An ceru So.8713465-72(20 urna Tetragonal-to-Monoclinic Transformation in Mg-PSZ Studied by in Situ Neutron Diffraction Yuxiang ma and Erich H. Kisi Department of Mechanical Engineering. The University of Newcastle. Callaghan, NSW 2308, Australia Shane J. Kennedy and Andrew J Studer Neutron Scattering Group, Australian Nuclear Science and Technology Organization, Private Mail Bag I Menai NSW 2234, Australia The deformation of 9.4 mol% magnesia-partially-stabilized reviews. 0-1 The dominant mechanism responsible for the high zirconia under compressive loads up to 1225 MPa was studied toughness of Mg-PSZ is widely held to be transformation tough- using mechanical testing with in situ neutron diffraction. The ening from the r-m transformation. -Transformation toughen- material shows obvious plastic deformation at applied stresses ng was first proposed by Garvie et al. in Ca-PSZ. Since then, in excess of an estimated critical stress of 925 t 20 MPa. Most significant new toughening mechanisms, such as micro-crack of the accumulated strain occurred by transient room. shielding and crack deflection have been proposed to make an temperature creep. Plastic deformation was associated with important contribution to the process. The bulk of the experimental considerable stress-induced tetragonal-to-monoclinie transfor work has been conducted by examining samples using TEM and mation. The volume change calculated from the strain gauges XRD after they have been mechanically tested. -Stress- correlates well with the amount of t- m transformation induced martensitic transformations, such as 1-m, are highly observed. Unlike previous studies of Ce-TZP and Y-TZP, dependent on the local stress-strain state of the sample. It is well ferroelasticity was not observed, nor was the t-o transfor- mation observed. Minor microstructural changes were noted, known that different surface preparation techniques give quite cluding an increase in the root mean square internal strain of 0.05%, commensurate with an increase in internal stress of tion, and so it can be used to give the bulk-phase proportions in the -100 MPa. It would appear that transformation selectivity ceramic. Er situ neutron diffraction has bee en success ul in solving was exercised with the transformation occurring first in tet- ragonal crystallites favorably oriented to the applied stress. such problems as the phase composition, low temperature he st strong preferred orientation. Comparison is made with the Mg-PSZs. Room-temperature creep has been observed using strain-gauge techniques in samples of 9. 4Mg-PSZ subjected to other commercially interesting zirconia ceramics, Ce-TZP and tensile stresses. An overall volume increase in the sample Y-TZP, which have been studied using the same techniques. fi.e, negative Poisson's ratio) suggested that the creep and duced t-m transformation were related. This was lat L. Introduction confirmed by ex situ neutron diffraction measurements on regions taken from the gauge volume and the(low stress) gripped region AGNESIA-PARTIALLY-STABILIZED ZIRCONIA (Mg-PSZ) has re- of the samples. When a long period of time had elapsed between ceived considerable attention in the literature for its high the tensile testing and neutron diffraction, discrepancies were fracture toughness. The material is a complex mixture of the noted between the amount of m phase observed and the measured ambient pressure cubic (c), tetragonal (0), and monoclinic (m volume increase in the samp 28.79As the strain gauge and phases of pure zirconia. The active microstructural component is neutron observations were conducted separately, it was suggested lenticular precipitates of the t phase, which is present in a largely that stored elastic stresses in the microstructure had triggered the c phase matrix. Under an applied stress, the I phase is able to reverse m-I transformation on relaxation of the applied stress undergo a martensitic transformation to the m phase, accompanied Hence, in addition to the surface sensitivity highlighted above, the by a volume increase of.9%. In its most refined form, results from ex situ experiments may not reflect the real transient controlled cooling from the solution treatment temperature at behavior of the material. In situ neutron diffraction has the 1700C followed by an aging treatment leads to precipitation of capacity to overcome both problems at once and hence to capture dynamic changes in the phase proportions, structural parameter the anion-ordered 8-phase(Mg2Zr, O 2), which replaces a major and internal stress distribution of each phase at different stresses proportion of the c matrix. The 8-phase precipitates at the the interface, leading to localized strain, metastability, and destabili- Recently, in situ neutron diffraction studies have been con- zation of the I phase. . 9 ducted on the microstructurally simpler tetragonal zirconia poly The phase-equilibrium, microstructure, and mechanical proper- crystals 12Ce-TZP 0.3 and 3Y-TZP. 2 There it was observed that the stress-induced I- m transformation is both time-dependent and reversible in 12Ce-TZP and that both materials show a degree of ferroelasticity under compressive loads. 3Y-TZP did not un- dergo any observable t-m transformation under compressive R. Hannink--contnbuting editor loads up to 2.3 GPa. The results of similar in situ loading experiments have not previously been reported for Mg-PSZ. It is important to comment briefly on the creep observed in Mg-PsZ by ex situ measurements and that observed in 12Ce-TZP 87072 Received March 19, 2002: approved December 30, 202. and 3Y-TZP by in situ diffraction measurements. The high scien y d e hunterian Research Couneil and the Australian temperature creep of ceramics is not a new phenomenon, and it is an Ceramic society well understood using the theoretical framework developed for
