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Titanium Alloys

The density of Titanium is roughly 55% that of steel.  Titanium alloys are extensively utilized for significantly loaded aerospace components.  Titanium is used in applications requiring somewhat elevated temperatures .  The good corrosion resistance experienced in many environments is based on titaniumĺs ability to form a stable oxide protective layer.  This makes titanium useful in surgical implants and some chemical plant equipment applications.

 Unalloyed (commercially pure) titanium can be found in two crystallographic forms:

  •   Hexagonal close-packed (hcp) or alpha (α) phase is found at room temperature

  •   Body centered cubic (bcc) or beta (▀) phase is found above 883 ░C (1621 ░F)

The control of alpha (α) and beta (▀) phases through alloying additions and thermomechanical processing is the basis for the titanium alloys used by industry today.  It is also the primary method for classifying titanium alloys.  Titanium alloys are categorized as either alpha (α) alloys, beta (▀) alloys, or alpha+beta (α+▀) alloys.  Some common titanium alloys are listed below according to these categories.

Alpha and near alpha alloys Alpha + Beta alloys Beta alloys
Ti-2.5Cu

Ti-6Al-4V

Ti-13V-11Cr-3Al

Ti-5Al-2.5Sn

Ti-6Al-6V-2Sn

Ti-8Mo-8V-2Fe-3Al

Ti-8Al-1V-1Mo

Ti-6Al-2Sn-2Zr-2Cr-2Mo

Ti-10V-2Fe-3Al

Ti-6242

Ti-3Al-2.5V

Ti-15-3

Ti-6Al-2Nb-1Ta-0.8 Mo

Ti-8Al-1Mo-1V

 

Ti-5Al-5Sn-2Zr-2Mo

   

One of the primary effect of alloying elements used in titanium production is the affect on the alpha to beta transformation temperature.  Some elements raise the alpha to beta transformation temperature thereby stabilizing the alpha crystal structure.  While other elements lower the alpha to beta transformation temperature thereby stabilizing the beta crystal structure.  The effect of some elements is shown below:

    Element Effect
    Aluminum  alpha stabilizer
    Tin alpha stabilizer
    Vanadium Beta stabilizer
    Molybdenum Beta stabilizer

    Chromium

Beta stabilizer

    Copper

Beta stabilizer

Alpha alloys commonly have creep resistance superior to beta alloys.  Alpha alloys are suitable for somewhat elevated temperature applications.  They are also sometimes used for cryogenic applications.  Alpha alloys have adequate strength, toughness, and weldability for various applications, but are not as readily forged as many beta alloys.  Alpha alloys cannot be strengthened by heat treatment.

Beta alloys have good forging capability.   Beta alloy sheet is cold formable when in the solution treated condition.  Beta alloys are prone to a ductile to brittle transition temperature.  Beta alloys can be strengthened by heat treatment.  Typically beta alloys are solutioned followed by aging to form finely dispersed particles in a beta phase matrix.

Alpha + beta alloys have chemical compositions that result in a mixture of alpha and beta phases.  The beta phase is normally in the range of 10 to 50% at room temperature.  Alloys with beta contents less than 20% are weldable.  The most commonly used titanium alloy is Ti-6Al-4V, an alpha + beta alloy.  While Ti-6Al-4V is fairly difficult to form other alpha + beta alloys normally have better formability. 

Alpha + beta alloys can be strengthened by heat treatment.  When strengthening alpha + beta alloys the components are normally quickly cooled from a temperature high in the alpha-beta range or even above the beta transus.  Solution treatment is then followed by aging to generate an proper mixture of alpha and transformed beta.  Heat treatment is dependent on the cooling rate from the solution temperature and can be affected by the size of the component.

Some of the uses of titanium alloys are listed below:

  •   Surgical Implants

  •   Prosthetic devices

  •   Jet engines

  •   Chemical processing plants

  •   Pulp and paper industry

  •   Marine applications

  •   Sports equipment

 

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Last modified: January 25, 2012