Availableonlineatwww.sciencedirect.com DIRECT E噩≈3S SEVIER Journal of the European Ceramic Society 25(2005)301-311 www.elsevier.com/locate/jeurceramsoc Mechanical and microstructural characterization of calcium aluminosilicate(CAS)and Sio2/Cas composites deformed at high temperature and high pressure Shaocheng Jia, d, * Erik Rybackib, Richard Wirth, Zhenting Jiang, Bin Xia d a departement des Genies Civil, Geologique et des Mines, Ecole Polytechnique de montreal, Montreal, Canada H3C 3A7 b GeoForschungsZentrum Potsdam, D-14473 Potsdam, germany e Department of Earth Sciences, University of Liverpool. Liverpool L69 3BX, Uk d laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry and South China Sea Institute of oceanography, Received 16 October 2003: received in revised form 18 February 2004; accepted 25 February 2004 Available online 21 July 2004 Abstract We performed axial compression experiments on polycrystalline calcium aluminosilicate(CAs or anorthite)and on particulate and layered composites with equal volume fractions of CAs and sio2(quartz) at a confining pressure of 300 MPa, temperatures of 1173-1473 K, and strain rates of 10-to 10-4s-I. The dense samples were fabricated from quartz crystalline and CAs glass powders by hot isostatic pressing (HIP). Under the experimental conditions, triclinic CAS, regardless in monolithic aggregates or composites, deforms by dislocation creep as indicated by TEM microstructures, intensive grain boundary migration recrystallization and strong crystallographic preferred orientation (CPO). Dislocation creep of CAs is characterized by dominant glide on a single slip system(0 10)[100] while mechanical twinning, anisotropic growth and recrystallization play an role to relieve the strain incompatibilities which would otherwise result from such limited slip systems. Particulate and particularly layered composites are significantly stronger than monolithic CAS aggregates, indicating that quartz is an effective reinforcement to the CAs matrix even when the material is used at high temperature and high pressure. Under layer-normal compression, the flow strength of layered composites increases remarkably with decreasing the thickness of the layers, and the thin-layered composites are significantly stronger than particulate counterparts with the same composition. The observed layering- induced stiffening is due to constraint effects of rigid quartz on plastic flow of CAs O2004 Elsevier Ltd. All rights reserved eywords: Composites; Mechanical properties; Hot isostatic pressing: Plasticity; Anorthite; SiO2 1. Introduction strain rates of 10-5 to 10-4s-I and a constant confining pressure of 300 MPa. Two main considerations on the merit In this paper we present our experimental results on the of this study should be mentioned in the following mechanical properties and microstructures of monolithic alcium aluminosilicate(CAs or anorthite: CaAl2Si3Og) aggregates, particulate and layered composites with equal (D)The CAS has been widely used as a matrix in fibre-or olume fractions of quartz(SiO2) and CAs, deformed in particle-reinforced ceramic composites that are excel axial compression(o1 >02=03>0, where o1, 02, 03 are lent prospective materials for application as mechanical the maximum, intermediate and least compressive princi- components in aerospace and automobile propulsion and pal stresses, respectively)at temperatures of 1173-1473K power systems. For better fabrication and application of such composites, it is essential to understand the rhe- ological properties, microstructures and textures of the author.Tel:+1-514-3404711x5134; CAS and various CAS-based ceramic composites de- fax:+1-514-3403970. formed under various conditions(T, P, flow strength and E-mail address: sji(apolymtl ca(. Ji) strain rate). Although a significant amount of work has 0955-2219/s-see front matter O 2004 Elsevier Ltd. All rights reserved doi: 10.1016/j jeurceramsoc 2004.02.018
