Determination of residual stresses in thin sheet titanium aluminide composites
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I.
INTRODUCTION
I N metal matrix composites, the significant difference in the coefficients of thermal expansion of the fibers and the matrix generates thermal mismatch strains during fabrication. The magnitude, distribution, and stability in service of the resulting residual stresses have long been known to influence the mechanical and physical properties of the composite. ~ Since the microscopic inhomogeneity of the residual stresses defies determination by conventional experimental methods, it has become popular to predict their spatial variation by computer simulations, especially finite element calculations, of the process of their formation. This generally involves a detailed account of the various mechanisms of relaxation of thermal mismatch and transformation strains during cooldown from processing. The results of these calculations are often used to predict failure of the composite. Unfortunately, it has become far too common to rely on such calculations without sufficient checking against experimental measurements. In this paper, some effort is made to measure the spatial variation of residual stresses in two thin sheet titanium aluminide/SiC composites. The results are surprising in some aspects and highlight the danger of oversimplifying the description of residual stress formation in numerical simulations. II.
SPECIMENS
Two composites were studied, one with Ti-24AI-11Nb (atomic percent) as the matrix and the other with Ti25Al-10Nb-3V-1Mo (super-a2), both reinforced with SCS-6 SiC fibers and consolidated by Textron Specialty Metals, Lowell, MA. The composites were three plies thick with an average of 48 fibers/cm/ply. The super-a2 B.N. COX, Member of Technical Staff, M.R. JAMES, Manager of Mechanics of Materials, D.B. MARSHALL, Manager of Structural Ceramics, and R.C. ADDISON, Jr., Member of Technical Staff, are with Rockwell International Science Center, Thousand Oaks, CA 91360. Manuscript submitted August 8, 1989. METALLURGICAL TRANSACTIONS A
composite was consolidated from a stack of three fiber plies and four alloy foils laid down alternately. Its final average thickness was 650/zm, and its fiber volume fraction, VI, was 0.36 (Figure l(a)). The Ti-24AI-11Nb composite was consolidated from a similar layup, except that the starting foils were thicker. Its final average thickness was 1000/zm, with = 1 6 0 / x m of alloy remaining outside the outermost fibers (Figure l(b)), and its fiber volume fraction was 0.25. Consolidation of both specimens was effected by holding them at - 1 0 0 0 ~ under a pressure of - 1 0 0 MPa normal to the layers. Total consolidation time was of the order of a few hours. The X-ray diffraction experiments revealed that the Ti24A1-11Nb matrix was single-phase ordered hexagonal a2 structure with lattice parameters a = 5.779 and c = 4.664. The super-a2 matrix showed significant/3 phase, having a strong (200)/3 reflection, but no significant difference in a~ lattice parameters from the Ti-24Al-11Nb matrix. III.
X-RAY DIFFRACTION MEASUREMENTS
A. Experimental Method
Matrix re
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