SEI TERIALS ENGE& ENGIEERN ELSEVIER Materials Science and Engineering A300(2001)68-79 www.elsevier.com/locate/msea High-temperature creep of a bi-directional continuous-SiC-fiber-reinforced glass-ceramic composite B.G. Nair a, *, R F. Cooper a, M.E. Plesha b Materials Science Program, Unirersity of Wisconsin-Madison Madison, WI 53706, US.A Department of Nuclear Engineering and Engineering Physics, Unirersity of wisconsin-Madison Madison, WI 53706, US.A Received 9 May 2000: received in revised form 6 September 2000 Abstract The ' off-axis, high-temperature compression creep behavior of bidirectionally(2D, 0/90) reinforced CAS-l/SiC(Nicalon fiber) composites was studied experimentally in the stress-temperature regime of 1275-1325.C and 15-50 MPa. The results indicated that the overall, high-T rheologic response of the 2D composites was intermediate to the properties of ID composites with fiber orientations corresponding to the constituent plies in the 2D material. This behavior strongly suggested that the 2D material behaved as an isostrain laminate during creep. A simple analysis, treating the 2D material as a three-phase laminate, where the constituent plies were assigned the viscoelastic properties of the corresponding ID materials and separated by thin layers of unreinforced matrix, fit the experimental data. In the case of 2D composites with the plies misoriented at 20 and 70%to the applied stress(20/-700composites), however, microstructural study suggested that growth of cracks in directions perpendic ular to the applied stress due to the poisson effect would have made a significant contribution to the bulk strain. Hence, such crack growth acts as a limitation to the universal applicability of the laminate model. c 2001 Elsevier Science B v. All rights Keywords: Ceramic composite: Fiber-reinforced; Bidirectional; Creep; Modeling: Laminate 1. Introduction limited by their creep, fracture and fatigue properties at high temperatures. Multidirectional fiber reinforcement is often sug While fracture properties of 2D ceramic composites gested as a possible solution to the anisotropic mechan- have been studied in some detail [1, 2 ], high-temperature observe creep is not commonly considered as a limiting factor (D) ceramic composites. Conventional wisdom sug for their applicability. This is primarily due to a ten- gests that a simple, multi-ply, symmetrical, bidirection- to overestimate the creep performance of 2D ally reinforced (2D, 0/90)composite would exhibit composites based on experiments conducted in ge superior, near-isotropic, inplane mechanical properties ometries that maximize the creepresistance offered by as compared to ID composites. However, a detailed the reinforcing fibers, e.g. tensile creep experiments with investigation of the mechanical properties of such 2D the direction of the applied stress parallel to one set of composites as a function of misorientation of the ap- reinforcing fibers 3-5] or flexural creep experiments plied load with respect to the fiber reinforcement has where one set of fibers are parallel to the direction of not been undertaken. At the typical operating condi- the maximum principal stress [6-8]. In these loading tions of high temperature and low differential stress. geometries, a large portion of the load is transferred to the possible application of 2D composite components is the fibers and so the steady-state creep response of the composite strain rates result(typically less than 10-8 Corresponding author. present address: Energ y Tecnolgy Divi: s-). Our work on the creep of unidirectionally rein- +1-630-2524193;fax:+1-630-2523604 forced (ID) composites [9-11] suggests, however, that E-mail address: bgnair@pop.et anl. gov(B.G. Nair ). the 'off-axis' geometry might be the real limiti 0921-5093/01/s- see front matter o 2001 Elsevier Science B.V. All rights reserved PI:S0921-509300)01778-0
Materials Science and Engineering A300 (2001) 68–79 High-temperature creep of a bi-directional, continuous-SiC-fiber-reinforced glass-ceramic composite B.G. Nair a,*, R.F. Cooper a , M.E. Plesha b a Materials Science Program, Uni6ersity of Wisconsin-Madison Madison, WI 53706, USA b Department of Nuclear Engineering and Engineering Physics, Uni6ersity of Wisconsin-Madison Madison, WI 53706, USA Received 9 May 2000; received in revised form 6 September 2000 Abstract The ‘off-axis’, high-temperature compression creep behavior of bidirectionally (2D, 0/90°) reinforced CAS–II/SiC (Nicalon® fiber) composites was studied experimentally in the stress–temperature regime of 1275–1325°C and 15–50 MPa. The results indicated that the