B2 Phase Research Articles

Variation of BCC/B2 coherent microstructure and resultant improvement in strength of Al0.7CrxFe3−xCoNi high-entropy alloys

The effects of Cr and Fe contents on the microstructure and mechanical properties of B2 nanoparticle-enhanced BCC-based Al0.7CrxFe3−xCoNi (x = 1.5–3.0) high-entropy alloys (HEAs) were studied. It was indicated that the addition of Cr and reduction of Fe in the alloys can increase the lattice misfits between the BCC and B2 phases, inducing the variations of BCC/B2 coherent microstructure and the significant improvements of the compressive yield strengths from ∼ 1485 MPa to ∼ 2252 MPa. The calculations of the yield strengths based on the microstructure of the alloys suggested that the improvements of the strengths were mainly due to the enhancements of the precipitation-strengthening effects. The interactions between dislocations and B2 particles with various shapes and sizes were also discussed on the basis of the calculations. It is demonstrated that adjusting the contents of Cr and Fe is an effective way to control BCC/B2 coherent microstructure and improve the mechanical properties of Al–Cr–Fe–Co–Ni HEAs.

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.

Microstructural Evolution and Mechanical Behaviour of Near-Eutectic High Entropy Alloy

In the current work, the thermal stability of the eutectic regions and precipitation and mechanical behaviour of near eutectic high-entropy alloy is studied. The alloy solidified as FCC and underwent sequential ordering to L12 with order–disorder transition temperature between 700°C and 1000°C. Cooling rate, variation in the concentration boundary layer, thermal gradient and interfacial energy were attributed to define the eutectic morphology (L12 and B2). Thermal exposure resulted in homogeneous precipitation of acicular Al-Ni-rich B2 phase within the proeutectic and irregular eutectic regions. The preferential B2 precipitation within the irregular eutectic regions was attributed to the lower stability and higher lattice strain associated with these regions. The FCC: B2 phase fraction after heat treatment was similar to that of the eutectic alloy. Furthermore, lamellar degradation, globularization and Ostwald ripening of the B2 phase were observed after heat treatment. Lamellar degradation occurred via mechanisms like cylinderization, edge spheroidization, termination migration and boundary splitting. The experimental observations on the phase evolution, stability and order–disorder transformation matched well with the CALPHAD predictions except for the σ phase formation. B2 precipitation during heat treatment enhanced the mechanical behaviour of the alloy. However, the work hardening rate was superior for as-cast alloy.

Microstructural evolution during annealing of a powder metallurgical TiAl–Nb composite and its effect on mechanical properties

Ductile Nb particles have been applied to toughen TiAl-based intermetallic alloys due to the positive effects of the Nb element on their mechanical and processing properties. However, the room-temperature (RT) fracture toughness of powder metallurgical (PM) TiAl–Nb composites needs further improvement to avoid micro-cracks in complex components prepared by PM near-net shape technologies. Herein, a facile heat treatment—annealing in the (α+γ) phase region for a short duration and air-cooling—was applied to strengthen and toughen a PM TiAl–Nb composite. The phase and microstructure evolution of the PM composite during annealing, and their effects on mechanical properties and fracture behaviors, were investigated specifically. It is found that after annealing, the volume fraction of α2 phase increased markedly, while that of the B2 phase decreased significantly, and the Nb solid solution (Nbss) phase almost completely disappeared; the microstructures of the TiAl–Nb composite became significantly finer and more uniform overall, and the microstructure type of the TiAl matrix transformed from near-gamma (NG) to duplex (DP). Both the RT tensile strength and fracture toughness of the TiAl–Nb composite improved markedly after annealing. The strengthening mechanisms for the as-annealed composite were primarily based on the phase and microstructure evolution. The toughening mechanisms for the as-annealed composite included the generation of shear ligaments, delaminations, crack deflection and branching. The improvement in tensile strength and the reduction in work-hardening exponent made the composite tougher through facile annealing. The incorporation of Nb particles and further heat treatment can efficiently facilitate the improvement in fracture toughness of PM TiAl-based alloy.

