Availableonlineatwww.sciencedirect.com SCIENCE Acta materialia ELSEVIER Acta Materialia 52(2004)5709-5721 Martensitic transformation in zirconia Part Il martensite wth Sylvain deville gerard Guenin, Jerome Chevalier Associate Research Unit 5510, Materials Science Department, National Institute of Applied Science(GEMPPM-INSA), Bat B Received 13 July 2004: received in revised form 23 August 2004: accepted 26 August 2004 Available online 25 September 2004 Abstract Though the martensitic transformation in zirconia has been the object of a very large number of studies for the last decades, qualitative and quantitative observations of the formation and growth of relief induced by low temperature treatments has hardly ever been reported. In the first part of the study(Martensitic transformation in zirconia, Part D), we have demonstrated the excellent agreement between the atomic force microscopy quantitative observations and the outputs of the calculations derived from the phe- nomenological theory of martensitic transformation. The intermediate stages of transformation were nonetheless not considered In this second part, the growth mechanisms of monoclinic phase resulting from the martensitic transformation in ceria-stabilized conia(10 mol% CeO2)are investigated. Surface transformation is induced by aging treatments in water vapor at 413 K. The obser- vations are rationalized by the recent analysis proposed for the crystallographic ABCl correspondence choice, where the c, axis transforms to the cm axis. Three growth modes are observed and interpreted in terms of transformation strains accommodation is the largest. The influence of grain boundary paths on the surface relief features is demonstrated. Overall, our results strong, stains Microcracks formation is observed, explaining grain pop-out where the crystallographic disorientation between two adjacent grains port the non-existence of a critical grain size for low temperature transformation, confirmed by the classical thermodynamics theory applied to this particular case. o 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Zirconia; Martensite transformation; Atomic force microscopy; Microcracks 1. Introduction phenomenon is know as aging, and has been widely investigated for its detrimental consequences on the long Martensitic transformations occur in a number of term performance of zirconia components. Several con metal alloys but also minerals and ceramics like zirconia clusions were drawn from the experimental observa- 2 ] In order to stabilize zirconia and retain it in its tions, performed mostly by tEm, sEM and XRD. metastable tetragonal structure at ambient temperature, among which the existence of a critical grain size was it is possible alloying it with various oxides, among observed, size varying from 0. 1 to 0.5 um in the case which yttria(Y2O3) and ceria(CeO2). If the metastable of 3Y-TZP [5, 6]. This existence of a critical grain size tetragonal structure is indeed retained at ambient tem- was demonstrated by the application of the classical perature, the transformation can however occur during thermodynamics theory [7, 8]. Moreover, it was argued low temperature treatment in water vapor [3, 4]. This the transformation was very different in the case ofY TZP and Ce-TZP, because of the respective trivalent Corresponding author. Tel: +334 7243 63 57; fax: +33 4 72 4385 and quadrivalent nature of the stabilizE ing specie, leading to a different oxygen vacancies concentration []. As a E-mail address: sylvain. deville(ainsa-lyon fr(S. Deville) consequence, the aging sensitivity of these two materials 1359-6454/$30.00 C 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi: 10. 1016/j.actamat. 2004.08.03
Martensitic transformation in zirconia Part II. Martensite growth Sylvain Deville *, Ge´rard Gue´nin, Je´roˆme Chevalier Associate Research Unit 5510, Materials Science Department, National Institute of Applied Science (GEMPPM-INSA), Baˆt B. Pascal, 20 av. A. Einstein, 69621 Villeurbanne Cedex, France Received 13 July 2004; received in revised form 23 August 2004; accepted 26 August 2004 Available online 25 September 2004 Abstract Though the martensitic transformation in zirconia has been the object of a very large number of studies for the last decades, qualitative and quantitative observations of the formation and growth of relief induced by low temperature treatments has hardly ever been reported. In the first part of the study (Martensitic transformation in zirconia, Part I), we have demonstrated the excellent agreement between the atomic force microscopy quantitative observations and the outputs of the calculations derived from the phenomenological theory of martensitic transformation. The intermediate stages of transformation were nonetheless not considered. In this second part, the growth mechanisms of monoclinic phase resulting from the martensitic transformation in ceria-stabilized zirconia (10 mol% CeO2) are investigated. Surface transformation is induced by aging treatments in water vapor at 413 K. The observations are rationalized by the recent analysis proposed for the crystallographic ABC1 correspondence choice, where the ct axis transforms to the cm axis. Three growth modes are observed and interpreted in terms of transformation strains accommodation. Microcracks formation is observed, explaining grain pop-out where the crystallographic disorientation between two adjacent grains is the largest. The influence of grain boundary paths on the surface relief features is demonstrated. Overall, our results strongly support the non-existence of a critical grain size for low temperature transformation, confirmed by the classical thermodynamics theory applied to this particular case. 