Transcript Document
Titanium is widely distributed throughout the whole universe such as stars and interstellar dust but,
after Al; Fe and Mg
, titanium is the fourth most abundant of
structural metals
and is the ninth most abundant element on the earth.
Although the
commercial production of titanium did not begin till 1950's
by the Titanium Metals Company of America (TMCA), this element has been recognized over at least 200 years, which is first discovered in minerals now known as rutile.
Titanium exists in most minerals such as ilmenite (FeTiO 3 ); rutile (TiO 2 ); arizonite (Fe 2 Ti 3 O 9 ); perovskite (CaTiO 3 ) and titanite (CaTiSiO 5 ).
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Titanium was first discovered by the
Reverend William Gregor in 1790
who was a clergyman and amateur mineralogist.
Little interest was shown in the discovery by Martin Heinrich
Klaproth, a German chemist, in 1795.
There was a close agreement between Gregor’s discovery and his investigations on a black sand contained 51% iron oxide; 42.25% titanium oxide; 3.5% silicon oxide and 0.25 magnesium oxide (ilmenite) and Klaproth’s investigations on a wine-red crystal which is known as rutile (titanium oxide).
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The identity of two substances established soon and Klaproth applied the temporary name of "
Titanium" after the Titans, the powerful sons of the earth in Greek mythology.
Interests in the properties of titanium started after the Second World War, in the late 1940s and the early 1950s, Especially in USA, Government sponsored programs led to the installation of large capacity titanium sponge (the product type of kroll process) production plants, for example at TIMET (1951) and RMI (1958).
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In Europe, large scale sponge production started in 1951 in UK. In France, titanium sponge was produced for several years but discontinued in 1963.
In Japan sponge production started in 1952 and two companies, Osaka Titanium and Toho Titanium had relatively large capacities by 1954.
By 1979, The Soviet Union became the world's largest titanium sponge producer.
Worldwide capacity of titanium sponge increased steadily from
because of the aerospace industry and military market
. But it
1980 till 1990, dropped sharply from 1990 to 1995 due to the military budget decrease in USA
and finally, after the minimum
in 1994 it increased again which was the result of the pick up in commercial aero planes sales
.
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The production of ductile, high purity titanium still proved to be difficult, because of the strong tendency of this metal to react with oxygen and nitrogen .
There are some commercial methods for producing titanium like: sodium reduction process (or Hunter process); direct oxygen reduction process; electrolytic process. But, the most famous titanium production method is Kroll process.
spongy and porous , “titanium sponge”
2𝑀𝑔 + 𝑇𝑖𝐶𝑙
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→ 2𝑀𝑔𝐶𝑙
2
+ 𝑇𝑖
It is removed from the titanium by distillation under very low pressure at a high temperature
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Due to main features of titanium:
High strength to weight ratio, Low density, High corrosion resistance, Biocompatible (non-toxic and it is not rejected by the body), This metal is a very applicable material for many uses.
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Titanium applications generally are classified into several main groups :
Aerospace Applications:
airframes.
such as engines and
Chemical Processing:
Many chemical processing operations specify titanium to increase equipment lifetime.
Petroleum:
In petroleum exploration and production, flexible titanium pipe's light weight, makes it an excellent material for deep sea production risers.
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Automotive applications:
Particularly in motorcycling racing, This area is extremely challenging because of its cost sensitivity.
Consumer products:
such as spectacle frames; cameras; watches; jewelries and various kinds of sporting goods.
Biomedical field:
Such as surgical implements and implants.
Architectural applications:
Such as exterior walls and roofing materials.
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Pure titanium crystalline structure undergoes a transformation from hcp (α – at lower temperature) to bcc (β – at higher temperature) by increasing the temperature up to 882 o C and The mentioned single-phase regions are separated by two-phase region of α+β.
Alloying elements in titanium are usually classified in two groups of α and β stabilizing additions depending on whether increase or decrease α/β transformation temperature of 882 o C.
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Effect of alloying additions on equilibrium phase diagrams of titanium alloys (schematically)
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α stabilizers
: Substitutional elements such as Al, Sn, Ga, Ge and etc. ; Interstitial elements such as O;N and C. Thus, unalloyed titanium and titanium alloys with α stabilizers (either singly or in combination) are called α- alloys which have hcp crystalline structure. Al is the main alloying addition in this kind of alloys and increases the transformation temperature.
there is another group of α-alloys in which there is a small amount of ductile β-phase (1 to 2 percentage of Mo or Si exist) is called
Near α-alloy
.
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Alloying elements in both mentioned group provide solid solution strengthening. α- alloys and Near α- alloys have moderate mechanical strength , good fracture toughness and good creep resistance . They can be easily welded and they don not need heat treatment. But, due to the presence of some amount of ductile β phase in Near α alloys, they may be heat treated and are
hot forged
.
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β stabilizers
: are categorized into two groups of β isomorphous elements (which are mentioned as fully stabilized β phase) and β eutectoid forming elements (which are mentioned as partially stabilized β phase) .
β isomorphous elements such as Mo; V; Nb and Ta. β eutectoid stabilizers such as Fe; Cr; Mn; Co; Cu; Si and H.
.
There are some elements such as Zr, Hf and Sn which are neutral. They lower the α/β transformation temperature slightly and then increase it again at higher concentration.
This kind of titanium alloy is heat treatable and All β alloys contain small amount of aluminum which is an alpha stabilizer.
