The Effects of the Oxygen-enriched Surface Layer on Mechanical Properties of .ALPHA.+.BETA. Type Titanium Alloys

Abstract

The effects of heat treating conditions on thickness of the oxygen-enriched layer or a-case in Ti-4.5%Al-3%V-2%Fe-2%Mo alloy were studied in comparison with Ti-6%Al-4%V alloy, and then the effects of these layers on the mechanical properties were investigated. The higher heating temperature and an extended heating time increased oxygen-enriched layer or a-case thickness in Ti-4.5%Al-3%V-2%Fe-2%Mo alloy, and atmospheric heating using high purity argon gas with 99.999% purity could not prevent formation of the oxygen-enriched layer even by heating at such a low temperature as 998 K. The 30 mm thick oxygenenriched layer without a-case was formed by heat treating at 1 048 K for 3.6 ks in use of 99.9 % purity argon gas which corresponded to the typical heating conditions adopted in a practical superplastic forming (SPF) operation using Ti-4.5%Al-3%V-2%Fe-2%Mo alloy. The thickness of both surface layers in Ti-6%Al-4%V alloy was much thicker than Ti-4.5%Al-3%V-2%Fe-2%Mo alloy because of its higher heating temperature. Tensile elongation, fatigue strength and bendability were deteriorated with the increase of oxygen-enriched layer or a-case thickness, and removal of these layers by pickling treatment fully recovered these properties to an original level. Deterioration of the mechanical properties was exerted by existence of the surface layer with high hardness, and in particular, the oxygen-enriched layer with a-case decreased fatigue strength more pronouncedly compared with the surface layer without a-case. Existence of brittle surface layer with high hardness and the coarse grain size in a-case appeared to be responsible for accelerated formation of microcrack at the surface of the mechanical testing specimens, resulting in deterioration of various properties.

STEM-EELS identification of TiOXNY, TiN, Ti2N and O, N dissolution in the Ti2642S alloy oxidized in synthetic air at 650 °C

“…However, at high temperatures and under oxidizing conditions, Ti-based alloys undergo oxygen dissolution in addition to the growth of an oxide scale. Depending on time and duration, the oxygen dissolution can affect large depths, which can greatly alter mechanical properties [1,2]. Nitrogen, main air constituent, can also dissolve in titanium and its alloys [3].…”

Hydrogen-Affected Tensile and Fatigue Properties of Ti-6Al-4V Sheets by Superplastic Forming

“…An SEM fractographic analysis was conducted on the tested specimens to examine the fracture mode. titanium alloys [7][8][9][10][17][18][19], so it is typically removed by machining and/or chemical milling process [20]. The α-case formed during the SPF process in this study was also removed by mechanical polishing before mechanical testing.…”

“…During high temperature exposure for SPF, the α-case (an oxygen-enriched phase) is formed on the surface of the titanium and its alloys [4][5][6]. This detrimental phase on the surface needs to be carefully removed after SPF, otherwise a significant reduction in mechanical properties of titanium and its alloys is expected [7][8][9][10]. Previous studies on the effect of the a case formed during the high temperature exposure of Ti64 alloys have shown that it can significantly reduce the resistance to high cycle fatigue of Ti64 alloys, as well as *Corresponding Author: Sangshik Kim [Tel: +82-55-772-1667, E-mail: sang@gnu.ac.kr] Copyright ⓒ The Korean Institute of Metals and Materials tensile ductility [11].…”

Kinetic regularities of high-temperature interaction of titanium alloys with oxygen-medium

“…To predict the parameters of surface hardening of titanium alloys as a function of thermodiffusion saturation by interstitial impurities the improved physico-mathematical model of gas saturation of titanium alloys in rarefied gas medium is proposed for using [6].…”

“…However, some of their natural physical-mechanical properties (increased reactivity to interact with oxygen and nitrogen, low antifriction and unstable fatigue properties, etc.) narrow the spectrum of use [5][6][7][8]. One of the ways to improve the above-mentioned properties is to engineering surface of titanium alloys, including by thermodiffusion saturation from a controlled gaseous medium [9][10].…”