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Zirconia; Martensite transformation; Atomic force microscopy; Microcracks 1. Introduction Martensitic transformations occur in a number of metal alloys but also minerals and ceramics like zirconia [1,2]. In order to stabilize zirconia and retain it in its metastable tetragonal structure at ambient temperature, it is possible alloying it with various oxides, among which yttria (Y2O3) and ceria (CeO2). If the metastable tetragonal structure is indeed retained at ambient temperature, the transformation can however occur during low temperature treatment in water vapor [3,4]. This phenomenon is know as aging, and has been widely investigated for its detrimental consequences on the long term performance of zirconia components. Several conclusions were drawn from the experimental observations, performed mostly by TEM, SEM and XRD, among which the existence of a critical grain size was observed, size varying from 0.1 to 0.5 lm in the case of 3Y–TZP [5,6]. This existence of a critical grain size was demonstrated by the application of the classical thermodynamics theory [7,8]. Moreover, it was argued the transformation was very different in the case of Y– TZP and Ce–TZP, because of the respective trivalent and quadrivalent nature of the stabilizing specie, leading to a different oxygen vacancies concentration [4]. As a consequence, the aging sensitivity of these two materials 1359-6454/$30.00 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2004.08.036 * Corresponding author. Tel.: +33 4 72 43 63 57; fax: +33 4 72 43 85 28. E-mail address: sylvain.deville@insa-lyon.fr (S. Deville). Acta Materialia 52 (2004) 5709–5721 www.actamat-journals.com
S. Deville et al. Acta Materialia 52(2004)5709-5721 was expected to be very different. Finally, microcracking ence ABCI was demonstrated. For this correspondence, was observed in the surroundings of transformed grains, it was shown the characteristics of the habit planes and and related to the volume and shear components of the transformation matrix allow a complete accommoda transformation. However, no direct observation and tion of transformation strains by a surface uplift outside clear interpretation of the microcracks formation has of the free surface. Minimal residual stresses should be been reported. The observation of martensite formed expected in that case. a particular interest of this anal during aging treatment is very appealing in regards of ysis relies in the interpretation of martensitic variants the lack of validation of the various theories, for several arrangement in the volume of the material from the reasons. The transformation is occurring at surface so experimental observations of relief features at the that it may be observed straightaway, and the influence surface of a free surface on the transformation can potentially On the other hand, if the martensitic phenomeno- be assessed. Moreover, the transformation is propagat- logical theory was successfully applied, it is worth ing step by step when performing several aging treat- remembering the theory does only describe the trans- ment in autoclave, so that intermediate stages of the formation in mathematical terms, and by no means ransformation should be observed. Then, the less stable in physical terms. No informations on the chemical parts(from an energetical point of view) of the surface mechanism of transformation are brought by the the- are the first one to transform: the transformation ory. If the spatial arrangement was well understood not triggered by the experimental observations, like in terms of transformation strain accommodation grains transforming under the beam during TEM obser the remaining questions were thus: how are these vations. Finally, the sample geometry has no effect on arrangements formed? How do they grow and how he transformation behavior, as opposed to thin foils fast? where the very low thickness induces some peculiar fea In this study, this approach is applied to investigate tures. It should therefore be possible determining some and analyze the nucleation and growth of martensite of the factors affecting the stability in ceria stabilized zirconia and subsequent consequences Considering the scale at which the transformation like microcracking. It is shown how the combination of occurring, very few qualitative and quantitative reports AFM observations and of the outputs of the phenome- of transformation induced relief can be found in the lit nological analysis can provide new insights on both the erature. Considering the dilatational(0.04)and shear physical and the chemical mechanisms of the transfor- (0.16)components