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 (FeTiO3); rutile (TiO2); arizonite
(Fe2Ti3O9); perovskite (CaTiO3) and titanite (CaTiSiO5).
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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 1980 till 1990,
because of the aerospace industry and military market. But it 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 + 𝑇𝑖𝐶𝑙4 → 2𝑀𝑔𝐶𝑙2 + 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: such as engines and
airframes.
 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
882oC 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
882oC.
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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 αalloys, 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-15V3Cr-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 Al content causes good strength
characteristic and oxidation resistance up to
600°C.
Al is soluble up to ~16 wt% in α- Ti - and raises
.the α/β transformation temperature from
882 to 1172 oC
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 excellent corrosion resistance in most
environments, except those media that contain fluoride ions.
 Titanium alloys show less resistance to corrosion than CP titanium and the main
problem with them appears to be crevice corrosion which occurs in locations where
the corroding media are virtually stagnant.
 Titanium has limited oxidation resistance in air at temperatures above
approximately 650oC, Titanium and its alloys resist H2S and CO2 gases at
temperatures up to 260oC .
 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 weak in very strong oxidation
environments; presence of fluoride ions; continuous wear or sliding contact
conditions with other metals.
 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 less than 10nm thick and are invisible to
the eyes.
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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 into Titanium. If it does, the dissolved
hydrogen can severely embrittle titanium. The potential for embrittlement is
increased where hydrogen flow rates are high or where the coating on
titanium becomes damaged.
 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 in β phase can reach as high as 50% at
elevated temperatures above 600oC. However, in α phase the solubility is only
7% at 300oC and decreases rapidly by decreasing temperature.
Why?
 the higher solubility in β phase results from the relatively open body centered
cubic structure which consists of 12 tetrahedral and six octahedral interstices.
In comparison, the hexagonal close packed lattice of α phase exhibits only 4
tetrahedral and 2 octahedral interstitial sites.
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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. The δ – hydrides (TiHx)
which has 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 37oC).
 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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http://www.knovel.com/web/portal/basic_search/display?_EXT_KNOVEL_DIS
PLAY_bookid=3144
http://www.springerlink.com/content/t77602/#section=343739&page=1
http://www.worldscibooks.com/etextbook/4311/4311_chap01.pdf
http://www.springerlink.com/content/p24707u210853575/
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