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Indentation-induced twinning in LaAlO3 single crystals: An atomic force microscopy study
Scripta Materialia 49 (2003) 903–908 www.actamat-journals.com
Indentation-induced twinning in LaAlO3 single crystals: An atomic force microscopy study P.D. Tall a, C. Coupeau
b,* ,
J. Rabier
b
a
b
D epartement de Physique, University Cheikh Anta Diop, Dakar, BP 5005, Senegal Laboratorie de Metallurgie-Physique UFR Sciences-Bat., LMP-SP2MI/Teleport 2, Department of Materials Science, University of Poitiers, Bd Pierre et Marie CURIE/BP 30179, 86962 Futuroscope, Chasseneuil Cedex, France Received 14 April 2003; accepted 30 June 2003
Abstract Micro-indentation tests performed from room temperature to 700 °C on LaAlO3 single crystals show a plastic anisotropy in the rhombohedral phase, depending on the orientation of the Vickers indenter. The indentation-induced twins were investigated by atomic force microscopy. Their geometry appears to be different from those resulting usually from the cubic to rhombohedral phase transition. Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ceramics; Twinning; Atomic force microscopy; Dislocation; Surface structure
1. Introduction Lanthanum monoaluminate LaAlO3 single crystals are promising substrates for the epitaxy of thin oxide films [1–3]. Many authors have reported different synthesis methods and potential applications using these crystals [4–7]. As in many other materials which undergo a second order phase transition between the cubic high-temperature phase and the rhombohedral one at lower temperature, ferroelastic twins are observed in both {1 1 0} and {1 0 0} crystallographic habit planes. The temperature of this structural transformation has been determined to be T ¼ 544 °C [8]. How* Corresponding author. Tel.: +33-05-4949-6652; fax: +3305-4949-6692. E-mail address: [email protected] (C. Coupeau).
ever, few data are available about the mechanical properties of such single crystals. In this paper, the temperature dependence of micro-hardness is investigated. The micro-structures of defects such as pre-existing or indentationinduced twins have been characterized by atomic force microscopy (AFM). The effect of indenter orientation in the formation of twins near imprints is described.
2. Experimental As-grown LaAlO3 single crystals were obtained from 0.5 mm-thick wafers with a (0 0 1) surface orientation. Indentation tests were performed from room temperature (RT) up to 700 °C using a HT5 Nikon High Temperature Micro-Hardness Tester, with a Vickers indenter operating in an
1359-6462/$ - see front matter Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/S1359-6462(03)00409-3
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P.D. Tall et al. / Scripta Materialia 49 (2003) 903–908
argon environment. Micro-hardness measurements were performed as a function of temperature using a 100 g load, with a dwell time fixed at 30 s. For AFM analysis of RT indented samples, a lower charge (50 g) was used in order to minimize fracture damage. The thermal cycle was as follows: the samples were heated for 1 h at the temperature chosen for indentation in order to achieve thermal equilibrium and cooled down to room temperature after the indentation. The Vickers micro-hardness Hv was measured using optical microscopy, using the relation Hv ¼ 1:854 P =d 2 , where P is the applied load and d the mean size of two diagonals. Owing to the brittleness of ceramic compounds, irregular cracks were formed in the vicinity of the indentations but the diagonals were sufficiently reproducible to yield reliable values of microhardness when averaged over 10 indents. The AFM images shown below have been mainly taken in contact signal error mode. These images are of course not calibrated in the z-axis but are useful due to fine detail enhancement. Moreover this AFM image mode is particularly suited to the investigation of constant slope surface features such as those resulting from twinning mechanisms.
3. Results and discussion As expected at room temperature, the as-grown samples show pre-existing twins both of the {1 0 0} and {1 1 0} type (Fig. 1). It has been reported that {1 0 0} rather than {1 1 0} twins are predominant in LaAlO3 single crystals at room temperature [9] and during thermal cycling through temperatures close to those used for film growth [10–12]. These results were confirmed also in polycrystals [13,14]. {1 1 0} twins are difficult to observe by optical microscopy whereas both types of twins can be readily characterized by AFM through their habit plane, as well as the dihedral angle with the surface. As an example, Fig. 1 shows {1 0 0} and {1 1 0} twins in the as-grown material in the image mode as well as by the measurement of surface profile, with an angular misfit of only few percent degree. For this reason AFM investigations were carried out to characterize the indented surfaces.
