Microstructure and Texture Development in Ti-5Al-5Mo-5V-3Cr Alloy during Cold Rolling and Annealing

Microstructure and texture development in Ti-5Al-5Mo-

5V-3Cr alloy during cold rolling and annealing

Alireza Ghaderi1,2, a, Peter D Hodgson1,2, b and Matthew R Barnett1,2, c

1ARC Centre of Excellence for Design in Light Metals

2 Institute for Frontier Materials, Deakin University, Geelong, VIC 3216, Australia

[email protected], [email protected], [email protected]

Keywords: Ti-5Al-5Mo-5V-3Cr, cold rolling, texture, recrystallization

This study focuses on the microstructure and texture evolution of a Ti-5Al-5Mo-5V-3Cr alloy

during cold rolling and annealing treatments. Three samples with different initial microstructures

were cold rolled to a 40% reduction in thickness. The starting microstructure of one sample was

single β phase while two other specimens were α+β phases with different α particle sizes,

distributed in β grains. For all three samples, the average size of primary β grains was 150 µm. The

cold rolled specimens were then annealed at 860 °C (10 °C above the β transus temperature) for 5

minutes followed by water quenching. Microstructure development during cold rolling and

recrystallization was studied by scanning electron microscopy (SEM) and electron backscatter

diffraction (EBSD) technique. Microstructure investigations showed that massive amount of shear

bands occurred during the cold rolling of the single β phase sample while only a few shear bands

were observed in the α+β cold rolled microstructures. The cold rolled texture of the sample

comprised of a single β phase contains a gamma fibre (<111>//ND) and a partial alpha fibre

(<110>//RD). Annealing treatment decreased the intensity of the cold rolled texture in the single β

phase sample. Also, it was found that the presence of α precipitates changes the common annealing

texture observed in the single β phase specimen.

Introduction

The alloy Ti-5Al-5Mo-5V-3Cr (Ti5553) has been recently used in heavy section aerospace

components due to its high specific strength and excellent fracture toughness [1, 2]. This alloy is in

the category of metastable β alloys that contain single β phase at room temperature upon quenching

from the β phase field. They can be then aged, below the transus temperature, to precipitate a high

volume fraction of α particles throughout the β grains. The α particles inside the β grains act as

obstacles for dislocation glide resulting in strengthening of the material [3].

The average size of the β grains and their overall textures are important in terms of the strength

and ductility. In general in these alloys, grain size and bulk textures are optimized during hot

deformation processing such as forging or rolling. However, the processing window for stimulating

recrystallization is narrow and the stored energy is often released by dynamic recovery rather than

recrystallization, which causes non-uniform microstructures [4, 5].

The aim of this study is to investigate the evolution of microstructure in Ti5553 during cold

rolling and subsequent annealing. In particular, the main focus is to understand the effect of alpha

phase on the recrystallization behaviour and texture evolution of β phase.

Experimental

The material employed in this study was a 3.5 mm thickness sheet with the chemical

composition given in Table 1. The material was first annealed in a tube furnace at 900 °C for 30

minutes followed by quenching in water. Fig. 1 shows the typical microstructure of the annealed

sample, which contains equiaxed β grains with an average size of 150 µm measured by intercept

line method. Two pieces were cut from of the annealed sample and aged at two different

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temperatures and times. One piece was held at 670 °C for 30 minutes followed by quenching in

water (named as AG1) and the other one was aged at 770 °C for 300 minutes then water quenching

(named as AG2). Fig. 2 shows the typical morphology of α particles in the two aged samples. The

morphology and size of the α particles in the two aged samples are not uniform but the average size

of the alpha particles in sample AG1 is approximately one order of magnitude less than the average

size of the alpha particles in AG2. Also, β grain boundaries are covered by a thick layer of α

precipitation. This layer is thicker in sample AG2.

Table 1. Chemical composition of the raw material (wt.%).

Al Mo V Cr Ti

5.28 5.31 4.83 2.65 Balance

Fig. 1: Microstructure of the initial annealed sample, heat treated at 900 °C for 30 minutes followed

by water quenching (ST).

Fig. 2: Microstructure of the aged samples, a) at 670 °C for 30 minutes (AG1), b) at 770 °C for 300

minutes (AG2) then quenching in water.

