•TiAl alloys with similar compositions but different constituents have been designed.•Both β/B2-rich and β/B2-lean alloys shows impressive superplasticity at 1000 °C.•The β/B2-rich alloy shows much larger cavity sizes than β/B2-lean alloy.•Cavities in β/B2-rich alloy are more likely to elongate along the tensile axis.•The cavity growth rate of γ-TiAl is much higher than that of ordinary alloys.Fine-grained Ti-(42.5–43.5)Al-8Nb-0.2W-0.2B-(0–0.1)Y (in at.%) alloys with two different phase constituents (i.e., (α2+γ) alloy and (β/B2+γ) alloy) have been prepared by means of thermo-mechanical processing. Tensile tests were performed at 1000 °C under an initial strain-rate of 10−4 s−1 to evaluate the superplastic properties, and it was found that both alloys exhibited impressive superplasticity so that the elongations reached ∼400%. Texture measurements preliminarily revealed that intragranular deformation and the subsequent dynamic recrystallization was the predominant superplastic deformation mechanism in (β/B2+γ) alloy, while grain boundary sliding was responsible for the large elongation of (α2+γ) alloy. Cavities caused by superplastic deformation have been quantitatively evaluated by using SEM and X-ray tomography to reveal the effect of β/B2 phase on cavitation behavior. It was found that in (α2+γ) alloy the cavities preferentially nucleated in the triple-junctions of interphase boundaries, whereas most of the small cavities in (β/B2+γ) alloy were located along the β/γ interfaces. Quantitative analysis revealed that the alloys had comparable cavity number density after deformation, but the (β/B2+γ) alloy was characterized by larger cavity sizes, and thus by higher cavitation volume fraction and coalescence parameter in comparison with that of (α2+γ) alloy. Moreover, the large cavities in (β/B2+γ) alloy preferentially elongated along the direction parallel to the tensile axis, whereas the large cavities were more equiaxed in (α2+γ) alloy. The different cavity nucleation–growth–coalescence behavior of the two alloys was clarified based on the divergence in superplastic deformation mechanisms. Besides, theoretical evaluations were performed and revealed that the cavity growth rate of γ-TiAl was much higher than ordinary Al alloys under similar deformation conditions. This was believed to be the main reason for the relatively lower superplastic elongations of TiAl-based intermetallics.

•Surface relief accompanying formation of lamellar microstructure was investigated.•There are single-tilt, regular and irregular terrace surface reliefs of γ lamellae.•Formation of surface relief is mainly based on diffusional growth mechanism.•Kink density and size control lateral growth and thus determine γ lamellar width.Surface relief produced during lamellar transformation in a Ti48Al2Cr2Nb (at.%) alloy was quantitatively studied. Three-dimensional representations and quantitative data exhibit there are three kinds of surface reliefs, single-tilt, regular-terrace and irregular-strip, corresponding to narrow-width, moderate-width and large-width lamellae, respectively. Observation on fine structure reveals that kinks and ledges of the three kinds of surface reliefs are different. All the surface reliefs are related to diffusional terrace-ledge-kink growth mechanism and only single-tilt surface relief combines displacive character. The γ lamellae grow forward by nucleation of growth islands one upon the other and increase length and width by kink migration. The kink density and size control the lateral growth and further determine the lamellar width.Fine structures of different kinds of surface reliefs produced by formation of γ lamellae. (a) Pyramid terraces configuration with growth islands in regular-terrace surface relief; (b) Single-tilt surface; (c) Terraces with irregular edges in irregular-strip surface relief.

In this Reply we demonstrate that the nano domains reported in our paper commented by D. Banerjee cannot be the artifacts induced by hydrogen absorption from electropolishing, as assumed by D. Banerjee. The effects of two different TEM sample preparation methods (ion milling and electropholishing) were verified and compared with some results in the literature. The formation of the interface α domains in Ti-7333 in our published work has the same origin as the secondary α domains formed on the existing α in Ti-6Al-4V predicted by the 3D phase field simulations by R. Shi and Y. Z. Wang.

