《复合材料 Composites》课程教学资源（学习资料）第五章陶瓷基复合材料_In-situ TEM Observations of Tetragonal.pdf_大学文库

P1:S0272.8842(97)00016.3 884298$19.00+00 In-situ TEM Observations of Tetragonal to Monoclinic Phase Transformation in ZrO2-2 mol%Y2O3 Ceramics G.Y. Lin Institute of Materials Science and Engineering, South China University of Technology, Guangzhou 510641 T.C. Lei&Y Zhou School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001 People's Republic of China (Received 16 September 1996. accepted 3 December 1996) In-situ tEM observations of tetragonal to mation in ZrOr 2 mol%Y203 ceramics were performed under the action of eiectron beam heating. The results show that the nucleation and growth of m- Zro2 martensite and its final morphology are dependent upon the microscopic content and distribution of Y2O3 In microareas with lower Y203 content and omogeneous distribution, the m-ZrO2 martensite nucleates and grows by twins, heterogellcous a parallel laths. But in microareas with higher Y203 content and finally forming istribution, single plate of m-ZrO2 martensite Is first formed, and simulates the appearance of other martensite plates, finally forming n or Z shaped skeleton.@ 1998 Elsevicr Science Limited and Techna S r L. All rights rescrved INTRODUCTION ceramics are carefully examined and the effecti factors of transformation method and final pro Phase transformation toughening via the marten- duct morphology are discussed in this paper itic tetragonal(t) to monoclinic(m) phase transfor mation in ZrO is one of the most effective ways of improving the fracture toughness of ceramics.22 EXPERIMENTAL and results in the development of the most hopeful zirconia-toughened ceramics used in engineering. The ZrO2-2 mol%Y2O3 ceramics was prepared by The effects and mechanisms of phase transforma- hot-pressing ZrO2-2 mol%Y203 powders fabri tion toughening are directly dependent on the nat- cated by coprecipitation-filtration with an average ure of t-m ZrO2 transformation and the related particle size of about 0.65 um(having 34% t-zrO2 microstructure. However, careful studies on the and 66% m-ZrO2)at 1600oC under 25 MPa for I h process and mechanisms of t-m phase transforma- The selected temperature for hot-pressing lies in tion have been very scarcely reported because the the single tetragonal phase region of Zroz Y203 martensitic transformation is usually too rapid to diagram. b The tEm foils were prepared as follow- observe clearly. For this reason, the t-ZrO2 parti- ing: sheets with a thickness of about 0. 5 mm were cles are partially stabilized by adding proper stabi- cut from the hot-pressed specimen and then izer and by selecting suitable t-Zro2 grain and ground to a thickness of about 50 um and divided adjusting the operating voltage and current of into discs of 3 mm diameter. The ground discs were TEM so that the t-m transformation speed can be gripped between two pieces of copper nets and controlled and in-situ TEM observations of the glued togcthcr with resign. The well-glued speci whole process of this transformation become mens were thinned by an ion-thinner of Gatan possible. 4, The nucleation and growth of m-ZrO2 modcl-600 at a small angle of 10. The centre of the martensite in hot-pressed ZrO2-2 mol%Y2O3 copper net was denuded and a copper ring was

Ceramics Internaricmal 24 (I 998) 307 -3 I2 :C 1998 Elsewer Science Limited and Techna S.r.1. PIl:SO272-8842(97)00016-3 Printed in Great Btitain. All rights reserved 0272-8842/98 $19.00+ .OO In-situ TEM Observations of Tetragonal to Monoclinic Phase Transformation in ZrOT2 mol% Y203 Ceramics G. Y. Lin Institute of Materials Science and Engineering, South China University of Technology, Guangzhou 5 1064 I, Peopfe’s Republic of China T. C. Lei &Y. Zhou School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001. People’s Republic of China (Received 16 September 1996; accepted 3 December 1996) Abstract: In-situ TEM observations of tetragonal to monoclinic phase transformation in Zr02-2mol%Y203 ceramics were performed under the action of electron beam heating. The results show that the nucleation and growth of mZrOz martensite and its final morphology are dependent upon the microscopic content and distribution of Y,03. In microareas with lower Y203 content and homogeneous distribution, the m-ZrOz martensite nucleates and grows by twins, finally forming parallel laths. But in microareas with higher YzO3 content and heterogeneous distribution, single plate of m-ZrOl martensite is first formed, and simulates the appearance of other