The solution treated sample (named as ST) and two aged specimens were then cold rolled. For

each sample, the thickness was reduced from 3.5 mm to 2.1 mm (~40% reduction). The rolling was

carried out in several passes with a reduction of ~0.05 mm in each pass. It should be noted that

Key Engineering Materials Vol. 551 211

during cold rolling, several transverse cracks appeared at the edges of the AG1 sample. Annealing

treatment was conducted on the cold rolled plates in a fluid bed furnace at 860 °C (10 °C above the

β transus temperature) for 5 minutes followed by water quenching. The heating rate was

approximately 14 °C.s-1

. In order to minimize oxidation during the high temperature treatments, all

samples were coated with a glass based coating (Acheson’s Deltaglaze FB-412) as well as purging

Argon gas into the furnaces.

Microstructure investigation was carried out on the midsection of samples parallel to the rolling

direction, using the scanning electron microscope (SEM) equipped with different detectors. An

angular selective backscatter (ASB) detector was employed to examine the microstructure and an

electron backscatter diffraction (EBSD) detector equipped with the HKL Technology CHANNEL 5

EBSD software was used for data acquisition, orientation mapping and texture measurements.

Samples examined with the SEM were prepared by mechanical polishing finished with a mixture of

colloidal alumina slurry and hydrogen peroxide.

Results and Discussion

SEM image from the longitudinal section of the cold rolled sample ST is shown in Fig. 3. The β

grains are slightly elongated and prolific micro and shear banding can be seen in their interiors.

Small offsets suggest that at least some of the micro-bands carry shear. The microstructure image

with higher magnification (Fig.3 b) indicates large β grains are divided into small regions (~ 15 by

15 µm) due to formation of intersecting shear bands. The offsets at the intersections are

considerable but few large offsets were seen at grain boundaries. These observation are common for

deformed metastable β titanium alloys e.g. [6-8] as well as in other rolled bcc alloys e.g. [9-11]

indicating that strain is highly localized within the grains.

Fig. 3: a) SEM image of the cold rolled sample ST, the primary beta grain boundaries are

highlighted by dashed lines to guide the reader, b) higher magnification image showing shear and

micro-bands.

In contrast to the rolled ST sample, intersecting shear bands were not observed in the two other

specimens (i.e. AG1 and AG2). Fewer shear bands overall were observed in these samples but

significant shear offsets were seen at grain boundaries. This shows shear banding to be co-ordinated

in neighbouring grains and also suggests that the shear bands carried more shear in these samples.

(see AG2 in Fig. 4).

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Fig. 4: Shear bands (arrowed) in cold rolled samples AG1 and AG2.

The EBSD orientation maps of the samples annealed at 860 °C for 5 minutes are shown in Fig. 5.

Grain boundaries (black lines) are defined for those misorientations larger than 15°. Also, deformed

and recrystallized areas are indicated. These were determined using the “recrystallization fraction”

function in HKL CHANNEL 5 software and are superimposed on the orientation maps. For all three

annealed samples, the fraction of alpha phase detected by EBSD acquisition was less than 0.5%. As

can be seen, the unrecrystallized area (black regions) for sample ST was the highest with value of ~

20%. This fraction for AG1 and AG2 samples was approximately 5.5 and 9 precent, respectively.

Also, the average grain sizes of the two annealed AG1and AG2 samples are similar and are less

than the annealed ST sample.

Fig. 5: Unrecrystallized area (black regions) superimposed on EBSD orientation maps of annealed

samples at 860 °C for 5 minutes. Unrecrystallized regions are estimated by “recrystallization

fraction” function in HKL CHANNEL 5 software. Grain boundaries (black lines) are defined for

those misorientations larger than 15°.

The presence of α particles during the cold rolling clearly changes the deformation behaviour of

the β grains. This may account for the slightly accelerated recrystallization in the alpha containing

samples. However, in these cases the transformation of alpha to beta during annealing may impact

on the recrystallization kinetics. Certainly the finer grain size in these samples suggests that

nucleation was more prolific.

