Deformation of whisker-reinforced metal-matrix composites under changing temperature conditions
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I.
INTRODUCTION
M E T A L , ceramic, and intermetallie matrix composites are receiving much attention as potential high-temperature materials. However, several studies on metal-matrix composites have shown that thermal expansion differences between the matrix and a reinforcing phase can dramatically accelerate deformation, t~'2'31 The key features of the deformation under sufficiently large temperature cycles and low stresses are that: (1) the material will flow at stresses far below the yield stress at the high temperature of the cycle, and (2) at low applied stress, the material deforms with a high effective strain rate sensitivity. This gives rise to high tensile elongation, as in conventional fine-structure superplasticity. At sufficiently high stresses (or imposed strain rate), thermal cycling has very little effect on deformation. The deformation of composites under thermal-cycling conditions may have potential benefits and imposes some limitations. As a benefit, a thermal-cycling process may possibly be developed into a useful new technique for the superplastic forming. The apparent drawback to such a process, however, is that the maximum superplastic strain rate appears to be about 10 4 s-I,[1,2,4] which may be too low for commercial production applications. However, this effect may be useful for some specialized applications. The enhanced composite deformation seen under changing temperature conditions may prove to be a drawback in many cases. Many advanced composites are now being developed for elevated temperature service, tSj and it has been reported that these materials will creep and exhibit shape instabilities at unusually low stresses under thermal-cycling conditions. I61 Therefore, it is important to be able to predict how temperature fluctuations will influence the deformation behavior of these materials in service. GLENN S. DAEHN. Assistant Professor, is with the Department of Materials Science and Engineering, Ohio State University, Columbus, OH 43210. GASPAR GONZALEZ-DONCEL, Research Scientist, is with the Departamento de Metalurgia Fisica, Centro Nacional de Investigaciones Metaltirgicas (CENIM), Madrid, Spain. Manuscript submitted October 18, 1988. METALLURGICAL TRANSACTIONS A
This analysis examines how the stress and strain developed by thermal expansion mismatch and temperature change can influence the axial deformation of aligned whisker-reinforced composites. This simplified analysis is intended to elucidate considerations which must be made when predicting service life for composites in changing temperature environments. Also, the issues considered should have relevance in the design of composites which will resist the problems associated with temperature changes.
II.
BACKGROUND
A. Prior Experimental Results There are two important conditions under which stress and strain can develop within a material, in the absense of any external forces: (1) Phase changes., If a phase change has an accompanying volume change, mismatch stresses and strains are developed at the transformation fron
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