Journal of the European Ceramic Society 25 (2005) 301–311 Mechanical and microstructural characterization of calcium aluminosilicate (CAS) and SiO2/CAS composites deformed at high temperature and high pressure Shaocheng Ji a,d,∗, Erik Rybacki b, Richard Wirth b, Zhenting Jiang c, Bin Xia d a Département des Génies Civil, Géologique et des Mines, École Polytechnique de Montréal, Montréal, Canada H3C 3A7 b GeoForschungsZentrum Potsdam, D-14473 Potsdam, Germany c Department of Earth Sciences, University of Liverpool, Liverpool L69 3BX, UK d Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry and South China Sea Institute of Oceanography, Chinese Academy of Sciences, Wushan, Guangzhou 510640, PR China Received 16 October 2003; received in revised form 18 February 2004; accepted 25 February 2004 Available online 21 July 2004 Abstract We performed axial compression experiments on polycrystalline calcium aluminosilicate (CAS or anorthite) and on particulate and layered composites with equal volume fractions of CAS and SiO2 (quartz) at a confining pressure of 300 MPa, temperatures of 1173–1473 K, and strain rates of 10−5 to 10−4 s−1. The dense samples were fabricated from quartz crystalline and CAS glass powders by hot isostatic pressing (HIP). Under the experimental conditions, triclinic CAS, regardless in monolithic aggregates or composites, deforms by dislocation creep as indicated by TEM microstructures, intensive grain boundary migration recrystallization and strong crystallographic preferred orientation (CPO). Dislocation creep of CAS is characterized by dominant glide on a single slip system (0 1 0)[1 0 0] while mechanical twinning, anisotropic growth and recrystallization play an role to relieve the strain incompatibilities which would otherwise result from such limited slip systems. Particulate and particularly layered composites are significantly stronger than monolithic CAS aggregates, indicating that quartz is an effective reinforcement to the CAS matrix even when the material is used at high temperature and high pressure. Under layer-normal compression, the flow strength of layered composites increases remarkably with decreasing the thickness of the layers, and the thin-layered composites are significantly stronger than particulate counterparts with the same composition. The observed layering-induced stiffening is due to constraint effects of rigid quartz on plastic flow of CAS. © 2004 Elsevier Ltd. All rights reserved. Keywords: Composites; Mechanical properties; Hot isostatic pressing; Plasticity; Anorthite; SiO2 1. Introduction In this paper we present our experimental results on the mechanical properties and microstructures of monolithic calcium aluminosilicate (CAS or anorthite: CaAl2Si3O8) aggregates, particulate and layered composites with equal volume fractions of quartz (SiO2) and CAS, deformed in axial compression (σ1 > σ2 = σ3 > 0, where σ1, σ2, σ3 are the maximum, intermediate and least compressive principal stresses, respectively) at temperatures of 1173–1473 K, ∗ Corresponding author. Tel.: +1-514-3404711x5134; fax: +1-514-3403970. E-mail address: sji@polymtl.ca (S. Ji). strain rates of 10−5 to 10−4 s−1 and a constant confining pressure of 300 MPa. Two main considerations on the merit of this study should be mentioned in the following: (1) The CAS has been widely used as a matrix in fibre- or particle-reinforced ceramic composites that are excellent prospective materials for application as mechanical components in aerospace and automobile propulsion and power systems.1–4 For better fabrication and application of such composites, it is essential to understand the rheological properties, microstructures and textures of the CAS and various CAS-based ceramic composites deformed under various conditions (T, P, flow strength and strain rate). Although a significant amount of work has 0955-2219/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2004.02.018
SJi et al. / Journal of the European Ceramic Society 25(2005)301-317 been carried out on monolithIc CAS and other plagio- clase feldspars under uniaxial compression(o1>02 o3=0)at ambient pressure, -ll it remains uncertain if the results from small strain creep tests are able to be apolated to other conditions because a small amount of strain is usually insufficient for steady-state microstruc ture to occur. 12 Cavitation often occurs in the sam- ples deformed at ambient pressure. -Furthermore the application of CAs-based ceramic composites to high temperature and high pressure environment was largely hinder ab aural any y little knowledge of me- CAS-based composites from laboratory tests. (2)CAS crystal is triclinic with the lowest crystallographic symmetry. The relative activity of different slip systems and dynamic recrystallization in dislocation creeping 目 CAS under varying physical and chemical conditions are still poorly known. The investigation of CAs mi- crostructure and texture can further our understanding of plastic deformation mechanism and textural devel- opment in triclinic crystalline materials. In spite of its importance, CAS has a much smaller textural database