Ductile High-entropy Alloys Research Articles

Thermal expansion-adjustable carbon-doped FeCoCrNiMn high-entropy alloys for electronic packaging

Application of an electronic packaging material is always limited by its established coefficient of thermal expansivity (CTE) and poor plastic working capacity. Here, we explored that a ductile high-entropy alloy (HEA) could achieve adjustable CTE values via decomposing at 500 °C. The carbon-doped equiatomic FeCoCrNiMn HEAs (0.8, 1.0, and 1.3 at% C) decomposed into B2, L10, M23C6 and σ phases during annealing. The adjustable CTE of the carbon-doped HEAs can be ascribed to the relatively low CTE values of the formed B2, L10, M23C6, and σ phases. The CTE value of a single face-centered cubic (FCC)-structured FeCoCrNiMn-1.3 at% C HEA is 16.7 × 10–6 °C−1, yet it can be continuously adjusted to 11.3 × 10–6 °C−1 when the FCC matrix is gradually decomposed into the B2, L10, M23C6, and σ phases. Furthermore, the decomposition rate and fractions of the B2, L10, M23C6, and σ phases can be controlled via changing carbon concentration and rolling reduction. More importantly, the CTE range of the 1.3C HEAs meets the requirements of electronic packaging. This work provides a way to design HEAs with desirable CTE for electronic industry via annealing at intermediate temperature.

A powder-metallurgy-based fabrication route towards achieving high tensile strength with ultra-high ductility in high-entropy alloy

The strength-ductility trade-off dilemma is perennially problematic in the materials science community. In particular, the attainability of high tensile strength and large elongation is ambitious in alloys fabricated by powder metallurgy. Here, we demonstrate a powder-metallurgy-based fabrication route to achieve a high synergy of tensile strength and ductility through cold-consolidation of CoCrFeMnNi high-entropy alloy powder using high-pressure torsion followed by annealing. This approach has resulted in an exceptional synergy of high yield strength of 754 MPa with an ultra-high tensile elongation of 58%% which has never been achieved in alloys fabricated by powder metallurgy routes. Additionally, the microstructure can be tuned by annealing treatment to achieve a range of strength and ductility that are highly sought after in industries for a specific application. The present fabrication route can be applied for fabrication of high-entropy alloy-matrix composites using alloys, metals, and ceramic powders to achieve controllable microstructure and eminent tensile properties.

A Powder-Metallurgy-Based Fabrication Route Towards Achieving High Tensile Strength with Ultra-High Ductility in High-Entropy Alloy

The strength-ductility trade-off dilemma is perennially problematic in the materials science community. In particular, the attainability of high tensile strength and large elongation is ambitious on alloys fabricated by powder metallurgy. Here we demonstrate a powder-metallurgy-based fabrication route to achieve high synergy of tensile strength and ductility through cold-consolidation of CoCrFeMnNi high-entropy alloy powder using high-pressure torsion followed by annealing. This approach has resulted in an exceptional synergy of high yield strength of 754 MPa with an ultra-high tensile elongation of 58% that has never been achieved in alloys fabricated by powder metallurgy routes. Additionally, the microstructure can be tuned by annealing treatment to achieve a range of strength and ductility that are highly sought after in industries for a specific application. The present fabrication route can be applied to fabricate high-entropy alloy-matrix composites using alloys, metals, and ceramic powders leading to a controllable microstructure and eminent tensile properties.

Snoek-type damping performance in strong and ductile high-entropy alloys.

