Influence of oxygenation time on crack growth in titanium alloy under cyclic bending
The paper contains the results of tests on the fatigue crack growth in Ti–6Al–4V titanium alloy. Plane specimens of 4 mm in thickness with a unilateral notch were used. An analysis of crack growth was carried out for specimens without and after 2 and 4 h oxygenation process as well as fatigue crack...
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Фізико-механічний інститут ім. Г.В. Карпенка НАН України
2011
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irk-123456789-1381292018-06-19T03:07:10Z Influence of oxygenation time on crack growth in titanium alloy under cyclic bending Rozumek, D. Hepner, M. The paper contains the results of tests on the fatigue crack growth in Ti–6Al–4V titanium alloy. Plane specimens of 4 mm in thickness with a unilateral notch were used. An analysis of crack growth was carried out for specimens without and after 2 and 4 h oxygenation process as well as fatigue crack growth rate was determined. From the test results it appears that the fatigue crack growth in the specimens without thermo-chemical treatment differs from the fatigue crack growth in the specimens after oxygenation. It has been also found that the applied surface treatment increases fatigue life of the considered alloy. Досліджено втомний ріст тріщин у титановому сплаві Ti–6Al–4V у вихідному стані та після високотемпературного насичення киснем на плоских зразках товщиною 4 mm з одностороннім боковим надрізом за навантаження згином. Виконано фрактографічний аналіз особливостей втомного росту тріщини і визначено її швидкість для різних станів металу: у вихідному та після 2 і 4 h насичення киснем. Виявлено, що хімікотермічна обробка впливає на втомний ріст тріщини, при цьому підвищується втомна довговічність. Исследован усталостный рост трещин в титановом сплаве Ti–6Al–4V в исходном состоянии и после высокотемпературного насыщения кислородом на плоских образцах толщиной 4 mm с односторонним боковым надрезом при нагружении изгибом. Проведен фрактографический анализ особенностей усталостного роста тещин и определена её скорость для разных состояний металла: в исходном и после 2 и 4 h насыщения кислородом. Выявлено, что химико-термическая обработка влияет на усталостный рост трещин, при этом повышается усталостная долговечность. 2011 Article Influence of oxygenation time on crack growth in titanium alloy under cyclic bending / D. Rozumek, M. Hepner // Фізико-хімічна механіка матеріалів. — 2011. — Т. 47, № 1. — С. 82-86. — Бібліогр.: 8 назв. — англ. 0430-6252 http://dspace.nbuv.gov.ua/handle/123456789/138129 en Фізико-хімічна механіка матеріалів Фізико-механічний інститут ім. Г.В. Карпенка НАН України |
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The paper contains the results of tests on the fatigue crack growth in Ti–6Al–4V titanium alloy. Plane specimens of 4 mm in thickness with a unilateral notch were used. An analysis of crack growth was carried out for specimens without and after 2 and 4 h oxygenation process as well as fatigue crack growth rate was determined. From the test results it appears that the fatigue crack growth in the specimens without thermo-chemical treatment differs from the fatigue crack growth in the specimens after oxygenation. It has been also found that the applied surface treatment increases fatigue life of the considered alloy. |
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Article |
| author |
Rozumek, D. Hepner, M. |
| spellingShingle |
Rozumek, D. Hepner, M. Influence of oxygenation time on crack growth in titanium alloy under cyclic bending Фізико-хімічна механіка матеріалів |
| author_facet |
Rozumek, D. Hepner, M. |
| author_sort |
Rozumek, D. |
| title |
Influence of oxygenation time on crack growth in titanium alloy under cyclic bending |
| title_short |
Influence of oxygenation time on crack growth in titanium alloy under cyclic bending |
| title_full |
Influence of oxygenation time on crack growth in titanium alloy under cyclic bending |
| title_fullStr |
Influence of oxygenation time on crack growth in titanium alloy under cyclic bending |
| title_full_unstemmed |
Influence of oxygenation time on crack growth in titanium alloy under cyclic bending |