than other crystalline materials such as metals and min- erals(mainly olivine, quartz and calcite). Reasons for this are purely technical because CAs is triclinic crystallographic orientations of CAs cannot be deter- mined using conventional X-ray due to the large number of overlapping diffraction peaks. The texture of triclinic albite(NaAlSi3Os) has been measured by employing synchrotron X-ray goniometry, 7 but this technique is expensive and not widely available. Neutron diffraction goniometry has been applied to the measurements of ICAS plagioclase texture, 18,19 however, relatively large vol- umes of sample material (l cm' which is often larger 的数山m than most of samples deformed experimentally Fig 1. Photograph(a) and photo raph(b)of a hot isostatical pressed are needed for this type of measurements because layered Qtz-CAS composite neutron flux densities are generally low. CAS texture an be determined but its grain size should be larger than 20-30 um and and sharp interfaces(Fig. 1), which was created during cold the measurement is time-consuming. Recently, it was pressing and subsequently thinned during HIP. The layering shown that the most powerful technique for success- in cylindrical LC samples is characterized by the ratio of the fully measuring texture of triclinic crystals is electron diameter() to the thickness(h)of material layers. The PC backscattering diffraction(EBSD) equipped in a scan- is a homogeneous mixture of equal volume fraction of Qtz ning electron microscope(SEM).21-23 Thus, this new and CAS(Fig 2a) technique was used in collecting representative CAs Commercial powders of CAS(An98. Oro.2Abo. ) glass texture from our deformed samples (Schott GmbH, Germany) and quartz Johnson-Matthey GmbH, Germany) were used as the starting material The same Cas glass powder has been used in previous 2. Samples studies. 16,22 The CAS glass powder with particle size less than <60 um was first predried in an oven at a constant Four categories of samples were prepared, using hot iso- temperature of 393k for at least 100 h to drive off ad- atic pressing(HIP)techniques, for mechanical tests. They sorbed water. The powder was then encapsulated into a are layered composites (LC, Fig. 1), particulate composites steel jacket(= 15 mm, I= 25 mm) and cold-pressed (PC, Fig 2a) of quartz(Qtz)and CAs, and the pure CAs under an axial stress of about 150 MPa. Each cold-pressed (Fig 2b)and Qtz(Fig. 2c)polycrystalline aggregates. The pellet was HIPed and statistically annealed at 1123K for LC contains alternating Qtz and CAS layers with strong I h, 1323 K for I h and then 1473 K for 3 h at a confin-
302 S. Ji et al. / Journal of the European Ceramic Society 25 (2005) 301–311 been carried out on monolithic CAS and other plagioclase feldspars under uniaxial compression (σ1 > σ2 = σ3 = 0) at ambient pressure,5–11 it remains uncertain if the results from small strain creep tests are able to be extrapolated to other conditions because a small amount of strain is usually insufficient for steady-state microstructure to occur.12 Cavitation often occurs in the samples deformed at ambient pressure.5–11 Furthermore, the application of CAS-based ceramic composites to a high temperature and high pressure environment was largely hindered by the relatively little knowledge of mechanical, microstructural and textural data of CAS and CAS-based composites from laboratory tests.11,13–16 (2) CAS crystal is triclinic with the lowest crystallographic symmetry. The relative activity of different slip systems and dynamic recrystallization in dislocation creeping CAS under varying physical and chemical conditions are still poorly known. The investigation of CAS microstructure and texture can further our understanding of plastic deformation mechanism and textural development in triclinic crystalline materials. In spite of its importance, CAS has a much smaller textural database than other crystalline materials such as metals and minerals (mainly olivine, quartz and calcite). Reasons for this are purely technical because CAS is triclinic. Full crystallographic orientations of CAS cannot be determined using conventional X-ray due to the large number of overlapping diffraction peaks. The texture of triclinic albite (NaAlSi3O8) has been measured by employing synchrotron X-ray goniometry,17 but this technique is expensive and not widely available. Neutron diffraction goniometry has been applied to the measurements of plagioclase texture,18,19 however, relatively large volumes of sample material (>1 cm3 which is often larger than most of samples deformed experimentally8–11) are needed for this type of measurements because neutron flux densities are generally low. CAS texture can be determined using optical U-stage method18,20, but its grain size should be larger than 20–30m and the measurement is time-consuming. Recently, it was shown that the most powerful technique for successfully measuring texture of triclinic crystals is electron backscattering diffraction (EBSD) equipped in a scanning electron microscope (SEM).21–23 Thus, this new technique was used in collecting representative CAS texture from our deformed samples. 