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Single-pass thermal compression simulation experiments were performed on a dual-phase Al0.9FeCoNiCr high-entropy alloy at strain rates of 1–0.001 s−1, temperatures of 850–1050°C (true strain of 0.916). According to a model of the dynamic DMM processing map proposed by Prasad et al, the hot-deformation behaviour of the alloy was analysed. The flow-instability regions were obtained as follows: temperature 850–1040°C with a strain rate 1–0.1 s−1 and 850–900°C with a strain rate of 1–0.001 s−1. The microstructure was obtained by EBSD characterisation. It was found that under certain strain-rate conditions, the FCC phase in the dual-phase Al0.9FeCoNiCr high-entropy alloy did not change significantly at low temperatures but increased significantly when the temperature was over 1000°C. The hot-working properties of the alloy are affected when the FCC phase exceeds 80%. [ABSTRACT FROM AUTHOR]

The thermal compression experiments of the GH99 alloy were carried out at different strains from 1020 °C to 1170 °C and 0.001 s−1–1 s−1 conditions using a Gleeble-3800 thermal compression simulation tester. Construction of thermal processing maps with four instability criteria were superimposed on Murty, Prasad, Gegel, and Malas at different strains based on stress-strain data. Based on the theoretical basis, prediction results, and EBSD microstructure characterization method of four instability criteria, the suitable forming processing region and rheological instability region of the alloy were predicted. It was found that the Prasad instability criterion had the most accurate prediction results. The instability range predicted by Murty was accurate under minor strains, but as the strain increased, the expected instability range slightly increased compared to the actual range. However, the Gegel and Malas criteria have biases in predicting alloys under low-rate conditions at different strains. A scientific and rational optimization was carried out to select hot working process parameters for GH99 alloy in response to the influence of strain on its hot deformation behavior. [ABSTRACT FROM AUTHOR]

Focal research has been conducted on medium-entropy alloys (MEAs) that exhibit a balanced combination of strength and plasticity. In this study, the microstructure, dynamic mechanical properties, and texture evolution of an as-cast medium-entropy CoCr0.4NiSi0.3 alloy were investigated through dynamic compression tests at strain rates ranging from 2100 to 5100 s−1 using the Split Hopkinson Pressure Bar in order to elucidate the underlying dynamic deformation mechanism. The results revealed a significant strain rate effect with dynamic compressive yield strengths of 811 MPa at 2100 s−1, 849 MPa at 3000 s−1, 919 MPa at 3900 s−1, and 942 MPa at 5100 s−1. Grains were dynamically refined from 19.73 to 3.35 μm with increasing strain rates. The correlation between adiabatic temperature rise induced by dynamic compression and dynamic recrystallization was examined, revealing that the latter is not associated with adiabatic heating but rather with phase transition triggered by the dynamic stress during compression. The proportion of Σ3n (1 ≤ n ≤ 3) grain boundaries in deformation specimens increases with increasing strain rates during dynamic compression. The formation of specific three-node structures enhances both strength and plasticity by impeding crack propagation and resisting higher mechanical stress. In the as-cast state, significant anisotropy was observed in the MEA. As strain rates increased, it transited into a stable {111}<112> F texture. The exceptional dynamic properties of strength and plasticity observed in the as-cast state of the MEA can be attributed to a deformation mechanism involving a transition from dislocation slip to the formation of intricate arrangements, accompanied by interactions encompassing deformation nanotwins, stacking faults, Lomer–Cottrell locks, stair-rods, and displacive phase transformations at elevated strain rates. [ABSTRACT FROM AUTHOR]

