简介概要

Hot deformation behavior and globularization mechanism of Ti-6Al-4V-0.1B alloy with lamellar microstructure

来源期刊：Rare Metals2013年第2期

论文作者：Yang Yu Bai-Qing Xiong Song-Xiao Hui Wen-Jun Ye

文章页码：122 - 128

摘要：Hot deformation behavior and globularization mechanism of Ti-6Al-4V-0.1B alloy with lamellar microstructure were quantitatively studied through isothermal compression tests with the temperature Hot deformation behavior and globularization mechanism of Ti-6Al-4V-0.1B alloy with lamellar microstructure were quantitatively studied through isothermal compression tests with the temperature range of 850-950 °C and strain rate range of 0.01-1.00s-1 . The results show that the peak flow stress and steady stress are sensitive to the strain rate and temperature. The value of deformation activation energy is 890.49 kJ mol-1 in （α+β） region. Dynamic recrystallization is the major deformation mechanism. Flow softening is dominated by dynamic recrystallization at 850-950 °C. TiB particles promote the recrystallization of a laths. Globularization processes consist of four steps: formation of subgrain after dynamic recovery in α plates; subgrain boundary migration caused by α/β interfacial instability; interfacial migration promoting β phase wedge into α phase; disintegrating of α laths by diffusion processes; and grainboundary sliding. Globularization mechanisms during hot deformation processes of the Ti-6Al-4V-0.1B alloy with lamellar structure are continuous dynamic recrystallization.

Rare Metals 2013,32(02),122-128+2

Hot deformation behavior and globularization mechanism of Ti-6Al-4V-0.1B alloy with lamellar microstructure

Yang Yu Bai-Qing Xiong Song-Xiao Hui Wen-Jun Ye

State Key Laboratory of Nonferrous Metals & Processes,General Research Institute for Nonferrous Metals

作者简介：Yang Yu e-mail:yuyang@grinm.com;

收稿日期：22 February 2012

基金：supported by the International Science and Technology Corporation Foundation of China(No.2012DFG51540);

Hot deformation behavior and globularization mechanism of Ti-6Al-4V-0.1B alloy with lamellar microstructure

Abstract：

Hot deformation behavior and globularization mechanism of Ti-6Al-4V-0.1B alloy with lamellar microstructure were quantitatively studied through isothermal compression tests with the temperature range of 850-950 °C and strain rate range of 0.01-1.00s-1 . The results show that the peak flow stress and steady stress are sensitive to the strain rate and temperature. The value of deformation activation energy is 890.49 kJ mol-1 in (α+β) region. Dynamic recrystallization is the major deformation mechanism. Flow softening is dominated by dynamic recrystallization at 850-950 °C. TiB particles promote the recrystallization of a laths. Globularization processes consist of four steps: formation of subgrain after dynamic recovery in α plates; subgrain boundary migration caused by α/β interfacial instability; interfacial migration promoting β phase wedge into α phase; disintegrating of α laths by diffusion processes; and grainboundary sliding. Globularization mechanisms during hot deformation processes of the Ti-6Al-4V-0.1B alloy with lamellar structure are continuous dynamic recrystallization.

Keyword：

Ti-6Al-4V-0.1B; Hot deformation; Lamellar microstructure; Globularization mechanism;

Received： 22 February 2012

Abstract Hot deformation behavior and globularization mechanism of Ti-6Al-4V-0.1B alloy with lamellar microstructure were quantitatively studied through isothermal compression tests with the temperature range of 850–950°C and strain rate range of 0.01–1.00 s^-1.The results show that the peak flow stress and steady stress are sensitive to the strain rate and temperature.The value of deformation activation energy is 890.49 k J?mol^-1in(a?b)region.Dynamic recrystallization is the major deformation mechanism.Flow softening is dominated by dynamic recrystallization at 850–950°C.Ti B particles promote the recrystallization of a laths.Globularization processes consist of four steps:formation of subgrain after dynamic recovery in a plates;subgrain boundary migration caused by a/b interfacial instability;interfacial migration promoting b phase wedge into a phase;disintegrating of a laths by diffusion processes;and grainboundary sliding.Globularization mechanisms during hot deformation processes of the Ti-6Al-4V-0.1B alloy with lamellar structure are continuous dynamic recrystallization.Keywords Ti-6Al-4V-0.1B;Hot deformation;Lamellar microstructure;Globularization mechanism

