Superplasticity in ceramic materials—I. The observation of a “superplastic partition” in ceramics

Acra maker. Vol. 44, No. 6. pp. 2373-2382. 1996 Elsevier Science Ltd Acta Metallurgica Inc. Printed in Great Britain 1359-6454196 $15.00 + 0.00

Pergamon 0956-7151(95)00359-2

SUPERPLASTICITY IN CERAMIC MATERIALS-I. THE OBSERVATION OF A “SUPERPLASTIC PARTITION” IN CERAMICS T. J. DAVIES, Manchester

Materials

A. A. OGWU,

N. RIDLEY

and Z. C. WANG

Science Centre, Manchester University/UMIST, Ml 7HS, England

Grosvenor Street, Manchester

(Received IO May 1995; in revised form 22 August 1995)

Abstract-Recent developments

in the understanding of the mechanical behaviour of ceramics assessed for superplastic deformation indicate that stoichiometric changes at interfaces and an associated “metallic” behaviour can account for the superplastic behaviour in the absence of a grain boundary glassy phase. Based on the experimental work carried out in Manchester and elsewhere, a “superplastic partition” is observed in ceramics which corresponds to a direct relationship with stoichiometric changes and a related “metallic” behaviour in ceramics. These changes in stoichiometry and the associated “metallic” behaviour can provide mechanisms for resistance to cavity nucleation and propagation, respectively. When this observation is linked with the Gifkins core-mantle concept and the findings of Mott, Cottrell and Gilman on insulator-metal transitions, this yields an improved appreciation of the superplastic partition in ceramics.

1. INTRODUCTION Raj [I] has reported that whilst stoichiometric ceramics usually cavitate in tension, non-stoichiometric ceramics exhibit a resistance to cavitation thereby enhancing their ability to undergo superplastic deformation. The above observation is supported by experimental studies on a variety of materials, including hydroxyapatite, zinc sulphide and other functional ceramics [l]. The deductions of Raj are based on the fact that interfaces in non-stoichiometric ceramics harbour an excess charge, leading to a space charge layer adjacent to the interface, resulting in a greater resistance to intergranular cavitation thereby enhancing superplastic flow. Functional ceramics are generally classified as mixed ionic and electronic conductors (MIEC) [2], wherein the bonding leads to charge delocalization involving both ions and electrons. Raj’s model describes the role of delocalized ions in superplastic flow as a contribution to grain boundary cohesion, whilst another model presented by Davies and Ogwu [3], which is complementary to that of Raj, explains the contribution of delocalized electrons to a metallization of the matrices thereby enhancing the capacity for resistance to cavity and crack propagation. It is anticipated that a general model can be proposed consisting in part of Raj’s model, which is concerned with resistance to cavity nucleation, and the Davies and Ogwu model, which requires accommodation to be rooted in the charge

delocalization concept for ions and electrons. Essentially, both models are different manifestations of the same phenomena, i.e. charge delocalization. The details of our proposed model are now treated below. Transition/rare earth metal oxides have properties that vary from being “metallic” in the early members of the periodic table of elements through to semi-conductors and insulators in the later members. The details behind these broad ranges of behaviour are given later. Many of the early transition/rare earth metal oxides such as Y-TZP have exhibited or aided superplastic flow as either matrices or additives to matrices. In the present investigation, the effect of additions of the later transition metal oxides NiO and CuO on the deformation in alumina has been investigated. A diagrammatic representation of the bonding types in oxides is shown in Fig. 1, with the vertical axis representing oxygen electronegativities and the horizontal axis representing a tendency for metallization expressed in terms of molar refractivity R, and volume V,,,. Based on Fig. 1, we would expect CuO to contribute more towards the metallization of an alumina matrix than NiO. This was investigated experimentally, as is reported in this paper. As one moves away from the metallic regime in Fig. 1, for testing under similar conditions, superplastic flow is less likely to occur when these oxides are used as dopants. The relationship expressed as 0 = K[i exp(Q,/RT)r

Q Crown copyright (1996). 2373

(1)

2314

DAVIES et al.: Covalent

SUPERPLASTICITY

IN CERAMICS

regime

0.7 0.6 0.5 0.4 ;___,___-,____T___i--_7_-_,____,___

0.3

0.2

0.1 -I

I 3.5k

I P2Os

I I I t 1 I 3.0’+

,

‘*

?? B203 SiOz

??