overall, high-T rheologic response of the 2D composites was intermediate to the properties of 1D composites with fiber orientations corresponding to the constituent plies in the 2D material. This behavior strongly suggested that the 2D material behaved as an isostrain laminate during creep. A simple analysis, treating the 2D material as a three-phase laminate, where the constituent plies were assigned the viscoelastic properties of the corresponding 1D materials and separated by thin layers of unreinforced matrix, fit the experimental data. In the case of 2D composites with the plies misoriented at 20 and 70° to the applied stress (20/–70° composites), however, microstructural study suggested that growth of cracks in directions perpendicular to the applied stress due to the Poisson effect would have made a significant contribution to the bulk strain. Hence, such crack growth acts as a limitation to the universal applicability of the laminate model. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Ceramic composite; Fiber-reinforced; Bidirectional; Creep; Modeling; Laminate www.elsevier.com/locate/msea 1. Introduction Multidirectional fiber reinforcement is often suggested as a possible solution to the anisotropic mechanical properties observed in unidirectionally reinforced (1D) ceramic composites. Conventional wisdom suggests that a simple, multi-ply, symmetrical, bidirectionally reinforced (2D, 0/90°) composite would exhibit superior, near-isotropic, inplane mechanical properties as compared to 1D composites. However, a detailed investigation of the mechanical properties of such 2D composites as a function of misorientation of the applied load with respect to the fiber reinforcement has not been undertaken. At the typical operating conditions of high temperature and low differential stress, the possible application of 2D composite components is limited by their creep, fracture and fatigue properties at high temperatures. While fracture properties of 2D ceramic composites have been studied in some detail [1,2], high-temperature creep is not commonly considered as a limiting factor for their applicability. This is primarily due to a tendency to overestimate the creep performance of 2D composites based on experiments conducted in geometries that maximize the creepresistance offered by the reinforcing fibers, e.g. tensile creep experiments with the direction of the applied stress parallel to one set of reinforcing fibers [3–5] or flexural creep experiments where one set of fibers are parallel to the direction of the maximum principal stress [6–8]. In these loading geometries, a large portion of the load is transferred to the fibers and so the steady-state creep response of the composite is rate-limited by creep of the fibers low composite strain rates result (typically less than 10−8 s−1 ). Our work on the creep of unidirectionally reinforced (1D) composites [9–11] suggests, however, that the ‘off-axis’ geometry might be the real limiting case * Corresponding author. Present address: Energy Technology Division, Argonne National Laboratory, Argonne, IL 60439, USA. Tel.: +1-630-2524193; fax: +1-630-2523604. E-mail address: bgnair@pop.et.anl.gov (B.G. Nair). 0921-5093/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S0921-5093(00)01778-0
B G. Nair et al. /Materials Science and Engineering 4300(2001)68-79 for such 2D composites. In ID ceramic matrix com- tial transient due to load transfer from the matrix to the posites with a distinctly thin(<O I um) viscoelastic fibers, leading to steady-state creep that is rate limited nterface separating the fiber and matrix, the loading by flow of the fibers. As o increases, the shear forces at geometry that produces the highest strain-rate is one the fiber-matrix interface increase, these result in a hat optimizes slip at the interface, i.e. that have fibers substantial contribution to the bulk composite strain by oriented at approximately 45 to the applied stress, or. interface sliding (or, depending on the composite sys- Thus, it is of fundamental interest to extend the study tem, interphase filow). The contribution due to this of fiber-orientation effects to the creep deformation of relative displacement at the interface is maximized at 2D composites and so investigate the feasibility of 45. However, geometrical constraints dictate that modeling high-T creep of these materials the matrix would still be rate-limiting as slip at the The specific rheologic response of the individual plies interphase must be accommodated by matrix shear in 2D composites depends on the stress-distribution flow. Further increase