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Tensile tests of Fe70Al30 in a TEM in the temperature range of the yield stress anomaly
MATERIALS SCIENCE & ElltlllEERlNG Materials Science and Engineering
A
Al 92/l 93 (1995) 793-798
Tensile tests of Fe,,Al,,, in a TEM in the temperature stress anomaly
range of the yield
H. Rijsnera, G. Mo16natb, M. Kolbe”, E. Nembach” aInstitut fiir Metallforschung,
“CEMES-LOEICNRS,
Wilhelm-Klemm-Strasse IO, D-48149 M&steer, Germany 29 rue Jeanne Marvig, BP 4347, F-31055 Toulouse Cedex, France
Abstract Fe,,,Al,,, is a long-range ordered material, which exhibits a yield stress anomaly. Thin single-crystal foils of Fe,,,Al,,, have been strained inside a transmission electron microscope in the temperature range of the yield stress anomaly. The structure in this range is DO,. The dislocations had Burgers vectors parallel to the (111) directions and were predominantly of screw character. All dislocations were two-fold dissociated; no four-fold dissociations have been found. Below the peak temperature TP glide on { 1 lO}-planes has been observed. The dislocations propagate by jumps. Once emitted from a source, they glide very rapidly over long distances before locking. In addition to glide on { 1 IO}-planes, close to T, glide on (1 12}-planes has been observed. In these planes the glide is viscous. It is concluded that the anomalous temperature variation of the CRSS in Fe,,,AI,,, can probably not be explained on the basis of the temperature dependence of the state of order. Keywords: Iron; Aluminium;
Stress; Tensile testing; Transmission
electron microscopy
1. Introduction
Fe,Al-based alloys have numerous potential applications [l] because of a combination of good oxidation resistance, high strength, low density and low cost. The high strength at elevated temperatures is related to a yield stress anomaly. On the basis of the Kubaschewski phase diagram [2], Fe,,,A13,, has a DO,-ordered structure below 793 K, a B2-ordered structure between 793 K and 1273 K and an A2-disordered structure between 1273 K and 1753 K, which is the melting temperature. The DO, and B2 crystal structures can be derived from the bodycentred cubic structure. The Curie temperature is 473 K. It is worth noting that Hilfrich et al. [3] proposed to revise the current Fe-Al phase diagram on the basis of their investigations by neutron scattering, because above 793 K there is still DO,-long-range order, but antiphase domains exist in thermal equilibrium. The critical resolved shear stress (CRSS) z, of Fe,,,A13,, has an anomalous temperature dependence, which can be described for middle orientated single 0921-.5093/95/$9.50 0 1995 - Elsevier Science S.A. All rights reserved SSDlO921-5093(94)03316-l
crystals as follows [4]: 270-550
K: at,/T<
0
(1)
550-750
K: at,/T>
0
(2)
T> 750 K: az,lT< 0
(3)
Dislocations have Burgers vectors b of the type a&l 11). The most important dissociations are described schematically in Fig. 1 [S]. APB 1 indicates an antiphase boundary with a change of nearest neighbour and APB2 of next-nearest neighbour relations, respectively. The types d-f are imperfect dislocations which can be explained by the decoupling of the perfect dislocations (types a and b).
2. Experimental Single crystals were grown by a modified melting zone method from Fe,,,A!,,, ingots. Most of the specimens were cut with a [321] tensile axis. The Schmid factors of the glide systems ( i 0 1)[1 i l] and (2 i 1 )[1 i l] were nearly the same: 0.35 and 0.30. These systems are
/
Materials Science and Engineering Al 92/l 93 (1995) 793- 798
Double tilt and employed.
heating
double
tilt holders
were
3. Results 3.1. 573 K (minimum
of the stress anomaly)
ii = 2.2<111>
6 = 2,111,
ii = %
Fig. 1. Schematic structure.
