J.Am. Ceran.Soe.,88[]3104-310902005 Dol:lo.l.1551-2916.2005.00559x journal C) 2005 The American Ceramic Society Investigation of Anelastic Creep Recovery in SiC Whisker- Reinforced Alumina Composites G. C. Quan, K. T. Conlon, and D.S. Wilkinson Department of Materials Science and Engineering, McMaster University, Hamilton, oN, Canada LSS 4L7 Anelastic strain recovery has been studied in SiC whisker-rei Il. Materials and Experimental Procedures forced alumina composites following tensile creep The magni tude of the recovered strains were 10-3 with 20 vol% SiC Alumina matrix composites with 10, 20, and 30 vol% SiC whiskers upon unloading from 74 MPa at 1673 K, while no such hikers were prepared from premixed powders supplied by behavior was observed with 10 vol% SiC whiskers. A strain re- ACMC(see Acknowledg using ceramIc powder covery mechanism based on hertzian contact deformation with- rocessing followed by uniaxial hot The nixed (or particle)network is wders were first dispersed in distilled water at which appears to be consistent with the experimental observa- oH=2 and ball milled for 0.5 h. The heteroflocculated slurries tions. Two models (for whiskers and particles) have been devel- were then slip cast into rectangular-shaped billets for the sub- oped which seem to predict the magnitude of recoverable strain quent hot pressing. The details of the processing routes can be reasonably well found elsewhere. cReep experiments were conducted in tension Coading direction perpendicular to the hot-pressing axis)using a methodology similar to that followed by Carroll et al. L. Introduction S IGNIFICANT anelastic strain recovery following creep has long been observed in some ceramic materials, the presence of Ill. Estimation of Peak broadening because of whisker which can be used to better understand creep mechanisms. -9 Suppose that whisker bending is indeed the origin of anelastic vol%SiC whisker-reinforced alumina in four-point bending tests strain recovery and that the change in the (lll) Sic peak where the recovered strain was as large width is determined primarily by the extent of whisker bending loading from 50.5 MPa at 1773 K. However, no such behavior among all possible causes. It follows that during forward creep, vas observed in 5 vol% whisker-reinforced alumina. Anelasti- whiskers would undergo bending deformation, and thus, result city in whisker-reinforced ceramics is commonly attributed to the formation of a whisker network that exhibits a low effective ever, the reverse process takes place wherein the extent of whis elastic stiffness. In order to sustain the high degree of anelastic ker bending would decrease with a concomitant decrease in peal strain recovery upon load removal, strain energy has to be ac- width. This hypothesis is schematically illustrated in Fig. 1. It is cumulated in the constrained whisker network by the elastic portant to recognize the reversibility of this process, which bending of whiskers that are not free to rotate about their con- requires that the amount of whisker bending gained during for tact points. 6. 8.9 Gu et al. 10-13 reported that a composite with ward creep be the same as that reduced during recover 15 vol% whiskers showed significant anelastic recovery(10-3) The extent of peak broadening upon a certain degree of was essentially independent of the total forward creep stran lo o pon load removal at 1723 K, and that the recoverable strain whisker bending is estimated with the followi 1. There is only hydrostatic (or dilatational) distortion of the strain at which the load was removed ) They also found that unit cells after bending and Braggs law is still applicable for the a smaller whisker aspect ratio resulted in more strain recovery bent crystal (1.5x 10)and more extensive transient creep behavior. This 2. The amount of bending is small and the outer fiber mac. is a somewhat perplexing result as a high aspect ratio should be restrains are equal to the extreme lattice strains. associated with more whisker-bending strain according to por- Broadening of the line profile of diffraction peaks from po- ter's model, thus resulting in greater strain recovery. Furthe lycrystals can be caused by the small size of coherently diffract sic particulate-reinforced alumina but not with dilute (5 volo microstrains within the grains(strain effect). This is termed composites. This was ascribed to an inclusion network effect but physical broadening. In addition to this there is an inherent in- the underlying physical reason was not sought. As no bending strumental broadening. because of the effect of slit widths effect is associated with spherical particles, there must be a dif- sample size, penetration into the sample, and imperfect focusing, ferent mechanism responsible for the observed strain recovery covery in tensile mode, assessing the peak broadening effect as- tering angle, Wze for a pure diffraction peak (i.e. withow at- behavior therein. This study is aimed at studying the creep re- In the absence of crystallite size effect, the variance in scat sociated with whisker-bending deformation, and estimating the strumental broadening) is given as magnitude of the recoverable creep strain based on a newly pro- H3=(20-(0)24m() S M. Wiederhorncontributing editor This uscript No 20101. Received July 9, 2004: approved February 2, 2005. Council of Can ade rseg unding from the Natural Science and Engi- the [lll] direction. Because of the high density tion. the long-range periodicity along the other three off-axis <lll) directions has been Author to whom correspondence should be addressed. e-mail: wilkinson mcmaster.ca destroyed. 3104Investigation of Anelastic Creep Recovery in SiC Whisker-Reinforced Alumina Composites G. C. Quan, K. T. Conlon, and D. S. Wilkinsonw Department of Materials Science and Engineering, McMaster University, Hamilton, ON, Canada L8S 4L7 Anelastic strain recovery has been studied in SiC whisker-rein￾forced alumina composites following tensile creep. The magni￾tude of the recovered strains were B10
