Effect of Dislocation Slip Mechanism under the Control of Oxygen Concentration in Alpha-Case on Strength and Ductility of TC4 Alloy

Xin Feng,Shu Wang,Yilong Liang,Hao Sun

doi:10.3390/met11071057

Abstract

The oxygen diffusion layer (alpha-case) is generally considered to have a negative impact on the mechanical properties and applications of titanium alloys. In this study, TC4 alloy specimens with four types of different oxygen concentrations in alpha-case were obtained by controlling the oxygen diffusion process parameters. Scanning electron microscopy and glow discharge spectrometry were employed to characterize the microstructure and oxygen concentration of alpha-case. The effect of alpha-case on strength and ductility of TC4 alloy was investigated via tensile test and new insights were provided. The results indicate that with the increase in the oxygen concentration in the alpha-case, the ductility of the TC4 alloy gradually decreased. Interestingly, the strength of TC4 alloy with the alpha-case first increased and then decreased, resulting in the existence of a peak corresponding to a lower oxygen concentration before the decline of strength. Furthermore, a relatively good ductility match was also observed at the peak. When the oxygen concentration was relatively high, both the strength and ductility decreased. This phenomenon is attributed to the fact that dislocations in the alpha-case controlled by the oxygen concentration were modified from wavy slip to planar slip. Finally, the dislocation’s slip morphology was characterized by transmission electron microscopy.

Highlights

TC4 alloy is one of the main components employed in the aerospace field, and it accounts for more than half of the global titanium alloy production [1,2]
At relatively high temperatures, the increased oxidation of titanium leads to an increase in the thickness of surface oxide film as well as the formation of an oxygen diffusion layer under the oxide film [7]
After hot working, hot forming, and high-temperature applications, the surface layer of titanium alloys is often accompanied by the formation of alpha-case [8,9]

Summary

Introduction

TC4 alloy is one of the main components employed in the aerospace field, and it accounts for more than half of the global titanium alloy production [1,2]. Higher chemical affinity allows titanium to form a natural oxide film (usually less than 10 nm in thickness) even at room temperature [6]. At relatively high temperatures (usually higher than 480 ◦ C), the increased oxidation of titanium leads to an increase in the thickness of surface oxide film as well as the formation of an oxygen diffusion layer (alpha-case) under the oxide film [7]. After hot working, hot forming, and high-temperature applications, the surface layer of titanium alloys is often accompanied by the formation of alpha-case [8,9]. The alpha-case has an important impact on the mechanical properties of titanium alloys. It can even limit the application of titanium alloys. For gas turbine engines employed in the aerospace industry, the service life of titanium alloys can be affected by alpha-case [10]

Methods

Results

Discussion

Conclusion