MICROSTRUCTURAL FEATURES OF THE HYDROGENATION-DEHYDROGENATION PROCESS IN TI ALLOYS

Temporary alloying of Ti alloys with hydrogen is used to modify microstructure and hence enhance mechanical properties. In the present work the influence of the thermal history of the Ti-6Al-4V alloy on the final microstructure was studied. Two different kinds of microstructures were investigated: (a) as-hot isostatic pressed (HIP) having an alpha + beta microstructure; (b) beta-solution-treated and subsequently quenched (alpha'-microstructure). After each stage of the hydrogenation-dehydrogenation (HDH) process the material was analyzed by optical, scanning electron and transmission electron microscopy, and by x-ray diffractometry. It was found that fcc (gamma) hydrides precipitate in both the alpha and the alpha' matrix. The orientation relationships were found to be the same in both cases. Hydrides precipitate from the beta-phase when the hydrogen content of the alloy is sufficiently high. The minimum concentration that still led to precipitation was 1.4 wt.% H. The beta/gamma orientation relationship was determined. The HDH process resulted in either of two structures: (1) a fine, broken-up microstructure containing a fine mixture of alpha' + (alpha + beta) when the starting microstructure was alpha', or (2) a modified alpha + beta microstructure containing fine alpha and beta precipitates and a high density of microdefects within the primary alpha in the as-HIP alpha + beta microstructure specimens.