A. G. ILLARIONOV et al.: EFFECT OF YTTRIUM AND ZIRCONIUM MICROALLOYING ON THE STRUCTURE ...
855–859
EFFECT OF YTTRIUM AND ZIRCONIUM MICROALLOYING ON
THE STRUCTURE AND PROPERTIES OF WELD JOINTS OF A
TWO-PHASE TITANIUM ALLOY
U^INEK MIKROLEGIRANJA ITRIJA IN CIRKONIJA NA
STRUKTURO IN LASTNOSTI NA SPOJE ZAVROV DVOFAZNE
ZLITINE TITANA
Anatoly Illarionov, Artemy Popov, Svetlana Illarionova, Dmitry Gadeev
Ural Federal University, 620034, 28 Mira str., Yekaterinburg, Russia
d.v.gadeev@urfu.ru
Prejem rokopisa – received: 2016-11-10; sprejem za objavo – accepted for publication: 2017-04-19
doi:10.17222/mit.2016.317
The effect of microalloying of the Ti-4.8Al-1.2Mo-2.6V-0.6Cr–0.25Fe titanium alloy with yttrium and zirconium on the phase
composition, structure and mechanical properties of welded sheets was studied using light and transmission electron
microscopy, X-ray diffraction analysis and microindentation-hardness testing. A differential thermal analysis was employed to
model the welding thermal cycle. It was found that alloying with yttrium led to the precipitation of Y2O3 oxide particles
resulting in refining the microstructure of the alloy. In addition, yttrium additions were shown to stabilise the -phase of the
alloy that decreases the hardness of the alloy.
Keywords: microalloying, rare-earth elements, titanium alloy, welding
U~inek mikrolegiranja zlitine titana Ti-4.8Al-1.2Mo-2.6V-0.6Cr-0.25Fe z itrijem in cirkonijem na sestavo faze, strukturne in
mehanske lastnosti je bil raziskovan s svetlobno elektronsko mikroskopijo, z rentgensko difrakcijsko analizo in s testiranjem
trdote mikroindentacije. Uporabili smo diferencialno termi~no analizo za pridobitev modela za varilni termi~ni cikel.
Ugotovljeno je bilo, da legiranje z itrijem vodi k razpr{enosti Y2O3 oksidnih delcev, kar se ka`e pri rafiniranju mikrostrukture
zlitine. Poleg tega je bilo dokazano, da so dodatki itrija stabilizirali -fazo v zlitini, ki zmanj{uje trdoto zlitine.
Klju~ne besede: mikrolegiranje, elementi redke zemlje, zlitina titana, varjenje
1 INTRODUCTION
Titanium alloys, due to their high specific strength
and good corrosion resistance, are widely used in the
aerospace and marine industries as structural materials
and it is desirable to further enhance their properties. The
latter is possible by means of complex multicomponent
alloying.1,2 In addition to the specific strength, good
weldability is usually necessary to maximise the material
utilisation.
Since -titanium and + alloys are considered to
have a better weldability compared to metastable -tita-
nium alloys,3 a number of +-alloys were developed in
Russia to meet these requirements. One of them is
Ti-4.8Al-1.2Mo-2.6V-0.6Cr–0.25Fe, which is a typical
martensite-type two-phase alloy.4
Despite good weldability, the welding usually
negatively affects the mechanical properties of titanium
alloys.5 This is partly due to the oxygen absorbed from
the shield atmosphere followed by its diffusion to the
weld-fusion zone (FZ), partly due to a high heat input
resulting in significant grain coarsening. One way to
overcome such a problem is to refine the prior grain size
by the grain-boundary pinning effect of second-phase
particles and promote heterogeneous nucleation in the
FZ.6 It was shown that microalloying with transition
elements, especially rare-earth metals, is a promising
way to refine the structure of titanium alloys.7 One of the
most frequently employed alloying elements is
yttrium.8–11
Although it was already shown what microalloying of
the Ti-4.8Al-1.2Mo-2.6V-0.6Cr–0.25Fe alloy with
0.06 % mass fraction of yttrium decreases the oxygen
concentration in - and -solid solutions due to the
formation of Y2O3 particles,4 detailed studies of a weld
structure were not conducted. This is why our study
aimed at investigating the influence of microalloying
titanium alloys with zirconium and yttrium on the
microstructure formation and mechanical properties of
the weld joints of this alloy.
