Note: Descriptions are shown in the official language in which they were submitted.
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Title of invention
High strength, oxidation and wear resistant titanium-silicon alloy
Field of invention
The present invention concerns high strength silicon-containing titanium-based
alloys with optionally additives of aluminium, boron, chromium, scandium and
rare
earth metals (Y, Er, and Ce and La containing misch metal).
Background art
A variety of two phase a/p-titanium and near a-titanium alloys, such as Ti-6AI-
4V,
~ Tm
IMI 834 (Ti-5.8-AI-4Sn-3Zr-0.7Nb-0.5Mo-0.35Si-0.06C) and TIMET 1100 (Ti-6AI-
2.7Sn-4Zr-0.4Mo-0.45Si) show great potential application in the air plane and
space industry.
Among them Ti-6AI-4V exhibits the broadest application due to an optimum
combination of high strength and fracture toughness and excellent fiatigue
properties at room and elevated temperature. These alloys have, however, some
disadvantages such as a poor oxidation resistance above 475 C (a-case
formation), insufficient creep strength at 600 C and higher temperatures and a
poor wear resistance at room and elevated temperatures. The a-case causes
crevice formation on the oxidised surface and has a detrimental effect on the
fatigue properties. The arc melting process of these relatively high melting
point
alloy of about 1660 C) and the necessary melt overheating to about 1750 to
1770 C is a very energy consuming procedure for the manufacture of investment
castings for the air plane and automotive industry, and engineering purposes
in
general.
Low silicon-containing titanium-based alloys are well known. Thus JP
2002060871 A describes a titanium alloy containing 0.2 - 2.3 wt % Si, 0.1 -
0.7 wt
% 0 (total content oxygen), and 0.16 - 1.12 wt % N and 0.001 - 0.3 wt % B and
remainder of titanium including unavoidable impurities, used for as cast
products.
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These are e.g. golf club heads, fishing tackles and medical components such as
tooth root, implants, bone plates, joints and crowns. The low silicon-
containing
titanium-based alloy does, however, suffer from a disadvantage, by forming
small
needle like Ti3Si precipates along grain boundaries, which decrease the
fracture
toughness and ductility of this material.
From the paper "Structures and properties of the refractory silicides, Ti5Si3
and
TiS2 and Ti-Si-(Al) eutectic alloys", by Frommeyer et. al. published in May
2004, it
is on page 301 described a hypereutectic Ti-Si7.5-All alloy. It is further
stated
that with increasing silicon content up to about 9% by weight, the
microstructure of
the cast samples consists of fine dispersion of Ti5Si3 silicide particles
within the a-
Ti(Si) solid solution matrix.
The alloys described by Frommeyer et. al. have excellent hardness and flow
strength. The warm strength of the Ti-Si-Al alloys is, however, moderate and
there is no indication of the oxidation resistance at high temperature.
There is thus a need for an alloy that has a high strength at high
temperatures,
has a lower melting point than the Ti-Al-V alloys and has good casting
properties.
Description of invention
By the present invention it is provided Ti-Si alloys with relatively high
silicon
contents which exhibit a relatively low melting point due to their eutectic
constitution, good casting properties and high strength at higher temperatures
as
well as a very high resistance to oxidation and creep deformation at high
temperatures.
The present invention thus relates to a Ti-Si alloy comprising 2.5 - 12 wt %
Si, 0-
5wt loA1,0-5wt%Cr,0-0.5wt%B,0-1 wt%rareearth metals and/or
yttrium and/or Sc, the remaining except for impurities being Ti.
According to a preferred embodiment the alloy contains 0.3 - 3 wt % Al.
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According to another preferred embodiment the Ti-Si alloy contains 3 - 6 wt %
Si
and 1.2-2.5wt%AI.
According to yet another preferred embodiment the alloy contains 0.001 to 0.15
wt % rare earth metals and/or scandium.
It has been found that the addition of rare earth metals and/or yttrium and/or
scandium improves the warm strength and creep strength of the Ti-Si alloy up
to
at least 675 C,
The rare earths yttriym and scandium additions form a fine dispersion of
thermo-
dynamically stable oxides, such as Er203, Y203 etc. in the Ti-Si alloy.
The alloy preferably contains 0.1 to 1.5 wt % Cr. The addition of Cr enhances
solid solution hardening and therefore increases the strength and increase the
oxidation resistance of the alloy.
In the as cast state, the Ti-Si alloy possesses fine-grained hypoeutectic,
eutectic
or slightly hypereutectic microstructures depending upon the silicon content.
The
microstructure of the eutectic Ti-Si alloy consists of finely dispersed Ti5Si3
silicide
particles of discontinuous rod like shape within the hexagonal close-packed a-
Ti(Si) solid solution matrix. The hypoeutectic microstructure consists of
primary
solidified a-Ti(Si) crystals and the surrounding eutectic.
The Ti-Si alloy according to the invention has with a yield stress of at least
800
MPa, a Brinell hardness of 350-400 HB and sufficient ductility and fracture
toughness -stress intensity factor Kic . of more than 23 MPa.,Fm- at room
temperature and up to 500 C.
The Ti-Si alloy according to the invention further exhibits excellent
oxidation
resistance up to 650 C and above depending upon the Si content and improved
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wear resistance both at room and elevated temperature. The yield strength at
650 C will be of at least RPo 2 >250 MPa and the tensile strength exceeds Rm =
450 MPa.
The hypereutectic microstructures consist of primary solidified Ti5Si3
crystals of
hexagonal shape within the fine-grained eutectic microstructure.
In the as cast state the hypoeutectic Ti-Si alloys exhibit at room temperature
fractures toughness -ICic-values- of more than 23 MPa [m--, yield stress of
more
than 500 MPa with a plastic strain of more than 1.5 to 3 %.
