EFFECT OF HEAT-TREATMENT ON THE MICROSTRUCTURE OF A METASTABLE BETA-TITANIUM ALLOY

Metastable beta-titanium alloys have improved formability and ductility compared to high-strength alpha- and alpha+beta titanium alloys; this can be attributed to their body-centered cubic structure in the solution-treated condition. In addition, a high strength level can be achieved by a simple aging treatment. During manufacturing, components are subjected to a variety of thermal cycles at temperatures ranging from 650 to 925 degrees C, as well as to cooling rates that vary from air cooling to furnace cooling. Consequently, various microstructures are produced that influence the mechanical properties of the products. The present study was undertaken to characterize the precipitation behavior of Timetal 21S at various heat treatment conditions by employing x-ray diffraction techniques combined with optical and scanning electron microscopy. It was observed that alpha precipitated preferentially on the grain boundaries during high-temperature aging (650 degrees C) and within the grains during low-temperature aging (400 degrees C). High-temperature solutioning produced a coarse grain size, and at the same time enhanced finer a precipitation during aging. The amount of alpha precipitate attained after the standard aging treatment was about 33%. Re-solution treatment followed by slow cooling, such as that which occurred during brazing of the alloy, resulted in alpha precipitation during cooling; however, an aging treatment was necessary to precipitate an alpha content greater than 20%.