3 with 20 vol% SiC whiskers upon unloading from 74 MPa at 1673 K, while no such behavior was observed with 10 vol% SiC whiskers. A strain re￾covery mechanism based on Hertzian contact deformation with￾in a percolating whisker (or particle) network is suggested, which appears to be consistent with the experimental observa￾tions. Two models (for whiskers and particles) have been devel￾oped which seem to predict the magnitude of recoverable strain reasonably well. I. Introduction SIGNIFICANT anelastic strain recovery following creep has long been observed in some ceramic materials,1–7 the presence of which can be used to better understand creep mechanisms.7–9 Porter6 first reported significant anelastic strain recovery in 15 vol% SiC whisker-reinforced alumina in four-point bending tests where the recovered strain was as large as B2 10
3 upon un￾loading from 50.5 MPa at 1773 K. However, no such behavior was observed in 5 vol% whisker-reinforced alumina. Anelasti￾city in whisker-reinforced ceramics is commonly attributed to the formation of a whisker network that exhibits a low effective elastic stiffness. In order to sustain the high degree of anelastic strain recovery upon load removal, strain energy has to be ac￾cumulated in the constrained whisker network by the elastic bending of whiskers that are not free to rotate about their con￾tact points.6,8,9 Gu et al. 10–13 reported that a composite with 15 vol% whiskers showed significant anelastic recovery (B10
3 ) upon load removal at 1723 K, and that the recoverable strain was essentially independent of the total forward creep strain (i.e. the strain at which the load was removed). They also found that a smaller whisker aspect ratio resulted in more strain recovery (B1.5 10
3 ) and more extensive transient creep behavior. This is a somewhat perplexing result as a high aspect ratio should be associated with more whisker-bending strain according to Por￾ter’s model, thus resulting in greater strain recovery. Further￾more, they observed similar strain recovery behavior in 15 vol% SiC particulate-reinforced alumina but not with dilute (5 vol%) composites. This was ascribed to an inclusion network effect but the underlying physical reason was not sought. As no bending effect is associated with spherical particles, there must be a dif￾ferent mechanism responsible for the observed strain recovery behavior therein. This study is aimed at studying the creep re￾covery in tensile mode, assessing the peak broadening effect as￾sociated with whisker-bending deformation, and estimating the magnitude of the recoverable creep strain based on a newly pro￾posed mechanism. II. Materials and Experimental Procedures Alumina matrix composites with 10, 20, and 30 vol% SiC whiskers were prepared from premixed powders supplied by ACMC (see Acknowledgments) using ceramic powder colloidal processing followed by uniaxial hot pressing. The pre-mixed composite powders were first dispersed in distilled water at pH 5 2 and ball milled for 0.5 h. The heteroflocculated slurries were then slip cast into rectangular-shaped billets for the sub￾sequent hot pressing. The details of the processing routes can be found elsewhere.14 Creep experiments were conducted in tension (loading direction perpendicular to the hot-pressing axis) using a methodology similar to that followed by Carroll et al. 15 III. Estimation of Peak Broadening Because of Whisker Bending Suppose that whisker bending is indeed the origin of anelastic strain recovery and that the change in the (111) SiCz16 peak width is determined primarily by the extent of whisker bending among all possible causes. It follows that during forward creep, whiskers would undergo bending deformation, and thus, result in (111) SiC peak broadening. When the load is removed, how￾ever, the reverse process takes place wherein the extent of whis￾ker bending would decrease with a concomitant decrease in peak width. This hypothesis is schematically illustrated in Fig. 1. It is important to recognize the reversibility of this process, which requires that the amount of whisker bending gained during for￾ward creep be the same as that reduced during recovery. The extent of peak broadening upon a certain degree of whisker bending is estimated with the following assumptions: 1. There is only hydrostatic (or dilatational) distortion of unit cells after bending and Bragg’s law is still applicable for the bent crystal. 2. The amount of bending is small and the outer fiber mac￾rostrains are equal to the extreme lattice strains. Broadening of the line profile of diffraction peaks from po￾lycrystals can be caused by the small size of coherently diffract￾ing domains (size effect) and by a non-uniform distribution of microstrains within the grains (strain effect). This is termed physical broadening. In addition to this there is an inherent in￾strumental broadening,17 because of the effect of slit widths, sample size, penetration into the sample, and imperfect focusing, etc. In the absence of crystallite size effect, the variance in scat￾tering angle, W2y for a pure diffraction peak (i.e. without in￾strumental broadening) is given as18,19 W2y ¼ ð Þ 2y
h i 2y 2 D E ¼ 4 tan2 y e2 hkl
(1) Journal J. Am. Ceram. Soc., 88 [11] 3104–3109 (2005) DOI: 10.1111/j.1551-2916.2005.00559.x r 2005 The American Ceramic Society 3104 S. M. Wiederhorn—contributing editor This work was supported by the research funding from the Natural Science and Engi￾neering Research Council of Canada (NSERC). w Author to whom correspondence should be addressed. e-mail: wilkinso@mcmaster.ca Manuscript No. 20101. Received July 9, 2004; approved February 2, 2005. z Each SiC whisker is a single crystal with cubic structure and the whisker axis is parallel to the [111] direction. Because of the high density of planar defects along this growth di￾rection, the long-range periodicity along the other three off-axis /111S directions has been destroyed.