops, dislocation pinning, and dislocation tangles were observed at Al/Ti interface, resulting in an increased back stress. Large Al grains contributes the highest bending strength of 734.8 MPa, tensile strength of 753.2 MPa, and fracture strain of 62 %. The effect of grain size on work hardening was attributed to the fraction of LAGBs, dislocation storage capacity and additional HDI strengthening.

Simultaneously improving the strength and ductility of an oxide dispersion-strengthened high-entropy alloy by employing innovative precursors for oxide formation

The engineering of a composite microstructure, achieved by coupling heterogeneous grain distribution with oxide dispersion, has been demonstrated as an effective strategy for enhancing the strength-ductility synergy of alloys at both room and elevated temperatures. The effectiveness of this approach is strongly correlated with the micro-features of dispersed oxides. In this study, we selected the Ni38Co20Cr20Fe18Ti4 high-entropy alloy (HEA) as the base material, which could achieve the desired composite structure through preparation using mechanical alloying and spark plasma sintering methods. Our primary objective was to investigate the effects of incorporating Y2O3 or utilizing hydrides (TiH2 and YH3) as alternative precursors on the modification of oxide precipitates, evolution of a bimodal grain microstructure, and alteration in mechanical properties for the oxide dispersion strengthened (ODS) HEA. The results show that the incorporation of Y2O3 promotes the formation of ultrafine and semi-coherent Y2Ti2O7 ternary oxide particles, in addition to the pre-existing coarse binary TiO in the HEA matrix. Moreover, this leads to a significant increase in the fraction and a decrease in the average size of ultrafine grains (UFGs) within the bimodal microstructure. Notably, utilizing Ti- and Y-hydrides instead of Y2O3 and Ti as oxide-forming precursors within an equivalent composition remarkably amplifies the precipitation proportion of Y2Ti2O7 among dispersoids while further refining UFGs. The ODS-HEA synthesized using hydrides exhibits a simultaneous enhancement of 12 % in yield strength and 13 % in elongation to fracture, compared to that prepared from Ti and Y2O3. This intriguing phenomenon primarily arises from the heightened strengthening and toughening contributions, facilitated by the increased precipitation of advantageous Y2Ti2O7 nanoparticles.

Multi-scale structural heterogeneity promoting hydrogen embrittlement in additively manufactured pure nickel

In this study, how physical differences in the microstructure of additively manufactured (AM) pure Ni affect its susceptibility to hydrogen embrittlement (HE) are investigated. The presence of high-density dislocation cells (DCs) and grain boundaries in AM Ni could absorb more hydrogen therein, with the DC boundaries serving as reversible hydrogen trapping sites. The DC boundaries overlapped with the grain boundaries might lead to an enhanced hydrogen diffusion therein for AM Ni. Consequently, an increased depth of hydrogen-affected region and accelerated failure process were observed in AM Ni during tensile testing due to intergranular cracking. Furthermore, the HE resistance can be slightly improved by a dislocation-cell elimination heat treatment for AM Ni-800, but it remains lower than that of traditionally manufactured Ni, primarily due to the residue of bi-modal grains with small grains along the molten pool boundaries. The underlying HE mechanism based on the structural heterogeneities at different scales for pure Ni is discussed.

Fabricating a pure titanium foil with excellent strength-ductility combination via bimodal grain structure coupling with deformation twins

Preparation of pure titanium foil by cold rolling deformation often confronts the strength-ductility trade-off dilemma, how to obtain an excellent strength-ductility combination remains a great challenge. In the present work, we have prepared a pure titanium foil with a bimodal grain (BG) structure consisting of coarse grain (CG) and ultrafine grain (UFG), coupling with high-density deformation twins (DT) via a cryogenic-temperature rolling (CTR) and partial recrystallization annealing (PRA) process. Notably, a high strength-ductility synergy with YS of ∼483 MPa, UTS of ∼517 MPa and elongation of ∼21 % is achieved. The high strength is mainly caused by pronounced fine grain strengthening from the UFG region and twin boundary strengthening from strong interactions between DT and dislocations activated in the CG region. The GND caused by the hetero-deformation between CG and UFG, along with the activation of pyramidal <c+a> dislocation via DT in the CG region endows the pure titanium foil with excellent ductility.

