Alpha-Case Kinetics and Surface Crack Growth in the High-Temperature Alloy TIMETAL 834 Under Creep Conditions
- PDF / 617,892 Bytes
- 8 Pages / 593.972 x 792 pts Page_size
- 84 Downloads / 187 Views
INTRODUCTION
TIMETAL 834 is a near-alpha alloy that consists of 15 pct equiaxed primary alpha in a fine lamellar alpha/ beta phase matrix.[1,2] This unique microstructural composition of the alloy offers an increased tensile strength and creep resistance up to 873 K (600 °C).[3] Besides, the alloy derives its properties from the solidsolution strengthening and heat treatment in the alpha/ beta phase field, which makes it able to retain a good level of properties up to around 75 cm diameter, with small reductions in strength in larger sections. It is also reasonably forgeable and effectively weldable using all the established titanium welding techniques. This alloy is mainly used for the rings, compressor disks, and blades of gas turbine applications.[3] The chemical composition of this alloy consists primarily of Ti-5.8Al-4.0Sn-3.5Zr-0.7Nb-0.5Mo-0.35Si0.06C.[3] The different elements that comprise this alloy provide, altogether, a stable composition and superior properties necessary for the high-temperature applications. For instance, aluminum (Al), tin (Sn), and zirconium (Zr) are considered to be alpha a stabilizing elements that act to stabilize the hexagonal alpha phase, whereas niobium (Nb) and molybdenum (Mo) are beta b stabilizing elements that act to stabilize the bodycentered cubic beta phase. Other significant alpha stabilizing elements include oxygen (O), nitrogen (N), and carbon (C), which in addition to extending the ZAKARIA ABDALLAH, Postdoctoral Research Associate, and KAREN PERKINS, Lecturer, are with the Materials Research Centre, School of Engineering, Swansea University, Swansea SA2 8PP, U.K. Contact e-mail: [email protected] STEVE WILLIAMS, Engineering Associate Fellow, is with Rolls-Royce plc, Derby DE24 8BJ, U.K. Manuscript submitted February 16, 2012. Article published online July 3, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
a-phase field to higher temperatures, the a-stabilizers develop a two-phase a+b field.[3] The addition of silicon (Si) is to enhance the creep performance of the alloy.[4] The alpha stabilizers have the effect of increasing the transformation temperature between the alpha and the beta phases in contrast to the beta stabilizers, which tend to decrease this transformation temperature.[4] Another effect of the alpha stabilizers, aluminum (Al) for instance, is that their atoms are slightly smaller and less dense than titanium atoms, resulting in a lower density of this alloy in comparison with pure titanium.[4] Moreover, the hexagonal arrangement of atoms, i.e., the alpha phase, increases the hardness of the alloy in comparison to the body-centered cubic arrangement, i.e., the beta phase, and thus, it decreases the deformation under the effect of high stresses and temperatures.[4] This alloy has been developed for the high-temperature applications of gas turbines in order to replace the heavy nickel-base superalloys and increase the payload.[5] It has an added advantage of developing a wide variety of microstructures depending on the solution treatment and the subseque
Data Loading...
