Why is there Aluminium in common grade 5 Titanium?

Pure titanium undergoes an allotropic transformation from the hexagonal close-packed alpha phase to the body-centered cubic beta phase at a temperature of 882.5°C (1620.5°F). Alloying elements can act to stabilize either the alpha or beta phase. Through the use of alloying additions, the beta phase can be sufficiently stabilized to coexist with alpha at room temperature. This fact forms the basis for creation of titanium alloys that can be strengthened by heat treating.

Titanium alloys are generally classified into three main categories: Alpha alloys, which contain neutral alloying elements (such as Sn) and/or alpha stabilizers (such as Al, O) only and are not heat treatable; Alpha + beta alloys, which generally contain a combination of alpha and beta stabilizers and are heat treatable to various degrees; and Beta alloys, which are metastable and contain sufficient beta stabilizers (such as Mo, V) to completely retain the beta phase upon quenching, and can be solution treated and aged to achieve significant increases in strength.
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https://cartech.ides.com/datasheet.aspx?i=101&E=269
 
Vicarious Reality said:
Very interesting, thank you

Why is both alpha and beta used? What is the use for the different forms?
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Alpha Titanium Alloys
Commercially pure titanium and alpha alloys of titanium are non-heat treatable and are genarally very weldable . They have:

• Low to medium strength

• Good notch toughness

• Reasonably good ductility

• Excellent mechanical properties at cryogenic temperatures

Pure titanium and alpha alloys possess the highest corrosion resistance.

More highly alloyed near-alpha alloys offer optimum high temperature creep strength and oxidation resistance.

Alpha Beta Titanium Alloys
Alpha Beta alloys are heat treatable and most are weldable. Typical properties include:

• Strength levels are medium to high

• High temperature creep strength is not as good as most alpha alloys

• Cold forming may be limited but hot forming qualities are normally good

• Many alloys can be superplastically formed

The most commonly used alpha beta alloy is Ti6Al4V, developed in many variations of the basic formulation for the widest possible choice of key properties and for many widely differing applications.

Beta Titanium Alloys
Beta or near beta alloys are:

• Fully heat treatable

• Generally weldable

• Capable of high strengths

• Possess good creep resistance up to intermediate temperatures

• Excellent formability can be expected in the beta alloys in the solution treated condition

Beta-type alloys have good combinations of properties in sheet, as fasteners and are ideal for sporing applications. Some alloys offer uniform property levels through heavy sections.

The higher alloy content of beta alloys increases the density of beta alloys typically by 7-10% compared to Ti6Al4V.

Typical beta alloys include:

Ti3Al8V6Cr4Mo4Zr ASTM Grade 19

Ti10V2Fe3Al AMS 4983, 4984, 4987

Ti15Mo3Nb3Al2Si ASTM Grade 21

Ti15V3Cr3Sn3Al AMS 4914

Correctly, it is only the metastable beta alloys which are heat treatable by solution treatment and ageing. Fully stable beta alloys can only be annealed.

Source: http://www.azom.com/article.aspx?ArticleID=2591
 
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