??

Moo,

GazOs

??

uo3

. A1203

??

MgO.

Cr203

Sn%* Fe0

CaO

sro.

MnO ?? pb . NiO t ??

ZnO

TiO2

??

?? ??

TiO

Fez03

. cue . cu*o

??

v203

CoO.PhO J

A 800 A 725 0 ! 103

......J w-w__ 104 105

regime

Fig. 1. A schematic which depicts variations in bonding for a range of oxides: after Duffy [4], with vertical axis representing oxygen electronegativities (X0) and horizontal axis representing a tendency for metallization expressed in terms of molar refractivity R, and molar volume V,.

is found to describe flow stress and strain rate relationships during superplastic deformation of metals and ceramics. In this relationship, 0 is the flow stress, R is the gas constant, T is the absolute temperature, K is a material constant that contains a grain size dependence and m is the strain rate sensitivity index. The term [i exp (QJRT)] is referred to as the Zener-Hollomon parameter, Q, is the activation energy for superplastic flow and i is the strain rate. One of the major differences between superplastic deformation in ceramics and metals could be that superplasticity in ceramics is very sensitive to changes in the Zener-Hollomon parameter while superplastic deformation in metals is relatively insensitive to this parameter [S], provided that other requirements like ultra-fine grain sizes and a high strain rate sensitivity index are obtained. Fine grained ceramics are known to exhibit an increase in tensile ductility with a decrease in t exp (QJRZ), even when the strain-rate sensitivity exponent remains high [5]. These observations may be explained by superplastic flow occurring at the tip of a crack so inhibiting its propagation and subsequent failure [S], consistent with the Gifkins [6] core-mantel concept. If the strain rate is increased or temperature is decreased, i.e. as i exp (Q,/RT’) is increased, the amount of recovery at the crack tip through superplastic flow is diminished Table I. Aqueous

Code AA AACM AANM TZP

Composition 99.99% AI,O, AI,O, + O.lwt%MgO + 0.9wt%CuO AI,O, + O.lwt%MgO + 0.9wt%NiO zro, + 5.3wt%Y,O,

10’

106

S-’

.+ exp(Q,/RT),

Ionic

106

Fig. 2. Comparison of the tensile ductility behaviour of a metallic alloy (Zn22Al) with a ceramic alloy (Fe&-2OFe) as a function of 1 exp (Q,/RT) in the range where m = 0.5-0.6 (after Kim et al. [5]). and this results in a rapid propagation of cracks leading to early (tensile) failure. Ceramic materials assessed for superplastic deformation may be divided or partitioned into two categories, namely those that undergo extensive superplasticity and those that are relatively brittle. The argument in this paper is that this “superplastic partitioning” in ceramics could be associated with the fact that the ceramic materials observed (to date) to exhibit extensive super-plastic deformation have an intrinsic “metallic” character in their bonds. This could lead to less sensitivity to changes in the Zener-Hollomon parameter [i exp (QJRT)] which would produce behaviour similar to that found in metals and metal alloys. Brittle ceramics that lack this “metallic” character, and are generally insulator type materials, would have a high sensitivity to changes in the Zener-Hollomon parameter. A comparison of the tensile ductility behaviour of a metallic alloy with that of a material described by Kim et al. [5] as a ceramic alloy, as a function of the Zener-Hollomon parameter, is shown in Fig. 2. As a basis for discussion, the results of experimental studies of superplastic behaviour of pure A&O,, pure alumina doped with Cuo or NiO, and tetragonal zirconia polycrystals (TZP) are presented. 2. EXPERIMENTAL Tensile specimens for superplastic testing were produced by a novel net shape procedure based on

suspension

for slip casting

Particle size (rm)