| title_sort |
influence of oxygenation time on crack growth in titanium alloy under cyclic bending |
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Фізико-механічний інститут ім. Г.В. Карпенка НАН України |
| publishDate |
2011 |
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http://dspace.nbuv.gov.ua/handle/123456789/138129 |
| citation_txt |
Influence of oxygenation time on crack growth in titanium alloy under cyclic bending / D. Rozumek, M. Hepner // Фізико-хімічна механіка матеріалів. — 2011. — Т. 47, № 1. — С. 82-86. — Бібліогр.: 8 назв. — англ. |
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Фізико-хімічна механіка матеріалів |
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AT rozumekd influenceofoxygenationtimeoncrackgrowthintitaniumalloyundercyclicbending AT hepnerm influenceofoxygenationtimeoncrackgrowthintitaniumalloyundercyclicbending |
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2025-07-10T05:10:00Z |
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| fulltext |
82
Ô³çèêî-õ³ì³÷íà ìåõàí³êà ìàòåð³àë³â. – 2011. – ¹ 1. – Physicochemical Mechanics of Materials
INFLUENCE OF OXYGENATION TIME ON CRACK GROWTH
IN TITANIUM ALLOY UNDER CYCLIC BENDING
D. ROZUMEK, M. HEPNER
Opole University of Technology, Poland
The paper contains the results of tests on the fatigue crack growth in Ti–6Al–4V titanium
alloy. Plane specimens of 4 mm in thickness with a unilateral notch were used. An analysis
of crack growth was carried out for specimens without and after 2 and 4 h oxygenation
process as well as fatigue crack growth rate was determined. From the test results it
appears that the fatigue crack growth in the specimens without thermo-chemical treatment
differs from the fatigue crack growth in the specimens after oxygenation. It has been also
found that the applied surface treatment increases fatigue life of the considered alloy.
Keywords: titanium alloy, oxygenation, fatigue crack growth, bending, notch.
Titanium and its alloys are the materials having very good mechanical, physical and
chemical properties. Titanium alloys are mainly used in aerospace, armaments, shipbuil-
ding and medical industries, or other fields where the structure weight, high strength and
high corrosion resistance are very important. They are applied in production of heat ex-
changers, water desalting lines and many other products. However, application of titani-
um is limited, because its wear resistance is low, and its susceptibility to seizing under
friction is high. Thus, development and modification of titanium-based materials [1, 2]
should include not only a proper selection of the alloying elements, but changes of the
surface layer properties by suitable surface treatment, as well. Oxygenation of titanium is
one of such methods. It uses susceptibility of oxygen to formation of the solid solution in
Tiα under high temperature. The authors of this paper discussed that problem [3]. The
regularities of titanium alloys surface hardening under saturation in oxygen-containing
media were described in [4, 5]. It is also important, how the applied modification of the
surface layer influences fatigue strength and crack growth in the material. Tests of the
fatigue crack growth in two-phase titanium alloys are described [6]. Influence of micro-
structure on the fatigue crack growth was analyzed for turbine blades made of Ti–6Al–4V
titanium alloy. It was found that the grain size strongly influenced the crack growth.
The aim of this paper is to determine the influence of the oxygenation time on
changes in the material and the fatigue crack growth in Ti–6Al–4V titanium alloy.
Material and testing procedures. The crack growth was considered in Ti–6Al–4V
alloy [3, 7] (brand name IMI318, σy = 930 MPa, σu = 977 MPa, micro-hardness is
373HV0.1) in the structural aspect of the material. This alloy is a typical two-phase α+β
structure. Aluminium is dissolving in α-phase and stabilizing it, and vanadium stabilizes
β-phase. Plane specimens were made of the as softened bar (hot rolled and after
stabilizing annealing), 16 mm in diameter with a width of 16 mm, thickness of 4 mm
and the unilateral side notches of the root rounding 0.2 mm and 2 mm in depth.
