Microstructural Control of Ti-Al-Nb-W-B Alloys
- PDF / 551,197 Bytes
- 8 Pages / 593.972 x 792 pts Page_size
- 28 Downloads / 189 Views
TRODUCTION
THE mechanical properties of TiAl-based alloys with lamellar structures depend on three factors: the colony size, interlamellar spacing, and alloying addition.[1] The tensile elongation at room temperature is strongly dependent on the lamellar-colony size, showing increased ductility with decreasing colony size. The strength at room and elevated temperatures is sensitive to the interlamellar spacing, showing increased strength with decreasing interlamellar spacing.[1,2] As-cast TiAl-based alloys usually have a coarse grain size.[3] A refinement in the microstructures is necessary to meet the desirable applications. Additional elements, such as W and B, added into the base alloy can help refine the grain size.[3-8] Related heat treatments can facilitate the development of the desirable microstructures. For TiAl-based alloys, the fully lamellar (FL) structure, composed of a2/c lamellae, has a low tensile ductility at room temperature but higher fracture toughness than the other microstructures. On the other hand, the duplex (DP) structure, composed of the lamellar colony and primary c phase, tends to have a good room-temperature tensile ductility but low fracture toughness.[1] By both refining the colony size and the lamellar spacing through the cyclic heat treatment,[9] thermomechanical treatment,[10] and alloying,[12] the FL microstructure can exhibit both good ductility and fracture toughness. L. HUANG, Graduate Assistant, and P.K. LIAW and C.T. LIU, Professors, are with the Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 379962200, USA. Contact e-mail: [email protected] This article is based on a presentation given in the symposium entitled ‘‘Deformation and Fracture from Nano to Macro: A Symposium Honoring W.W. Gerberich’s 70th Birthday,’’ which occurred during the TMS Annual Meeting, March 12–16, 2006 in San Antonio, Texas and was sponsored by the Mechanical Behavior of Materials and Nanomechanical Behavior Committees of TMS. Article published online April 11, 2007. 2290—VOLUME 38A, SEPTEMBER 2007
In this study, a refinement in the colony size of the TiAl-based alloy has been observed merely through alloying, and a desirable microstructure can be obtained through related heat treatments without any additional thermomechanical processing or cyclic heat treating. An investigation regarding the effects of the tungsten addition on the phase formation, the microstructural features, and the mechanical properties of the alloy has been conducted. It has been shown that W is a betaphase stabilizer; with the increasing amount of W, both the grain size and the interlamellar spacing of the alloy can be reduced. Hardness of the alloy can also be improved with the increasing amount of W.
II.
EXPERIMENTAL PROCEDURES
In this investigation, the compositions of the TiAlbased samples fabricated at Oak Ridge National Laboratory are as follows: (1) Ti-45 at. pct Al-7 at. pct Nb-0.15 at. pct B, (2) Ti-45 at. pct Al-7 at. pct Nb-0.2 at. pct W-0.15 at. pct B, (3) Ti-45 at. pct Al-7 at
Data Loading...
