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 ment70 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, pre￾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￾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￾racy 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 mon￾itor 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 collec￾tion rate was between one and six readings per minute depending on the strain rate displayed by in￾dividual specimens. Given the length of the speci￾mens, the apparatus could easily resolve strain rates as low as 10−8 s−1 ; little drift in the room tempera￾ture aided the resolution. A typical displacement-time plot obtained from a creep experiment on a 2D com￾posite 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 strain￾rate 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 in￾elastic creep data were fit by a regression analysis to the Burgers solid model so as to discern the steady￾state 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-expo￾nential) 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 rela￾tively high; as such, the possible error in cal￾culation of steady-state strain-rate due to temperature uncertainty was greater by far than any systematic error related to measurement of the creep displace￾ment. 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 crys￾tallized 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) inter￾phases, one of graphite against the fiber and the other of amorphous calcium aluminosilicate contact￾ing the matrix. These interphases, each 100 nm thick, are formed by a fiber oxidation/displacement reaction at the interface during composite pro￾cessing [13]. The densities of both the 2D and 1D composites were estimated directly by precise mass and dimensional measurements of polished, rectangu￾lar 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 di￾mensions of each test specimen were precisely mea￾sured with a micrometer after polishing each of the faces to 600 grit. 2D composite specimens with a sur￾face-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° symme￾try; a c/(c−90°) specimen is expected to have iden￾tical 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 com￾posite 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 pres￾sure +100 Pa; flow rate 30 cm3 min−1 ). The apparatus is described in detail elsewhere [15]. Creep