466 Journal of the American Ceramic Society-Ma et al. Vol 87. No. 3 creep in metallic materials. The mechanisms involved are not active at room temperature and cannot explain the observed creep in structural zirconia ceramics. The behavior ously observed in Mg-PSZ was transient creep and the strains(both longitudinal and transverse)approximated a power law similar to that proposed by Andrade ∈=Br It was observed that the stress also approximated a power b 38≥ leading to a composite equation of the form III II III In I! III IlIl El Il The creep parameters were quite variabl 2000 It I lI1 1101011IIa sensitively on the processing and thermal history of the samples. Although creep in 12Ce-TZP was also observed to follow a 2e(degree er law such as Eq (1). the observations were made during a stepwise loading experiment and it was accompanied by large Fig. I. Rietveld refinement result for 9.4Mg-PSz(sample 2).The bursts of transformation that prevented a systematic study of the observed data are indicated by (+)and the calculated pattern by the solid stress dependence. Very much smaller creep strains were observed in 3Y-TZP 32.37 and the time constant for reversal was much he observed and calculated patterns on ame scale. The rows of shorter. The only analysis that could be conducted was in the final markers below the pattern show(from the top down) the positions of peaks unloading step from 500 MPa to 0 MPa, during which the strains from cubic. tetragonal, and monoclinic zirconia and the 6 phase decayed to zero in good accord with a creep-relaxation model (Mg2 Zr,O1) This paper presents the results of an in situ neutron diffraction study of time-dependent stress-induced microstructural changes in 9.4 mol% Mg-PSZ during uniaxial compression testing shown in Fig. 1. Observed data are shown as(+) and the calculated profile as a solid line, The curve below is a difference plot between the observed data and the calculation, Reflection IL. Experimental Methods markers are indicated with small bars for the cubic, tetragonal monoclinic, and 8-rhombohedral phases(from the top down). Samples used for this study were Ts grade 9. 4 mol%o Mg-PSZ ICI Advanced Ceramics, Melbourne, Australia). The ceramic preparation is a proprietary process, the precise details of which IlL. Experimental Results are commercially sensitive. The process involves firing at -1700 C followed by controlled cooling and an aging treatment at (1) Stress-Strain Curves 100C. Sample I was cut to a height of 15 10 mm from a The stress-strain curves of the second 9. 4Mg-PSZ are shown in 90-mm-diameter rod with a diamond-impregnated saw Sample Fig. 2. Here, the strains are those at the end of the constant-stress 2 was cut into a prism having a cross section of 9. 70 mm X 9.80 holds and have been corrected for gauge off-set and an apparently mm at both ends. After cutting, the samples were polished with erroneous value in the longitudinal gauge at low stress. In both 1200-grit emery paper to remove surface micro-cracks, which may samples, when the stress was smaller than 800 MPa, the longitu- become the potential source of collapse. In addition the edges and dinal and transverse strains were linear functions of the applied corners were ground with fine SiC paper to have smooth 0.5 mm stress Sample I gave a Youngs modulus of 205+8 GPa, and radius rounded edges. Poissons ratio 0.33+ 0.06 in the range -100-1000 MPa. For The experiments were conducted with the medium-resolution Sample 2, Youngs modulus was determined to be 217+3 GPa powder diffractometer (MRPD) at HIFAR(high-flux Australian and Poissons ratio is 0.35=0.01, measured in the range 200-800 reactor of the Australian Nuclear Science and Technology Orga MPa. When the applied stress was >1000 MPa, nonlinearity nization). The angle range for neutron diffraction was 4-104 2 indicates that the sample had begun to plastically deform. Some with a step size of 0. 1. Neutron wavelengths were 1.6663A for nonlinearity in the transverse strain data were already apparent Sample I and 1. 6676A for Sample 2, as determined from a between 800 and 1000 MPa. We conclude that the critical stress is standard rutile specimen marginally <1000 MPa. Unlike in the elastic region, where the Compression testing of sample I began with a holding stress of transverse strain was much smaller than the longitudinal strain after the onset of plasticity, the transverse strains were much 2. 8 MPa, then 100 MPa, followed by 50 MPa increments up to larger. The plastic strains were estimated by extrapolating the 000 MPa For sample 2, the applied stress sequence was 5.2. 