illustration
of possible dissociations
in the DO,-
expected to become operable below and above the peak temperature, respectively [4]. The specimens used for these in situ tests in a TEM had a 7 x 2 mm2 rectangular shape with two anchoring holes for the direct fixation on the two jaws of the straining holder. The central part of the samples was reduced from 2 mm to 1.2 mm in order to lower the required straining forces. This complex shape was obtained by spark cutting. After that the samples were mechanically thinned and electrochemically polished. The single tilt heating and straining holder used for these experiments was built in the CEMES laboratory (Toulouse). It is used in a JEOL 200CX side entry TEM, operated at 200 kV. The current in the objective lens was only switched on when the temperature was above the Curie temperature to avoid magnetic straining effects due to the field of the lens. Observations were made under bright field (BF ) as well as under weak beam (WB) conditions. The dynamic sequences were recorded with a video system (low dose camera, amplifier, 3/4 inch U-matic video recorder), with a frame speed of 50 s l. Therefore the time resolution is 20 ms. Plates were taken under WB conditions if the stability of the image was sufficient. The video images were digitized and processed with the MASSRAM and SEMPER 6P software, respectively. More information about the recent developments of in situ deformation in a TEM and in situ straining experiments in WB conditions can be found in Refs. [6] and [7] respectively. If necessary, the strained, unloaded samples were subsequently observed in an H 800 NA Hitachi TEM.
The dislocations appear in groups. Once in motion, they go far away rapidly, gliding over long distances, typically more than 1 pm in less than 20 ms. After that they become locked. A decrease in the strain rate does not modify the glide characteristics, but increases the waiting time between two events. In Fig. 2 all the dislocations have suddenly appeared. They leave traces behind which correspond to the intersections of the slip planes with the surfaces of the thin foil. The dislocations have glided in different closely spaced (i0 1 )-planes. Using the classic extinction criterion gb = 0,the Burgers vector has been identified to be parallel to the [ 1 i l]-direction. The dislocations have thus a large dominant screw character, because the edge parts of the expanding loops are much more mobile and have therefore left the thin foil. Open loops labelled L in Fig. 2 can be observed on some dislocations. The slip traces in Fig. 2 are evidence for crossslip from (iO1) onto (ii 1) and for double cross-slip from (101) onto (211) and back to (101). (111) screw dislocations were also observed in WB conditions during the in situ experiments. Fig. 3 shows two-fold dissociated dislocations. This may correspond either to perfect dislocations (Fig. l(b)) or to imperfect dislocations (Fig. l(d)) in the DO,structure. In the case of Fig. l(d) it should be possible to image APB2 in the TEM with a DO,-superlattice reflection. Although there are some indications for an APB2 contrast, our results are not free from doubt up to now. No definitive conclusion can thus be drawn about the length of the Burgers vector. It is either a,( 111) or a,/2( 111). The dissociation plane has not yet been identified either. Work is progressing towards the solution of these questions. Figs. 4(a) and (b) show another area which has been observed in WB conditions under stress at 573 K with two different tilting conditions. The apparent dissociation widths of the dislocations in Figs. 4(a) and (b) and the widths of their bow-outs have been measured under full load. The dislocations were predominantly of screw character with [ii l] Burgers vector. Six dissociation planes-three { 110) and three { 112}-are possible for this Burgers vector. The true dissociation plane can be determined by calculating the ratio of the apparent dissociation widths for the two tilting conditions (Figs. 4(a) and (b)). The same method was used to
H. Riisner et al.
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Materials Science and Engineering Al9211 93 (1995) 793- 798
795
Fig. 2. Panorama of a strained zone. Bright field, g = [220]. The Burgers vector is b =[ li 11, L are open loops, Tr. ( i 0 1) and Tr. (2 i 1) show the slip traces left by the dislocations after gliding in the (iO1 )- and (2 i 1 )-planes, respectively. CS = cross-slip of dislocation D 1 from (iO1) onto (ii 1) and DCS =double cross-slip of dislocation D2 from (iOl)onto (ii 1) and back to (iO1). T=573K.
determine the plane of the dislocations’ bow-outs. The result is that these dislocations are bent and dissociated in the (2i 1 )-plane. The true dissociation width on the screw part is 7 nm. Dipoles (labelled D in Fig. 4) and switch-over anchoring points can be observed on these dislocations. Switch-over anchoring points (labelled S in Fig. 4) correspond to an inversion of the sequence of the two partial dislocations. Lastly, no motion of these dislocations is observed upon unloading; they stay locked. 3.2. 693 K
Fig. 3. [ 1 i I] dislocations in weak beam conditions 573 K after gliding in the (iOl)-plane. g=[i20].
under stress at
The characteristic features of the deformation are identical to those at 573 K: the deformation is heterogenous and sudden. Mobile dislocations have Burgers vectors parallel to the (1 1 1)-directions. They have a dominant screw character, they glide on { 1 lo}-planes and some of them exhibit open loops (Fig. 5).
