Microstructure/texture evolution maps to optimize hot deformation process of near-α titanium alloy

The microstructure/texture evolution(MTE,for short) map and processing map of a new near a titanium alloy Ti65 were constructed in order to investigate the workability and microstructure evolution of hot deformation.The processing map illustrated four domains,two summit domains and two instability domains.The morphologies of the a phase changed from the spheroidization(α+β region) to the deformed and elongated β grains(near the βtransus temperature T_β),and then to the obvious dynamic recrystallization(DRX)(β region) with the temperature rising from 930 ℃ to 1140℃.Deformation in the α+β field mainly generated the texture component with [0001]or [0223] parallel to radial directions(RDs).While deformation in the β phase field formed two types of texture component with [0001] parallel to RDs and [2110] parallel to compression direction.An optimized processing map was summarized by overlaying the macro-instability map on the original processing map,and the instability domain of Ti65 alloy was confirmed in the area with the strain rate higher than 0.01 s~(-1).     

Microstructure and texture evolution of Mg–3Zn–1Al magnesium alloy during large-strain electroplastic rolling

Large-strain deformation by single electroplastic rolling(EPR) was imposed on AZ31 magnesium alloy strips. During EPR at low temperature(150–250°C), numerous twins formed in the alloy. After EPR at a high temperature(350°C), the number of twins reduced and some dynamic recrystallization(DRX) grains formed at grain boundaries and twinned regions. The synergic thermal and athermal effects generated by electropulsing, which promoted dislocation motion, induced a few small DRX grains, and ductile bandings were mainly responsible for large-strain deformation during EPR. The inclination angle of the basal pole stemmed from the counterbalance of the inclination direction of the basal pole between the DRX grains and deformed coarse grains.     

Study on the formation mechanism of α lamellae in a near β titanium alloy

In order to obtain more insight in the two existing mechanisms on the nucleation, i.e., sympathetic nucleation and interface instability nucleation, of α lamellae in the near β Ti alloys, the micro-texture ofα lamellae in Ti-7Mo-3Nb-3Cr-3Al has been investigated by using electron backscatter diffraction(EBSD).The results show that the α_(WGB) lamellae can not only grow up from α_(GB) grains by inheriting the orientations but also sympathetically nucleate at the α_(GB)/β interface during the slow cooling process. These observations provide the first direct experimental evidence that the formation mode of α lamellae in Ti-7333 alloy consists of sympathetic nucleation and interface instability nucleation. Based on the present results together with some previous studies on α phase transformation in Ti-based alloys the influence of β-stabilizers parameter on the change of formation mechanism of α lamellae was discussed.     

Mechanical behavior of a near α titanium alloy under dynamic compression: Characterization and modeling

Dynamic compression tests under strain rates from 870 s^?1 to 2100 s^?1 were conducted for a near α Ti–8Al–1Mo–1V titanium alloy with equiaxed microstructure. Compression behavior, adiabatic shearing and band microstructure were investigated via characterization and calculation. The results demonstrate that all dynamic constitutive curves exhibited obvious stress fluctuation phenomenon with double increase-decrease changing stages at the primary stage of compression. The dislocation multiplication theory can be used to explain this phenomenon. After the stress fluctuation period, work hardening coexisted with the thermal softening, resulting in the slow hardening tendency in constitutive curves. J-C model was utilized to quantify the dynamic constitutive curves. The deviations between the predicted and experimental curves under high strain rates may be attributed to the over-consideration of thermal softening effect in J-C model. Adiabatic shearing band (ASB) began to form under the strain rate of 2100 s^?1. A total shearing strain of 8.1 within ASB achieved in 8.9 μs, corresponding to a local strain rate of about 9.1 × 10⁵ s^?1 and is over 430 times of the macro strain rate. Post annealing was conducted on ASB before EBSD characterization. Due to the static recrystallization during annealing, the α phase within ASB generally presented as ultra-fine grains less than 1 μm in diameter.     

Tensile deformation behavior of a solution-treated Ti-33Nb-4Sn alloy with a dual β and α" phases under cyclic loading-unloading

The cyclical tensile deformation behavior of a solution-treated(ST) Ti-33Nb-4Sn alloy with a dual β and α "phases was investigated in this study.Experimental results indicated that the ST Ti-33Nb-4Sn alloy exhibited different deformation behavior during two different loading-unloading cycles,and that the deformation behavior was closely related to the characteristics of stress-induced martensitic(SIM) transformation,mainly including the extent and the reversibility of SIM transformation.During the first cycle,the extensive and incompletely reversible SIM transformation occurred,resulting in notable "double yielding" during loading and residual permanent strain after unloading.In the second cycle,however,a slight and reverse SIM transformation,together with pure elastic deformation,taken place concurrently,giving rise to a recoverable nonlinear deformation behavior.Our results also imply that by tailing the characteristics of SIM transformation,the Ti-based alloys with a mixture of α" and β phases might perform recoverable deformation and possess a promising potential for engineering applications.     