The specimens with notches were subjected to oxygenation at a temperature of
900°C by 2 and 4 h in the electric furnace in hot air then the specimens were cooled in
the air. Under high-temperature oxidation of titanium and its alloys, the scale including
mainly TiO2 and Al2O3 is formed on the surface, and a part of oxygen diffuses through
Corresponding author: D. ROZUMEK, e-mail: d.rozumek@po.opole.pl
83
a layer of oxides and dissolves in the metallic base, causing significant increase of
hardness dependent on an amount of dissolved oxygen. Titanium and its alloy are able
to form oxygen solid solutions (at temperatures above 500°C) in phases Tiα and Tiβ,
and this is a very important feature. In the case of other metals and alloys, oxidation
usually causes formation only oxides without oxygenation of the metallic base. Titanium
α of the hexagonal compact structure is able to dissolve no more than 34% at. (14% by
weight) of oxygen, and β-phase – less oxygen, no more than 3.8% by weight. After 2 h
of oxygenation at 900°C, micro-hardness equal to about 800HV0.1 was obtained on the
specimen surface (the surface was measured after removal of the layer of oxides). After
4 h of oxygenation, further increase of micro-hardness to about 890HV0.1 was reported.
The material hardening decreased together with the distance from the external surface
of the specimen, and it was found for 2 h at the depth 100 µm, and for 4 h at the depth
165 µm, although the alloy structure changes were seen at the greater depth (Fig. 1).
The dissolved oxygen stabilizes α-phase, so the top layer contains less β-phase.
Fig. 1. Cross section of the specimen made of the Ti–6Al–4V alloy after:
a – 2 h oxygenation; b – 4 h oxygenation.
The tests were performed on the fatigue test stand MZGS – 100 under the loading
change frequency of 28.8 Hz [6, 7]. The specimens were subjected to cyclic bending
with the constant stress ratio R = σmin / σmax = 0 and amplitude of moment Ma = 11.2 Nm
(Mmax = 22.4 Nm), which corresponded to the nominal amplitude of normal stress
σa = 74.6 MPa before crack initiation. Synchronous sinusoidal variable loadings inclu-
ding the mean value were applied. Crack growth on the specimen surface was observed
with the optical method. The fatigue crack increments were measured with the micro-
meter located in the portable microscope with magnification of 20 times and accuracy
up to 0.01 mm. At the same time, a number of loading cycles N was recorded. The
results of measurements are shown in the graphs of crack length “a” versus a number
of cycles N, and the graphs of fatigue crack growth rate da/dN versus the variability
range of the stress intensity factor ∆K.
Results and discussion. Structures without and after oxygenation. Since the
plane specimens were cut off along the bar axis, their surfaces contained elongated
grains of α-phase (with a length of about 50 µm) with precipitations at the boundaries
of the disintegrated β-phase, forming a band structure perpendicular to the applied
loading. Fig. 2 shows the specimen surface without thermo-chemical treatment, tested
with a number of failure cycles Nf = 9.000. The main crack grows on the planes of the
greatest normal stresses. At the fractures, transcrystalline cracks through the grains of
α-phase predominated, but cracks along the grain boundaries were also observed, espe-
cially in the clusters of β-phase. In the tested material, there were also secondary short
cracks, developing from the main crack. In the alloy without oxygenation, short cracks
were located at different directions with respect to the specimen axis. These secondary
cracks running from the main (long) are often generated at the phase boundary α and β,
forming slips which run along β-phase precipitations. Also the cracks which stop in
84
α-phase and do not reach the phase bo-
undary can be observed. The cracks de-
veloping in the grains of α-phase are often
stop at the phase boundary α and β (see
Fig. 2). In the case of specimens after 2
and 4 h oxygenation, a network of
developing semi-elliptic cracks could be
seen in the oxygenated layer near the
main crack (Figs. 3, 4). Fig. 3a presents
the specimen surface after 2 h oxygena-
tion, tested during Nf = 28.000 cycles.
Fig. 3b illustrates the specimen surface after 4 h oxygenation under the same loading
and Nf = 19.000 cycles. The scale residue was removed from the surface by grinding,
so the metallic oxygenated base was exposed. The semi-elliptic network of the fatigue
cracks occurs only in the oxygenated layer, over all its thickness, and progressively
disappears as a degree of hardening decreases (see Fig. 3c, d). Fig. 3 gives the structure
in deeper layers of the specimen after 2 h oxygenation, after removing about 60 µm.
