B2 Phase Research Articles

Spinodal Zr–Nb alloys with ultrahigh elastic admissible strain and low magnetic susceptibility for orthopedic applications

Metallic biomaterials, such as stainless steels, cobalt–chromium–molybdenum (Co–Cr–Mo) alloys, and titanium (Ti) alloys, have long been used as load-bearing implant materials due to their metallic mechanical strength, corrosion resistance, and biocompatibility. However, their magnetic susceptibility and elastic modulus of more than 100 GPa significantly restrict their therapeutic applicability. In this study, spinodal Zr60Nb40, Zr50Nb50, and Zr40Nb60 (at.%) alloys were selected from the miscibility gap based on the Zr–Nb binary phase diagram and prepared by casting, cold rolling, and aging. Their microstructure, mechanical properties, corrosion resistance, magnetic susceptibility, and biocompatibility were systematically evaluated. Spinodal decomposition to alternating nanoscale Zr-rich β1 and Nb-rich β2 phases occurred in the cold-rolled Zr–Nb alloys during aging treatment at 650 °C. In addition, a minor amount of α phase was precipitated in Zr60Nb40 due to the thermodynamic instability of the Zr-rich β1 phase. Spinodal decomposition significantly improved the mechanical strength of the alloys due to nanosized dual-cubic reinforcement. The Zr–Nb alloys showed an electrochemical corrosion rate of 94–262 nm per year in Hanks’ solution because of formation of dense passive films composed of ZrO2 and Nb2O5 during the polarization process. The magnetic susceptibilities of the Zr–Nb alloys were significantly lower than those of commercial Co–Cr–Mo and Ti alloys. The cell viability of the Zr–Nb alloys was more than 98 % toward MC3T3-E1 cells. Overall, the spinodal Zr–Nb alloys have enormous potential as bone-implant materials due to their outstanding overall mechanical properties, extraordinary corrosion resistance, low magnetic susceptibility, and sufficient bicompatibility. Statement of significanceThis work reports on spinodal Zr–Nb alloys with heterostructure. Spinodal decomposition significantly improved their mechanical strength due to the nanosized dual-cubic reinforcement. The Zr–Nb alloys showed large corrosion resistance in Hanks’ solution because of formation of dense passivation films composed of ZrO2 and Nb2O5 during the polarization process. The magnetic susceptibilities of the Zr–Nb alloys were significantly lower than those of commercial Co–Cr–Mo and Ti alloys. The cell viability of the Zr–Nb alloys was more than 98 % toward MC3T3-E1 cells. The results demonstrate that spinodal Zr–Nb alloys have enormous potential as bone-implant materials due to their outstanding overall mechanical properties, high corrosion resistance, low magnetic susceptibility, and sufficient biocompatibility.

Microstructure Evolution and Mechanical Properties of AlCoCrFeNi2.1 Eutectic High-Entropy Alloys Processed by High-Pressure Torsion.

High-entropy alloys (HEAs) have garnered significant attention for their exceptional properties, with eutectic high-entropy alloys (EHEAs) emerging as particularly notable due to their incorporation of eutectic structures comprising soft and hard phases. This study investigated the influence of shear strain on the microstructural refinement and mechanical properties of AlCoCrFeNi2.1 EHEAs, which were subjected to high-pressure torsion (HPT) at room temperature under a pressure of 6 GPa across 0.5 to 3 turns, compared to the initial material. After HPT treatment, significant grain refinement occurred due to strong shear strain, evidenced by the absence of B2 phase peaks in X-ray diffraction (XRD) analysis. Microhardness increased substantially post-HPT, reaching a saturation point at approximately 575 HV after three turns, significantly higher than that of the original sample. Moreover, the ultimate tensile strength of HPT-treated specimens reached around 1900 MPa after three revolutions, compared to approximately 1100 MPa for the as-cast alloy, with a mixed fracture mode maintained. This investigation underscores the efficacy of HPT in enhancing the mechanical properties of AlCoCrFeNi2.1 EHEAs through microstructural refinement induced by shear deformation, offering insights into the design and optimization of advanced HEAs for various engineering applications.