1 Introduction

Ti-6Al-4V alloy,which is usually used in aerospace,power generation,and chemical and biomedical applications[1,2],possesses a number of characteristics,such as high specific strength and excellent corrosion resistance[1,3].In practical application,Ti-6Al-4V is also the most commonly used titanium alloy,which occupied about 50%of total weight of titanium.However,Ti-6Al-4V alloy is more expensive than other structural metal materials,such as aluminum alloys,irons,and steels.Recent research shows that the additions of trace amounts(*0.1 wt%)of B can decrease the grain size of the ingot by approximately an order of magnitude[4].Trace amounts of B additions can also reduce the thickness of the brittle grain-boundary a phase,which will improve the hot workability[5].It also has the potential to reduce the thermo-mechanical processing step compared with the traditional work with the improvement of ingot microstructure.In previous research papers,most studies focused on the effect of B addition on the microstructure and properties[6–10],whereas a few systematic investigation on the hot deformation behavior has been reported.In fact,because of high cost and working complexity of titanium alloys,experimental testing of material plays a significant role for process simulation and optimization.

The objective of the present work was to obtain a more quantitative understanding of the flow behavior during subtransus hot working and globularization process in Ti-6Al-4V-0.1B alloy with lamellar microstructure.Isothermal hot compression tests were conducted on Ti-6Al-4V-0.1B with lamellar structure to obtain data for flow behavior modeling.Quantitative microscopies were used to quantify microstructure evolution and globularization mechanism.

2 Experimental

A 16.5 kg Ti-6Al-4V-0.1B ingot was melted through twice vacuum arc furnace.The corresponding electrode was made by Al-55V,Al-22Ti-7B,aluminum foil,and sponge titanium.The chemical composition of ingot is listed in Table 1.The b transus temperature measured was about(970±10)°C.The ingot was forged and rolled into U12 mm bars and then heat treated at 1020°C for 30 min,followed by air cooling.The microstructure of the heat treated alloy is given in Fig.1.The microstructure shows a colony a structure consisting of lamellar a colony.It was found that Ti B particles were randomly oriented in the matrix.

Table 1 Chemical composition of Ti-6Al-4V-0.1B alloy(wt%) 下载原图

Table 1 Chemical composition of Ti-6Al-4V-0.1B alloy(wt%)

Fig.1 Initial microstructure of Ti-6Al-4V-0.1B alloy

Compression specimens were machined from the heat treated bars with 8 mm in diameter and 15 mm in height.The isothermal compression tests were performed on Gleeble1500D thermal simulator in the temperature range850–950°C,with the initial strain rates of 0.01–1.00 s^-1and height reduction of 15%–60%.The specimens were heated to test temperature at heating speed of 5°C?s^-1,held for 3 min,compressed to the given reduction,and then water quenched to room temperature.

The deformed specimens were sectioned parallel to the compression axis for microstructure analysis.Characters of microstructure during hot deformation globularization processes of Ti-6Al-4V-0.1B alloy with lamellar structure were investigated by scanning electron microscopy(SEM),electron backscatter diffraction(EBSD),and transmission electron microscopy(TEM).

3 Results and discussion

3.1 True stress-true strain curves

The true stress-true strain curves of Ti-6Al-4V-0.1B alloy compressed at 850,900,and 950°C under different strain rates are shown in Fig.2.The flow stress decreases with increasing deformation temperatures for the given strain rate while the flow stress increases with increasing strain rate for the given temperature.Flow stress is shown to be sensitive to deformation temperature and strain rate.The curves exhibit a flow softening behavior,in which the flow stress reaches a peak caused by work hardening at critical strain and then decreases with further straining.The flow softening may be relative to deformation heating and the change of microstructure including dynamic recovery and recrystallization.

Fig.2 True stress–strain curves of Ti-6Al-4V-0.1B alloy compressed at a 850°C,b 900°C,and c 950°C with different strain rate

The kinetic equation for hot deformation related with the flow stress(σ),deformation temperature(T),and strain rate is

where A is constant,n is the stress exponent,a is constant Q is the activation energy of deformation(J?mol^-1),and R is gas constant.

Fig.3 Relationship between ln r and ln_e at different temperatures for Ti-6Al-4V-0.1B alloy

After doing the partial differential to Eq.(1),the following equation is obtained:

Figure 3 shows the relationship between ln r and ln_e at different temperatures.From the plot,a linear relationship is observed,suggesting that the equation is valid,and its slope represents the stress exponent(n)[11,12].

