Titanium Alloys - Classifications

Topics Covered

Metallurgy of Titanium

Alpha Alloys

Alpha-Beta Alloys

Beta Alloys

Classification by Strength

Metallurgy of Titanium

Pure titanium is normally of a hexagonal (alpha - α) structure but transforms to a body centred cubic (beta - β) form when heated above 882°C. The addition of alloying elements to titanium influences this transformation temperature and in many alloys results in beta being retained at room temperature, thus producing a material containing both alpha and beta phases or even one which is wholly beta. The relative amounts of alpha and beta phases in any particular alloy have a significant effect on the properties of that material in terms of tensile strength, ductility, creep properties, weldability and ease of formability. It is common practice in the metallurgical industry to refer to titanium alloys by their structure, hence alpha, alpha-beta, and beta alloys, examples of whichare given in table 1.

Table 1. Titanium alloys classified by metallurgical structure

Alpha Alloys

Commercially pure titanium is, in fact, alloyed with small amounts of oxygen which increase the hardness and tensile strength. By varying the amounts added it is possible to produce a range of commercially pure grades of titanium with strength levels varying between 290 and 740 MPa These materials are nominally all alpha in structure although small amounts of beta phase are possible if the impurity levels of beta stabilisers such as iron are high. While the alpha alloys cannot be heat treated to increase strength, the addition of 2.5% copper to titanium results in a material which responds to solution treatment and ageing in a similar way to aluminium-copper alloys. Aluminium as an alloying addition to titanium is an alpha stabiliser and is found in many of the commercially available alloys.

Alpha-Beta Alloys

Elements such as vanadium, molybdenum, iron and chromium stabilise the beta phase and many alpha-beta alloys have been developed. These are generally medium to high strength materials with tensile strengths from 620 to 1250 MPa and having useful creep resistance up to 350 to 400°C. In addition to tensile properties, low and high cycle fatigue and fracture toughness are now critical design parameters and so thermomechanical and heat treatment procedures have been developed to ensure that the alloys provide an optimum balance of mechanical properties for a wide range of applications.

For maximum creep resistance at temperatures above 450°C, alloys of the near alpha type are now used. These offer acceptable creep strength at temperatures up to 600°C.

Beta Alloys

The other class of titanium materials is that of the beta alloys. When sufficient beta stabilising elements are added to titanium, all-beta alloys can be produced. These materials have been available for many years but have come into prominence recently. They are generally more easily cold workable than the alpha-beta alloys, are heat treatable to high strengths, and some have superior corrosion resistance to the commercially pure grades.

International and national specifications exist for titanium materials used in aerospace but there is not similar coverage for materials for non-aircraft applications. Generally, for this sector, the ASTM series of specifications are used.

Classification by Strength

The preceding classification of titanium alloys according to metallographic structures has been included because a knowledge of the terminology is useful. A classification system more relevant to the designer, however, is one based on tensile. The classification system is given in table 2, which does not provide a complete list of titanium alloys but includes the more common ones in use in each of the strength ranges.

Table 2. Classification of titanium alloys by strength.

Source: Materials Information Service – The Selection and Use of Titanium, A Design Guide

For more information on this source please visit The Institute of Materials.