of the transformation, this dearth mation, with particular attention being paid on the of experimental observations is not surprising; modifica- influence of the free surface on the variants growth tions of the surface relief occur below the micrometer modes. AFM observations were performed at different range, typically from 10 to 100 nm for a zirconia poly- steps of the aging treatment in order to follow locally crystal of typical grain size(0.5-3 um). In absence of the transformation at the surface of the sample with experimental validation of the relief features, the predic tions of transformation induced relief rely on the valid ity and relevance f the phenomenological theory [9, 10] of martensite transfor- 2. Experimental methods mation. Validation of these predictions requires precise quantitative measurements of martensite relief. The ef- Ceria stabilized zirconia (Ce-TZP) materials were fect of free surface on the transformation local charas processed by classical powder processing route, using teristics relies mostly so far on the outputs of Zirconia Sales Ltd powders, with uniaxial pressing and calculations [11-13]. Preliminary reports [14-16] have sintering at 1823 K for 2 h Residual porosity was neg- nonetheless drawn the attention of the potentialities of- ligible Samples were mirror polished with standard dia fered by atomic force microscopy(AFM), technique mond based products. offering a vertical resolution below the nanometer range The martensitic transformation was induced by a AFM allows straightforward observations of trans- thermal treatment in water vapor autoclave. This kind formed surfaces of bulk samples. The observation of of treatment is known to induce the tetragonal to mond partial transformation of the grains [16] already ques- clinic phase transformation in zirconia [3, 18, 19].Hence, tioned the existence of a critical size for the transforma these treatments were conducted in autoclave at 413K tion. The technique has just been applied [17] to in saturated water vapor atmosphere, with a 2 bar pres- investigate the martensitic relief at the end of the trans- sure, inducing phase transformation at the surface of the formation, and compared with the outputs of the crys- samples with time. Thermal treatment steps were tallographic theory of martensitic transformation. An bounded to the thermal activation of the transformation excellent quantitative agreement was found between and the technical limits of the autoclave. These steps he experimental observations and theoretical predic- could have been reduced by several decades if an higher tions. In particular, the peculiar behavior of correspond- treatment temperature had been chosen
was expected to be very different. Finally, microcracking was observed in the surroundings of transformed grains, and related to the volume and shear components of the transformation. However, no direct observation and clear interpretation of the microcracks formation has been reported. The observation of martensite formed during aging treatment is very appealing in regards of the lack of validation of the various theories, for several reasons. The transformation is occurring at surface so that it may be observed straightaway, and the influence of a free surface on the transformation can potentially be assessed. Moreover, the transformation is propagating step by step when performing several aging treatment in autoclave, so that intermediate stages of the transformation should be observed. Then, the less stable parts (from an energetical point of view) of the surface are the first one to transform; the transformation is not triggered by the experimental observations, like grains transforming under the beam during TEM observations. Finally, the sample geometry has no effect on the transformation behavior, as opposed to thin foils where the very low thickness induces some peculiar features. It should therefore be possible determining some of the factors affecting the stability. Considering the scale at which the transformation is occurring, very few qualitative and quantitative reports of transformation induced relief can be found in the literature. Considering the dilatational (0.04) and shear (0.16) components of the transformation, this dearth of experimental observations is not surprising; modifications of the surface relief occur below the micrometer range, typically from 10 to 100 nm for a zirconia polycrystal of typical grain size (0.5–3 lm). In absence of experimental validation of the relief features, the predictions of transformation induced relief rely on the validity and relevance of the outputs of the phenomenological theory [9,10] of martensite transformation. Validation of these predictions requires precise quantitative measurements of martensite relief. The effect of free surface on the transformation local characteristics relies mostly so far on the outputs of calculations [11–13]. Preliminary reports [14–16] have nonetheless drawn the attention of the potentialities offered by atomic force microscopy (AFM), technique offering a vertical resolution below the nanometer range. AFM allows straightforward observations of transformed surfaces of