2. Samples Four categories of samples were prepared, using hot isostatic pressing (HIP) techniques, for mechanical tests. They are layered composites (LC, Fig. 1), particulate composites (PC, Fig. 2a) of quartz (Qtz) and CAS, and the pure CAS (Fig. 2b) and Qtz (Fig. 2c) polycrystalline aggregates. The LC contains alternating Qtz and CAS layers with strong Fig. 1. Photograph (a) and photomicrograph (b) of a hot isostatical pressed layered Qtz–CAS composite. and sharp interfaces (Fig. 1), which was created during cold pressing and subsequently thinned during HIP. The layering in cylindrical LC samples is characterized by the ratio of the diameter (d) to the thickness (h) of material layers. The PC is a homogeneous mixture of equal volume fraction of Qtz and CAS (Fig. 2a). Commercial powders of CAS (An98.9Or0.2Ab0.9) glass (Schott GmbH, Germany) and quartz (Johnson-Matthey GmbH, Germany) were used as the starting materials. The same CAS glass powder has been used in previous studies.16,22 The CAS glass powder with particle size less than <60m was first predried in an oven at a constant temperature of 393 K for at least 100 h to drive off adsorbed water. The powder was then encapsulated into a steel jacket (φ = 15 mm, l = 25 mm) and cold-pressed under an axial stress of about 150 MPa. Each cold-pressed pellet was HIPed and statistically annealed at 1123 K for 1 h, 1323 K for 1 h and then 1473 K for 3 h at a confin-
S. i et al. / Journal of the European Ceramic Society 25(2005)301-311 ate composite (50%Qtz, 50% CAS) J12-QA N=200 Mean grain size: 45.3 om Grain size J12-QA Mean aspect ratio: 2.0 distribution of quartz in a hot pressed particulate com- posite (J1 onsisting of 50 vol. Qtz and 50 vol %CAS. Measure- ments w aggregates and Qtz-CAs particulate composites, 3-5%in the layered composites and 5-6% in the monolithic Qtz aggregates. From two typical HIPed samples from each category, sev eral polished thin sections were made for characterizing grain size, microstructure and water content of undeformed Fig. 2. Hot isostatical pressed but undeformed Qtz-CAS particulate com materials using optical microscope, SEM, transmission elec posite (a), pure CAS aggregate(b) and pure quartz (Qtz) aggregate(c) tron microscope(TEM), Fourier transform infrared spec- (a and c)Optical micrograph of petrographic thin sections. (b)SEM m trometer(FTIR)and EBSD analyses ograph of a spherulite-free area from a polished and thermally etched (1373K, 30h)sample (1)Grain sizes of Qtz and CAs in the Hiped sample were measured using the linear intercept method24 from petrographic sections and SEM(Zeiss DSM 962, ing pressure of 300 MPa to maximize the densification GFZ-Potsdam, Germany) photographs of polished sec- and to allow the polycrystalline aggregate starting toward tions, respectively. Qtz displays a normal distribution microstructural equilibrium. After samples were retrieved of grain size ranging from 15 to 80 um with a mean from the vessel, the steel jacket was dissolved in a mixture size of 45 um(Fig. 3a). The distribution of Qtz grain of 50/50 vol. HCI/HNO3 acids. Density of each specimen aspect ratios is shown in Fig. 3b, yielding a mean value as determined using Archimedes' method with the accu- of 2.0. The CAS crystals, with a mean aspect ratio of racy of +0.003 g/em. Porosity is <1% in the HIPed CAS 2.2(Fig. 4b), display a log-normal distribution of grain