Deformation behavior and microstructural evolution during hot compression of an α+β Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy

The effect of processing parameters on the flow response and microstructural evolution of the α+β titanium alloy Ti-6.5Al-3.5Mo-1.5Zr-0.3Si has been studied by conducting isothermal hot compressive tests at a strain rate of 0.01–10 s-1 at 860–1100℃. The true stress-true strain curves of the sample hot-compressed in the α+β phase region exhibit a peak stress followed by continuous flow softening, whereas in the β region, the flow stress attains a steady-state regime. At a strain rate of 10 s-1, the alloy exhibits plastic flow instabilities. According to the kinetic rate equation, the apparent activation energies are estimated to be about 674–705 kJ/mol in the α+β region and 308–335 kJ/mol in the β region, respectively. When deformed in the α+β region, the globularization process of the α colony structure occurs, and α dynamic recrystallized microstructures are observed to show bimodal. Dynamic recrystallization can take place in the β region irrespective of starting deformed structures.     

Microstructure evolution of a recycled Al-Fe-Si-Cu alloy processed by tube channel pressing

Although excellent recyclability is one of the advantages of Al alloys, a recycling process can reduce different properties of these alloys by adding coarse AlFeSi particles into the alloys' microstructures. One of the well-known methods for modifying the microstructure of metallic materials is the imposition of severe plastic deformation (SPD). Nevertheless, the microstructure evolutions of recycled Al alloys containing extraordinary fractions of AlFeSi particles during SPD processing have seldom been considered. The aim of the present work is to study the microstructure evolution of a recycled Al-Fe-Si-Cu alloy during SPD processing. For this purpose, tubular specimens of the mentioned alloy were subjected to different numbers of passes of a recently developed SPD process called tube channel pressing (TCP); their microstructures were then studied using different techniques. The results show that coarse AlFeSi particles are fragmented into finer particles after processing by TCP. However, decomposition and dissolution of AlFeSi particles through TCP processing are negligible. In addition, TCP processing results in an increase in hardness of the alloy, which is attributed to the refinement of grains, to an increase of the dislocation density, and to the fragmentation of AlFeSi particles.     

Microstructure,mechanical properties and deformation mechanisms of an Al-Mg alloy processed by the cyclical continuous expanded extrusion and drawing approach

Al-Mg alloys are an important class of non-heat treatable alloys in which Mg solute and grain size play essential role in their mechanical properties and plastic deformation behaviors.In this work,a cyclical continuous expanded extrusion and drawing(CCEED)process was proposed and implemented on an Al-3Mg alloy to introduce large plastic deformation.The results showed that the continuous expanded extrusion mainly improved the ductility,while the cold drawing enhanced the strength of the alloy.With the increased processing CCEED passes,the multi-pass cross shear deformation mechanism progressively improved the homogeneity of the hardness distributions and refined grain size.Continuous dynamic recrystallization played an important role in the grain refinement of the processed Al-3Mg alloy rods.Besides,the microstructural evolution was basically influenced by the special thermomechanical deformation conditions during the CCEED process.     

Microstructure evolution during reheating of a Nb-bearing steel Ⅰ. Metallographic examination

Cooled in water after the isothermal relaxation of deformed austenite for different timea, a Nb-bearing microalloyed steel always exhibits synthetic microstructures of bainitic ferrite, granular bainite and acicular ferrite. When these samples were reheated to and held at 650 or 700℃, the non-equilibrious microstructures tend to evolve into equilibrious ones. The sample relaxed for 60 s displays the highest thermostability, while the microstructure evolution is the quickest in the sample relaxed for 1000 s even though it is the softest before reheating. Softening is not a single process occurring during reheating, in which the hardness fluctuates with time. There are two peaks in the hardness-time curve of each sample having undergone relaxation, while a single peak occurs in the curve of the sample having not been relaxed. Pre-strain accelerates the evolution process. These results indicate that the thermostability of microstructures is determined by their history of formation to a considerable degree.     

Microstructure evolution during reheating of a Nb-bearing steel Ⅱ. Dislocation-precipitate interaction

Cooled in water after the isothermal relaxation of deformed austenite for different times, a Nb-bearing microalloyed steel always exhibits the synthetic microstructure in which bainitic ferrite dominates. Strain-induced precipitates do not occur in an unre-laxed sample while they distribute outside dislocations in the sample relaxed for long time. Most of the strain induced precipitates distribute along dislocations in the sample relaxed for proper time. After bainitic transformation, the dislocations formed in the deformed austenite remain to be pinned by the precipitates so that the thermostability of the bainitic ferrite is improved. Coarsening of the precipitates accompanied by their distribution density change has caused the first hardness peak of bainite during reheating. The second hardness peak is attributed to the precipitates, which nucleate in bainite. Dislocations inside the laths getting rid of the pinning of precipitates and their polygonization play the precursor to the evolution of microstructures during reheating.     