2 EXPERIMENTAL PART
Two-millimeter-thick hot-rolled sheets of an (+)
martensitic titanium alloy alloyed with yttrium (alloy 1)
and zirconium (alloy 2) were used in this study. The
chemical compositions of the alloys are shown in
Table 1. Prior to their use, the sheets were subjected to
vacuum annealing at 750 °C for 1 h followed by arc
welding using non-consumable tungsten electrodes.
Materiali in tehnologije / Materials and technology 51 (2017) 5, 855–859 855
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 669.794:669.296:67.017:66.046.51 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(5)855(2017)
Table 1: Chemical compositions of the test materials, in mass
fractions (w/%)
Materials
Alloying elements, in mass fractions (w/%)
Ti Al Mo V Cr Fe Y Zr
Alloy 1 Bal 4.80 1.20 2.60 0.60 0.60 0.06 -
Alloy 2 - 0.07
The microstructures of the alloys were analyzed with
light optical (LOM) and transmission electron (TEM)
microscopy on Olympus GX51 and JEM 200C micro-
scopes, respectively. A qualitative phase analysis was
carried out using X-ray diffraction (XRD) with Cu-K
radiation on a DRON-3M powder diffractometer. A
differential thermal analysis (DTA) was performed on a
Du Pont appliance in DTA-1600 crucibles. The average
grain size and grain-size distribution were determined
with an Epiquant optical microscope.
Vickers microhardness was measured at a 1 N load in
accordance with the formula below (ISO 6507) and
multiplied with the standard gravity (g = 9.81) to get the
values in MPa in Equation (1):
HV = 0.1891· (F/d2) (1)
with F being the applied load (Newton) and d being the
average length of the diagonal of the residual indent
(millimeters).
Metallographic samples were prepared according to
the standard procedures. The steps consisted of grinding
with 120–2400 grit SiC paper, polishing with 0.3 μm
colloidal alumina, followed by the final polishing with a
0.05 μm colloidal silica suspension. Kroll’ s reagent with
a composition of 100 mL water + 2 mL HF + 5 mL
HNO3 was used to etch the specimens.12 Thin foils for
the TEM investigation were prepared with electrolytic
polishing in a methanol-containing electrolyte (300 mL
methanol, 175 mL butanol, 30 mL perchloric acid
(70–72 %)) at -30 °C.12
3 RESULTS AND DISCUSSION
With respect to the differences in the microstructure,
weld joints generally consist of three distinct zones, i.e.,
the base metal, the fusion zone and the heat-affected
zone. In the fusion zone (FZ), materials melt during the
welding and crystallize during the subsequent cooling.
The heat-affected zone (HAZ) corresponds to the narrow
transition region between the base metal and the fusion
zone where the material is heated up to sub-transus or
even superheated above the critical temperature. The
latter might obviously cause significant microstructure
changes in comparison with the base metal.
On the present samples, the zone width was
measured and it was around 12 mm and 4.5 mm for the
fusion and heat-affected zones, respectively (Figure 1).
The base-metal microstructure of both alloys is
represented by primary -phase precipitates of a mixed,
globular and lamellar, morphology in a -phase matrix
(Figure 2a). The yttrium-containing alloy 1 is dis-
tinguished by a dispersive oxide precipitation of Y2O3
particles with the average diameter of up to 120 nm
(Figure 2b). These particles were found to be almost
spherical and to precipitate mainly near the grain
boundaries. The TEM micrograph (Figure 2b) shows a
specific 'comet-tail-like' contrast around these particles,
which is attributed to the elastic-stress field caused by
the difference between the thermal-expansion coeffi-
cients of the oxides and the matrix.