Fig. 1. {1 0 0} and {1 1 0} twin structures emerging on a (0 0 1) crystallographic plane in an as-grown LaAlO3 single crystal. The dihedral angle / at the surface is / ¼ 0:29° for {1 1 0} twins and / ¼ 0:21° for {1 0 0}.
Using this technique it is thus emphasized that in the as-grown materials both types of twins show a constant thickness all along their length. The hardness Hv as a function of temperature between RT and 700 °C, shown in Fig. 2, was obtained on the (0 0 1) surface with the diagonals of the indenter oriented along h1 0 0i crystallographic directions. These measurements were performed in the same run starting conventionally from the highest temperature in order to prevent any relaxation effects induced by higher tempera-
P.D. Tall et al. / Scripta Materialia 49 (2003) 903–908 1200
H V (Kg.mm -2 )
1100 1000 900 800 700 Sample LaAlO3 600 0
200
400 Temperature (°C)
600
800
Fig. 2. Vickers hardness Hv as a function of temperature T . The slope changes drastically at T ¼ 500 °C, near the phase transition temperature.
ture on surface features resulting from lower one. It is found that Hv is approximately constant up to approximately 500 °C and then decreases abruptly;
905
this behaviour has to be related to the rhombohedral to cubic phase transition. A sharp hardness drop is however also observed near room temperature with no relevant explanation about. If the plasticity of the rhombohedral phase is partly accommodated by twinning, the deformation mechanism in the cubic phase should be different. Although twinning appears in the vicinity of all the indentations, regardless of temperature, around high temperature imprints performed in the cubic phase, it is the result of stress relief as the material is cooled through the phase transition temperature. Since the high temperature deformation signature is blurred by the phase transition, we have focussed in this paper on the observation of surfaces deformed at low temperatures. Furthermore since surface pollution precludes any good observation by AFM, the AFM investigations reported here have been performed on room temperature indented samples.
Fig. 3. Deformation features on the (0 0 1) surface of an as-grown LaAlO3 single crystal, resulting from 50 g Vickers indentations at room temperature and investigated by AFM. The indenter diagonals are aligned in the: (a) h1 0 0i direction. a: long radial {1 0 0} cracks propagating in mode III; the enclosed cross-section has been taken along the black dotted arrow (uplifts of few tens of nanometers for the two cracks considered). d and c: {1 0 0} and {1 1 0} lenticular twins nucleated near and at the crack edges respectively. (b) h1 1 0i direction. b: small radial cracks associated with mater extrusions all around the imprint. g: {1 0 0} and {1 1 0} induced-twins with a concave lenticular shape.
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P.D. Tall et al. / Scripta Materialia 49 (2003) 903–908
At room temperature indentations have been performed with the indenter diagonals both along h1 0 0i (‘‘h1 0 0i indentations’’ in the following) and h1 1 0i (‘‘h1 1 0i indentations’’) crystallographic directions, in order to see whether different types of twins can be selectively nucleated depending on the stress orientation. Such indentations are imaged by AFM in Fig. 3. For the two different orientations of the indenter the indentation sizes are about the same, which suggests from hardness measurements a similar plastic mechanism. However, although twin structures appear in the two cases, deformation features are very different. h1 0 0i indentations are associated with long radial cracks located on the {1 0 0} planes that appear to be propagating in mode III (a in Fig. 3(a)). Close to the indentation site, heights of few tens of nanometers can be thus measured between the surfaces around cracks with no observations (in the limit of the AFM lateral resolution) of opened cracks (mode I) in the surface plane (see cross-section in Fig. 3(a)). h1 1 0i indentations result in some material extruding from the indentation site. These extrusions are bordered by the sides of the indents (b in Fig. 3(b)) and by small radial cracks running close to the h1 1 0i direction. A few long h1 0 0i cracks coexist with these features. From these observations, it appears that h1 1 0i indentations are associated with a more brittle character; instead of longer cracks in h1 0 0i indentation configuration, more complex fracture mechanisms seem to be activated leading to very uneven surface features close to the imprints. Lenticular twins are associated with the deformation induced by the two types of imprints: {1 0 0} and {1 1 0} twins are found with such lenticular shapes. In both cases {1 1 0} twins are less numerous but their mean thickness appears to be larger than that of the {1 0 0} twins. This suggests two different nucleation mechanisms for these two types of twins. Anyway, far enough from the imprints, the density of twins induced during the indentation tests is mainly of type {1 0 0}, in agreement with previous calculations in the frame of elasticity showing that the formation of {1 0 0} twins is energetically favourable [14]. For h1 0 0i indentations, twins are nucleated near the edge of h1 0 0i cracks (c and d areas in Fig.