200 400,600.800.1000,100.1200.1225,1200.1100.1000.900 800.700,600.530.480.420.380,250.180.100,0MPa, followed by post-loading neutron diffraction patterns to detect microstruc- tural changes immediately after the load was released Transverse Two strain gauges were glued to the surfaces of the two Horzontal amples. One, placed vertically, measured the longitudinal strain 4000 and the other, placed horizontally, measured the transverse strain. Strain data were recorded every 30 s(Datataker DT50, Data 0 Electronics, Victoria, Australia) and downloaded to a computer. Rietveld analysis was used to extract information from leutron diffraction data as outlined in previous publications. 3/5 Four phases were found in these les, cubic zirconia (c) tetragonal zirconia(n), monoclinic zirconia (m), and the rhomb- hedral 8-phase(Mg, Zr, O,2). The refined parameters were global 1200 parameters and scale factors of all the four phases. In addition, for Applied stress(MPa the major phases, lattice parameters, the breadth of the internal strain distribution, and the March coefficient for preferred orien Fig. 2. Stress-strain curves of a 9. 4Mg-PSZ sample during uniaxial tation were also refined. An example refinement for sample 2 is compression testing
March 2004 Tetragonal-to-Monoclinic Transformation in Mg-PSZ Studied by in Situ Neutron Diffraction 467 elastic part of the stress-strain curves to the maximum stress. In Table 1, wt%o monoclinic phase estimated sample 2, the longitudinal strain deviated from the straight line by from Macroscopic Volume Change a maximum of-1331(He), whereas the transverse strain deviated from the straight line by-5887(He tress(MPa) Hold time (min) (2) Diffraction Patterns 1000 4.66 The diffraction patterns recorded during compression testing of sample 2 are shown in Fig. 3. On close scrutiny, in the loading 1150 1200 154 2336 process, the(I 11) reflection at% 26, grew gradually as the 1225 109 applied stress increased. The reflection appears to increase in size Total 989920.2 ven at relatively low loads. An obvious increase occurred when the applied stress was >1000 MPa. There is a corresponding decrease in the(013)and (121) peaks of tetragonal phase near 65 20 especially after o exceeded 1 100 MPa. At higher stresses, the To provide an independent measure of the real phase comp (I1I)m peak continued to increase as the load increased, until the tion In 9. 4Mg-PSZ under each applied stress, quantitative phase maximum stress, 1225 MPa. During the unloading process, no change in the size of the (11 1)m peak was observed In addition to analysis was conducted using the Rietveld refinement scale fac- peak intensity changes, some additional line broadening can be tors where the wt% of phase p is given by observed, e.g., in the double peak near 6520, where there is a (SZMV progressive loss of resolution y (SZMV 3)t→ m Transformation where Wp is the weight fraction of phase p, S is the scale factor, Z From the strain data, it is clear that the sample volume increased after the applied stress exceeded 800 MPa. It is well known that the one unit cell, and V is the unit cell volume of each phase. In total tetragonal-to-monoclinic phase transformation will give rise to a four phases were considered: the cubic, tetragonal, monoclinic 4.9%e volume increase. If it is presumed that the volume increase and 8-rhombohedral phases. The phase composition for the start in this experiment came totally from the f-m phase transforma- ing material is listed in Table II with error estimates. tion, the volume increase can be used to estimate the percentage of The calculated weight percentage of tetragonal phase and new monoclinic phase, The estimated fraction of monoclinic phase corresponding weight percentage of monoclinic phase during the produced during each hold at constant load was calculated and is whole mechanical testing process are shown in Figs. 4(a) and(b) shown in Table L. These results neglect a small amount of The arrows indicate the sequence in which the external stress was transformation that occurred essentially instantaneously during applied. When the load was between 0 and 800 MPa. the fraction application of the load increment In the loading half-cycle. a total of monoclinic phase was constant to within the estimated errors. of -20 wt% of new monoclinic phase was predicted by this As expected from the raw neutron data( Fig 3), during loading, the calculation greatest increase in the monoclinic phase was observed at stresses 20000 MPa 15000 420 058 4567 120 1200 5000}1150 1000 400 20(degree) Fig 3. A portion of the diffraction patterns recorded during compression testing of the 94Mg- PSZ. Note the growth of the monoclinic-phase peaks, e.g. at31°20