Achieving a superior combination of strength and ductility by adjusting heterogeneous structure in a Fe–Mn–Al–Mo–C lightweight steel

A combination of high strength and good ductility was achieved by adjusting heterogeneous structure through varying annealing temperatures ranging from 750 to 900 °C after cold rolling in a Fe–Mn–Al–Mo–C lightweight steel. The microstructure consists of heterogeneous recrystallized, unrecrystallized grains containing nanoscale Mo2C precipitates, and intergranular Mo-enriched carbides. As annealing temperature increases, the fraction of recrystallization, and the average Schmid factor increase, while the average grain sizes and the volume fraction of Mo2C decrease. Annealing at 825 °C forms a desirable heterogeneous structure characterized by a normal bimodal grain distribution and diffuse precipitation of intragranular Mo2C particles. Tensile test results indicated that higher annealing temperatures decrease strength but enhance ductility. However, the yield strength of the 825 °C annealed sample only slightly decreases compared to the 800 °C annealed sample owing to the synergistic contributions of various strengthening mechanisms, including grain boundary, solid solution, precipitation, and heterogeneous deformation-induced strengthening. Furthermore, its ductility significantly improves, approaching that of the 850 °C annealed sample, facilitated by sustained heterogeneous deformation-induced strengthening effects and significant refinement in grain structure. The high strain hardening rate of the C825 sample is attributed to dynamic slip band refinement and local stress adjustments during later deformation stages. The heterogeneous grain structure induces a strong HDI strengthening effect due to uneven deformation, which also contributes to the high strain hardening rate. These findings highlight the potential of Fe–26Mn–8Al-1.2C–3Mo lightweight steel for applications requiring a balance of strength and ductility.

Comprehensive study of microstructural evolution and strengthening mechanism of high-performance pure copper prepared by blue laser metal deposition (B-LMD)

The high absorption of the copper to the blue laser provides an alternative method to circumvent the laser reflection of laser metal deposition (LMD) of pure copper. In this work, blue laser metal deposition (B-LMD) was used to prepare pure copper parts. The B-LMD pure copper tracks fabricated using the optimum processing parameter have well bonding with the substrate, consisting of fine equiaxed grains with an average size of 2.8 μm, and exhibit the highest microhardness of 133.21 HV0.2. The pure copper deposit fabricated by the optimum processing parameter exhibited the highest relative density (97.9 % ± 0.2 %) and microhardness (103.05 ± 4.37 HV0.2). Additionally, the microstructure of the B-LMD pure copper deposit is showing a bimodal grain structure with columnar grains at the melt pool boundary and fine equiaxed grains at the center. The B-LMD pure copper deposit exhibits excellent mechanical properties, with an ultimate tensile strength of 244 ± 9 MPa, yield strength of 158 ± 6 MPa, and a fracture elongation of 14.7 ± 0.8 %. The superior mechanical properties are primarily attributed to grain refinement strengthening and dislocation strengthening. The high performance of B-LMD pure copper demonstrate the potential of the blue laser as an effective alternative energy source for fabricating high-performance pure copper components.

Strong and ductile low carbon low alloy steels with multiphase bimodal microstructure

Restrained by the strength-ductility tradeoff, it is still challenging to develop advanced high-strength low carbon low alloy (LCLA) steels with superior strength-ductility combinations and cost-effectiveness to satisfy industry demands. In this study, an innovative 2-cyclic quenching and partitioning (Q&P) heat treatment was developed to produce a novel LCLA steel with the optimized microstructure, in which a bimodal grain size distribution across various constituent phases was achieved. Tensile test results show that the 2-cyclic Q&P LCLA steel exhibits excellent mechanical properties with a uniform elongation, close to 18 %, nearly triple that of conventional Q&P LCLA steel while maintaining a tensile strength above 1 GPa. To reveal the underlying mechanisms of such exceptional strength-elongation synergy, the detailed deformation behaviors of the developed LCLA steel were characterized while the evolution of hetero-deformation-induced (HDI) stress and effective stress was investigated from the perspective of the dislocation model. It is indicated that, with increasing strain, the heterogeneous structures promote strong strain partitioning which leads to extensive geometrically necessary dislocations (GNDs) pile-ups at hetero-interface and persistently strong HDI strengthening effect, and produce the coordinated deformation among constituent phases to realize dislocation forest strengthening, collectively contributing to the enhanced work hardening capacity and hence overcoming the strength-ductility tradeoff. This study provides a new processing strategy for developing strong and ductile LCLA steels.

