All language subtitles for [English] The Problem with Titanium [DownSub.com]

On September 30th, 2017, an Airbus A380 cruising
high over the North Atlantic suffered one of the

most dramatic engine failures in recent
aviation history. Air France Flight 066

was flying from Paris to Los Angeles when a
loud bang shook the cabin. One of the four

engines had torn itself apart - the entire
front fan assembly had separated in flight,

shredding the nacelle and scattering debris into
the sky. The crew diverted to Goose Bay in Canada,

and landed safely, but the engine was destroyed.
The failure was traced to the engine's fan hub,

the massive rotating component that carries the
fan blades. It was forged from a titanium alloy,

Ti-6-4. Nothing unusual there - this is the
kind of high performance application where

titanium alloys excel. But when investigators
finally pinpointed the failure mechanism,

it wasn't overstress, poor design, or conventional
fatigue. It was something far more subtle,

a hidden weakness of the material that only
appears under specific conditions. And it forced

the industry to reconsider how it uses titanium.
To understand why, we need to look at what's

happening inside titanium alloys - all the way
down at the atomic scale. If you zoom all the way

into a block of pure titanium, you'll find that,
like all metals, its atoms pack together in a very

regular, grid-like lattice. The underlying unit
of this structure is a repeating hexagonal cell,

called the hexagonal close-packed structure, HCP.
This is how titanium is structured on the atomic

scale, at room temperature. Heat it up though,
and at 882 degrees Celsius the atoms suddenly

become more stable in a different configuration
and undergo a phase transformation, re-arranging

from hexagonal close-packed to a body-centred
cubic structure, where the atoms are less tightly

packed. The lower temperature HCP structure is
called the Alpha phase, the higher temperature

BCC structure is called the Beta phase, and the
temperature at which the transformation occurs

is called the Beta transus temperature. The Alpha
and Beta phases behave quite differently, because

of their different geometries. The HCP Alpha phase
has fewer easily activated slip systems - specific

planes and directions along which defects
in the crystal lattice can easily move.

This means the Alpha phase tends to be stronger
and stiffer but less ductile than the Beta phase.

Its closer-packed structure also makes it more
difficult for atoms to move through the lattice,

making the Alpha phase more resistant to
time-dependent deformation like creep,

and better able to maintain its structure at high
temperatures. You can't actually get completely

pure titanium. Even when highly purified it always
contains trace amounts of impurity elements,

like Oxygen, Iron, Nitrogen, Carbon and Hydrogen.
The Oxygen atoms in particular have a big impact

on the properties of titanium. They sit in
the gaps between titanium atoms, distorting

the lattice structure and making it harder for
defects to move through it, which increases

the material's strength. Titanium is said to be
"commercially pure" if made up of less than 1%

of these impurities, and there are actually four
different grades of commercially pure titanium,

depending on the level of impurity elements.
Grades with higher impurity levels tend to have

higher yield strength, but reduced ductility,
mainly because they contain more oxygen.

Commercially pure grades are highly
corrosion-resistant and reasonably ductile, but

have quite low strength, making them well suited
to applications like chemical processing equipment

or medical devices, where durability in aggressive
environments matters more than mechanical

performance. But when high strength is essential,
like in fasteners, pressure vessels or aerospace

components, Titanium is alloyed with other
elements to produce materials with much better

mechanical performance. Some alloying elements,
including aluminum and oxygen, act as Alpha

stabilisers - they make the Alpha phase stable at
higher temperatures. The more Alpha stabilisers

you add, the higher the temperature needed
to transform Alpha into Beta. The transition

is also less abrupt - it now happens over a range
of temperatures, where both Alpha and Beta phases

can co-exist. Other elements like Vanadium and
Molybdenum act as Beta stabilisers, lowering the

transformation temperature. Add enough of them,
and the Beta transus temperature can be lowered to

well below room temperature. This leads to three
main categories of Titanium alloys - Alpha, Beta,

and Alpha-Beta alloys - depending on which phases
are present at room temperature. Alpha-Beta alloys

offer a good balance of properties, combining the
strength and creep resistance of the Alpha phase

with the ductility of the Beta phase. It's no
surprise, then, that the most widely used Titanium

alloy falls into this category. Ti-6-4 contains
6% aluminum - an Alpha stabiliser - and 4%

vanadium - a Beta stabiliser. It accounts for more
than half of all commercially used titanium, and

is often used in demanding applications, including
the fan hub that failed on Air France Flight 066.