Microstructure and Mechanical Properties of Ti-6Al-4V In Situ Alloyed with 3 wt% Cr by Laser Powder Bed Fusion

This work studied the microstructure and mechanical properties of Ti-6Al-4V in situ alloyed with 3 wt% Cr by laser powder bed fusion (LPBF). Specimens with a relative density of 99.14 ± 0.11% were produced, showing keyhole and lack of fusion pores. Due to incomplete mixing of the components during melting, chemical inhomogeneities were observed in the solidified material. The addition of Cr promoted thermal supercooling during solidification and induced a reduction in the primary β grain size in the longitudinal direction and a weakening of the otherwise strong ⟨100⟩β texture, both typical issues for Ti-6Al-4V produced by LPBF. The primary β at first transformed martensitically to α’, but by preheating the substrate plate to 500 °C and cyclically reheating the material by melting subsequent layers, in situ martensite decomposition was achieved, resulting in a fine lamellar α + β microstructure. In addition, the B19 phase was detected in the β matrix, presumably caused by Fe impurities in the Cr powder feedstock. Specimens exhibited a hardness of 402 ± 18 HV10, and an excellent ultimate tensile strength of 1450 ± 22 MPa at an elongation at break of 4.5 ± 0.2%.

Enhancing atomic ordering, magnetic and transport properties of Mn2VGa Heusler alloy thin films toward negatively spin-polarized charge injection

Magnetic materials with negative spin polarization have attracted attention for their potential to increase the design freedom of spintronic devices. This study investigated the effects of off-stoichiometry on the atomic ordering, microstructure, and magneto-transport properties in Mn2+xV1−xGa (x = −0.2, 0, +0.2, +0.4) Heusler alloy films, which are predicted to have large negative spin polarization derived from a pseudo band gap in the majority spin channel. The Mn2+xV1−xGa films epitaxially grown on MgO(001) substrates exhibits variations of B2 and L21 order with the Mn concentration. A high-quality L21 ordered film was achieved in the Mn-rich composition (x = +0.2) with B2 and L21 order parameters of 0.97 and 0.86, respectively, and a saturation magnetization of 1.4 μB/f.u., which agrees with the Slater-Pauling rule. Scanning transmission electron microscopy observations showed that B2 and L21 phases coexist in Mn-poor and stoichiometric films, while the L21 phase is dominant in the Mn-rich film with small amounts of Mn–V and Mn–Ga disorders, as revealed by laboratory and anomalous X-ray diffraction. Combined first-principles calculations and anisotropic magnetoresistance analysis confirms that the addition of excess Mn preserves the high spin polarization by suppressing the formation of detrimental antisites of V atoms occupying Mn sites. Therefore, the Mn-rich composition is promising for negatively spin-polarized charge injection in Mn2VGa-based spintronic applications.

Synthesis and properties of (Hf,Mo,Ti,W,Zr)B2–(Hf,Mo,Ti,W,Zr)C dual phase ceramics

Dense, dual phase (Hf,Mo,Ti,W,Zr)B2–(Hf,Mo,Ti,W,Zr)C ceramics were synthesized with varying contents of Mo and W. The final (Hf0.317Mo0.025Ti0.317W0.025Zr0.317)C–(Hf0.317Mo0.025Ti0.317W0.025Zr0.317)B2 was a nominally pure, dual phase ceramic, while compositions with higher amounts of Mo and W contained multiple phases. The final microstructures had submicron grains due to pinning effects. Vickers hardness values were up to 48.6 ± 2.2 GPa for an applied load of 0.49 N for the ceramic with optimized composition and densification. The solubility limits for Mo and W into the (Hf,Mo,Ti,W,Zr)B2–(Hf,Mo,Ti,W,Zr)C ceramics were mitigated by decreasing their contents to 2.5 at% each, which produced a nominally pure dual phase ceramic that could be densified by spark plasma sintering or hot pressing. A synergistic hardening effect was observed for optimized ceramic with 5 at% of Mo and W, whereby it had a higher hardness than individual high entropy carbide and high entropy boride phases containing the same transition metals.