Tailoring the radiation tolerance of eutectic high-entropy alloy via phase-composition control

Eutectic high-entropy alloys (EHEAs) take advantage of heterogeneous crystal structures, numerous interfaces, and multicomponent composition designs, implying a potential radiation tolerance in the nuclear environment. In this work, we employed the He ion irradiation to study the irradiation behavior of two types EHEAs (Fe42Ni31Al17Cr10, abbreviated as Fe42, and Fe47Ni26Al17Cr10, abbreviated as Fe47) with different phase designs. The results showed that the FCC + B2 constituted Fe42 EHEA and BCC + B2 constituted Fe47 EHEA retained their general phase-composition features up to ∼ 1.2 dpa at 1073 K, while the long-range order of B2 phase was slightly disrupted in both alloys and more severe in the Fe42 EHEA. The quantification on He bubbles formation in different phases further indicated that the ordered B2 phase in Fe47 EHEA has a superior radiation resistance with smaller bubble size and less population. The differences in phase compositions and structures were believed to influence the vacancy-mediated helium diffusion during irradiation, which can account for the He bubble behaviors in the two kinds of alloys. The analysis on the width of bubble denuded zone along the phase boundary further suggested a higher energy barrier for the vacancy migration in the B2 phase, which consistently supported our inference on the underlying mechanism of bubble formation resistance of Fe47 EHEA. We believe that this work will provide a new strategy for designing EHEAs with excellent resistance to irradiation at elevated temperatures.

Tuning microstructure via cold deformation and annealing for superb mechanical properties in Al0.5CoFeCrNiSi0.25 dual-phase high-entropy alloys

In the field of metal materials, deformation and annealing play an irreplaceable role in improving the microstructure and optimizing the properties. In this study, we prepared Al0.5CoFeCrNiSi0.25 dual-phase high-entropy alloys (DHEAs) by vacuum arc melting and investigated their microstructure evolution and mechanical properties at different rolling and annealing temperatures. The results showed that the volume ratio of the FCC phase remained largely unchanged with increasing annealing temperature, with only small recovery for 10% reduction alloy. In contrast, the volume fraction and recrystallization ratio of the FCC phase in a 40% reduction alloy increased, and its recrystallization rate was higher than that of the BCC phase. Annealing the alloys at 900 °C formed the FCC, BCC, σ, L12, and B2 phases. As the annealing temperature increased to 1100 °C, the lamellar structure was changed, and the L12 and σ phases dissolved, leading to the gradual increase in the spacing size and volume fraction of the FCC phase. Increasing the annealing temperature reduced the yield strength but enhanced the ductility of DHEAs. Annealing for 1 h at 900 °C after 40% cold rolling enhanced their strength to 1360.61 MPa due to the high dislocation density and presence of σ phases but led to poor ductility. Annealing for 1 h at 1100 °C after 40% cold rolling produced a good combination of tensile strength (∼1267.8 MPa) and ductility (uniform elongation of ∼34.4%). Such remarkable strength and ductility may be attributed to the increased volume fraction of the FCC phase and the dual-phase heterogeneous deformation induction strain hardening effect.

Novel eutectic high-entropy alloy surfacing of flower-pot structure with outstanding wear resistance manufactured by laser cladding

Eutectic-high entropy alloys (EHEA) have been a hot topic of study in recent years due to their excellent thermal stability and great potential for industrial applications. In this study, a novel “flowerpot-seed” structure 19Al-24Cr-19Ti-37Ni-1Y eutectic high-entropy surfacing, which was manufactured by laser cladding successfully, is reported. The structure of EHEA surfacing consists of stable Heusler-type order phase (L21 phase) and B2 phase (interlamellar spacing of ∼320 nm). The structure near the interface resembles a "flowerpot-seed" with a large number of uniform rod-like nanoprecipitates inside the "seed", while the structure away from the interface resembles a "flower". The nanoprecipitates and low lattice misfit eutectic-phase interface with a lattice misfit of only 2.51% has the effect of strengthening and improving the hardness and wear resistance (WR) of EHEA surfacing. This alloy displays higher hot hardness and WR than most reported high-entropy alloys, EHEAs, and conventional Ni- and Co-based alloys widely used in industry.