Journal of the American Ceramic Sociery-Ma er al Vol. 87. No. 3 Table I. Phase Content of 9, 4Mg-/ at 5.2 MPa (4) Structural and Microstructural Effects Determined by Neutron Diffraction In addition to phase quantification, the in situ neutron diffrac z M(amu) V(A') wt ErTwt tion and Rietveld analysis gives data on changes to the structure and microstructure of the ceramic during loading and un 4I1.61131.520.5 9 particular, changes were noted in the axial ratio, c/a, of the 2123.2267.0412.162.61 tetragonal phase, preferred orientation in the tetragonal and mon m-Zr( 4123.22139.660.36.50.7 aclinic phases, and the internal strain distribution in the tetragonal Mg2Zr3O123696.72679060.0492180.9 phase. A composite diagram summarizing these results is shown in (A) Tetragonality: The c/a ratio of the tetragonal phase >1000 MPa(Fig 4(b). Likewise, when the load was between 0 adopting the pseudocubic unit cell, is shown in Fig. 5(a). Overall c/a increases as a function of applied stress and the slopes of the within one standard deviation(Fig. 4(a)). in good correspondence curves during loading and unloading are the same at stresses with the weight fraction of monoclinic phase. The amount of <1100 MPa. The lattice parameters observed were the average phase began to decrease after the load reached 800 MPa, and the alues of those crystallites oriented so as to satisfy Braggs law. So rate accelerated after the stress exceeded 1 100 MPa In agreement from-0-1 100 MPa, tetragonal particles with c axes perpendicu- with the strain gauge data, the critical stress for the stress-induced lar to the applied stress expanded(Poisson strains) faster than transformation would appear to be-1000 MPa in this material those with the a axes perpendicular to the applied stress or more These results confirm that the f-m phase transformation is responsible for the bulk of the strains observed In the unloading half of the stress cycle, the fraction of monoclinic phase did not stop increasing, but rose quickly until 1.0215 1000 MPa. Between 1000 MPa and 0 MPa. the fraction of monoclinic phase continued to increase but at a much smaller rate. The apparent tetragonal phase content too continued to decrease ith a stable rate during unloading 1.0210 From the observations shown in Figs. 4 (a) and(b). it was estimated that in the loading half cycle (i.e, <1225 MPa). the tetragonal phase decreased 10.9=1, 2 wt% and the monoclinic the tetragonal phase was 190+1.5 wt% and total increase in the o10205 hase increased 12.2+ lI wt%. The total estimated decrease in monoclinic phase was 16.9+ 1. I wt%. 1.0200 0.35 0.20 1.04 Applied stress(MPa) 1200 Applied stress(MPa) a)Tetragonal-phase content in 9.4Mg-PSZ during a compression loading cycle calculated from Rietveld refinement scale factors: Fig. 5. (a) Tetragonality of the I phase: (b) internal strain distribution in rved monoclinic-phase content during the same the loading. I phase; (c) March coefficient of t phase, during loading and unloading of cycle. Error bars indicate one estimated standard deviation. 9.4Mg-PSZ. Error bars indicate one estimated standard deviation
March 2004 Tetragonal-to-Monoclinic Transformation in Mg-PSZ Studied by in Situ Neutron Diffraction simply, for the tetragonal phase s13 >$12. This behavior contrasts load, were averaged over every 30 s. The averaged strain data were with 3Y-TZP and 12Ce-TZP37 with simpler phase compositions. hen plotted as a function of time at each applied stress. An where c/a was unchanged below the critical stress At >1100 MPa example of such curves is given in Fig. 6(a), showing th the c/a ratio in Mg-PSZ decreased by approximately three standard accumulation of vertical, horizontal, and volume strains at 800 deviations. Reasons for the decrease may be: (i) removal from the MPa, below the gross critical stress. Creep curves for samples diffraction pattern by transformation of the most highly stressed I loaded >800 MPa were fitted to functions using the model given articles, causing the average to shift to a lower value: (i)removal by Finlayson et al. 20 rom the diffraction pattern of the least-stabilized t particles, so Unlike the strain from the tensile creep samples. the strain in again the average shifts to lower values: or(ii) residual strain this compression test was not a direct function of applied stress imposed on I-phase particles by the transformed monoclinic phas from the step-wise loading. The fitting was to a modified Andrade If the latter effect is responsible, the residual stresses can be function estimated to be of order I 10 MPa (B) Internal Strain Distribution in the t Phas ∈=∈n+Brn uare(rms) width of the internal strain dis lculated from the tane component of the The fitted creep curves are shown in Fig. 6(b). Uobs. using the following equation: 1000 MPa mation >1000 MPa. The plastic deformation is accompanied by a accompanied the r-m phase transformation. The overall increase volume expansion. In situ neutron diffraction has shown that the in internal strain of 0.05% is equivalent to an increase in rms plastic deformation is accompanied by a substantial amount of internal stress of-lo0 MPa stress-induced t- m transformation within the ceramic. No (C) Preferred Orientation: Previous uniaxial compression evidence was found in this material for ferroelastic switching of t ests with in situ neutron diffraction 1.2 on 3Y-TZP and 12Ce- crystallites nor was the competing t-o transformation that has TZP have shown considerable changes to the orientation distribu- been previously reported in tensile room temperature creep sam convenient measure of the degree of preferred orientation and from the volume change (assuming a 4.9% local volume change on density function due to March"obtained during the Rietveld tron diffraction data. The two estimates for the total amour refinements, In both cases referred to above, the ferroelastically transformed agree to within two standard deviations. It can switched domains oriented to diffract from the shorter a crystal therefore be concluded that the plastic strains are primarily due to lographic axes out of the neutron beam and switched the longer c the t-m transformation. coefficient of the t phase above the critical stress. By contrast, for as The transition from elastic to plastic deformation in Fig. 2 is not axes into the beam. This resulted in a strong decrease in the March 9. 