The average -phase grain size decreased with the
increasing distance from the FZ (Figure 1) due to the
higher temperatures achieved in the HAZ close to the
A. G. ILLARIONOV et al.: EFFECT OF YTTRIUM AND ZIRCONIUM MICROALLOYING ON THE STRUCTURE ...
856 Materiali in tehnologije / Materials and technology 51 (2017) 5, 855–859
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: Microstructures of base metals: a) alloy 2 (LOM); b) alloy 1
(TEM)
Figure 1: Structures of welded joints: a) alloy 1 with Y; b) alloy 2
with Zr
FZ. The coarsest grains of alloy 2 reached about 400 μm
in diameter, whereas in alloy 1, they did not exceed 330
μm, i.e., the yttrium-containing alloy 1 was characterized
by a 1.2 times finer grain size. A similar relationship was
previously observed in the solution-treated and water-
quenched alloys of the same composition4 and it was
attributed to the presence of the yttrium-oxide particles
in the Y-containing alloy that inhibited the grain-boun-
dary movement.
The X-ray diffraction analysis showed that the vo-
lume fractions of the - and -phases varied significantly
in different parts of the HAZ (Figure 3). The intensity of
-phase peaks in the XRD patterns decreased with the
increasing distance from the FZ, clearly indicating that
the volume fraction of the -phase was significantly
higher in the near-FZ areas. At the same time, the a
lattice parameter changed in the opposite direction.
It can be seen that the (200)-peak intensity ratio for
alloy 1 was considerably higher than that for alloy 2.
This confirms the assumption that yttrium binds oxygen,
thus increasing the stability of the supercooled -phase
upon cooling.4,8–9 Furthermore, such a behaviour resulted
in a higher -phase volume fraction, decreasing the hard-
ness of the alloy. This was confirmed with the micro-
hardness-indentation tests. Alloy 1 had a lower micro-
hardness (2800±50 MPa) than alloy 2 (3300±50 MPa). It
was also found that the -phase morphology along the
HAZ changed from a complex one (Figure 2) to a
lamellar one consisting of thin platelets (Figure 4).
The analysis of the microstructures of the alloys
revealed a noticeable feature of both alloys, i.e., a jagged
shape of the grain boundaries. A similar phenomenon
was seen previously, for instance, in Ni–Mn–In-based
alloys after thermal cycling13 and near-alpha titanium
alloys14 and it was attributed to the formation of marten-
site crystals, which grow towards the grain boundaries
curving them. Figure 4a shows that such a shape coin-
cides closely with the lamellar -phase precipitations on
both sides of the grain boundaries. Thin -lamellae were
formed during the cooling from the single-phase
-region. The stress field next to the growing -lamellae
could interact with the grain boundaries curving them.
The same process occurred in the neighbouring grains
resulting in the jag formation. Since this process can
occur in preferentially oriented grains, the jags were
found only on the corresponding parts of the grains. On
the whole, we suppose that the jagged shape of the grain
boundaries is associated with the interaction of the
boundaries with the products of the -transformation
occurring upon the cooling.
To summarize the above, it can be concluded that the
welding thermal cycle results in significant changes of
the grain structure, phase composition and morphology
of the phases. Yttrium binds oxygen forming oxide
particles that inhibit the grain growth and improve the
-phase stability. In turn, microalloying the alloy with
zirconium does not result in the formation of additional
phases and leads to the solution hardening of the alloy.
The average grain sizes for the weld joints of the
samples of alloys 1 and 2 did not show any significant
difference and was about 480–600 μm (Figure 1). The
fine structure of the alloys mainly consists of the
-phase, with dislocations perpendicular to the lamellae
(Figure 5b).