796
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et al.
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Materials Science and Engineering A 192/I 93 (I 995) 793- 798
Fig. 5. Panorama of a strained zone. b = I 1111, L is an open loop, Trr( i i0) corresponds to the trace left by the dislocations after gliding in the ( i 10)-plane. g =[?20] T= 693 K.
Fig. 6 shows dislocations i:n WB conditions under stress at 693 K. As at 573 K, they appear as two-told dissociated dislocations. 3.3. 753 K (close to Tp ) As observed at 573 K and 693 K, dislocations move by jumps on { 1 lO}-planes. They have a dominant screw character. Dislocations Dl and D2 in Fig. 7 have suddenly arrived (the time between Figs. 7(a) and (b) is 20 ms) in the observed area. They glide on two distinct closely spaced ( i 0 1 )-planes. In contrast to the results obtained at 573 K and 693 K, (111) screw dislocations are also observed to glide on { 112}-planes. They move individually in a viscous way. Fig. 8 shows a sequence of such a glide process. The dislocation labelled D is dissociated. Between Figs. S(a) and (b) (7.88 s) the dislocation glides steadily over a distance of about 70 nm. In the glide plane, the dissociation width on the screw part is 7 nm. A change in the strain rate modifies the velocity of these dislocations. Such a viscous motion is similar to what occurs above the peak temperature in the case of { 112)-glide
[81.
4. Discussion F lg. 4. An area in weak beam contrast for two tilting conditl Ions is the und ler stress at 573 K. (a) g=[220], (b) g=[OAO]. b=[iil] BLlr‘gers vector. D are dipoles, and at S the segments switch OFfer.
The observed dislocations have a large dominant screw character, because the edge parts of the expand-
H. R&wzer et al.
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Materials Science and Engineering Al 92/l 93 (1995) 793- 798
797
Fig. 7. Two dislocations D 1 and D2 have suddenly arrived in the observed area in the time interval between (a) and (b). The activated system is the (iOl)[l i I] one. g=[400], T= 753K.
Fig. 6. ( 101 )[11 l] dislocations stress at 693 K, g=[220].
in weak beam
contrast
under
ing loops are much more mobile and have therefore left the thin foil. The screw dislocations thus control the expansion of the loop. Screw dislocations have been reported by several authors [4, S-101. Such behaviour can be related to the high friction forces of the screw parts. In b.c.c. metals friction forces are interpreted by a non-planar spreading of the core of screw dislocations [ 111. The concept of friction forces is related to the Peierls mechanism. In terms of the Peierls mechanism the dislocation could jump between adjacent Peierls valleys, leading to an apparently continuous motion. It corresponds to the observation of the glide on { 112}-planes. On the contrary, our in situ experiments have shown a jerky motion of dislocations on { llO}-planes in the temperature range of the stress anomaly. They glide very rapidly over long distances before locking. These observations could be related to an extension of the Peierls mechanism called the locking-unlocking mechanism [ 121. It should be mentioned, that the jump widths in Fe,,,A13,, are much larger ( = 1 pm) than those that are usually interpreted in terms of the locking-unlocking mechanism ( < 0.25 pm) [121. Two-fold dissociated dislocations have been found in all our experiments; four-fold dissociations have
Fig. 8. Viscous motion of a [ 11 l] dislocation D in a (112)-plane. Between (a) and (b) the dislocation has glided continuously over a distance of 70 nm. g=[O40], T=753 K.