Semi-elliptic cracks forming the network run through the grains of Tiα(O)-phase and
along the boundaries of grains. In order to analyze crack growth, the chosen crack interval
in Fig. 3c was magnified to ×500 (Fig. 3d). In Fig. 3d, one can see of the crack network
development in deeper layers with poor content of oxygen and rich β-phase. The
development stops in the β-phase grains (in the ground layer the semi-elliptic crack
network is not visible).
Fig. 3. Fatigue crack growth in the Ti–6Al–4V alloy after
2 h (a, c, d) oxygenation and 4 h oxygenation (b):
c, d – removal of the layer of about 60 µm.
The fatigue failure process of the oxygenated
alloy starts after initiation of the main crack at the
notch root from semi-elliptic cracks in the brittle
hardened top layer (Fig. 4). The structure after 4 h oxygenation is also shown. The
semi-elliptic cracks precede and strongly determine propagation of the main crack,
developing on the planes of the greatest normal stresses. Thus, the main crack becomes
a big, zigzag-shaped, irregular crack (Fig. 4c) and it differs from the path of the main
crack after 2 h oxygenation (Fig. 3a). Fig. 4b presents the magnified crack area from
Fig. 4a. In Fig. 4b one can see the development of short cracks initiating in the network
of semi-elliptic cracks blocked in the β-phase grains. Character and growth of fatigue
cracks shown in Fig. 3 depend on the thermo-chemical treatment time. The network of
semi-elliptic cracks for a specimen after 4 h oxygenation requires much more area than
such network for a specimen after 2 h oxygenation. The main crack growth after 2 h
oxygenation is uniform, and after 4 h oxygenation the crack displaces at the intervals of
the semi-elliptic cracks network (Fig. 4).
Fig. 2. Fatigue crack growth in the Ti–6Al–4V
alloy without thermo-chemical treatment.
85
Fig. 4. Network of the fatigue cracks in the specimen after 4 h oxygenation: visible front of the
main crack and the preceding elliptic cracks (a, b) and zigzag-shaped course of the main crack
determined by the network of semi-elliptic cracks (c).
Characteristics of fatigue crack growth. Variation of the crack length depending
on a number of loading cycles a = f (N) for specimens without oxygenation, after 2 and
4 h oxygenation is presented in Fig. 5. Comparing fatigue life of the considered alloy
before oxygenation and after 2 h oxygenation, we can notice its treble increase and for
4 h oxygenation, a double increase can be noticed. Graphs of the fatigue crack growth
rate da/dN = f (∆K) for specimens before and after 2 and 4 h oxygenation are shown in
Fig. 6. From analysis of the test results it appears that after 2 or 4 h oxygenation the
fatigue crack growth rate decreases (more after 4 h) in all amplitude range of ∆K as
compared with the fatigue crack growth rate in the specimens without oxygenation. For
example, for ∆K = 28 MPa⋅m1/2, we can observe decrease of the fatigue crack growth
rate about three times after 2 h and about two times after 4 h oxygenation (from
6.70⋅10–7 m/cycle without oxygenation to 2.73⋅10–7 m/cycle after 2 h oxygenation and
3.50⋅10–7 m/cycle after 4 h oxygenation).
Coefficients C, m and correlation coefficients r for the graphs shown in Fig. 6
Graph in Fig. 6 C, m(MPa⋅m1/2)–m/cycle m r
1 7.656 ⋅ 10–9 1.230 0.946
2 3.669 ⋅ 10–9 1.267 0.980
3 1.089 ⋅ 10–8 1.015 0.945
Fig. 5. Fig. 6.
Fig. 5. Fatigue crack length, a, versus number of cycles, N:
– without oxygenation; – 2 h oxygenation; – 4 h oxygenation.
