The Kinetics of Isothermal Martensitic Transformation of Zirconia Containing a Small Amount of Yttria
Jae-Hwan Pee 1;*,Takahiro Akao 1,Shigeru Ohtsuka 2and Motozo Hayakawa 1
1Department of Mechanical Engineering,Tottori University,Tottori 680-8552,Japan
2
Department of Mechanical Engineering,Yonago National College of Technology,Yonago 683-8502,Japan
The high temperature tetragonal pha of zirconia containing 1.40–1.60mol%Y 2O 3were successfully quenched in at room temperature by rapid cooling of small spherical specimens.On the metastable specimens,the isothermal transformation behavior into the monoclinic pha was studied by dilatometric method.The transformation start time exhibited a C-curve in a TTT diagram.The C-curve shifted toward short time side as well as toward high temperature side with decreasing yttria content or increasing grain sizes.A study of the pre-aging effect during the incubation period on the subquent isothermal transformation suggested concutive growth of em
bryos,but alternative interpretation was also possible.The activation energy was evaluated to be $50kJ/mol.(Received May 22,2003;Accepted July 22,2003)
Keywords:zirconia-yttria alloy,martensitic transformation,isothermal transformation,time-temperature-transformation diagram,compo-sition dependence,grain-size dependence,activation energy,dilatometric measurement
1.Introduction
The pha transformation from the high temperature tetragonal to the monoclinic pha of zirconia has long been considered to be a typical athermal type martensitic trans-formation.Historically,Wolten 1)was the first to suggest the athermal nature of the transformation.He conducted a high temperature X-ray experiment and obrved a large hyster-esis between the forward and backward transformation as well as stagnation of the transformation when the temper-ature was halted during the transformation.The obrva-tions led him to conclude that the transformation was an athermal type martensitic transformation.
Since zirconia doped with a small amount of yttria behaves similarly except for the decreasing transformation temper-ature with increasing yttria content,the transformation mode has also been co
nsidered to be the same.As illustrated in Fig.1after Ref.2),the M s (martensite start temperature)
decreas almost linearly with increasing yttria content up to 1.75mol%Y 2O 3,but beyond which no transformation takes place not only during cooling to room temperature but also by a subquent cooling to the liquid nitrogen temperature.2)This disappearance of the transformation is so sudden that it is difficult to be accounted for by a reduction of M s with increasing amount of yttria.
On the other hand,as also shown in Fig.1,Y-TZP (Yttria doped Tetragonal Zirconia Polycrystals),which contain 2.6–3.0mol%Y 2O 3,are well known to exhibit isothermal trans-formation when annealed at around 550K causing a ver degradation in the mechanical properties.3–5)The isothermal transformation takes place faster with decreasing yttria content.Thus,it was suggested that if the athermal nature of the transformation obrved in the low yttria composition range is interpreted as a result of incread transformation rate due to the reduced yttria content so that the C-curve becomes intercted by the cooling curve in the TTT (Time Temperature Transformation)diagram,the above mentioned sudden disappearance of the transformation beyond 1.75mol%Y 2O 3can be accounted for.
Evidence for the validity of such interpretation,namely the isothermal transformation behavior in the l
ower yttria region,can be found in the literature.2,6,7)Among all,Hayakawa et al.2)found not only that the transformation temperature was dependent on the cooling rate but also the parent pha could be fully retained at room temperature for a certain compo-sition range below 1.75mol%Y 2O 3by a rapid cooling.The metastable tetragonal pha obtained by such a fast cooling readily transformed into the stable monoclinic pha by heating at around 550K as expected from the isothermal nature of the transformation.More interestingly,the quenched metastable tetragonal pha underwent a burst type martensitic transformation into the monoclinic pha during a subzero cooling to the liquid nitrogen temperature.The occurrence of both isothermal and athermal modes of transformation in an alloy of a composition is very rare and no other system is known to transform at so different
300
5007009001100130015000.0
1.0
2.0
3.0
Y 2O 3 content, x /mol%
M s a n d A s , T /K
Fig.1The variation of M s and A s (rever transformation start temper-ature)with Y 2O 3composition for bulk specimens measured with a continuous cooling and heating of 200K/h 2)along with the region of the isothermal t Àm transformation 4)after Ref.2).
*Graduate
Student,Tottori University.
Materials Transactions ,Vol.44,No.9(2003)pp.1783to 1789#2003The Japan Institute of Metals
概况temperatures as in the prent ca.
