Journal of Materials Processing Technology 78 (1998) 117 – 121
Development of the microstructure and fatigue strength of two
phase titanium alloys in the processes of forging and heat
treatment
K. Kubiak *, J. Sieniawski
Department of Materials Science, Rzeszów Uni6ersity of Technology, ul. W.Pola 2, 35 -959 Rzeszów, Poland
Abstract
The paper reports results of the studies on the influence of deformation degree and temperature in the die forging process and
annealing temperature on the fatigue strength of forgings made of two phase martensitic titanium alloys Ti – 6Al –4V and
Ti–6Al–2Mo–2Cr. Dilatometric and metallographic studies have been carried out along with X-ray phase analysis and fatigue
tests. The influence of the phase composition and microstructure obtained after cooling with different rates on the fatigue strength
have been determined. © 1998 Elsevier Science S.A. All rights reserved.
Keywords: Two phase titanium alloys; Die forging; Microstructure; Fatigue strength
the possibility of using the alloy in structural elements
[4,5]. Therefore an effort has been made in this paper to
determine the effect of plastic working conditions
(strain rate and degree, forging method and temperature) on the fatigue strength of two phase, martensitic
titanium alloys, Ti–6Al–4V and Ti–6Al–2Mo–2Cr.
1. Introduction
The larger part of the elements made of two phase,
martensitic, titanium alloys is fabricated in the process
of plastic working. The service life and reliability of
these alloys depend on the microstructure obtained
after working and heat treatment and on the surface
layer condition [1]. Development of different microstructure and phase composition in various regions
of the forging is the result of the high sensitivity of two
phase titanium alloys to the plastic deformation rate,
degree and temperature [2,3]. Proper selection of these
parameters allows control of the phase composition of
the alloy (as a result of a’, a¦, b, v metastable phases
forming and decomposition) and its mechanical properties. Dynamic strength is a basic criterion determining
2. Material and research methodology
The material tested was two phase a+ b, martensitic
titanium alloys Ti–6Al–4V and Ti–6Al–2Mo–2Cr
having the chemical composition presented in Table 1.
Dilatometric tests were performed using an absolute
dilatometer LS-4. Changes in specimens elongation
were measured by means of an inductive converter and
Table 1
Chemical composition of the Ti–6Al–4V and Ti–6Al–2Mo– 2Cr alloys
Alloy
Ti–4Al – 4V
Ti–6Al – 2Mo – 2Cr
Stability factor
Alloying elements and impurities content (%wt.)
Kb
Al
Mo
V
Cr
Fe
C
Si
H
N
O
Ti
0.3
0.6
6.1
6.3
—
2.6
4.3
—
—
2.1
0.16
0.40
0.01
0.05
—
0.2
0.015
0.016
0.06
0.016
0.12
0.09
balance
balance
* Corresponding author.
0924-0136/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.
PII S0924-0136(97)00472-X
118
K. Kubiak, J. Sieniawski / Journal of Materials Processing Technology 78 (1998) 117–121
X –Y recorder (Riken Denshi). Temperature was measured using a Ni–CrNi thermocouple. Rounded specimens 4 mm in diameter and 15 mm long with a ¥
1.6 ×7.5 hole for the thermocouple were used for the
tests. A protective argon atmosphere was used.
The start and finish temperature of the a + b ? b
phase transformation was determined for a heating rate
6h = 0.08 K s − 1. The specimens were soaked at the
b-range temperature for half an hour and cooled at
controlled average rate in the range of 6c =48 } 0.004
K s − 1.
X-ray phase analysis and lattice constants measurements were carried out using a Philips diffractometer
with a copper tube and nickel filter. Characteristic
radiation CuKa was applied at a wavelength of l=
0.1542 nm.
The microstructure of the specimens was examined
using an optical microscope Neophot 2 and a transmission electron microscope Tesla BS613. Specimens were
ground, polished and etched using etchants having following chemical composition: 50% HNO3 +40% HF+
10% H2O or 10% HNO3 +2% HF +88% H2O at 270
K. Thin foil specimens were prepared by two-sided
mechanical thinning and electrolytic polishing at 230 K
in 5%HClO4 +35% C4H9OH +60% CH3OH electrolyte
at 15 V at 0.05 A · cm − 2.
