Light alloys

Aluminium alloys are the main type of light alloys. However, other types of low density metals are also considered as light alloys. They are titanium and magnesium alloys.

Titanium alloys

Titanium alloys are used in various applications such as medical, sporting, consumer, transportation, chemical and down-hole. They were first used in aircraft in the 1950's by Pratt & Whitney and Rolls Royce in the first fan-type turbine engines. These days, Ti alloys account for 25-35% of the total weight of modern turbine engines. They are used where high mechanical properties at elevated temperatures are required, namely:

• Large front fan
• Stator
• Rotor blades & discs of low to moderate temperature sections of the compressor (up to 600ºC)
• Compressor casing
• Fan frames
• Bearing housings
• Ducts
• Vanes

The use of Ti alloys in the airframe structures has increased in the mid-1980's but it still represents only a small amount of the whole composition (4-6% in non-military aircraft). However, some military aircraft such as the F-14 Tomcat and F-15 Eagle use large amounts of Ti alloys (25-26% of the whole structure).

Ti alloys are used because they have the following properties:

• High specific Young's modulus and specific yield strength (up to 1 400 MN.m^-2)
• Relatively high specific gravity compared to Al alloys (4.505 Mg.m^-3)
• Some alloys retain good strength up to 400-500ºC
• Good corrosion and contamination resistance due to its protective TiO₂ film (which breaks down at temperatures above 535ºC)

However, mechanical properties are affected by high temperatures and stress in corrosive environments, and they also are expensive. In fact, a tonne of Ti alloy costs around $AU100-120K (click here to convert to your currency).

Pure titanium is allotropic with an HCP crystal structure at low temperatures and a BCC structure above 882ºC. Titanium alloys occur most commonly in , or allotropic forms. They have similar chemical composition but different crystal structure. is an HCP crystal structure which does not respond to heat treatment but can be welded. is a BCC crystal structure which is age-hardenable and has a high strength. contains grains of alpha and beta and is strengthened by heat treatment and ageing.

Aluminium and Oxygen are alpha stabilising elements while Vanadium, Manganese and Molybdenum are beta stabilising elements. Because both allotropic forms have different mechanical properties, varying the amount of alloying element has an influence on the mechanical properties of the resultant Ti alloy. Other elements such as Tin and Zirkonium can also be added but for strengthening purposes, not promotion of alpha or beta.

Out of all alloying elements used for making of Ti alloys, Aluminium is the most widely used (and important) one. It stabilises alpha which increases strength with a minor reduction to ductility (ductility and strength are always counteracting), reduces the density and improves creep and oxidation resistance. As a general rule of thumb, Aluminium addition should not exceed 9% as it forms a brittle intermetalic Ti₃Al precipitate.

Some of the most common Ti alloys and their properties are listed below:

The last alloy in the table above (6% Al-4% V) is used in airframes, rockets, jet engines and landing gear and accounts for more than 50% of Ti alloys.

Magnesium alloys

Magnesium alloys are among the lightest traditional alloys with only 1.74 Mg.m^-3 density. Apart from its low density, it also has a good specific Young's modulus, it can be easily machined into complex aircraft components and it's weldable. However, it also has some disadvantages such as:

• Poor resistance to fatigue, creep and wear
• Relatively small improvements in stiffness and strength when alloyed
• Low Young's modulus compared with Al and Ti alloys
• High cost
• Poses a hazard during casting and machining as it easily combines with oxygen and burns

As with Ti alloys, main alloying element is Aluminium. Other elements such as Zinc and Manganese are also alloyed. Mg alloys are designated a four-part system with the first two capital letters indicating main alloying elements followed by two digits giving the rounded percentage of alloying element present in the Mg alloy in the same order as preceding letters. In other words, AZ63 would be Mg alloy with 6% Aluminium and 3% Zinc.

Pure Magnesium has an HCP structure and is less ductile than Al. Mg alloys also have an HCP crystal structure but are more ductile due to the increased number of active slip lanes caused by the alloying elements. They are strengthened by precipitation strengthening or age hardening.

Properties of typical Mg alloys are listed in the following table:

References

• Askeland D.R. 1996, The Science and Engineering of Materials, 3^rd S.I. edn, Chapman & Hall, London