Electrochemical corrosion behavior of nanostructured BCC-Al0.5CoCrFeNi0.4 high-entropy alloy in sodium chloride solution

The electrochemical corrosion behavior of Al0.5CoCrFeNi0.4 high-entropy alloy (HEA) in 3.5 wt.% NaCl solution was investigated by electrochemical measurements coupled with comprehensive characterization of its surface films. The results demonstrate that the Al0.5CoCrFeNi0.4 HEA primarily consists of (Al,Ni)-rich B2 phase and (Cr,Fe)-rich A2 phase. When subjected to electrochemical polarization, an ultrathin passive film rich in Cr is homogeneously formed on the surface. As the applied potential increases, the protective film tends to break down at the interface of two phases, followed by the preferential dissolution of B2 phase. Finally, the surface film with a triple-layer structure is formed on the HEA surface. Specifically, the inner layer and the middle layer are enriched in Cr and Fe, respectively, due to the high stability of Cr species and the selective dissolution of Fe. The (Al,Ni)-rich corrosion products are found to deposit at the outmost position ascribed to the dissolution of B2 phase.

Mg-Sc-Nd-xZn series alloys with medium strength and high plasticity

Different application scenarios put forward diverse requirements of mechanical properties for magnesium alloys. In this study, the extruded Mg-Sc-Nd-Zn alloy series with high plasticity and medium strength was developed. The uniformly distributed Mg3Nd (β1) phase, high-density twins, and the activated non-basal dislocation slips contribute to the high plasticity of Mg-Sc-Nd alloy. With the addition of Zn element, the appearance of coarse ScZn phases, which prevents the activation of {101¯ 2} tensile twins, leads to an increase in strength but a simultaneous decrease in the plasticity of the extruded alloys. Adjusting the Zn content in the Mg-Sc-Nd-xZn allows a series of alloys with various strengths and plasticity matching. The results obtained in this work are of great importance for developing a regulatable magnesium alloy series that aims at different application scenarios.

Superstrengthening effect of beyond-solid-solution laser powder bed fused WE43 magnesium alloy triggered by direct aging treatment

Laser powder bed fusion (LPBF) technology exhibits the potential to develop magnesium alloys as lightweight, load-bearing components in critical industries such as aerospace and automotive. This study investigated the effects of various direct aging treatments on the microstructural evolution and mechanical properties of the beyond-solid-solution WE43 magnesium alloy fabricated via LPBF. The results show that the limited kinetic conditions provided by aging at 175°C lead to the decomposition of the beyond-solid-solution to form the β' metastable phase. The original β1 phase transformed to β' and β'' along the direction of least resistance. These nanoscale coherent phases increased the stored deformation energy, acting as nucleation sites for dynamic recrystallization and stimulating the formation of low-angle grain boundaries. This contributed to segmenting grains into multiple zones and subcrystals. Therefore, after aging at 175°C for 64 h, the grain size decreased from 3.39 to 1.96 μm. Meanwhile, the ultimate tensile strength (UTS) increased from 313 to 353 MPa, yield strength (YS) from 236 to 297 MPa, and elongation (EL) from 7.6 % to 12.4 %. Additionally, the aging treatment at 175°C for 128 h shifted the dominant strengthening mechanism from grain boundary strengthening to Orowan strengthening. As a result, the strengths of the alloy aged at 175°C/128 h exceeded the as-built alloy by 53 and 80 MPa for UTS and YS, respectively.