Dilatometric examination was applied to determine
the start and finish temperature of the a +b “ b phase
transformation during alloy heating and cooling. On
the basis of the phase transformation temperature the
conditions of the plastic working of the alloys were
defined. Forging was accomplished in the b-range
(1320 K) and in the a +b-range (1170 K). Semifinished products (¥ 65 ×120) were forged on the
crank operated power press LKM 4000 and then annealed according to the scheme presented in Table 2.
Fatigue behaviour was examined in a rotational
bending test according to the PN-76/H-04326 standard
on the UMBS-4 machine with a frequency of 50 Hz.
From 6 to 10 specimens were tested on each stress level
and the terminal number of cycles was set to N =2 · 107.
Specimens 7 and 3.5 mm in diameter were made of
both as forged and heat treated forgings. Results of the
Table 2
Heat treatment of Ti – 6Al–4V and Ti–6Al–2Mo–2Cr alloys forgings
Forging temperature (K)
Anealing parameters
1170
1320
1170
1320
1170
1320
1170
1320
—
—
950
950
1060
1060
1250
1250
K
K
K
K
K
K
3
3
3
3
1
1
h−1
h−1
h−1
h−1
h−1
h−1
air
air
air
air
air
air
Table 3
The start and finish temperature of the a+b“b phase transformation for Ti – 6Al – 4V and Ti – 6Al – 2Mo – 2Cr alloys (6h =6c =0.08 K
s−1)
Phase transformation temperature (K)
Alloy
Ti – 6Al--4V
Ti – 6Al – 2Mo
– 2Cr
T ns
a+a “ b
T ps
a+a “ b
T fa+a “ b
T sa+a “ b
T fa+a “ b
1090
1170
1260
1220
970
1110
1190
1260
1210
810
ns, Nucleation start; ps, precipitation start; s, start; f, finish.
fatigue tests were statistically analysed in order to determine: average life Nav, S.D. s of average life, and
confidence intervals for fatigue limit and fatigue
strength.
3. Results and their analysis
On the basis of the a+b ? b-phase transformation
temperature (Table 3) the forging temperature was
defined. Dilatometric and metallographic examination
and structural analysis of the alloys (Table 4) after
continuous cooling from the b-range with a rate in the
range of 48–0.004 K s − 1 enabled the determination of
model changes in their phase composition and microstructure. That in turn permitted the evaluation of
quality of different regions of the forging.
It was found that at the cooling rate 6c \ 18 K s − 1
formation of martensitic phases a%(a¦) takes place exclusively in both alloys. The start and finish temperature of the martensitic transformation b“ a%(a¦) or
Table 4
Phase composition of the Ti – 6Al – 4V and Ti – 6Al – 2Mo – 2Cr alloys
Cooling rate (K s−1)
48
40
18
9
7
3.5
1.2
0.08
0.04
0.024
0.008
0.004
Phase composition
Ti– 6Al– 4V
Ti– 6Al– 2Mo –2Cr
a%(a¦)
a%(a¦)
a%(a¦)
a+a%(a¦)
a+a%(a¦)
a+a%(a¦)trace+b
a+b
a+b
a+b
a+b
a+b
a+b
a%(a¦)
a%(a¦)
a%(a¦)
a+a%(a¦)+b
a+a%(a¦)trace+b
a+a%(a¦)trace+b
a+b
a+b
a+b+TiCr2
a+b+TiCr2
a+b+TiCr2
a+b+TiCr2
K. Kubiak, J. Sieniawski / Journal of Materials Processing Technology 78 (1998) 117–121
119
Table 5
Fatigue strength of Ti – 6Al–4V and Ti–6Al–2Mo–2Cr alloys as a function of temperature and degree of deformation and annealing temperature
of the forging
Alloy
Forging temperature (K)
Fatigue strength (MPa)
Plastic deformation (o1 =0.35)
Plastic deformation (o2 =2.0)
Annealing temperature (K)
Ti–6Al – 2Mo – 2Cr
Ti–6Al – 4V
1170
1320
1170
1320
—
950
1250
—
950
1060
1250
546
482
511
426
605
—
558
—
—
465
—
417
261
351
262
340
285
372
283
362
232
—
221
—
—
326
—
322
b “ a¦ does not depend on the cooling rate but on the
b-stabilizing elements content. The bigger b-stabilizer
content (Kb ) the smaller start and finish temperature of
martensitic transformation.
