Interdiffusion and Phase Behavior in Polysynthetically Twinned (PST) TiAl / Ti Diffusion Couples
- PDF / 703,135 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 40 Downloads / 236 Views
Interdiffusion and Phase Behavior in Polysynthetically Twinned (PST) TiAl / Ti Diffusion Couples Ling Pan, David E. Luzzi Department of Materials Science and Engineering, University of Pennsylvania 3231 Walnut Street, Philadelphia, PA 19104, U.S.A ABSTRACT Diffusion couples of pure Ti and polysynthetically twinned (PST) TiAl (49.3 at.% Al) were prepared by high vacuum hot-pressing, with the bonding interface perpendicular to the lamellar planes. Diffusion experiments were carried out by annealing the couples in the same furnace at 650, 700 and 850°C for various times. The cross-section of the couple was studied using scanning electron microscopy (SEM) and quantitative wavelength-dispersive x-ray spectroscopy (WDS). A reaction layer whose composition is close to that of the stoichiometric α2–Ti3Al phase formed along the PST TiAl / Ti bonding interface in PST TiAl side. Direct measurements of the thickness of the reaction zone were performed at different phase regions and various boundaries. By assuming the thickness of the reaction zone increases as (Dt)1/2, where D is the diffusion coefficient and t is the annealing time, the diffusion coefficients at these temperatures were calculated. Composition profiles in the reaction zone, along the lamellae and at the lamellar interfaces were obtained by WDS analyses. INTRODUCTION Intermetallic compounds made of the light elements Ti and Al are promising candidates for aerospace, automotive and turbine power generation applications[1-5]. In the past decade, special interest has been paid to poly-synthetically twinned (PST) TiAl, composed of alternate lamellae of the γ-TiAl phase and the α2-Ti3Al phase with the orientation relationship {111}γ // (0001)α2 and γ // α2 [6]. PST TiAl exhibits low temperature ductility [7, 8] and higher toughness and high-temperature strength than TiAl alloys with other microstructures [9, 10]. At high temperatures the physical and mechanical properties of materials are generally associated with diffusion. In the Ti-Al alloy system, the formation and high-temperature stability of the lamellar structure are controlled by diffusion processes within the two phases and along the γ/γ and γ/α2 lamellar boundaries. Moreover, diffusion is an important determinant of the creep resistance of the lamellar structure [11, 12]. Therefore, a fundamental understanding of the diffusion mechanisms in PST TiAl alloys is of great importance for the development of titanium aluminide alloys. Current understanding of the diffusion processes in the Ti-Al system is largely based on the studies by Herzig et al. [13-16], who performed a series of tracer diffusion experiments, including self-diffusion, in polycrystalline γ-TiAl and α2-Ti3Al. The penetration profiles for diffusion in large grain size (> 1 mm) materials exhibit a c ∝ exp (-x2 / 4Dt) behavior for instantaneous sources and c ∝ erfc [x / 2(Dt)1/2] behavior for constant sources, as in semi-infinite materials. c is the average layered concentration of the diffusant and x is the penetration depth. In smaller grain si
Data Loading...
