The effect of plastic deformation on Al 2 CuLi ( T 1 ) precipitation
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INTRODUCTION
THE previous paper tll gave a detailed explanation of TI nucleation and growth. Although the improved behavior of A1-Li-Cu alloys with plastic deformation prior to heat treatment is well-known, t2-7] the precise mechanism is not. Ashton e t al. t21 have demonstrated that stretching 2090 and 8091 alloys increases the toughness of the materials. They were able to demonstrate qualitatively that stretching results in a higher density of smaller TL and S' precipitates. The effect of stretch on dislocation density was measured in an A1-Li-Cu alloy by Rioja e t al. t3~ They found that it increased by a bit less than two orders of magnitude when the stretch level was increased from 0 to 6 pct. This translated into a higher density of smaller precipitates. Although they incorrectly identified the precipitate as T'1, their data can be directly interpreted as T 1. Sainfort and GuyottaJ stretched their AI-2.7Li-1.9Cu alloy 2 pct and felt that the T~ precipitates were unshearable during deformation and were subsequently bypassed by dislocations. More recently, Huang and Ardell t7J employed reversion experiments to differentiate the contributions of 3' and Tt precipitates. They found the strengthening effects to be additive according to their generalized addition rule. The present study was undertaken to quantify the relationship between plastic deformation and T~ precipitation. The detailed mechanism of T~ nucleation and growth described in the previous paper f~l will be employed to explain the efficacy of plastic deformation on the dramatic increase in Tl density with stretch. It is the relationship between the amount of plastic deformation and TI size and distribution that is considered in this paper. T~ plate distributions are related to matrix dislocations using both conventional transmission electron microscopy (CTEM) and high-resolution TEM (HRTEM). These considerations are motivated by the possible use of T~ as a primary hardening agent in A1-Li-Cu-X alloys. For opW.A. CASSADA, formerly Graduate Student, Department of Materials Science, University of Virginia, is Research Scientist, Reynolds Metals Company, Richmond, VA 23219. G.J. SHIFLET, Professor, Department of Materials Science, and E.A. STARKE, Jr., Earnest J. Oglesby Professor and Dean, School of Engineering and Applied Science, are with the University of Virginia, Charlottesville, VA 22903. Manuscript submitted December 18, 1989. METALLURGICAL TRANSACTIONS A
timum benefit of T1 precipitation, it is necessary to obtain a uniform distribution of particles. Without plastic deformation prior to heat treatment, precipitation of T1 is distributed heterogeneously, primarily on subgrain boundaries. Such a fundamental understanding of Tl plate nucleation and growth will allow a more complete interpretation of the effect of plastic strain prior to aging at 463 K (190 ~ on the resultant T~ distribution. II.
EXPERIMENTAL PROCEDURE
The A1-2.45Li-2.45Cu-0.18Zr alloy used in this work was prepared by the Reynolds Metals Company from highpurity materials. The al
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