MSE 258:Physical Metallurgy of Non-Ferrous Metals

Dr. Emmanuel Kwesi Arthur

Department of Materials Engineering,

Kwame Nkrumah University of Science and Technology, Kumasi

2019

Lecture Eleven

Refractory Metals and Alloys

Refractory metals include tungsten (W), molybdenum (Mo), tantalum (Ta) and niobium (Nb).

Called refractory because of their high melting points.

Used in steels and superalloys because they maintain their strength at high temperatures.

Their metallic bonds are weak resulting in low yield strengths.

They have exceptionally high melting temperature (>1925 oC) and so have high-temperature service.

They have a BCC structure so they display a ductile to brittle transition temperature. Hot forming these materials aids their fabrication into useable products.

Applications include filaments for light bulbs (tungsten), rocket nozzles, nuclear power generators (niobium), electronic capacitors (tantalum and niobium) and chemical processing equipment.

Properties of Some Refractory Metals

T=1000oC

Metal Melting temperature [oC]

Density [g/cm3]

Tensile strength[MPa]

Yield strength[MPa]

TransitionTemperature [oC]

Nb 2468 8.57 117 55 -140

Mo 2610 10.22 345 207 30

Ta 2996 16.6 186 165 -270

W 3410 19.25 455 103 300

Niobium (Nb)

First identified in 1801.

Also known as Columbium.

Has good ductility and formability and has greater oxidation resistance than other refractory metals.

Used in rockets and missiles and in nuclear, chemical, and superconductor applications.

Processed from ores by reduction and refinement and from powder by melting and shaping into ingots.

Molybdenum (Mo)

A silvery-white metal.

Discovered in the 18th century.

Has high melting point, high modulus of elasticity, good resistance to thermal shock, and good electrical and thermal conductivity.

Needs a protective coating because of low resistance to oxidation at high temperatures.

Used in solid-propellant rockets, jet engines, honeycomb structures, electronic computers, heating elements, and dies for die casting.

Principle alloying element for titanium and zirconium.

Tantalum (Ta)

Characterized by its high melting point (3000°C), high density, good ductility and resistance to corrosion.

Used mainly in electrolytic capacitors and various electrical, electronic and chemical industries.

Sometimes used in thermal applications such as in furnaces and acid-resistant heat exchanges.

Processed from ores by reduction and refinement and from powder by melting and shaping into ingots.

Tungsten (W)

First identified in 1781.

Most abundant of all refractory metals.

Highest melting point of any metal at 3410° C

High strength at high temperatures.

Has high density (which makes it brittle at low temperatures).

Used in hottest part of missiles and rockets, weldinging electrodes, spark-plug electrodes, and the wire filament in incadescent bulbs.

Processed from ore concentrates by chemical decomposition and is then reduced.

Titanium and Titanium Alloys Discovered in 1791 not produced commercially

until 1950.

Titanium is the forth abundant metal on earth crust (~ 0.86%) after aluminium, iron and magnesium.

Not found in its free, pure metal form in nature but as oxides, i.e., ilmenite (FeTiO3) and rutile (TiO2).

Highly expensive, but posses a very high strength to weight ratio, and corrosion resistance at room and elevated temperatures.

Very extensive process for production which adds to the cost of titanium.

Must be handled carefully while being produced to ensure quality of final product.

Titanium Alloys While there are numerous other nonferrous alloys such as Mg, Zn,

and Zr, which can be considered for special applications, the alloys that has received considerable attention is Titanium, Ti.

Ti is relatively light weight (4.54 g/cm3)

Ti has excellent corrosion resistance, especially in sea water

It has a high strength to weight ratio

Some Ti alloys have a strength of over 1000 MPa

It’s surface oxide (TiO2) breaks down at ~500oC, which limits the range of its use, but surface coatings help to increase it’s range of use to much higher temperatures (~1000oC).

Ti has a HCP structure at room temperature and changes to BCC at 882oC – alloying with niobium stabilizes the BCC phase to room temperature, which enhances fabricability.

Typically, other elements added to Ti increase its strength and machinability.

Density of selected metals

Specific strength vs temperature

•Has the highest strength to weight ratio

Disadvantages High cost Difficulty in extraction Limited in high performance applications

Advantages

Advantages and Disadvantages of Titanium alloys

Titanium and its alloys

Excellent Corrosion Resistance- Titanium is immune to corrosive attack by salt water or marine atmospheres. It also exhibits exceptional resistance to a broad range of acids, alkalis, natural waters and industrial chemicals.

Superior Erosion Resistance- Titanium offers superior resistance to erosion, cavitation or impingement attack. Titanium is at least twenty times more erosion resistant than the copper-nickel alloys.

High Heat Transfer Efficiency- Under ‘in service’ conditions, the heat transfer properties of titanium approximate those of admiralty brass and copper-nickel.

• Used mainly inaerospace, marine,chemical, biomedicalapplications and sports.

Turbine blades

Hip-joint component

National science centre, Scotland

Shape memory alloy

Material Tensile

Strength

[MPa]

Yield Strength

[MPa]

% Elongation

Commercial pure Ti

99.5% Ti 241 172 24

99.0% Ti 552 482 15

Alpha Ti alloys

5% Al-2.5% Sn 862 779 15

Beta Ti alloys

13% V-11% Cr-3% Al 1289 1213 5

Alpha-beta Ti alloys

6% Al-4% V 1034 965 8

Properties of Selected Titanium Alloys

Have similar strength as steel but with a weight nearly half of steel.

Titanium Aluminium Iron Mg

Density, g/cm3 4.54 2.70 7.87 1.77

Modulus of Elasticity MPa 115,831.92 62,052.82 196,500.58 44,815.92

Melting point, oC 1668 660 1536 649

Crystal structure at room temperature HCP FCC BCC HCP

Strength, MPa 1034 414 1517 241

Selected Properties of Titanium as Compared to Aluminium, Iron and Magnesium