Thermohydrogen processing (THP) of titanium-(6)aluminum-(4)vanadium and titanium-aluminum alloys

The microstructures, phases and phase transformations occurring in cast and Hot Isostatic Pressed (HIP'd) Ti-6Al-4V-H and the blended elemental (BE) TiAl-H systems were investigated.; In this work, the existing Ti-6Al-4V-H phase diagram was revised and the time-temperature-transformation (TTT) diagrams for beta-phase (isothermal) and martensite (quench plus aging) decomposition were determined at different hydrogen concentrations. Alloying with hydrogen decreases the nose temperatures for the start of both the beta/martensite decompositions in a linear fashion and increases the nose times for both of these in a non-linear fashion. During aging at temperatures below the beta transus temperature, the martensite decomposes into α+β_M (metastable beta) and on quenching, from the aging temperature, the β_M transforms to martensite + β _R (residual beta) with the amount of latter increasing with increasing hydrogen content. Microstructures varying from α-lamellar laths to fine equiaxed α-grains were produced in the Ti-6Al-4V alloy, by using different thermohydrogen processing (THP) treatments. A microstructure consisting of mixed equiaxed and elongated α-grains were only produced in samples containing 30at.%H after the complete decomposition of the beta/martensite below a critical temperature (T_c), followed by dehydrogenation. A mixture consisting of partially equiaxed α-grains thus produced by THP, increased the tensile strength from 841MPa (starting Ti-6Al-4V) to 965MPa after THP and also increased the % elongation from 7.5% to 10.5%. In addition to other THP parameters, the final microstructure also depends on the starting microstructure and recommendations are made for future work in this regard. Initial results of temperature cycling treatments, which involved heat treating of Ti-6Al-4V samples containing 30at.%H at 680°C for 5 minutes followed by water quenching and repetition of the same treatment for 10 cycles, did not show a decrease in the average prior beta grain size; recommendations have been made for future work in this area.; Fully dense hydrogenated nano-crystalline TiAl compacts were produced from BE powders. HIP'ing of the mechanically alloyed hydrogenated powders at 850°C resulted in a homogenous microstructure, whereas prior powder particle boundaries were visible in the samples produced from non-hydrogenated powders. The hydrogen was removed by vacuum annealing resulting in nano-size γ-TiAl.