S. Ji et al. / Journal of the European Ceramic Society 25 (2005) 301–311 303 Fig. 2. Hot isostatical pressed but undeformed Qtz–CAS particulate composite (a), pure CAS aggregate (b) and pure quartz (Qtz) aggregate (c). (a and c) Optical micrograph of petrographic thin sections. (b) SEM micrograph of a spherulite-free area from a polished and thermally etched (1373 K, 30 h) sample. ing pressure of 300 MPa to maximize the densification and to allow the polycrystalline aggregate starting toward microstructural equilibrium. After samples were retrieved from the vessel, the steel jacket was dissolved in a mixture of 50/50 vol.% HCl/HNO3 acids. Density of each specimen was determined using Archimedes’ method with the accuracy of ±0.003 g/cm3. Porosity is <1% in the HIPed CAS Hot-pressed particulate composite (50% Qtz, 50% CAS) 0 10 20 30 40 50 60 Grain size, m Number of measurements 0 10 20 30 40 50 60 70 80 90 100 N=200 Mean grain size: 45.3 ∝m J12-QA Qtz 0 10 20 30 40 50 Aspect ratio Number of measurements 0 1.0 2.0 3.0 4.0 5.0 6.0 N=200 Mean aspect ratio: 2.0 J12-QA Qtz (a) (b) Fig. 3. Grain size distribution of quartz in a hot pressed particulate composite (J12-QA) consisting of 50 vol.% Qtz and 50 vol.% CAS. Measurements were made from optical photomicrographs. aggregates and Qtz–CAS particulate composites, 3–5% in the layered composites and 5–6% in the monolithic Qtz aggregates. From two typical HIPed samples from each category, several polished thin sections were made for characterizing grain size, microstructure and water content of undeformed materials using optical microscope, SEM, transmission electron microscope (TEM), Fourier transform infrared spectrometer (FTIR) and EBSD analyses. (1) Grain sizes of Qtz and CAS in the HIPed samples were measured using the linear intercept method24 from petrographic sections and SEM (Zeiss DSM 962, GFZ-Potsdam, Germany) photographs of polished sections, respectively. Qtz displays a normal distribution of grain size ranging from 15 to 80 m with a mean size of 45m (Fig. 3a). The distribution of Qtz grain aspect ratios is shown in Fig. 3b, yielding a mean value of 2.0. The CAS crystals, with a mean aspect ratio of 2.2 (Fig. 4b), display a log-normal distribution of grain
S Ji et al. /Journal of the European Ceramic Society 25(2005)301-317 Hot-pressed particulate composite 001 [100] (50%Qt,50%CAS J12-QA N=300 Mean grain size: 2.1 om L。ower L001] [100 Fig. 5. Preferred orientations of triclinic CAS(0 10)[100] and [001] Grain size, dm for undeformed, hot isostatical pressed, pure CAS aggregate(sample J7) Notice that the whole sphere, rather than a hemisphere, is necessary to J12-QA represent the distribution of the positive directions. Projections on the lower(a)and upper(b) hemispheres. Stereonets are equal-area plots: 130 measurements are used N=300 Mean aspect ratio: 2.2 (3)TEM(Philips CM200, GFZ-Potsdam, Germany)op- erating at 200 kV shows that the grain boundaries in the CAs aggregates are coherent and high-angle ones They are straight and clean, suggesting that the crystal lization and compaction of samples were well done16 Very little melt(<<0.5%)were found to occur in the triple- junctions. CAS grains in the HIPed samples are characterized by closely spaced growth twin lamellae with low dislocation densities(10 m-).The twins Aspect ratio have their composition planes parallel to(0 10) and are Fig. 4. Grain size distribution of CAs in hot pressed particulate com mainly albite, Carlsbad and Carlsbad-albite types. The posite(sample J12-QA)consisting of 50 vol. Qtz and 50 vol. CAS). ge fibres in CAS spherulites are actually composed Measurements were made from SEM micrographs. of very small grains (4) EBSD patterns of CAs and Qtz were measured and indexed using a SEM(Philips XL30)at Liverpool University, and the software package Channel + from size ranging from 0. 4 to 9 um with a mean value of HKL Software Company. The patterns were recorded at 2. 1 um(Fig 4a) 30kV acceleration voltage and nominal beam currents (2)Both optical and sEM observations show that Qtz grains of 80 HA. No carbon coat was used on the thin sec- in the PC aggregates form almost rigid clasts dispersed tions, which were chemically-mechanically polished to homogeneously within a relatively continuous matrix of emove specimen surface damage, because the coat de- CAS(Fig 2a). Spherulites with radial fibres of CAs(not teriorated the eBsd image quality. In most cases, more shown in Fig. 2)are occasionally observed in the pure than five or six bands were detected, allowing the bands CAS aggregates and the CAs layers of laminated com- indexed unambiguously by the computer simulation posites. In the spherulites, CAS fibres are generally tab- The measurement uncertainty was given by the software ular on 010) with an elongation mainly along [001 as a mean angular deviation(MAD) between detected and to a lesser extent along [100]. However, no CAs bands and simulated patterns. The indexing was not spherulites occur in particulate composites(Fig. 2a) accepted if the MAd value