Strength-ductility Trade-off Research Articles

Enhancing strength–ductility synergy in a Mg–Gd–Y–Zr alloy at sub-zero temperatures via high dislocation density and shearable precipitates

The strength–ductility trade-off dilemma is hard to be evaded in high-strength Mg alloys at sub-zero temperatures, especially in the Mg alloys containing a high volume fraction of precipitates. In this paper, we report an enhanced strength–ductility synergy at sub-zero temperatures in an aged Mg–7.37Gd–3.1Y–0.27Zr alloy. The tensile stress–strain curves at room temperature (RT), −70 °C and −196 °C show that the strength increases monotonically with decreasing temperature, but the elongation increases first from RT to −70 °C then declines from −70 °C to −196 °C. After systematic investigation of the microstructure evolutions at different deformation temperatures via synchrotron X-ray diffraction, electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM), it is found that a high dislocation density with sufficient <c+a> dislocations promotes good tensile ductility at −70 °C, which is attributed to the minimized critical resolved shear stress (CRSS) ratio of non-basal <c+a> to basal <a> dislocations. In addition, more shearable precipitates can further improve the ductility via lengthening the mean free path of dislocation glide. The present work demonstrates that an excellent strength–ductility synergy at sub-zero temperatures can be achieved by introducing a high dislocation density and shearable precipitates in high-strength Mg alloys.

Dynamic precipitation and deformation behaviors of a bimodal-grained WE43 alloy with enhanced mechanical properties

Achieving a high strength-ductility synergy in rolled Mg alloys remains a major challenge, especially for Mg alloys with high contents of rare earth (RE) elements, which are hard-to-deform and exhibit poor formability. In the present work, a bimodal grain structure consisting of fine grains (FGs) and coarse grains (CGs) has been achieved in a novel WE43 alloy sheet prepared by the hard-plate rolling (HPR) technique, which breaks the strength-ductility trade-off dilemma of the conventional WE43 alloy. Notably, the yield strength (YS) reaches as high as ∼312 MPa, presenting a remarkable improvement of YS (∼60 MPa) in comparison to conventionally rolled counterparts. Meanwhile, the ultimate tensile strength (UTS) and elongation-to-failure (EF) are ∼332 MPa and ∼11.8%, respectively, much higher than those of the wrought WE43 alloy reported in literature. Specifically, the disparate dislocation slips and dynamic precipitation behaviors in FGs and CGs have been studied in detail by combining EBSD and TEM analysis. The discrepancy of dynamic precipitation behavior in FGs and CGs is originated from the heterogeneous dislocation slip-dominated deformation behavior during HPR, related to initial grain orientations. It reveals that basal slips are dominant in the FG area, where equilibrium β-Mg14Nd2Y phase has precipitated along grain boundaries. In contrast, non-basal slips are prevalent and the metastable short rod-shaped β1 phase has formed within CGs. The enhanced YS of the present HPRed WE43 alloy sheet is mainly attributed to the formation of large amounts of extremely fine grains with an average size of ∼0.94 μm. The enhanced Orowan strengthening comes from the interaction between short rod-shaped β1 particles and non-basal dislocations (prismatic <a>, pyramidal <c + a> and pyramidal <a> dislocations) in CGs. In particular, the interactions between β1 particles and pyramidal <a> dislocations dominate the Orowan strengthening, leading to an increase of ∼34 MPa in the YS. Basal to basal slip transfers have been observed in FGs while non-basal slips are prevalent in CGs, which could be helpful for release the stress concentration near GBs. It is beneficial for preventing earlier fracture and thus enhancing the tensile strength combined with decent ductility.