The eutectic alloy shows a fracture toughness of ICic of 15 - 18 MPa -~m and
the
yield stress exceeds 850 MPa at room temperature. At 600 C and above the
fracture toughness is increased to 30 MPa -\rm and the strength is of the
order of
at least Rm = 450 MPa.
Oxidation tests with exposure to air at 600 C have resulted in an increase in
mass
of less than 5 mg/cm2 after 500 hours. In comparison the conventional Ti-AI6-
V4 alloy exhibits alpha case formation at 475 C during long term exposure on
air.
The creep stress (applied stress at given temperature where the creep rate is
107 s"1) of the eutectic Ti-Si alloy according to the invention is higher than
200 MPa
at 600 C. In contrast the Ti-AI6-V4 alloy with potential application in the
air plane
and space industry exhibits a creep stress of about 150 MPa at 450 C.
The Ti-Si alloy according to the invention has a low melting point of between
about 1330 and about 1380 C. The alloy according to the invention has further
excellent casting properties making it possible to cast virtually any size and
shape.
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As a result of its spectrum of characteristics properties presented above, the
Ti-Si alloy according to this invention are advantageously suitable for the
manufacture of diverse components, subjected to high temperature, such as:
5 connecting rods, piston crowns, piston pins, inlet and outlet valves and
manifolds
of exhaust gas mains in internal combustion engines and diesel engines;
static blades in axial flow compressors and fan blades in jet engines;
wear resistant parts in textile machines -weaving looms- like shuttles and
connecting shafts;
surgical implants, bone plates, joints;
hard facings and surface alloys used as coatings in surface engineering for
improving wear resistance and to avoid fretting;
watch cases;
pump cases and impellers for the chemical and oil industry.
The Ti-Si alloy according to the invention is particularly suitable for as
cast
components because of their relatively low melting temperatures of about 1330
to
1380 C and excellent castability.
The Ti-Si alloy according to the invention can be produced in conventional
way,
such as by arc melting in a water cooled copper hearth.
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Detailed description of invention
Example 1
A hypoeutectic Ti-6Si-2AI alloy according to the invention was produced by arc
melting using a non consumable tungsten electrode. Titanium sponge with a
purity of more than 99.8 wt %, metallurgical grade silicon and aluminium
granules
with a purity of more than 99.8 wt % were used as starting materials. The
alloy
was kept during arc melting in a water cooled copper hearth by forming a thin
solid skull on the copper hearth and was then cast into a copper mould in
order to
achieve rod like ingots. These were machined by turning and grinding to
cylindrical compression and tensile test samples exhibiting a smooth surface
finish.
The Brinell hardness was determined to be about 336 3 HB 187.5/2.5 applying
a testing load of 187.5 kp. The flow stress was determined at room temperature
in
compression test to be about RPo Z':~' 725 to 750 MPa and the plastic strain
exceeds -gpi 10 %. The fracture toughness was measured in a four point bend
test. The stress intensity factor Kic varies between 19 <_ Kic <_ 21 MPa 4m.
At
higher temperature of 650 C the flow stress is still 260 RPo Z 275 MPa and the
fracture toughness is about 32<_ Kic <_ 34 MPa qm. The weight gain in an
oxidation
test on air at 600 C was 4.5 mg/cm2 after 525 hrs.
Example 2
A hypereutectic Ti-10Si alloy containing 0.2 wt % Al was also produced by arc
melting technique as described above in Example 1.
The macrohardness -Brinell- of this alloy was determined to be about 365 HB
187.5/2.5 and the yield stress at room temperature ranges between 930 <_
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RPo 2 <_965 MPa depending upon the grain size of the alloy. The plastic strain
in
compression is about 6 to 8 % and the fracture toughness is in between Kic =
16
and 19 MPa ~m.
At higher temperature of 650 C the yield stress is about 330 to 360 MPa. The
fracture toughness is in between 25 and 28 MPa 4m. The creep strength was
determined at 600 C and exhibits values of 215 to 230 MPa in the coarse-
grained
state.
The oxidation on air at 650 C leads to a weight gain of about 3.8 mg/cm3 at
500 hrs exposure time.
Example 3
A hypoeutectic (near eutectic) oxide dispersion strengthened Ti-7Si-2AI alloy
with
addition of 0.07 mass-% Y was also produced by the arc melting technique
described in example 1. Metallic Yttrium was added to the melt and formed Y203
with the dissolved oxygen of about 1200 ppm. The Brinell hardness was
determined to be 347 2 HB 187.5/2.5. The measured yield strength was about
960 to 990 MPa. First creep experiments at 600 C with the creep rate of 10-7s
1
showed a creep strength in between 235 and 255 MPa.
Example 4
A hypoeutectic oxide dispersion strengthened Ti-5.5Si-3.5AI.-1.5Cr-0.1Y alloy
was
produced by the melting method technique described in Example 1. Metallic
yttrium was added to the melt and formed Y203 with oxygen dissolved in the
melt.
The Brinell hardness was measured to 373 2 HB at a load of 187.5 Kp at room
temperature and the fracture toughness stress intensity was measured to Kic =
21
MPa,[m--. At 650 C the tensile strength was measured to about Rm = 360 MPa,
the fracture toughness was between 35 and 40 MPaVm and the creep strength at
the strain rate of s=10-7 s"1 excerted 270 MPa.
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Oxidation tests at 600 C in air exhibits a mass gain of less than 8 mg/cm3
after an
exposure time of 500 hours. For comparison, the oxidation tests of the
commercial Ti-6AI-4V alloy shows a mass gain of more than 20 mg/cm3 after 500
hours exposure in air at 600 C.
These examples show that the Ti-Si alloys of the present invention have a
surprisingly high warm strength and very good oxidation resistance at high
temperatures.