bulk samples. The observation of partial transformation of the grains [16] already questioned the existence of a critical size for the transformation. The technique has just been applied [17] to investigate the martensitic relief at the end of the transformation, and compared with the outputs of the crystallographic theory of martensitic transformation. An excellent quantitative agreement was found between the experimental observations and theoretical predictions. In particular, the peculiar behavior of correspondence ABC1 was demonstrated. For this correspondence, it was shown the characteristics of the habit planes and transformation matrix allow a complete accommodation of transformation strains by a surface uplift outside of the free surface. Minimal residual stresses should be expected in that case. A particular interest of this analysis relies in the interpretation of martensitic variants arrangement in the volume of the material from the experimental observations of relief features at the surface. On the other hand, if the martensitic phenomenological theory was successfully applied, it is worth remembering the theory does only describe the transformation in mathematical terms, and by no means in physical terms. No informations on the chemical mechanism of transformation are brought by the theory. If the spatial arrangement was well understood, in terms of transformation strain accommodation, the remaining questions were thus: how are these arrangements formed? How do they grow and how fast? In this study, this approach is applied to investigate and analyze the nucleation and growth of martensite in ceria stabilized zirconia and subsequent consequences like microcracking. It is shown how the combination of AFM observations and of the outputs of the phenomenological analysis can provide new insights on both the physical and the chemical mechanisms of the transformation, with particular attention being paid on the influence of the free surface on the variants growth modes. AFM observations were performed at different steps of the aging treatment in order to follow locally the transformation at the surface of the sample with time. 2. Experimental methods Ceria stabilized zirconia (Ce–TZP) materials were processed by classical powder processing route, using Zirconia Sales Ltd powders, with uniaxial pressing and sintering at 1823 K for 2 h. Residual porosity was negligible. Samples were mirror polished with standard diamond based products. The martensitic transformation was induced by a thermal treatment in water vapor autoclave. This kind of treatment is known to induce the tetragonal to monoclinic phase transformation in zirconia [3,18,19]. Hence, these treatments were conducted in autoclave at 413 K, in saturated water vapor atmosphere, with a 2 bar pressure, inducing phase transformation at the surface of the samples with time. Thermal treatment steps were bounded to the thermal activation of the transformation and the technical limits of the autoclave. These steps could have been reduced by several decades if an higher treatment temperature had been chosen. 5710 S. Deville et al. / Acta Materialia 52 (2004) 5709–5721
S. Deville et al Acta Materialia 52(2004)5709-5721 AFM experiments were carried out with a D3100 how obtaining a stack of four variants of corre nanoscope from Digital Instruments Inc, using oxide spondence ABCI was possible and energetically sta sharpened silicon nitride probes(Nanosensor, CONT-R ble, since all interfaces (inside and outside of the model)in contact mode, with an average scanning speed arrangement) between tetragonal and monoclinic of 10 um s. Since the I-m phase transformation is phases were habit planes of the same type. All the accompanied by a large strain, surface relief is modified transformation strain is accommodated by this con by the formation of monoclinic phase. The vertical res- figuration. Fig. 1(a) provides such an observation, olution of AFM allows following very precisely the with its evolution as a function of the aging treat- transformation induced relief. ment time. Treatments steps of 20 h at 413 K in autoclave were performed between each image. The progressive transformation of the inner tetragonal 3. Experimental results part of the arrangement is clearly observed. Thi behavior is interpreted in Fig. 1(b), with the progres 3. Variants growth modes sive growth of variants, toward the inside of the grain. The potential future habit planes are symbol Three different modes of variants growth are experi ized by the dashed line, the transformation strain mentally observed for the correspondence ABCl, lead- being constant along these planes. Hence, the vari- ing to a fourfold symmetry final arrangement. These ants can grow until the transformation is completed three modes will be, respectively, referred to as internal without being restrained by any transformation in growth, external growth and needle growth duced stresses. The way the primary variants can grow to form the initial fourfold symmetry arrange- 3.1.1. Internal growth ment will be discussed later, in regards of the other This first mode of transformation is related to the variants growth mode. This mode will be called"in partial transformation, reported in [17]. It was shown ternal growth, in regards of its peculiar features Fig. 1.(a) Observation of partial transform and internal growth. Aging steps of 20 h. Horizo tal scale: I um/div, vertical scale: 250 nm/div (b) nterpretation of the arrangement observed in(a). The vertical scale is not respected for clarity. The dashed lines represent the location of the inner habit planes before transformation
