Microstructure after deformation(a)• 763°C: 0.001 s-1 and 10 s-1• Dynamic recovery• Geometric dynamic recrystallization at the

grain boundary markedly for 0.001 s-1• High angle grain boundaries become

closer, subgrain size depends on T,(b)• 843°C: 0.001 s-1 and 10 s-1• Subgrain structure formation decreases

with increasing strain rate• Misorientation within each grain increases

with increasing strain rate(c)• Small grains on the beta grain boundary

at high strain rates are due to recrystallization by lattice rotation

(d) • New small grains = recrystallized = blue• Grain consist of subgrains = red =

recrystallized• Grain: really misorientation = red

Flow behaviour

Processing maps

• Near beta alloy Ti-5Al-5Mo-5V-3Cr-1Zr (Ti55531) with a globular starting microstructure• Deformation tests carried out using a Gleeble®1500 machine • Beta transformation temperature: 803°C• Strain rates from 0.001 – 10 s-1 up to a true strain of 0.7 • Temperatures from 763 – 843°C controlled with a welded K-thermocouple• Argon atmosphere and air quenching• Lubricant: graphite and molybdenum foil between sample and anvil• EBSD measurements carried out for samples deformed in α/β- and β-field• Processing maps were implemented using Matlab

EXPERIMENTAL

Hot deformation and processing maps of a near β - Ti alloyM. Dikovits1, C. Poletti1, F. Warchomicka2

1. Graz University of Technology, Institute for Materials Science and Welding, Kopernikusgasse 24, 8010 Graz, Austria2. Vienna University of Technology, Institute of Materials Science and Technology, Karlsplatz 13, 1040 Wien, Austria

Institute for Materials Science and Welding (IWS)Graz University of Technology, Austria

RESULTS & DISCUSSION

Processing maps of different metal alloys can be used to describe the deformation behaviour as a function of and σ, to optimize thermomechanicalprocessing parameters and to avoid defects in the products.• Processing maps consist of 2 superimposed maps: dissipation efficiency- (η) and instability- (κ) map• General model was defined by Prasad using dynamic material model [1], modified model developed by Murty and Rao [2]

• The η value is defined for both models as η = J/Jmax = 2J/P but calculated differently. In the modified model η = 2

• Instability parameter κ defined by Murty and Rao is given as: κ = 2m/η – 1 and predict instability by κ < 0

The aim of this work was to study the deformation behaviour of Ti-5Al-5Mo-5V-3Cr-1Zr by means of compression tests, metallography and to correlate themicrostructure with the processing maps.

ACKNOWLEDGEMENTS TOFWF (Austrian Science Fund) for supporting project P22238-N22, Böhler Schmiedetechnik for the provision of the material and to USTEM (Vienna University of Technology) for the provision of the FEG-SEM EBSD facilities.

REFERENCES[1] Y.V.R.K. Prasad, H.L. Gegel, Metallurgical Transactions A, pp. 1883-1892, Vol. 15A, 1984.[2] S.V.S. Narayana Murty, B. Nageswara Rao, Journal of Materials Science Letters, pp. 1203-1205, Vol.17, 1998.[-] Y.V.R.K. Prasad, Metallurgical and Materials Science and Engineering A243, pp. 82-88, 1998.[-] S.V.S. Narayana Murty, B. Nageswara Rao, Journal of Materials Processing Technology, pp. 279-285, 2005.[-] M. Peters, C. Leyens, Titan und Titanlegierungen, pp.139-161, 2002.[-] F.J. Humphreys, M. Hantherly, Recrystallization and related annealing phenomena, pp. 415-467, 2004.

Fig. 1: Scheme of the Gleeble simulator

Fig. 2: Flow curves at (a) 763°C and (b) 843°C

• Influence of the temperature and strain rate on the flow curves• At strain rates higher than 1 s-1 softening, due to adiabatic flow occurs• At low strain rates: steady state takes place• Increasing the deformation temperature: stress values decrease

β-Ti

• The efficiency map shows a stronger dependence of the flow behaviouron the strain rate in the α/β-field than in the β-field.

• At moderate strain rates the highest η value can be seen and η remainsconstant with increasing strain.

• For low and high strain rates the η value is smaller and stronglydependent on the temperature and strain rate. η is increasing with thestrain.

• A big area of flow instabilities is related to inhomogeneous β graindeformation and not to damage.

A = 0.001 s-1B = 0.01 s-1C = 0.1 s-1D = 1 s-1E = 10 s-1

(a) α/β-field (b) β-field

Fig. 3: EBSD Measurements of β-phase (α in black), IPF (for a, b and c), grain boundaries: black, subgrain boundaries: grey(a) 763°C: 1) 0.001 s-1, 2) 10 s-1; (b) 843°C: 1) 0.001 s-1, 2) 10 s-1; (c) Small grains on grain boundary (823°C, 1 s-1);(d) Grain Spread Misorientation 843°C: 1) 0.001 s-1, 2) 0.01 s-1, 3) 1 s-1, 4) 10 s-1

(a) - 1 (b) - 1

50 µm

(c) (d) - 1

Load

Anv

il

Jaw

C-GaugeClamp

Sample

Lubricant Thermocouple

(a) - 2 (b) - 2

0-4° 4-9° 9-13° 13-17° 17-22°

(d) - 2 (d) - 4(d) - 3

MOTIVATION

Scale:

Fig. 4: Efficiency of power dissipation η with κ (shaded) for (a) 0.3 and (b) 0.5 strain

(a) (b)