Particle content of slurry (vol%)

Dispex A40 (“A)

Separation time (min)

0.4 0.4

40 40

0.15 0.3

7 4

0.4

40

0.3

4

0.2

30

0.5

0

DAVIES et al.: SUPERPLASTICITY

IN CERAMICS

the slip casting of a slurry prepared from fine grain high purity powders, with subsequent processing involving cold isostatic pressing and pressureless sintering [7]. This gives a specimen of good surface finish with a density of between 92 and 100% theoretically. The starting materials were a 3 mol% Y,O, partially stabilized zirconia (TZP) powder and a high-purity (> 99.99%) u-alumina powder, supplied by Mandoval Ltd, Zirconia Sales (U.K.) Ltd. The average particle size was 0.2 pm for TZP and 0.4 pm for a-alumina. The specimen preparation procedure [7] included dispersing the powder in distilled water with different concentrations of surfactant (Dispex A40, supplied by Allied Colloids, U.K.); the solid particle concentration was 30 ~01% for TZP and 40 ~01% for alumina-based ceramics. Dopants were introduced into the primary powders by conventional precipitation methods. The details of the casting slurries are listed in Table 1 and the slip-casting rig is shown in Fig. 3. The specially designed rig for tensile testing has been described previously [8]. The sintering conditions, sinter densities and grain sizes are given in Table 2. 3. EXPERIMENTAL

pressure u

Fig. 3. Schematic

RESULTS

casting

Experimental results for alumina-based ceramics are shown in Fig. 4, where elongations in excess of 100% were observed; a specimen of yttria-stabilized zirconia showing an elongation to failure approaching 500% is shown in Fig. 4(e). True stress vs true strain curves for CuO doped alumina are shown in Fig. 5. No definable steady state was observed and the strain hardening is believed to be caused by dynamic grain growth. Figure 6 shows the stressstrain relationships for TZP tested at a relatively high strain rate (2 x lo-‘S-I) when strains in excess of 100% can be easily achieved without necking. In the CuO doped alumina, the observed reduction in flow stress for superplasticity cannot be fully described by models based on the influence of porosity. As discussed in Part II of this paper, a “metallization” of the alumina matrix by CuO is proposed to account for this stress reduction. NiO oxide doped alumina (reported in Part II) was less effective in metallization; this is understandable from the position of NiO relative to CuO in Fig. 1. The early transition/rare earth oxides are more effective and contributory towards superplastic deformation than later ones like CuO or NiO for reasons discussed in the next section.

AA AACM AANM TZPa

drawing of the mould assembly for slip of ceramic tensile specimens (Ref. [7]).

4. THEORETICAL

CONSIDERATIONS

4.1. “Metallic” behaviour The mechanical and electrical properties of a solid are both related to the nature of the bonding in the solid. The bonding determines the type of cohesion that exists, as well as whether the solid is an electrical insulator, semi-conductor or metallic conductor. A good example of the inter-relationship between the mechanical state and electrical properties of a material can be found in the report of shear induced metallization of silicon, reported by Pharr et al. [9], and discussed by Gilman [lo] and Cahn [ll]. Gilman’s proposal is that the observed transformation of silicon from the insulating to the metallic state under an indenter [9] occurs because compression causes narrowing of the band gap until electrons can tunnel from the non-conducting valence band into the conduction band, leading to metallization. Alternatively, this could be viewed as a change in the overlap of atomic wave functions as

Table 2. The effect of CIPing

Code

2375

on sintered density

Green density (%), as-cast

Green density (%) cast + CIPed

Sintering condition temp. (K)

Sintering condition time (min)

Sintered density (%) as-cast

Sintered density (%) cast + CIPed

Mean grain size (pm)

63 54 54 42

66 62 62 51

1573 I553 1653 1593

60 60 60 60

90.92 90.92 9&92 > 98

92.95 94.98 94-98 > 98

1.2 1.0 1.3 < 0.3

‘Sinkred specimens containing 100% tetragonal phase. CIPed at 280 MPa for 2 min; heating and cooling rates of 200 K/h were used for all sintering.