Composition optimization of cobalt-free Fe-Cr-Ni-Al/Ti multi-principal element alloys for strength-ductility trade-off based on machine learning

Multi-principal element alloys (MPEAs) represent a novel class of advanced structural materials. However, due to the vast composition space, traditional experimental methods and first-principles calculation are not efficient for exploring this space. In recent years, machine learning (ML) has emerged as a promising tool for alloy design, offering an effective approach to develop high-strength and high-toughness MPEAs. In this work, we proposed a random-exhaustive feature selection (REFS) method to select an appropriate feature subset from a high-dimensional feature for model training. Moreover, we compared the metrics of models trained by various ML algorithms using either composition input or parameter input strategies. The results indicated that the models trained by using the Gradient Boosting Regression (GBR) algorithm with the parameter input strategy exhibited the high R2 values of yield strength (0.947), ultimate tensile strength (0.922), and fracture elongation (0.844). Finally, we conducted a two-step composition design for cobalt-free Fe-Cr-Ni-Al/Ti MPEAs. The optimized models designed Al5(Fe10Cr35Ni55)95 and Al2Ti1(Fe10Cr35Ni55)97 MPEAs, which demonstrated the significant improvements of 56.42%, 29.45%, and 5.82% in yield strength (YS), ultimate tensile strength (UTS), and fracture elongation (FE), respectively, compared to equiatomic FeCrNi MPEA. In addition, by using the association rule mining (ARM) on the model predictions, the compositional dependence of mechanical properties and the possibility of data-driven alloy design were revealed.

A new transformation-induced plasticity-assisted dual-phase medium-entropy alloy with ultra-high cryogenic mechanical properties

A novel maraging Fe68Ni10Mn10Co10Ti1.5Si0.5 (at%) medium-entropy alloy (MEA) was designed and microstructurally engineered to obtain a superior combination of tensile strength and uniform elongation at liquid nitrogen temperature. To this end, short-time martensite-to-austenite reversion treatment was conducted on an aged specimen to gain a dual-phase microstructure decorated by needle-like (NiMn)3−xTix and the elliptical-shaped Ni2SiTi nano-precipitates. The alloy exhibited an ultra-high yield strength of 1.41 GPa and ultimate tensile strength of 1.88 GPa, with a uniform elongation of ∼14% in the reverted condition. These superior properties are attributed to the transformation-induced plasticity (TRIP)-assisted heterogeneous dual-phase microstructure strengthened by well-distributed nano-precipitates. The metastability-engineering approach to achieve TRIP-assisted maraging MEA can usefully guide design to overcome the strength–ductility trade-off in extreme environments.

Achieving ultrahigh strength and ductility in high-entropy alloys via dual precipitation

The strength-ductility trade-off has been a longstanding dilemma in metallic materials. Here we report an innovative approach to achieve a high strength-ductility synergy via dual precipitation of sheared and bypassed precipitates. (Ni2Co2FeCr)96–xAl4Nbx (at.%) alloys strengthened by nanoscale L12 particles and Laves precipitates were selected as a model for this study, and their precipitate microstructures and mechanical properties were thoroughly investigated. The dual-precipitation-strengthened alloys exhibit a yield strength of more than 1400 MPa, an ultimate tensile strength of over 1800 MPa, and a uniform elongation of 18%, thus achieving a high strength-ductility synergy. Our analysis reveals that the nanoscale L12 precipitates contribute to the strength via the particle shearing mechanism, whereas the Laves phase provides the strengthening through the Orowan bypass mechanism. The study of deformation microstructures shows that the L12 precipitates are sheared by stacking faults, which facilitates long-range dislocation gliding through the matrix. As a result, deformation induces the formation of hierarchical stacking fault networks and immobile Lomer–Cottrell locks, which effectively enhance the work hardening capability and plastic stability, thereby resulting in a high ductility at high strength levels. Dislocations are piled-up against the interface between the Laves precipitates and matrix, which increases the work hardening capability at the early stages of plastic deformation but causes stress concentrations. The dual precipitation strategy may be useful for many other alloys for achieving superior mechanical properties for technological applications.