Influence of high-strain-rate compression and subsequent heat treatment on (TiNbZr)89(AlTa)11 refractory high-entropy alloys: Dynamic-mechanical behavior and microstructural changes

This study explored the dynamic-mechanical behavior of a novel low-density (TiNbZr)89(AlTa)11 refractory high-entropy alloy (RHEA) across strain rates ranging from 1.0 × 103 to 3.5 × 103 s−1. A significant increase in the yield and ultimate compressive strengths with rising strain rates up to 3.0 × 103 s−1 was observed and attributed to enhanced dislocation activities and stress-induced microstructural transformations. The formation of the B2 phase and Zr5Al3 precipitates was found to be crucial in bolstering the alloy strength at high strain rates. Beyond strain rates of 3.0 × 103 s−1, a decrease in strength occurred due to thermal softening and strain localization. Microstructural analyses at 3.5 × 103 s−1 revealed grain refinement, the development of micro shear bands, and dislocation tangles, which were indicative of dynamic recrystallization. Besides, the findings also revealed that the post-dynamic compression heat treatment further enhanced the hardness and microstructural stability of the alloy. These results highlight the potential of the (TiNbZr)89(AlTa)11 RHEA for applications requiring materials with high strength-to-weight ratios, particularly in dynamically loaded environments. It is expected that the results of this study will further advance our fundamental understanding of the behavior of RHEAs under extreme conditions, thereby opening new avenues for material innovation.

Microstructure, mechanical properties and corrosion resistance of direct energy deposited AlCu0.25CoCrFeNi2.1 high entropy alloy: Tailored by annealing heat treatment

In this study, a novel high entropy alloy AlCu0.25CoCrFeNi2.1 is additively manufactured by laser direct energy deposition process. The microstructure, mechanical properties and corrosion resistance of the prepared AlCu0.25CoCrFeNi2.1 high entropy alloy are investigated and tailored by annealing heat treatment. The results indicate the alloy exhibits a dual-phase microstructure with face-centered cubic (FCC) columnar dendrites and body-centered cubic (BCC) interdendritic structures. The annealing heat treatment facilitates the precipitation and coarsening of Cu-rich short rod-like B2 phase within the incipient FCC phase and the dissolution of incipient BCC phase, synergistically varying the proportion of FCC/BCC + B2 contents and tailoring the properties. The hardness of microstructure fluctuates under the coupling effect of precipitation strengthening and lattice distortion reduction and shows an overall downward trend with the increase of annealing time. The tensile properties are noteworthily improved within 6-h annealing and achieve a maximum ultimate tensile strength of 1156.4 MPa. After 8-h annealing, the ductility of the alloy obtains a maximum value of 30.3% with minor loss of ultimate tensile strength. The corrosion resistance is dramatically enhanced and obtains the best value after 2-h annealing, and then gradually deteriorates with the destroying of FCC matrix passive film phase by coarsened B2 phases. These results are of great importance in rapidly designing/manufacturing novel high entropy alloys by laser direct energy deposition process and tailoring microstructure, mechanical properties and corrosion resistance by annealing heat treatment.

A novel AlCo1.2Cr0.8FeNi2.1 eutectic high entropy alloy with excellent corrosion resistance

A novel AlCo1.2Cr0.8FeNi2.1 eutectic high entropy alloy (EHEA) was designed and prepared. The microstructure of the as-cast alloy was characterized that a fine lamellar structure composed of FCC (L12) phase and B2 phase. The mechanical of the EHEA was studies by the tensile tests, and the corrosion properties of the EHEA in 3.5 wt% NaCl solution at room temperature was investigated. The results shown that the alloy exhibits synergistic effect of strength- plasticity with a high tensile strength of 1048.42 MPa and an excellent elongation of 18.92%. Potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS) indicated that the EHEA exhibits spontaneous passivation in 3.5 wt% NaCl solution, forming a protective passive film that exhibits p-type and n-type semiconductor properties, with the main components of Al2O3, CoO, Cr2O3, Fe(OH)3, and NiO. Furthermore, the corrosion mechanism of EHEA was discussed.

Severe plastic deformation of two-phase TiNiCuNb shape memory alloy

(Ti54Ni34Cu12)90Nb10 alloy with initial two-phase structure of B19 martensite and β-Nb phase was subjected to high-pressure torsion (HPT) process at room temperature. HPT leads to the formation of nanocrystalline bands of β-Nb phase alternating with amorphous matrix. Nanocrystalline debris of B2 phase was found to be embedded into the amorphous matrix. Refinement of β-Nb phase upon HPT is much greater compared to that for pure Nb alloy.

