Dynamic recrystallization and phase transformation behavior of a wrought β-γ TiAl alloy during hot compression

The dynamic recrystallization (DRX) and phase transformation (PT) behavior of a wrought β-γ TiAl alloy during hot compression under various deformation temperatures were investigated. The typical work hardening and flow softening features indicated that DRX was the dominating softening mechanism. Both γ-DRX and β-DRX took place during the hot compression. γ-DRX was triggered at all compression temperatures, while the β-DRX was induced when the compression temperature was above 1000 °C. The hot deformation kinetics was calculated, which showed that DRX behavior existed in the whole hot compression process, and the DRX volume fraction increased with the increase of the compression temperature. Combined with the microstructure observation, it concluded that the β/B2+α₂→γ PT occurred at 850 °C and 1000 °C, while the γ→β/B2 PT happened at 1050 °C during hot compression, which is important to optimize microstructure. Moreover, the hot compression mechanism changed from dislocation gliding to grain-boundary sliding was discussed.     

Hot deformation behavior of 51.1Zr–40.2Ti–4.5Al–4.2V alloy in the single β phase field

The hot deformation behavior of a newly developed 51.1Zr–40.2Ti–4.5Al–4.2 V alloy was investigated by compression tests in the deformation temperature range from 800 to 1050 ℃ and strain rate range from 10-3to 100 s-1. At low temperatures and high strain rates, the flow curves exhibited a pronounced stress drop at the very beginning of deformation, followed by a slow decrease in flow stress with increasing strain. The magnitude of the stress drop increased with decreasing deformation temperature and increasing strain rate. At high temperatures and low strain rates, the flow curves exhibited typical characteristics of dynamic recrystallization. A hyperbolic-sine Arrhenius-type equation was used to characterize the dependences of the flow stress on deformation temperature and strain rate. The activation energy for hot deformation decreased slightly with increasing strain and then tended to be a constant value. A microstructural mechanism map was presented to help visualize the microstructure of this alloy under different deformation conditions.     

Effect of Zr and Ti on isothermal oxidation behavior of Nb–Si based alloys

The isothermal oxidation behavior of 56Nb-16Si-(20-x)Ti–3Cr–3Al-2Hf-xZr (x ?= ?0, 2, 5, 10 ?at. %) alloys was investigated at 800 ?°C and 1250 ?°C, respectively. The results show that increasing the Zr content evidently increased the oxidation rates at 800 ?°C, accompanied by the obvious occurrence of pesting oxidation. The alloys showed alike linear oxidation kinetics at 1250 ?°C. With the increase of Zr content, the adherence and integrity of oxide scales were improved, but the overall oxidation resistance was slightly deteriorated. The observed oxidation behavior may be attributed to the composition variation of Zr and Ti in the alloys. The oxidation mechanism associated with the composition variation is discussed in this study.     

Dependence of mechanical behavior on grain size of metastable Ti–Nb–O titanium alloy

Cold-rolled metastable β-type Ti–38Nb-0.2O alloy was subjected to annealing treatment to obtain different precipitates and grain sizes. The influence of annealing on microstructure and mechanical properties was investigated. The alloy annealed at 673 ?K or 773 ?K exhibited a single-stage yielding with high strength and low uniform elongation, due to the residual work hardening and the precipitation of ω or α phases. The alloy annealed at above 873 ?K exhibited an obvious double yielding behavior resulting from the stress-induced martensitic transformation. The grain growth kinetics of single β phase alloy is sensitive to temperature, and it is suggested that the existence of oxygen decreases the grain growth exponent and increases the required activation energy for grain growth. The critical stress for slip decreased monotonously with the increase of grain size, following the classic Hall-Petch relationship. However, the critical stress for martensitic transformation decreased to a minimum and then increased again, as the grain size increased. The results are worth for design of the heat-treatment parameters of the Ti–38Nb-0.2O alloy for engineering applications.     