The -lamellae in the FZs of the alloys tend to form
small colonies that may cross differently oriented
lamellae (Figure 5c). Interlamellar / spacings in
A. G. ILLARIONOV et al.: EFFECT OF YTTRIUM AND ZIRCONIUM MICROALLOYING ON THE STRUCTURE ...
Materiali in tehnologije / Materials and technology 51 (2017) 5, 855–859 857
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 3: -phase lattice parameter (a) and (200) XRD-peak inten-
sity ratio (IHAZ /IFZ) as a function of distance from the fusion zone
of a welded joint
Figure 4: Microstructures of heat-affected zones of weld joints:
a) alloy 1, b) alloy 2
Table 1: Chemical compositions of the test materials, in mass
fractions (w/%)
Materials
Alloying elements, in mass fractions (w/%)
Ti Al Mo V Cr Fe Y Zr
Alloy 1 Bal 4.80 1.20 2.60 0.60 0.60 0.06 -
Alloy 2 - 0.07
The microstructures of the alloys were analyzed with
light optical (LOM) and transmission electron (TEM)
microscopy on Olympus GX51 and JEM 200C micro-
scopes, respectively. A qualitative phase analysis was
carried out using X-ray diffraction (XRD) with Cu-K
radiation on a DRON-3M powder diffractometer. A
differential thermal analysis (DTA) was performed on a
Du Pont appliance in DTA-1600 crucibles. The average
grain size and grain-size distribution were determined
with an Epiquant optical microscope.
Vickers microhardness was measured at a 1 N load in
accordance with the formula below (ISO 6507) and
multiplied with the standard gravity (g = 9.81) to get the
values in MPa in Equation (1):
HV = 0.1891· (F/d2) (1)
with F being the applied load (Newton) and d being the
average length of the diagonal of the residual indent
(millimeters).
Metallographic samples were prepared according to
the standard procedures. The steps consisted of grinding
with 120–2400 grit SiC paper, polishing with 0.3 μm
colloidal alumina, followed by the final polishing with a
0.05 μm colloidal silica suspension. Kroll’ s reagent with
a composition of 100 mL water + 2 mL HF + 5 mL
HNO3 was used to etch the specimens.12 Thin foils for
the TEM investigation were prepared with electrolytic
polishing in a methanol-containing electrolyte (300 mL
methanol, 175 mL butanol, 30 mL perchloric acid
(70–72 %)) at -30 °C.12
3 RESULTS AND DISCUSSION
With respect to the differences in the microstructure,
weld joints generally consist of three distinct zones, i.e.,
the base metal, the fusion zone and the heat-affected
zone. In the fusion zone (FZ), materials melt during the
welding and crystallize during the subsequent cooling.
The heat-affected zone (HAZ) corresponds to the narrow
transition region between the base metal and the fusion
zone where the material is heated up to sub-transus or
even superheated above the critical temperature. The
latter might obviously cause significant microstructure
changes in comparison with the base metal.
On the present samples, the zone width was
measured and it was around 12 mm and 4.5 mm for the
fusion and heat-affected zones, respectively (Figure 1).
The base-metal microstructure of both alloys is
represented by primary -phase precipitates of a mixed,
globular and lamellar, morphology in a -phase matrix
(Figure 2a). The yttrium-containing alloy 1 is dis-
tinguished by a dispersive oxide precipitation of Y2O3
particles with the average diameter of up to 120 nm
(Figure 2b). These particles were found to be almost
spherical and to precipitate mainly near the grain
boundaries. The TEM micrograph (Figure 2b) shows a
specific 'comet-tail-like' contrast around these particles,
which is attributed to the elastic-stress field caused by
the difference between the thermal-expansion coeffi-
cients of the oxides and the matrix.
The average -phase grain size decreased with the
increasing distance from the FZ (Figure 1) due to the
higher temperatures achieved in the HAZ close to the
A. G. ILLARIONOV et al.: EFFECT OF YTTRIUM AND ZIRCONIUM MICROALLOYING ON THE STRUCTURE ...
856 Materiali in tehnologije / Materials and technology 51 (2017) 5, 855–859
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: Microstructures of base metals: a) alloy 2 (LOM); b) alloy 1
(TEM)
Figure 1: Structures of welded joints: a) alloy 1 with Y; b) alloy 2
with Zr
FZ. The coarsest grains of alloy 2 reached about 400 μm
in diameter, whereas in alloy 1, they did not exceed 330
μm, i.e., the yttrium-containing alloy 1 was characterized
by a 1.2 times finer grain size. A similar relationship was
previously observed in the solution-treated and water-
quenched alloys of the same composition4 and it was
attributed to the presence of the yttrium-oxide particles
in the Y-containing alloy that inhibited the grain-boun-
dary movement.