In situ observation is carried out in Ti48Al2Cr2Nb (at%) alloy to obtain a comprehensive understanding on the growth mechanisms of γ lamellae. The results reveal that the grain-boundary nucleated γ embryos occur first and grow into the adjacent α grains. Both the γ lamellae nucleated at the grain boundary and within α grain prefer to grow in pair by nucleation of the new γ lamella adjacent to the precedent γ lamella or branching of the growth tip. The widening of γ lamellae is controlled by the migration of α/γ interface, and the distance between growth pairs directly influences the lamellar width. The EBSD results exhibit that the two γ lamellae in growth pairs have a true-twin or pseudo-twin relationship. The underlying growth mechanism of γ lamellae is discussed.

The β to α phase transformation of Ti alloys progresses in a displacive-diffusive mixed-mode. The associated transformation strain has important influence on the resultant microstructure. In this work, the microstructural features of α precipitates in a metastable β Ti alloy, Ti-7333, were thoroughly investigated. Special attention was paid to the intragranular α for the advantage of a stress-free transformation environment. Results show that the constituents of each α precipitate is not single. Two kinds of nano-sized α domains exist. One is situated on the broad face of the major α precipitate (named interface α), and the other goes through the major α (termed penetrating α). The interface α is related with the β matrix by the Burgers orientation relationship (BOR) and with the major α by a 60°/〈112¯0〉α rotation. The nucleation of such α particles is induced by the largest shear strain generated by the formation of the major α. They act as stress-associated sympathetic nuclei of the neighboring α precipitates and eventually contribute to the formation of the triangular α cluster. The penetrating α does not obey the BOR with the β matrix but is related with the major α by a 60° rotation around another 〈112¯0〉α axis. The nucleation of such α is induced by the largest normal strain generated by the formation of the main α plate. This work provides comprehensive information on the displacive characters of the β to α transformation and their impact on the microstructure of metastable β Ti alloys.Download high-res image (212KB)Download full-size image

Serrated flow in Al0.5CoCrFeNi high entropy alloy (HEA) was studied at the temperature range of 200–500 °C with strain rates from 10−3 s−1 to 10−4 s−1. Results showed that serration type would transform from type A to type A+B, and eventually evolved into type B+C with the increasing temperature and decreasing strain rate. The dynamic strain aging was the reason for the Portevin-Le Chatelier (PLC) effect, and Al played a crucial role in the interaction between solute atoms and dislocation. The characteristics of serration are revealed through statistics firstly in HEAs.

Different amounts of nitrogen from 1000 to 14,000 wt ppm were added into the Ti48Zr20Nb12Cu5Be15 bulk metallic glass composites. Results show that addition of nitrogen can increase the yield strength of composites from 1400 to 2350 MPa due to the solid solution strengthening effect. The composites still owns certain plastic strain (above 10% when contains 14,000 ppm nitrogen) before fracture under compressive test. The reason is attribute to nitrogen shows to be a much weaker activator for the formation of brittle α-Ti phase as compared with oxygen, which make the β-Ti dendrite still softer than that of the glassy phase, thus can enhance the strength of the materials but do not deteriorate the plasticity too much. Our research show that nitrogen can be added into the Ti-based metallic glass composites in a very large range to effectively tune the suitable mechanical properties of Ti-based metallic glass composites, without worrying about the catastrophic failure in the process of deformation.

•ω phase contributes to the nucleation of α precipitates.•The ω/β interface acts as a preferred channel for the development of α precipitates.•α precipitates with fine size and high density are obtained within β matrix.•The morphological development of α precipitates during continuous heating is summarized.In this work, β-quenched Ti-7333 was slowly heated to different temperatures to investigate the precipitation behavior of α phase. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were used to characterize the microstructures and orientation relationships (ORs) between the precipitates and the β matrix. The results show that isothermal ω (ωiso) phase with an ellipsoidal morphology holds the OR with α phase characterized by [0001]α // [21¯1¯0]ω, (101¯0)α // (011¯0)ω, and assists the nucleation of α phase. The lattice misfit of ω/β interface is determined to be about 2.824%, which contributes to the development of α phase. With the assistance of ω phase, fine-scaled α precipitates with high density can be obtained within β matrix. The α variants obey the Burgers orientation relationship (BOR) with respect to β matrix, and the lattice misfit at α/β interface is determined to be about 5.340%. Eventually, the morphological evolution of the α precipitates during continuous heating is summarized. The results of the present work could provide a practical information for a better understanding of the precipitation behavior of the α phase during heating in near β titanium alloys.