The isothermal nature of zirconia doped with a small amount of yttria has been recognized only recently.2,6,7)Even though it may be regarded as similar to the isothermal transformation of TZP,the esntial nature of the trans-formation is not fully understood.In fact some rearchers consider the isothermal transformation of TZP is bainite-like.8,9)In order to identify the rate controlling thermally activated process and clarify the transformation mode, quantitative measurements of the kinetics are necessary.In the prent study,the kinetics of the isothermal transforma-tion is studied with special interests in its dependence on the composition and grain size.
2.Experimental Procedures
Series of specimens of different compositions,ZrO2–1.30–1.55mol%Y2O3(1.30–1.55Y)were prepared from pure zirconia and ZrO2–3.0mol%Y2O3powders(TZ-0and TZ-3Y,TOSOH,Japan),both having99.9%purity.Weighed powders for a given composition were mixed in a zirconia ball mill with ethanol for50h.After being dried,the mixed powder was lightly ground with a mortar and pestle and pasd through a100mesh screen.
Small spherical specimens were prepared from the mixed powders so that a fast cooling was possibl
e to retain the high temperature tetragonal pha after sintering.A mixed powder was uniaxially presd into a1mm-thick disk of20mm diameter in a steel die under200MPa.The sheet was diced with a knife-edge into cubes of1mm size.The cubes were granulated in a rotating cylinder who inner walls were lined with emery paper.In about24h,beads of uniform diameter were obtained.They were placed in a platinum crucible and sintered at1723–1873K for1h in the ambient atmosphere. To avoid isothermal transformation of the specimens during
cooling,they were taken out of the furnace when the furnace temperature reached1273K and rapidly cooled by immers-ing the outer surface of the crucible in water.Figure2shows SEM micrographs of a sintered specimen.
The retention of the tetragonal pha after quenching was examined by X-ray diffraction method using Cu K radiation. For the specimens sintered at1723K,a full retention of the tetragonal pha was possible with the specimens containing 1.40mol%yttria and more.Specimens sintered at higher temperatures were less stable,but only specimens of fully tetragonal pha were ud for the subquent kinetic measurement.The densities of successfully quenched speci-mens were higher than98%as measured by the Archimedes method.An average grain size of a sample was measured using the linear intercept method;the conversion factor1.54 was multiplied to the average intercept le
ngth measured over 500grains on the SEM micrographs taken from the cross-ction of a sample.As en in Fig.2,equiaxed grains were uniformly distributed.The average grain sizes incread with increasing sintering temperature but nearly independent on the composition.For1.55Y specimens,the grain sizes of 0.51,0.63,0.80,and1.00m m were obtained by sintering at 1723,1773,1823,and1873K,respectively.
A handmade micro-dilatometer2)was ud for monitoring the transformation behavior.It was a push rod type made of thin quarts rods;the specimen cell welded on one end of the supporting rod was heated by focud infrared gold image furnace(Shinku-Riko,Model MR-55W,Yokohama,Japan) and the displacement of the push rod was measured by a capacitance type transducer(MTI Accumeasure System, Model AS-2021SAI,USA).The maximum heating rate up to veral hundred degree K was approximately1000K/min, but cooling rate was somewhat lower.Only specimens of good spherical shape and diameter between600and700m m were ud for measurements.
3.Results
Dilatometric data obtained from the isothermal holding of
1.50Y specimens sintered at1723K are shown in Figs.3(a)–
(f).To avoid overlapping of the dilatation curves,the data are plotted in three parate temperature ranges((a)–(c)),and for each temperature range the early stage up to300s is re-plotted with an expanded time scale((d)–(f)).Specimen’s temperature reached a pret temperature within a time between30and50s depending on the pret temperature.A small dilatation during this period was primarily due to the thermal expansion of the specimen,which incread with the holding temperature.Then dilatation due to the transforma-tion followed a sigmoidal curve saturating at about10m m
错别字的危害up Fig.2SEM micrographs of a den spherical specimen of ZrO2–1.6mol% Y2O3prepared for rapid quenching experiments:(a)low magnification and(b)high magnification.
1784J.-H.Pee,T.Akao,S.Ohtsuka and M.Hayakawa
to 653K.Note that a complete transformation of a 700m m diameter specimen would yield an 11.7m m linear expansion assuming the lattice volume expansion was 5%.A well defined plateau was obrved at low temperatures.But this became shorter and eventually disappeared when the dilata-tions due to the transformation and thermal expansion merge at higher temperatures,say 653and 673K.With further increa of the temperature,the plateau tended to reappear indicating a delay of the beginning of the transformation (at temperatures 683and 703K in Fig.3(b)).At the temper-atures,the transformation behavior was no longer sigmoidal shape and was quite different from tho obrved at lower temperatures (e 693and 703K in Fig.3(b)).With further increa of the holding temperature,the transformation diminished.As en in Fig.3(c),transformation emed to be still in progress at 713and 733K.On the contrary,the curve at 753K appeared to show a gradual shrinkage of the specimen.At first sight,this phenomenon might appear suggesting a partial transformation by intercting the no of C-curve during heating and a subquent rever trans-formation at the holding temperature.However,this was verified not the ca by the fact that a heatin
g of the same specimen up to 1173K with the same heating rate showed no sign of rever transformation around the expected A s point ($1073K)in ca a partial transformation had occurred during the heating.