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.     

The microstructure evolution and phase transformation behavior of a β-solidifying γ-TiAl alloy during creep

The microstructural evolution and creep behavior of the Ti-43.5Al–4Nb–1Mo-0.1B alloy have been investigated by scanning electron microscope(SEM) and transmission electron microscope(TEM). The excellent creep property was obtained with a fully lamellar(FL) microstructure containing the least grain boundary β_o phase(GB-β_o).TEM results revealed that after creep testing the α₂→βophase transformation was observed in the FL microstructure. The formation βophase is asso...     

Microstructure evolution and reaction behavior of Cu–Ni alloy and B_4C powder system

Microstructure evolution and reaction behavior of Cu–Ni alloy and B_4C power system was studied by in-situ and static experimental investigations along with theoretical calculations. The reaction process was as follows. Firstly,B_4C decomposed into B and C atoms, and then B atoms diffused into Cu–Ni matrix, leading to the formation of Ni_2B particles. Subsequently, Ni atoms diffused into B_4C, forming a loose mixture of Ni_2B and amorphous C at the initial powder boundary. Finally, with the completion of reaction, Ni_2B particles at the powder boundary grew into a monolithic block, and then C substance was excluded out and accumulated at the edge of this monolithic Ni_2B block. It is believed that the formation of Ni_2B phase is caused by the most negative change of Gibbs free energy among all the potential reactions between Ni–B and Ni–B_4C systems.     

Effect of cold rolling process on microstructure and mechanical properties of high strength β titanium alloy thin sheets

The microstructure and mechanical properties of Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fe high strength titanium alloy sheets prepared by unidirectional cold rolling and two-step cross cold rolling were investigated. Results showed that the β phase grains were refined significantly by cold rolling followed by solution treatment for a short time.Compared to unidirectional cold rolling, the short time solution treatment after two-step cross rolling could significantly reduce the non-uniformity of the microstructure of the alloy sheets. After aging treatment at 550 ℃,the anisotropy of the mechanical properties still existed in the unidirectional rolled sheets, and the tensile strength was highest along the rolling direction. After solution and aging treatment, the anisotropy of the mechanical properties of the two-step cross rolling process sheet was not obvious than unidirectional cold rolling,and alloy had good strength and plasticity matching.     

Dynamic recrystallization behavior of the Ti–48Al–2Cr–2Nb alloy during isothermal hot deformation

The hot deformation behavior of the as-cast Ti–48Al–2Cr–2Nb alloy was investigated by isothermal compression tests at deformation temperatures ranging from 1000℃ to 1200℃,and strain rates from 0.001 s~(-1)to 0.1 s~(-1).The single peak stress features common to all flow curves indicate that DRX is the dominating softening mechanism.The calculated values of the hot deformation activation energy Q and stress index n are 296.5 kJ mol~(-1)and 3.97,respectively.Based on this,the Arrhenius type constitutive equation was successfully established.The DRX critical condition model and relationship among DRX volume fractions,deformation temperatures and strain rates were obtained to optimize the process.Combined with microstructure analysis,it's concluded that 1200℃/0.01s~(-1)is the optimization parameter.Besides,both DDRX and CDRX were observed in theγphase evolution.The deformation mechanism from the inter-grain dislocation motion to the grain boundary migration and grain rotation was discussed.     

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.     

Influence of hot-rolling on the microstructure and mechanical properties of a near β-type Ti-5.2Mo-4.8Al-2.5Zr-1.7Cr alloy

In this work, the 90° clock rolling and the uni-directionally rolling processes at high temperature were carried out on the near β-type Ti-5.2Mo-4.8Al-2.5Zr-1.7Cr titanium alloy cutting from an ingot, respectively. The corresponding microstructures were quantitatively characterized, and its effect on the dynamic mechanical properties and fracture mechanism were emphatically investigated. It was found that after 90° clock rolling, the microstructure composed of equiaxed primary α phase(α_p) with an average size of about 2 ?μm and the β transformed regions containing the acicular secondary α phase(α_s) with an average thickness of about 50 ?nm and the separated β phase was obtained. However, in the uni-directionally rolled titanium alloy, no acicular α_s was observed, and the corresponding microstructure consisted elongated lamellar α phase (average thickness: about 1.3 ?μm), few equiaxed α phase (average grain size: about 300 ?nm) and the inlaid β phase. The microstructural difference of the hot-rolled titanium alloys was closely related to the deformation process. Moreover, a great number of α_p and α_s in the 90° clock rolled titanium alloy effectively enhanced the strength, and the dynamic compressive strength reached to 1730 ?MPa. Furthermore, equiaxed α_p was conducive to the homogeneous deformation, which counteracted the localized deformation caused by acicular α_s to a certain extent and made the 90° clock rolled titanium alloy exhibit an acceptable critical fracture strain of about 10.5%. Moreover, the fracture microstructures showed that the main failure mode of the 90° clock rolled and the uni-directionally rolled titanium alloy were ductile fracture and brittle fracture, respectively.     