4Mg-PSZ, the March coefficient of the t phase began very close stress takes. A value in excess of 1000 MPa is indicated by the to unity, indicating an initially random orientation of t crystallites. longitudinal strain in Fig. 2. whereas the transverse strain gives a The influence of the compressive stress is shown in Fig. 5(c)to be slight increase in the March coefficient. It can be concluded from this that ferroelastic reorientation of the I phase does not occur in observable amounts in 9.4Mg-PSZ during uniaxial compression. It is believed that the slight increase in the March coefficient is due to slight transformation-induced texture. I42 The March coefficient for a monoclinic structure is often x nadequate, as there is no a priori reason to select a particular pole axis or preferred orientation vector. It does, however, often serve as a useful correction to the calculated intensities by use of a vector chosen based on the observed intensities In these experiments, it was noted that the stress-induced m phase has a more prominent 2500 (II reflection than a random orientation. The number of other phases present and the initially small amount of m phase present made the refinement of meaningful preferred orientation coeffi cients as a function of applied stress difficult. Instead, a March coefficient was refined at the maximum m phase content, and this >1000 was used as a constant in all other refinements above the critical stress and during unloading. The preferred orientation was con firmed by one of the post-loading neutron diffraction patterns that was taken after the sample was turned 90 to put the compression 120160 axis of the sample in the diffraction plane. This pattern showed a Time(min far smaller( 11 1) peak in agreement with the above observations (5) Time-Dependent Strain/Creep Fig. 6. (a) Transverse strain. 800 MPa of 9 4Mg-PSZ showi As noted in the introduction, strains in zirconia ceramics have fitting of the volume strain at been reported to have a strong time-dependence. .- To assess the monoclinic phase fraction this, the strain data, recorded while the sample was held at each by 0. 049% for each I% increase in volume
Journal of the American Ceramic Society-Ma et al. Vol 87. No. 3 value between 800-1000 MPa. The phase proportions in Fig 4 do in the diffraction patterns. There is no such simple explanation of not clarify the matter greatly, other than to also suggest a value why the I-phase fraction determined from the neutron diffraction between 800-1000 MPa. Close examination of the creep data, or patterns does not exactly mirror the m-phase fraction in Fig. 4. It the other hand, shows that there is already significant transforma is possible that the stress-induced orthorhombic phase occurs, but tion occurring at 800 MPa(Fig. 6(a). The reason for these is obscured by the lower counting statistics and broader lines. This differences may well lie in the complexity of the microstructure cannot account for the entire discrepancy, and we must conclude and the residual stresses induced by processing. Earlier work on that intensity in some of the c- and 8-phase peaks has beer he role of the 8 phase in Mg-PSZ highlighted TEM strain contours mistakenly attributed to the t phase during Rietveld refinement. A ithin the tetragonal precipitates adjacent to 8-phase precipitates slight linear decline in the sum of the c and 8 fractions support and neutron diffraction line broadening supported this observa- this explanation. Unlike the earlier work on Ce-TZP and Y-TZP tion. Figure 5(b) shows that even before the application of the where only two phases were involved, the severe peak overlap in external stress, there is a considerable amount of residual strain, these patterns places some limitations on the analysis 0, 22% rms. Conversion to a residual rms microstress yields a Very similar in situ neutron diffraction studies of mechanical value of 440 MPa. Therefore, even in the absence of an externally response have now been conducted on three zirconia ceramics applied stress. some small fraction of the tetragonal precipitates Ce-TZP, Y-TZP, and Mg-PSZ. in this work. Despite some already lie close to the transformation stress. These local commonality in the general perception of these ceramics and their internal stresses, when supplemented by the externally applied behavior, each one has shown quite different behavior wher ess, can trigger the transformation at far lower stresses than the tudied in this manner. All three showed elastic deformation up to gross critical stress of -1000 MPa. In support of this idea, the low ome critical stress, beyond which some plastic deformation tress range of Figs. 4(a) and (b), from 0-800 MPa, appear to occurred, Table Ill summarizes the critical stresses and plastic show a small slope albeit of only approximately one standard strains observed in the three ceramics deviation, A pragmatic definition of the critical stress is that stress It is clear from Table III that the I phase in 3Y-TZP is the required to trigger the I-m transformation in I phase that least transformable by uniaxial compression and that in 9 4Mg experiences only the average long-range stress field of the ceramic PSZ it is the most transformable. Not only do the numerical body. Both an interpolating curve and linear extrapolation of the values differ, but the mechanisms of the deformation also differ last three points above and below the elastic-plastic transition on widely Mg-PSZ deformed largely by the r-m transformation Fig.4 gave o。