024*******
1000
20003000
Time, t /s
D i l a t a t i o n , ∆ L /µm
情侣保证书
24681012
010*******
Time, t /s
D i l a t a t i o n , ∆ L /µm
653
613
573
(d)
533
024*******
1000
20003000
Time, t /s
D i l a t a t i o n , ∆ L /µm
673683
693
703
(b)
246810
12
010*******
Time, t /s
D i l a t a t i o n , ∆ L /µm
673
683
693
703
(e)
024*******
1000
2000
3000
Time, t /s
D i l a t a t i o n , ∆ L /µm
713
733
753
(c)
246810120100
200300
Time, t /s
五指毛桃功效D i l a t a t i o n , ∆ L /µm
Fig.3Dilatation curves of the isothermal transformation at various temperatures for the rapidly cooled specimens of 1.50Y sintered at 1723K.(a)data for 493–653K,(b)data for 673–703K,(c)data for 713–753K;(d),(e)and (f)show the expanded early stage of (a),(b)and (c),respectively.
The Kinetics of Isothermal Martensitic Transformation of Zirconia Containing a Small Amount of Yttria 1785
To clarify the origin of this negative slope,a blank test was conducted with the dilatometer at some temperatures.The results at 493and 733K are shown in Fig.4.At 493K the initial expansion due to the heating of the holder was maintained at constant.But at 733K,after the initial expansion,gradual shrinkage was obrved for quite an extended time.This ems to be caud by the gradual temperature redistribution through the measuring system,where only specimen cell was heated by a focud beam.Since the apparent shrinking behavior of 753K curve in Fig.3(c)is the same as 733K curve in Fig.4,the former was interpreted as showing neither forward nor rever trans-formation.
The transformation-start times as well as the times for 10%and 50%transformation obtained from the above dilatomet-ric curves were plotted in a TTT diagram in Fig.5.The transformation start time was r
ead from the point of a definite increment (roughly 2%of transformation)from the plateau.But this criterion became more or less uncertain when a flat plateau was abnt.The times read from Fig.3were corrected for the time expended for heating to the pret temperature for isothermal holding.
The temperature dependence of the transformation kinetics was reprented by typical C-curves:flatter top and signifi-cant temperature dependence below the no.The horizontal
line at 743K reprents an approximate upper limit of the isothermal transformation of the prent sample.Although the low temperature data were limited by the time of measurement,a partial transformation to the monoclinic pha was obrved even at room temperature during an extended period of time.The isothermal transformation of this sample proceeds quite fast and the usual cooling rate employed for a bulk ceramic sample should have intercted the C-curve resulting in transformation,which ems to have been mistaken as athermal type transformation.
Similar dilatometric measurements were conducted for specimens of different compositions and grain sizes to e the effect of the composition and grain size on the transformation kinetics.The results are summarized as the transformation start time in TTT diagrams.Figure 6shows the effect of
yttria content on the C-curve for the specimens sintered at 1723K.Becau the average grain sizes were esntially the same,the effect of grain size could be ignored.The C curves shifted toward the long time side as well as toward low temperature with increasing yttria content.Although this trend is qualitatively well expected from the stabilizing effect of yttria,it is worth noting that only 0.05mol%addition of Y 2O 3content resulted in a significant delay in the trans-formation start time.It is also noted that the reduction in the no temperature between 1.45Y and 1.55Y in Fig.6is comparable with the reduction in the M s (transformation temperature during a continuous cooling)between the same compositions plotted in Fig.1.
Figure 7shows the effect of grain size on the C-curve for specimens of 1.55Y sintered at different temperatures.The C-curves shifted toward the short time side as well as toward higher temperature with increasing grain size.The behavior is consistent with often reported un-stabilizing effect of grain coarning.10–16)As an M s point is influenced by both T 0(equilibrium temperature between the chemical free energies of the parent and martensite phas)and a critical driving force (difference in the chemical free energies to trigger the nucleation of martensite),the position of a C-curve in a TTT diagram can be considered to be influenced by the same parameters.Although the stabilizing effect due to yttria content is expected through the reduction of T 0and that due to grain refinement must be through the incread driving force required for triggering the transformation,the effects
400
50060070080010
100
神经紧张怎么治疗
100010000
Time, t /s
T e m p e r a t u r e , T /K
Fig.5A TTT-diagram showing C-curves for 2%transformation,10%transformation and 50%transformation for 1.50Y specimen sintered at 1723K.