Figure 4 shows the relationship between ln_e and ln(sinh(ar)),ln(sinh(ar)),and 1/T.The average rate of the slope of linear fit in Fig.4a,b,respectively,represent two items of Eq.2.The activation energy of deformation can be calculated to be 890.49 k J?mol^-1.The activation energy of deformation is higher than the self-diffusion energy and lattice diffusion activation energy of pure b-Ti(153 k J?mol^-1)and a-Ti(169 k J?mol^-1).As known,when the activation energy is close to self-diffusion,dynamic recovery is the restoration mechanism during hot deformation.However,when the activation energy is higher than self-diffusion energy,it is considered that dynamic recrystallization occurs during hot deformation.Therefore,it is concluded that dynamic recrystallization is likely the main mechanism for deformation in the a?b region of Ti-6Al-4V-0.1B alloy.As reported by researchers,Ti-6Al-4V alloy,the activation energy of deformation is 453 k J?mol^-1[13].For Ti-6Al-4V-0.1B alloy higher activation energies are observed,which may be attributed to the presence of additional Ti B phase.More activation energy causes more recrystallization to occur.Recrystallization is more likely to occur in Ti-6Al-4V-0.1B alloy at the same deformation parameters.

3.2 Microstructure evolution

Figure 5 shows the SEM morphologies of Ti-6Al-4V-0.1B alloy after deformation at 850°C with strain rate of 1.00 s^-1.As shown in Fig.5a,a completely deformed microstructure with a lath bending and kinking as the primary deformation mechanism can be seen in small deformation zone.a laths are moved and rotated during compression,which will rotate the neighboring colonies to favorable orientations.The cavities around Ti B particles are also cleared visible a850°C,1 s^-1.It is suspected that Ti B particles,which are hard and brittle,are difficult to deform in the ductile metal matrix phase.Therefore,a strain accumulation due to the dislocation pile ups around the particles occur and leads to the cavity formation.The microstructure of the samples in large deformation zone is obviously different with that in small deformation zone,as shown in Fig.5b.After large deformation,the bigger a laths are broken into small equiaxed a grains surrounded by b phase.It is noted that almost all a laths structure is fully globularized.Globular a grains are not elongated along the deformation direction,indicating that dynamic recrystallization is the major deformation mechanism.

Fig.4 Curves of a ln_e?ln?sinh?ar??and b ln?sinh?ar???1=T for Ti-6Al-4V-0.1B alloy

Figure 6 shows the SEM-EBSD micrographs of Ti-6Al-4V-0.1B alloy with different strains and strain rate a900°C.It can be found that the micrograph in Fig.6a contains a large amount of lamellar structure and parts of a lath bending and kinking with strain of 0.163 at 900°C With the increase of strain,a laths are moved,rotated,and aligned in stress direction.Moreover,the a laths orientation trend is uniform.A small amount of kinked lamellae and a little of globular or equiaxed morphology are detected at the cross point of different a colony with strain of 0.357(Fig.6b).Micrographs in Figs.6c and d show the effect of strain rate on dynamic globularization development of lamellar structure.Comparing the microstructure at higher strain rate 1.00 s^-1(Fig.6c),there is almost full globularization with fine globularized grains and full globularized structure with coarser a grains at low strain rates 0.01 s^-1(Fig.6d)at same temperatures.The recrystallization equiaxed a grains growth remarkably at a lower strain rate.More fragmentation of bigger a laths is change into small equiax a grains surrounded with Ti B particles(Fig.6b).It is probably related with stress field,which forms during inconsistent deformation between Ti B and metal matrix.Stress field can promote the recrystallization of a laths.

Fig.5 Microstructure of specimens deformed at 850°C with strain rate of 1 s-1a small deformation zone and b large deformation zone

Distributions of a phase misorientation of Ti-6Al-4V-0.1B alloy with different strains at 900°C and strain rate of1.00 s^-1are shown in Fig.7.It can be calculated that misorientation distribution in Fig.7a shows lowering of the fraction of low-angle boundaries(misorientation less than15°)with initial microstructure.High-angle boundaries(misorientation more than 15°)have two peaks in 55°–65°and 85°–90°.A large number of low-angle boundaries are formed during high-temperature deformation.This process is driven mainly by the energy stored in the low-angle boundaries.With the increase of strain,the fraction of the low-angle boundaries is increasing from 0.1 to 0.4;highangle boundaries in 55°–65°is decreasing at the same time(Fig.7b).After deformation to large strains(Fig.7c),the fraction of low-angle boundaries is decreasing from 0.40 to0.26.Transformation from low-angle boundaries to highangle boundaries is continuous.Minor boundary movements during the deformation may result in a fine-grained microstructure,consisting mainly of crystallites that are surrounded by high-angle boundaries.Continuous recrystallization can be formulated in terms of continuous distribution of a phase misorientation at large strains.It is indicated that microstructure evolution mechanism is continuous dynamic recrystallization.