In situ investigation on densification mechanism of Ti-20Al-19Nb (at.%) alloy by TiH2-assisted pressureless sintering

Dense Ti-20Al-19Nb (at.%) alloys can be cost-effectively fabricated by TiH2-assisted pressureless sintering; nevertheless, the densification mechanism remains controversial without understanding the entire sintering process. By in situ observing the surface morphology of the Ti/Al/Nb and Ti/TiH2/Al/Nb compacts upon heating, the densification mechanism of the Ti-20Al-19Nb alloys was elucidated in this study. In addition to the reported reason that the dehydrogenation of TiH2 provided reactive Ti, the densification of Ti-20Al-19Nb alloy was found to be strongly associated with the phase transformations upon sintering. The TiH2 participated in the reverse eutectoid transformation, α-Ti + δ-TiH2 → β-Ti, inducing the α/β and TiH2/β grain boundaries for the rapid diffusion of Al. The reaction of Ti(s) + Al(s) → TiAl3(s) was then accelerated, and the majority of the Al phase was consumed in the solid state, which significantly reduced the pores from the transient liquid Al. The activation energy of the Ti-Al reaction decreased with the addition of TiH2, and the growth mode of the TiAl3 phase was changed. By removing the large pores at 700 °C, the Ti-Al intermetallic phases were well connected, forming the continuous interdiffusion route for Ti, Al, and Nb. The diffusion of Nb, as well as the phase transformation of α2 → B2, was then promoted, and the ripening time for the B2 phase was increased. As a result, the density and mechanical properties were improved. The initial results of this study provided a foundation for the cost-effective fabrication of high-strength Ti-Al alloys containing refractory elements.

Phase transformation and strengthening effect induced by Al doping and annealing in nanostructural medium entropy alloy films

In this work, we have investigated the effects of Al content and annealing on the phase transformation and mechanical properties of nanocrystalline (CoCrFe)100-xAlx medium entropy alloy films prepared by DC magnetron co-sputtering. It is found that the as-deposited films have the face-centered cubic (fcc) structure even if Al content is as high as 20 at.%. However, after the annealing treatment, the fcc phase structure transforms into body-centered cubic structure (bcc) plus nanoscale ordered b2 phase. The nanoindentation test results show that the contribution of Al doping has little contribution on the hardness of the as-deposited films. After annealing treatment, the maximum hardness of the films increased to 11.7 GPa, which is 1.7 times higher than that of the as-deposited ones. Both bcc phase and nanoscale ordered b2 phase precipitation cause the superb strengthening effect of (CoCrFe)100-xAlx films.

Significance of adiabatic heating on phase transformation in titanium-based alloys during severe plastic deformation

Experiments were conducted on Ti-based alloys including a commercially pure titanium (α phase), Ti-6Al-4 V (α + β phase) and TiNi (B19’-martensitic phase) alloys in order to investigate the phase transformation during high-pressure torsion (HPT) at room temperature. An allotropic α to ω phase transformation in the commercially pure titanium occurred during HPT processing but increasing the rotation speed led to a decrease in the volume fraction of the ω-phase. This procedure facilitates the loss of the β phase in the Ti-6Al-4 V alloy and promotes a B19’ to B2 (austenite) phase transformation in the TiNi alloy. The results show that the bulk temperature increment in the processed specimens was not sufficiently high to affect phase transformations. However, the calculations by assuming adiabatic conditions revealed that the local temperature increment is responsible to encourage or suppress the phase transformation in the Ti-based alloys. This fundamental understanding of phase transformations during severe plastic deformation is useful for conducting microstructure engineering of Ti-based alloys by tailoring the microstructures containing different phases and thereby producing different mechanical and functional behaviors.

Thermal stability of (HfCo)90(NbMo)10 and (HfCo)75(NbMo)25 refractory high entropy alloys with a bcc + B2 structure

The effect of long-term (700 h) annealing at 1000 °С on the structure and mechanical properties of the as-cast (HfCo)90(NbMo)10 and (HfCo)75(NbMo)25 (in at.%) refractory high entropy alloys was examined. The as-cast alloys consisted of a mixture of (Nb, Mo)-rich bcc and (Hf, Co)-rich B2 phases with bcc / B2 ratios close to NbMo / HfСo ratios. Annealing did not result in the formation of additional phases, which agreed with the Thermo-Calc predictions. However, annealing led to the formation of (i) B2 / bcc nanoprecipitates inside the initial bcc / B2 phases and (ii) separate Nb-rich and Mo-rich bcc phases. Annealing also caused significant strengthening of the alloys; for example, the room-temperature yield strength of the (HfCo)90(NbMo)10 alloy increased from 640 to 1340 MPa. Hardening was induced by the bcc nanoprecipitates in the B2 phase according to the Orowan mechanism. The results obtained suggest novel routes for the development of refractory high entropy alloys with a stable bcc + B2 structure.