3(a)) whereas they are found slightly distant from the imprints in the case of h1 1 0i indentations. Note that {1 1 0} twins nucleated at crack edges show more complex shapes and can deviate apparently from the usual {1 1 0} habit plane (c in Fig. 3(a)). A residual stress anisotropy which could result from the occurrence of twin boundaries in the vicinity of the indents seems also to be evidenced, since the cracks are longer along the transversal crystallographic direction than the longitudinal one (Fig. 3(a)). This effect has to be confirmed by further experiments with a specific apparatus which consists of a nanoindenter interfaced with an AFM; the surface areas of interest will be thus first investigated by AFM, before performing nanoindentations (lower loads inducing very localised plastic zones) at a chosen position (1 lm accuracy). It is also evident from AFM images shown in Fig. 4 that the uplift of the surface resulting from the crack mode III propagation is accommodated by a very fine twin structure, sometimes only a few tens of nanometer thick (Fig. 4(b)). Moreover, it is quite interesting to note that a given crack propagation length (see top and bottom parts in Fig. 4(a)) leads to a perfectly similar surface deformation structure. On the other hand, the surface features strongly differ from left to right. Since the Vickers indentation observed in Fig. 4(a) has been carried out at the junction (vertical line in the middle of the image) of two {1 1 0} twins of the asgrown single crystals, this shows that the plastic indentation-induced mechanisms strongly depend on the crystallographic surface orientations. Twins observed in the vicinity of imprints have a very different geometry than those found in asgrown materials and can be related unambiguously to an indentation-induced plastic mechanism. Although twins in as-grown materials have a constant thickness all along their length and terminate at obstacles (surfaces, intersecting twins. . .), mechanical twins terminate freely in the matrix and possess a lenticular shape (g in Fig. 3(b)). Few constant thickness twins can be seen but are clearly associated with a surface relaxation induced by the cracks (j in Fig. 3(b)). The formation of the lenticular twins can be seen as resulting from the emission of twinning
P.D. Tall et al. / Scripta Materialia 49 (2003) 903–908
907
(a)
(b)
Fig. 5. Schematic of twin tips and twinning dislocations in (a) YBCO (b) LaAlO3 single crystals.
Fig. 4. (a) Signal error mode AFM images of h1 0 0i indentation showing crack features as well as twins nucleated at the crack edges. The Vickers indentation has been performed just between two twinned {1 1 0} surfaces. Note the symmetrical (top/ bottom) surface structure induced by a given crack length. (b) Details of the geometry of the {1 0 0} and {1 1 0} mechanical twins, labelled X in (a), revealing a fine nano-twin structure.
dislocations gliding on continuous twin planes whose movement is stopped when the effective stress on the dislocation is zero, leading to twin tips [15]. However, in contrast to what is found in some materials, these twin tips have a concave shape rather than convex (Fig. 5). Convex shapes of twin tips have been found to be in agreement with calculations of equilibrium configurations of twinning dislocations when subjected to an applied
stress [16]. In the present case these concave twin tips may suggest an inverse pile-up configuration. They could result from a relaxation of the dislocation configuration when the local residual stress is relaxed, suggesting a low friction stress on twinning dislocations and/or stresses resulting from dislocation interaction. Numerical calculations are currently in progress to test this hypothesis.