DAVIES et al.:

2376

SUPERPLASTICITY

IN CERAMICS

4

(b)

AACM)

AANM)

(4

ef = 480%(TZP)

W

Fig. 4. Specimens before and after testing at a strain rate of 1 x lO-4 s-',(a) before test; (b) pure alumina (AA) tested at 1573 K; (c) alumina doped with CuO (AACM) tested at 1673 K; (d) TZP specimen tested at 1823 K strain rate I x IO-“ s-'. compression

takes place, leading to electron

delocal-

ization. This is consistent with an earlier finding by Gilman [12] that a linear correlation exists between glide activation energies (i.e. dislocation motion) and the average band gap energies in solids. Gilman [lo] suggests that since the metallization process has an electronic origin (i.e. tunnelling of electrons from the valence band to the conduction band), it can be influenced by photons, doping (donors enhance 60

,

I(a)

whereas acceptors inhibit the transfer), currents, surface states, etc. The underlying bonding principles

behind insulator-metal transitions have also been the subject of study by Edwards and Sienko [13], Mott [14] and Herzfeld [15]. Based on the findings of Gilman and others, and the previous work of the authors [3, l&18], it is suggested that the extensive superplastic deformation (up to 800% tensile elongation) in yttria-stabilized

,

Al@, +

0.9wtmJo+ O.lwt%ugo Strain rate

(a”)

R3 l.OxlO+

60

I

(b)

I

AI& + 0.9wt!xuo + 0.lwt%Mgo

50Temp. (to g40-

T2 1473 Tl 1523

j

T3 1623

30-

v)

T3

0.4 True Fig. 5.

0.6 Strain

0.0

0.2

0.4

0.6

0.8

True Strain

True stress vs true strain curves of AACM (a) strain rate effects: (b) temperature effects.

1.0

DAVIES et al.:

SUPERPLASTICITY

70% 60B v5oT4823K

I 04Qb v,303 g 20-

01

0.0

I

I

I

I

I

0.2

0.4

0.6

0.6

1.0

1.2

True strain Fig. 6. True stress vs true strain curves for TZP at a strain rate of 2 x IO-* s-‘. tetragonal

zirconia

polycrystals

reported

by Nieh

et al. [19] and Wakai et al. [20], could be due to the fact that this material has an intrinsic metallic bonding. This imparts the ability to resist cavitation failure, i.e. accommodation of grain boundary sliding by dislocation motion [21], and leads to continuous elongation without fracture. The difficulty of achieving such extensive deformation in S&N,/ SIC composites [21] and alumina [22] could be due to the absence of this metallic character in their bonds. 4.2. Bonding mechanism It is generally agreed that the band theory of solids [23,24] cannot be applied to certain compounds of transition/rare earth metals with partly filled d and f shells, including oxides such as yttria-stabilized tetragonal zirconia; such compounds have properties that range from metallic to semi-conducting and insulating. The bonding behaviour of transition/rare earth metal compounds has been accounted for in a model developed by Hubbard [25-271 which is based on a concept that the partial filling of the d and f orbitals