Simultaneously improved the strength and ductility of laser powder bed fused Al-Cr-Fe-Ni-V high-entropy alloy by hot isostatic pressing: Microcrack closure and precipitation strengthening

Hot isostatic pressing (HIP) is usually applied to reduce the defects including cracks and pores in the materials prepared by laser powder bed fusion (LPBF). In the present research, in order to improve the relative density and mechanical property, HIP was employed on the LPBF-processed Al-Cr-Fe-Ni-V high-entropy alloy (HEA) with microcracks and pores. The microstructure evolution and property improvement induced by HIP were investigated. In the LPBF-processed HEA, the microcracks were caused by residual stress and element segregation, and these microcracks as well as the pores reduced significantly after HIP treatments. Remarkably, HIP temperature has a more critical effect on the microcrack closure than the holding time, thus, microcracks and pores still existed after HIP-1 treatment (1273 K, 8 h), while HIP-2 treatment (1473 K, 4 h) could close the microcracks significantly. The crack closure was attributed to the interfacial diffusion of the alloying element under high temperature accompanied by high pressure, and the degree of element diffusion at both interfaces of the cracks determined the bonding strength after crack closure. Higher temperatures at high pressure induced more adequate element diffusion and higher bonding strength. The above high temperature and high pressure also induced the growth of the L12 phase and the precipitation of the B2 phase in HEA. Consequently, the tensile strength and elongation of the LPBF-processed HEA after HIP-2 treatment were simultaneously enhanced (80.7% and 222.5% higher than that of LPBF-processed HEA, respectively). This could be attributed to the combined effect of microcrack/pore closure and precipitation strengthening. The strengthening effect of the B2 phase and L12 phase accounted for 53% (dislocation by-pass mechanism) and 47% (dislocation shearing mechanism) of the total precipitation strengthening, respectively.

Effects of Al and Ti on microstructure, mechanical properties and wear resistance of TiXCrFe2Ni2 alloys

Individual effects of Al and Ti on microstructure, mechanical and tribological properties of CrFe2Ni2 alloy were investigated. Al addition changed the alloy from a FCC phase to a mixture of FCC, A2 and B2 phases, increasing the mechanical strength at slight expense of ductility. While Ti made the alloy composed of the FCC phase with L12-Ni3Ti nanoparticles, DO24η-Ni3Ti and C14 Fe2Ti phases. Higher Ti contents caused a brittle Chi phase. The hard phases considerably hardened the Ti-containing alloys but markedly decreased the ductility of the alloys. Al appeared to be unfavorable for the wear resistance possibly due to inadequate balance between strength and weakened ductility, while Ti enhanced the wear resistance thanks to the hard phases. The detailed characterization would help guide tuning relevant HEAs for optimal tribological applications.

Mechanical milling/alloying, characterization and phase formation prediction of Al0.1–0.5(Mn)CoCrCuFeNi-HEA powder feedstocks for cold spray deposition processing