4. Conclusion Hardness experiments have been used to study plastic deformation of LaAlO3 single crystals between room temperature and 700 °C. It has been shown that, besides cracking, plasticity occurs at room temperature through the nucleation of mechanical twins located on both {1 0 0} and {1 1 0} twin planes. AFM observations reveal that these twins possess concave tips which may result from elastic relaxation of the twinning dislocations when the residual stress is relaxed.
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P.D. Tall et al. / Scripta Materialia 49 (2003) 903–908
References [1] Wang ZL, Shapiro AJ. Surf Sci 1995;328:141. [2] Sum R, Lang HP, Guntherodt HJ. Phys C Supercond 1995;242:174. [3] Lebedev OI, Van Tendeloo G, Amelinckx S, Ju HL, Krishnan KM. Phil Mag A 2000;80:673. [4] Adak AK, Pramanik P. Mater Lett 1997;30:269. [5] Peschev P, Slavova V. Mater Res Bull 1994;29:255. [6] Taspinar E, Cuneyt A. J Amer Ceram Soc 1997;80:133. [7] Phillips JM, Siegal MP, Perry CL, Marshal JH. IEEE Trans Magn 1991;27:1006. [8] Bueble S, Knorr K, Brecht E, Schmahl W. Surf Sci 1998;400:345.
[9] Yao GD, Hou SY, Dudley M, Phillips JM. J Mater Res 1992;7:1847. [10] Fay H, Brandle CD. Crystal growth. Oxford: Pergamon press; 1967. p. 51. [11] Berkstresser GW, Valentino AJ, Brandle CD. J Crystal Growth 1991;109:467. [12] Norton MG, Biggers RR. Scripta Metall Mater 1995;32:481. [13] Kim CH, Jang JW, Cho SY, Kim IT, Hong KS. Phys B Condens Matter 1999;262:438. [14] Kim CH, Jang JW, Cho SY, Kim IT, Hong KS. Mater Res Bull 2001;36:1561. [15] VanTendeloo G, Amelinckx S. J Less Common Metals 1990;164:92. [16] Kamat SV, Hirth JP. Phil Mag A 1996;73:669.
Indentation-induced twinning in LaAlO3 single crystals: An atomic force microscopy study P.D. Tall a, C. Coupeau
b,* ,
J. Rabier
b
a
b
D epartement de Physique, University Cheikh Anta Diop, Dakar, BP 5005, Senegal Laboratorie de Metallurgie-Physique UFR Sciences-Bat., LMP-SP2MI/Teleport 2, Department of Materials Science, University of Poitiers, Bd Pierre et Marie CURIE/BP 30179, 86962 Futuroscope, Chasseneuil Cedex, France Received 14 April 2003; accepted 30 June 2003
Abstract Micro-indentation tests performed from room temperature to 700 °C on LaAlO3 single crystals show a plastic anisotropy in the rhombohedral phase, depending on the orientation of the Vickers indenter. The indentation-induced twins were investigated by atomic force microscopy. Their geometry appears to be different from those resulting usually from the cubic to rhombohedral phase transition. Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Ceramics; Twinning; Atomic force microscopy; Dislocation; Surface structure
1. Introduction Lanthanum monoaluminate LaAlO3 single crystals are promising substrates for the epitaxy of thin oxide films [1–3]. Many authors have reported different synthesis methods and potential applications using these crystals [4–7]. As in many other materials which undergo a second order phase transition between the cubic high-temperature phase and the rhombohedral one at lower temperature, ferroelastic twins are observed in both {1 1 0} and {1 0 0} crystallographic habit planes. The temperature of this structural transformation has been determined to be T ¼ 544 °C [8]. How* Corresponding author. Tel.: +33-05-4949-6652; fax: +3305-4949-6692. E-mail address: [email protected] (C. Coupeau).
ever, few data are available about the mechanical properties of such single crystals. In this paper, the temperature dependence of micro-hardness is investigated. The micro-structures of defects such as pre-existing or indentationinduced twins have been characterized by atomic force microscopy (AFM). The effect of indenter orientation in the formation of twins near imprints is described.