IN CERAMICS

in transition/rare earth metal compounds (sufficient for metallic behaviour according to the band theory) must be accompanied by a considerable overlap of the orbitals of the metal ions to give the metallic character; when the overlap is absent or severely limited, electrons are localized on individual metal ions and the materials are insulators. Oxides of transition metals often adopt a NaCl structure; the 3d band in transition metal ions splits into two states, the lower designated t,, (i.e. d,,, dXZ,dJ can take 6N electrons and the upper one designated er (i.e. dr2, dX2_J can take 4N electrons. For example, early transition metal oxides, such as TiO and VO, show a metallic conductivity, because the symmetry of the NaCl structure allows three of the five d orbitals on different metal atoms to overlap, i.e. d,, d,; and d,., overlap as shown in Fig. 7(a), to form a broad t,, band. On moving towards the right-hand end of the transition metal series in the periodic table of elements, a contraction of the d orbitals occurs as the nuclear charge increases, leading to a localization of the d-orbital electrons on the metal ions, i.e. the orbitals dX2_+ would point directly at the oxygen ions, making orbital overlap difficult, with a resultant low electrical conductivity, as shown for NiO in Fig. 7(b); oxides such as MnO, Co0 and NiO do not show a metallic character. The Hubbard model can also be applied to rare earth metals and their compounds. The 4f orbitals are highly contracted in rare earth compounds (such as their oxides) with little or no overlap between them. However, for the early rare earths, e.g. cerium and its compounds, the 4f orbitals are more likely to overlap. Empirical rules for d orbital overlap in transition metal compounds have been proposed by Phillips and Williams [28] as follows: (i) the formal charge on the cations is small; (ii) the cation occurs early in the transition metal series; (iii) the cation is in the second or third transition metal series; (iv) the anion is reasonably electropositive.

Fig. 7. (a) Section through TiO structure, parallel to unit cell face showing Ti’+ positions only-overlap of d, orbitals on adjacent Ti*+ Ions, with similar overlap of d,, and d,: orbitals leads to i2, band. (b) Structure of NiO, showing d,,_,, orbitals pointing directly at oxide ions and therefore unable to overlap and form eR band

2371

(after Ref. [28]).

2378

DAVIES

et ul.:

SUPERPLASTICITY

It is interesting that yttria and zirconia, the two compounds associated with extensive superplastic deformation in structural ceramics, fall into the group of compounds covered by these empirical rules. Insulator-metal transitions in early transition/rare earth metal oxides and their temperature and pressure dependence have been discussed by Mott [14]. The metallic behaviour of materials that fit this category has been shown by the evident plasticity revealed in recent transmission electron microscopy observations of dislocation structures around Vickers indentations in a 9.4mol% Y,O, stabilised cubic ZrO, single crystal [29] at elevated temperatures, as shown in Fig. 8. It is known that dislocation loops can form in ceramics and other materials by processes such as the condensation of a super-saturated concentration of vacancies [30]. The dislocations observed in yttriastabilized cubic zirconia (quoted above) occurred under an indenter, i.e. under high local pressure. Materials frequently crack when an indenter is forced into them; the case of whether elastic strains generated in the vicinity of such cracks are relieved by the movement of dislocations or by crack extension relates to a classification of materials into ductile and brittle categories. The Peierls stress for dislocation motion in solids is relatively high for bound electron systems with directional bonding (e.g. insulators) and low for delocalized electron systems (e.g. metals) i.e. at the insulator-metal transition point a mixed mode or transitional behaviour occurs. Insulator-metal transitions are well documented in ceramics [14] and as stated by Cottrell [31], all elements in the periodic table are expected to be metallic when the right conditions exist, such as sufficiently high pressures; Cottrell [3 l] cites the example of hydrogen and iodine becoming metals at appropriate pressures. Some other conditions that could lead to insulator-metal transitions have been identified by Mott [14] and