•A new long-period stacking ordered phase is observed in Mg-Ni-Ce ternary alloy.•Nanocrystals with grain size in the range of 3–5 nm are observed after ball milling.•CeH2.73 reacts with oxygen spontaneously to form CeO2 during air exposure.•Anti-oxidation properties of Mg-based alloys are improved with the addition of Ce.On the basis of catalytic actions of transition metals and rare earth metals on Mg-based hydrogen storage alloys and aiming at alleviating the adverse influence of Mg oxidation, Mg-Ni-Ce alloys with different Ni and Ce contents were prepared by near equilibrium solidification. A new 18R-type long-period stacking ordered phase (LPSO) was formed coherently with Mg, Ni-substituted Mg12Ce and Mg2Ni in Mg-rich Mg-Ni-Ce ternary alloys. Distinct from the reported LPSO structures in other Mg-based alloys, in which the LPSO structures were fundamentally long period stacking variants of hexagonal close-packed structure of Mg, the LPSO structure found in the present work was a variant of Mg12Ce. Nanocrystalline alloys were obtained by high-energy ball milling the as-cast alloys. Nanocrystals of Mg, Mg2Ni and Mg12Ce with grain size in the range of 3–5 nm were observed in ball milled samples. The activation performance, isothermal hydrogenation behavior and anti-oxidation properties of the ball milled samples were systematically investigated and corresponding mechanisms were discussed based on detailed microstructural characteristics. CeH2.73 was formed after hydrogenation and spontaneously transformed into CeO2 during air exposure. The anti-oxidation properties of Mg-based hydrogen storage alloy were substantially improved with the addition of Ce by forming CeH2.73/CeO2 composite.

•The phase transformation can be divided into three stages in Al0.5CoCrFeNi alloy.•The activation energies, Q, determined by KAS model show an increasing trend.•The FCC-BCC phase transition firstly is interface controlled and moved to diffusion controlled.•We get the good combination of strength and plasticity for heat-treated alloy.In this paper, thermal expansion method is used to characterize the phase transformation process of Al0.5CoCrFeNi high entropy alloy (HEA), and the FCC-BCC phase transformation kinetics, microstructure and mechanical properties are investigated. Results show that during continuous heating process, three stages of phase transformation process for Al0.5CoCrFeNi HEA are characterized by thermal expansion curves, and the third stage is corresponding to FCC-BCC phase transformation. The FCC-BCC phase transformation kinetics is analyzed using the thermal expansion experiments carried out at different heating rates. The FCC-BCC phase transformed fraction, f, as a function of temperature is determined from thermal expansion curve. The activation energies, Q, determined by KAS model show an increasing trend which is from 144.4 kJ/mol at initial stage (f = 0.2) to 209.2 kJ/mol when f = 0.8, indicating the barrier for transformation is increasing. The Avrami exponent, n, determined by modified JMA model, varies with transformed fraction, indicating the FCC-BCC phase transition firstly is interface controlled with decreasing nucleation rate (f < 0.05), and changed to diffusion controlled with an increasing nucleation rate (f < 0.15), then diffusion controlled with decreasing nucleation rate. The microstructure and mechanical properties of the corresponding FCC-BCC phase transformation process are also studied.Download high-res image (203KB)Download full-size image

•Dynamic behavior of in-situ Ti-based BMG composites was investigated in SLR.•Transition from viscous flow to shear-band-dominated deformation was observed.•Constitutive equation is established for the quasi-static deformation behavior.The quasi-static and dynamic deformation behaviors of the Ti-based bulk metallic glass (BMG) composite (Ti48Zr20Nb12Cu5Be15) were investigated in the supercooled liquid region. The present BMG composite exhibits homogenous viscous flow under quasi-static deformation, but shear-band dominated inhomogeneous plastic deformation upon dynamic loading. With the increase of the temperature, the flow stress decreases in both quasi-static and dynamic conditions. Due to the effective plastic accommodation of dendrites to the glass matrix, characteristic of the formation of shear steps and multiple dislocations, the remarkable plasticity under dynamic deformation has been obtained in the present BMG composite in SLR. The constitutive equation for the present BMG composite upon quasi-static loading is obtained. Upon dynamic loading, the flow stress approximately follows a linear relationship with the temperature: σ (MPa) ≈ 2,791–2.11T(K).