=925±20MPa but with a strong time dependence. Ce-TZP deformed primarily This critical stress is appropriate only for uniaxial compression by the f-m transformation but with some associated ferro- and for the long-term loading used here. The majority of the 1- elasticity. The t-m transformation in Ce-TZP was mostly in m transformation in this material occurred slowly by creep, as quite rapid bursts followed by much smaller amounts of previously highlighted by Finlayson et al. Hence, mechanical time-dependent transformation or creep. No r-m transforma ests conducted at more usual laboratory strain rates would give a tion was observed in 3Y-TZP, and the plastic deformation in much higher critical stress and much smaller plastic strains In uniaxial compression was attributed to ferroelasticity alone. In contrast, previous creep testing was conducted in tension and a all cases, there would appear to be a tendency to restore the significant amount of creep was observed at stresses as low as 240 original microstructure(and shape )of the material, albeit with MPa. Unlike the earlier creep tests, where samples were taken widely differing time constants. Mg-PSZ shows evidence of stepwise. Hence, each creep curve represents the transformation of several hours and the 3Y-TZP studied had completely relaxed a different subset of r crystals. There appear to be two types of within 3 h of removal of the load behavior. At 800-1000 MPa, the creep is relatively slow and the It is interesting to consider the chemical and microstructural terminal slope of the curves is low. Above this level, i.e., onditions that make these disparate responses possible. The ceramics ignificantly above the estimated bulk critical stress, the slope is studied are stabilized quite differently. The interplay of stabilizer and much greater and the curves lie almost on top of one another. The transformation behavior is not known. In an earlier publication, we last curve, at 1225 MPa, again has a lower slope(see Fig. 6(b)) highlighted that the shear strains (measured using stress-induced shifts This may be caused by the smaller stress increment of 25 MPa n neutron diffraction peak positions)in Ce-TZP are greater than those nstead of 50 MPa. or it may be that the supply of read in Y-TZP. It is possible that this is caused by stabilizer-induced ransformed t crystallites under uniaxial compression in this softening of the elastic shear constants in the former. No comparable direction is declining In the absence of results at a range of data could be extracted for Mg-PSZ because of the microstructural temperatures and in view of the very complex microstructure, little complexity. The behavior of these three ceramics is so different that additional information concerning the creep mechanism can be the influence of the stabilizer on the I phase cannot be solely derived from the fitted creep constants responsible. In systems other than zirconia(e.g, in shape-memory It is interesting to note that the transverse plastic strains are five alloys), stress-induced Martensitic-phase transformations have first times as great as the longitudinal strains, owing to constraint of the been studied extensively in the temperature domain. In Ca-PSZ and Ce-TZP, the t- m transformation is readily induced by cooling transversely. The I- m transformation is triggered by shear below room temperature: however, in 3Y-TZP. the ceramic remains stresses,and so the orientation of individual t particles with in its original majority I-phase state, and in Mg-PSZ. the respect to the stress(applied and internal) is a critical factor. The phase transforms to an orthorhombic (o) phase on cooling. The t-m initial March coefficient indicates a quasi-random distribution for transformation is driven by shear stresses. it is unknown whether the t phase. Only a subset of the I phase will be oriented for the ferroelasticity is driven by shear or dilatational stresses, and the t- maximum shear stress to lie along a potential transformation o transformation is thought to be driven by dilatational stresses. A direction. Therefore, a degree of transformation selectivity is exercised. This is confirmed by the high degree of preferred orientation in the m phase: a clear case of transformation-induced Table lll. Critical Stresses and Plastic texture as predicted by Bowman and Chen. 42 From the limited Strains Observed for Zirconia Ceramics in pole data that may be inferred from these results, the preferred Uniaxial Compression orientation observed in this work agrees well with the calculated pole figures of Bowman and Chen. 12 Preferred orientation may Critieal Plastic Strain (9) influence the small discrepancy between the m-phase fractio derived from the macroscopic volume change and the neutron diffraction phase analysis at high stress. Then, as the stress 0.1330.589 changes, e. g. during unloading, some of the m phase may undergo 12Ce-TZP 1200 0.14 0.375 ferroelastic reorientation into orientations more readily observable 3Y-TZP 70 0.1160.100