was larger than 2. EBSD It is generally accepted that spherulite texture results measurements showed a random crystallographic pre where the rate of crystal growth exceeds that of crystal ferred orientation(CPO)of either CAS(Fig. 5)or Qtz in nucleation2-27. The spherulite is a typical texture for HIPed samples, as expected for hydrostatic conditions crystallization of Cas glass that generally starts from a (5)FTIR measurements using a Bruker IFS-66v(GFZ nucleation centre where the water content is relatively Potsdam, Germany) show that the HIPed samples have high. The volume fraction of spherulites in CAS aggre a water content ranging from 8000 to 20.000 H/106 gates is about 10% on average with an average value of 13,000 H/10oSi( 0.08 wt %
304 S. Ji et al. / Journal of the European Ceramic Society 25 (2005) 301–311 Hot-pressed particulate composite (50% Qtz, 50% CAS) 0 20 40 60 80 100 Grain size, m Number of measurements 0 1 2 3 4 5 6 7 8 9 10 N=300 Mean grain size: 2.1 ∝m CAS J12-QA 0 10 20 30 40 50 60 Aspect ratio Number of measurements 0 1.0 2.0 3.0 4.0 5.0 6.0 N=300 Mean aspect ratio: 2.2 J12-QA CAS (a) ∝ (b) Fig. 4. Grain size distribution of CAS in hot pressed particulate composite (sample J12-QA) consisting of 50 vol.% Qtz and 50 vol.% CAS). Measurements were made from SEM photomicrographs. size ranging from 0.4 to 9 m with a mean value of 2.1m (Fig. 4a). (2) Both optical and SEM observations show that Qtz grains in the PC aggregates form almost rigid clasts dispersed homogeneously within a relatively continuous matrix of CAS (Fig. 2a). Spherulites with radial fibres of CAS (not shown in Fig. 2) are occasionally observed in the pure CAS aggregates and the CAS layers of laminated composites. In the spherulites, CAS fibres are generally tabular on {010} with an elongation mainly along [0 0 1] and to a lesser extent along [1 0 0]. However, no CAS spherulites occur in particulate composites (Fig. 2a). It is generally accepted that spherulite texture results where the rate of crystal growth exceeds that of crystal nucleation25–27. The spherulite is a typical texture for crystallization of CAS glass that generally starts from a nucleation centre where the water content is relatively high. The volume fraction of spherulites in CAS aggregates is about 10% on average. Fig. 5. Preferred orientations of triclinic CAS (0 1 0), [1 0 0] and [0 0 1] for undeformed, hot isostatical pressed, pure CAS aggregate (sample J7). Notice that the whole sphere, rather than a hemisphere, is necessary to represent the distribution of the positive directions. Projections on the lower (a) and upper (b) hemispheres. Stereonets are equal-area plots; 130 measurements are used. (3) TEM (Philips CM200, GFZ-Potsdam, Germany) operating at 200 kV shows that the grain boundaries in the CAS aggregates are coherent and high-angle ones. They are straight and clean, suggesting that the crystallization and compaction of samples were well done.16 Very little melt (<<0.5%) were found to occur in the triple-junctions. CAS grains in the HIPed samples are characterized by closely spaced growth twin lamellae with low dislocation densities (∼1011 m−2). The twins have their composition planes parallel to (0 1 0) and are mainly albite, Carlsbad and Carlsbad-albite types. The large fibres in CAS spherulites are actually composed of very small grains.22 (4) EBSD patterns of CAS and Qtz were measured and indexed using a SEM (Philips XL30) at Liverpool University, and the software package Channel + from HKL Software Company. The patterns were recorded at 30 kV acceleration voltage and nominal beam currents of 80A. No carbon coat was used on the thin sections, which were chemically–mechanically polished to remove specimen surface damage, because the coat deteriorated the EBSD image quality. In most cases, more than five or six bands were detected, allowing the bands indexed unambiguously by the computer simulation. The measurement uncertainty was given by the software as a mean angular deviation (MAD) between detected bands and simulated patterns. The indexing was not accepted if the MAD value was larger than 2◦. EBSD measurements showed a random crystallographic preferred orientation (CPO) of either CAS (Fig. 5) or Qtz in HIPed samples, as expected for hydrostatic conditions. (5) FTIR measurements using a Bruker IFS-66v (GFZPotsdam, Germany) show that the HIPed samples have a water content ranging from 8000 to 20,000 H/106Si with an average value of 13,000 H/106Si (∼0.08 wt.%)