•The property optimization of the new alloy were investigated for the first time.•The microstructure and property correlation was evaluated using an orthogonal method.•The optimized heat treatment process and microstructure were provided.•Ti-7333 exhibits excellent combination in mechanical properties after STA treatment.As a newly developed near β titanium alloy, knowledge on heat treatment associated microstructural evolution and its influence on mechanical properties has not yet been established for Ti-7333. Thus mechanical property potential of this alloy is not fully explored. In view of such a situation, a detailed study on heat treatment dependent mechanical property evolution and its correlation with microstructure was made in the present work with an aim to explore the optimum strength–ductility combination. The results show that the optimal combination of strength and ductility of Ti-7333 is realized with a mixed microstructure possessing globular/ellipsoidal αp (about 3% in volume fraction and about 1 μm in average particle size) and fine lenticular/acicular αs (about 30 nm in width and about 500 nm in length) distributed dispersedly in the β matrix (about several microns in average grain size). With such a microstructure, an ultrahigh tensile strength (over 1400 MPa) with a reasonable ductility (over 10% in elongation) can be achieved. The corresponding heat treatment procedure is composed of a subtransus solution treatment at 820 °C for 50 min with air cooling plus an aging at 520 °C/540 °C for 6 h with air cooling. Compared with some other high strength Ti alloys (Ti-5553, Ti-1023, VT22, etc.), the newly developed Ti-7333 exhibits not only a more excellent strength–ductility combination but a more flexible heat treatment operation, rendering it more competitive. The results of the present work provide practical information for process optimization and to enrich the database of Ti alloys.

The tribological properties of AlCoCrFeNi and AlCoCrFeNiTi0.5 high entropy alloys under gear oil and multiply alkylated cyclopentanes (MACs) lubrication condition have been studied. The equiaxed crystal structure of AlCoCrFeNi alloy is obtained after heat-treatment. The AlCoCrFeNiTi0.5 alloy keeps dendrite structure. Under the gear oil with good lubrication action, AlCoCrFeNiTi0.5 alloy preserves better tribological properties than AlCoCrFeNi alloy. The delamination and crack behaviors tend to occur in the grain boundary of AlCoCrFeNi alloy and along the interdendrite region of AlCoCrFeNiTi0.5 alloy. Under the MACs with relatively poor lubrication action, the applied load slightly influences the wear behavior of AlCoCrFeNi alloy, but seriously impacts the wear mechanism of AlCoCrFeNiTi0.5 alloy. Compared with AlCoCrFeNi alloy, AlCoCrFeNiTi0.5 alloy keeps better wear-resistance at low applied load of 100 N, but preserves less wear-resistance at high applied load of 200 N.

•The superplastic deformation mechanism was revealed by texture evolution.•The rate-controlling mechanism was found to be lattice dislocation climb.•A unified rate-equation for various microcrystalline TiAl alloys was developed.In this paper, superplastic deformation behaviour of a high Nb containing TiAl alloy with fine (α2 + γ) microstructure, Ti–43.5Al–8Nb–0.2W–0.2B (at.%), has been examined and studied by means of hot tension from 850 °C to 1050 °C under an initial strain rate of 10−4 s−1. The mechanical behaviour and microstructure evolution have been characterized and analyzed. Besides, to gain insight into deformation mechanisms, the texture evolution during deformation at ordinary (non-superplastic) and superplastic conditions has been systematically studied. The results showed that, the alloy exhibited impressive superplastic elongation at 1000 °C with a strain-rate sensitivity exponent (m) of about 0.5 and an apparent activation energy (Qapp) value of about 390 kJ/mol. The microstructural characterization showed that, when the alloy was deformed at ordinary condition (850 °C), severe grain refinement occurred and the fraction of low-angle grain boundary notably increased. Meanwhile, the textures were characterized by <100> and <111> double-fiber components parallel to the tensile direction. All these observations suggested a dislocation slip and twinning mechanism. However, if deformed at the superplastic condition (1000 °C), it was found that the microstructure was fairly stable in terms of grain size, morphology and grain boundary characteristics during tension, but a continuous weakening of the initial <110> fiber texture (resulted from canned-forging) was observed. This was believed to be an indication of grain boundary sliding mechanism. Moreover, the deformation texture (<100> + <111>)—though is very weak—was simultaneously appeared. According to a detailed discussion on the deformation kinetics and microstructure evolution, it was believed that the slip/twinning-accommodated grain boundary sliding was responsible for superplastic deformation and the dislocation climb inside of γ grains was the rate-controlling step.