Kinetic analysis and strain-compensated constitutive models of Ti-42.9Al-4.6Nb–2Cr during isothermal compression

The hot deformation behavior of Ti-42.9Al-4.6Nb–2Cr (at. %) was investigated by isothermal compression tests at the deformation temperature range of 1373–1573 K, strain rate range of 0.001–1.0 s⁻¹, up to the strain of 0.69. The flow stress test results of Ti-42.9Al-4.6Nb–2Cr showed negative temperature and positive strain rate sensitivity. Besides, strain had a great effect on the hot deformation behavior of Ti-42.9Al-4.6Nb–2Cr. Kinetic analysis was adopted to assess the hot workability of Ti-42.9Al-4.6Nb–2Cr via apparent activation energy (Q) of hot deformation, strain-rate sensitivity index (m) and strain hardening index (n). The Q value varied from 607.1 ± 0.7 kJ·mol⁻¹ to 512.6 ± 10.8 kJ mol⁻¹ with the increasing of strain from 0.1 to 0.6. The effect of strain on the Q value at the deformation temperatures below 1473 K was mainly related to dynamic recrystallization of γ phase and kinking of γ lamellae, while the Q value at the deformation temperature above 1473 K might be linked to γ→α phase transformation and DRV of α phase. Based on the kinetic analysis, strain-compensated Arrhenius model and Hensel-Spittel model were successfully established to predict the hot workability (flow stress). Average absolute relative errors of established strain-compensated Arrhenius model and Hensel-Spittel model were 7.52% and 11.95%, respectively. Moreover, both established constitutive models can be extrapolated for predicting the flow stress of Ti-42.9Al-4.6Nb–2Cr to larger strain levels.     

Unique cyclic deformation behavior of a heavily alloyed Al–Si piston alloy at different temperatures

The cyclic deformation behavior of a near-eutectic Al–Si piston alloy with a variety of alloying elements is investigated. Due to the distinctive microstructure, the alloy exhibited various cyclic deformation characteristics at different temperatures. Unique double-stage cyclic hardening occurred at room temperature, while the first hardening was mainly caused by the interaction between the dislocations and the finely dispersive precipitates, and the dislocation/dislocation interaction resulted in the second hardening. At elevated temperatures (200 1C and 350 1C), the alloy mainly presented cyclic softening because of the fact that the thermally activated dislocations could bypass or shear the obstructive precipitates.     

Modeling the hot working behavior of near-α tita nium alloy IMI 834

The hot working behavior of near-a titanium alloy IMI 834 with a duplex starting microstructure was studied using the technique of processing map.The processing map was interpreted in terms of the microstrucmral processes occurring during deformation,based on the values of dimensionless parameter η which represents the energy dissipation through microstructural processes.An instability criterion（ξ<0） was also applied to demarcate the flow instability regions in the processing map.Both the parameters（η and ξ） were computed using the experimental data generated by carrying out hot compression tests over a range of temperatures（850-1060℃） and strain rates(3×10^-4-1/s).The deterministic domains observed under the investigated temperature and strain rate conditions were attributed to continuous dynamic recrystallization or globularisation of a lamellae,dynamic recrystallization and growth of β grains through microstructural observations.An unified strain compensated constitutive equation was established to describe the hot working behavior of the material in the selected temperature-strain rate range.The established constitutive equation was validated using standard statistical parameters such as correlation coefficient and average absolute relative error.     

Hot deformation behavior and the processing map of Zr–1.0Be alloy in single α phase

The hot deformation behavior,hot workability and dynamic recrystallization evolution of Zr-1.0(wt%) Be alloy in single a phase were investigated by conducting hot compression tests.The strain rates ranging from 10~(-3) s~(-1) to 10° s~(-1) and testing temperatures varying from 650 ℃to 850℃ were used.Flow stress was found to increase with increasing strain rate and decrease with the increment of the deformation temperature.A constitutive equation of flow behavior was established to describe the dependence of flow stress on strain rate and deformation temperature.The activation energy for deformation of Zr-1.0Be alloy was determined to be Q= 301 kJ/mol.The processing map of Zr-1.0Be alloy was constructed at strain rates ranging from 10~(-3) s~(-1) to 10° s~(-1) and deformation temperatures varying from 650 ℃ to 850 ℃ at the true strain of 0.7.A processing map was used to identify the best domains of thermal processing,including a domain at a temperature of 650 ℃ and strain rate of 10~(-3) s~(-1) as well as another domain at deformation temperatures ranging from 800 ℃ to 850 ℃ and strain rates varying from 10~(-3) s~-~(-1) to 10~(-1) s~(-1).Microscopic analysis of Zr-l.OBe alloy showed that the flow instability and kink were very obvious at low temperatures and high strain rates.At high temperatures and low strain rates,the dynamic recrystallization became the main softening mechanism during hot working.     

