Characterization of the Effect of Cr Additions to TiAl-Base Alloys
- PDF / 1,640,118 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 30 Downloads / 180 Views
CHARACTERIZATION OF THE EFFECT OF Cr ADDITIONS TO TiAl-BASE ALLOYS SHYH-CHIN HUANG AND ERNEST L. HALL General Electric Research and Development Center, Schenectady, NY 12301. ABSTRACT Ternary TiAl-base alloys containing Cr have been studied, using rapidsolidification processed materials. The mechanical behavior has been characterized and related to other results on microstructure, site occupancy, phase stability, and deformation mode. It was found that Cr additions enhance the plasticity of duplex y+a 2 alloys, but not single-phase alloys. The ductilization effect of Cr is partially due to its ability to occupy Al lattice sites and modify the Ti-Al bond. It is also partially due to its ability to promote twin formation, by modifying the Al partitioning and therefore the a 2/7 volume ratio in transformed regions. INTRODUCTION The lightweight compound TiAl has limited plasticity (-1%) at low temperatures, which is a major concern for structural applications. Recently, alloying additions of V, Mn or Cr have been reported to result in plasticity enhancement [1,2]. The effect of Mn additions was studied extensively [3-5]. Hanamura et al. [3] concluded that Mn additions enhance the plasticity of TiAl (the y phase in the Ti-Al system) by promoting twin deformation. Two specific ,factors were cited: 1) A Mn addition stabilizes the annealing twin structure, which provides nucleation sites for twinning dislocations; and 2) it lowers stacking fault energy and thus favors twin deformation over superdislocation slip. Indeed, stacking fault energy has been measured to decrease with Mn additions [4]. Hashimoto and coworkers [5] also noted the promotion of twinning by Mn additions, citing that Mn mainly occupies the Al sites and thus weakens the covalency of Ti-Al bonds. This appears to be supported by a model based on bond order calculation [6]. However, twin deformation and plasticity enhancement have already been observed to result from decreases in Al content in binary alloys without any ternary addition [7,8]. The third element addition effects claimed above thus need to be characterized in detail. This study was aimed at understanding the effect of Cr additions to y-base alloys. Systematic variations in Cr and Al concentrations were carried out, and their effects on ductility characterized by bending and tensile tests at room temperatures. Their effects on microstructure, phase stability, deformation mechanism, and lattice substitution were also characterized and related to the ductility results. In order to correctly assess the Cr effect, it was critical to characterize the off-stoichiometry effect in binary alloys. A thorough study of binary alloys has recently been completed [9], and the results are used as the base to evaluate the Cr effect. All of the materials studied here have been processed via the rapid solidification route to ensure microstructural homogeneity. EXPERIMENTAL Several y-base ternary alloys containing 45-54 at.% Al and 1-4 at.% Cr were prepared by rapid solidification processing for this investigation.
Data Loading...
