Trace element effects on precipitation processes and mechanical properties in an Al-Cu-Li alloy
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I.
INTRODUCTION AND BACKGROUND
MANY age-hardenable aluminum alloys are deformed prior to aging in order to introduce dislocation structures which will act as heterogeneous nucleation sites for precipitation. A stretch may also be applied to redistribute residual stresses. However, some product forms do not lend themselves readily to the use of this T8 temper. Alternatively, certain trace alloying additions have been shown to aid in the nucleation and/or growth of the strengthening phases. There are several ways in which trace elements might alter the nucleation process:[1,2] they may shift the solvus or metastable solvus boundaries; they may reduce the interfacial energy associated with an embryo by segregating to the precipitate-matrix interface; or if the additions form a new particle, they may reduce the surface energy of a nucleating phase by providing a heterogeneous nucleation site. By interaction with vacancies, the solute may affect diffusion and the formation of clusters and dislocation loops which serve as nucleation sites. Finally, trace additions may affect the stability of a phase by changing the electron-toatom ratio. Uniaxial yield strength is not the only mechanical property of a material that may change with variations in the precipitation process. In textured products such as sheet, phases may have different effects on the anisotropy due to their orientation relationship with the matrix.[3–7] Thus, if a trace element changes the volume fraction of a precipitate, D.L. GILMORE, Research Fellow, is with the Department of Materials, Oxford Centre for Advanced Materials and Composites, Oxford University, Oxford OX1 3PH, United Kingdom. E.A. STARKE, Jr., University Professor, is with the Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903. Manuscript submitted December 13, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
it may increase or decrease the anisotropy. Also, changes in precipitate size or morphology may lead to a change in deformation mechanism from shearing to looping. This will affect strain localization and related properties such as fatigue.[8] The first work on trace additions in age-hardenable aluminum alloys was published by Sully et al.[9] They reported that the addition of 0.05 wt pct Sn to a commercial-purity Al-4.5Cu alloy resulted in a much lower response to natural aging but a marked increase in mechanical properties after artificial aging. Hardy[10,11] went on to examine the influence of the elements closely adjacent to tin on the periodic table. He found that small additions (;0.012 at. pct) of Cd or In had the same general effects in high-purity Al-4Cu and Al4Cu-0.15Ti alloys. The increase in both the rate of aging and the peak strength was found to be significant at both 130 7C (403 K) and 190 7C (463 K), although the effect was much larger for the higher aging temperature. Cold work was found to interfere with trace alloying effects. Cadmium was also shown to reduce the specific interfacial enthalpy for u ' by a factor of ro
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