never been observed. The absence of four-fold dissociations in the DO,-structure has already been reported for Fe,,,Al,,, [9]. Crawford and Ray [9] have interpreted the two-fold dissociated dislocations as a,,/4( 111) partials (Fig. l(d)), b ecause they have found that the inner separation (Fig. l(a)) of the four-fold dislocations increases with increasing concentration of Al, whereas the outer separation was nearly constant. Therefore they concluded an increasing tendency for four-fold
H. Riisner et al.
798
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Materials Science and Engineering Al 92/l 93 (1995) 793-798
dislocations to uncouple with increasing concentration of Al. But it should be mentioned that it has not been possible to determine the length of the Burgers vector of the partials, which may have been either a,/4(111) (Fig. l(d)) or a,/2(111) (Fig. l(b)), up to now. The observation of two-fold dissociated dislocations in the whole temperature range of the yield stress anomaly (573-753 K) shows that there is no straightforward way to apply Stoloff and Davies’ explanation of the anomaly [ 131. Stoloff and Davies concluded that the increase of the yield stress with increasing temperatures is due to the decoupling of the four-fold dissociated dislocations as a consequence of the decrease of the ordering parameter at elevated temperatures.
{112}-glide (753 K): the dislocations have a steady motion. It is interpreted to be controlled by the Peierls mechanism. A change in the strain rate modifies the speed. It is questionable whether the temperature dependence of the state of order governs the yield stress anomaly. Acknowledgements
We would like to thank Professor Dr Frommeyer, Dusseldorf, for providing the polycrystals of Fe-30 at.% Al and the Deutsche Forschungsgemeinschaft for financial support.
5. Conclusions 0 The dislocations had Burgers vectors parallel to the (111) directions and were predominantly of screw character. These screw dislocations are less mobile than the edge dislocations because of friction forces. These screw dislocations are believed to govern the macroscopic critical resolved shear stress. ?? In
all cases two-fold dissociated dislocations have been observed; no four-fold dissociations have been found.
?? The
dislocations glide on { 1 lO}-planes in the whole temperature range of the anomaly. At 753 K ( - T,) they can also glide on { 112}-planes.
?? At
573 K and 693 K cross-slip from (110) onto
{ 1121 and double cross-slip from { 1 lo} onto { 1121 and back onto { 110) has been observed. After the
cross-slip event the dislocation can locally glide on a { 112}-plane or bow out in it. ?? { 1 lO}-glide:
the dislocations are generated in groups and glide very rapidly over long distances (typically 1 pm in less than 20 ms) before locking. Once locked no unlocking events of the dislocations were generally observed. A change in the strain rate does not modify these features.
References [II V.K. Sikka, S. Viswanathan
and C.G. McKamey, in R. Darolia, J.J. Lewandowski, C.T. Liu, P.L. Martin, D.B. Miracle and M.V. Nathal, Proc. Int. Syrnp. Intermetallics, TMS, Seven Springs PA, USA, 1993, pp. 483-490. PI 0. Kubaschewski, Iron Binary Phase Diagrams, Springer, Berlin, 1982. [31 K. Hilfrich, W. Petry, 0. Scharpf and E. Nembach, Acta Metall. Mater., 42 (1994) 731. [41 W. Schroer, C. Hartig and H. Mecking, Z. Metallkd., 84 (1993) 294. [51 M.G. Mendiratta, SK. Ehlers, D.K. Chatterjee and H.A. Lipsitt, Metall. Trans. A, 18 (1987) 283. bl A. Couret, J. Crestou, S. Farenc, G. Molenat, N. Clement, A. Coujou and D. Caillard, Microsc. Microanal. Microstrucc., 4 (1993) 153-170. [71 D. Caillard, A. Couret, G. Molenat and J. Crestou, Microsc. Microanal. Microstruct., 4( 1993) 183-190. kJ1 H. Rosner, G. Molenat and E. Nembach, in H. Oikawa, K. Maruyama, S. Takeuchi and H. Yamaguchi (eds.), Strength of Materials, Proc. ICSM AlO. Jpn. Inst. Met., 1994, p. 717. [91 R.C. Crawford and I.L.F. Ray, Philos. Mag. A, 3.5 (1977) 549. 1101 L.P. Kubin, A. Fourdeux, J.Y. Guedou and J. Rieu, Philos. Msg. A, 46 (1982) 357. V. Vitek, Cryst. Lattice Dejects, 5 (1974) 1. t:Y A. Couret and D. Caillard, in Proc. Int. Conf Dislocation Mechanisms and the Strength of Advanced Materials, Aussois, France, 1990, J. Phys. ZZI,88.5 (1991). [I31 N.S. Stoloff and R.G. Davies, Acta Metall., 12 (1964) 473.
