An Electron Microscope Study of Diffusion Assisted Dislocation Processes in Intermetallic Gamma TiAl
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significant structural changes. Thus, for the high-temperature capability of the materials dislocation climb may be important in several different ways. Investigation of these processes is the subject matter of the present paper. EXPERIMENTAL INVESTIGATIONS The investigations were performed on a Ti-48A1-2 Cr alloy (in at.%) with a nearly-lamellar microstructure [4], which might be considered as a model material for engineering two-phase titanium aluminide alloys. Long-term creep studies were performed at T = 973 K and applied stresses aa= 80 to 140 MPa [6]. These conditions led to very low creep rates, which are certainly conducive to the initiation of diffusion assisted climb processes. Electron transparent foils were prepared from deformed samples by mechanical grinding and electrolytic polishing [3, 4]. Conventional and high-resolution electron microscope (TEM) observations were performed in order to characterize deformation induced defect structures. These investigations were supplemented by TEM in situ heating experiments. In these cases the thin foils were prepared from samples which had been deformed at room temperature to strain , = 3 %. This produced a suitable density of dislocations and certainly a small supersaturation of point defects because jog dragging occurs [4]. Thus, during in situ heating inside the TEM the dislocations are probably subjected to osmotic forces due to excess point defects. The kinematics and dynamics of the observed defect processes might therefore be affected by 105 Mat. Res. Soc. Symp. Proc. Vol. 589 © 2001 Materials Research Society
the applied method. However, in order to establish desired microstructures, processing routes of TiAl alloys often involve high temperature annealing followed by rapid cooling. This certainly leads to a significant point defect supersaturation. The dislocation behaviour occurring under such conditions is therefore not only a scientific matter but also of technical significance. The TEM in situ experiments were carried out at low acceleration voltages of 120 and 200 kV, respectively, in order to avoid radiation damage. For the TEM investigations described here the Philips instruments CM30, CM200 and 400T were used. CLIMB ASSISTED DISLOCATION MULTIPLICATION Dislocation climb often occurs at low stresses and strain rates. Under such conditions dislocation multiplication by conventional glide sources is difficult because the applied stress is too low to initiate cross glide and to by-pass the dislocation segments trailed at jogged screw dislocations. In the present study information about climb assisted multiplication has been deduced from in situ heating experiments performed inside the TEM. Climb was preferentially observed on 1/2 [R b LA/ L 2n(1 - v) kT] In (L a/1.8b),
(1)
where Q is the atomic volume, c = 4 and v is Poissons' ratio. For the present experimental conditions T = 820 K and L = 150 b to 350 b, the values c/co = 3 to 1.7 were obtained. These supersaturations are small in comparison to those produced initially after rapid cooling, whi
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