IN CERAMICS

Gilman [lo]. It is important to remark that the metallization described above (under insulator-metal transition behaviour) is due to an overlap of the bonding and anti-bonding orbitals in materials under pressure rather than to a suppression of cracking and void growth. This effect is over and above that which could be contributed by the influence of hydrostatic compression on crack nucleation and growth. This issue has also been treated by Gilman [lo]. The good electrical conductivity found in the early rare earth and transition metal oxides is further evidence for the existence of metallic type bonding in these compounds. The compounds are referred to as mixed ionic and electronic conductors as discussed by Weber rt al. [22] and Kaneko et al. [32]. Recently, Reidy and Simkovich [33] made electrical conductivity measurements on ceria-stabilized zirconia and found relatively large electronic conduction compared with the ionic conduction component: an electron hopping mechanism is thought to dominate the electrical conduction in the ceria-doped zirconia at low oxygen partial pressures. The proposition regarding the electronic contribution to conduction is consistent with the arguments on orbital overlap and electron delocalization in the early rare earth and transition metal oxides. Contrary to a fairly commonly held opinion, the presence of a glassy grain boundary phase is not an absolute requirement for superplastic deformation in ceramics, although in certain conditions it may contribute to it. Sherby and Wadsworth [34] reporting on high resolution analytical transmission electron microscopy studies on yttria-stabilized tetragonal zirconia ceramics, stated that superplastic deformation could occur without the presence of a glassy grain boundary phase. Similarly, Lakki et al. [35] carried out creep tests in compression and internal friction measurements on yttria-stabilized tetragonal zirconia ceramics, and have stressed that in pure compositions

Fig. 8. Bright field transmission electron micrograph of dislocation structure 50 pm below the surface of a { Ill} surface indentation in cubic zirconia single crystal: Vickers hardness test carried out at 800°C using 4.9 N load (after Ref. [29]).

DAVIES et al.:

SUPERPLASTICITY

which do not contain an amorphous glassy grain boundary layer, the accommodation of grain boundary sliding (which is the main mechanism involved in superplastic deformation) could be explained in terms of a substantial increase in the grain boundary dislocation mobility; this is in agreement with transmission electron microscopy studies which revealed the presence of dislocations under an indenter (see Fig. 8). Further evidence for metallic deformation behaviour was found in the work of Davies et al. [36, 371 on superplastic deformation in ceramics, in which yttria-stabilized tetragonal zirconia polycrystals were found to deform with cavities aligned parallel to the tensile axis (see Fig. 9), a behaviour typical of superplastic deformation in metals. Ceramics tested in tension usually have their cavities aligned perpendicular to the tensile axis as shown in Fig. 9(b) for A&O, doped with CuO and MgO. It is deduced that, in the absence of a glassy grain boundary phase, a common feature of ceramic alloys (such as 3Y-TZP) and metallic alloys that show superplastic extension is the occurrence of cavities aligned in the tensile testing direction: there is currently no satisfactory explanation of this purely phenomenological observation. It is evident that the materials that exhibit a metallic behaviour during superplastic deformation do so owing to an intrinsic property of their bonding. An example of an “externally” induced metallic

2319

character in superplastic ceramics can be found in a magnesium aluminate spine1 (MgO.nAl,O,). Lappalainen et al. [38] recently observed that thin film tensile specimens of this spine1 failed at 3% tensile strain in a superplastic deformation test conducted at 1200°C and at a strain rate of 1 x 10m5 s-‘. However, when the magnesium aluminate spine1 was doped with platinum (platinum has a relatively high electron density at the boundaries of its Wigner-Seitz cell [39]), superplastic flow (of a type previously associated with yttria-stabilized tetragonal zirconia) was observed, including the presence of a serrated stress-strain curve, as shown in Fig. 10. Based on an earlier observation by Chiang and Kingery [40] that a space-charge segregation of intrinsic lattice defects occurs in the grain boundaries in the magnesium aluminate spine1 resulting in a net grain boundary charge, Lappalainen et al., [38] proposed that the possible existence of an attractive stress at the grain boundaries in MgO.nAl,O, spine1 (due to the electrical potential difference at the grain boundaries, relative to the interior of the grains) could be responsible for an increased grain boundary resistance to cavitation which would allow extensive superplastic flow. However, Lappalainen et al. stated that although they thought platinum could influence the development of a cavitation resistant stress at the grain boundaries, they could not identify the mechanism, since the results of Chiang and Kingery had previously indicated that the ratio of Al/Mg in the