400
500600700800
10100
100010000
Time, t /s
张曼娟T e m p e r a t u r e , T /K
Fig.6A TTT diagram showing C-curves for the transformation start time for specimens with different yttria contents and sintered at 1723K.
-20
2
460
500
1000
1500
Time, t /s
D i l a t a t i o n , ∆ L /µm
200
400600800
T e m p e r a t u r e ,
T /K Fig.4Blank tests of the dilatometer at two different temperatures:493and 733K.
1786J.-H.Pee,T.Akao,S.Ohtsuka and M.Hayakawa
on the C-curves were indistinguishable.
As en in Fig.3(a),the isothermal transformation behavior at lower temperatures was characterized by a well defined incubation period before a detectable amount of transformation was obrved.To e if any significant change was occurring during this incubation period,the effect of pre-aging on the subquent isothermal transformation was studied.Figure 8(a)shows the effect of the aging treatments at 523K for 300and 600s on the subquent transformation start times.Although the aging treatments were well within the incubation stage,they definitely shorten the transforma-
tion start times for the subquent aging as en from the shifted C-curves in Fig.8(a).The result ems to imply that some sort of preparation toward transformation or even subtle transformation below detection limit was in progress during the incubation period;certainly not a stage of simple waiting for a nucleation event governed by a statistical probability.It was then examined if such a change during the incubation period proceeded linearly with time until it reached the critical state at the transformation start time.If this hypothesis is correct,the effect of pre-aging for a certain period (300or 600s)at 523K can be regarded as equivalent
to the aging for t 0
T at temperature T .That is:
t 0
T
¼300
or 600
523
T
ð1Þ
where 523and T are the incubation times (in unit s)at 523K and T K,respectively.Thus,the effective transformation start
time at T ,t total
T ,for the specimens pre-aged at 523K can be evaluated by adding the effective contribution by the aging,namely:
t total T ¼t T þt 0T
ð2Þ
where t T is the obrved transformation start time at T for the
specimen pre-aged at 523K for 300or 600s.
魏斌说The corrected transformation start times are plotted in Fig.8(b),where we e that the effective time
expended before the transformation started after the pre-aging agreed well with tho for the as sintered specimens at all temperatures.The result supports the initial hypothesis,namely a change toward transformation proceeded linearly with time during the incubation period.
Finally,activation energies were evaluated from the prently obtained C-curves.Figures 9(a)and (b)show Arrhenius plots of the reaction rates,which are taken as the inver of the transformation start time in Figs.6and 7.An activation energy was evaluated from the slope of the linear portion of the Arrhenius plot which corresponds to the data well below the no temperature of C-curve.17,18)The data prented by open symbols which denote the data near and above no of the C-curve were not ud for the linear fit.Activation energies were about 50kJ/mol,irrespective of yttria content and grain sizes.4.
Discussion
Isothermal nature of the tetragonal to monoclinic pha transformation was clearly shown for zirconia doped with 1.40–1.55mol%Y 2O 3.The isothermal nature is the same as tho obrved in Y-TZP,which contains 2.5–3.0mol%Y 2O 3.The isothermal nature ems to extend to pure zirconia as verified by the time dependence of the transformation obrved through the transformation tempe
rature.6)The reason why the t-m transformation of this composition range had long been considered to be typical athermal martensite ems to be due to its fast reaction.In fact,the time intervals of X-ray measurement ud for monitoring the transforma-tion by Wolten 1)were longer than 30min,by which time the amount of transformation must have well saturated to the value prescribed by the temperature.When the specimen’s temperature was reduced by a step,the amount of the m-
400
50060070080010100
100010000
Time, t /s
T e m p e r a t u r e , T /K
400
50060070080010100
100010000
Time, t /s
T e m p e r a t u r e , T /K
Fig.8(a)C-curves of the transformation start time for 1.55Y specimens pre-aged at 523K for 300and 600s in the incubation period as compared with that of no pre-aging.(b)Same as (a)but the transformation start times were corrected for the pre-aging effect (e text).
400
50060070080010
100
1000
10000
Time, t /s
T e m p e r a t u r e , T /K
Fig.7A TTT diagram showing C-curves for the transformation start time
哄男朋友睡觉的话
for 1.55Y specimens with different grain size.
The Kinetics of Isothermal Martensitic Transformation of Zirconia Containing a Small Amount of Yttria 1787