2. Experimental As-grown LaAlO3 single crystals were obtained from 0.5 mm-thick wafers with a (0 0 1) surface orientation. Indentation tests were performed from room temperature (RT) up to 700 °C using a HT5 Nikon High Temperature Micro-Hardness Tester, with a Vickers indenter operating in an
1359-6462/$ - see front matter Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/S1359-6462(03)00409-3
904
P.D. Tall et al. / Scripta Materialia 49 (2003) 903–908
argon environment. Micro-hardness measurements were performed as a function of temperature using a 100 g load, with a dwell time fixed at 30 s. For AFM analysis of RT indented samples, a lower charge (50 g) was used in order to minimize fracture damage. The thermal cycle was as follows: the samples were heated for 1 h at the temperature chosen for indentation in order to achieve thermal equilibrium and cooled down to room temperature after the indentation. The Vickers micro-hardness Hv was measured using optical microscopy, using the relation Hv ¼ 1:854 P =d 2 , where P is the applied load and d the mean size of two diagonals. Owing to the brittleness of ceramic compounds, irregular cracks were formed in the vicinity of the indentations but the diagonals were sufficiently reproducible to yield reliable values of microhardness when averaged over 10 indents. The AFM images shown below have been mainly taken in contact signal error mode. These images are of course not calibrated in the z-axis but are useful due to fine detail enhancement. Moreover this AFM image mode is particularly suited to the investigation of constant slope surface features such as those resulting from twinning mechanisms.
3. Results and discussion As expected at room temperature, the as-grown samples show pre-existing twins both of the {1 0 0} and {1 1 0} type (Fig. 1). It has been reported that {1 0 0} rather than {1 1 0} twins are predominant in LaAlO3 single crystals at room temperature [9] and during thermal cycling through temperatures close to those used for film growth [10–12]. These results were confirmed also in polycrystals [13,14]. {1 1 0} twins are difficult to observe by optical microscopy whereas both types of twins can be readily characterized by AFM through their habit plane, as well as the dihedral angle with the surface. As an example, Fig. 1 shows {1 0 0} and {1 1 0} twins in the as-grown material in the image mode as well as by the measurement of surface profile, with an angular misfit of only few percent degree. For this reason AFM investigations were carried out to characterize the indented surfaces.
Fig. 1. {1 0 0} and {1 1 0} twin structures emerging on a (0 0 1) crystallographic plane in an as-grown LaAlO3 single crystal. The dihedral angle / at the surface is / ¼ 0:29° for {1 1 0} twins and / ¼ 0:21° for {1 0 0}.
Using this technique it is thus emphasized that in the as-grown materials both types of twins show a constant thickness all along their length. The hardness Hv as a function of temperature between RT and 700 °C, shown in Fig. 2, was obtained on the (0 0 1) surface with the diagonals of the indenter oriented along h1 0 0i crystallographic directions. These measurements were performed in the same run starting conventionally from the highest temperature in order to prevent any relaxation effects induced by higher tempera-
P.D. Tall et al. / Scripta Materialia 49 (2003) 903–908 1200
H V (Kg.mm -2 )
1100 1000 900 800 700 Sample LaAlO3 600 0
200
400 Temperature (°C)
600
800
Fig. 2. Vickers hardness Hv as a function of temperature T . The slope changes drastically at T ¼ 500 °C, near the phase transition temperature.
ture on surface features resulting from lower one. It is found that Hv is approximately constant up to approximately 500 °C and then decreases abruptly;
905
this behaviour has to be related to the rhombohedral to cubic phase transition. A sharp hardness drop is however also observed near room temperature with no relevant explanation about. If the plasticity of the rhombohedral phase is partly accommodated by twinning, the deformation mechanism in the cubic phase should be different. Although twinning appears in the vicinity of all the indentations, regardless of temperature, around high temperature imprints performed in the cubic phase, it is the result of stress relief as the material is cooled through the phase transition temperature. Since the high temperature deformation signature is blurred by the phase transition, we have focussed in this paper on the observation of surfaces deformed at low temperatures. Furthermore since surface pollution precludes any good observation by AFM, the AFM investigations reported here have been performed on room temperature indented samples.