March 2004 Tetragonal-to-Monoclinic Transformation in Mg-PSZ Studied by in Situ Neutron Diffraction shear transformation is strongly dependent on the degree of coherence References between the transforming region and its surrounding In Mg-PSZ, it has been postulated that a very strong coherent Advances in Ceramics, Vol. 3. Science and Technology of Zirconia boundary between the transforming t crystals and the surrounding nces in Ceramics, Vol. 12, Science and Techmology of Zirconia II c and 8 phases, and considerable dilatational thermal mismatch Claussen, M. Ruhle, and A. H. Heuer. American Ceramie Society, Columbus. stresses, are responsible for the I-o transformation replacing the Advances in Ceramics, Vol 24A and B, Science and Technology of r-m transformation at low temperature. 4>However, external Edited by S. Somiya, N, Yamamoto, and H. Yanagida, American Ce loads generate shear stresses that are able to overcome the Westerville. OH. 1988 coherence and initiate the I- m transformation, The transforma- 4A. G. Evans and A: H. Heuer, "Transformation Toughening in Ceramics Martensitic Transformations in Cracks Tip Stress Fields. "J. Am. Ceram. Soe, ion is restricted in extent by the microstructural complexity in 15-61 241-48(1981) Mg-PSZ. The t phase is present as extremely small crystallites and R. M. MeMccking and A G. Evans."Mechanics of Transformation-Toughening the transformation is necessarily impeded or arrested at the in Brittle Materials. "J, Am. Ceram Soc. 65 151 242(1982 interphase boundary. This is not the case in Ce-TZP, where large R. H J. Hannink, and R. C, Garvie. ""Sub- Eutectoid Aged Mg-PSZ Alloy with Enhanced Thermal Up-Shock Resistance. "J, Mater. Sci. 17, 2637-43(1982) crystals of the t phase form the entire microstructure. It is well Microstructure Evolution and Ordering in documented that the t- m transformation in that material occurs Commercial Mg-PSZ.J. Mater. Sel, 19. 2934-42(1984 in large bursts that are auto-catalytic Unlike Mg-PSZ, there is little crystallographically oriented 290AMuM数14 ment of Sub- Eutectoid Aged Mgo. R. H. J. Hannink. C. J. Howard, E. H. Kisi, and M. V. Swain, "Relationship residual stress in the microstructure of Ce-TZP, as it is comprised ween Fracture Toughness and Phase Assemblage in Mg-PSZ" J, Am. Ceram Soc. from a single phase and therefore the t-o transformation is not seen on cooling or under load. Some ferroelasticity was ob- R. Stevens, Zirconia and Zirconia Ceranics. Magnesium Elektron Ltd, Twick served; however, this was strongly associated with the 1-m enham U. K D J. Green, R. H J. Hannink, and M. V, Swain, Transformation Toughening e transformation (i.e. the same critical stress and reversion under the Ceramics, CRC Press, Boca Raton, Florida, 1989 same conditions ). The microstructure of Ce-TZP does not rou R. H. J. Hannink and M. V. Swain, "Progress in Transformation Toughening of tinely contain many twin domains and so ferroelasticity is not ct,24.359-408(1994 accessible as a major deformation mechanism 3Y-TZP has the R. H J. Hannink, P. M. Kelly, and B C. Muddle, "Transformation Toughening in Zirconia-Containing Ceramics. "J. Am. Cera, Soc, 83 [31 461-87(2000 highest critical stress and does not transform on cooling. It must IR. C. Garvie. R. H. J Hannink, and R. T. Pascoe, " Ceramic Steel?"Natur therefore be concluded that the t phase that makes up 80-95 wt% London.258.703-704(1975 of the microstructure is highly stabilized. The presence of 5-20 M. Ruhle and A H. Heuer, "Phase Transformation in ZrO, Containing Cera wt% of c phase" reinforces this notion. However, the micro- Science and Technology of Zirconia /I, Edited by N. Claussen, M. Ruhle, and A. H structure does routinely contain a high density of twin do- Heuer, Amencan Ceramic Society, Columbus, OH, 1985 mans6、474t and ferroelastic reorientation can occur at lower D. Michel, L. Mazerolles, and M. Perez Qqy Jorba, "Polydomain stresses than the t-m transformation under uniaxial compressio It must be remembered that the in situ neutron diffraction lL. Edited by N. Claussen. M. R(hle, and A H. Heuer. American Ceran xperiments were conducted under compressive loads. The critical Columbi resses observed for Mg-PSZ are much higher than previously M. V Swain and R H J. Hannink."R-Curve Behaviour in Zirconia Ceramics" observed in tensile room temperature creep samples, presumabl 224-239 in Advances in Ceramics, Vol 12. Science and Technology of irconia because of interaction of the volume expansion associated with the Edited by N. Claussen, M. Ruhle, and A H. Heuer. American Ceramic Society Columbus, OH, 198 I- m transformation and the constraint exercised on the sample P E Reyes-Morel and 1-w. Chen, "Transformation Plasticity of CeO, Stabilized by the compression platens. Although it is felt that the general Tetragonal Zirconia Polycrystals, 1: Stress Assistance and Autocatalysis, "/.Am. differences between the ceramics would also be present under tensile loading, the details may differ. P. E. Reyes. Morel. J-S. Cherng, and 1-w. Chen, "Transformation Plasticity of cOr Stabilized Tetragonal Zirconia Polycrystals, Il: Pseudoelaticity and Sha Memory Effect, "J. Am. Ceram. Soc. 71 18] 648-57(1988) T.R. Finlayson, K. A Gross, J. R, Griffiths, and E. H, Kisi. "Creep of Mg-PSZ C.J. Howard and R J Hill. "The Polymorphs of Zirconia: Phase Abundance and rom the experimental results and the analysis, the following / Mater. Sci. 26, 127-34(1991) (1) Stress-induced tetragonal-to-monoclinic transformation is "C, J. Howard, R, J, Hill, and B. E Reichert. "Structures of the Zro Polymorphs at Room Temperature by High-Resolution Neutron Powder Diffraction. "Acta Cryst the toughening mechanism in 9.4Mg-PSZ, even when the defor- B44.116-20(1988 mation is conducted in uniaxial compression. This behavior is C.J. Howard and E H. Kisi. "POlymorph Method: Determination of Monoclinic similar to that for Ce-TZP but unlike that for Y-TZP where no Zirconia in Partially Stabilized Zirconia Ceramics. "J, Am. Ceram. Soc., 73 [101 transformation was observed 3096-99(1990 E H Kisi and C. Howard. "Neutron Diffraction Studies of Zirconia- (2) Most of the stress-induced monoclinic phase in 9. 4M Toughened Eng PSZ was produced during creep at constant stress and not during E. H. Kisi. C. J. Howard, and R. J. Hill. Crystal Structure of Orthorhombic the initial stress change, as was the case in 12Ce-TZP. Thi difference is considered to be from the microstructure of Mg-PSZ rhombic Zirconias. "J. An. Ceram. Soc. 74 [91 2321-23(1991) separated from each other and can communicate only via trans. Din D. N Argyriou. C: J. Howard, and R, I. Smith,"Al-Temperature Neutron where small precipitates of the tetragonal phase are physically formation stress fields transmitted by the c +8 matrix Zirconia. J. A, Ceram. Soc. 77 [121 3073-76(1994) (3) The stress-induced monoclinic phase exhibited strong E H. Kisi, T.R. Finlayson and J. R, Griffiths, "Phase Determination in Partially Stabilised Zirconia Creep Specimens, "Muter. Set Foram, 56-58. 351-56(1990 transformation-induced texture during loading and appeared to T. R. Finlayson, J. R. Griffiths, A. K. Gross, and E. Tsesmelis reorient partially during the initial unloading steps Partially- Stabilized Zirconia", Pp. 99-104 in The Material Wealth of the Nation Proceedings of the Bicentennial Conference of the Institute of Metals and Materials 山 ing the transformation. This provides the driving force for tl The reversion is considerably slower than that in Ce-TZP, presum of Ferroelastic Domain Switching and Tetragonal-to-Monoclinie Transformation in ably for the reasons outlined in No. 2 above Ce-TZP.J Am, Ceram. So, 80 31 621-28(1997). JE H. Kisi. Y. Ma, and S. J. Kennedy. "In-situ Neutron Diffraction Study of Ceramics under Load": pp. 693-9% in Materials gs, Proceedings of the Biennial Materials Conference of The Institute of Materials Engineering, Australia(Wollen nt gong, July 1998). Edited by Michael Ferry, University of Wollongong, Wollongong. Australia, 1998 The authors wish to thank Professor P. M. Kelly for helpful discussion during th Y Ma, E H Kisi, and S, J. Kennedy, "Neutron Diffraction Study of Ferroelas. ticity in a 3 mol% Y,OrZO."JAm. Cera. Soe. 84[ 399-405(2001)
4 Journal of the American Ceramic Sociery-Ma et al. Vol 87. No. 3 w. R Cannow and T. G. Langdon. "Review: Creep of Ceramics. 1: Mechanical 4K. 1. Bowman and 1-w. Chen. "Transformation Textures in Zirconias. "/Arm Characterisics,"/ Mater. Sci. 18. 1-50(1983). cmm.Soc,76m1322(199 G E Dieter, Mechanical Metallurgy, 2nd ed, Ch 13. McGraw-Hill. New York. B. C, Muddle and R. H. J. Hanninck, "Crystallography of the Tetragonal to H J. Frost and M. F. Ashby. DeformationMechanisn Maps, Ch 2. Pergamon onoclinie Transformation in MgO- Partially-Stabilized Zirconia. "J. Am. Ceram. Soc,69171547-55(19%6 ess, Oxford, U. K. 1982. aE N. da C. Andrade, On the Viscous Flow in Metals and Applied D. B. Marshall, M. R. James, and J. R. Porter, "Structure and Mechanieal phenomena," Proc. R Soc. London. A. 84, 1-12(1910). (by"The Flow in Metals operty Changes in Toughened Magnesia-Partially-Stabilized Zirconia at Low nder Large Constant Stresses, ibid., 90. 329-42(1914). Temperatures,J. Am. Ceram. Soe, 72[2]218-27(198 Y. Ma,"Toughening Mechanism in Zirconia Ceramics Studied by in-Sita 4'C J. Howard, E H. Kisi R. B. Roberts, and R Neutron Diffraction eutron Diffraction Ph. D. Thesis. The Univeristy of Newcastle, Australia, 1999 H Kisi. " Influence of Hydrostatic Pressure on the t-n Transformation in Mg-PSZ Studied by in Situ Neutron Diffraction. "J, An Ceram. Sec. 81 131741-4 (1990 RJ. Hill and C.. Howard Diffraction Data Using the Rietveld w. A. Dollase. "Correction Diffractometry: Application of the of Intensties f Phase Analysis from Neutron Powder SD. N, Argyriou and C. J. Howard, " Re-investigation of Yitria-Tetragonal irconia Polycrystal (Y-TZP) by Neutron Powder Diffraction-a Cautionary Tale. for Preferred Orientation in powd 11a M. Peres Y Jorba,""Fracture of Metastable el," Appl Crystallogr. 19, 267-72 Tetragonal Zirconia Crystals. "J. Muter. Sovc. 18, 2618-28(1983) +A. V. Virkar,Role of Ferroelasticity in Toughening of Zirconia Ceramics K. J. Bowman, Texture from Domain Switching of Tetragonal Zirconias." pp18.3 -210 in Zirconia Engineering Ceramics: Old Challenges -New Ideas. Tran JAm.Ceam.Soc,741102690-92(1991) Tech Publications, Switzerland, 1998
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