The most highly β stabilized alloys are alloys such as Ti-3Al-8V-6Cr-4Mo-4Zr and Ti-15V 3Cr-3Al-3Sn.
β alloys are exceedingly formable and they are not suitable for low temperature applications (unlike α-alloys which are suitable for cryogenic applications.)
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α+β alloys:
α+β alloys support a mixture of α and β at room temperature They may contain (10-50)% β stabilizers at room temperature. If they contain more than 20% β stabilizers, the weld ability decreases. Because :
On quenching –
b
decomposes to hcp martensite
Aluminum (Al) is added to the alloy as α-phase stabilizer and hardener due its solid solution strengthening effect. Vanadium (V) stabilizes ductile β-phase, providing hot workability of the alloy.
The most important alpha-beta alloy is Ti-6Al-4V. High strength alpha-beta alloys include Ti-6Al-6V-2Sn and Ti-6Al-2Sn-4Zr-6Mo. They are stronger and more readily heat treated than Ti-6Al-4V.
Titanium α-β Alloys have
high tensile strength and fatigue strength, good hot formability and creep resistance up to 425 ° C.
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Ti-Al Alloy System
(Aluminum is the most widely used alloying element in titanium alloys, because it is the only common metal raising the transformation temperature and having large solubility in α and β phases.)
High characteristic and oxidation resistance up to 600°C.
Al content causes good strength Al is soluble up to ~16 wt% in α- Ti - and raises .
the α/β transformation temperature from 882 to 1172 o C An alloy with 16 wt% Al will precipitate the brittle d-phase on cooling – so a-phase solid solution alloys are usually limited to <7 wt % Al
CP (commercially pure) titanium offers
environments
, except those media that
excellent corrosion resistance in most contain fluoride ions.
Titanium alloys show less resistance to corrosion than CP titanium
problem with them appears to be
crevice corrosion
and the main which occurs in locations where the corroding media are virtually stagnant.
Titanium has
limited oxidation resistance in air at temperatures above approximately 650 o C
, Titanium and its alloys resist H 2 S and CO 2 temperatures up to 260 o C .
gases at
Unalloyed titanium
is highly resistant to the corrosion normally associated with many natural liquid environments including
seawater (almost 18 years) ; body fluids and fruit and vegetables juices.
Molten sulfur; many organic compounds (including acids and chlorinated compounds) and most oxidizing acids have essentially no effect on this metal.
How?
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The excellent corrosion resistance of titanium alloys results from the formation of
very stable; continuous highly adherent and protective oxide film.
Titanium corrosion resistance becomes
environments
; presence of
fluoride ions
; conditions with other metals.
weak in very strong oxidation continuous wear
or sliding contact In such situations, the protective nature of the oxide film and its stability and integrity can be improved substantially
by adding inhibitors to the environment
.
These naturally formed films are typically the eyes.
less than 10nm thick
and are invisible to
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A
corrosion inhibitor
is a chemical compound that, when added to a liquid or gas, decreases the corrosion rate of a material, typically a metal or an alloy. The effectiveness of a corrosion inhibitor depends on fluid composition, quantity of water, and flow regime. A common mechanism for inhibiting corrosion involves formation of a coating, often a passivation layer, which prevents access of the corrosive substance to the metal.
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Hydrogen
chemically reacts
with a constituent of the metal
to form a new microstructural phase such as hydride
which
accumulates on the grain boundaries of metallic components .Thus , makes it brittle
α+ Hydride
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Hydrogen
can be
absorbed and diffuse titanium becomes damaged.
into Titanium. If it does, the dissolved hydrogen can severely increased where
embrittle titanium
. The potential for embrittlement is
hydrogen flow rates are high or where the coating on
The strong stabilizing effect of hydrogen on the β phase field results in a
decrease of the alpha-to-beta transformation temperature from 882°C to a eutectoid temperature of 300°C.
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The maximum hydrogen solubility
elevated temperatures above 600 o C.
in β phase
However, can reach as high as
in α phase 50% at
the solubility is only
7% at 300 o C
and decreases rapidly by decreasing temperature.
Why?
the higher solubility in cubic structure which
β phase
results from the relatively open body centered
consists of 12 tetrahedral and six octahedral interstices
.
In comparison, the hexagonal close packed lattice of
tetrahedral and 2 octahedral interstitial sites
.
α phase exhibits only 4
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When
only α phase
is present,
degradation is insensitive to external hydrogen pressure
, since hydride formation in α phase can occur at virtually any reasonable hydrogen partial pressure.
In
alpha + beta alloys
, when
a significant amount of β phase
is present, hydrogen can be preferentially transported within β lattice and will react with α phase along the α/β boundaries.
Since
β alloys
exhibit very high terminal hydrogen solubility and
do not readily form hydrides
, until lately they were considered to be fairly resistant to hydrogen, except possibly at very
high hydrogen pressures
.
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Three different kinds of hydrides have been observed around
room temperature
which has . The
δ – hydrides (TiH x ) fcc structure
with hydrogen atoms occupying tetrahedral interstitial sites. (
X = 1.55 to 1.99
).
At
high hydrogen concentrations
(
X≥1.99
), δ hydride transforms to the diffusion-less ε- hydride with
fct (face centered tetragonal)
structure (c/a≤1 at temperature below 37 o C).
At
% low hydrogen concentration of (1-3)
the
metastable γ-hydride forms
, with
fct structure
of c/a higher than 1.
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