martensite plates, finally forming N or Z shaped skeleton. ~_izl 1998 Elsevier Science Limited and Techna S.r.1. All rights reserved 1 INTRODUCTION Phase transformation toughening via the martensitic tetragonal(t) to monoclinic(m) phase transformation in ZrOz is one of the most effective ways of improving the fracture toughness of ceramics132 and results in the development of the most hopeful zirconia-toughened ceramics used in engineering.3 The effects and mechanisms of phase transformation toughening are directly dependent on the nature of t-m ZrO;! transformation and the related microstructure. However, careful studies on the process and mechanisms of t-m phase transformation have been very scarcely reported because the martensitic transformation is usually too rapid to observe clearly. For this reason, the t-ZrOz particles are partially stabilized by adding proper stabilizer and by selecting suitable t-ZrO, grain and adjusting the operating voltage and current of TEM so that the t--m transformation speed can be controlled and in-situ TEM observations of the whole process of this transformation become possible. ‘J The nucleation and growth of m-ZrOz martensite in hot-pressed Z&-2 mol%YzOX ceramics are carefully examined and the effecting factors of transformation method and final product morphology are discussed in this paper. 2 EXPERIMENTAL The Zr02-2mol%Y203 ceramics was prepared by hot-pressing Zr02-2 mol%Y203 powders fabricated by coprecipitation-filtration with an average particle size of about 0.65pm (having 34% t-ZrO, and 66% m-Zr02) at 1600°C under 25 MPa for 1 h. The selected temperature for hot-pressing lies in the single tetragonal phase region of ZrOTY203 diagram.6 The TEM foils were prepared as following: sheets with a thickness of about 0.5mm were cut from the hot-pressed specimen and then ground to a thickness of about 50 pm and divided into discs of 3 mm diameter. The ground discs were gripped between two pieces of copper nets and glued together with resign. The well-glued specimens were thinned by an ion-thinner of Gatan model-600 at a small angle of IO”. The centre of the copper net was denuded and a copper ring was 307

G. y. Lin et al remained. This copper ring then becomes a the dynamic phase transformation in Fig. I(a). It supporting skeleton which protects the foil from was probably caused by the t-m phase transfor- breakage during operation The t-m ZrO2 transfor- mation of near other grains. Figure 1(d)-(n show mation in ZrOx-2 mol%Y2O3 ceramics was induced the dynamic process of the nucleation and growth and observed by using a Philips CM-12 transmission of microcracks. The microcracks were formed in clectron microscopy operated at 120KV the grain boundary induccd by centration caused by the volume expansion and shear as the martensite plate nucleates and grows 3 RESULTS AND DISCUSSION The microcracks always nucleated at the cross top of two plates, which indicated that there was higher The XRD examinations'show that there is 27% stress intensity at the cross top. The microcracks m-zro2 in the ZrO2-2 mol%Y2O3 ceramics after could further grow in length and width as the hot pressing although hot pressing was performed martensite phase transformation went on. The sizes Lt 1600 C, which lies in the single t-phase region of this kind of microcracks are small compared ccording to the zrO2-Y2O3 phase diagram. 6 This with the of ZrO2 particles so that they may indicates that part of the t-phase may transform play a role in toughening ceramics into m-phase after cooling to room temperature Figure 2 shows another pattern of the nucleation (but to an amount much less than 66%). the and growth of m-zro2 martensite in ZrO2- ntercept mcthod reveals that the avcrage particle 2 mol%Y2O3 ceram he martensite laths size of Zro2 in the ZrO2-2 mol%Y2O3 ceramics is formed first also at grain boundaries, but it was 2.35 um after hot-pressing different froIm Fi that two martensite laths A series of TEM photographs of the whole pro-(Fig. 2(a)), not only one, nucleated simultaneously nucleation and growth of a typical and the two martensite laths continually grew in type martensite were shown in Fig. 1. It can be both length and width by means of shear along a found that a plate of m-zrO2 martensite nucleated certain plane(Fig. 2(b)). At the same time, new bound martensite laths continually nucleated and nucleation under electron beam bombardment. and always two and two nucleated simultaneously When the martensite plate grew completely across grew together(Fig. 