Fig. 6. Fatigue crack growth rate da/dN versus changes of stress intensinty factor range ∆K:
1 – without oxygenation; 2 – 2 h; 3 – 4 h oxygenation.
86
The test results (Fig. 6) were approximated with the Paris equation [8]. The coef-
ficients C and m were determined with the least square method and shown in the Table.
It contains also the correlation coefficients r which were determined for the significance
level α = 0.05. The coefficients r are high (close to unity), so there is strong linear depen-
dence between variables da/dN and ∆K described by experimental results. The relative
error of test results for cyclic bending does not exceed 20% when using ∆K parameter.
CONCLUSIONS
The presented results of the fatigue crack growth in the plane specimens subjected
to bending loading allow to formulate the following conclusions: the network of semi-
elliptic cracks can be seen in the specimens after oxygenation; the network of semi-ellip-
tic cracks occurs in the layers rich in oxygen; micro-hardness of the oxygenated layers
is more than two times greater than that of the basic material; lifetime of the tested alloy
without oxygenation was compared with its lifetime after oxygenation. The lifetime
increased about three times after 2 h, and about two times after 4 h oxygenation.
РЕЗЮМЕ. Досліджено втомний ріст тріщин у титановому сплаві Ti–6Al–4V у вихід-
ному стані та після високотемпературного насичення киснем на плоских зразках товщи-
ною 4 mm з одностороннім боковим надрізом за навантаження згином. Виконано фрак-
тографічний аналіз особливостей втомного росту тріщини і визначено її швидкість для
різних станів металу: у вихідному та після 2 і 4 h насичення киснем. Виявлено, що хіміко-
термічна обробка впливає на втомний ріст тріщини, при цьому підвищується втомна дов-
говічність.
РЕЗЮМЕ. Исследован усталостный рост трещин в титановом сплаве Ti–6Al–4V в
исходном состоянии и после высокотемпературного насыщения кислородом на плоских
образцах толщиной 4 mm с односторонним боковым надрезом при нагружении изгибом.
Проведен фрактографический анализ особенностей усталостного роста тещин и опреде-
лена её скорость для разных состояний металла: в исходном и после 2 и 4 h насыщения
кислородом. Выявлено, что химико-термическая обработка влияет на усталостный рост
трещин, при этом повышается усталостная долговечность.
1. Rozumek D. and Hepner M. Influence of the Ti–6Al–4V titanium alloy microstructure on the
fatigue crack growth // Mater. Engn. – 2005. – 2. – P. 81–83 (in Polish).
2. Rozumek D. and Macha E. Fatigue crack growth in titanium and aluminum alloys under
bending // Mater. Science Forum. – 2007. – 567–568. – P. 317–320.
3. Rozumek D. and Hepner M. Analysis of the fatigue crack growth in Ti–6Al–4V alloy
without and after oxygenation // Mater. Engn. – 2008. – 3. – P. 154–156 (in Polish).
4. Interstitial hardening of the surface of titanium alloys by vacuum heat treatment / V. M Fe-
dirko, A. T. Pichuhin, O. H. Luk’yanenko, and V. S. Onuferko // Materials Science. – 2005.
– 41, № 2. – P. 208–216.
5. Endurance of VT1-0 titanium alloy subjected to solid-solution surface hardening / A. T. Pi-
chuhin, V. M. Fedirko, O. H. Luk’yanenko, and V. S. Onuferko // Ibid. – № 3. – P. 418–422.
6. Nalla R. K., Campbell J. P., and Ritchie R. O. Mixed-mode, high-cycle fatigue-crack growth
thresholds in Ti–6Al–4V: Role of small cracks // Int. J. of Fatigue – 2002. – 23. – P. 1047–1062.
7. Fatigue and microstructure of two-phase titanium alloys // W. Ziaja, J. Sieniawski, K. Kubiak,
and M. Motyka // Mater. Engn. – 2001. – 22. – P. 981–984 (in Polish).
8. Paris P. C. and Erdogan F. A critical analysis of crack propagation laws // J. Basic Eng.,
Trans. ASME. – 1960. – 85. – P. 528–534.
Received 05.01.2011
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