Timetal 21S has been selected as a baseline for the development of a new high temperature beta titanium alloy. A combinatorial approach employing directed laser deposition of elemental powders has been used to produce a number of test coupons with controlled variations of composition. In addition to the variation of the baseline elements (Ti, Mo, Nb, Al and Si), the alloys contain varying amounts of neutral elements (Zr and Sn), beta-stabilizers (W) and dispersoid formers (B, C and Ge). Subsequently, the creep properties, represented by their minimum creep rates, have been assessed using an Instron Electrothermal Mechanical Tester (ETMT). The microstructures of the test coupons have been characterized using a range of techniques and have been quantified using rigorous stereological techniques to populate databases and subsequently train and test Bayesian Neural Network models for the prediction of creep properties. Additionally, advanced characterization techniques and computation tools have been employed to aid in the identification of the creep rate-limiting microstructural features. For example, SEM and TEM studies show a critical dependence of the size of alpha-denuded beta regions on the creep properties in these beta-Ti alloys. The most important microstructural features (volume fraction alpha, alpha lath thickness and beta mean free path) and alloying additions (Sn and Ge) have been identified and are discussed.
The ETMT, used to investigate creep properties in the work, has also been characterized and compared with traditional tensile and creep testing methods. Computational models incorporating heat transfer and electrostatics were used to investigate the temperature profiles that result from the interaction of joule heating, conductive cooling and radiative cooling in subscale Ti-6Al-4V samples at five current densities in the ETMT. The tensile properties, including YS, UTS, E and total elongation, of sub-scale specimens have been evaluated over a range of temperatures and a variation of microstructural features in alpha+beta and beta-processed Ti-6Al-4V using the ETMT and traditional methods. The creep properties for Timetal 21S, alpha+beta and beta-processed Ti-6242 are compared with legacy data and traditional means. It was found that the applied direct current increases the minimum creep rate.