2(c)and(d). another differ a grain, it was stopped at a grain boundary and the ence from Fig. 1 is that the new martensite laths resulting stress concentration caused by volume nucleated independently by means of twins expansion and shear, induced a differently oriented (Fig. 2(a)and( c)), but didn t form at the top of old plate to nucleate and begin to grow. Sometimes the laths by self-triggering. Of course stress conce length of a plate stopped growing before it grew tion caused by the volume expansion and completely across a grain and a ncw oricnted plate could enhance the nucleation and growth. The two was induced to nucleate and grow(Fig. 1(b) and laths nucleated simultaneously should grow to (e)), which location in the grain constitute twins on their common plane as twin hinders the martensite transformation. However, plane. The two neighbouring laths, but not nucle the plate could grow across the subgrain boundary ated simultaneously, would also grow to constitute (Fig. 1(c). The old plates also grew in width by twins along a certain habit plane by means of shear means of shear at the same moment of forming a (as shown in Fig. 2). Therefore, the final morphol- new one, but the speed was very low. Every plate ogy of m-Zro2 was parallel lath-shaped martensite almost stopped growing in width after 7 min and with twin relationship in this grain( Fig. 2(). This the t-m Zro 2 phase transformation in this grain kind of transformation nucleating and growing by wasn't complete under electron beam bombard- means of twins could decrease the total change of ment even after removal of the condenser aperture shape and release elastic strain because of the self and a further condensation of the electron beam co-ordination effe that its transformation onto the grain for 20 min. The reversal transfor- speed was faster under the action of electron beam mation didnt happen, either, which was observed heating and didn,'t easily cause microcracks to be in ZrO2 2.5 mol%Y2O3 ceramics by Mecartney nucleated at the grain boundary. The microcrack and Ruhle. 4 and Heuer et al. Finally, the grain in Fig. 3(b)may be due to the larger volume was composed of t+ m dual phases, the plates of expansion caused by t-m phase transformation of m-ZrO2 martensite formed N or Z shaped skeleton. some neighbouring particles and the end product Figure I also shows the dynamic process of the of some particles also forming N or Z shaped nucleation and growth of microcracks in ZrO2 skeleton 2 mol%Y203 ceramics. There was already a long It is found that there are two kinds of typical microcrack in the triangular grain boundary before types of m-ZrO2 martensite from t-m phase

308 G. Y. Lin et al. remained. This copper ring then becomes a supporting skeleton which protects the foil from breakage during operation. The t-m ZrO;! transformation in ZrOT2 mol%Y203 ceramics was induced and observed by using a Philips CM- 12 transmission electron microscopy operated at 120 KV. 3 RESULTS AND DISCUSSION The XRD examinations’ show that there is 27% m-Zr02 in the ZrOT2mol%YzOj ceramics after hot pressing although hot pressing was performed at 16OO”C, which lies in the single t-phase region according to the Zr02-Y203 phase diagram.6 This indicates that part of the t-phase may transform into m-phase after cooling to room temperature (but to an amount much less than 66%). The intercept method reveals that the average particle size of ZrOs in the ZrO*-2mol%Y203 ceramics is 2.35 pm after hot-pressing. A series of TEM photographs of the whole process of nucleation and growth of a typical platetype martensite were shown in Fig. 1. It can be found that a plate of m-ZrOz martensite nucleated at grain boundaries by means of heterogeneous nucleation under electron beam bombardment. When the martensite plate grew completely across a grain, it was stopped at a grain boundary and the resulting stress concentration caused by volume expansion and shear, induced a differently oriented plate to nucleate and begin to grow. Sometimes the length of a plate stopped growing before it grew completely across a grain and a new oriented plate was induced to nucleate and grow (Fig. l(b) and (e)), which meant that some location in the grain hinders the martensite transformation. However, the plate could grow across the subgrain boundary (Fig. l(c)). The old plates also grew in width by means of shear at the same moment of forming a new one, but the speed was very low. Every plate almost stopped growing in width after 7min and the t-m ZrOa phase