Microstructure and texture evolution of a near β titanium alloy Ti-7333 during continuous cooling hot deformation

The microstructure and texture evolution during continuous cooling hot deformation(CCHD) in a near β titanium alloy, named Ti-7Mo-3Nb-3Cr-3Al(Ti-7333), were investigated by using the electron backscattered diffraction(EBSD). The results indicate that the precipitation of secondary α phase was restricted by CCHD, and the morphology of primary α phase nearly had no change with the deformation and temperature drop. In contrast, βphase underwent more deformation and the grains tended to refine. This may be due to the dynamic recrystallization(DRX) of β phase, including continuous dynamic recrystallization(CDRX) and discontinuous dynamic recrystallization(DDRX). In addition, the textures of {110} 110 , {225} 520 and{115} 123 transformed to {100} 110 and {001} 100 during CCHD. Among these, the η-fiber component of {001} 100 was the dominant deformation texture in the deformed Ti-7333 alloy. Finally,continuous cooling has an important effect on the work hardening and softening during CCHD, contributing to the different flow behaviors at different cooling rates.     

Improvement of oxidation resistance of Nb–Ti–Si based alloys with additions of Al,Cr and B at different temperatures

The effects of Cr,Al and B addition on the microstructure and high-temperature oxidation behaviors(at 1200,1250 and 1300℃) of Nb-Ti-Si based alloys were investigated.The results showed that the addition of Cr stabilized α-Nb₅ Si₃,while Al promoted the formation of β-Nb₅ Si₃ and adding B promoted the formation of γ-Nb₅ Si₃.Among the three elements,Al and Cr were beneficial to oxidation resistance at 1200℃,and B was favorable to the...     

Creep behavior and effect factors of a TiAl–Nb alloy at high temperature

The effect of solution treatment on the microstructure and creep properties of forged TiAl-Nb alloys was investigated.The results showed that the microstructure of forged alloy mainly consisted of γ/α₂ lamellar colonies and fine equiaxed recrystallized γ/α₂ grains.During the solution treatment the microstructure of the alloy transformed into a fully lamellar structure due to the lamellar colonies growth by consuming equiaxed grains.Compared with the forged alloy the creep l...     

Peculiar aging response of near b Ti–25 Nb–2Mo–4Sn alloy for biomedical applications

In this paper,aging response of a recently developed near β Ti-25Nb-2Mo-4Sn(wt%) alloy with high strength and low modulus was investigated intensively.The experimental results from X-ray diffraction and transmission electron microscopy showed that the aging production of the Ti-2524 alloy was(β+ω) or(β+α) even under the same aging treatment condition,depending on the pre-treatments prior to the aging.Solid evidence confirmed the competition between stable α phase and metastable ω phase during the decomposition of β phase on aging.Different aging response of Ti-2524 alloy can be attributed to high-density dislocations and grain boundaries which suppress the formation of ω,and alternatively promote a phase formation.This provides a thermo-mechanical approach to inhibit deleterious ω phase formation and assist fine α phase precipitation.Upon an appropriate aging treatment,superior mechanical properties of high ultimate tensile strength(1233 MPa) and low elastic modulus(77 GPa) were achieved in Ti-2524 alloy.     

Oxidation behaviours of Nb–22Ti–15Si–2Al–2Hf–2V–(2, 14)Cr alloys with Al and Y modified silicide coatings prepared by pack cementation