Microstructure evolution and mechanical properties of a large-sized ingot of Mg-9Gd-3Y-1.5Zn-0.5Zr (wt%) alloy after a lower-temperature homogenization treatment

In this paper, a large-sized ingot of Mg-9Gd-3Y-1.5Zn-0.5Zr (wt%) alloy with a diameter of 600 mm was successfully prepared by the semi-continuous casting method. The alloy was subsequently annealed at a relatively low temperature of 430℃ for 12 h as a homogenization treatment. The microstructure and room-temperature mechanical properties of the alloy were investigated systematically. The results show that the as-cast alloy contained a mass of discontinuous lamellar-shaped 18R long-period stacking ordered (LPSO) phases with a composition of Mg₁₀ZnY and an α-Mg matrix, along with net-shaped Mg₅(Y,Gd) eutectic compounds at the grain boundaries. Most of the eutectic compounds dissolved after the homogenization treatment. Moreover, the amount and dimensions of the lamellar-shaped LPSO phase obviously increased after the homogenization treatment. The structure of the phase transformed into 14H-type LPSO with composition Mg₁₂Zn(Y,Gd). The mechanical properties of the heat-treated large-sized alloy ingot are uniform. The ultimate tensile strength (UTS) and tensile yield strength (TYS) of the alloy reached 207.2 MPa and 134.8 MPa, respectively, and the elongation was 3.4%. The high performances of the large-sized alloy ingot after the homogenization treatment is attributed to the strengthening of the α-Mg solid solution and to the plentiful LPSO phase distributed over the α-Mg matrix.     

Microstructure evolution and mechanical properties of a large-sized ingot of Mg–9Gd–3Y–1.5Zn–0.5Zr(wt%) alloy after a lower-temperature homogenization treatment

In this paper,a large-sized ingot of Mg–9Gd–3Y–1.5Zn–0.5Zr(wt%) alloy with a diameter of 600 mm was successfully prepared by the semi-continuous casting method.The alloy was subsequently annealed at a relatively low temperature of 430°C for 12 h as a homogenization treatment.The microstructure and room-temperature mechanical properties of the alloy were investigated systematically.The results show that the as-cast alloy contained a mass of discontinuous lamellar-shaped 18 R long-period stacking ordered(LPSO) phases with a composition of Mg10 Zn Y and an α-Mg matrix,along with net-shaped Mg5(Y,Gd) eutectic compounds at the grain boundaries.Most of the eutectic compounds dissolved after the homogenization treatment.Moreover,the amount and dimensions of the lamellar-shaped LPSO phase obviously increased after the homogenization treatment.The structure of the phase transformed into 14H-type LPSO with composition Mg12Zn(Y,Gd).The mechanical properties of the heat-treated large-sized alloy ingot are uniform.The ultimate tensile strength(UTS) and tensile yield strength(TYS) of the alloy reached 207.2 MPa and 134.8 MPa,respectively,and the elongation was 3.4%.The high performances of the large-sized alloy ingot after the homogenization treatment is attributed to the strengthening of the α-Mg solid solution and to the plentiful LPSO phase distributed over the α-Mg matrix.     

Microstructure evolution and mechanical properties of Ti_2AlNb/TiAl brazed joint using newly-developed Ti–Ni–Nb–Zr filler alloy

Joining of Ti_2AlNb alloy to TiAl intermetallics was conducted by the newly-developed Ti–Ni–Nb–Zr brazing filler alloy.The microstructure evolution of the joints was investigated by scanning electron microscope (SEM),energy dispersive spectrometer (EDS) and electron backscatter diffraction (EBSD).The macro-micro mechanical properties were studied by shear test and nano-indentation test.Typical interfacial microstructures across the brazing seam were Ti_2AlNb substrate,α_2-Ti_3Al+β-Ti,γ-TiAl+Ti_2Ni+TiNi+α_2-Ti_3Al,α_2-Ti_3Al+β-Ti,TiAl substrate.The Ti_2Ni phase were firstly dissolved in the joints brazed at 1000°C for 10 min and then precipitated after a prolonged holding time of 15 min.The nano-indentation test revealed that Ti_2Ni phase exhibited the highest hardness of 12.60 GPa.The joints brazed at 1000°C/15 min presented the maximum shear strength of271 MPa.The dissolution and precipitation behavior of Ti_2Ni phase was also discussed.