Overcoming the strength-ductility trade-off and anisotropy of mechanical properties of Ti6Al4V with electron beam powder bed fusion

Achieving a combination of excellent ductility and mechanical property isotropy is highly desirable for conventional biomedical materials, although this has been a long-standing problem. Here, we present a novel approach, in situ alloying of Cu (3.5 wt%) into Ti6Al4V through electron beam powder bed fusion (EB-PBF), through which the aforementioned issues can be addressed. The ultimate tensile strength (UTS) and elongation (EL) to failure of the Ti6Al4V-3.5Cu (TAVC) alloy fabricated by EB-PBF were found to reach values of 1053 MPa and 15.3%, respectively, which are markedly superior to those of the PBF-EB/Ti6Al4V (UTS: 902 MPa; EL: 13.5%). However, an alloy with identical composition made by spark plasma sintering exhibited a significant brittleness characteristic (UTS: 1051 MPa; EL). Moreover, the generation of multiple micro/nanosized Ti2Cu phases (1–5 μm, 50–100 nm, and 5–10 nm) in the TAVC alloy samples is thought to contribute a significant strengthening (46.24%), which effectively prevents plastic deterioration caused by brittle lamellar (Ti2Cu + α-Ti) structures. Further investigation revealed that the ductile enhancement of the TVAC alloy can be attributed to the microstructural evolution that encompasses the generation of twin structures and spheroidization of α-Ti and results from the addition of Cu coupled with the intrinsic heat treatment during the EB-PBF manufacturing process. Our findings not only provide an in-depth understanding of the formation mechanism of multi-scale Ti2Cu precipitates but also offer critical guidance for improving the overall mechanical properties of Cu-containing biomedical titanium alloys.

Ultrahigh strength-ductility of nanocrystalline Cr2AlC coating under micropillar compression

MAX phases gain increasing attention as protective coatings due to superior anti-corrosion and oxidation resistance for harsh high-temperature applications. However, the lower strength and strength-ductility trade-off in bulk MAX phases still pose an enormous challenge arising from the large ratio of c/a lattice parameters and less activated slips. Here, nanocrystalline Cr2AlC coating with high-purity was fabricated using a hybrid arc/sputtering technique with subsequent thermal annealing. Cylindrical micropillar compression test was conducted by FIB milling to identify the mechanical properties of the coating under deformation. The stress-strain curves demonstrated that an ultrahigh compressive strength of 5.3 GPa with strain beyond 12.5% was surprisingly achieved for Cr2AlC coating, which was 5 times larger than all the reported bulk MAX phases before. Specifically, the extraordinary ductility at RT is ascribed to the synergy of the fundamental basal slip, additional non-basal slip and deformation twinning stimulated by ultra-refinement effect of nano-crystalline Cr2AlC coating.

Microstructural engineering through high enthalpy states: implications for far-from-equilibrium processing of structural alloys

Metallic materials derive their strength and ductility from their microstructural features. The general principle of alloying is not only to control the phases present in the alloys, but also how the alloying elements can help in microstructural control during the processing of materials. The overall thermodynamic framework of using free energy to explain the stability of an alloy has two significant terms, enthalpy, and entropy. Engineering alloys are processed by several far-from-equilibrium processes. During the processing, the materials are in metastable states. The metastability can also be purposely enhanced through alloy design. This paper provides a perspective on how “high enthalpy states” can be used to tailor the microstructure to overcome the conventional strength-ductility tradeoff. The emergence of new manufacturing processes also provides unique opportunities to design alloys to maximize the potential of such processes. A few illustrative examples are presented to tie the historical use of high enthalpy states and point to future opportunities. Co-development of advanced materials for disruptive new manufacturing processes can be enhanced through integrated computational materials engineering approaches.