Fig. 3. Deformation features on the (0 0 1) surface of an as-grown LaAlO3 single crystal, resulting from 50 g Vickers indentations at room temperature and investigated by AFM. The indenter diagonals are aligned in the: (a) h1 0 0i direction. a: long radial {1 0 0} cracks propagating in mode III; the enclosed cross-section has been taken along the black dotted arrow (uplifts of few tens of nanometers for the two cracks considered). d and c: {1 0 0} and {1 1 0} lenticular twins nucleated near and at the crack edges respectively. (b) h1 1 0i direction. b: small radial cracks associated with mater extrusions all around the imprint. g: {1 0 0} and {1 1 0} induced-twins with a concave lenticular shape.
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P.D. Tall et al. / Scripta Materialia 49 (2003) 903–908
At room temperature indentations have been performed with the indenter diagonals both along h1 0 0i (‘‘h1 0 0i indentations’’ in the following) and h1 1 0i (‘‘h1 1 0i indentations’’) crystallographic directions, in order to see whether different types of twins can be selectively nucleated depending on the stress orientation. Such indentations are imaged by AFM in Fig. 3. For the two different orientations of the indenter the indentation sizes are about the same, which suggests from hardness measurements a similar plastic mechanism. However, although twin structures appear in the two cases, deformation features are very different. h1 0 0i indentations are associated with long radial cracks located on the {1 0 0} planes that appear to be propagating in mode III (a in Fig. 3(a)). Close to the indentation site, heights of few tens of nanometers can be thus measured between the surfaces around cracks with no observations (in the limit of the AFM lateral resolution) of opened cracks (mode I) in the surface plane (see cross-section in Fig. 3(a)). h1 1 0i indentations result in some material extruding from the indentation site. These extrusions are bordered by the sides of the indents (b in Fig. 3(b)) and by small radial cracks running close to the h1 1 0i direction. A few long h1 0 0i cracks coexist with these features. From these observations, it appears that h1 1 0i indentations are associated with a more brittle character; instead of longer cracks in h1 0 0i indentation configuration, more complex fracture mechanisms seem to be activated leading to very uneven surface features close to the imprints. Lenticular twins are associated with the deformation induced by the two types of imprints: {1 0 0} and {1 1 0} twins are found with such lenticular shapes. In both cases {1 1 0} twins are less numerous but their mean thickness appears to be larger than that of the {1 0 0} twins. This suggests two different nucleation mechanisms for these two types of twins. Anyway, far enough from the imprints, the density of twins induced during the indentation tests is mainly of type {1 0 0}, in agreement with previous calculations in the frame of elasticity showing that the formation of {1 0 0} twins is energetically favourable [14]. For h1 0 0i indentations, twins are nucleated near the edge of h1 0 0i cracks (c and d areas in Fig.
3(a)) whereas they are found slightly distant from the imprints in the case of h1 1 0i indentations. Note that {1 1 0} twins nucleated at crack edges show more complex shapes and can deviate apparently from the usual {1 1 0} habit plane (c in Fig. 3(a)). A residual stress anisotropy which could result from the occurrence of twin boundaries in the vicinity of the indents seems also to be evidenced, since the cracks are longer along the transversal crystallographic direction than the longitudinal one (Fig. 3(a)). This effect has to be confirmed by further experiments with a specific apparatus which consists of a nanoindenter interfaced with an AFM; the surface areas of interest will be thus first investigated by AFM, before performing nanoindentations (lower loads inducing very localised plastic zones) at a chosen position (1 lm accuracy). It is also evident from AFM images shown in Fig. 4 that the uplift of the surface resulting from the crack mode III propagation is accommodated by a very fine twin structure, sometimes only a few tens of nanometer thick (Fig. 4(b)). Moreover, it is quite interesting to note that a given crack propagation length (see top and bottom parts in Fig. 4(a)) leads to a perfectly similar surface deformation structure. On the other hand, the surface features strongly differ from left to right. Since the Vickers indentation observed in Fig. 4(a) has been carried out at the junction (vertical line in the middle of the image) of two {1 1 0} twins of the asgrown single crystals, this shows that the plastic indentation-induced mechanisms strongly depend on the crystallographic surface orientations. Twins observed in the vicinity of imprints have a very different geometry than those found in asgrown materials and can be related unambiguously to an indentation-induced plastic mechanism. Although twins in as-grown materials have a constant thickness all along their length and terminate at obstacles (surfaces, intersecting twins. . .), mechanical twins terminate freely in the matrix and possess a lenticular shape (g in Fig. 3(b)). Few constant thickness twins can be seen but are clearly associated with a surface relaxation induced by the cracks (j in Fig. 3(b)). The formation of the lenticular twins can be seen as resulting from the emission of twinning
P.D. Tall et al. / Scripta Materialia 49 (2003) 903–908
907
(a)
(b)
Fig. 5. Schematic of twin tips and twinning dislocations in (a) YBCO (b) LaAlO3 single crystals.