transformation in this grain wasn’t complete under electron beam bombardment even after removal of the condenser aperture and a further condensation of the electron beam onto the grain for 20min. The reversal transformation didn’t happen, either, which was observed in ZrOT2.5 mol%YzOS ceramics by Mecartney and Ri.ihle.4 and Heuer et aL8 Finally, the grain was composed of t + m dual phases, the plates of m-ZrOz martensite formed N or Z shaped skeleton. Figure 1 also shows the dynamic process of the nucleation and growth of microcracks in Zr02- 2mol%Y203 ceramics. There was already a long microcrack in the triangular grain boundary before the dynamic phase transformation in Fig. l(a). It was probably caused by the t-m phase transformation of near other grains. Figure l(d)-(f) show the dynamic process of the nucleation and growth of microcracks. The microcracks were formed in the grain boundary induced by the stress concentration caused by the volume expansion and shear as the martensite plate nucleates and grows. The microcracks always nucleated at the cross top of two plates, which indicated that there was higher stress intensity at the cross top. The microcracks could further grow in length and width as the martensite phase transformation went on. The sizes of this kind of microcracks are small compared with the sizes of Zr02 particles so that they may play a role in toughening ceramics.9 Figure 2 shows another pattern of the nucleation and growth of m-ZrOz martensite in ZrOr 2 mol%Y~Oj ceramics. The martensite laths formed first also at grain boundaries, but it was different from Fig. 1 that two martensite laths (Fig. 2(a)), not only one, nucleated simultaneously and the two martensite laths continually grew in both length and width by means of shear along a certain plane (Fig. 2(b)). At the same time, new martensite laths continually nucleated and grew, and always two and two nucleated simultaneously, grew together (Fig. 2(c) and (d)). Another difference from Fig. 1 is that the new martensite laths nucleated independently by means of twins (Fig. 2(a) and (c)), but didn’t form at the top of old laths by self-triggering. Of course stress concentration caused by the volume expansion and shear could enhance the nucleation and growth. The two laths nucleated simultaneously should grow to constitute twins on their common plane as twin plane. The two neighbouring laths, but not nucleated simultaneously, would also grow to constitute twins along a certain habit plane by means of shear (as shown in Fig. 2). Therefore, the final morphology of m-ZrOz was parallel lath-shaped martensite with twin relationship in this grain (Fig. 2(f)). This kind of transformation nucleating and growing by means of twins could decrease the total change of shape and release elastic strain because of the selfco-ordination effect so that its transformation speed was faster under the action of electron beam heating and didn’t easily cause microcracks to be nucleated at the grain boundary. The microcrack in Fig. 3(b) may be due to the larger volume expansion caused by t-m phase transformation of some neighbouring particles and the end product of some particles also forming N or Z shaped skeleton. It is found that there are two kinds of typical types of m-ZrOz martensite from t-m phase

In-situ TEM observations 31 However, m-ZrO, can nucleate and grow by means of microarea of the t-ZrO, grains with lower Y20 t and ho forms parallel lath type of martensite. Yoshizawa and Sakuma have also discovered that the y,o3 contents in microareas are very heterogeneous in ZrOr4 mol%Y03 ceramics. they may be from 5.6 mol%. 0 Finally, it should pointed out that the effect of Y,O3 content on the m-ZrO, morphology in ZrOx-Y,O3 ceramics is very that of carbe morphology in steels. The martensite in low carbon steels always forms parallel lath type of martensite structure and the martensite in middle and high carbon steels always forms n or Z shaped skeleton plate type of martensite structure after quenching. I 4 CONCLUSIONS The t-m ZrO, pha clectron beam bom bardment of TEM. the m- ZrO, always nucleates at the [-ZrO2 grain 2. The patterns of nucleation and growth of m 9 Zro orpl %Y,O3 dependent upon the microscopic YO3 content and its distribution. In the t-ZrO,gr ith low Y,O, content and homogeneous distribution, the m- ZrO, martensite nucleates and grows by twins in a higher spccd and finally forms par- llel lath type of martensitc. But in the t-Z1 th higher y,o; content and he geneous distribution, single plate of m-ZrO