One of the huge advantages of Alpha-Beta
alloys like Ti-6-4 is that they can be

heat treated - meaning we can use carefully
controlled heating and cooling to manipulate

the balance and arrangement of the two phases.
In fact, Titanium alloys are highly sensitive

to how they've been processed, which opens
up a wide range of possible microstructures

and corresponding material properties.
The three variables that matter most are

the temperature the alloy is heated to,
the soaking time - which is how long the

alloy is held at temperature - and the cooling
rate. If the alloy is heated above the transus

temperature - a process called Beta-annealing -
any existing Alpha-Beta microstructure dissolves

to a uniform Beta phase. When subsequently cooled,
plates of Alpha form inside the prior Beta grains,

separated by thin films of retained Beta.
This is called a lamellar microstructure.

If cooled faster, the Alpha plates form more
densely and with a greater range of orientations

within each prior Beta grain. This creates a
distinctive basketweave microstructure, where the

plates intersect in a criss-crossing pattern.
If the alloy is heated to below the transus

temperature instead of above it, both the Alpha
and Beta phases are present before cooling begins.

The existing Alpha grains remain stable, while
the Beta phase transforms during cooling into

fine plates of Alpha - called secondary Alpha
- separated by thin films of retained Beta. The

result is called an "equiaxed" microstructure
- it has roughly spherical primary Alpha grains

distributed within regions of transformed Beta.
This is called the mill-annealed condition - it's

the state Ti-6-4 is typically supplied in from
the mill, and the most common microstructure

of in-service Ti-6-4 components. By heating
to just below the transus temperature you can

retain some equiaxed Alpha while still forming
lamellar regions during cooling. The result is

a "bimodal" microstructure that has a deliberate
mix of equiaxed grains and lamellar plates. If

cooling from above the transus temperature is done
very rapidly, by quenching in water for example,

the Beta phase transforms almost instantaneously
into a very fine needle-like structure called

Alpha-prime. Alpha-prime has the same underlying
hexagonal close-packed structure as normal Alpha,

but because cooling is too fast
for the atoms to fully re-arrange,

the lattice is trapped in a strained, distorted
configuration. This is called martensite. The

alloy can then be re-heated to around 500 degrees
Celsius in a process called "aging". This causes

the strained Alpha-prime phase to break down
into a fine mix of Alpha and Beta. The very

fine scale of the resulting microstructure
creates numerous phase boundaries that hugely

impede the motion of dislocations through the
lattice, which significantly increases strength.

Each of these microstructures - lamellar,
basketweave, equiaxed, bimodal, and martensite

- can be either fine or coarse, depending mainly
on how long the alloy is held at high temperature.

This makes titanium alloys very versatile.
With the same chemical composition, you can

produce a huge range of different microstructures
just by changing the thermal processing route.

These different microstructures lead to different
material properties - an equiaxed structure might

be chosen for conventional fatigue resistance,
a lamellar or basketweave structure for fracture

toughness or high-temperature performance,
and a martensitic structure for high

strength. — Titanium alloys, including Ti-6-4,
are mainly used in high performance applications,

where their high cost can be justified by
three key advantages - good performance at high

temperatures, outstanding corrosion resistance,
and an unmatched strength-to-weight ratio. When

SpaceX engineers were first designing the Falcon
9 rocket, a key challenge was figuring out how to

guide the booster back down to earth. They decided
to use grid fins, small gridded control surfaces

that help steer the booster during descent and
landing. At first these were made from an aluminum

alloy. But engineers soon realised the material
wasn't quite up to the job. Although it was

very light, it only barely withstood the intense
aerodynamic heating experienced during descent.

