Solutionizing effects on deformation-induced phase transformations in 2014 aluminum composite
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I. INTRODUCTION
IT has been shown that the microstructures evolved during the aging process and the age-hardening curves depend on the solutionizing time and temperature in 6061 and 2014 aluminum alloys reinforced with various volume fractions of alumina particles (VFAP).[1–6] The composites show the formation of a series of Guinier-Preston (GP) zones leading to the formation of metastable phases b8 and l8 in 6061 and 2014 aluminum alloy composites, respectively, as a function of aging time in a range of temperatures from 160 8C to 220 8C besides another phase u8 in 2014 aluminum alloy composite usually nucleated at the dislocation lines.[1–12] The effect of increasing solutionizing time at a given temperature on the age-hardening curves has been found to be such that the time required to achieve the peak hardness value decreases in the monoliths, while either a (1) decrease and then increase or (2) decrease and then stabilized values have been observed for the composites. The generation of dislocations due to the differences in the coefficient of thermal expansion (CTE) values between matrices and reinforcements and the concentration of quenched-in vacancies can account for the observed aging behavior of composites and their monoliths.[1–6] The quenched-in vacancies and CTE dislocations have been observed to be the nucleation sites for the precipitates originating from the GP zone series (for both composites), while CTE dislocations promote the u8 precipitation (for 2014 aluminum alloy composite only) during aging. An accelerated aging response from the composites containing age hardenable matrix has been well documented.[11] However, Suresh and Chawla[12] have shown that the aging response from a 6061 aluminum alloy in monolith form after a 0.36 pct cold work is similar to the composite containing 10 pct SiC particles in the unstrained condition. This clearly suggests that the dislocations play a very important role in the nucleation of precipitates during aging treatments.
S.K. VARMA, Professor, and ERICA CORRAL, ERIKA ESQUIVEL, and DANIEL SALAS, Students, are with the Department of Metallurgical and Materials Engineering, The University of Texas at El Paso, El Paso, TX 79968-0520. Manuscript submitted July 27, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
Monolith of 2014 aluminum alloys indicates the phase transformations during aging, which involve the formation of several kinds of precipitates, including the usual sequence of GP zones.[8] The presence of u phase with a stoichiometry of CuAl2, which has a tetragonal structure, is well known, along with its metastable forms u9 and u8. Another phase in which four metals are present has a chemical formula given by Al5Cu2Mg8Si5, which has a hexagonal crystal structure and has been assigned a symbol of l. This l phase also has its metastable counterpart known as l8 .The third type of phase in this system is called the S phase, which has the chemical composition given by Al2CuMg with an orthorhombic crystal structure. Thus, the purpose of this article is to r
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