Oxidation experiments of a Ti-47.5Al-2.5V-1.0Cr-0.2Zr alloy were carried out at 950, 1000 and 1050 °C. The alloy's oxidation kinetics curve, oxidation products, oxide film structure and oxidation mechanism at different oxidation temperatures and times were studied systematically. The oxide film has an obvious multilayer structure. Oxygen diffuses into the matrix alloy, and two oxides (TiO2 and Al2O3) grow alternately in layers to produce multilayer structures. As the oxidation process continues, oxygen atoms continue to diffuse in the matrix, there are holes and cracks in the oxide film, and the oxidation resistance of the oxide film is weakened. [ABSTRACT FROM AUTHOR]

The martensite phase with low crystal symmetry in NiTi alloy consists of internal twins and nano-layered compounds. The martensite phase has various inelastic deformation modes, such as twinning, reorientation, and nanocrystallization, during the deformation process, which leads to the further understanding of this deformation mechanism is unclear. Therefore, this research performed hot compression tests on the as-cast martensitic NiTi alloy, focusing on the analysis of the hot deformation behavior of the flow curves with abnormally increased stress value under high strain. The results showed that the peak stress, peak strain and softening stage of the flow curve decrease with the increases in deformation temperature, and the stress value of the flow curve increases at the high strain stage. According to the microstructure analysis of the alloy, the Ti2Ni phase is extruded and broken into small particles with the compression progress. Further spheroidization of the Ti2Ni phase with the compression temperature increases. In the process of hot compression, the martensite variants in the B′19 phase reorient, and the deformation twins and nanocrystalline structures are formed. The hindering effect of the broken Ti2Ni particles on dislocation movement and the strengthening role of nanocrystalline grains are the main reasons for the abnormal increase in stress value at high strain. [ABSTRACT FROM AUTHOR]

The effects of casting and different heat treatment processes on the corrosion resistance of AlFeCoNiMo0.2 high-entropy alloy in 3.5% NaCl (mass fraction) and 0.5 mol/L H2SO4 solutions were investigated using dynamic potential polarization curves, SEM, XRD, XPS, and other test methods. The results show that in the Cl− environment, the cast alloy has the lowest corrosion current density and higher corrosion resistance compared to the annealed alloy. The elements Al and Mo are severely segregated in the crystal and in the grain boundaries, where galvanic corrosion occurs, and the Al-rich phase produces pitting corrosion in the crystal. The main components of its passive film are oxides of Al, Fe, Co, and Mo, and oxides and hydroxides of Ni. In the SO42− environment, the best corrosion resistance is achieved in the 900 °C annealed state of the alloy. Electrochemical test results show that the alloys all undergo secondary passivation, producing two successive product films to protect the metal matrix. Preferential corrosion areas are concentrated in the molybdenum-rich grain boundaries and nearby dendritic regions, reducing the corrosion resistance of the alloy. The main components of the passive film are oxides of Al and Mo; oxides of Fe, Co, Ni; and hydroxides. The Mo element in the passive film prevents the activated dissolution of Fe and produces the protective component MoO3, which inhibits the dissolution of the alloy and improves the stability of the passive film. The presence of Mo elements increases the selective dissolution of Fe, and the aggregation of Mo elements at grain boundaries after annealing weakens the corrosion resistance of the alloy and leads to the dissolution of the passive film. The main components of the passive film are oxides of Al and Mo; oxides of Fe, Co, Ni; and hydroxides. [ABSTRACT FROM AUTHOR]

The hot compression tests of Ti–47.5Al–2.5V–1.0Cr–0.2Zr alloy were achieved by a Gleeble thermal simulator. The results show that the residual lamellar microstructure has sufficient energy and sufficient time to decompose, which is conducive to the evolution of the dynamic recrystallisation (DRX) microstructure. The dislocation density decreases significantly owing to the generation of a DRX. The deformation mechanism during hot deformation is dominated by the DRX of the γ-phase. Meantime, the increment of dislocations will induce to the generation of sub-grain boundaries. In addition to the mechanism of dislocation-induced recrystallised grain formation, twinning deformation is also a typical deformation mechanism in TiAl alloy. The transformation of lamellar microstructure into uniform and fine DRX microstructure can improve the microstructural uniformity of the alloy. [ABSTRACT FROM AUTHOR]