Mechanical properties and deformation behavior of equiatomic CoCrFeMnNi high-entropy alloy foam: A molecular dynamics study

In recent years, the aerospace and mechanical industries have employed metal foams for High Energy Capacity applications. However, the porous nature of such metallic foams results in a drop in tensile properties. High Entropy Alloys (HEAs) have been reported to overcome the strength-ductility trade-off as opposed to conventional metals and their alloys. Based on this, an equiatomic nano porous CoCrFeMnNi HEA is investigated using molecular dynamics simulation to elucidate the deformation behavior and mechanical performance at different atom distributions, percent porosity, and pore locations. The findings indicated that the mechanical properties are independent of atom distributions. However, with an increase in percent porosity, a decline in the ultimate tensile strength was observed. Up to 66% reduction in tensile strength was recorded. A similar trend was observed under temperature variations. By varying pore location, either within or on the structure, the pores within the foam structure led to a significant reduction in properties. The deformation mechanism of the HEA foam was established via analyses of the dislocation distributions, stacking faults, crack nucleation, and propagation. The findings offer new insights for material design to produce high-performance HEA foams and a comprehension of its failure mechanism under loading.

Effect of interstitial oxygen /nitrogen on mechanical and wear properties of TiZrHfNb refractory high-entropy alloy

Refractory high-entropy alloy (RHEA), as a kind of promising alloy, has received particular attention in high-temperature applications because of its stability and excellent high-temperature softening resistance. However, most RHEAs are in the strength-ductility trade-off dilemma, which leads to limited applications. In this study, the TiZrHfNb alloy with better ductility is selected as the base alloy, and the strength of the alloy is enhanced by interstitial solid-solution strengthening of O and N atoms, thereby improving the wear resistance of the alloy. All alloys exhibit a body-centered cubic (BCC) structure. The O-doped alloy exhibits a remarkable balance between strength and ductility, while the N-doped alloy displays the highest levels of hardness and yield strength, attributed to the grain boundary strengthening and solid-solution strengthening effects of interstitial atoms. The N-doped alloy has the lowest wear rate at room temperature and 500 ℃, which is attributed to the hardening effect of N atoms. At 25 ℃, the main wear mechanisms are severe abrasive wear, adhesive wear, and oxidative wear. At 500 ℃, the dominant wear mechanisms change to oxidative wear and slight abrasive wear. The oxide layer on the worn surface can decrease the coefficient of friction (COF) and improve the wear resistance.

Overcoming strength-ductility trade-off in CoCrNi medium entropy alloy by forming fine crescent grains in laser powder-bed fusion

This study investigated the influence of microstructure on the strengthening mechanism in CoCrNi medium entropy alloys (MEAs) fabricated by laser powder-bed fusion (L-PBF). An excellent combination of high strength and elongation was achieved in L-PBFed CoCrNi MEA. The yield strength of L-PBFed CoCrNi MEA was significantly improved because of the combined effect of grain boundary strengthening and dislocation strengthening. Due to the temperature gradient difference during L-PBF process, the crescent shape grain boundaries were evident in L-PBFed samples, which increased the mechanical properties.

Enhancing the strength and ductility in an Al–5Mg alloy via accumulative continuous extrusion forming at room and cryogenic temperatures

In this work, the microstructural evolution of Al–5Mg (wt%) alloy fabricated by accumulative continuous extrusion forming (ACEF) at room temperature (RT) and cryogenic temperature (CT) was investigated. The grain refinement mechanism during ACEF was revealed. The strengthen and ductility enhancements of the alloy were discussed. The results showed that good grain refinement effect was achieved in the alloy after multi-pass RT-ACEF, and was further enhanced in the alloy fabricated by 1 pass CT-ACEF. High-density dislocations, as the driven force of continuous dynamic recrystallization, were achieved in the 1 pass CT-ACEFed alloy, and the growth of recrystallized grains was limited at CT. As a result, superior grain refinement effect was obtained in the alloy after CT-ACEF. Simultaneous strength and ductility improvements were found in the ACEFed alloy. The ultimate tensile strength and elongation were enhanced to 294.3 MPa and 55.1 % of the alloy after 1 pass CT-ACEF. Grain boundary strengthening and dislocation strengthening dominated the strength enhancement of the ACEFed alloy. The high work hardening exponent and wide strain interval were responsible for the increase in ductility of the CT-ACEFed alloy. The purpose of this study is to provide a high-efficiency strategy to achieve Al–Mg alloys with superior mechanical performance and overcome the strength-ductility trade-off.