S. i et al. / Journal of the European Ceramic Society 25(2005)301-311 No significant difference in water content of samples Qtz aggregates before and after experimental deformation, indicat- 1000 P=300 MPa ing no detected loss of water species such as hy Strain rate =10-/s drogen through the Fe jacket during the mechanical tests. 6 If the water content were higher than 0.5 wt% creep mechanism in fine-grained feldspar / s ate as a solution-precipitation processes might oper 巴巴 3. Mechanical data All axial compressive tests(oI >02= 03>0)were 0.10 performed at 300 MPa confining pressure in a Paterson-type gas-medium apparatus(GFZ-Potsdam, Germany). Temper- ature varied from 1173 to 1473K and axial strain rate from Fig. 7. Stress-strain curves for pure quartz aggregates deformed under 10- to 10-s. Cylindrical specimens of 10 mm diameter at a confining pressure of 300 MPa, a constant strain nd temperatures of 1373 and 1473 K. Note that the and 20 mm length, fabricated from HIP, were jacketed in iron quartz could not yield at 1373K or lower temperatures under with 0.23 mm thick wall. The tests were carried out at con- the expe conditions stant strain rates. In this case, the sample flow strength cor- responds to a differential stress of magnitude(o=01-o3) hich is superimposed upon a state of hydrostatic stress or 300 MPa at temperatures from 1273 to 1473K. The shape confining (o2=03). In other words, a differential of the stress-strain curves is characterized by an initial rapid stress(o) is the difference between the maximum and min- strength increase followed by a slow strain hardening. At an imum compressive principal stresses(o1 -o3). Thus it is axial strain of 0.25, the CAs polycrystal has flow strength always a positive scalar quantity. The axial compression is of 16.6, 60.8 and 115.4 MPa at 1473, 1373 and 1273K, re- frequently used in laboratory experiments on the high tem- spectively rocks. Differential stresses and axial strains were derived, porosity up to 5-6%, do not yield at 1273 and 1373K at respectively, from measured loads and displacements after the conditions of confining pressure 300 MPa and strain correcting for the load supported by the Fe jacket, rig distor- rate 10-5s-l(Fig. 7). Even at a temperature as high as tion and change in sample cross-sectional area and length. 1473 K, the quartz aggregate still has its strength higher than The uncertainty in stress measurements is estimated within 600 MPa. Under the same conditions(1473 K, 300 MPa and regates from the sequence of axial compression tests at CAS ("soft"component). It is ern ("hardo, thus a clor t5 MPa. Temperature control was +3 K along the gauge 10-3s-), quartz is stronger than CAS by a factor of 40 length of specimens (E=0.15)to 51(E =0.05). There is thus a large Fig 6 shows differential stress-strain curves for CAS ag- ological contrast between quartz ("hard"component)and a constant rate of 10-s and a confining pressure of hard quartz into a soft matrix of Cas should produce sig- nificant effects on improving the mechanical properties of CAS-matrix ceramic composite Fig 8 displays typical stress-strain curves for particulate CAS aggregates composite that is a homogeneous mixture of equal volume Strain rate 10-s fractions of Qtz and CAs. Three aspects of the mechanical P=300 MPa data are striking: (i) Steady-state flow is only att in sample J9 which deformed at 1473 K and 10-5s-l.(i) J31(1273K a drastic drop in stress occurs immediately after a strength peak at a strain of approximately 0.04 for sample J26 that J5(1373K deformed at 1373 K and 10-s-I. The abrupt decrease in the level of stress supported from 389 MPa at E=0.04 to 160 MPa at e=0.29 is due to strong strain localization into a J38(1473K semi-brittle shear zone aligned at about 300 to the maximum compression stress(o1).(iii)st rain so 0.000.050.10 and 1.0 x 10-to 2.5 10-s. For example, the strength Fig. 6. Stress-strain curves for pure CAS ag 9Binuau-20 0 25 0.30 .3s peak takes place in all samples deformed at 1173-1373K of sample J25, deformed at 1373 K and 2.5 x 10->s-is ompression at a confining pres nstant strain rate o 59,214,183andl54 MPa at o.10,0.35,0.50and0.65 10-Ss-I and temperatures of 1273, 1373 and 1473K strain, respectively. From 0. 10 to 0.65 shortening strain, the
