the bcc refractory metals from the table of t max temperatures – the bcc refractory metals provide...

Mech 473 Lectures

Professor Rodney Herring

The bcc Refractory MetalsFrom the table of Tmax temperatures – the bcc refractory metals provide the only

option for high temperature applications between 1200-1500 oC

Nb Tmax =1210 oC Tm = 2742 oC

Ta Tmax = 1375 oC Tm = 3293 oC

W Tmax = 1390 oC Tm = 3695 oC

Mo Tmax = 1460 oC Tm = 2896 oC

The temperature, Tmax is a temperature at which the material can maintain a stress of 69 MPa (10 ksi) for a period of 100 h without fracturing.

Tungsten AlloysTungsten (W) has the highest melting point temperature of

all the metals – which is the basis of its use for filaments for incandescent light bulbs – as cathodes in electronic and X-ray tubes – and as electrodes for welding and other arc sources

In common with other refractory bcc metals – W reacts with oxygen at high temperatures – but it is unique in not absorbing or reacting with hydrogen – and can thus be treated in a hydrogen (reducing) atmosphere

Tungsten Alloys

The recrystallization temperature of W is raised by fine dispersions of aluminum-potassium silicate and thoria (ThO2) – but the later must be used with care – as it mildly radioactive!

W light bulb filaments use ThO2 to prevent grain boundary sliding, which significantly extends their lifetime.

Why is recrystallization temperature important?

Tungsten Alloys

W is strengthened by alloying with Rhenium, Re – which raises its recrystallization temperature and hence its high temperature rupture strength

Re also lowers the ductile-brittle transition temperature of W – so that compacts formed by powder metallurgy can be cold drawn into wire – or cold rolled into sheet

The prime high temperature use of these tungsten alloys is for supports and radiation shields for furnaces

Thermocouples of W-3Re and W-25Re have also been developed for use at temperatures up to 2000 oC – in protected atmospheres

Tungsten

Due to its high mass density

– W is usually not considered to be a good material for moving components in heat engines

Specific Strength of bcc Refractory Alloys

A comparison of the specific strength (ultimate strength/mass density) – (lbs.in-2/lbs.in-3 = in) for refractory alloys shows:

1. Ta-based alloys have the lowest specific strength at all temperatures

2. Nb-based alloys have intermediate specific strength at all temperatues

3. Mo is the strongest at temperatures between 1100-1500 oC – and hence shows the best potential for raising the temperature of gas turbines

4. Thoriated W becomes the strongest at temperatures above 1500 oC – which is beyond present designs for input temperatures for gas turbines

Specific Strength of bcc Refractory Alloys

Tungsten – New Applicationas Penetrators

Penetrators used for defense purposes consisted of depleted Uranium-0.75Ti alloy and U2O3, which have small amounts of radioactive Uranium.

U was used because of it high atomic weight, which gives a strong impact when it hits the target as a penetrator.

Upon hitting the target, the penetrator explodes giving off a fine dust that escapes into the environment making it radioactive

Afterwards combat forces must either wait for the dust to blow away or enter the battle ground with special gear, however, even so, the troops are exposed to radioactivity, which is not desired.

W-based Penetrators

What is desired by the defense industry is a penetrator with equal or better performance to replace U-based alloys

W has a slightly higher density than U, 19.3 g/cm3 vs 18.9 g/cm3.

A high density is required for transferring impact force.

W-based Penetrators

At the US Army Research Laboratory, Aberdeen Proving Grounds, the Ballistic Research Laboratory tested W’s ballistic properties versus Uranium’s.

Single crystal W penetrators having orientations of [100], [110] and [111] were made to test their penetration ability by firing into semi-infinite steel blocks, standard armor material.

Their performance were compared to the standard U penetrators, U-0.75Ti alloy, with the following results (next slide)

W-based Penetrators

W-based Penetrators

The W-based penetrators performed poorer than the U-0.75Ti alloy in the [110] and [111] single crystal orientations but were slightly superior in the [100].

To understand why, metallography, transmission electron microscopy (TEM), and x-ray diffraction analysis were used to study the resulting microstructures.

Why did the [100]-oriented penetrator perform the best?

The metallography showed that the penetrators undergo extrusion and deposition of their material from the tip of the penetrator around to the sides of the hole that is created.

