Work Hardening Characteristics and Recovery of Gamma Base Titanium Aluminides
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+ G* (0)= ao + g (0) + 1/fVd(E)
(AFd* + kTlnt/to).
(I)
The parameters involved in eq. (1) are , strain rate, •o constant pre-exponential factor, k Boltzmann constant, T absolute temperature. cro is considered to be independent of strain E and represents a stress contribution from thermal and athermal mechanisms which are operating at the onset of yielding. Micromechanisms associated with a. have been investigated in a previous study [2]. o-(E) is an athermal stress contribution to work hardening representing long-range dislocation interactions. cr*(E) is an effective or thermal stress component due to thermally assisted overcoming of deformation KK1.7.1 Mat. Res. Soc. Symp. Proc. Vol. 552 © 1999 Materials Research Society
induced short-range glide obstacles. Vd(E) and AFd* are the activation volume and the free energy of activation, respectively, of this thermally activated process. Thus, the variation of the reciprocal activation volume with E will serve as a measure for the contribution of thermal glide obstacles to work hardening. V was determined by strain rate cycling tests as described in [1]. For these tests compression samples of 4 mm in diameter and 8 mm long were used. The flow stresses u whand reciprocal activation volumes 1/Vwh measured after room temperature compression to strain E = 7.5 % are listed in Table II. The deformed samples were subjected to isochronal annealings for an annealing time tr = 120 min at different temperatures Tr = 673 - 1133 K,
in order to recover the deformation induced defect structure gradually.
TABLE I: Alloy compositions and microstructures Symbol
Alloy I1
Composition (at. %) Ti - 48A1 - 2Cr
Microstructure nearly-lamellar, colony size 1000 gim, lamellar spacings 0.05-1.0 [am
o
2a
Ti - 47A1 - 2Cr - 0.2Si near gamma, grain size 11 [Im
o
2b
Ti - 47A1 - 2Cr - 0.2Si nearly-lamellar, colony size 330 gam,
lamellar spacings 0.05 -1.0 9tm 3
Ti - 52A1
4
Ti - 54A1
-
2Cr
near gamma, grain size 5.2 gam near gamma, grain size 188 gim
TABLE II: Flow stresses Gwh and reciprocal activation volumes /Vwh determined after compression to strain e = 7.5 %; T = 295 K, ý= 4.16* 104 s'l (MPa) WYh
Alloy 1 ......
2a 2b 3 4
10-18Nwh (mm -3) 0.53
_1029
947 1065 1037 901
0.7 1.12 0.7 0.75
Standard methods of conventional transmission electron microscopy (TEM) have been combined with high resolution imaging in order to characterize the defect structure generated during the predeformation at room temperature. The investigations were supplemented by TEM in situ heating experiments. In these cases electron transparent foils were prepared from samples which had been deformed at room temperature to strain E= 3%. This produced a suitable density of dislocations and probably a supersaturation of point defects. During in situ heating inside the TEM the changes of the defect structure could directly be observed. Work Hardening Phenomena and Defect Structures
The variation of the work hardening coefficients W/p. = (i/p.) du/dE with strain is shown in Figure 1.Work hardening is alway
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