3.3 Globularization mechanism of lamellar

Typical TEM observation microstructures obtained in a specimen deformed at 900°C with the strain rate of 1 s^-1are shown in Fig.8.It is found that the a phase exhibits high dislocation density(Fig.8a).Curved and pinned dislocations at the a/b boundaries and complex three-dimensional dislocation network in the interior a phase are observed.More dislocations are produced,and the dislocation walls are denser due to poorer recovery rate[14]in Fig.8b.When the deformation continues,the dislocation walls transfer to subgrains.As a result,subgrain aggregates at boundaries of a/b.The balance of composition is broken at a/b boundaries and thermal groove appears there[15]Thermal groove indicates balance operations by subgrain boundaries and interfacial force.Therefore,this shows the depth of thermal groove associated with boundary energy c_a/aand a/b interface energy.The angle h at therma groove,as shown in Fig.9,is determined by the balance between the force arising from the boundary energy c_a/aand interface energy c_a/b;it is given by:

According to Fig.9 and Eq.4,with the decrease of the angle of 2h,the depth of thermal groove becomes larger.A the same time,b phase wedged into a phase and energy of c_a/ainterface is higher.The value of c_a/bis constant;c_a/aand cosh are positively correlated.When a/a boundary is a small angle boundary,with the increasing of a/a boundary angle,the value of c_a/awill increase.However,when a/a boundary is a large angle boundary,the value of c_a/ais unrelated with the angle of a/a interface[15].The curvature radius of the a/b interface,which is located near the thermal groove,is smaller than that of the a/b interface,which is located at the groove,resulting from the presence of nearby a phase stabilizing elements Al and b phase stabilizing elements V concentration gradient,and thus,we have the concentration gradient of the drive a phase dissolution,so the phase interface is moved.Interfacial migration promotes b phase to wedge into a phase(Fig.8c).Then,the b phase will throw out a phase,resulting in lamellar microstructure disintegration.The breakup of the lamellar begins at formation of interphase boundary in the lamellar.Migration of the interfaces to form a globular shape occurs through diffusion processes driven by the trend to minimize the interfacial energy or the surface tension(Fig.8d).To sum up,globularization process may consist of four steps:(1)formation of subgrain after dynamic recovery in a plates,(2)subgrain boundary migration caused by a/b interfacial instability,(3)interfacial migration promoting b phase wedge into a phase,and(4)disintegration of a laths by diffusion processes and grain-boundary sliding.Globularization mechanisms during hot deformation processes of the alloy with lamellar structure are continuous dynamic recrystallization occurring in a lath.

Fig.6 SEM-EBSD microstructure observation in specimens deformed at 900°C,a e=0.163,1 s-1,b e=0.357,1 s-1,c e=0.916,1 s-1,d e=0.916,0.01 s-1

Fig.7 Misorientation angles distribution of a phase in microstructure of lamellar deformed at 900°C,1 s-1and with strain of a e=0,b e=0.357,and c e=0.916

Fig.8 Globularization processes of a lamellar.a accumulation of dislocation in a phase;b formation of subgrain in a plates;c b phase wedge into a phase;d formation of globular a phase in structure

Fig.9 Thermal groove on a/b interface[13]

4 Conclusion

The flow behavior of Ti-6Al-4V-0.1B alloy is sensitive to the deformation temperature and strain rate.The activation energy for hot deformation in a–b range is 890.49 k J?mol^-1Ti B phase improves the activation energy for hot deformation of Ti-6Al-4V alloy.

Softening mechanism is dominated by dynamic recrystallization at 850–950°C.Ti B particle promotes the recrystallization of a laths.

Globularization mechanisms during hot deformation processes of the Ti-6Al-4V-0.1B alloy with lamellar structure are continuous dynamic recrystallization.