Tensile yield behavior of high strength-ductility 24CrNiMoY alloy steel by dual-field assisted laser direct energy deposition

Laser direct energy deposition (LDED) high strength-ductility matching alloy steel is promising for the manufacturing of anti-deformation and deformation parts. However, there are differences in strength-ductility matching and tensile yield behavior of parts with different deformation requirements. It is essential to understand how to control the microstructure and prepare alloy steel with either continuous or discontinuous yield phenomena to meet manufacturing requirements of the deformation or anti-deformation parts. To address this issue, the microstructure evolution and tensile yield behavior of 24CrNiMoY alloy steel deposited by preheating-ultrasonic dual-field assisted LDED using a multi-factor optimized-design response surface method combined with the experiment is investigated. The experimental results indicate that the relationship between microstructure evolution and strength-ductility of alloy steel prepared by dual-field-assisted LDED was clarified. Interestingly, we found that the various dual-field synergistic parameters help to prepare tensile samples with either continuous or discontinuous yield behavior to meet the requirements applicable to the alloy steel parts with different working conditions. It is discovered that the characteristics of bainite and ferrite, including their quantity, distribution, and morphology, can influence the continuous or discontinuous tensile yield behavior of alloy steel. The advanced technology enabled the preparation of high strength-ductility 24CrNiMoY alloy steel with tensile strength up to 1060 MPa, elongation up to 13.8%, and a strength-plastic product up to 14.6 GPa%. This work breakthrough has overcome the technical bottleneck associated with the strength-ductility trade-off of LDED alloy steel, which makes it possible to directly obtain continuous or discontinuous yield behavior through process control. This study provides a useful theoretical and technical guideline for LDED preparation of high strength-ductility alloy steel parts meeting anti-deformation or deformation requirements.

Ballistic impact response of a heat-treated dual-phase Ti–5.2Mo–4.8Al–2.5Zr–1.7Cr alloy with hierarchical microstructure

In the present study, a dual-phase Ti-5.2Mo-4.8Al-2.5Zr-1.7Cr alloy was hot-rolled at 980 °C with a thickness reduction of 65% and then heat-treated with the strategy of 920 °C/1 h/water quenching +550 °C/6 h/air cooling, and a hierarchical microstructure was prepared, which contained micro-scale equiaxed primary α phase (αp), sub-micro scale rod-like α phase (αr), nano-scale acicular secondary α phase (αs) and β matrix segmented by αs and αr. In addition, the dislocation densities of α phase and β phase were determined as 0.3652 × 1015/m2 and 2.2502 × 1015/m2, respectively. Contributing to αr and αs, the hierarchical microstructure exhibited higher strength (yield strength: 1228 MPa, ultimate tensile strength: 1389 MPa, dynamic compressive strength: 1661 ± 27 MPa). Simultaneously, αp and αr were helpful to the strain transfer, and thus the plasticity was maintained at a considerable level (elongation: 13.4 ± 0.2%, critical fracture strain: 18.9 ± 0.2%). Such hierarchical microstructure overcame the limitation of the strength-ductility trade-off to a certain extent and exhibited a superior combination of strength and ductility. The ballistic impact behavior of the titanium alloy plates with the thickness of 20.3 mm (1#), 19.3 mm (2#) and 18.4 mm (3#) against 7.62 mm armour piercing projectiles illustrated that as the titanium alloy thicknesses decreased from 20.3 mm to 18.4 mm, more ASB-induced cracks were formed near the rear face and connected to form catastrophic cracks in the 2# and the 3# titanium alloy plates, even resulting in the failure for the 3# titanium alloy plate. Ultimately, the 1# and 2# titanium alloy plates exhibited preferable ballistic impact properties.

