Flow behaviour and microstructure during hot deformation of Ti-5Al-5Mo-5V-3Cr-1Zr F. Warchomicka1, M. Stockinger2, H. P. Degischer1 1 Institute of Materials Science and Technology, Vienna University of Technology, Karlsplatz 13/E308, A-1040 Vienna, Austria 2 BÖHLER Schmiedetechnik GmbH & Co KG, Mariazellerstraße 25, A-8605 Kapfenberg, Austria The near beta titanium alloys are widely used for aerospace applications in structural parts. They also present a better forgeability at lower temperature than the α + β titanium alloys. In this work, a modified version of alloy VT22, Ti-5Al-5Mo-5V-3Cr-1Zr (Ti-5-5-5-3-1), is studied to determine the flow behaviour in the α+β and β field and the effect of the temperature, time, strain and strain rate on the microstructure. Compression tests and heat treatments were performed by means of Gleeble® 1500 machine around β-transus temperature. Compression tests were carried out at 0.01, 0.1 and 1s-1 of constant strain rate. In the α+β field, the flow curves show a continuous softening at 0.1 and 1s-1 and steady flow at 0.01s-1 of strain rate. Above β-transus temperature, a steady state flow for all the strain rates is observed. The evolution of the globular α phase and the β sub-grains before and after the deformation are examined by quantitative analysis of SEM images to identify the deformation mechanisms dominating during hot working. Keywords: Near beta titanium alloys; Ti-5-5-5-3-1; Hot deformation; Flow Curves; Microstructure. 1. Introduction

The new generation of commercial planes are changing the concept of material selection for aerospace application. Alloys with good specific stiffness and strength are required to replace structural steel parts with the intention to reduce the total weight and thus the fuel consumption per flight. In addition, they should show high fracture toughness, good fatigue and corrosion properties to increase lifetime of the part.

Near beta titanium alloys are very attractive in the aerospace industry. They have a very good hot workability and low β-transus temperature (Tβ). A wide range of mechanical properties can be obtained by heat treatments. These alloys are currently used for landing gear components and flap tracks and in near future also for parts in the fuselage, nacelles and wing1).

The modified versions of titanium alloy VT 22 present better strength, toughness ductility and hardenability than Ti 1023 and VT 222,3).

For the alloy Ti-5Al-5Mo-5V-3Cr-1Zr (Ti-5-5-5-3-1) Zirconium was added to reduce the β-transus temperature in comparison with similar alloys (TIMETAL 555 and VT 22). The objective of this work was to study the behaviour of the Ti-5-5-5-3-1 during deformation below and above β-transus temperature and determine the mechanisms involved. Compression tests were carried out to examine the flow behaviour of the material and to analyse the microstructure for different strain rates. Therefore, these results will help to develop constitutive models that relate the forging parameters to the microstructure evolution and thus allow to improve the simulation procedures of the forging process. 2. Material and Procedure

The nominal chemical composition of the investigated Ti-5-5-5-3-1 in wt% is: 5.04V, 5.51Al, 5.01Mo, 2.85Cr 0.32Fe, 1.125Zr, 0.0655O, 0.0045C, 0.0085N and Ti. The Tβ determined by the supplier for this alloy is 803°C3). The double melt material has been pre-forged (by

cogging) to billet dimensions with final step in the α+β field.

Cylindrical specimens of 15 mm length and 10 mm diameter were machined with the cylinder compression axis perpendicular to the cogging direction.

Figure 1. Scheme of the compression test machine.

Hot compression tests were carried out using a Gleeble®1500 servo hydraulic system machine (Figure 1) at constant strain rates of 0.01, 0.1 and 1s-1 at temperatures between Tβ±40°C, up to a true strain of 0.7. The specimens were heated to the compression temperature at a heating rate of 10K/s and were held at the deformation temperature to reach the equilibrium of α phase below the Tβ, and a full α transformation into β field above Tβ. The temperature was controlled by a type K thermocouple welded in the middle of the specimen. Three additional thermocouples were used to record the temperature gradient along the specimen. A combination of graphite and molybdenum foils was used to reduce the friction between the anvils and the specimen as well as the gradient of temperature along the specimen. The deformation for all the tests was controlled by the stroke and measured by means of a transverse strain gauge located at the middle of each specimen to avoid any effect of both gradient of temperature and friction on the flow curves. Furthermore, heat treatments at different holding

11 th World Conference on Titanium, Kyoto/ Japan; 03.06.2007 - 07.06.2007; in: "Proceedings of Ti-2007, The 11th World Conference on Titanum", (2007), ISBN: 978-4-88903-406-6; S. 511 - 514.

times were carried out in the Gleeble®1500 machine to determine the alpha phase transformations. The tests were carried out in argon atmosphere. In order to analyse the microstructure, the samples were water quenched.