The tensile property of Al0.5CoCrFeNi high entropy alloys heat-treated at 650 °C for 0.5–8 h respectively was executed. The yield strength and ultimate tensile strength reached to 834 MPa, 1220 MPa for 650 °C/8 h heat-treated condition, which was attributed to the nano-sized B2 phase in the interdendritic region and precipitates in the dendritic region.

γ-TiAl-based alloys have attracted intense attentions in the past two decades and are regarded as promising candidate materials for high-temperature applications due to their low density, high specific strength, superior creep and oxidation resistance, etc. It is well known that there are four typical microstructures in TiAl-based alloys, namely, full-lamellar (FL), near-lamellar (NL), duplex (DP) and near-gamma (NG). The FL/NL-TiAl alloys possess superior high temperature mechanical performance such as creep resistance, but their deformability is relatively lower. In contrast, the DP/NG-TiAl alloys have much better formability while the creep resistance is reduced. Hence the DP/NG microstructure is preferable for hot-working and thought to the prerequisite for secondary processing, and the present paper is primarily focused on the high temperature deformation behavior of DP/NG-TiAl alloys. Till date, considerable research efforts have been directed towards the deformation kinetics or mechanical properties of various DP/NG-TiAl alloys, and a mass of experimental data has been accumulated. In order to explore their general features of high temperature deformation kinetics, in this paper the tensile/compression mechanical data of various DP/NG-TiAl alloys from the literature published in the past twenty years has been summarized and reviewed. Three deformation mechanisms (dislocation creep, grain boundary sliding and diffusion creep) have been identified, and for each the effects of major factors (i.e., alloy composition, microstructure and grain size) on deformation kinetics have been analyzed and discussed in detail. The results revealed that the high temperature deformation kinetics of DP/NG-TiAl alloys is less sensitive to composition except some dispersion-hardened alloys such as carbon-containing TiAl alloys. Instead, the γ grain size became the major limiting factor. Based on this observation, a series of unified rate-equations has been successfully developed.

•Dynamic flow behavior of in-situ Ti-based BMG composites was investigated.•Positive to negative transition of SRS can be observed in the present composite.•Constitutive equations are used to describe the positive to negative SRS transition.The deformation behavior of in-situ Ti48Zr20Nb12Cu5Be15 bulk-metallic-glass (BMG) composites was investigated upon dynamic deformation. The present BMG composite exhibits good dynamic mechanical properties, combining high fracture strength (1850 MPa) with remarkable plasticity (>10%) at the strain rate of 1.3 × 103 s−1. Ductile to brittle transition occurs with the increase of strain rates, which can be ascribed to the deteriorated ability of dendrites to impede the propagation of shear bands at higher strain rates. An obvious positive to negative transition on strain rate dependence of flow stresses can be observed with the increasing strain rates. Detailed analysis reveals that the variation from the dendrite-dominated mechanism associated with dislocation movement to the matrix-dominated fracture related to thermal softening is responsible for the present transition. The constitutive equations based on their deformation mechanisms are established for describing the present transition.