Microstructure and mechanical properties of high-entropy diboride-based ceramic assisted by Ti3AlC2 additive

Ti3AlC2 was used as the additive to fabricate a novel (Zr0·2Ta0·2Nb0.2Hf0.2Mo0.2)B2-based (HEBTAC) ceramic using spark plasma sintering at 1900 °C. The as-prepared HEBTAC ceramic sample consists of (Ti, Zr, Hf, Ta, Nb, Mo)B2, multiscale (Zr, Hf, Ta, Nb, Mo)C, and Al2O3 phases, which are produced by the decomposition of Ti3AlC2 and subsequent exchange reaction & solid solution reactions during the sintering process. The relative density, hardness, fracture toughness, and flexural strength of the HEBTAC ceramic sample are 99.43 %, 27.92 ± 1.87 GPa, 5.84 ± 0.14 MPa m1/2, and 629 ± 8.7 MPa, respectively, which are even better than the same class of high-entropy diboride ceramics previously published. Solid-solution strengthening, dislocation strengthening and nano-HEC strengthening existed in the ceramic sample contribute to its excellent mechanical properties. The in situ formed Al2O3 and (Zr, Hf, Ta, Nb, Mo)C effectively enhances the toughness of the HEBTAC ceramic sample through the synergistic effect of intrinsic and extrinsic toughening. This study presents a new approach for reactive sintering densification of high-entropy diborides with both high strength and high toughness.

Phase transformation behavior and mechanical properties of NiTi shape memory alloys fabricated by directed laser deposition

Directed laser deposition shows promise as a method for fabricating complex structure NiTi shape memory alloy parts in engineering, aerospace, automobile, and biomedical fields. The dislocation density has an impact on phase transformation behavior and mechanical properties of NiTi manufactured by directed laser deposition. In this work, NiTi samples were produced by directed laser deposition, and the mechanism of martensitic phase transformation and its interactions with dislocation density were revealed through molecular dynamics. Laser power and scanning speed are the main factors affecting dislocation density, the phase compositions of the samples are B2, B19′, and Ti2Ni phase, and the samples with dislocation density below 2 × 1015 m−2 show single-step B2–B19′ martensitic transformation, stress plateau and improved elongation, the samples with dislocation density above the value exhibit no significant transformation peak and no stress plateau. The movement of (100) twin planes and the growth of advantageous variants are observed in the simulation which can explain stress-induced martensitic reorientation and the stress plateau phenomenon. The bending test suggested that the sample under the optimal parameters has an 85 % recovery ratio within 8 % pre-strain.

Unraveling the role of the BCC-B2 transition and V occupancies in the contradictory magnetism-ductility relationship of FeCoV alloys

The contradictory relationship between magnetism and ductility restricts further applications of FeCoV alloys in high-performance electrical machines. The role of the BCC-B2 transition, accompanied by vanadium (V) site occupancies, in magnetic moments and ductility has been explored using first-principles calculations. The variations in magnetism and ductility of FeCoV alloys are attributed to the coupling of the BCC-B2 transition and V occupancies. When V replaces Fe atoms in the equiatomic B2-FeCo alloy, the superior magnetism observed in B2-Fe50-cCo50Vc alloys is a consequence of the enhanced local magnetic moment of Fe and the ferrimagnetic-ferromagnetic transition in the magnetic state. Moreover, due to the preferential V occupancy in the B2 phase, the B2-Fe46Co50V4 alloy exhibits comparable ductility to the BCC-Fe50Co46V4 alloy. The results indicate that the increased brittleness in the B2 phase arises from the raised Peierls stress and the enhanced covalent component in interatomic bonding, which is caused by the strong hybridization between Fe and Co atoms. Pearson correlation analysis illustrates that valence electron concentration (VEC) and V content are significant factors in the contradictory relationship between magnetization and ductility. The theoretical results demonstrate that tuning the V content and atomic occupancies is helpful to achieve a trade-off between magnetization and ductility in B2-FeCoV alloys.