Extrusion Structures of W- Penetrators

[111]Single crystalorientation

[110] [100]

PenetrationDirection

PenetrationDirection

W-based Penetrators

W-material deposition

The [111]-oriented penetratror had a feathery-structured material deposition on the walls and a broad nose, which indicated large-energy and high-temperature deposition.

The [110]-oriented penetrator had an uneven material deposition, some thin and some thin areas, and a broad nose which indicated a cork-screw deposition mechanism that would require large-energy useage.

The [100] penetrator, which penetrator the furthest, had an even-layer material deposition and a pointed nose, which indicated a low-energy, efficient mechanism of material deposition.

W-based Penetrators

A small amount of penetrator material was left at the end of the penetration depth.

The structures were as follows.

W-based PenetratorsThe residual penetrator material at the end of the hole showed:

For [111] – a mushroom shape

For [110] – a tilted-orientation

For [100] – a symmetric orientation

[111] [110] [100]

[110]orienta-tion

Why?

W-based PenetratorsIn addition, the [100]-penetrator was broken into small crystals blocks made from

cracks having <100>{001} that were flowing from the tip around to the sides.

W-based PenetratorsMass transport from the tip to the sides by dislocations appears to be the main operative mechanism for the [111] and [110]

penetrators.

The deformation mechanism was determined by electron microscopy as follows.

Initially the W penetrators were single crystals.Due to deformation, dislocations interacted to form low-angle grain boundaries, which enabled mass transport of the material.

Optically these appeared as shear bands.

The dislocations were determined for their Burgers vectors and habit planes by examining the contrast of the dislocations A, B, C and D at various W crystal-electron beam orientations (<hkl>{u,v,w}), as seen in the micrographs.

W-based Penetrators

More dislocations structuresWhat is the deformation slip system for bcc W?

Recrystallized grains

Critical Resolved Shear Stress (CRSS)

• We can determine the deformation behaviour in materials using Schmid’s Law, which takes into account the orientation of an applied force and the possible slip directions and slip planes.

• The unidirectional stress, applied to the cylinder and its resolved shear stress, in the slip direction is given by:

coscos ; 0 A

F

A

F r

W-based Penetrators

There are no close packed planes in BCC materials.

The major slip system in W is <111>{110}.

A minor system is <111>{112}.

Another possibility is <111>{113}

W-based Penetrators

The CRSS analysis determined:

4 major slip systems were found to be operating during the deformation for [100] penetrator,

3 major slip systems were found to be operating

during the deformation for [111] penetrator and,

2 major slip systems were found to be operating

during the deformation for [110] penetrator.

The more slip systems operative, the easier and more efficient the mass transport.

W-based Penetrators

The CRSS determined for [100], [111] and [110] orientations. Note that CRSS is zero for 90 degrees applied stress.

CRSS = cosx cos

W-based Penetrators

Mass transport for the [100] penetrator was the most efficient because it occurred not only by dislocations but also by the flow of small crystals.

What could be the mechanism for the creation of the cracks to form the small crystals?

Dislocations, what else!

W-based PenetratorsSmall crystals possibly formed by a [100] crack formation mechanism from the

following dislocation slip system:

00110011011120111112

ooo aaa

This dislocation mechanism requires a much larger energy to form because its Burgers vector, B=ao[001] is much larger than the dislocations having B=ao/2[111].

Some evidence by TEM supports this mechanism but it is not yet definitive.

Note that the habit plane of the dislocations forming cracks is (001) and not the (011) or (021).

W-based Penetrators

The formation of the crack on the {001} by the interaction of <111> dislocations on the {011}.

Formation of Small W Crystals

The cracks formed by dislocations having B=ao[001] on the {001}:

1) enable cubic crystals to be formed from the 6 surfaces having cracks

2) the small cubes of material haven’t deformed and slide past each other during the mass transfer from the tip to the sides of the penetrator

3) enable less energy for the deformation

The more slip systems operative, the easier and more efficient the mass transport.

Summary

W-based penetrators are the ideal material to replace U-based penetrators as they have done since they can penetrate steel armor as well as U-based penetrators.

W-based penetrators solved the contamination problem of radioactivity of the battle field.

Still to be solved is verifying the mechanism to make the small single crystals from the starting [100] oriented W crystal penetrator.

The End

Any questions or comments?

W-based Penetrators

The interaction of two ao/2<111>{011} dislocations to form a B=ao[001] dislocation on the (001).