in op results in a high plasticity developed in the composite at steady state. Thus, the e.g. von Mises) potential le matrix around the recognition of a suitable paradigm that describes the fibers, leading to matrix flow around the fibers. This is elastic flow of 2D composites is predicated on under- accompanied by cavitation at the fiber-matrix interface standing how the stresses are partitioned between the due to the development of tensile tractions normal to various plies. This line of thought leads to the question the interface of whether a 2D composite could behave as a simple The present work focuses on characterizing the hi twophase laminate with each ply behaving as a thin temperature, low-differential-stress rheology of 2D(O/ section of a ID composite. In the remainder of this 90, cross-ply) laminates in off-axis loading geometries paper, any reference to 2D composites should be con- and comparing/contrasting their response with the case sidered to be meant for composites with 0/90o fiber for ID reinforcement. As a necessary requirement for reinforcement unless otherwise mentioned 2D composite specimens for the creep study under composites with an identical matrix composition. Be- taken here are designed such that the applied stress, a1 cause, the matrix chemical composition, phase distribu is parallel to the planes defined by the fiber directions in tion and morphology(and hence it is viscoelastic both sets of plies. These 2D specimens can be charac rheology) were different from that of the composites terized by a misorientation angle, y, which is the acute used in our previous experimental work [9], these ID angle between the direction of the applied stress and the composite baseline experiments also provided addi- set of plies is thus misoriented at(y-90%) from the oped earli cation to the Id rheological model devel- orientation of any one set of fibers(Fig. la); the other loading direction. Similarly, in ID composites, off-axis geometry can be characterized by a misorientation an- 2. Experimental design and procedure gle, o (Fig. 1b). Characterizing flow in 2D composites requires an analysis of their behavior in the context of low-well-understood creep behavior of ID composites; 2.1. Material specifications possible that fiber orientations in the 2D material The materials used in this study were Sic fiber orrespond behaviorally with ID material, i.e. with =vand90°一ψ. For ID composites with~0°,the Nicalon)-reinforced calcium aluminosilicate com- rheologic response is characterized by a significant ini posites fabricated and supplied by Coming, Inc. Both the bidirectionally reinforced(2D) and unidirectionally reinforced(ID)composite sheets were fabricated by uniaxial hot-pressing of prepreg plies at 1350C and 90°v 15 MPa [12]. Both ID and 2D materials consisted of 16 plies, each with a fiber volume fraction, Vr, of 0.3 the 2D composite had a [o/90s(sy D)lay-up The matrix was an anorthite-based glassceramic(Com- ing Code CAS-Il) with a grain-size of 3 um. The estimated oxide composition of the CAS-II matrix based on X-ray fluorescence spectroscopy (XRAL Labs, Ont., Canada)is shown in Table 1. X-ray diffrac tion of powder specimens indicated that the primary phases were anorthite (Cao: Al,O3: 2SiO2) and mullite (ALO3: 2SiO2 ): a calculation based on peak intensities Fig.1.Specimen geometry for off-axis'compression creep exp: indicated 85% anorthite and 11% mullite by weight ments.(a) 2D composites: The outer ply ()is cut away to reveal its complementary ply (D);(b) ID composites Electron-microprobe analysis(Cameca SX51) showed
B.G. Nair et al. / Materials Science and Engineering A300 (2001) 68–79 69 for such 2D composites. In 1D ceramic matrix composites with a distinctly thin (B0.1 mm) viscoelastic interface separating the fiber and matrix, the loading geometry that produces the highest strain-rate is one that optimizes slip at the interface, i.e. that have fibers oriented at approximately 45° to the applied stress, s1. Thus, it is of fundamental interest to extend the study of fiber-orientation effects to the creep deformation of 2D composites and so investigate the feasibility of modeling high-T creep of these materials. The specific rheologic response of the individual plies in 2D composites depends on the stress-distribution developed in the composite at steady state. Thus, the recognition of a suitable paradigm that describes the inelastic flow of 2D composites is predicated on understanding how the stresses are partitioned between the various plies. This line of thought leads to the question of whether a 2D composite could behave as a simple twophase laminate with each ply behaving as a thin section of a 1D composite. In the remainder of this paper, any reference to 2D composites should be considered to be meant for composites with 0/90° fiber reinforcement unless otherwise mentioned. 