A
Al 92/l 93 (1995) 793-798
Tensile tests of Fe,,Al,,, in a TEM in the temperature stress anomaly
range of the yield
H. Rijsnera, G. Mo16natb, M. Kolbe”, E. Nembach” aInstitut fiir Metallforschung,
“CEMES-LOEICNRS,
Wilhelm-Klemm-Strasse IO, D-48149 M&steer, Germany 29 rue Jeanne Marvig, BP 4347, F-31055 Toulouse Cedex, France
Abstract Fe,,,Al,,, is a long-range ordered material, which exhibits a yield stress anomaly. Thin single-crystal foils of Fe,,,Al,,, have been strained inside a transmission electron microscope in the temperature range of the yield stress anomaly. The structure in this range is DO,. The dislocations had Burgers vectors parallel to the (111) directions and were predominantly of screw character. All dislocations were two-fold dissociated; no four-fold dissociations have been found. Below the peak temperature TP glide on { 1 lO}-planes has been observed. The dislocations propagate by jumps. Once emitted from a source, they glide very rapidly over long distances before locking. In addition to glide on { 1 IO}-planes, close to T, glide on (1 12}-planes has been observed. In these planes the glide is viscous. It is concluded that the anomalous temperature variation of the CRSS in Fe,,,AI,,, can probably not be explained on the basis of the temperature dependence of the state of order. Keywords: Iron; Aluminium;
Stress; Tensile testing; Transmission
electron microscopy
1. Introduction
Fe,Al-based alloys have numerous potential applications [l] because of a combination of good oxidation resistance, high strength, low density and low cost. The high strength at elevated temperatures is related to a yield stress anomaly. On the basis of the Kubaschewski phase diagram [2], Fe,,,A13,, has a DO,-ordered structure below 793 K, a B2-ordered structure between 793 K and 1273 K and an A2-disordered structure between 1273 K and 1753 K, which is the melting temperature. The DO, and B2 crystal structures can be derived from the bodycentred cubic structure. The Curie temperature is 473 K. It is worth noting that Hilfrich et al. [3] proposed to revise the current Fe-Al phase diagram on the basis of their investigations by neutron scattering, because above 793 K there is still DO,-long-range order, but antiphase domains exist in thermal equilibrium. The critical resolved shear stress (CRSS) z, of Fe,,,A13,, has an anomalous temperature dependence, which can be described for middle orientated single 0921-.5093/95/$9.50 0 1995 - Elsevier Science S.A. All rights reserved SSDlO921-5093(94)03316-l
crystals as follows [4]: 270-550
K: at,/T<
0
(1)
550-750
K: at,/T>
0
(2)
T> 750 K: az,lT< 0
(3)
Dislocations have Burgers vectors b of the type a&l 11). The most important dissociations are described schematically in Fig. 1 [S]. APB 1 indicates an antiphase boundary with a change of nearest neighbour and APB2 of next-nearest neighbour relations, respectively. The types d-f are imperfect dislocations which can be explained by the decoupling of the perfect dislocations (types a and b).
2. Experimental Single crystals were grown by a modified melting zone method from Fe,,,A!,,, ingots. Most of the specimens were cut with a [321] tensile axis. The Schmid factors of the glide systems ( i 0 1)[1 i l] and (2 i 1 )[1 i l] were nearly the same: 0.35 and 0.30. These systems are
/
Materials Science and Engineering Al 92/l 93 (1995) 793- 798
Double tilt and employed.
heating
double
tilt holders
were
3. Results 3.1. 573 K (minimum
of the stress anomaly)
ii = 2.2<111>
6 = 2,111,
ii = %
Fig. 1. Schematic structure.