ormed at the beneficial microareas and Fig. 3. TEM photographs showing the morphology ol m stimulates the appearance of other martensite ZrO, martensite in ZrO, 2 moloYO, ceramics and Y,O3 contents(in mol"a)in the microareas: (a)n or Z shaped ske plates. the plates grow in width hy shear at a leton plate type of m-ZrO. martensite: (b) parallel lath tvpe shaped skeleton throughout the gr 3. The effect of y,o content on the m-ZrO the t-ZrO, grains with higher Y,O3 content and het morphology in ZrOr-Y2O3 ceramics Is very erogeneous distribution then the nucleation and similar to that of carbon content on the mar- growth of m-ZrO, were stopped at the microareas tensite morphology in steels with higher Y,O3 content and some martensite 4. The microcracks are formed in the grai plates couldnt grow across the whole grain boundary induced by the 1-m phase transfor Fig. I(b)and (c) or grow in width by shear lo mation, which may play a role in toughening transform the whole grain into m-ZrO,, so that the ceramics grain Is composed of t+m dual phases and m ZrO finally forms N or Z shaped skeleton. This is because the higher the y,O3 stabilizer in Zro, the REFERENCES larger the difference between tetragonal phasc structure and monoclinic phase structure and more difficulty in inducing ohase transformation 3405)(1986)761-800

Fig. 3. TEM photographs 5howmg the morphology of n- &-02 martensitc in Zr02 3 mol%Y20~ ceramics and YzO1 contents (in mol”‘;) III the mwoareas: (a) N or Z shaped skeleton plate type ~lt‘ m-7x0? martensite: (h) parallel lath typt: of m-Z-O2 murtensite. the t-Zr02 grains with higher Y20i content and heterogcneous distribution then the nucleation and growth of m-%1-O? were stopped at the microareas with higher Y,03 content and some martcnsite plates couldn’t grow across the whole zrain (Fig. l(b) and (c)) or grow in width by shear to transform the whole grain into m-Zr02, so that the grain is composed of t + m dual phases and mZr02 finally forms N or Z shaped skeleton. This is because the higher the Y203 stabilizer in ZrO?. the larger the difference between tetragonal phase structure and monoclinic phase structure and more difficulty in inducing t m phase transformation. However. m-ZrO? can nucleate and grow by means of twins with lower energy rampart in every microarea of the t-ZrO, grains with lower Y203 content and homogeneous distribution and finally forms parallel lath type of martensite. Yoshizawa and Sakuma have also discovered that the Y203 contents in microareas are very h~etcrogeneous in Zr02Jmol%Y203 ceramics, the;y may be from 1.9 mol% to 5.6mol%.‘” Finally, it should be pointed out that the effect of YIOq content on the m-Zr02 morphology in Zr02--Y?O-, ceramics is very similar to that of carbon content on the martensite morphology in steels. The martensite in low carbon steels always forms parallel lath type of martensite structure and the martensite in middle and high carbon steels always forms N or Z shaped skeleton plate type of martensite structure after quenching.” 4 CONCLUSIONS The t-m ZrO? phase transformation in ZrO?- 2 mol%Y203 ceramics can be induced by the electron beam bombardment of TEM. The mZr02 always nucleates at the i-ZrO, grain - c boundaries. The patterns of nucleation and growth of mZrOl martensite and its final morphology in ZrO:- 2 mol%Y203 ceramics are dependent upon the microscopic Y20i content and its distribution. In the t-ZrO, grains with lower Y203 content and homogeneous distribution the m-ZrO? martcnsite nucleates and grows by twins in a higher speed and finally forms parallel lath type of martensite. But in the t-ZrO, grains with higher Y701 content and heterogeneous distribution, single plate of m-ZrOz is first formed at the beneficial microareas and stimulates the appearance of other martensite plates. the plates grow in width by shear at a very low speed, and finally form N or Z shaped skeleton throughout the grains. The effect of Y203 content on the m-Zr02 morphology in Zr02PY203 ceramics is very similar to that of carbon content on the martensite morphology in steels. The microcracks are formed in the grain boundary induced by the t-m phase transformation, which may play a role in toughening ceramics. REFERENCES I. EVANS. A. G. & CANNOK, R. M., Toughening of brittle solids by martensite transformai 1(x. -1~ tu nlr/u//.. .34(S) (19X6) 761 +ao

《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_In-situ TEM Observations of Tetragonal

《复合材料 Composites》课程教学资源（学习资料）第五章陶瓷基复合材料_In-situ TEM Observations of Tetragonal