2D composite specimens for the creep study undertaken here are designed such that the applied stress, s1 is parallel to the planes defined by the fiber directions in both sets of plies. These 2D specimens can be characterized by a misorientation angle, c, which is the acute angle between the direction of the applied stress and the orientation of any one set of fibers (Fig. 1a); the other set of plies is thus misoriented at (c−90°) from the loading direction. Similarly, in 1D composites, off-axis geometry can be characterized by a misorientation angle, 8 (Fig. 1b). Characterizing flow in 2D composites requires an analysis of their behavior in the context of now-well-understood creep behavior of 1D composites; it is possible that fiber orientations in the 2D material correspond behaviorally with 1D material, i.e. with 8=c and 90°−c. For 1D composites with 80°, the rheologic response is characterized by a significant initial transient due to load transfer from the matrix to the fibers, leading to steady-state creep that is rate limited by flow of the fibers. As 8 increases, the shear forces at the fiber-matrix interface increase, these result in a substantial contribution to the bulk composite strain by interface sliding (or, depending on the composite system, interphase flow). The contribution due to this relative displacement at the interface is maximized at 845°. However, geometrical constraints dictate that the matrix would still be rate-limiting as slip at the interphase must be accommodated by matrix shear flow. Further increase in 8 results in a high plasticity (e.g. von Mises) potential in the matrix around the fibers, leading to matrix flow around the fibers. This is accompanied by cavitation at the fiber-matrix interface due to the development of tensile tractions normal to the interface. The present work focuses on characterizing the hightemperature, low-differential-stress rheology of 2D (0/ 90°, cross-ply) laminates in off-axis loading geometries and comparing/contrasting their response with the case for 1D reinforcement. As a necessary requirement for such a comparison, experiments were done on 1D composites with an identical matrix composition. Because, the matrix chemical composition, phase distribution and morphology (and hence it is viscoelastic rheology) were different from that of the composites used in our previous experimental work [9], these 1D composite baseline experiments also provided additional verification to the 1D rheological model developed earlier. 2. Experimental design and procedure 2.1. Material specifications The materials used in this study were SiC fiber (Nicalon)-reinforced, calcium aluminosilicate composites fabricated and supplied by Coming, Inc. Both the bidirectionally reinforced (2D) and unidirectionally reinforced (1D) composite sheets were fabricated by uniaxial hot-pressing of prepreg plies at 1350°C and 15 MPa [12]. Both 1D and 2D materials consisted of 16 plies, each with a fiber volume fraction, Vf , of 0.3; the 2D composite had a [0/90°]4S (symmetrical) lay-up. The matrix was an anorthite-based glassceramic (Coming Code CAS-II) with a grain-size of 3 mm. The estimated oxide composition of the CAS-II matrix based on X-ray fluorescence spectroscopy (XRAL Labs, Ont., Canada) is shown in Table 1. X-ray diffraction of powder specimens indicated that the primary phases were anorthite (CaO:Al2O3:2SiO2) and mullite (3Al2O3:2SiO2); a calculation based on peak intensities indicated 85% anorthite and 11% mullite by weight. Electron-microprobe analysis (Cameca SX51) showed Fig. 1. Specimen geometry for ‘off-axis’ compression creep experiments. (a) 2D composites: The outer ply (I) is cut away to reveal its complementary ply (II); (b) 1D composites.