They eventually made a change, swapping out
aluminum for a titanium alloy that could survive

the extreme heat over and over again, unlocking
true reusability. Titanium's corrosion resistance

is just as impressive. To understand why, we need
to look closely at what happens at the surface

of the material. The moment a fresh titanium
surface is exposed to air, oxygen atoms react

instantly on contact with the surface, forming an
extremely thin layer of a tightly bonded material

called titanium dioxide. This layer grows as
oxygen migrates inwards, until it reaches a

thickness of around 5 nanometres, at which point
the oxygen can no longer move through it and the

layer is fully established. Despite being so thin,
this layer "passivates" the titanium, acting as

a very effective barrier that prevents the atoms
underneath it from reacting with the environment.

This allows titanium to resist attack in a wide
range of environments with very little degradation

compared to most structural metals. This is even
true inside the body. The corrosion resistance

and chemical stability provided by the oxide layer
give titanium exceptional biocompatibility, making

it an excellent material for medical applications,
like dental implants, hip replacements, and bone

plates. Even more remarkably, bone can actually
bond directly to, and grow on, the titanium oxide

layer in a process called osseointegration. This
creates a strong, lasting connection between the

bone and the implant that can actually get
stronger over time, as the bone remodels and

densifies in response to mechanical stress. While
the naturally formed oxide layer already provides

excellent corrosion resistance, it's sometimes
beneficial to increase the thickness of the layer

in a controlled way, to increase wear resistance
in high contact areas, for example, or to provide

electrical insulation, since the passive layer
is non-conductive. This is done using a process

called anodisation. The titanium part is placed
in an electrolytic bath, a solution that contains

a dissolved electrolyte, allowing it to conduct
electricity. The part is connected to the positive

terminal of a power supply, making it the anode.
A separate part - usually a piece of stainless

steel - is connected to the negative terminal and
used as a cathode, completing the circuit. When

a voltage is applied, the electric field drives
oxygen from the electrolyte through the existing

oxide layer, where it reacts with the titanium
underneath. This can push the titanium oxide

layer well beyond its naturally self-limiting
thickness of 5 nanometres. The thickness can

be precisely controlled by adjusting the applied
voltage, allowing it to reach over 200 nanometres.

This brings the thickness of the oxide layer
closer to the wavelength of visible light,

causing it to interact with light waves
in interesting ways. When light reaches

the titanium surface, some of it reflects
immediately, and some enters the oxide layer,

reflecting off the underlying titanium instead.
The extra distance travelled by the light that

passes through the oxide layer causes a phase
shift between the two sets of reflected waves.

When they recombine, they interfere with each
other - wavelengths that are in phase reinforce

one another, and wavelengths that are out of
phase cancel out. This effect - called "thin

film interference" - gives the anodised part
color. It's the exact same effect you see in

soap bubbles or on the surface of an oil slick.
The specific color of the titanium part depends

directly on the oxide layer thickness,
which is controlled by the applied voltage.

So anodisation isn't just used for protection
- it's also a practical way to add those

characteristic vibrant colors to titanium parts
without any dyes or paints. Corrosion resistance

and high-temperature performance make titanium
valuable in a lot of different applications.

But the real reason it's become such an important
material in aerospace is another of its defining

properties - its incredible strength-to-weight
ratio. Titanium has a density that's roughly

halfway between steel and aluminum. If a titanium
bar has a mass of 100 grams, a steel bar of

the same volume would weigh 175 grams, while an
aluminum bar would weigh just 60 grams. Stiffness

follows the same trend. The Young's modulus of
titanium sits between the two other materials,

so for the same applied force the aluminum bar
would stretch more and the steel bar would stretch

less. It's when we look at strength that titanium
starts to get really interesting though. A common

aerospace alloy like Ti-6-4 has a yield strength
of around 900 Megapascals in typical heat-treated

conditions. That's much higher than high-strength
aluminum alloys and comparable to many structural

steels, at less than 60% of the mass. On a
strength-to-weight basis, titanium alloys

outperform pretty much all traditional structural
metals. That's why they're used extensively in

aircraft landing gear - where you need high
strength, but can't accept the mass penalty of

steel, or the thicker cross-sections you would
need with aluminum. In fact titanium accounts

for around 15% of the mass of modern commercial
aircraft. You'll find it in the engines too - in