illustration
of possible dissociations
in the DO,-
expected to become operable below and above the peak temperature, respectively [4]. The specimens used for these in situ tests in a TEM had a 7 x 2 mm2 rectangular shape with two anchoring holes for the direct fixation on the two jaws of the straining holder. The central part of the samples was reduced from 2 mm to 1.2 mm in order to lower the required straining forces. This complex shape was obtained by spark cutting. After that the samples were mechanically thinned and electrochemically polished. The single tilt heating and straining holder used for these experiments was built in the CEMES laboratory (Toulouse). It is used in a JEOL 200CX side entry TEM, operated at 200 kV. The current in the objective lens was only switched on when the temperature was above the Curie temperature to avoid magnetic straining effects due to the field of the lens. Observations were made under bright field (BF ) as well as under weak beam (WB) conditions. The dynamic sequences were recorded with a video system (low dose camera, amplifier, 3/4 inch U-matic video recorder), with a frame speed of 50 s l. Therefore the time resolution is 20 ms. Plates were taken under WB conditions if the stability of the image was sufficient. The video images were digitized and processed with the MASSRAM and SEMPER 6P software, respectively. More information about the recent developments of in situ deformation in a TEM and in situ straining experiments in WB conditions can be found in Refs. [6] and [7] respectively. If necessary, the strained, unloaded samples were subsequently observed in an H 800 NA Hitachi TEM.
The dislocations appear in groups. Once in motion, they go far away rapidly, gliding over long distances, typically more than 1 pm in less than 20 ms. After that they become locked. A decrease in the strain rate does not modify the glide characteristics, but increases the waiting time between two events. In Fig. 2 all the dislocations have suddenly appeared. They leave traces behind which correspond to the intersections of the slip planes with the surfaces of the thin foil. The dislocations have glided in different closely spaced (i0 1 )-planes. Using the classic extinction criterion gb = 0,the Burgers vector has been identified to be parallel to the [ 1 i l]-direction. The dislocations have thus a large dominant screw character, because the edge parts of the expanding loops are much more mobile and have therefore left the thin foil. Open loops labelled L in Fig. 2 can be observed on some dislocations. The slip traces in Fig. 2 are evidence for crossslip from (iO1) onto (ii 1) and for double cross-slip from (101) onto (211) and back to (101). (111) screw dislocations were also observed in WB conditions during the in situ experiments. Fig. 3 shows two-fold dissociated dislocations. This may correspond either to perfect dislocations (Fig. l(b)) or to imperfect dislocations (Fig. l(d)) in the DO,structure. In the case of Fig. l(d) it should be possible to image APB2 in the TEM with a DO,-superlattice reflection. Although there are some indications for an APB2 contrast, our results are not free from doubt up to now. No definitive conclusion can thus be drawn about the length of the Burgers vector. It is either a,( 111) or a,/2( 111). The dissociation plane has not yet been identified either. Work is progressing towards the solution of these questions. Figs. 4(a) and (b) show another area which has been observed in WB conditions under stress at 573 K with two different tilting conditions. The apparent dissociation widths of the dislocations in Figs. 4(a) and (b) and the widths of their bow-outs have been measured under full load. The dislocations were predominantly of screw character with [ii l] Burgers vector. Six dissociation planes-three { 110) and three { 112}-are possible for this Burgers vector. The true dissociation plane can be determined by calculating the ratio of the apparent dissociation widths for the two tilting conditions (Figs. 4(a) and (b)). The same method was used to
H. Riisner et al.
/
Materials Science and Engineering Al9211 93 (1995) 793- 798
795
Fig. 2. Panorama of a strained zone. Bright field, g = [220]. The Burgers vector is b =[ li 11, L are open loops, Tr. ( i 0 1) and Tr. (2 i 1) show the slip traces left by the dislocations after gliding in the (iO1 )- and (2 i 1 )-planes, respectively. CS = cross-slip of dislocation D 1 from (iO1) onto (ii 1) and DCS =double cross-slip of dislocation D2 from (iOl)onto (ii 1) and back to (iO1). T=573K.
determine the plane of the dislocations’ bow-outs. The result is that these dislocations are bent and dissociated in the (2i 1 )-plane. The true dissociation width on the screw part is 7 nm. Dipoles (labelled D in Fig. 4) and switch-over anchoring points can be observed on these dislocations. Switch-over anchoring points (labelled S in Fig. 4) correspond to an inversion of the sequence of the two partial dislocations. Lastly, no motion of these dislocations is observed upon unloading; they stay locked. 3.2. 693 K
Fig. 3. [ 1 i I] dislocations in weak beam conditions 573 K after gliding in the (iOl)-plane. g=[i20].
under stress at
The characteristic features of the deformation are identical to those at 573 K: the deformation is heterogenous and sudden. Mobile dislocations have Burgers vectors parallel to the (1 1 1)-directions. They have a dominant screw character, they glide on { 1 lo}-planes and some of them exhibit open loops (Fig. 5).