B G. Nair et al. Materials Science and Engineering 4300 (2001)68-79 Table I Composition of CAS-lI Estimated by X-Ray Fluorescence (i.e. constant-stress)tests were performed, based on the assumption of constant-volume deformation, pre- Oxide Mol%b cision adjustments were made to the total load ap- plied to the specimen; these adjustments accompanied each inelastic strain increment of 0.001. The tempera- 16.9 ture was controlled and monitored during a test using a type-C(alloy w/w-26% Re) thermocouple located 2 mm from the center of the specimen. The accu- As,O, racy of the temperature measurement is 1C, the drift in temperature during any experiment was also X-ray fluorescence spectroscopy(XRAL Labs, Hamilton, Ont less than±1°C Two DCDTs connected in series were used to mon- itor the displacement of the top-piston during traces of free silica(Sio2) and very small particles test;analog-to-digital conversion and data storage <0.2 um) of zircon (ZrSiO4)finely distributed were done with a personal computer. The data collec throughout the matrix tion rate was between one and six readings per The mean diameter of the Nicalon Sic fibers is minute depending on the strain rate displayed by in- 15 um. The fibers in the composite are fully crys- dividual specimens. Given the length of the speci allized with a very fine grain-size of 1.5 nm mens, the apparatus could easily resolve strain rates [13, 14]. The fiber-matrix interface in these composites as low as 10-8s-l little drift in the room tempera- consists of two planar (i.e. cylindrical sheath) inter- ture aided the resolution. A typical displacement-time phases, one of graphite against the fiber and the plot obtained from a creep experiment on a 2D com other of amorphous calcium aluminosilicate contact- posite specimen(40/-500, 1275oC)is shown in Fig ng the matrix. These interphases, each <100 nn 2a. At each level of stress, the specimen is allowed to thick, are formed by a fiber oxidation/displacement reach a nominal steady shown in the strain- action at the interface during composite pro- rate versus strain plot of the same experiment shown cessing [13]. The densities of both the 2D and Id in Fig. 2b. Fig. 2c and d show similar plots for a ID composites were estimated directly by precise mass composite specimen(=400, T=1300oC and dimensional measurements of polished, rectangu lar specimens. The 2D composites had a density of 2.3. Data analysis 2.57g cm; the ID materials density was 2.64 g cm-3 For individual segments of an experiment, the in- elastic creep data were fit by a regression analysis to 2. 2. Experimental methodology the Burgers solid model so as to discern the steady state strain-rate at each level of applied stress, the All experimental specimens had nominal dimensions functional form employed was 3×3×6 mm and were cut from composite sheets using a diamond saw with one pair of 3 x 6 mm a[t-t]= K exp[ -A(t-t)]+Ess(t-t) (1) faces being parallel to the component plies. The di where e[t-t] is the inelastic strain, with t denoting mensions of each test specimen were precisely mea- the starting time at each particular level of an, and Ess sured with a micrometer after polishing each of the is the steady-state strain-rate. The first (negative-expo- faces to 600 grit. 2D composite specimens with a sur- nential) term describes the transient strain at each face-ply misorientation angle y(Fig. la)are referred level of applied stress. The constant K is a geometric to as y /(y-90%) specimens. The 2D specimens, for factor that defines the load-transfer characteristics of our purposes, can be considered to have 90 symme- the composite for a given fiber orientation(); it is a try:a y/(y-90%) specimen is expected to have iden- function of the modulii of elasticity of the fiber(E) tical mechanical properties as(90%-y)/-y specimen and matrix(Em), the respective Poisson ratios (ur and neglecting end effects. For this study, 0/-90%, 20/-70 Um) and the volume fraction of the fibers (va and 40/-50 2D specimens were prepared. ID com- Error bars for the creep data were estimated base posite specimens were made with =0, 20, 40, 50, 70 on the uncertainty in temperature ( 1C). The ind 90 orientations(Fig. Ib) activation energy for creep in these composites is rela High-temperature deformation experiments were tively high; as such, the possible error in cal performed on a dead-weight compression apparatus culation of steady-state strain-rate due to temperature controlled atmosphere of flowing Ar(gauge pres- uncertainty was greater by far than any systemat tIc sure flow rate 30 cm min-). The error related to measurement of the creep displace apparatus is described in detail elsewhere [15]. Creep ment
