Oxygen diffusion in Ti₃Al single crystals

Abstract

Diffusion coefficients of oxygen in directions parallel to the c-(D c ) and a-axis (D a ) of Ti 3 Al single crystals were measured by oxygen implantation and Auger electron spectroscopy. The temperature dependence of D c and D a in the temperature range 723 À 1073 K is expressed as D c ¼ 7:3 þ3:1 À2:2  10 À9 expðÀ187:3 AE 2:7½kJ=mol=RTÞ ½m 2 =s and D a ¼ 4:7 þ3:3 À2:0  10 À9 expðÀ184:8 AE 4:0 ½kJ=mol=RTÞ ½m 2 =s, respectively. The diffusion coefficients are three orders of magnitude less than those in -Ti. The value of D c /D a is within the range 0.94 À 1.11 and the diffusion anisotropy is weak, despite the anisotropic arrangement of octahedral sites. The diffusion mechanism is discussed in terms of the ordered arrangement of two types of octahedral sites occupied by oxygen atoms. IntroductionOver the last two or three decades, 2 -Ti 3 Al intermetallic compounds have been widely investigated as lightweight, heat-resistant structural materials, and numerous research results have been published on their high-temperature strength and oxidation behavior [1,2]. Recently, Ti 3 Al has attracted attention as a diffusion barrier between Si substrate and Al wiring in integrated circuits [3], and as an electrode/diffusion barrier between the ferroelectric oxide layer and Si in ferroelectric memory cells [4,5]. The knowledge of diffusion is essential to understanding the high-temperature deformation, oxidation behavior and diffusion-barrier properties. In the last decade, the self-diffusion [6], interdiffusion [6] and impurity diffusion of some metallic and semimetallic elements, such as Fe [7], Ni [7], Nb [7], Ga [8] and Si [9] in Ti 3 Al, have been reported. However, oxygen diffusion in Ti 3 Al has never been studied despite its importance in understanding the oxidation behavior and diffusion barrier property of Ti 3 Al.Ti 3 Al has a D0 19 -type ordered structure, which is based on the hexagonal close packed (hcp) structure shown in Figure 1. In the ordered structure, Ti atoms and Al atoms are arranged alternatively along the a-axis directions. The a-axis length of the Ti 3 Al is larger by a factor of two or more than that of -Ti with an ordinary hcp structure, while the c-axes lengths of both structures are almost identical. Effects of this ordered structure

Impact of β-phase in TiAl alloys on mechanical properties after high temperature air exposure

“…Unfortunately, only few data exist on diffusion of O in the different phases of the TiAl alloys. Experimental data can be found for diffusion of O in 2 [32,36], with some discrepancy. At 800°C for example, values of 4-6×10 -18 m 2 /s [36] and 1.1×10 -16 m 2 /s [32], are reported.…”

“…Experimental data can be found for diffusion of O in 2 [32,36], with some discrepancy. At 800°C for example, values of 4-6×10 -18 m 2 /s [36] and 1.1×10 -16 m 2 /s [32], are reported. Diffusion coefficient of O in -TiAl is unknown, probably because of the very limited solubility.…”

Diffusion of Au in the Intermetallic Compound Ti3Al

“…Diffusion mechanisms of impurity atoms in Ti 3 Al have previously been investigated for Ni, [7] Fe, [7] Nb, [7] Si, [8] O, [9] and Ga. [10] Their diffusivities are discussed mainly from the standpoint of relative atomic size and chemical interaction of impurity atoms with host atoms. The atomic radii of host atoms Ti and Al are 0.146 and 0.143 nm, respectively.…”

“…Ru¨sing et al [6] have measured self-diffusivity of Ti and interdiffusivity in binary Ti 3 Al. Although the diffusivities of some impurity elements contained in Ti 3 Al alloys for high-temperature structural application have been measured, [7] only limited data are available for diffusion of constituent elements of electrics devises; the diffusivities of Si and O have been measured by Koizumi et al [8,9] and that of Ga, by Herzig et al [10] Further study for diffusivity of elements used in electronic devises such as Au, Ag, and Cu is desirable to understand the implications for device reliability.…”

Effects of substitutional impurity Au and Si atoms on antiphase boundary energies in Ti3Al: A first principles study

“…The authors have measured the diffusivities of Au [14], Si [15] and O atoms [16], which are the major possible impurities in Ti 3 Al as conductive diffusion barriers. However, all the measured diffusivities were bulk diffusivities in coarse-grained crystals or single crystals, and therefore the diffusivities in Ti 3 Al containing a high density of lattice defects still remain unknown.…”