Microstructure evolution and mechanical properties of β/γ-TiAl alloy during high-rate near-isothermal multidirectional forging

The microstructure of β/γ-TiAl alloy was characterized and analyzed in various deformation regions during a high-rate near-isothermal multidirectional forging (HR-NIMDF) process in this study. The results indicate that the localized precipitation of the B2 phase occurs within the large lamella microstructure through DRX and heterogeneous exsolution decomposition mechanism in the stagnant zone. The majority of mixed fine-grained microstructures (α2/γ/B2), with a grain diameter smaller than 1 μm, were formed under an appropriate stress-temperature coupling field. In the central region of the large deformation zone, a small twisted lamellar microstructure with low B2 phase content is observed, suggesting that HR-NIMDF can efficiently decompose the (α2/γ) lamellar structure and achieve refined grain microstructures. After undergoing HR-NIMDF, the β/γ-TiAl alloy exhibits a low tough-brittle transition temperature and an elongation of 7 % at 650 °C, with an ultimate tensile strength of 862.6 MPa. At 700 °C, the elongation increases to 55.4 %, while the ultimate tensile strength decreases to 653.8 MPa. Moreover, this study demonstrates that when the plastic deformation degree is insufficient and the adiabatic warming effect is relatively pronounced, significant precipitation of the B2 phase leads to a reduction in DRX extent within the material, ultimately resulting in the formation of large residual (α2/γ) lamellar microstructure. The unsynchronized thermal-force coupling relationship between the precipitation of the B2 phase and DRX holds significant implications for advancing and refining the thermoplastic deformation process of β/γ-TiAl alloy.

Role of (Al, Ti) alloying and annealing temperature in precipitation, recrystallization, and mechanical properties of ferrous multi-component alloys

L12-precipitate-strengthened multi-component alloys (MCAs) possess exceptional combination of strength and ductility. However, the high production costs of L12-containing MCAs impede their industrial application. This study introduces a novel approach for designing cost-effective L12-containing MCAs by microalloying Al and Ti with a base composition of Fe50Ni30Cr20 (at%). Three types of MCAs were investigated in this study: Fe50.13Ni28.84Cr18.56Al1.39Ti1.08 (1Ti alloy), Fe46.53Ni27.32Cr19.91Al3.44Ti2.80 (3Ti alloy), and Fe48.51Ni25.93Cr15.75Al5.10Ti4.71 (5Ti alloy) (at%). An increase in the (Al, Ti) content from 1 to 3 at% accelerates the formation of L12 particles, whereas an increase in the (Al, Ti) content from 3 to 5 at% leads to the activation of the brittle B2 and D024 phases. The rapid co-precipitation of the L12, B2, and D024 phases in the 5Ti alloys substantially hinders recrystallization during annealing, whereas the 3Ti alloys exhibit the highest rate of recrystallization upon annealing at 600 and 700 °C, mainly due to the reduction in the stacking fault energy of the matrix. The 5Ti alloys exhibit the highest hardness and the lowest plasticity owing to the presence of brittle B2 and D024 precipitates. By contrast, the 3Ti alloy annealed at 600 °C, in which dense and relatively fine L12 particles precipitated, exhibits a superior combination of hardness and plasticity. This study provides insights into the composition and annealing condition-dependent precipitation, recrystallization, and mechanical properties of ferrous MCAs.

Electropulsing of low-density duplex steel for strengthening and ductilization by microstructural refinement