Ti–6Al–4V fine-grained plates were manufactured using a rolling method and then subjected to superplastic tensile tests at varying temperatures and strain rates on an AG 250KNE electronic tensile testing machine. The superplastic behaviours of the plates were also tested. A cavity-growth model was established and the changing laws of energy during cavity growth and microstructure evolution of superplastic deformation were predicted. The Ti–6Al–4V alloy possessed the maximum elongation rate of 886% at 840°C and a strain rate of 5 × 10−4 s−1. The strain-rate sensitivity index m for this alloy was 0.54. The mechanism of superplastic deformation was established to be strain-induced grain-boundary slip, and the mechanism of cavity growth to be plasticity-controlled cavity coalescence and growth. [ABSTRACT FROM AUTHOR]

Based on high-temperature compression tests and focusing on the mechanical behaviour characteristics and microstructure evolution laws during high-temperature compression, laws relating to the strain-rate sensitivity index m and the activation energy of deformation Q-value at varying strain rates and deformation temperatures are clarified. Deformation mechanism maps involving dislocation quantity are plotted. Dislocation evolution laws and deformation mechanisms of the grain size with Burgers vector compensation, and the flow stress with modulus compensation during high-temperature deformation are predicted using the deformation mechanism maps. [ABSTRACT FROM AUTHOR]

High-temperature compression tests, electron backscatter diffraction (EBSD) and transmission electron microscope (TEM) observations are performed to investigate the effect of energy dissipation changes on the dynamic recrystallization microstructure of NiTi alloy during hot deformation. The Strain-Compensated Arrhenius-Type (SCAT) model and hot processing map of NiTi alloy are developed to predict the optimal processing window. The energy analysis of the dynamic recrystallization process is realized by combining the change of power dissipation rate (η) and the evolution of dynamic recrystallization. The results show that the dynamic recrystallization fraction of the alloy increased with the increase of energy dissipation. Rheological instability is mainly observed in the low η region, and the lowest dynamic recrystallization fraction is 23.49%. However, the η values fluctuate abnormally in the medium η region and the high η region. At 1000 °C, 1 × 10−2 s−1, the highest dynamic recrystallization fraction is 38.59%. At 950 °C, 1 s−1, the dynamic recrystallization fraction decreased to 28.87%. The analysis shows that the dynamic recrystallization fraction is 33.68% and the microstructure is uniform at 850 °C, 1 × 10−2 s−1, which is the optimal processing parameter. In addition, the critical strain kinetic model is developed, and the critical strain is 0.298 at 850 °C, 1 × 10−2 s−1. The microstructure evolution of dynamic recrystallization under different strains (0.3–0.9) shows that discontinuous dynamic recrystallization and continuous dynamic recrystallization participate in the softening of NiTi alloy with the increase of strain. Different η areas have different energy for microstructure evolution. In the low η area, the main softening mechanism is discontinuous dynamic recrystallization. With the increase of η, the energy for microstructure evolution increases, and continuous dynamic recrystallization occurs. In the high η area, the main softening mechanism is continuous dynamic recrystallization. • The SCAT model hot processing map with theDRX fraction were developed to predict the optimal processing window. • The critical strain kinetic model of NiTi alloy was established, and the evolution process of DRX was obtained. • Combined with the power dissipation rate andDRX fraction, the softening mechanism in different η areas was revealed. [ABSTRACT FROM AUTHOR]