The samples were cut parallel to the axis of load and mechanical ground and polished. They were examined using a Scanning Electron Microscope (SEM) in the Backscattered Electron (BSE) mode to distinguish the α from the β phase. Microstructure images for each specimen were analysed by means of the software AxioVs40 v4.4 and the β sub-grain size was calculated using the mean linear intercept method according to the ASTM Standard4). A mean β sub-grain diameter was determined from the measurement in both directions: perpendicular and parallel to the compression axis. 3. Results and Discussion 3.1 Microstructure Figure 2 shows the microstructure in as received condition. It presents a globular alpha phase in a fine grained β field as product of the heat treatment carried out yielding a content of alpha phase of around 25%.

Figure 2. Microstructure of the as received material, where the globular alpha phase is dark and the beta sub-grains are bright, but differently shaded.

During heat treatment at different holding times in the α+β field at 763°C, the content of alpha phase decreased till the equilibrium was reached. Immediately when the temperature was attained, the content of alpha was 26%. It decreased to ~14% after 300 seconds there after it remains constant. The area of alpha grains remained also almost constant in same period but their shape became more rounded with time and temperature as Figure 3 shows.

In the beta field at 843°C, the α phase was completely transformed into the β phase after 180 seconds. The beta sub-grains (originally ~ 2 µm in diameter) started to grow abnormally and after 900 seconds, fully recrystallized microstructure with a mean β grain diameter of 79 ± 3µm was observed. Figure 4 shows the beta grain growth at 843°C.

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Figure 4. Microstructure of specimens heat treated at 843°C. a) After 180sec. Where there is not indication of alpha phase and abnormal grain growth is observed locally. b) After 900sec. The beta phase is fully recrystallized 3.2 Hot deformation 3.2.1 Flow behaviour

In the α+β field the flow curves showed a softening behaviour at 0.1 and 1s-1, while the tests carried out at 0.01s-1 of strain rate showed steady state flow. In the β field, all the curves presented steady state flow. Flow curves at 763 and 843°C are shown in Figure 5. A strong influence of the strain rate on the flow behaviour was observed.

11 th World Conference on Titanium, Kyoto/ Japan; 03.06.2007 - 07.06.2007; in: "Proceedings of Ti-2007, The 11th World Conference on Titanum", (2007), ISBN: 978-4-88903-406-6; S. 511 - 514.

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Figure 5. Flow curves obtained for different strain rates at a) 763 and b) 843°C.

After an initial sharp peak, discontinuous yielding was

observed at 0.1s-1 for all the temperatures, and at 1s-1 in the β field (see Figure 5). This phenomenon was reported5) as to be produced by a fast generation of mobile dislocation from grain boundary sources, where hot deformation proceeds from the grain boundaries inwards. The ratio of the stress drop depends on the strain rate, temperature and content of β−stabilizers5,6). Figure 6 shows the variation of yield drop in percentage ((σpeak - σdrop)/ σpeak). It is larger at temperatures in β field.

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Figure 6. Yield drop percentage of Ti-5-5-5-3-1 for different temperatures and strain rates.

3.2.2 Microstructure after deformation Figure 7 shows the microstructural features of the

specimens before and after deformation at 0.01 and 1s-1 and 0.7 of true strain in the α+β field, 40°C below Tβ. Along the radius in the middle of the cylindrical specimen, the area of alpha grains decreased towards the centre of the specimen as a consequence of the Joule effect during heating. After deformation, the alpha grains did not change significantly in area and Feret ratio (Feret Min / Feret Max lengths) but a significant difference in the β sub-grains size between both strain rates.