Fig. 4. (a) Signal error mode AFM images of h1 0 0i indentation showing crack features as well as twins nucleated at the crack edges. The Vickers indentation has been performed just between two twinned {1 1 0} surfaces. Note the symmetrical (top/ bottom) surface structure induced by a given crack length. (b) Details of the geometry of the {1 0 0} and {1 1 0} mechanical twins, labelled X in (a), revealing a fine nano-twin structure.
dislocations gliding on continuous twin planes whose movement is stopped when the effective stress on the dislocation is zero, leading to twin tips [15]. However, in contrast to what is found in some materials, these twin tips have a concave shape rather than convex (Fig. 5). Convex shapes of twin tips have been found to be in agreement with calculations of equilibrium configurations of twinning dislocations when subjected to an applied
stress [16]. In the present case these concave twin tips may suggest an inverse pile-up configuration. They could result from a relaxation of the dislocation configuration when the local residual stress is relaxed, suggesting a low friction stress on twinning dislocations and/or stresses resulting from dislocation interaction. Numerical calculations are currently in progress to test this hypothesis.
4. Conclusion Hardness experiments have been used to study plastic deformation of LaAlO3 single crystals between room temperature and 700 °C. It has been shown that, besides cracking, plasticity occurs at room temperature through the nucleation of mechanical twins located on both {1 0 0} and {1 1 0} twin planes. AFM observations reveal that these twins possess concave tips which may result from elastic relaxation of the twinning dislocations when the residual stress is relaxed.
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P.D. Tall et al. / Scripta Materialia 49 (2003) 903–908
References [1] Wang ZL, Shapiro AJ. Surf Sci 1995;328:141. [2] Sum R, Lang HP, Guntherodt HJ. Phys C Supercond 1995;242:174. [3] Lebedev OI, Van Tendeloo G, Amelinckx S, Ju HL, Krishnan KM. Phil Mag A 2000;80:673. [4] Adak AK, Pramanik P. Mater Lett 1997;30:269. [5] Peschev P, Slavova V. Mater Res Bull 1994;29:255. [6] Taspinar E, Cuneyt A. J Amer Ceram Soc 1997;80:133. [7] Phillips JM, Siegal MP, Perry CL, Marshal JH. IEEE Trans Magn 1991;27:1006. [8] Bueble S, Knorr K, Brecht E, Schmahl W. Surf Sci 1998;400:345.
[9] Yao GD, Hou SY, Dudley M, Phillips JM. J Mater Res 1992;7:1847. [10] Fay H, Brandle CD. Crystal growth. Oxford: Pergamon press; 1967. p. 51. [11] Berkstresser GW, Valentino AJ, Brandle CD. J Crystal Growth 1991;109:467. [12] Norton MG, Biggers RR. Scripta Metall Mater 1995;32:481. [13] Kim CH, Jang JW, Cho SY, Kim IT, Hong KS. Phys B Condens Matter 1999;262:438. [14] Kim CH, Jang JW, Cho SY, Kim IT, Hong KS. Mater Res Bull 2001;36:1561. [15] VanTendeloo G, Amelinckx S. J Less Common Metals 1990;164:92. [16] Kamat SV, Hirth JP. Phil Mag A 1996;73:669.