The X-ray diffraction analysis showed that the vo-
lume fractions of the - and -phases varied significantly
in different parts of the HAZ (Figure 3). The intensity of
-phase peaks in the XRD patterns decreased with the
increasing distance from the FZ, clearly indicating that
the volume fraction of the -phase was significantly
higher in the near-FZ areas. At the same time, the a
lattice parameter changed in the opposite direction.
It can be seen that the (200)-peak intensity ratio for
alloy 1 was considerably higher than that for alloy 2.
This confirms the assumption that yttrium binds oxygen,
thus increasing the stability of the supercooled -phase
upon cooling.4,8–9 Furthermore, such a behaviour resulted
in a higher -phase volume fraction, decreasing the hard-
ness of the alloy. This was confirmed with the micro-
hardness-indentation tests. Alloy 1 had a lower micro-
hardness (2800±50 MPa) than alloy 2 (3300±50 MPa). It
was also found that the -phase morphology along the
HAZ changed from a complex one (Figure 2) to a
lamellar one consisting of thin platelets (Figure 4).
The analysis of the microstructures of the alloys
revealed a noticeable feature of both alloys, i.e., a jagged
shape of the grain boundaries. A similar phenomenon
was seen previously, for instance, in Ni–Mn–In-based
alloys after thermal cycling13 and near-alpha titanium
alloys14 and it was attributed to the formation of marten-
site crystals, which grow towards the grain boundaries
curving them. Figure 4a shows that such a shape coin-
cides closely with the lamellar -phase precipitations on
both sides of the grain boundaries. Thin -lamellae were
formed during the cooling from the single-phase
-region. The stress field next to the growing -lamellae
could interact with the grain boundaries curving them.
The same process occurred in the neighbouring grains
resulting in the jag formation. Since this process can
occur in preferentially oriented grains, the jags were
found only on the corresponding parts of the grains. On
the whole, we suppose that the jagged shape of the grain
boundaries is associated with the interaction of the
boundaries with the products of the -transformation
occurring upon the cooling.
To summarize the above, it can be concluded that the
welding thermal cycle results in significant changes of
the grain structure, phase composition and morphology
of the phases. Yttrium binds oxygen forming oxide
particles that inhibit the grain growth and improve the
-phase stability. In turn, microalloying the alloy with
zirconium does not result in the formation of additional
phases and leads to the solution hardening of the alloy.
The average grain sizes for the weld joints of the
samples of alloys 1 and 2 did not show any significant
difference and was about 480–600 μm (Figure 1). The
fine structure of the alloys mainly consists of the
-phase, with dislocations perpendicular to the lamellae
(Figure 5b).
The -lamellae in the FZs of the alloys tend to form
small colonies that may cross differently oriented
lamellae (Figure 5c). Interlamellar / spacings in
A. G. ILLARIONOV et al.: EFFECT OF YTTRIUM AND ZIRCONIUM MICROALLOYING ON THE STRUCTURE ...
Materiali in tehnologije / Materials and technology 51 (2017) 5, 855–859 857
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 3: -phase lattice parameter (a) and (200) XRD-peak inten-
sity ratio (IHAZ /IFZ) as a function of distance from the fusion zone
of a welded joint
Figure 4: Microstructures of heat-affected zones of weld joints:
a) alloy 1, b) alloy 2
alloy 2 have a complex structure and high dislocation
density (Figure 5d). -phase interlayers are relatively
wide (up to 200 nm) and non-uniform throughout the
bulk of the alloy. Secondary -phase precipitations were
observed in the interlamellar spacings, both along and
across the  interlayers as well as at the /-interphase
boundaries (Figure 5d).
Generally, the coarse -platelets with a small dislo-
cation density prevail in both yttrium- and zirconium-
containing alloys (Figures 5e, 5f). However, alloy 1 is
distinguished by a more conventional structure of inter-
lamellar spacing. Although the thin lenticular platelets of
the secondary -phase were still present, no highly
dispersive precipitates were found on the interphase
boundaries and within the -layers. It should be noted
that no Y2O3 particles were detected in the FZ. This
might be connected with the intense overheating of the
alloy caused by welding that resulted in a dissociation of
the oxide.