Fig. 9. (a) Cavity alignment in a (3Y-TZP) yttria-stabilized (b) A&O, doped

IN CERAMICS

with CuO and MgO (ceramic-like):

tetragonal double

zirconia polycrystal (metal-like) arrows indicates tensile axis.

and

2380

DAVIES

8 wt.%

el al.:

SUPERPLASTICITY

IN CERAMICS

2:6

Pt/spinel

0.00 TRUE

mol.%

0.15

0.10

0.05

TRUE

STRAIN

Y,OJ--ZrO,

0.20

STRAIN

Fig. 10. Stress-strain curves for (a) a platinum doped magnesium aluminate spine], (b) yttria-stabilized zirconia (after Ref. [38]). (a) Grain size of specimens in range 35-90 nm--curves are serrated and exhibit no strain hardening; (b) grain size of specimens in range 70-290 nm--curves are very similar to those in (a).

grain boundary regions of MgO.nAl,O, spine1 is relatively insensitive to variations in stoichiometry and that pure MgO.nAl,O, is brittle. The present authors suspect that a metallization of the MgO.nAl,O, spine1 could occur in the presence of platinum (with a relatively high electron density), as described above. The possibility of electron transfer into antibonding levels in the spine1 is feasible according to recent proposals by Li [41] based on his studies of the wettability of ceramics by metals. Gilman’s recent association of the activation of dislocation glide in silicon with the tunnelling of electrons into antibonding levels by processes that include alloying would also seem to be consistent with metallization of the MgO.nAl,O, spine1 by the addition of platinum. It is suggested that the processes responsible for resistance to grain boundary crack growth in metals, such as dislocation glide activated processes which are known to contribute to superplastic flow in

Table 3. Experimental

data for fine grained

Material

Investigators Wakai ef al. [ZO] Nieh ef al. 1191 Hermansson [42] Wakai et al. [43]

Y.TZP + 20 wt% Al,O,

Nieh et al. [44]

Y .,TZP + 40 wt% AI,O,

Wakai

et al. [20]

Y.TZP + 60 wt% A&O,

Wakai

ef al. [20]

Y.TZP + 80 wt”/ AI,O,

Wakai

el al. (201

A&O, AI,O, A&O, ALO,

Gruffel Gruffel Gruffel Gruffel

er et rr er

+ + + +

MgO MgO + C&O, MgO + Y,O, Me0 + TLO,

al. al. al. al.

[22] (221 [22] 1221

become

(pm)

in

platinum

doped

On the other hand, insulator type ceramics, like MgO with a band gap energy of 7.3 eV, and AllO, with 8.3 eV [4], show limited superplastic deformation even when they meet other requirements for superplasticity, e.g. ultrafine grain size; this is further highlighted in Fig. 1. A similar reason will account for the limited superplasticity obtained in fine grained S&N, and SIC. It should be mentioned that SIC has a structure consisting of a layered tetrahedral stacking sequence, with strong covalently bonded tetrahedral units with minimal possibility for plasticity. Typical examples of ceramic materials that exhibit a metallic character in their superplastic behaviour compared with insulating ceramics are presented in Table 3. A simple comparison will illustrate the presence of the metallic character: whereas Nieh et rrl. obtained tensile elongations of up to 800% (Table 3)

size

0.34.4 - 0.3 - 0.3 Zr0,:0.5 AI,O,:0.5 zro,: 0.5 Al20,:0.5 Zr0,:0.51 AI,O,:0.61 zro,: 0.59 A1,0,:0.99 zro, :0.47 A&O,: 1.0 0.77-1.51 0.83 0.66 0.72

operative

Mg0.n Al,O, spinel.