70 B.G. Nair et al. / Materials Science and Engineering A300 (2001) 68–79 Table 1 Composition of CAS-II Estimated by X-Ray Fluorescence Oxide Mol.% Wt.%a b SiO2 39.8 47.7 21.716.9CaO 28.5Al2O3 40.3 2 1.32.3ZrO 0.4MgO 0.7 0.10.3As2O3 a X-ray fluorescence spectroscopy (XRAL Labs, Hamilton, Ont.). b Calculated from weight-percent data. (i.e. constant-stress) tests were performed, based on the assumption of constant-volume deformation, precision adjustments were made to the total load applied to the specimen; these adjustments accompanied each inelastic strain increment of 0.001. The temperature was controlled and monitored during a test using a type-C (alloy W/W-26% Re) thermocouple located 2 mm from the center of the specimen. The accuracy of the temperature measurement is 91°C; the drift in temperature during any experiment was also less than 91°C. Two DCDTs connected in series were used to monitor the displacement of the top-piston during a creep test; analog-to-digital conversion and data storage were done with a personal computer. The data collection rate was between one and six readings per minute depending on the strain rate displayed by individual specimens. Given the length of the specimens, the apparatus could easily resolve strain rates as low as 10−8 s−1 ; little drift in the room temperature aided the resolution. A typical displacement-time plot obtained from a creep experiment on a 2D composite specimen (40/–50°; 1275°C) is shown in Fig. 2a. At each level of stress, the specimen is allowed to reach a nominal steady-state as shown in the strainrate versus strain plot of the same experiment shown in Fig. 2b. Fig. 2c and d show similar plots for a 1D composite specimen (8=40°, T=1300°C). 2.3. Data analysis For individual segments of an experiment, the inelastic creep data were fit by a regression analysis to the Burgers solid model so as to discern the steadystate strain-rate at each level of applied stress, the functional form employed was o[t−t]=K exp[−A(t−t)]+o; ss(t−t) (1) where o[t−t] is the inelastic strain, with t denoting the starting time at each particular level of s1, and o; ss is the steady-state strain-rate. The first (negative-exponential) term describes the transient strain at each level of applied stress. The constant K is a geometric factor that defines the load-transfer characteristics of the composite for a given fiber orientation(s); it is a function of the modulii of elasticity of the fiber (Ef ) and matrix (Em), the respective Poisson ratios (6f and 6m) and the volume fraction of the fibers (Vf ). Error bars for the creep data were estimated based on the uncertainty in temperature (91°C). The activation energy for creep in these composites is relatively high; as such, the possible error in calculation of steady-state strain-rate due to temperature uncertainty was greater by far than any systematic error related to measurement of the creep displacement. traces of free silica (SiO2) and very small particles (B0.2 mm) of zircon (ZrSiO4) finely distributed throughout the matrix. The mean diameter of the Nicalon SiC fibers is 15 mm. The fibers in the composite are fully crystallized with a very fine grain-size of 1.5 nm [13,14]. The fiber-matrix interface in these composites consists of two planar (i.e. cylindrical sheath) interphases, one of graphite against the fiber and the other of amorphous calcium aluminosilicate contacting the matrix. These interphases, each 100 nm thick, are formed by a fiber oxidation/displacement reaction at the interface during composite processing [13]. The densities of both the 2D and 1D composites were estimated directly by precise mass and dimensional measurements of polished, rectangular specimens. The 2D composites had a density of 2.57 g cm−3 ; the 1D material’s density was 2.64 g cm−3 . 2.2. Experimental methodology All experimental specimens had nominal dimensions 3×3×6 mm and were cut from composite sheets using a diamond saw with one pair of 3×6 mm faces being parallel to the component plies. The dimensions of each test specimen were precisely measured with a micrometer after polishing each of the faces to 600 grit. 2D composite specimens with a surface-ply misorientation angle c (Fig. 1a) are referred to as c/(c−90°) specimens. The 2D specimens, for our purposes, can be considered to have 90° symmetry; a c/(c−90°) specimen is expected to have identical mechanical properties as (90°−c)/–c specimen neglecting end effects. For this study, 0/–90°, 20/–70° and 40/–50° 2D specimens were prepared. 1D composite specimens were made with 8=0, 20, 40, 50, 70 and 90° orientations (Fig. 1b). High-temperature deformation experiments were performed on a dead-weight compression apparatus in a controlled atmosphere of flowing Ar (gauge pressure +100 Pa; flow rate 30 cm3 min−1 ). The apparatus is described in detail elsewhere [15]. Creep