At the cooling rate in the range 6c =9 – 7 K s − 1
diffusion transformation b“a and martensitic transformation b “a¦ take place in the Ti – 6Al – 4V alloy
resulting in the formation of a, a¦ and b phase mixtures
[3]. Cooling of the alloy at the rate 6c B1.2 K s − 1 leads
to the development of stable a and b phases in the
shape of colonies of parallel a-phase lamellae in primary b-phase grains. However in the Ti – 6Al – 2Mo–
2Cr alloy at the cooling rate 6c \3.5 K s − 1 martensitic
transformation b “a%(a¦) was observed. Cooling of this
alloy at the rate 6c B 0.024 K s − 1 results in eutectoid
decomposition of the part of b-phase: b “ a +TiCr2.
The forging temperature, the degree of deformation
and the annealing temperature of the Ti – 6Al – 4V and
Ti – 6Al–2Mo–2Cr alloys have very pronounced effect
on the fatigue strength of the alloys studied (Table 5). It
applies especially to Ti – 6Al – 2Mo – 2Cr alloy as the
presence of chromium raises its susceptibility to phase
composition changes (martensitic transformation b“
a‘‘).
Metallographic examination revealed the significant
effect of the forging temperature on the microstructure
of Ti–6Al–4V and Ti – 6Al – 2Mo – 2Cr alloys (Figs.
1 – 4).
After forging in the a +b-range (1170 K) a globular
microstructure was formed with grain deformation depending on the degree of plastic strain. A characteristic
feature of that microstructure was significant irregularity, i.e. various distortions of the a-phase grains (Fig.
1). Microstructure irregularity is related to the strain
degree. The strain degree was calculated by numeric
simulation using FORGE – 2 software [6,7]. The higher
deformation degree the lower microstructure irregularity and the larger the a-phase grains distortion.
Forging in the b-range (1320 K) generated lamellar
microstructure (Fig. 2). Parallel lamellae of a-phase
formed colonies in primary b-phase grains. Forging in
the b-range leads to a primary b-phase grain refinement that follows the dynamic recrystallization
processes.
The annealing temperature of the forgings after plastic working has an influence on the fatigue strength.
The maximum fatigue strength of the Ti–6Al–4V and
Ti–6Al–2Mo–2Cr alloys was obtained after annealing
at 950 K for 3 h. Increase in fatigue strength after
annealing (950 K for 3 h) compared with the strength in
as forged condition was:
“ Ti–6Al–4V alloy
“ forging temperature 1170K 9%
forging temperature 1320 K 4%
“ Ti–6Al–2Mo–2Cr alloy
forging temperature 1170K 10%
forging temperature 1320 K 6%
Examination of the microstructure revealed that a
recovery process took place in the alloys during annealing at 950 K for 3 h. A much lower dislocation density
was observed in the a-grains and lamellae (Fig. 3).
Annealing at 1250 K for 1 h led to a decrease in the
fatigue strength comparing with the alloys both in as
forged condition and annealed at 950 K for 3 h. For
Fig. 1. Microstructure of the Ti – 6Al – 2Mo – 2Cr alloy after forging at
1170 K.
120
K. Kubiak, J. Sieniawski / Journal of Materials Processing Technology 78 (1998) 117–121
Fig. 2. Microstructure of the Ti–6Al–2Mo–2Cr alloy after forging at
1320 K.
Fig. 4. Microstructure of the Ti – 6Al – 2Mo – 2Cr alloy after forging at
1170 K and annealing at 1250 K for 1 h.
forging temperature 1320 K reduction in fatigue
strength was:
“ Ti –6Al–4V alloy
strain degree o1 =0.35 2%
strain degree o2 =2.0 5%
“ Ti –6Al–2Mo–2Cr alloy
strain degree o1 =0.35 4%
strain degree o2 =2.0 8%
Decrease in the fatigue strength is related to the
volume fraction of the recrystallized microstructure
which depends on the degree of deformation. Hardness
measurements of the alloys forged at 1170 and 1320 K
showed that the annealing temperature is a critical
factor in considering the fraction of recrystallized microstructure and yet the degree of deformation has a
minor effect (Table 6). Annealing of Ti – 6Al – 4V and
Ti – 6Al–2Mo–2Cr alloys at 1250 K for 1 h promoted
the formation of new recrystallized grains and lamellae
of the a-phase and an increase in the fraction of
recrystallized regions in the microstructure to 66 and
67% respectively. This was also confirmed by micro-
scopic examination. Annealing at 1250 K resulted in
the formation of new recrystallized grains and lamellae
of the a-phase (Fig. 4)
4. Summary
Analysis of the results of the Ti–6Al–4V and Ti–
6Al–2Mo–2Cr titanium alloys examined allowed us to
determine: (1) the effect of the forging conditions, i.e.