The single-phase body-centered cubic (BCC) structured high-entropy alloys are considered to be typically difficult-to-deform processing alloy because they can still crack during hot deformation at high temperatures and low strain rates. However, we found that the hot working formability of this type of alloy is very sensitive to temperature, and there is a unique microscopic transformation. In this paper, the as-cast single-phase BCC-structured AlFeCoNiMo 0.2 high-entropy alloy was investigated with the single-pass hot-compression simulation experiment (deformation of 0.6), at temperatures and strain-rate ranges of 900 – 1150 ℃ and 0.001 – 0.1 s−1, respectively. The hot-deformation behavior and microstructure-evolution mechanisms were studied. The Arrhenius constitutive relation model was revised and established. The processing maps of Prasad, Gegel, Malas, and Murty with different instability criteria were constructed. The optimal thermal-processing parameters (temperatures of 1070 – 1150 ℃ and strain rates of 0.001 – 0.1 s−1) were provided. It was great to find that the alloy has a narrow temperature window effect of hot working with the Ruano-Wadsworth-Sherby (R-W-S) deformation-mechanism map established by incorporating the dislocation quantity. The deformation mechanisms of the alloy at 900 °C, 1000 °C, 1050 °C, and 1100 °C were predicted. The single-phase (BCC) structure of the alloy has strong stability during hot deformation. In a narrow range of hot-working temperatures, the microstructure has a necklace-like structure, and its substructure has a strong textured effect, impeding grain-boundary slip. In the optimized processing interval, discontinuous dynamic recrystallization (DDRX) occurs, and the necklace-like structure disappears. The recrystallization mechanism is related to grain-boundary sliding, caused by dislocation sliding. • The processing map under the four instability criteria was constructed, the accurate results were obtained to analyze the hot-forming properties of the alloy. • The reason for the difficulty in hot forming of the AlFeCoNiMo0.2 HEA is the narrow temperature window of the hot-working process. • The dislocation-quantity factor is introduced into the R-W-S deformation mechanism map through the model to obtain its deformation mechanism. [ABSTRACT FROM AUTHOR]

The steady-state rheological stress, critical strain capacity, structure evolution dynamics, and energy dissipation during dynamic recrystallization of Ni60Ti40 alloy were characterized through single-pass isothermal compression experiments at 980–1070 °C and strain rate of 5 × 10−3 - 5 s−1. The underlying structural evolution mechanism was revealed via microstructure characterization. The critical strain capacity of Ni60Ti40 during dynamic recrystallization decreased and the structure transformation volume fraction increased with the rise of deformation temperature or the decline of strain rate. The critical power dissipation rate upon dynamic recrystallization was 0.15. At low temperature and high strain rate, the structure evolution was dominated by geometrical dynamic recrystallization. At high temperature and low strain rate, non-continuous dynamic recrystallization was dominant. The nucleation mechanism was grain boundary slip induced by dislocation motion. • The rheological stress, critical strain capacity and structure evolution dynamics of Ni60Ti40 during dynamic recrystallization were revealed. • The KM model, Poliak-Jonas critical strain model, and Avrami recrystallization volume fraction evolution model were introduced into the power dissipation rate model. Thereby, the energy changing rules of Ni60Ti40 during dynamic recrystallization were clarified. • Single-pass thermal compression simulation tests under different deformation conditions were designed. The structure evolution mechanism of Ni60Ti40 during dynamic recrystallization was uncovered through microstructure characterization. [ABSTRACT FROM AUTHOR]

The hot deformation behavior and processing maps of the casting NiTi alloy were studied at the deformation temperature of 650–1050 °C and the strain rate of 5 × 10−3–1 s−1 by Gleeble-3800 thermal simulating tester. The variation of the strain rate sensitivity exponent m and the activation energy Q under different deformation conditions (T = 650–1050 °C, ε ˙ = 0.005–1 s−1) were obtained. The formability of the NiTi alloy was the best from 800 °C to 950 °C. The constitutive equation of the casting NiTi alloy was constructed by the Arrhenius model. The processing map of the casting NiTi alloy was plotted according to the dynamic material model (DMM) based on the Prasad instability criterion. The optimal processing areas were at 800–950 °C and 0.005–0.05 s−1. The microstructure of the casting NiTi alloy was analyzed by TEM, SEM and EBSD. The softening mechanisms of the casting NiTi alloy were mainly dynamic recrystallization of the Ti2Ni phase and the nucleation and growth of fine martensite. [ABSTRACT FROM AUTHOR]