In the present work the synthesis of multicomponent nanocrystalline Al0.1- 0.5(Mn)CoCrFeNi HEA powder feedstocks by mechanical alloying (MA) through high energy ball milling to produce HEA coatings with cold spraying were studied. Pure (>99.5) elemental powders of Al (Mn), Co, Cr, Cu, Fe & Ni were mixed in predetermined ratios and mechanically alloyed under Ar gas in a Planetary Ball Mill. MA HEA powder feedstocks were characterized by optical microscopy, SEM+EDAX, XRD & XPS to determine powder particle size, shape & distribution, particle surface condition, chemical composition, crystal & phase structures & microstructures. MA-processed Al-HEA powder feedstocks exhibited tri-modal particle size distribution (PSD) while Mn-HEA had four distinct PSD modes. Fine size fraction of the MA- HEA powders contained A2/B2 (BCC) harder phases with lattice parameters of 2.8831 and .2.8690 Å, respectively and coarse size fraction contained (ductile) softer A1 phase/Cu-rich regions combined (FCC) having a lattice parameter of 3.6181 Å. The ratio of the phases varied depending on the Al and Mn contents in the powder. For higher aluminium contents, there was more disordered A2/B2 phases and less disordered A1 phase in Al0.1–0.5 HEA powder feedstocks. A1 phase contained Fe, Cr, Co and Ni; A2 phase contained Fe, Cr; and B2 phase contained Al, Ni elements in Al0.1–0.5 HEA powder feedstocks. There were passive surface oxide films on the rough surfaces of MA HEA powder feedstocks consisting of a main mixture of surface oxides such as Al2O3, MnO, Cr2O3, Mn2O3, and minor oxides such as CuO, Fe2O3, NiO, Fe3O4, and a few CoCr2O4 spinel oxide. Dual FCC-dual BCC phase prediction of thermodynamic stable phase formation criteria reported in the literature on MA-HEA powder feedstocks were consistent with the experimental results of the present study.

Effect of Cu content on the microstructure and corrosion resistance of AlCrFeNi3Cux high entropy alloys

The FCC+B2/BCC structure of AlCrFeNi3Cux (x = 0 eutectic, x = 0.2, 0.4 hypo-eutectic, x = 0.6, 0.8, 1.0 dendritic) high-entropy alloys were fabricated and corrosion behaviours were investigated. In 3.5 wt% NaCl solution, pitting occurred initially in the B2 phases of all alloys. The values of corrosion rate, icorr and ipass first increased and then decreased with increasing Cu content. The AlCrFeNi3Cu0.4 alloy exhibited optimal corrosion resistance attributed to the increased passive film protection and reduced the volume fraction of B2 phases. The Cr-rich BCC precipitate in B2 phase generated dense Cr2O3 passive film improving localized corrosion resistance.

Revealing the microstructure evolution of the deformed superelastic NiTi wire by EBSD

The generation of irreversible macroscopic strains in superelastic NiTi alloys is an important issue closely related to the crystallography of the transformation from B2 austenite to B19’ martensite under stress. Although widely investigated at micro and nano scales by transmission electron microscopy (TEM), the observation of this phase transformation at the mesoscale is still lacking. In the present study, a superelastic NiTi wire was maintained in bending condition and investigated by electron backscattered diffraction (EBSD) technique. A strong variant selection and a formation of martensite texture were observed and quantified using the interaction work (IW) based on the distortion matrix of a single variant. A new habit plane between martensite and parent B2, {114}B2 || (201¯)M, was experimentally measured. It was explained by the Phenomenological Theory of Martensite Crystallography (PTMC) using dislocation slip {110}<001>B2 as lattice invariant shear (LIS). Some new B2 domains were also found in {114}B2 twinning relation with primary B2 grain. Two scenarios are proposed to explain them: a) the direct formation of {114}B2 deformation twin in B2, and b) a more complex one in which a primary B2 grain is transformed into a martensite variant, a (201¯)M deformation twin is formed inside this variant; then the (201¯)M twinned martensite reversely transforms into a new {114}B2 twinned B2 domain. The scenario b) permits to explain some uncommon (130)M, (201)M, and (101¯)M twins observed by EBSD between the martensite variants of the two {114}B2 twin-related B2 phases.

Influence of sulfur addition on the glass formation, phase transformation and mechanical properties of Cu50Zr50 alloy