The present investigation finds the effect of electropulsing treatment on the mechanisms of refining and reprecipitation of ordered L20 structure (B2) and its impact on tensile properties of low-density duplex steel of Fe-18Mn-10.5Al-1 C-6Ni composition. The selected composition without manganese is melted at 1600°C by induction heating in a vacuum but Mn is added into the melt at 1560°C, in the argon atmosphere to reduce the loss. The melt is cast into a plate form in a copper mold. The cast steel is homogenized, hot rolled, and annealed (PD1-A sample) to get elongated bands, spheroids, and platelets of B2 phase with an austenitic matrix which results in a yield strength of 1015 MPa, and tensile strength of 1285 MPa with a plastic elongation of 16.3%. Electropulsing partially dissolves the B2 phase at a temperature much lower than the equilibrium solvus by reducing barrier energy because of electron wind. As a result, the sizes of bands and spheroids are reduced, and platelets are disintegrated. Electron wind force deforms coarse precipitates, and fragments as well as spheroidized it. Electropulsing of annealed steel (PD1-AE) mobilizes dislocations, and recovers it in B2, but induces localized recrystallization of austenite. At a later time of the signal, the temperature rapidly falls and the matrix becomes supersaturated but electropulsing accelerates low-temperature precipitation of the B2 phase. Therefore, electropulsing can be adopted as a fast manufacturing process to dissolve, deform, fragment, spheroidize and reprecipitate high temperature phase at much lower temperature by dominant athermal effect of electron wind energy over thermal effect. Electropulsing also induces recovery and recrystallization at a relatively low temperature and reduces defect density. Reduction in the overall size of B2 and its distribution towards better uniformity provided an increased yield strength of 1077 MPa, tensile strength of 1355 MPa, and a plastic elongation of 21.6%. The selected duplex low-density steel follows two-stage work hardening of Ludwigson model with an additional stage of dynamic recovery. Fractography analysis confirms that both samples show mixed modes of ductile and brittle fracture, with an increase in size and area percentage of dimples.

Synergistic effect of twin consumption within lamellae and slip system activation at grain boundary on the formation of nano-spherical B2 phase

In this study, the spheroidization behavior of the B2 phase in TiAl alloys under isothermal compression is revealed for the first time, reducing the cut apart trend on matrix. Results show that the deformation stored energy under the 1050 °C–40 %–0.01s−1 condition is insufficient so that most B2 phase bends and twists with the γ lamellae rather than being spheroidized. Under the 1000 °C–80 %–0.01s−1 condition, reduced temperature and increased strain further promote B2 phase spheroidization. The B2 phase nucleates and becomes spheroidized by twin boundary consumption within the γ lamellae. At the γ recrystallized grain boundary, the B2 phase grows into a spherical crown shape due to favorable slip tendency activation, multiple dislocation shear, and microslip bands coordination. This work offers a significant approach to improving the hot workability of TiAl alloys.

Achieving excellent strength–ductility combination of Al-aided Mg–Gd–Al–Zr–Zn alloys fabricated by cold metal transfer-based wire-arc directed energy deposition via uniform equiaxed grains and multiple precipitates

Limited ductility consistently poses a challenge for high-strength Mg–RE (rare earth) structures prepared by additive manufacturing. Particularly following heat treatment, while the strength of the Mg–RE alloy experiences a substantial increase, there is a noteworthy decrease in its ductility. Herein, a novel Mg–Gd–Al–Zr–Zn thin wall was successfully fabricated by cold metal transfer-based wire-arc directed energy deposition (CMT-WA-DED) and Al alloying. The microstructural evolution and mechanical properties of deposited and heat-treatment alloys were elucidated systematically. By adding Al element, Al2Gd and Al–Zr phases with thermal stability were precipitated in the deposited alloy. During the solidification or heat treatment stage, Al2Gd and Al–Zr phases could pin grain boundaries to restrict grain growth. The as-deposited alloys exhibited fully equiaxed grains at different heights and orientations. Thus, the thin wall exhibited good isotropic performance. After the heat treatment, the grain size only slightly grew and no abnormal grain growth was observed. Various morphologies of β precipitation phases were discovered along grain boundaries and within the grains. Additionally, a high density of β' and β1 phases within the grains and β phases along the grain boundary played pivotal roles in enhancing the mechanical strength of the Mg–Gd–Al–Zr–Zn alloy. The alloys exhibited an excellent combination of strength and ductility with ultimate tensile strength of 340 ± 7 MPa, yield strength of 212 ± 5 MPa, and elongation of 12.7 ± 0.9 %. This work provides theoretical and practical reference value for the Mg–RE structures obtaining an excellent combination of strength and ductility prepared by additive manufacturing.