B G. Nair et al. Materials Science and Engineering 4300(2001)68- 2D:40/50 1275°c T:366 (a) 3 0 TIme TIme(h) 25F2D:409/50° 1Dφ=40° T=1275°c .50 T=1300c 5.7B 6.0 +;3Mm MPa 25 MPa 6.5 0 1.6 2.0 Strain (% Fig. 2. Typical creep responses of 2D and ID composite specimens. (a)and(b), strain vs time and strain-rate vs strain, respectively, for 40/-50o 2 D composite at I275°C.(c)and(d), same representation forφ=40° ID composite at1300°C 2.4.Optical microscopy 3. Experimental results Optical microscopy specimens were prepared from 3.1. 2D Composite creep experime h the deformed and undeformed 2D composite creep specimens. In an effort to study the creep-induced The composite creep data was evaluated relative to cavitation at the fiber-matrix interface as well as other the standard, semi-empirical equation for steady-state similar damage, all specimens were sectioned such that cre ep[161 one set of fibers was perpendicular to the plane of observation(the(y-90) ply)and the other parallel Es=Coiexpl-se (the y ply) To avoid possible variations in observed microstructure due to variations in specimen prepara- where a is the applied stress, Oapp is the apparent tion, the 2D specimens were ground and polished activation energy for composite creep and T is the lultaneously, on the same polishing block Polishing absolute temperature. The data for creep of CAS-Il was done to l-um diamond paste. For ID specimens Cr 2D composites are presented in Figs. 3 and 4.At ll sectioning was done such that the fibers were per- constant temperature, Ess increased with the 40/-50o pendicular to the plane of observation. The microstruc- composites showed the highest strain-rates. For exam- tures were recorded using a digital camera(Pixera PVc ple, at 1300%C, the steady-state strainrate for the 40/- 100C); image enhancement and analysis was performed 50 composite under a stress of 40 MPa wa using OPTIMAS software(FSI Automation, Bothell, approximately two orders of magnitude higher than WA that for the 0/-90 composite(Fig. 3b). Both the
B.G. Nair et al. / Materials Science and Engineering A300 (2001) 68–79 71 Fig. 2. Typical creep responses of 2D and 1D composite specimens. (a) and (b), strain vs time and strain-rate vs strain, respectively, for 40/–50° 2D composite at 1275°C. (c) and (d), same representation for 8=40° 1D composite at 1300°C. 2.4. Optical microscopy Optical microscopy specimens were prepared from both the deformed and undeformed 2D composite creep specimens. In an effort to study the creep-induced cavitation at the fiber-matrix interface as well as other similar damage, all specimens were sectioned such that one set of fibers was perpendicular to the plane of observation (the (c−90°) ply) and the other parallel (the c ply). To avoid possible variations in observed microstructure due to variations in specimen preparation, the 2D specimens were ground and polished simultaneously, on the same polishing block. Polishing was done to 1-mm diamond paste. For 1D specimens, all sectioning was done such that the fibers were perpendicular to the plane of observation. The microstructures were recorded using a digital camera (Pixera PVC 100C); image enhancement and analysis was performed using OPTIMAS software (FSI Automation, Bothell, WA). 3. Experimental results 3.1. 2D Composite creep experiments The composite creep data was evaluated relative to the standard, semi-empirical equation for steady-state creep [16]: o; ss=Cs1 n exp −Qapp RT (2) where s1 is the applied stress, Qapp is the apparent activation energy for composite creep and T is the absolute temperature. The data for creep of CAS-II/ SiCf 2D composites are presented in Figs. 3 and 4. At constant temperature, o; ss increased with c: the 40/–50° composites showed the highest strain-rates. For example, at 1300°C, the steady-state strainrate for the 40/– 50° composite under a stress of 40 MPa was approximately two orders of magnitude higher than that for the 0/–90° composite (Fig. 3b). Both the
B G. Nair et al. Materials Science and Engineering 4300 (2001)68-79 40/-500 and the 20/-700 composites showed non-New- tonian creep behavior with n increasing from about 2 at 2D 1275° c to about3atl300°C.Theo/-90°spec showed Newtonian behavior in the temperature range 200-+0/m0 1300-1325C. Fig. 4 illustrates the variation of @a with -a, for specimens with different values of y. The 1500 0/-90o composites had an activation energy of about 480 k mol-I. The increase in n with t for othe 。1000 loading configurations (y+O) results in @app having a 500 very strong dependence on the applied stress; @ a 2D 4. Variation of Oapp with for 2D T〓1275c different values of y 8.0 increased from 650 to 1400 kJ mol- for 40/ and from x 1500 to 2400 kj mol 20/-70° composites. 3. 2. Creep experiments on ID composites and on the 20°/70";n=21 unreinforced CAS- matrix 1.5 A comparison of the creep data of y(y-90%)2D Log F-o.