temperature, amount of deformation and cooling rate
after forging on the phase composition, microstructure
and fatigue strength of the alloys studied; (2) the effect
of the heat treatment on the forgings, i.e. annealing
temperature on their fatigue strength.
Continuous cooling of the Ti–6Al–4V and Ti–6Al–
2Mo–2Cr alloys from the b-range produces a microstructure in the shape of colonies of parallel a-phase
lamellae in primary b-phase grains with various phase
compositions (a+ a%(a¦), a+ a%(a¦)+ b, a+ b, a+
b+ TiCr2). As a result of the increased chromium
content (Kb = 0.6) the volume fraction of martensitic
a%-phase grows in the Ti–6Al–2Mo–2Cr alloy.
Table 6
Recrystallized microstructure fraction for Ti – 6Al – 4V and Ti–6Al–
2Mo – 2Cr alloys for various deformation conditions and heat treatment
Alloy
Recrystallized microstructure fraction (%)
Deformation degree and heat treatment
Fig. 3. Microstructure of the Ti–6Al–2Mo–2Cr alloy after forging at
1170 K and annealing at 950 K for 3 h.
Ti– 6Al– 4V
Ti – 6Al – 2Mo – 2Cr
o =2.0
950°C
3 h−1
o= 2.0
1250°C
1 h−1
o =0.35
950°C
3 h−1
o =0.35
1250°C
1 h−1
34
36
66
67
26
27
63
60
K. Kubiak, J. Sieniawski / Journal of Materials Processing Technology 78 (1998) 117–121
Forging of two phase titanium alloys at the b “ a+
b transformation temperature develops a globular microstructure. Lamellar microstructure is formed during
forging in the primary b-phase temperature range.
The fatigue strength of the alloys studied depends on
the forging temperature, which controls the type of
microstructure, and plastic deformation degree. The
maximum fatigue strength was obtained by forging at
1170 K (Ti–6Al–2Mo – 2Cr, 546 MPa; Ti – 6Al–4V,
511 MPa). Forging at 1320 K leads to a decrease in
fatigue strength (Ti – 6Al – 2Mo – 2Cr, 482 MPa; Ti–
6Al –4V, 426 MPa).
Increased plastic deformation degree lowers the fatigue strength. Decrease in the fatigue strength is
smaller in the case of forging in the b-range than in the
a+ b-range.
The fatigue strength of the alloys depends also on the
heat treatment of the forgings. During annealing microstructure recovery and recrystallization processes
take place and result in fatigue strength changes. For
example during annealing at 950 K for 3 h a recovery
process took place and the maximum fatigue strength
was obtained (Ti–6Al – 2Mo – 2Cr, 605 MPa; Ti –6Al–
.
121
4V, 558 MPa). After annealing at 1250 K for 1 h the
recrystallized microstructure fraction was found to be
as high as 67% which led to a decrease in the fatigue
strength.
Acknowledgements
This work was supported by the KBN under Grant
708A03809.
References
[1] I.J. Polmear, Light Alloys, Arnold, London, 1995.
[2] J. Sieniawski, Zeszyty Naukowe Politechniki Rzeszowskiej,
Mechanika 10 (1985).
[3] J. Sieniawski, Inz; . Materiałowa, 2 – 3 (1993) 64.
[4] H. Inagaki, Z. Metall. 6 (1990) 324.
[5] R.V. Miner, Fatigue of Superalloys, Wiley, New York, 1987.
[6] A. Piela, Arch. Metall. 1 (1993) 101.
[7] Program FORGE – 2, CMFM, Ecole Nationale Soperieure des
Mines, de Paris rue C. Daunesse, Sophia Antipolis, 06560 Valbonne cedex, France.