A. G. ILLARIONOV et al.: EFFECT OF YTTRIUM AND ZIRCONIUM MICROALLOYING ON THE STRUCTURE ...
858 Materiali in tehnologije / Materials and technology 51 (2017) 5, 855–859
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 5: Thin structures of the weld joints of alloys 1 and 2: a), c), d) alloy 1; b), e), f) alloy 2
Figure 6: DTA curves for continuous cooling (20 °C/min cooling rate)
from 1200 °C: a) alloy 1, b) alloy 2
In order to model the welding thermal cycle, a num-
ber of DTA experiments involving continuous heating up
to 1200 °C and the subsequent cooling to 20 °C/min
were carried out. Continuous-cooling DTA curves
allowed us to determine the temperature ranges of the
-transformation (Figure 6).
A comparative analysis of the curves showed that the
-phase decomposition process in alloy 2 consisted of
three distinctive stages, whereas only two exothermal
effects were found for alloy 1. Stages I and II (at
920–750 and 700–550 °C, respectively) were attributed
to the formation of coarse primary -colonies with a low
defect density and secondary -platelets within the
-matrix. Stage III (500–400 °C) was assigned to the
precipitation of the highly dispersed -precipitates
mentioned above (Figure 5 d).
The formation of the -phase upon supercooling
below the transus temperature of the titanium alloys is
initiated by the crystallographic defects of the structure
such as grain boundaries, dislocations, etc. Yttrium binds
oxygen and thus refines the alloy. The products of the
-phase decomposition were quite free of precipitates.
On the other hand, the precipitation of the secondary
-platelets was possible in both alloys studied during
supercooling of up to about 600 °C. At lower tem-
peratures, the -phase was stabilized in alloy 1, whereas
its further decomposition occurred in alloy 2 resulting in
the formation of highly dispersed -precipitates. We
believe that all these differences in the behaviour of the
alloys are associated with different -phase stability
values, which are primarily defined by the concentration
of impurities in the alloys. Specifically, the interstitial
elements in alloy 1, mainly oxygen, are bound to yttrium
in the form of oxides. Zirconium has a lower affinity for
oxygen and thus oxygen atoms remain in the solid
solution leading to a more complete -phase decom-
position upon the cooling (Figure 2) resulting in a lower
-phase lattice parameter. With an increase in the volume
fraction of the body-centred cubic -phase, the strength
of the titanium alloys usually decreases.12 This is why
the microhardness of Y-containing alloy 1 was found to
be lower (3300±100 MPa for alloy 2) and 3000±100 MPa
for alloy 1).
4 CONCLUSIONS
The influence of the microalloying of titanium alloy
Ti–4.8À1–1.2Ìî–2.6V–0.6Ñr–0.25Fe with yttrium and
zirconium on the structure and microhardness was
considered. It was found that the alloying with yttrium
has the most noticeable effect on the structure of a weld
joint and the phase transformation of the alloy.
A yttrium addition refines the alloy by means of
binding with oxygen atoms and forming Y2O3 particles.
These particles pin the grain boundaries in both the
heat-affected and fusion zones, inhibiting the grain
growth during a welding thermal cycle. In addition, due
to a lower concentration of oxygen in the -phase,
yttrium decreases the -transus temperature of the alloy
leading to the stabilization of the supercooled -phase.
Due to a higher volume fraction of the "soft" -phase
in the yttrium-containing alloy, its microhardness was
shown to be lower than that of the alloy with zirconium.
Acknowledgment
We hereby acknowledge the support of the Ministry
of Science and Education of the Russian Federation, in
accordance with the decree of the Government of 9 April
2010, No. 218, project No. 03.G25.31.0234.
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A. G. ILLARIONOV et al.: EFFECT OF YTTRIUM AND ZIRCONIUM MICROALLOYING ON THE STRUCTURE ...
Materiali in tehnologije / Materials and technology 51 (2017) 5, 855–859 859
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
alloy 2 have a complex structure and high dislocation
density (Figure 5d). -phase interlayers are relatively
wide (up to 200 nm) and non-uniform throughout the
bulk of the alloy. Secondary -phase precipitations were
observed in the interlamellar spacings, both along and
across the  interlayers as well as at the /-interphase
boundaries (Figure 5d).