• High cycle fatigue properties of a Ti-4Al-6Mo-2 V-5Cr-2Zr alloy were investigated. • Fatigue cracks tend to nucleate at the α p /β trans and GB α/β trans interfaces in HN. • Fatigue cracks of LM tend to nucleate on the GBs during cyclic loading. • Fatigue life data for HN are more scattered than for LM. To clarify effects of microstructure on fatigue crack initiation mechanism of Ti-4Al-6Mo-2V-5Cr-2Zr alloy, high-cycle fatigue behavior of hierarchical nanostructure (HN) and lamellar microstructure (LM) was investigated. Fatigue tests results show that the high-cycle fatigue property of LM (σ 0.1 (10 7) , 685 MPa) is better than that of HN (σ 0.1 (10 7) , 630 MPa). EBSD analysis and TEM observation demonstrate that with cyclic loading of stress, prism < a > slip is activated in α p phase, and basal < a > slip is activated in grain boundary α phase (GB α). Plastic deformation of HN mainly occurs in short rod α p phase and GB α. Fatigue cracks preferentially initiate at weak parts of these structures and propagate into a transformed β structure (β trans). In addition, the intrinsic reason for the random behavior of the fatigue life can be attributed to differences in the microstructure ahead of the main crack tips. Fatigue data for HN containing GB α and short rod α p shows high scattering. Dislocation slip in LM occurred mainly in β matrix and piled up at α s /β interfaces and grain boundaries (GBs). Local high stresses caused { 0 1 ¯ 11 } nano-scale twins in α s phase to harmonize local plastic deformation, which makes cracks more preferable to nucleate at β GBs. [ABSTRACT FROM AUTHOR]

Evolution of secondary α phase during aging treatment of a novel near β titanium alloy Ti-6Mo-5V-3Al-2Fe(wt.%) was studied by OM, SEM, and TEM. Results indicated that size and distribution of secondary α phase were strongly affected by aging temperature and time. Athermal ω phase formed after super-transus solution treatment followed by water quenching, and promoted nucleation of needle-like intragranular α in subsequent aging process. When aged at 480 °C, fine scaled intragranular α with small inter-particle spacing precipitated within β grains and high ultimate tensile strength above 1500 MPa was achieved. When the aging temperature increased, the size and inter-particle spacing of intragranular α increased and made the strength reduce, but the ductility got improved. When aging temperature reached as high as 600 °C, ω phase disappeared and intragranular α coarsened obviously, resulting in serious decrease of strength. While mutually parallel Widmanstätten α laths formed at the vicinity of β grain boundaries and grew into the internal area of β grains, and significant improvement of ductility was achieved. As the aging time increased from 4 h to 16 h at 600 °C, the intragranular α grew slightly and brought about minor change of mechanical properties. [ABSTRACT FROM AUTHOR]

Changes in the microstructure characteristics and fracture mechanism of Ti-47.5Al-2.5 V-1.0Cr-0.2Zr alloy during tensile deformation were studied by high temperature tensile tests at 750–900 °C and strain rate of 10−5–10−3 s−1. The experimental results indicate that when the tensile temperature increases or the strain rate decreases, the number of small dimples in the tensile fracture structure increases, the proportion of cross-layer fracture decreases, the dimples have torn edges, and the plastic deformation resistance increases. The higher tensile temperature corresponds to a higher dynamic recrystallization (DRX) ratio, and the number of dislocations near the DRX structure is reduced, which shows that the rheological softening effect is significantly enhanced. A lower strain rate corresponds to a higher proportion of DRX and a smaller proportion of lamellar microstructure. Dislocation slip is the main deformation mode; stacking faults can be observed in DRX grains. The changes in the hot deformation microstructure directly reflect decrease of high temperature tensile strength in macroscopic flow behavior. Under the action of tensile stress, holes usually form at the lamellar grain boundary, and the aggregation of multiple micro-holes will merge and connect to form micro-cracks. Cracks are generated and propagated between layers, and the tendency of holes or cracks generated at lower deformation temperatures or higher strain rates is greater. When the stress concentration reaches a certain critical value, the specimen will eventually fracture. • The relationship between the residual lamellar microstructure, dislocation and DRX was discussed. • The deformation mechanism during tension mainly includes dislocation, twin and recrystallization. • The schematic diagram of tensile fracture mechanism was obtained. • The crack propagation mode and path in the tensile process are obtained. [ABSTRACT FROM AUTHOR]

