EFFECTS OF HYDROGEN ON THE MICROSTRUCTURE AND MICROCHEMISTRY OF TI3AL-NB INTERMETALLICS AT HIGH-TEMPERATURES AND HIGH-PRESSURES

The microstructural and microchemical changes produced in a Ti-25Al-10 Nb-3V-1Mo alloy (at. %) by charging at high temperatures in high press ures of hydrogen gas have been studied using transmission electron mic roscopy (TEM) and x-ray methods. Hydrides incorporating all of the sub stitutional solutes that formed during charging have a face-centered c ubic (fee) structure and exhibit either a plate or fine-grained morpho logy. With increasing hydrogen content, the size of the hydrides decre ases and their microchemistry changes as they approach the stable bina ry hydride, TiH2. Rejection of substitutional solute elements from the hydride produces changes in the microchemistry, and consequently in t he crystal structure, of the surrounding matrix. In alloys containing 50 at. % H, this solute redistribution results in the formation of an orthohombic substitutional solid solution phase containing increased l evels of Nb. The driving force of this redistribution of solutes is th e reduction in the chemical potential of the system as the amount of t he most stable hydride, TiH2 forms. The hydrides reverted to a solid s olution on annealing in vacuum at 1073 K, and the original microchemis try of the alloy was restored. Reversion from the hydride structure to the original alpha(2) ordered DO19 structure proceeds via a disordere d HCP phase.