Noise and mechanical vibrations not only cause damage to devices, but also present major public health hazards. High-damping alloys that eliminate noise and mechanical vibrations are therefore required. Yet, low operating temperatures and insufficient strength/ductility ratios in currently available high-damping alloys limit their applicability. Using the concept of high-entropy alloy (HEA), we present a class of high-damping materials. The design is based on refractory HEAs, solid-solutions doped with either 2.0 atomic % oxygen or nitrogen, (Ta0.5Nb0.5HfZrTi)98O2 and (Ta0.5Nb0.5HfZrTi)98N2. Via Snoek relaxation and ordered interstitial complexes mediated strain hardening, the damping capacity of these HEAs is as high as 0.030, and the damping peak reaches up to 800 K. The model HEAs also exhibit a high tensile yield strength of ~1400 MPa combined with a large ductility of ~20%. The high-temperature damping properties, together with superb mechanical properties make these HEAs attractive for applications where noise and vibrations must be reduced.

A ductile high entropy alloy strengthened by nano sigma phase

A nanoscale sigma (σ) phase strengthened Co34.5Cr30Ni26.5Al5.4W3.6 (at, %) high entropy alloy was prepared and showed high strength and ductility. Dual phase microstructure that comprises face-centered cubic (FCC) matrix and nanoscale σ particles was obtained after thermomechanical procedures. The excellent ductility is attributed to its high strain hardening exponent FCC matrix that strongly favors uniform deformation and prevents crack propagation. The alloy shows an extremely high strain hardening rate and strength due to the interactions between the σ particles, deformation twins and dislocations. The design concept of “deformation twins + nano particles” should be considered for the design of additional double-high (strength and ductility) FCC HEAs in future work.

High-strain-rate void growth in high entropy alloys: Suppressed dislocation emission = suppressed void growth

Void growth, as a critical stage in ductile fracture, could play some important role in the ductility of high entropy alloys (HEAs). Under a high-strain-rate tension (strain rate: 0.1~0.4/ns), our atomistic simulations reveal a severely suppressed growth of void (initial diameter: 4.0~9.1 nm) in HEAs at a low temperature, compared with that in Ni metal or Ni-based binary alloys. The emission of unclosed glide loops is found critical to void growth. Void growth is suppressed, once dislocation emission is suppressed due to the solute drag effect. Regarding the alloying elements, we find the void suppression relevance as Fe>Co and Cr>Fe>Mn.

Enhanced strength and slightly reduced ductility in a high entropy alloy via cold rolling and annealing

Simultaneous enhancement of strength and ductility of high entropy alloys through plastic forming and heat treatment procedures is difficult to realize. Through cold rolling and annealing on an as cast Fe24Co23Ni24Cr23Ti2Al4 high entropy alloy (HEA), the crystallographic structure transforms from a single face-centered cubic (fcc) phase to a dual body-centered cube (bcc) and fcc phases accompanied by precipitated strip shaped bcc phase with enriched Al–Ni binary structure. Based on a significant reduction in grain size from 2.87 ± 0.25 μm to 0.45 ± 0.15 μm and grain boundary type change from predominant high-angle grain boundaries to combination of predominant low-angle grain boundaries and precipitated band-shaped high-angle grain boundaries, the grain size gradient is substantially improved. The fracture mode also transforms from typical microvoid accumulation ductile fracture to ductile in-plane-shear fracture. Accordingly, the tensile strength and Vickers hardness enhance by 105.8% (from 861.2 to 418.1 MPa) and 172.8% (from 487.3 to 178.6 HV), respectively. In especial, the excellent ductility is maintained, as the elongation after fracture merely reduces from 0.22 to 0.17.

Tribological behavior of an amorphous Zr20Ti20Cu20Ni20Be20 high-entropy alloy studied using a nanoscratch technique

In this study, friction and wear behaviors of an amorphous Zr20Ti20Cu20Ni20Be20 high-entropy alloy (a-HEA) were characterized using a nanoscratch method. Scratch tests were carried out under both ramping and constant loading conditions to measure the coefficient of friction (COF) and wear resistance of the amorphous alloy. The morphology of scratched surface and subsurface was further examined using scanning and transmission electron microscopes. It was found that friction behavior of the a-HEA can be divided into two regions: elastic and plastic. In the elastic region, the COF dramatically decreases with increasing normal force, however, it monotonically increases in the plastic region. Morphological observation indicated that brittle fracture dominated in the plastic region, which was consistent with the result that a series of load serrations were accompanied by the formation of brittle cracks along the scratched track. Wear performance of the a-HEA was also evaluated and compared with “conventional” metallic glasses and crystalline high-entropy alloys. As a result of high hardness and large elastic recovery, the current a-HEA exhibited good wear resistance and low COF, behaved more like brittle amorphous alloys rather than ductile high-entropy alloys.