Synchronous improvement in strength and ductility of Cu-bearing stainless steels through formation of bimodal grain structure induced by short-time electric pulses

Excellent strength and favorable formability are two important mechanical properties of stainless steel, but there is usually a trade-off between both properties. However, it has been suggested in recent studies that preparing microstructures with a non-homogeneous structure can effectively achieve strength-ductility synergy. Given these facts, a microstructure with bimodal grain structures was prepared in this study by short-time electric pulse treatment (EPT). Besides, the evolution of microstructures and mechanical properties of Cu-bearing stainless steel during EPT was analyzed. The results demonstrated that the non-uniform heating in the EPT process can rapidly promote localized grain growth, thus forming a bimodal grain structure compared with conventional heat treatment. The microstructure of the fine grains formed random textures, while the coarsened grains showed stronger textures. After EPT, the amount of S {123} < 634 < 634 >-type textures increased significantly, with the proportion reaching up to 30.1 %. There was also a certain amount of brass {110} < 112 >- and copper {112} < 111 >-type textures. Compared with the solution-treated samples, the best overall mechanical properties were detected under the optimal electric pulse parameters, which ultimately realized a synergistic increase of 11.8 % and 10.2 % in the ultimate tensile strength and ductility. The excellent strength-ductility synergy was closely related to heterogeneous deformation-induced (HDI) strengthening and textures induced by the bimodal grain structure. This finding may provide novel insights for enhancing the formability of biomedical metallic materials.

Trimodal Grain Structured Aluminum Matrix Composites Regulated by Transitional Hetero-Domains

Aluminum matrix composites (AMCs) with hetero-grains exhibit high strength with good ductility. A trimodal grain structure composed of ultrafine grains (UFGs), fine grains (FGs) and coarse grains (CGs) prevents the pre-mature cracking of hetero-zone boundaries in conventional bimodal grain structures; thus, it is favored by AMCs. However, the design of the size and distribution of hetero-domains in trimodal AMCs is tough, with complicated multi-scale deformation mechanisms. This study tunes the distribution of FG domains elaborately via altering the volume fraction of FG from 10 vol.% to 60 vol.% and investigates the distribution effect of FG domains on strength–ductility synergy. The optimized 2024 Al matrix composites with 30 vol.% FG exhibited a tensile strength of over 700 MPa and an elongation of 7.5%, respectively, realizing a good combination of high strength and ductility. This work enlightens the heterostructure design with a balance between heterogeneous deformation induced (HDI) strain hardening and high-content soft phase induced strain homogenization.

Regulating the bimodal structure and strength-ductility synergy of Zn-decorated Ti particles reinforced AZ91 composite through high-volume fraction Mg17Al12 precipitations

In this work, Mg-9Al-1Zn (AZ91) alloy reinforced with zinc-decorated titanium (Zn@Ti) particles was fabricated using the powder metallurgy method. Zn nanoparticles effectively dissolved into magnesium (Mg) matrix, which led to a reduction in the solid solubility of aluminum (Al) and precipitations of submicron sized Mg17Al12. As a result, the Zn@Ti/AZ91 composite displays a bimodal grain structure, achieving a remarkable balance between strength and ductility, with a yield strength of 248 ± 3.5 MPa, an ultimate tensile strength of 378 ± 5.3 MPa, and an elongation of 15.0 ± 2.8 %. The improved strength of Zn@Ti/AZ91 composite primarily stems from the synergistic effect of a significant volume fraction of submicron sized Mg17Al12 and Al8Mn5 precipitations strengthening, Zn solid solution strengthening, and grain boundary strengthening. Regarding ductility mechanisms, the presence of Mg17Al12 and Al8Mn5 precipitations effectively impede crack propagation and enhance ductility. This innovative approach represents a promising strategy for developing high strength and ductility of Mg composites.