•Tribological properties of high entropy alloys under 90% H2O2 are investigated.•Mechanical wear dominates the wear behavior against 1Cr18Ni9Ti and ZrO2.•Rubbing with SiC, lubricating colloidal film formed offers low COF and wear loss.•AlCoCrFeNiTi0.5/SiC shows better tribological properties than AlCoCrCuFeNi/SiC.Tribological properties of AlCoCrCuFeNi and AlCoCrFeNiTi0.5 alloys under 90% H2O2 solution rubbing against 1Cr18Ni9Ti steel, ZrO2 ceramic and SiC ceramic were investigated. The tribological behavior is dependent on the counterparts and the structure of high entropy alloys. The high entropy alloys exhibit significantly lower friction coefficient and wear loss sliding against SiC ceramic than 1Cr18Ni9Ti steel and ZrO2 ceramic, which can be attributed to the lubricating colloidal film formed by tribo-chemical reaction between H2O2 and SiC. Moreover, the AlCoCrFeNiTi0.5/SiC tribo-pair preserves better tribological properties than AlCoCrCuFeNi/SiC tribo-pair, as results of high strength and hardness of AlCoCrFeNiTi0.5 alloy.

•A novel cryogenic mechanical properties for the present composite are obtained.•Plastic dendrite is responsible for the present cryogenic plasticity of composite.•Continuous matrix other than the dendrites initiates the fracture behavior.•In situ bulk metallic glass composites could be promising cryogenic materials.Excellent mechanical properties as high as 2760 MPa fracture strength and 18.4% plastic strain are obtained in Ti48Zr20Nb12Cu5Be15 bulk-metallic-glass (BMG) composite at cryogenic temperature (77 K). The novel cryogenic plasticity of present composite is determined by good cryogenic plasticity of dendrites induced effective interaction between dendrites and shear bands. Improved cryogenic yield strength of dendrites is responsible for the increase of cryogenic yield strength of the present composite. Continuous matrix other than the dendrites is believed to initiate the fracture behavior. This finding demonstrates that in situ dendrite-reinforced BMG composites can be a kind of promising materials for low-temperature applications.

Tribological behavior of a Ti-46Al-2Cr-2Nb alloy, produced by hot-pressed sintering, was investigated using a home-built ball-on-disc tribotester against a Si3N4 ceramic ball at a constant speed of 0.188 m s−1 and an applied load of 10 N from 20 to 900 °C. It was found that the friction coefficient decreased slowly with increasing temperature from 20 to 600 °C, and then rose and reached the highest value at 800 °C, but slightly dropped at 900 °C. The wear rate of the alloy, in the magnitude of 10−4 mm3/Nm, increased mildly with increasing temperature to a maximum value at 400 °C and then dropped rapidly from 600 to 800 °C, and increased a little at 900 °C. The wear mechanism of the alloy transformed from mainly ploughing and small delamination wear at 20–700 °C to plastic deformation and adhesive wear at 800 and 900 °C. The transition of the wear mechanism occurred between 700 and 750 °C. The friction and wear behavior of the hot-pressed Ti-46Al-2Cr-2Nb alloy was comparable to that of a vacuum casting Ti-41.7Al-8.3Nb-0.09Y alloy measured under the same conditions for comparative purposes.Highlights► The tribological behavior of the Ti-46Al-2Cr-2Nb alloy was studied from 20 to 900 °C. ► The friction coefficient first decreases and then increases with the temperature. ► The wear rate of the alloy was strongly dependent on the test temperatures. ► There are two kinds of different wear mechanisms at below and above 750 °C.

•A CoCrFeNiMnPd eutectic high-entropy alloy was designed and prepared.•A unique eutectic-dendritic solidification microstructure was found.•The formation mechanism of the eutectic-dendritic solidification pattern was discussed.•Mere consideration of average physical properties of constituent elements was found to be not sufficient to design of high-entropy alloys.A CoCrFeNiMnPd eutectic high-entropy alloy was designed and prepared. A unique eutectic-dendritic solidification pattern was observed in the as-cast alloy. Heretofore, this kind of pattern was rarely reported no matter in binary or multi-component alloys. Through a series of experimental characterization and theoretical analysis, it could be concluded that the sluggish diffusion effect of high-entropy alloys makes the interface deviate considerably from the local non-equilibrium condition and the alternate arrangement of CoCrFeNiPd-rich FCC and Tetragonal Mn7Pd9 lamellae results in weak interface energy anisotropy, thus forming the seaweed eutectic-dendrite. Application of the designing rules of high-entropy alloys to the current work indicates that mere consideration of the weighted average physical properties of constituent elements is not sufficient to the design of high-entropy alloys with a single solid-solution phase.