A comparison between specimens before and after deformation revealed the deformation of the α grains and their orientation almost perpendicular to the compression axis. Near to the border, where the strain is lower than in the middle of the specimen, the shape of the alpha grains is quite similar to the non-deformed sample. While at the centre, the α grains become elongated. The β sub-grains became slightly elongated parallel to the α grains at the strain rate of 0.01s-1 while at high strain rate, a refinement of the grains was observed (mean value around 1.5 µm diameter). This suggests dynamic recrystallization (DRX). EBSD technique will be needed to verify this mechanism.

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Figure 7. Microstructural features comparison in function of a radius normalized in the middle of the specimens before and after deformed at 763°C. From top to bottom: Feret Ratio of α grains, the mean area of α grains and the mean area of β sub-grains.

11 th World Conference on Titanium, Kyoto/ Japan; 03.06.2007 - 07.06.2007; in: "Proceedings of Ti-2007, The 11th World Conference on Titanum", (2007), ISBN: 978-4-88903-406-6; S. 511 - 514.

Figure 8 shows an overview of the different microstructures at the centre of the specimens before and after deformation in both α+β and β field at 0.7 true strain and at different strain rates.

In the β field, Figures 8d) to 8f) compare the β grains. At strain rate of 1s-1 (8f)) a necklace structure is perceptible, indicating an incomplete DRX. At lower

strain rate DRX can be also observed with a higher amount of growth after the nucleation and therefore a larger mean of β grain size than at 1s-1.

11 th World Conference on Titanium, Kyoto/ Japan; 03.06.2007 - 07.06.2007; in: "Proceedings of Ti-2007, The 11th World Conference on Titanum", (2007), ISBN: 978-4-88903-406-6; S. 511 - 514.

Figure 8. Summary of the microstructure obtained before and after deformation at the centre of the specimen. a) b) and c) at 763°C without deformation, deformed to 0.7 at 0.01s-1 and 1s-1 respectively. d) at 843°C and 15min holding time without deformation, e) and f) necklace details (DRX) of specimens deformed at 843°C to 0.7 at 0.01s-1 and 1s-1 respectively. Compression axis is vertical. 4. Conclusion

This work has shown the sensitivity of the microstructure of Ti-5-5-5-3-1 to the strain rate and strain during deformation below and above Tβ: 1. Below β-transus temperature, softening behaviour was observed for those tests carried out at 0.1 and 1s-1 and steady state flow at 0.01s-1 of constant strain rate. During deformation, the α grains were deformed and orientated almost perpendicular to the load. Refinement of β sub-grains suggested dynamic recrystallization at 1s-1 2. Above β-transus temperature, steady state flow was observed for all the strain rates and temperature. Dynamic recrystallization took place during deformation. 3.Discontinuous yielding phenomenon was observed mainly at 0.1s-1 and 1s-1 during deformations in beta field.

Acknowledgement This work was carried out within the project

“Modelling of microstructure and microstructure-dependent mechanical properties of near β titanium alloys” sponsored by the Austrian Aeronautic programme “TAKE OFF” and Böhler Schmiedetechnik GmbH & Co KG. REFERENCES 1) R.R. Boyer, R.D. Briggs: JMEPEG 14 (2005) pp. 681-685. 2) J.C. Fanning, R.R. Boyer: Ti 2003: Science and Technology, Wiley VCH, Germany, 2004, pp 2643-2650. 3) Materials supplier receiving inspection Report, VSMPO, 2005 4) ASTM E112 Standards Test Method for determining average grain size, Annual Book of ASTM Standards, Vol. 03.01, pp. 243-266 5) M. Long, H. J. Rack: Titanium ‘95: Science and Technology, The Institute of Materials, London, 1996, pp. 316 – 323. 6) M. N. Vijayshankar, S. Ankem: Titanium ‘92: Science and Technology, TMS, Warrendale, PA, 1993, pp. 1733 – 1739.

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Necklace structure

d) e) f)100µm 20µm 20µm

Flow behaviour and microstructure during hot deformation of Ti-5Al-5Mo-5V-3Cr-1ZrKeywords: Near beta titanium alloys; Ti-5-5-5-3-1; Hot deformation; Flow Curves; Microstructure.

1. Introduction 2. Material and Procedure 3. Results and Discussion4. ConclusionAcknowledgementREFERENCES