Two Types of Hydrides Formed in γ Phase of Ti-47 mol%Al Alloy by Electrochemical Treatment in Sulfuric Acid

Abstract

Metal hydride (MH) formation in a Ti-47 mol%Al (47Al) alloy with the γ+α₂ lamellar structure through electrochemical treatment in sulfuric acid has been studied by X-ray diffractometry (XRD), thermal desorption spectrometry (TDS) combined with quadrupole mass spectrometry (QMS) and SEM.
It has been shown for the γ phase that cathodic polarization (CP) makes only one type of MH having a tetragonal structure (MH_TET) of about 10 μm thick at the reaction surface. An addition of anodic polarization (AP) after CP makes another type of MH having a hexagonal structure (MH_HEX) of 1-2 μm thick along the reaction surface. On the other hand, MH with HCP structure (MH_HCP) is formed in the α₂ phase by either CP or CP+AP. Each hydride gives an individual peak in TDS and QMS curves. The peaks of MH_TET, MH_HEX and MH_HCP are located at around 200, 400, 600°C, respectively. Heating in vacuum to the temperature above each peak decomposes the hydride corresponding to the peak.
The preferential dissolution of Al atom from the γ phase of 47Al alloy by anodic reaction makes compositional change and lattice defects near the reaction surface, and it is considered that MH_HEX is formed by movement of hydrogen atoms to the defects. The fact that the formation of two types of MH in the γ phase of 47Al alloy by CP+AP is strongly suggests the possibility that two kinds of MH are formed in the γ single phase alloy.