Key Engineering Materials Vol. 551 213

The textures of the starting material (i.e. annealed at 900 °C for 30 minutes) and the ST sample

after rolling and annealing are shown in the form of ODFs with ϕ2=45° sections in Fig. 6. The ideal

rolling texture components in bcc metals and alloys are {001}<110>, {112}<110>, {111}<110>,

{111}<112>, {554}<225>, and {110}<110> orientations [12]. Due to the spread of these ideal

orientations, it is very common in the literature to describe the rolling texture of bcc alloys with the

two main fibre textures [12]. The alpha fibre containing a group of components that have a <110>

axis parallel to the rolling direction and gamma fibre containing a group of orientations that their

<111> axis are parallel to the normal direction. For example, it has been shown that the cold rolled

textures of IF steel comprise a complete gamma fibre and a partial alpha fibre with the main

component of {112}<110> [12]. As can be seen in Fig. 6, the starting material has a weak texture

with a maximum intensity of 3 near the {001}<110> component. However, the cold rolled texture

can be described as a complete gamma fibre with a partial alpha fibre. This is consistent with the

cold rolled texture reported by Guro et al [13] on a Ti-10.5V-4.5Fe-1.5Al alloy. However, they

found this particular fibre (i.e. complete gamma fibre with a weak alpha fibre texture) during a

special rolling condition that involved 90 ° rotation around normal direction after each consecutive

pass. Conventional rolling only produced a Goss component {011}<100> and a weak gamma fibre

with localized maxima at {111}<110> with no trace of alpha fibre. Inoue et al [14] investigated the

texture development during cold rolling for other metastable β titanium alloys (Ti-15V-3Cr-3Al)

and they found that the alpha fibre texture is dominant.

Fig. 6: φ2=45 ° ODF sections showing texture for samples a) as received and annealed at 900 °C for

30 minutes, b) 40% cold rolled, c) annealed at 860 C for 5 minutes. Texture was measured

following using EBSD for more than 2000 grains (contour lines: 1,2,3,…. times random). Max. is

texture strength.

Fig. 6c presents the texture of the ST sample after 5 minutes annealing at 860 °C. It appears that

the overall texture of the recrystallized sample is similar to the cold rolled samples but with a

weaker texture and spreading the orientations over the alpha and gamma fibres. The

recrystallization texture of the metastable β titanium alloys is commonly seen to contain a gamma

fibre with a high intensity around the {111}<112> component [13-16]. However, it has been shown

that the annealing texture is sensitive to the time and temperature of annealing as well as the history

of the deformed texture [13-16].

The annealing textures of the AG1 and AG2 samples are shown in Fig. 7. As can be seen, both

samples have approximately similar textures, though the intensity of the texture in sample AG1 is

slightly higher than sample AG2. The alpha particles have a significant effect on texture evolution

during annealing. This may be due to the impact of alpha on shear banding. It is known that shear

banding impacts strongly on recrystallization. It is also known that occurrence of shear bands can be

affected by the second phase leading to a different texture. For example, it has been shown that the

presence of second phase in an Al-Al3Ni eutectic alloy has a significant effect on texture intensity

[17]. The related TEM investigation showed that second phase prevents formation of shear bands

and instead develops formation of subgrains. More work is required to test this hypothesis.

214 Cost-Affordable Titanium IV

Fig. 7: φ2=45 ° ODF sections showing annealing texture of AG1 and AG2 samples. Texture was

measured following using EBSD for more than 2000 grains (contour lines: 1,2,3,…. times random).

Max. is texture strength.

Summary

The effect of initial microstructure on cold rolling and annealing behaviour of Ti-5Al-5Mo-5V-

3Cr alloy was investigated and the conclusions are summarized as follows.

1- In the cold rolled microstructure of single β phase sample, massive amount of micro and shear

bands were observed while only a few shear bands were seen in the α+β cold rolled sample.

2- Recrystallization is slightly accelerated in the samples containing α phase.

3- Texture of 40% cold rolled sample with single β phase contains a complete gamma fibre with

a partial alpha fibre. Texture of the annealed sample is similar to the cold rolled sample but with a

weaker texture.

4- The presence of α precipitates during the cold rolling changes the annealing texture observed

in the single β phase specimen.

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