Nonbasal Slip Systems Enable a Strong and Ductile Hexagonal-Close-Packed High-Entropy Phase.

Linear defects, referred to as dislocations, determine the strength, formability, and toughness of crystalline metallic alloys. The associated deformation mechanisms are well understood for traditional metallic materials consisting of one or two prevalent matrix elements such as steels or aluminum alloys. In the recently developed high-entropy alloys (HEAs) containing multiple principal elements, the relationship between dislocations and the mechanical behavior is less understood. Particularly HEAs with a hexagonal close-packed (hcp) structure can suffer from intrinsic brittleness due to their insufficient number of slip systems. Here we report on the surprisingly high formability of a novel high-entropy phase with hcp structure. Through insitu tensile testing and postmortem characterization by transmission electron microscopy we reveal that the hcp phase in a dual-phase HEA (Fe_{50}Mn_{30}Co_{10}Cr_{10}, at. %) activates three types of dislocations, i.e., ⟨a⟩, ⟨c⟩, and ⟨c+a⟩. Specifically, nonbasal ⟨c+a⟩ dislocations occupy a high line fraction of ∼31% allowing for frequent double cross slip which explains the high deformability of this high-entropy phase. The hcp structure has a c/a ratio of 1.616, i.e., below the ideal value of 1.633. This modest change in the structure parameters promotes nonbasal ⟨c+a⟩ slip, suggesting that ductile HEAs with hcp structure can be designed by shifting the c/a ratio into regimes where nonbasal slip systems are activated. This simple alloy design principle is particularly suited for HEAs due to their characteristic massive solid solution content which readily allows tuning the c/a ratio of hcp phases into regimes promoting nonbasal slip activation.

Tailoring strength and ductility of high-entropy CrMnFeCoNi alloy by adding Al

High-entropy alloys with high strength and acceptable ductility at both room and elevated temperatures for high-temperature structural applications are desired. In this paper, CrMnFeCoNi alloy with outstanding ductility but low strength was selected as baseline alloy to study the formation of hard phase to strengthen this ductile alloy by adding Al into it. The results indicate that there is a phase structure transition from fcc to bcc when adding enough Al into CrMnFeCoNi. The yield strength and hardness increase with the content of Al increasing, due to the formation of hard bcc phase in the ductile CrMnFeCoNi alloy. The CrMnFeCoNiAl alloy with a bcc structure shows a high yield strength and adequate ductility at elevated temperatures. At 400 and 500 °C, the yield strength and ductility of CrMnFeCoNiAl alloy are 975.59 MPa and 6.39% and 989.48 MPa and 9.15%, respectively. An example of tailoring the strength and ductility of high-entropy alloy is demonstrated in this paper.

Boron doped ultrastrong and ductile high-entropy alloys

A new class of materials called high-entropy alloys (HEAs) constitutes multiple principal elements in similar compositional fractions. The equiatomic Fe20Mn20Cr20Co20Ni20 (at%) HEA shows attractive mechanical properties, particularly under cryogenic conditions. Yet, it lacks sufficient yield and ultimate tensile strengths at room temperature. To strengthen these materials, various strategies have been proposed mainly by tuning the composition of the bulk material while no efforts have been made to decorate and strengthen the grain boundaries. Here, we introduce a new HEA design approach that is based on compositionally conditioning the grain boundaries instead of the bulk. We found that as little as 30 ppm of boron doping in single-phase HEAs, more specific in an equiatomic FeMnCrCoNi and in a non-equiatomic Fe40Mn40Cr10Co10 (at%), improves dramatically their mechanical properties, increasing their yield strength by more than 100% and ultimate tensile strength by ∼40% at comparable or even better ductility. Boron decorates the grain boundaries and acts twofold, through interface strengthening and grain size reduction. These effects enhance grain boundary cohesion and retard capillary driven grain coarsening, thereby qualifying boron-induced grain boundary engineering as an ideal strategy for the development of advanced HEAs.