ceramics 111 (strain rate hardening

Grain Y.TZP Y.TZP Y.TZP Y.TZP + 20 wt% Al,O,

metals,

m

exponent

rn e 0.5 for all materials)

Strain rate range (s_‘)

Temperature range (‘C)

Tensile elongation range (%)

- 0.5 1.1 l-5.56 x 10m4 -0.5 8.3 x IO ‘-2.7 x 10 ’ - 0.5 4.8 x 10~ ’ -0.5 1.11 x IO ~~I.11 x IO 3

580 580 580 620

l40& 1500 1500-1530 1450 135~l500

60-180 40&800 250 3G-220

-0.5

8.33 x IO ’

620

1650

500

-0.5

1.11-2.78 x 10m4

720

145&-1550

145-250

- 0.5

1.11-2.78 x IO-’

700

145&-1530

7&l 40

-0.5

1.11 -2.78 x 10m4

753

145&1550

60-110

400 400 400 400

1450 1450 1450 1250

38-54 55 65 25-m70

-0.5 1.2x IO 4 - 0.5 8.8 x IO ’ - 0.5 3.4 x IO 3 -0.5 5.5-1.2 x IO ’

DAVIES

et al.:

SUPERPLASTICITY

for yttria-stabilized zirconia, confirming its metallic character, A&O, + MgO (both with large band gap energies) would only give 38-54% tensile ductility at 1450°C; this is in agreement with our predictions and the positions in Fig. 1. It is noticeable (Table 3) that when yttria-stabilized zirconia is alloyed with 20, 40, 60 and 80 wt% -A&O, and tested in a narrow temperature range, there is a corresponding decrease in the total tensile elongation obtainable with increasing alumina content. Alternatively, the addition of Cr,O, and Y,O, to A&O, + MgO leads to slight increments in the tensile elongations obtained at 1450°C (Table 3) indicating the the selection of alloying compounds from the left-hand end of the transition metal series in the periodic table of elements contributes to an increase in the metallic character of the compounds. Verification of some of the data in Table 3 is shown in Figs 5 and 9 based on recent work in Manchester. Interestingly, improvements in the ductility and resistance to cavitation have been reported for A&O, doped with the early rare earth and transition metal oxides such as yttria, zirconia or hafnia [4246] which satisfy the rules proposed by Phillips and Williams [23] for orbital overlap, leading to metallic type bonding. Based on the concepts treated here, a partial metallization of the alumina is expected to occur. A combination of the metallization effect, which we propose, and the effects of interfacial non-stoichiometry proposed by Raj, would be expected to inhibit cavity nucleation and accommodate grain boundary sliding. 5. CONCLUSIONS

The superplastic flow observed in ceramics seems to be directly related to their electronic state. Extensive superplastic flow can be associated with the fact that some ceramics have a high concentration of defects at grain boundaries which could improve the fracture resistance of interfaces. Alternatively, some ceramics may be classified as having an intrinsic “metallic” character, e.g. yttria-stabilized tetragonal zirconia polycrystal, based either on the nature of the orbitals responsible for their bonding, or due to external influences (extrinsic) such as alloying, e.g. platinum doped MgO.nAl,O, spinel. The presence of the metallic character is likely to assist dislocation motion, which is active in the accommodation of grain boundary sliding and in promoting extensive deformation. REFERENCES 1. R. Raj, Mufer. Sci. Engng A166, 89 (1993). 2. W. J. Weber, H. L. Tuller, T. 0. Mason and A. N. Cormack, Mufer. Sci. Engng B18, 62 (1993). 3. T. J. Davies and A. A. Ogwu, Mater. Sci. Technol. 10, 669 (1994). 4. J. A. Duffy, Bonding, Energy Levels and Bands in Inorganic Solids, pp. 1699170. Longman, London (1990).

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