• PDF / 1,330,726 Bytes
• 6 Pages / 420.48 x 639 pts Page_size
• 85 Downloads / 280 Views

DOWNLOAD

REPORT

---

COMPRESSION DEFORMATION STRUCTURES OF SINGLE CRYSTAL TiAl B. Y. HUANG*, B. F. OLIVER** AND W. C. OLIVER*** *Powder Metallurgy Institute, South Central University of Technology, Changsha, Huuan, People's Republic of China **Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996-2200 ***Oak Ridge National Laboratory, Oak Ridge, TN 37831-6116 ABSTRACT The compression deformation behavior of single crystalline TiAl was examined by transmission electron microscopy (T.E.M.). The relatively pure Ti-56 a/o Al crystal was containerless processed in ultrapure hydrogen. The crystal growth direction is 180 off (011) and 600 off [111] in the [0111[1111-[010] unit triangle. At low stresses, a/2 11101 type dislocations were observed. a/2 [1101 dislocations appeared at slightly higher stresses.

Additional plastic deformation initiates twinning.

Twinning plays an

important role at higher stresses. Diffraction results indicate that most of the twins have the (111) mirror plane.

A small amount of (Hi1) twins were also observed. Superdislocations of the a type were not observed to contribute to the plastic deformation in this crystal. The results indicate that plastic deformation by twinning follows the low density of ordinary dislocations. INTRODUCTION TiAl is known to be quite brittle at ambient temperatures. Considerable work has been devoted to understanding the mechanism of this brittle behavior (1-7). Single phase TiAl was tested in compression by Lipsitt and co-workers (2-3). Their material was polycrystalline and contained more carbon and oxygen than the single crystal material examined here. They observed a/2 ordinary dislocations and a superlattice dislocations. The a [0111 dislocations generate the a/6 [112] type partial dislocations producing stacking faults. They consider a/2 type

dislocations, a consitutent of the hard slip mode a superdislocations.

These results would indicate that the mobility of the superlattice dislocations controls the plasticity of Tial. In 1985 and more recently T. Kawabata et al. tested a single cyrstal in compression with [010] orientation (8). They found that the mobility of the a/2 dislocations did not differ much from that of the a superlattice dislocations below the peak yield stress temperature. In 1986 Hug et al. reported a/2 type dislocations in TiAl (9). They concluded that these a/2 superdislocations were important in the plastic deformation process. In 1987 T. Kawabata found that at temperatures of 770 K the a/2 ordinary dislocations are immoble but a and a/2 superdislocations contributed to the plastic deformation (10). In 1988 Kawabata et al. carried out bend tests on 56 a/o Al single cyrstals of TiAl and discussed a model for cleavage fracture involving dislocation pileup (17). In this research the samples, cut from a large single crystal, were compressed up to different final stresses along the growth direction. TEM was used to observe the compression deformation structures.

EXPERIMENTAL PRODCEDURE The TiAl used in this was a single