AFM experiments were carried out with a D3100 nanoscope from Digital Instruments Inc., using oxide sharpened silicon nitride probes (Nanosensor, CONT-R model) in contact mode, with an average scanning speed of 10 lm s1 . Since the t–m phase transformation is accompanied by a large strain, surface relief is modified by the formation of monoclinic phase. The vertical resolution of AFM allows following very precisely the transformation induced relief. 3. Experimental results 3.1. Variants growth modes Three different modes of variants growth are experimentally observed for the correspondence ABC1, leading to a fourfold symmetry final arrangement. These three modes will be, respectively, referred to as internal growth, external growth and needle growth. 3.1.1. Internal growth This first mode of transformation is related to the partial transformation, reported in [17]. It was shown how obtaining a stack of four variants of correspondence ABC1 was possible and energetically stable, since all interfaces (inside and outside of the arrangement) between tetragonal and monoclinic phases were habit planes of the same type. All the transformation strain is accommodated by this con- figuration. Fig. 1(a) provides such an observation, with its evolution as a function of the aging treatment time. Treatments steps of 20 h at 413 K in autoclave were performed between each image. The progressive transformation of the inner tetragonal part of the arrangement is clearly observed. This behavior is interpreted in Fig. 1(b), with the progressive growth of variants, toward the inside of the grain. The potential future habit planes are symbolized by the dashed line, the transformation strain being constant along these planes. Hence, the variants can grow until the transformation is completed, without being restrained by any transformation induced stresses. The way the primary variants can grow to form the initial fourfold symmetry arrangement will be discussed later, in regards of the other variants growth mode. This mode will be called ‘‘internal growth’’, in regards of its peculiar features. Fig. 1. (a) Observation of partial transformation and internal growth. Aging steps of 20 h. Horizontal scale: 1 lm/div, vertical scale: 250 nm/div. (b) Interpretation of the arrangement observed in (a). The vertical scale is not respected for clarity. The dashed lines represent the location of the inner habit planes before transformation. S. Deville et al. / Acta Materialia 52 (2004) 5709–5721 5711
S. Deville et al. Acta Materialia 52(2004)5709-5721 3.1.2. External growth surface, with the formation of a fully accommodated set This mode is the opposite of the first one. In this case of opposed variants, of fairly small size. The following (Fig. 2(a)), the transformation starts undoubtedly at the bservations show the progressive growth of the vari- y Fig. 2.(a)Observation of external growth. Aging steps of 20 h. Horizontal scale: I um/div, vertical scale: 250 nm/div(b) Interpretation of the rangement observed in(a). The position of the junction plane remains constant along the transformation. The vertical scale is not respected for clarity. The dashed lines represent the location of the habit planes at the previous transformation stage
3.1.2. External growth This mode is the opposite of the first one. In this case (Fig. 2(a)), the transformation starts undoubtedly at the surface, with the formation of a fully accommodated set of opposed variants, of fairly small size. The following observations show the progressive growth of the variFig. 2. (a) Observation of external growth. Aging steps of 20 h. Horizontal scale: 1 lm/div, vertical scale: 250 nm/div. (b) Interpretation of the arrangement observed in (a). The position of the junction plane remains constant along the transformation. The vertical scale is not respected for clarity. The dashed lines represent the location of the habit planes at the previous transformation stage. 5712 S. Deville et al. / Acta Materialia 52 (2004) 5709–5721