S. Ji et al. / Journal of the European Ceramic Society 25 (2005) 301–311 305 No significant difference in water content of samples before and after experimental deformation, indicating no detected loss of water species such as hydrogen through the Fe jacket during the mechanical tests.16 If the water content were higher than 0.5 wt.%, solution-precipitation processes might operate as a creep mechanism in fine-grained feldspar.15 3. Mechanical data All axial compressive tests (σ1 > σ2 = σ3 > 0) were performed at 300 MPa confining pressure in a Paterson-type gas-medium apparatus (GFZ-Potsdam, Germany). Temperature varied from 1173 to 1473 K and axial strain rate from 10−5 to 10−4 s−1. Cylindrical specimens of 10 mm diameter and 20 mm length, fabricated from HIP, were jacketed in iron with 0.23 mm thick wall. The tests were carried out at constant strain rates. In this case, the sample flow strength corresponds to a differential stress of magnitude (σ = σ1 − σ3) which is superimposed upon a state of hydrostatic stress or confining pressure (σ2 = σ3). In other words, a differential stress (σ) is the difference between the maximum and minimum compressive principal stresses (σ1 − σ3). Thus it is always a positive scalar quantity. The axial compression is frequently used in laboratory experiments on the high temperature, high pressure properties of materials, minerals and rocks. Differential stresses and axial strains were derived, respectively, from measured loads and displacements after correcting for the load supported by the Fe jacket, rig distortion and change in sample cross-sectional area and length. The uncertainty in stress measurements is estimated within ±5 MPa. Temperature control was ±3 K along the gauge length of specimens. Fig. 6 shows differential stress–strain curves for CAS aggregates from the sequence of axial compression tests at a constant rate of 10−5 s−1 and a confining pressure of 0 40 80 120 160 200 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 Axial strain Differential stress, MPa J31 (1273 K) CAS aggregates Strain rate = 10-5 /s P = 300 MPa J5 (1373 K) J38 (1473 K) Fig. 6. Stress–strain curves for pure CAS aggregates deformed under axial compression at a confining pressure of 300 MPa, a constant strain rate of 10−5 s−1 and temperatures of 1273, 1373 and 1473 K. 0 200 400 600 800 1000 0.00 0.05 0.10 0.15 0.20 0.25 Axial strain Differential stress, MPa Qtz aggregates P = 300 MPa Strain rate = 10-5 /s J39 (1473 K) J20 (1373 K) Fig. 7. Stress–strain curves for pure quartz aggregates deformed under axial compression at a confining pressure of 300 MPa, a constant strain rate of 10−5 s−1 and temperatures of 1373 and 1473 K. Note that the quartz aggregate could not yield at 1373 K or lower temperatures under the experimental conditions. 300 MPa at temperatures from 1273 to 1473 K. The shape of the stress–strain curves is characterized by an initial rapid strength increase followed by a slow strain hardening. At an axial strain of 0.25, the CAS polycrystal has flow strength of 16.6, 60.8 and 115.4 MPa at 1473, 1373 and 1273 K, respectively. The polycrystalline aggregates of quartz, in spite of its porosity up to 5–6%, do not yield at 1273 and 1373 K at the conditions of confining pressure 300 MPa and strain rate 10−5 s−1 (Fig. 7). Even at a temperature as high as 1473 K, the quartz aggregate still has its strength higher than 600 MPa. Under the same conditions (1473 K, 300 MPa and 10−5 s−1), quartz is stronger than CAS by a factor of 40 (ε = 0.15) to 51 (ε = 0.05). There is thus a large rheological contrast between quartz (“hard” component) and CAS (“soft” component). It is expected that the addition of hard quartz into a soft matrix of CAS should produce significant effects on improving the mechanical properties of CAS–matrix ceramic composites. Fig. 8 displays typical stress–strain curves for particulate composite that is a homogeneous mixture of equal volume fractions of Qtz and CAS. Three aspects of the mechanical data are striking: (i) Steady-state flow is only attained only in sample J9 which deformed at 1473 K and 10−5 s−1. (ii) A drastic drop in stress occurs immediately after a strength peak at a strain of approximately 0.04 for sample J26 that deformed at 1373 K and 10−4 s−1. The abrupt decrease in the level of stress supported from 389 MPa at ε = 0.04 to 160 MPa at ε = 0.29 is due to strong strain localization into a semi-brittle shear zone aligned at about 30◦ to the maximum compression stress (σ1). (iii) Strain softening after a strength peak takes place in all samples deformed at 1173–1373 K and 1.0 × 10−5 to 2.5 × 10−5 s−1. For example, the strength of sample J25, deformed at 1373 K and 2.5 × 10−5 s−1, is 259, 214, 183 and 154 MPa at 0.10, 0.35, 0.50 and 0.65 strain, respectively. From 0.10 to 0.65 shortening strain, the