Metastable high entropy alloys of TiZrHfTa with glass-like characteristics at low temperatures

Metastability engineering of high entropy alloys can widen the alloy-design window through the access of a potentially huge configuration space. At the same time, it has the great capacity to overcome the strength-ductility trade-off, where often one witnesses an actual performance improvement at low to cryogenic temperatures. An in-depth microscopic understanding of the underlying mechanism is now needed to drive the design effort. Here we investigate in detail the energetic, structural and chemical complexity of Ti–Zr–Hf–Ta system. The large configuration space with local chemical interaction heterogeneity is explored by a combination of structure-searching techniques and accurate energy models constructed by cluster expansion based on density functional theory data. We find that the potential energy landscape evolves towards displaying shallow mega-basins when decreasing the Ta content. This is accompanied by an expansion of the configurational density of states, which evolves to be clearly distinct from that of a homogeneous random solid-state solution. Furthermore, the energy-state variability and local configuration flexibility are identified from high throughput analysis of atomic structures and their energies. Such features remind closely what found in glass-like systems. This work suggests a theoretical connection between glass physics and metastability engineering of HEAs is quite promising.

Achieving strength-ductility combination and anisotropy elimination in additively manufactured TiB/Ti6Al4V by in-situ synthesized network architecture with fine grains

It is of significance, but still remains a key challenge to overcome the strength–ductility trade-off, and homogenize the mechanical performance of the additively manufactured titanium alloy. Here, we provided a new strategy of multi-scale architecture design to overcome this issue through in-situ synthesizing TiB-reinforced titanium matrix composites manufactured by laser-directed energy deposition (LDED). The β grains were remarkably refined from ∼200 μm to ∼70 μm and the aspect ratio of α-Ti reduced from ∼16.3 to ∼4.8 by planting TiB (≥2.5 vol%). The 2.5 vol%TiB/Ti6Al4V composite exhibited the significant transition of columnar to equiaxed grains and presented the multi-scale columnar and equiaxed network structure with favorable continuity, and achieved the good combination of strength (1153 MPa) and ductility (6.2%) thanks to the grain refinement and structure strengthening afforded by the network structure with different strain domains. The 5 vol%TiB/Ti6Al4V obtained the excellent ultimate tensile strength of 1174 MPa while maintaining good ductility. Both the apparent columnar to equiaxed grain transition which led to the reduction of the texture intensity and Schmid factor, and the special distribution of TiB with <010> texture along the building direction achieved the isotropic strength in 2.5 vol%TiB/Ti6Al4V composite.

Substructure development, martensitic transformation and rapid work-hardening in an as-cast interstitial substituted N atom in FeMnCoCr high-entropy alloy

The microstructural evolution and room-temperature mechanical properties of nitrogen-added Fe50Mn30Co10Cr10 as-cast high-entropy alloy were investigated. Interestingly, an excellent strength-ductility trade-off with a rapid hardening stage was achieved in the as-cast structure (tensile strength: 740 MPa, ductility: 42 %) that was comparable with those counterparts holding wrought structure. These excellent mechanical properties were attributed to the capability of metastable FCC microstructure enabling deformation-induced martensitic transformation even at low imposed strain at room temperature. Simultaneously, the occurrence of substructure development independent of martensitic transformation, significantly contribute to increase the capability of the material for strain accommodation.