compressor blades and discs, shafts, frames, and
- of course - in the fan blades and hubs. So what

happened to Air France Flight 066? In the months
after the incident, search teams began recovering

debris from the ice sheet in Greenland. This
was a huge operation. Some of the most critical

fragments were buried deep in the snow, but
were eventually found after almost two years of

searching thanks to experimental radar technology.
This meant the French crash investigators could

finally pinpoint the cause, and the results were
surprising. They attributed the failure to a crack

that initiated early in the life of the titanium
fan hub, and grew progressively until catastrophic

failure. But this wasn't a conventional fatigue
crack. The fan hub had failed just a quarter of

the way into its expected life, based on normal
fatigue assumptions. The real cause was a hidden

weakness in titanium alloys - their susceptibility
to a unique failure mechanism called "cold-dwell

fatigue". Unlike conventional fatigue,
which occurs when a component is subjected

to cyclic loading, allowing cracks to grow
incrementally with each stress reversal,

cold-dwell fatigue occurs when cyclic loading
includes a sustained hold at high stress. This

kind of loading is common in rotating components
in jet engines, which see high-stress dwell

periods at various points throughout the flight
cycle, including during takeoff and climb, where

engine thrust is high and components experience
high sustained centrifugal forces. In the fan hub,

spinning of the blades causes a high stress to
develop in the hoop direction. It's called "cold"

dwell fatigue because it occurs at temperatures
well below those where you'd normally expect

creep or other time-dependent deformation - and in
some cases it can even occur at room temperature.

Cold-dwell fatigue is a consequence of the Alpha
phase within the alloy. The hexagonal close-packed

structure is highly anisotropic - its resistance
to deformation depends on the direction of

loading. The material is much stiffer when a load
is applied perpendicular to the hexagonal plane,

than when the load is applied parallel to it.
In the equiaxed microstructure of Alpha-Beta

alloys like Ti-6-4, the hexagonal close-packed
structure has a consistent orientation within

each Alpha grain. Depending on the direction
of the applied load, some grains are oriented

such that they deform more easily - these are
"soft" grains. Others have an orientation that

better resists deformation - these are "hard"
grains. The softer regions deform more than

the hard ones when a load is applied, and they
keep deforming over time when the load is held,

causing the load to redistribute and gradually
shift onto the harder regions. This isn't a

problem when orientations are random from grain
to grain - the redistribution happens everywhere,

so no single region accumulates too much stress.
But the danger arises when the material contains

a large cluster of similarly oriented grains,
forming a so-called "micro-texture region",

or "macro-zone". These regions can form
when groups of Alpha grains inherit

similar orientations from the larger
prior Beta grains they formed within,

and deformation applied during forging isn't
sufficient to fully break up and randomise

these orientations. A cluster of grains that's
"hard" relative to the load direction doesn't

contain any "soft" grains within it to
redistribute the applied stress. Under

prolonged dwell loading the softer surrounding
material continues to deform, transferring more

and more load into the hard region, and creating
a stress concentration that can cause cracking

to initiate. And that's exactly what caused
the engine failure on Air France Flight 66.

The French investigators concluded that dwell
periods accelerated the initiation of a crack in a

micro-texture region in the fan hub, which cyclic
loading then grew to failure. Cold-dwell fatigue

failures had happened before, but it was thought
to mainly affect Alpha alloys. This was the first

time a component made of Ti-6-4 - the workhorse
titanium alloy of the aerospace industry - had

failed in-service due to cold-dwell fatigue.
This had huge implications. Two parts could have

identical compositions, identical strength and
ductility, pass every standard qualification test,

and yet have hugely different dwell fatigue lives
depending on their grain structure. The industry

responded immediately. Fabrication methods were
re-examined to minimise micro-texture regions.

New techniques were developed to map grain
orientations in forging samples. And inspection

regimes were tightened to check for early
cracking in the existing fleet. Titanium

is an extraordinary material, and millions of
titanium components fly safely every day. But

the failure of Air France Flight 66 showed
that even the most well-understood materials

can behave in unexpected ways. And sometimes it
takes a failure to push our understanding forward.

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