(MPa) composite specimens to the data for ID composites with y and p=(90%-p) is presented in Fig. 5 4.5F2D For all values of y, the values of n for 2D composites are intermediate to those for the ID composites, where pp corresponds to either y or(90-y) As such a result suggests an application of laminate theory to the under tanding of 2D behavior, it is nessasary to provide here comprehensive results for the ID material -65 Creep data for ID composite specimens with differ ent values of at 1300 and 1275C are shown in Fig 6a and b respectively. For =0, the observed theol- v0"/90°;n=13 ogy was Newtonian (n A 1). For specimens with values 1112131.41.51.61.718 of op ranging from 40 to 90, n was consistently between Log I-o, (MPa)l 1.9 and 2.7 in the temperature range 1275-1300oC. Fig 7 shows the variation of @app with applied stress for vanous specimen ge metre es. For off-axis'geometries with (p=20-90, @app was significantly higher(> 1000 kJ mol-)than for =0(@app 400-440 kJ mol-) 1300°c The behavior of specimens with 20, both in 。1275°c terms of stress and temperature sensitivity is in striking 6.0 contrast to the general trend. The p= 20 specimens displayed the highest values of n for ID composites Further. n decreased from 5.3 at 1300oc to 3.6 at 1325C(Fig. 8). @app decreased from 2200 kJ mol-I at -a1=20 MPa to 900 kJ mol- at -a1=40 MPa(Fig. 7) Fig.9 illustrates the dependence of iss on at 1275 DEgrees and 1300oC. The =50 specimens consistently dis- Fig3.Stress/strain-rate relationships for 2D composites as a function played the highest strain-rates at all temperatures and of v(a)1275oC:(b)1300C(c) Strain-rate as a function of y for a stresses- at 1300C and 40 MPa, the steady-state constant stress of 40 MPa(the 0/90 data point is extrapolated based strain-rate for p= 50 was two orders of magnitude Eq.(1) higher than the strain-rate of the on-axis, =0 speci
72 B.G. Nair et al. / Materials Science and Engineering A300 (2001) 68–79 40/–50° and the 20/–70° composites showed non-Newtonian creep behavior with n increasing from about 2 at 1275°C to about 3 at 1300°C. The 0/–90° specimens showed Newtonian behavior in the temperature range 1300–1325°C. Fig. 4 illustrates the variation of Qapp with −s1 for specimens with different values of c. The 0/–90° composites had an activation energy of about 480 kJ mol−1 . The increase in n with T for other loading configurations (c"0) results in Qapp having a very strong dependence on the applied stress; Qapp Fig. 4. Variation of Qapp with −s1 for 2D composite specimens for different values of c. Fig. 3. Stress/strain-rate relationships for 2D composites as a function of c. (a) 1275°C; (b) 1300°C. (c) Strain-rate as a function of c for a constant stress of 40 MPa (the 0/90° data point is extrapolated based on Eq. (1)). increased from 650 to 1400 kJ mol−1 for 40/–50° compsites and from 1500 to 2400 kJ mol−1 for 20/–70° composites. 3.2. Creep experiments on 1D composites and on the unreinforced CAS-II matrix A comparison of the creep data of c(c−90°) 2D composite specimens to the data for 1D composites with 8=c and 8=(90°−c) is presented in Fig. 5. For all values of c, the values of n for 2D composites are intermediate to those for the 1D composites, where 8 corresponds to either c or (90°−c). As such a result suggests an application of laminate theory to the understanding of 2D behavior, it is nessasary to provide here comprehensive results for the 1D material. Creep data for 1D composite specimens with different values of 8 at 1300 and 1275°C are shown in Fig. 6a and b respectively. For 8=0°, the observed theology was Newtonian (n1). For specimens with values of 8 ranging from 40 to 90°, n was consistently between 1.9 and 2.7 in the temperature range 1275–1300°C. Fig. 7 shows the variation of Qapp with applied stress for various specimen geometries. For ‘off-axis’ geometries with 8=20–90°, Qapp was significantly higher (\1000 kJ mol−1 ) than for 8=0° (Qapp400–440 kJ mol−1 ). The behavior of specimens with 8=20°, both in terms of stress and temperature sensitivity is in striking contrast to the general trend. The 8=20° specimens displayed the highest values of n for 1D composites. Further, n decreased from 5.3 at 1300°C to 3.6 at 1325°C (Fig. 8). Qapp decreased from 2200 kJ mol−1 at −s1=20 MPa to 900 kJ mol−1 at −s1=40 MPa (Fig. 7). Fig. 9 illustrates the dependence of o; ss on 8 at 1275 and 1300°C. The 8=50° specimens consistently displayed the highest strain-rates at all temperatures and stresses — at 1300°C and 40 MPa, the steady-state strain-rate for 8=50° was two orders of magnitude higher than the strain-rate of the on-axis, 8=0° speci-