Abstract
This is a screening study using high-energy X-ray diffraction measurements to determine whether an orthorhombic phase forms in γ-based TiAl alloys of different compositions. The 13 alloy compositions investigated were chosen to be either close to commercial alloys or to identify the effects of different single alloying elements on the formation of orthorhombic phase. The orthorhombic O phase with Cmcm structure was found in several of those γ-TiAl alloys after an aging heat treatment at 550 °C for 20 h. The presence of different β-stabilising elements such as niobium, tantalum, molybdenum or vanadium did promote the formation of orthorhombic phase, while micro alloying elements such as carbon or boron were neutral in this respect. Furthermore, a limit for aluminium was also found, below which the orthorhombic O phase is formed in the alloys investigated. This limit lies between 46 at.% and 47 at.%.
Highlights
In γ-based TiAl alloys β-stabilising elements such as niobium, mo lybdenum or vanadium, or minor alloying elements such as carbon, silicium or boron are added to improve the mechanical properties, oxidation resistance or processability [1]
It was shown by the use of in situ high-energy X-ray diffraction (HEXRD) and high-resolution HAADF- STEM investigations that the orthorhombic O phase forms out of the hexagonal α2 phase with a D019 structure in the Ti-42Al-8.5Nb alloy
There, the splitting of the former α2 20-20 diffraction peak due to the formation of an orthorhombic phase is clearly visible after heat treatment at 550 ◦C/20 h/AC
Summary
In γ-based TiAl alloys β-stabilising elements such as niobium, mo lybdenum or vanadium, or minor alloying elements such as carbon, silicium or boron are added to improve the mechanical properties, oxidation resistance or processability [1]. The formation of an orthorhombic phase that is structural similar to the O phase with a Cmcm structure was reported by Rackel et al [7] and Gabrisch et al [8] in a high-niobium containing γ-based TiAl alloy. The emergence of the O phase is clearly evident in XRD or HEXRD measurements by the splitting of former α2 20-20 and α2 20-21 peaks. If both phases (α2 phase and O phase) are represented in the lowest common crystallographic space group i.e. Cmcm, as shown in Ref. The parameters for the heat treatment were taken from previous studies on the alloy Ti-42Al-8.5Nb in Ref. [7]
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