Laser Additive Manufacturing of Layered Zr-Based Bulk Metallic Glass Composite

As a potential functional material, much effort has been devoted to enhancing the mechanical properties of bulk metallic glass composites (BMGCs). Among them, layered BMGCs are regarded as effective for achieving a strength–ductility synergistic effect. However, it is difficult with the existing metallic glass (MG) preparation technologies to obtain a decent layered structure. In addition, the fragile interface between layers formed using the traditional fabricating method always exacerbates the deterioration of mechanical properties, which restricts the wide application of layered BMGCs. In the case of laser additive manufacturing (LAM), the cooperation of coarse grains in the hot affected zone (HAZ) and fine grains in the remelting zone induced by a unique thermal history is of key importance in eliminating the fragile interface and therefore overcoming premature cracking. Thus, we successfully synthesized Nb-Zr48Cu46.5Al4Nb1.5 layered material with a yield strength of 1332 (±91) MPa and a compression ductility of 4.17 (±0.14)% via LAM. The results of the compressive curves of Nb and BMGC prepared by LAM decisively demonstrate that the layered material obtains a certain degree of plasticity while maintaining relatively high strength. This remarkable mechanical property is mainly attributed to the asynchronous deformation and the interaction of the adjacent Nb and MG layers. It is worth emphasizing that a distinctive round-way crack extension is discovered during the deformation process, which plays a significant role in breaking through the strength ductility trade-off. In addition, the source of yield strength is calculated theoretically using the rule of the mixture and the dislocation strengthening principle. The results indicate that the strength contributed by geometrically necessary dislocations is around 101.7 MPa. In addition, the strength calculated by the rule of the mixture is ~1201.9 MPa. This work offers a new paradigm for BMGCs with excellent strength and ductility as practical engineering materials.

Hetero-Deformation Induced Hardening in a CoCrFeNiMn High-Entropy Alloy

One of the most important issues in materials science is to overcome the strength–ductility trade-off in engineering alloys. The formation of heterogeneous and complex microstructures is a useful approach to achieving this purpose. In this investigation, a CoCrFeNiMn high-entropy alloy was processed via cold rolling followed by post-deformation annealing over a temperature range of 650–750 °C, which led to a wide range of grain sizes. Annealing at 650 °C led to the formation of a heterogeneous structure containing recrystallized areas with ultrafine and fine grains and non-recrystallized areas with an average size of ~75 μm. The processed material showed strength–ductility synergy with very high strengths of over ~1 GPa and uniform elongations of over 12%. Different deformation mechanisms such as dislocation slip, deformation twinning and hetero-deformation-induced hardening were responsible for achieving this mechanical property. Increasing the annealing temperature up to 700 °C facilitated the acquisition of bimodal grain size distributions of ~1.5 and ~6 μm, and the heterogeneous structure was eliminated via annealing at higher temperatures, which led to a significant decrease in strength.

Regulation of bimodal structure and mechanical properties of powder-thixoformed GO/ZK60 magnesium matrix composite through adjusting GO content

Although remarkable strength enhancements can be achieved in graphene oxide (GO)/ graphene nanoplatelets (GNPs) reinforced Mg matrix composites by using the available techniques, their ductility is always quite poor due to the difficultly avoided strength-ductility trade-off. To conquer this dilemma, GO/ZK60 composites with bimodal-grain structure were fabricated using powder thixoforming in this work. The results indicate that the grain size and volume fraction of coarse grains (CGs) first decrease as the GO content increases to 0.2 wt.% and then increase again as the content increases to 0.3 wt.%, while the grain size in the fine grains (FGs) almost does not change. Consequently, the strength of the composites is improved with increasing GO content and reaches the peak values at the content of 0.2 wt.%. The composite with 0.1 wt.% GO content exhibits significantly increased tensile yield strength up to 177 ± 2 MPa while maintaining a high elongation of 23.1% ± 2.5%, being equivalent to that of the ZK60 matrix alloy. The increased FGs volume fraction, together with the promoted dislocation accumulation and storage via GO and grain refinement of large-sized CGs lead to the improvement of strain hardening ability, thus rendering the composite an excellent ductility. Furthermore, the deformation of the GO/ZK60 composites occurs progressively from the FGs to the CGs, which is opposite to the status of the milled ZK60 matrix alloy. In view of the microstructure characteristics of the composites, a new complex calculation model was proposed and it could well predict the strength of the bimodal GO/ZK60 composites. This study provides a new insight into the microstructure design and fabrication technology of GO/GNPs reinforced metal-based composites with high strength and ductility.