Al and Y modified silicide coatings on the Nb-15Si-22Ti-(2,14)Cr-2Al-2Hf-2V alloys(where the alloy with 2 at%Cr or 14 at%Cr is hereafter referred to as 2Cr and 14 Cr alloy,respectively) was prepared by pack cementation.The microstructural evolution and the oxidation behaviours of the coating 2Cr and 14 Cr samples at 1250 ℃ were studied.The 2Cr alloy consists of Nb solid solution(Nb_(SS)) and α-Nb_5Si_3 silicide,while the Laves C15-Cr_2Nb phase arised in the 14 Cr alloy.The coating structure of the coating 2Cr sample contained the outer(Nb,X)Si_2+(Nb,X)_5Si_3 layer,the middle(Nb,X)_5Si_3 layer and the inner undeveloped intermetallic(Nb,Ti)_3(Al,X) layer,the structure of the coating 14 Cr sample consisted of the outer single(Nb,X)Si_2 layer,the middle(Nb,X)_5Si_3 layer,the transition(Nb,Ti)(Cr,Al) layer and the inner(Cr,Al)_2(Nb,Ti) layer.The coating 14 Cr sample exhibited better oxidation resistance than the coating 2Cr sample.With an outer single(Nb,X)Si_2 layer,a compact oxide scale consisting of SiO_2 and TiO_2 formed on the coating14 Cr sample,which can efficiently prevent the substrate from oxidising.For the coating 2Cr sample with an outer(Nb,X)Si_2 +(Nb,X)_5Si_3 layer,the oxide scale of the SiO_2,TiO_2,Nb_2O_5 and CrNbO_4 mixture generated,and the scale spalled out from the surface of the sample,resulting in disastrous failure.     

Further mechanism of α/β interphase boundary evolution and dynamic globularization of Ti–5Al–2Sn–2Zr–4Mo–4Cr at elevated temperature deformation

The evolution characterization of the α/β interphase boundaries of the isothermally compressed Ti–5Al–2Sn–2Zr–4Mo–4Cr with lamellar microstructure was carried out via electron backscatter diffraction (EBSD) and transmission electron microscope (TEM). The effect of the α/β interphase boundary evolution on the dynamic globularization was discussed, and the nucleation model for the β recrystallized grain at the cusp of α lath in the late stage of deformation in terms of the classic nucleation theory was established. The mean α/β interface deviation angle from Burgers orientation relationship (BOR) increased continuously up to 18.9° with the increasing strain from 0.1 to 0.5, which was dominated by the continuous dynamic recrystallization. Restoration of BOR between α and β phases at the strains of 0.7 and 0.9 occurred due to the occurrence of recrystallized α and β grains following BOR, which was rationalized by a nucleation model considering the stored energy of deformation provided the driving force for nucleation at the cusp of α lath. Loss of coherency of α/β interphase boundaries at a strain about 0.3 was responsible for accelerating the dynamic globularization of α lamellae since the energy of α/β interphase boundaries increased up to the maximum value when the non-coherent α/β interphase boundaries were formed. Restoration of coherency of α/β interphase boundaries due to the recrystallization nucleation in the late stage of deformation did not substantially affect the dynamic globularization since the α/α intraphase high-angle boundaries (HAGBs) were formed and the most α/β interphase boundaries were non-coherent.     

Effect of thermo-mechanical treatment on mechanical and elastic properties of Ti–36Nb–5Zr alloy

The evolutions of phase constitutions and mechanical properties of a β-phase Ti–36Nb–5Zr(wt%) alloy during thermo-mechanical treatment were investigated. The alloy consisted of dual(β t α″) phase and exhibited a double yielding phenomenon in solution treated state. After cold rolling and subsequent annealing at 698 K for 20 min, an excellent combination of high strength(833 MPa) and low modulus(46 GPa) was obtained. The high strength can be attributed to high density of dislocations, nanosized α phase and grain refinement. On the other hand, the low Young's modulus originates from the suppression of chemical stabilization of β phase during annealing, which guarantees the low β-phase stability. Furthermore, the single-crystal elastic constants of the annealed Ti–36Nb–5Zr alloy were extracted from polycrystalline alloy using an in-situ synchrotron X-ray technique. The results indicated that the low shear modulus C44 contributes to the low Young's modulus for the Ti–36Nb–5Zr alloy, suggesting that reducing C44 through thermo-mechanical treatment might be an efficient approach to realize low Young's modulus in β-phase Ti alloys. The results achieved in this study could be helpful to elucidate the origin of low modulus and sheds light on developing novel biomedical Ti alloys with both low modulus and high strength.     