S. Deville et al Acta Materialia 52(2004)5709-5721 ants, the trace of the junction plane being in a constant transformation completion of the grain cannot proceed location. This behavior is well understood considering anymore, until an external event can provide supple the variants progressively grow into the volume of the mentary stresses to overcome the energy barrier opposed mple, while the junction plane remains in the same to transformation completion. The remaining untrans position(Fig. 2(b). All the transformation strain is formed part is transformed very rapidly. The grain goes again accommodated; the variants are at equilibrium from very partial transformation to transformation during all the growth steps. More complex arrange- completion during the last treatment step ments, like the so-called"L-arrangement[17 can be Though no statistical analysis was performed on the obtained by this mode, as shown in the last micrographs different growth modes, it seems from our experimental f Fig. 2(a). Since the variants grow in a direction op- observations the internal growth mode is the first mod posed to the junction plane, this mode will be called"ex- activated, after an apparent initial incubation period of ternal growth about 60 h, during which no surface transformation at all seemed to be observed. The external growth mode 3.1.3. Needle growth follows, and finally the isolated needle growth appears The last growth mode observed in these experiments Hence, the controlling factors of each growth mode corresponds to the transformation sequence observed in must be different Fig. 3. Again, transformation is very likely nucleating at the surface. An isolated martensite needle is formed and 3. 2. Variants growth kinetics grows in size. As soon as it is formed, it begins to gener ate opposite stresses(back stresses) in the surrounding sing experimental ob ons like in Figs. I(a). matrix, opposing continued transformation of the initial 2(a) and 3, the size of eac nt can be measured as variant. Since no complementary variants that could a function of their age, so the kinetic of variants accommodate the transformation strain are observe growth are obtained for every growth mode. These in the first steps, the back stresses are building up as kinetics are given, respectively, in Figs. 4-6. The width the variant thickens, until the formation of a second is normalized to the width of the fully transformed var variant is triggered by the shear components of the back ants for comparison between the three modes. Growth stresses. This second variant will again grow in size and speeds in each case were measured before normalization the formation of a third variant with this last Differences are observed between the three modes configuration, the added stresses are so high that the though similarities are also found. The simplest case is Fig 3. Observation of consecutive isolated needles growth. Aging steps of 20 h
ants, the trace of the junction plane being in a constant location. This behavior is well understood considering the variants progressively grow into the volume of the sample, while the junction plane remains in the same position (Fig. 2(b)). All the transformation strain is again accommodated; the variants are at equilibrium during all the growth steps. More complex arrangements, like the so-called ‘‘L-arrangement’’ [17] can be obtained by this mode, as shown in the last micrographs of Fig. 2(a). Since the variants grow in a direction opposed to the junction plane, this mode will be called ‘‘external growth’’. 3.1.3. Needle growth The last growth mode observed in these experiments corresponds to the transformation sequence observed in Fig. 3. Again, transformation is very likely nucleating at the surface. An isolated martensite needle is formed and grows in size. As soon as it is formed, it begins to generate opposite stresses (back stresses) in the surrounding matrix, opposing continued transformation of the initial variant. Since no complementary variants that could accommodate the transformation strain are observed in the first steps, the back stresses are building up as the variant thickens, until the formation of a second variant is triggered by the shear components of the back stresses. This second variant will again grow in size and trigger the formation of a third variant. With this last configuration, the added stresses are so high that the transformation completion of the grain cannot proceed anymore, until an external event can provide supplementary stresses to overcome the energy barrier opposed to transformation completion. The remaining untransformed part is transformed very rapidly. The grain goes from very partial transformation to transformation completion during the last treatment step. Though no statistical analysis was performed on the different growth modes, it seems from our experimental observations the internal growth mode is the first mode activated, after an apparent initial incubation period of about 60 h, during which no surface transformation at all seemed to be observed. The external growth mode follows, and finally the isolated needle growth appears. Hence, the controlling factors of each growth mode must be different. 3.2. Variants growth kinetics Using experimental observations like in Figs. 1(a), 2(a) and 3, the size of each variant can be measured as a function of their age, so that the kinetic of variants growth are obtained for every growth mode. These kinetics are given, respectively, in Figs. 4–6. The width is normalized to the width of the fully transformed variants for comparison between the three modes. Growth speeds in each case were measured before normalization. Differences are observed between the three modes, though similarities are also found. The simplest case is Fig. 3. Observation of consecutive isolated needles growth. Aging steps of 20 h. S. Deville et al. / Acta Materialia 52 (2004) 5709–5721 5713