• The deformation behavior is systematically studied by using constitutive model and processing map. • The influence of strain and the prediction accuracy were included in the constitutive analysis. • The deformation mechanism of the Ti-47.5Al-2.5 V-1.0Cr-0.2Zr alloy was dominated by DRX. [Display omitted] The hot deformation behavior of a Ti-47.5Al-2.5V-1.0Cr-0.2Zr alloy was investigated through thermal simulation experiments. The thermal deformation temperature range was 1050–1200 °C with a strain rate of 0.001–1 s−1. The connections between the stress, strain rate, and temperature were established using a constitutive equation, which included the influence of strain. The correlation coefficient between the calculated stress and the experimental data was 0.991, which indicated that the predicted stress curve was consistent with the experimental results and that the constitutive relationship could accurately predict the deformation behavior. The processing map indicated the optimum hot deformation ranges of temperatures and strain rates were determined to be 1140–1180 °C and 0.001–0.006 s−1, respectively. The proportion of dynamic recrystallization(DRX) and low dislocation density regions was higher in the optimum hot deformation region than in the instability region, and no instability phenomena, such as cracking, were found; hence, the thermal processing map predictions agreed well with the microstructure. The dominated deformation mechanism of the TiAl alloy mainly depended on DRX, and the movement of dislocations and twins were the typical deformation mechanisms. [ABSTRACT FROM AUTHOR]

The hot compression test of a Ti-47.5Al-2.5 V-1.0Cr-0.2Zr alloy was carried out at 1050–1200 °C and a strain rate of 0.001–1 s−1 by a Gleeble3800 thermal simulation testing machine. The hot processing maps at the true strains of 0.2, 0.4, 0.6, and 0.8 were established, and the optimal thermal processing parameter region and the instability region were determined. The microstructures corresponding to different regions of the processing maps were systematically studied, and the validity of the established processing maps were verified. The results indicated that when the true strain is 0.8, the optimal hot working parameter region are 1132–1185 °C and 0.001–0.002 s−1, respectively. The instability region are 1050–1125 °C/0.04–1 s−1 and 1125–1200 °C/0.7–1 s−1, respectively. The corresponding microstructures were lamellar microstructures with a bending and twisting deformation. At the same time, the instability phenomena, such as micro-cracks and voids, were found, and a large number of dislocation pile-ups appeared in the lamellar crystals. The dynamic recrystallization (DRX) degree of the microstructures corresponding to the optimal hot working parameter region is relatively complete, and the recrystallized grain size is relatively uniform. There is almost no residual lamellar microstructure, and no micro-cracks were found. The microstructures in the optimal hot working condition is obviously better than that in the instability region. The thermal processing safety region obtained in this work is authentic, and the processing map has a certain instructive role in the choice of thermal processing parameters. • The strain rate sensitivity index, power dissipation rate and instability parameters varies with deformation conditions. • The deformation characteristics between stable and instable regions are compared via the systematic microstructure analysis. • The deformation mechanism of the Ti-47.5Al-2.5 V-1.0Cr-0.2Zr alloy is mainly DRX. [ABSTRACT FROM AUTHOR]