High‐Temperature Mechanical Properties of Ceramic Materials: II, Beta‐Eucryptite

Abstract

High stresses arise in sintered B-eucryptite(Li20.Al2OI-2SiO2) during cooling from the 5ring temperature because of its extreme anisotropy of thermal expansion. These stresses cause extensive fracturing of the crystals, resulting in abnormally low mechanical property values at room temperature. During reheating, many fractures recombine, causing increase in strength and in elastic modulus with increasing temperature. Beta-eucryptite creeps under load at room temperature because of additional fracturing and the propagation of fractures. Equations are presented for calculating intend stresses in sintered singlephase aggregates of anisotropic crystah. lnhwluctionETA-EUCXYPTITE, L i d -Al&.2SiO,, is extremely anisotropic in thermal expansion, with coefficients of thermal expansion in the a and c crystallographic directions of 82 and -176 X lo-' per OC., respectively (Fig. 1).1 When a sintered aggregate of 8-eucryptite cools from the liring temperature, high tensile stresses arise in the a crystallographic direction. When these stresses exceed the strength of the crystals or the crystal interfaces, fractures occur.The influence of these internal fractures on the thermal expansion of 8-eucryptite aggregates has been previously described.' The object of this investigation was to determine what d e c t the fractures have upon mechanical properties. It was found that the mechanical properties are greatly influenced bN.the internal fractures, and are very similar to those of magnesium dititanate which is also highly anisotropic in thermal expansion.; II. Calculation of Internal StressThe magnitude and orientation of stresses in a crystal in a single-phase aggregate of anisotropic crystals depend on the orientation of the surrounding crystals, and an exact analysis of the state of stress would involve consideration of all possible crystal configurations. The problem is greatly simplified if it is assumed that the crystals in an aggregate must conform to the same strain as the bulk material during a temperature change.' When the elastic compliance constants are known, the stresses may be calculated using equations given by L L Z I~.~(~) If the elastic modulus, E, and Poisson's ratio, Y , are assumed to be the same in all crystallographic directions and constant with temperature and if shear stresses are assumed to be negligible, the following equations relate stress and strain for a temperature change TI-TI:1 Q = (a -a) AT = j j [a -~( u e + 0o)l where t . , us, and a . are the principal normal strains, stresses, and thermal-expansion coefiicients, respectively, in the indicated crystallographic directions and a is the thermalexpansion coefficient of the aggregate. The simultaneous solution of equations (1) gives the following equations for the principal normal stresses: When (3) a = '/a (a. + rq + a c ) which is not generally exact because of the internal stresses,' equations (2) become EAT ( a . -a ) l + v Ua = EAT (a -a ) 1 + v a = (4) EAT (ac -a ) l + r uc = In this case (5)i.e., the hydrostatic pressure on each crystal ...

Thermal shock behaviour of magnesia–spinel composites

“…14 An established view, supported by experimental evidence from BeO-SiC and MgO-W composites containing three-dimensional crack networks, 15 and single phase magnesium dititanate and b-eucryptite materials with internal thermal expansion coefficient anisotropy 16,17 is that microcracking is beneficial. This is attributed primarily to the fact that the microcrack networks permit accommodation of thermal strain and reduce the thermal stresses; elastic moduli are reduced, and strain to failure is increased.…”

Thermal expansion of β-eucryptite in oxide-based ceramic composites

“…Among them, cubic-AM 2 O 8 3-6 (A = Zr, Hf; M = Mo, W), orthorhombic-A 2 M 3 O 12 7-10 (A = trivalent metal; M = W, Mo), hexagonal lithium alumino silicates (LAS) such as ␤-eucryptite, [11][12][13][14] hexagonal NZP NaZr 2 P 3 O 12 15-18 and perovskite-type AMO 3 19,20 (A = Ba, Sr, Pb; M = Ti, Zr). Usually, intrinsic negative coefficients of thermal expansion (CTE) values are small and limited to a narrow range of temperatures.…”

Effect of grain (crystal) size on the microstructure and the properties of ceramics

“…I) were grown from a melt consisting of 97% zirconium dioxide and 3% yttrium oxide according to the method of directional solidification (specimens were cut out from them using a diamond tool). The well known three-stage method of direct high-frequency melting of dielectrics in a cold container [6,7] was used (Fig. 2 shows the scheme of melting and crystallization).…”

“…The maximum attainable tensile stress o was evaluated according to the equation derived by Bush and Hymell [7] (%>%>%):…”