The following is a short synopsis of basic titanium metallurgy. Both commercially pure and alloy grades of titanium can be grouped into three categories according to the predominant phase or phases in their microstructure…alpha, alpha-beta, and beta. In pure titanium, the alpha phase characterized by a hexagonal close-packed crystalline structure…is stable from room temperature to approximately 882 deg.C (1620 deg.F). The beta phase in pure titanium has a body-centered cubic structure and is stable from approximately 882 deg. C (1620 deg.F) to the melting point of about 1688 deg. C (3040 deg.F).
The selective addition of alloying elements to titanium enables a wide range of physical and mechanical properties to be obtained. Basic effects of a number of alloying elements are as follows:
Single-phase and near single-phase alloys of titanium known as alpha alloys, generally contain high amounts of aluminum . This provides excellent strength characteristics and oxidation resistance at elevated temperatures. Single phase unalloyed titanium alloys such as Ti-CP Gr.1 & 2, Ti-Pd GR.7 exhibit superior corrosion resistance and are commonly poured at Ti Squared.
The addition of controlled amounts of beta alloying elements stabilizes some beta phase to exist below the beta transus temperature at room temperature. The result is a two phase system. Two-phase titanium alloys can be strengthened by heat treatment then quenching from a temperature high in the alpha-beta range followed by an aging cycle at a somewhat lower temperature. The transformation of the beta phase and precipitation of fine alpha particles from the metastable beta is a strengthening process similar to that of precipitation hardening of steel alloys. The result is a structure that is slightly stronger than the as-cast alpha-beta structure. Alloys that Ti Squared pours in this group are Ti 6Al-4V and Ti 3-2.5
The high percentage of beta-stabilizing elements in this group of titanium alloys results in a microstructure that is metastable beta after solution annealing. Extensive strengthening can occur by the precipitation of alpha during aging. Alloys that Ti Squared pours in this group are Ti 15-3-3-3 and Beta-C. These alloys compete well with high strength stainless steels such as 17-4Ph and are excellent for spring applications.
A case hardening can be formed on the surface of titanium castings known as alpha case. This layer is an alpha stabilized phase at the surface caused by mold-metal reactions during pouring. Case depth is a function of time at temperature and will form at greater depths on heavy walled castings.
At hardness values of HRc 45, alpha case will reduce cutter wear during machining and may act as a stress riser for crack propagation. Case hardening can be useful for wear resistance in applications that are prone to excessive erosion.