Deformation mechanisms of Mo alloyed FeCoCrNi high entropy alloy: In situ neutron diffraction

A FeCoCrNiMo0.23 high entropy alloy was processed by powder metallurgy with two conditions: hot extruded and annealed. In situ neutron diffraction, together with electron microscopy, was used to study the deformation mechanisms and concomitant microstructural evolution for both conditions. The as-extruded alloy has a single face-centered-cubic structure with a calculated stacking fault energy of ∼19 mJ/m2. When the alloy is tensile deformed, nano-twins and microbands are induced, resulting in an excellent combination of strength and ductility (784 MPa ultimate tensile strength and over 50% elongation). Annealing at 800 °C for 72 h increases the strength of the alloy but decreases its ductility. This is due to the decomposition of the alloy after annealing, causing the formation of Mo-rich intermetallic particles and a decrease of the stacking fault probability. These results highlight that combined mechanisms (i.e. solute strengthening and twin/microband induced plasticity) can effectively improve both the strength and ductility of high entropy alloys.

Interstitial atoms enable joint twinning and transformation induced plasticity in strong and ductile high-entropy alloys

High-entropy alloys (HEAs) consisting of multiple principle elements provide an avenue for realizing exceptional mechanical, physical and chemical properties. We report a novel strategy for designing a new class of HEAs incorporating the additional interstitial element carbon. This results in joint activation of twinning- and transformation-induced plasticity (TWIP and TRIP) by tuning the matrix phase’s instability in a metastable TRIP-assisted dual-phase HEA. Besides TWIP and TRIP, such alloys benefit from massive substitutional and interstitial solid solution strengthening as well as from the composite effect associated with its dual-phase structure. Nanosize particle formation and grain size reduction are also utilized. The new interstitial TWIP-TRIP-HEA thus unifies all metallic strengthening mechanisms in one material, leading to twice the tensile strength compared to a single-phase HEA with similar composition, yet, at identical ductility.

A ductile high entropy alloy with attractive magnetic properties

FeCoNiMn0.25Al0.25 high entropy alloy was investigated thoroughly in this paper from crystal structure to magnetic and mechanical properties. We found that this alloy formed simple face centered cubic structure which was very stable to deformation and heat treatment. Moreover, this alloy showed high saturated magnetization, low coercivity, high Curie temperature as well as good tensile ductility. This high entropy alloy shows soft magnetic properties much better than most high entropy alloys reported previously. This study suggests that the FeCoNiMn0.25Al0.25 high entropy alloy has the potential for use as soft magnets in the future.

Deformation and annealing behaviors of high-entropy alloy Al 0.5CoCrCuFeNi

This investigation explores the deformation and annealing behaviors of high-entropy alloys using a ductile high-entropy alloy of Al 0.5CoCrCuFeNi. This alloy had excellent workability and exhibited a large work hardening capacity in both hot forging and cold rolling. The main deformation and hardening mechanisms during cold work are uniquely associated with the nanotwinning deformation of this alloy. The easy formation of nanotwins appears to result from the blockage by the Widmanstätten Cu-rich precipitates of local slip deformation in a space of several tens nanometers, and the low stacking fault energy, which promotes the nucleation of nanotwins. This alloy was fully annealed in 5 h at 900 °C, revealing its significantly higher resistances to static anneal softening than traditional alloys with comparable melting points. This resistance is attributable to extensive solution hardening, low stacking fault energy, and the effect of sluggish diffusion on high-entropy alloys.