796
H. Rhner
et al.
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Materials Science and Engineering A 192/I 93 (I 995) 793- 798
Fig. 5. Panorama of a strained zone. b = I 1111, L is an open loop, Trr( i i0) corresponds to the trace left by the dislocations after gliding in the ( i 10)-plane. g =[?20] T= 693 K.
Fig. 6 shows dislocations i:n WB conditions under stress at 693 K. As at 573 K, they appear as two-told dissociated dislocations. 3.3. 753 K (close to Tp ) As observed at 573 K and 693 K, dislocations move by jumps on { 1 lO}-planes. They have a dominant screw character. Dislocations Dl and D2 in Fig. 7 have suddenly arrived (the time between Figs. 7(a) and (b) is 20 ms) in the observed area. They glide on two distinct closely spaced ( i 0 1 )-planes. In contrast to the results obtained at 573 K and 693 K, (111) screw dislocations are also observed to glide on { 112}-planes. They move individually in a viscous way. Fig. 8 shows a sequence of such a glide process. The dislocation labelled D is dissociated. Between Figs. S(a) and (b) (7.88 s) the dislocation glides steadily over a distance of about 70 nm. In the glide plane, the dissociation width on the screw part is 7 nm. A change in the strain rate modifies the velocity of these dislocations. Such a viscous motion is similar to what occurs above the peak temperature in the case of { 112)-glide
[81.
4. Discussion F lg. 4. An area in weak beam contrast for two tilting conditl Ions is the und ler stress at 573 K. (a) g=[220], (b) g=[OAO]. b=[iil] BLlr‘gers vector. D are dipoles, and at S the segments switch OFfer.
The observed dislocations have a large dominant screw character, because the edge parts of the expand-
H. R&wzer et al.
/
Materials Science and Engineering Al 92/l 93 (1995) 793- 798
797
Fig. 7. Two dislocations D 1 and D2 have suddenly arrived in the observed area in the time interval between (a) and (b). The activated system is the (iOl)[l i I] one. g=[400], T= 753K.
Fig. 6. ( 101 )[11 l] dislocations stress at 693 K, g=[220].
in weak beam
contrast
under
ing loops are much more mobile and have therefore left the thin foil. The screw dislocations thus control the expansion of the loop. Screw dislocations have been reported by several authors [4, S-101. Such behaviour can be related to the high friction forces of the screw parts. In b.c.c. metals friction forces are interpreted by a non-planar spreading of the core of screw dislocations [ 111. The concept of friction forces is related to the Peierls mechanism. In terms of the Peierls mechanism the dislocation could jump between adjacent Peierls valleys, leading to an apparently continuous motion. It corresponds to the observation of the glide on { 112}-planes. On the contrary, our in situ experiments have shown a jerky motion of dislocations on { llO}-planes in the temperature range of the stress anomaly. They glide very rapidly over long distances before locking. These observations could be related to an extension of the Peierls mechanism called the locking-unlocking mechanism [ 121. It should be mentioned, that the jump widths in Fe,,,A13,, are much larger ( = 1 pm) than those that are usually interpreted in terms of the locking-unlocking mechanism ( < 0.25 pm) [121. Two-fold dissociated dislocations have been found in all our experiments; four-fold dissociations have
Fig. 8. Viscous motion of a [ 11 l] dislocation D in a (112)-plane. Between (a) and (b) the dislocation has glided continuously over a distance of 70 nm. g=[O40], T=753 K.