In this work, a thorough study on the effects of sulfur-addition in the Cu50Zr50 glass-forming system was conducted using laboratory-based X-ray diffraction (XRD), synchrotron-based high-energy XRD (HESXRD), differential scanning calorimetry, and uniaxial compression testing. We report that the glass-forming ability (GFA), thermal stability, phase transformation sequence during heating and cooling, as well as the mechanical properties of the Cu50Zr50 system are highly sensitive to the sulfur concentration. Proper addition of sulfur can not only stabilize the B2-CuZr phase, but also enhances the GFA of the system. This enables the formation of BMG composites that solely contain a B2 phase, which results in a combination of high strength and substantial plastic strain under compressive loading. The total strains of the newly developed BMG composites with sulfur contents of 0.5 and 1.0 at.% are as large as ∼10 at.%, exceeding the previously reported strains of the Cu50Zr50 BMG by far. These superior properties make the Cu-Zr-S BMG composites a promising candidate material for industrial applications. In addition, it is found that excessive addition of sulfur leads to the precipitation of the Cu10Zr7 phase upon cooling from the alloy melt and a sulfide phase during heating from the glassy state. HESXRD experiments suggest that the preferential bonding between Zr and S atoms could be the determining structural mechanism on the atomic scale, leading to the changing primary crystallization sequence for heating and cooling. The findings in this work further strengthen the high relevance of sulfur in alloy development and tuning of properties in BMGs, especially considering the low cost, good availability, and non-toxicity of sulfur in the relevant modifications.

Design of novel AlCoFeNiV high-entropy alloys with high-strength and high-ductility

Eutectic high-entropy alloys (EHEAs) are gaining interest due to their excellent mechanical properties, superior liquidity and castability, and reduced compositional segregation. Here, we developed and successfully prepared Al19Co23Fe21Ni37 EHEA using vacuum arc melting. By adjusting the Co content and introducing V, we also obtained near-eutectic high entropy alloys (Al18Co27Fe20Ni35 and Al18Co22Fe20Ni35V5). The Al19Co23Fe21Ni37 and Al18Co27Fe20Ni35 alloys consist of alternating face-centered cubic (FCC) and ordered body-centered cubic (B2) phases, whereas the Al18Co22Fe20Ni35V5 alloy consists of FCC, ordered face-centered cubic (L12) and B2 phases. The heterostructure not only restricts dislocation motion but also induces significant hetero-deformation-induced hardening, resulting in a high-strain hardening effect. Furthermore, the high volume fraction of the hard B2 phase contributes to the increased strength of the alloy due to its high-stress partition relative to the other phases. As such, Al18Co22Fe20Ni35V5 alloy exhibits an excellent tensile ductility of 16.1%, yield strength of 571 MPa, and ultimate tensile strength of 1157 MPa.

Precipitation of B2 and B19 phases in α2 lamella of Ti-44.5Al-4 Nb-1Mo-0.1B alloy

Precipitation of B2 and B19 phases in α2 lamella of Ti-44.5Al-4 Nb-1Mo-0.1B alloy

Inhibiting cracking and improving strength for additive manufactured Al CoCrFeNi high entropy alloy via changing crystal structure from BCC-to-FCC

Hot cracking in laser powder bed fused (LPBF) AlxCoCrFeNi high entropy alloys (HEAs) plague the production of high strength parts. Here we show that the body-centered cubic (BCC) phase with very high hardness (∼670 HV) in LPBF printed AlxCoCrFeNi(x = 1, 0.7) HEAs causes cracking due to the plastic incompatibility between grain boundaries. To mitigate this, we propose a BCC to face-centered cubic (FCC) transformation strategy in the AlxCoCrFeNi HEAs to inhibit cracking and found that the Al0.5CoCrFeNi alloy (i.e., when x = 0.5) with a single FCC crystal structure possess low cracking sensitivity and well intergranular plastic compatibility, accompanied with decreased residual stress confirmed by using X-ray diffraction testing. The mechanical properties of the LPBF printed Al0.5CoCrFeNi HEA can be further enhanced by precipitation heat treatment. Scanning electron microscope/ transmission electron microscopy analysis show that nano-sized B2, L12 and σ phases precipitated from FCC Al0.5CoCrFeNi HEA when aged at 700 °C for 1 h. The ultimate tensile strength (UTS) increases from 899 MPa (as-print) to 1552 MPa (aged) with the elongation decreasing from 35.6 % (as-print) to 7.1 % (aged) due to the Orowan bypass mechanism of nano-precipitates. An increased aging temperature to 800–1000 ℃, decreases the UTS while increases the elongation, accompanied by the L12-to-B2 transformation and B2 phase coarsening. The deformation mechanism is mainly dominated by dislocations and stacking faults in the Al0.5CoCrFeNi HEA sample.