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Phase Diagrams

Titanium-Aluminum (Ti-Al) Phase Diagram

In the solid state, the titanium alloys are arranged in either hexagonal close-packed (alpha) or body-centered cubic (beta) structure. Pure titanium undergoes an allotropic transformation from hexagonal close-packed (HCP) alpha titanium to body-centered cubic (BCC) beta titanium as its temperature is raised through 882 °C (1620 °F). The melting point of pure titanium is 1668 °C (3034 °F).

The titanium-aluminum binary system provides a model for solution strengthening. In addition, it is the prototype of technical α alloys, as well as the α components of α + β alloys.

Aluminum is the most widely used alloying element in titanium-based alloys. It is the only common metal that raises the beta transus temperature and have large solubilities in both the alpha and beta phases.

Figures 1 and 2 show the equilibrium titanium-aluminum phase diagrams (expressed in wt. % and at. %, respectively) calculated with Thermo-Calc, coupled with SSOL2 thermodynamic database.

Titanium-Aluminum (Ti-Al) Phase Diagram in Weight Percents

Figure 1. Ti-Al phase diagram shows which phases are to be expected at equilibrium for different combinations of aluminum content (expressed in weight %) and temperature (in °C). The Ti-Al phase diagram was calculated with Thermo-Calc, coupled with SSOL2 thermodynamic database.


Titanium-Aluminum (Ti-Al) Phase Diagram in Atomic Percents

Figure 2. Ti-Al phase diagram shows which phases are to be expected at equilibrium for different combinations of aluminum content (expressed in atomic %) and temperature (in °C). The Ti-Al phase diagram was calculated with Thermo-Calc, coupled with SSOL2 thermodynamic database.

Apart from the alpha and beta phases, the presence of Ti3Al phase (also referred to as α2) and TiAl (gamma) intermetallic phase in the titanium-aluminum binary system is also noteworthy. Both phases are of great technical importance (e.g., titanium-aluminide alloys, which are especially important for high-temperature applications).

Ti3Al is an ordered DO19 structure based on the hexagonal close-packed alpha phase. A unit cell of the DO19 structure is composed of four regular HCP cells supported by covalent-like directional bonds connecting the titanium and aluminum atoms.

Note how the α + Ti3Al field narrows with increasing temperature, passes through a maximum at 1179 °C (2155 °F) and approximately 21.0 wt. % Al (32.0 at. % Al), and ends in a three-phase equilibrium with TiAl.

TiAl is an ordered L10 face-centered cubic (FCC) phase whose homogeneity ranges from approximately 48 at. % Al (34.2 wt. % Al) to 68 at. % Al (54.5 wt. % Al).

According to SSOL2 thermodynamic database, the titanium-rich section of the Ti-Al binary system has two peritectic reactions, one eutectoid reaction, and two congruent reactions. The two peritectic reactions are:

beta + liquid —> alpha at 1503 °C (2737 °F) and 32.4 wt % Al (46.0 at % Al) and

alpha + liquid —> gamma at 1443 °C (2620 °F) and 40.0 wt % Al (54.2 at % Al).

The eutectoid reaction alpha —> gamma + Ti3Al takes place at 1111 °C (2032 °F) and 27.6 wt. % Al (40.4 at. % Al).

The two congruent reactions are:

liquid —> beta at 1715 °C (3120 °F) and approximately 12.3 wt % Al (20.0 at % Al) and

alpha —> Ti3Al at 1179 °C (2155 °F) and approximately 21.0 wt. % Al (32.0 at. % Al).

Alloying Elements

Elements such as aluminum, oxygen, nitrogen, carbon, gallium, germanium, lanthanum, and cerium stabilize the alpha phase to higher temperature and are thus referred to as alpha stabilizers.

In general, transition metals and noble metals (i.e., metals which, like titanium, have unfilled or just-filled d-electron bands) are stabilizers of the beta phase to lower temperatures and are thus referred to as beta stabilizers.

Beta stabilizers are subdivided into two groups: beta-isomorphous (e.g. vanadium, niobium, tantalum, molybdenum, and rhenium) and beta-eutectoid (e.g., copper, silver, gold, palladium, indium, lead, bismuth, chromium, tungsten, manganese, iron, cobalt, nickel, uranium, hydrogen, and silicon).

Vanadium, molybdenum, and niobium are the most frequently used beta-isomorphous forming elements in titanium-based alloys. These elements, when added in sufficient concentrations, can stabilize the beta phase to room temperature. Tantalum and rhenium, which are also beta-isomorphous forming elements, are rarely used, mainly due to their high densities.

Chromium, iron, and silicon are the only beta-eutectoid forming elements that are commonly used in many titanium-based alloys.

Zirconium, hafnium, and tin form a group of alloying elements known as neutral additions. These three elements are sometimes classified as beta stabilizers, as they depress the beta transus temperature (albeit only slightly) in their respective binary phase diagrams with titanium. Zirconium and hafnium are isomorphous with titanium and exhibit the same beta to alpha allotropic phase transformations. These two elements have complete solubilities in the alpha and beta phases of titanium. Tin is a beta-eutectoid forming element and its effect on the beta transus temperature is negligible for all practical purposes.

When added to alpha-matrix alloys, however, neutral additions such as zirconium and tin can take on the characteristics of alpha stabilizers (e.g., aluminum). This is because of the chemical similarity of zirconium to titanium and because tin can substitutionally replace aluminum in the Ti3Al phase.

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Titanium-Aluminum Phase Diagram expressed in wt. % and °C

Titanium-Aluminum Phase Diagram expressed in at. % and °C

Titanium-Aluminum Phase Diagram expressed in wt. % and °F

Titanium-Aluminum Phase Diagram expressed in at. % and °F

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