never been observed. The absence of four-fold dissociations in the DO,-structure has already been reported for Fe,,,Al,,, [9]. Crawford and Ray [9] have interpreted the two-fold dissociated dislocations as a,,/4( 111) partials (Fig. l(d)), b ecause they have found that the inner separation (Fig. l(a)) of the four-fold dislocations increases with increasing concentration of Al, whereas the outer separation was nearly constant. Therefore they concluded an increasing tendency for four-fold
H. Riisner et al.
798
1
Materials Science and Engineering Al 92/l 93 (1995) 793-798
dislocations to uncouple with increasing concentration of Al. But it should be mentioned that it has not been possible to determine the length of the Burgers vector of the partials, which may have been either a,/4(111) (Fig. l(d)) or a,/2(111) (Fig. l(b)), up to now. The observation of two-fold dissociated dislocations in the whole temperature range of the yield stress anomaly (573-753 K) shows that there is no straightforward way to apply Stoloff and Davies’ explanation of the anomaly [ 131. Stoloff and Davies concluded that the increase of the yield stress with increasing temperatures is due to the decoupling of the four-fold dissociated dislocations as a consequence of the decrease of the ordering parameter at elevated temperatures.
{112}-glide (753 K): the dislocations have a steady motion. It is interpreted to be controlled by the Peierls mechanism. A change in the strain rate modifies the speed. It is questionable whether the temperature dependence of the state of order governs the yield stress anomaly. Acknowledgements
We would like to thank Professor Dr Frommeyer, Dusseldorf, for providing the polycrystals of Fe-30 at.% Al and the Deutsche Forschungsgemeinschaft for financial support.
5. Conclusions 0 The dislocations had Burgers vectors parallel to the (111) directions and were predominantly of screw character. These screw dislocations are less mobile than the edge dislocations because of friction forces. These screw dislocations are believed to govern the macroscopic critical resolved shear stress. ?? In
all cases two-fold dissociated dislocations have been observed; no four-fold dissociations have been found.
?? The
dislocations glide on { 1 lO}-planes in the whole temperature range of the anomaly. At 753 K ( - T,) they can also glide on { 112}-planes.
?? At
573 K and 693 K cross-slip from (110) onto
{ 1121 and double cross-slip from { 1 lo} onto { 1121 and back onto { 110) has been observed. After the
cross-slip event the dislocation can locally glide on a { 112}-plane or bow out in it. ?? { 1 lO}-glide:
the dislocations are generated in groups and glide very rapidly over long distances (typically 1 pm in less than 20 ms) before locking. Once locked no unlocking events of the dislocations were generally observed. A change in the strain rate does not modify these features.
References [II V.K. Sikka, S. Viswanathan
and C.G. McKamey, in R. Darolia, J.J. Lewandowski, C.T. Liu, P.L. Martin, D.B. Miracle and M.V. Nathal, Proc. Int. Syrnp. Intermetallics, TMS, Seven Springs PA, USA, 1993, pp. 483-490. PI 0. Kubaschewski, Iron Binary Phase Diagrams, Springer, Berlin, 1982. [31 K. Hilfrich, W. Petry, 0. Scharpf and E. Nembach, Acta Metall. Mater., 42 (1994) 731. [41 W. Schroer, C. Hartig and H. Mecking, Z. Metallkd., 84 (1993) 294. [51 M.G. Mendiratta, SK. Ehlers, D.K. Chatterjee and H.A. Lipsitt, Metall. Trans. A, 18 (1987) 283. bl A. Couret, J. Crestou, S. Farenc, G. Molenat, N. Clement, A. Coujou and D. Caillard, Microsc. Microanal. Microstrucc., 4 (1993) 153-170. [71 D. Caillard, A. Couret, G. Molenat and J. Crestou, Microsc. Microanal. Microstruct., 4( 1993) 183-190. kJ1 H. Rosner, G. Molenat and E. Nembach, in H. Oikawa, K. Maruyama, S. Takeuchi and H. Yamaguchi (eds.), Strength of Materials, Proc. ICSM AlO. Jpn. Inst. Met., 1994, p. 717. [91 R.C. Crawford and I.L.F. Ray, Philos. Mag. A, 3.5 (1977) 549. 1101 L.P. Kubin, A. Fourdeux, J.Y. Guedou and J. Rieu, Philos. Msg. A, 46 (1982) 357. V. Vitek, Cryst. Lattice Dejects, 5 (1974) 1. t:Y A. Couret and D. Caillard, in Proc. Int. Conf Dislocation Mechanisms and the Strength of Advanced Materials, Aussois, France, 1990, J. Phys. ZZI,88.5 (1991). [I31 N.S. Stoloff and R.G. Davies, Acta Metall., 12 (1964) 473.