Generally, the coarse -platelets with a small dislo-
cation density prevail in both yttrium- and zirconium-
containing alloys (Figures 5e, 5f). However, alloy 1 is
distinguished by a more conventional structure of inter-
lamellar spacing. Although the thin lenticular platelets of
the secondary -phase were still present, no highly
dispersive precipitates were found on the interphase
boundaries and within the -layers. It should be noted
that no Y2O3 particles were detected in the FZ. This
might be connected with the intense overheating of the
alloy caused by welding that resulted in a dissociation of
the oxide.
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858 Materiali in tehnologije / Materials and technology 51 (2017) 5, 855–859
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Figure 5: Thin structures of the weld joints of alloys 1 and 2: a), c), d) alloy 1; b), e), f) alloy 2
Figure 6: DTA curves for continuous cooling (20 °C/min cooling rate)
from 1200 °C: a) alloy 1, b) alloy 2
In order to model the welding thermal cycle, a num-
ber of DTA experiments involving continuous heating up
to 1200 °C and the subsequent cooling to 20 °C/min
were carried out. Continuous-cooling DTA curves
allowed us to determine the temperature ranges of the
-transformation (Figure 6).
A comparative analysis of the curves showed that the
-phase decomposition process in alloy 2 consisted of
three distinctive stages, whereas only two exothermal
effects were found for alloy 1. Stages I and II (at
920–750 and 700–550 °C, respectively) were attributed
to the formation of coarse primary -colonies with a low
defect density and secondary -platelets within the
-matrix. Stage III (500–400 °C) was assigned to the
precipitation of the highly dispersed -precipitates
mentioned above (Figure 5 d).
The formation of the -phase upon supercooling
below the transus temperature of the titanium alloys is
initiated by the crystallographic defects of the structure
such as grain boundaries, dislocations, etc. Yttrium binds
oxygen and thus refines the alloy. The products of the
-phase decomposition were quite free of precipitates.
On the other hand, the precipitation of the secondary
-platelets was possible in both alloys studied during
supercooling of up to about 600 °C. At lower tem-
peratures, the -phase was stabilized in alloy 1, whereas
its further decomposition occurred in alloy 2 resulting in
the formation of highly dispersed -precipitates. We
believe that all these differences in the behaviour of the
alloys are associated with different -phase stability
values, which are primarily defined by the concentration
of impurities in the alloys. Specifically, the interstitial
elements in alloy 1, mainly oxygen, are bound to yttrium
in the form of oxides. Zirconium has a lower affinity for
oxygen and thus oxygen atoms remain in the solid
solution leading to a more complete -phase decom-
position upon the cooling (Figure 2) resulting in a lower
-phase lattice parameter. With an increase in the volume
fraction of the body-centred cubic -phase, the strength
of the titanium alloys usually decreases.12 This is why
the microhardness of Y-containing alloy 1 was found to
be lower (3300±100 MPa for alloy 2) and 3000±100 MPa
for alloy 1).
4 CONCLUSIONS
The influence of the microalloying of titanium alloy
Ti–4.8À1–1.2Ìî–2.6V–0.6Ñr–0.25Fe with yttrium and
zirconium on the structure and microhardness was
considered. It was found that the alloying with yttrium
has the most noticeable effect on the structure of a weld
joint and the phase transformation of the alloy.
A yttrium addition refines the alloy by means of
binding with oxygen atoms and forming Y2O3 particles.
These particles pin the grain boundaries in both the
heat-affected and fusion zones, inhibiting the grain
growth during a welding thermal cycle. In addition, due
to a lower concentration of oxygen in the -phase,
yttrium decreases the -transus temperature of the alloy
leading to the stabilization of the supercooled -phase.
Due to a higher volume fraction of the "soft" -phase
in the yttrium-containing alloy, its microhardness was
shown to be lower than that of the alloy with zirconium.
Acknowledgment
We hereby acknowledge the support of the Ministry
of Science and Education of the Russian Federation, in
accordance with the decree of the Government of 9 April
2010, No. 218, project No. 03.G25.31.0234.
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