A Ti–Zr–Cu–Ni–Co–Fe–Al–Sn amorphous filler metal for improving the strength of Ti–6Al–4V alloy brazing joint

Ti_(50)Zr_(27)Cu_8Ni_4Co_3Fe_2Al_3Sn_3(at%) amorphous filler metal with low Cu and Ni contents in a melt-spun ribbon form was developed for improving mechanical properties of Ti–6Al–4V alloy brazing joint through decreasing brittle intermetallics in the braze zone. Investigation on the crystallization behavior of the multicomponent Ti–Zr–Cu–Ni–Co–Fe–Al–Sn amorphous alloy indicates the high stability of the supercooled liquid against crystallization that favors the formation of amorphous structure. The Ti–6Al–4V joint brazed with this Ti-based amorphous filler metal with low total content of Cu and Ni at 1203K for 900s mainly consists of α-Ti, β-Ti,minor Ti–Zr-rich phase and only a small amount of Ti_3Cu intermetallics, leading to the high shear strength of the joint of about 460 MPa. Multicomponent composition design of amorphous alloys is an effective way of tailoring filler metals for improving the joint strength.     

Microstructural evolution and mechanical behavior of metastable β-type Ti–30Nb–1Mo–4Sn alloy with low modulus and high strength

A metastable P-type Ti-30Nb-lMo-4Sn alloy with ultralow elastic modulus and high strength was fabricated.Under the solution treatment state,the Ti-30Nb-1Mo-4Sn alloy possesses low yield strength of about 130 MPa owing to the presence of the coarse α " martensitic laths.Upon a cold rolling and annealing process,the martensitic transformation from β to α" is significantly retarded due to the inhibitory effect of grain boundaries and dislocations.As a result,the metastable β phase with low total amount of β-stabilizers is retained to room temperature,giving rise to a low modulus of 45 GPa.Meanwhile,nano-sized a precipitates and dislocation tangles play a key role in strengthening the Ti-30Nb-1Mo-4Sn alloy,resulting in a high tensile strength of ~ 1000 MPa.With low elastic modulus and high strength,the metastable P-type Ti-30Nb-1Mo-4Sn alloy could be a potential candidate for biomedical materials.     

Microstructural evolution and mechanical properties of an Fe–18Ni–16Cr–4Al base alloy during aging at 950℃

The development of Gen-IV nuclear systems and ultra-supercritical power plants proposes greater demands on structural materials used for key components. An Fe–18Ni–16Cr–4Al (316-base) alumina-forming austenitic steel was developed in our laboratory. Its microstructural evolution and mechanical properties during aging at 950℃ were investigated subsequently. Micro-structural changes were characterized by scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. Needle-shaped NiAl particles begin to precipitate in austenite after ageing for 10 h, whereas round NiAl particles in ferrite are coarsened during aging. Precipitates of NiAl with different shapes in different matrices result from differences in lattice misfits. The tensile plasticity increases by 32.4% after aging because of the improvement in the percentage of coincidence site lattice grain boundaries, whereas the tensile strength remains relatively high at approximately 790 MPa.     

Evolutions of diffusion activation energy and Zener–Hollomon parameter of ultra-high strength Al-Zn-Mg-Cu-Zr alloy during hot compression

The changes of stress level for the ultra-high strength Al-Zn-Mg-Cu-Zr alloy were described by constitutive equation with considering lattice diffusion of aluminum, zinc, magnesium and copper. Zener–Hollomon(Z)parameter expression based on the constitutive equation with considering lattice diffusion was used to reflect the changes of microstructure. The critical stress σcfor the initiation of dynamic recrystallization(DRX) was introduced to calculate the Z parameter. Steady-state dislocation density ρsatand critical dislocation density ρcfor the initiation of DRX decreased with the increase of deformation temperature. The dependence of diffusion activation energy Q on temperature and strain rate was given and the effects of deformation conditions on Q were discussed in detail. Microstructural evolution revealed that low angle boundaries(2–5°) created in the process of dynamic recovery(DRV) could convert into subgrain boundary, thus the original grains were divided into subgrains, and then subgrains transformed into DRX grains by the way of progressive rotation. When the Z value was high(ln Z 30.9), DRV was the main softening mechanism. With the decrease of Z value, both of DRV and DRX played an important roles in softening effect, while with the further decrease of Z value(ln Z 28.6), DRX became the main softening mechanism. Continuous dynamic recrystallization(CDRX) and discontinuous dynamic recrystallization(DDRX) operated together under the condition of lower Z value, but CDRX was confirmed as the dominant DRX mechanism.