852 Phase Diagrams

There have been a number of attempts to categorize titanium alloy phase diagrams [10, 11], all agreeing there are two major divisions: a-stabilized and P-stabilized systems. Of these probably the most convenient is that developed by Molchanova [10] (Fig. 8.2). Here the alpha stabilizers are divided into those having complete stability, in which the alpha phase can coexist with the liquid (e.g., Ti-O and

FIGURE 8.2 Classification scheme for binary titanium alloys [10].

Ti-N), and there is a simple peritectic reaction, and those that have limited alpha stability in which, with decreasing temperature, decomposition of the alpha takes place by a peritectoid reaction into beta plus a compound (beta peritectoid). Examples of the latter type of system are Ti-B, Ti-C, and Ti-AI. Molchanova [10] also divides the P stabilizers into two categories, P isomorphous and P eutectoid. In the former system an extensive P-solubility range exists with only a limited a-solubility range. Examples are Ti-Mo, Ti-Ta, Ti-V, with elements such as Zr and Hf occupying an intermediate position since they have complete mutual solubility in both the a and P phases. For the P-eutectoid systems the P phase has a restricted solubility range and decomposes into a and a compound (e.g., Ti-Cr and Ti-Cu). This class can also be further subdivided depending on whether the P transformation is rapid (the "active" eutectoid formers such as Ti-Si, Ti-Cu, and Ti-Ni) or slow (the "sluggish" eutectoid formers such as Ti-Cr and Ti-Fe).

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