Studies on the Work-Hardening Behavior of AA2219 under Different Aging Treatments

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MINUM alloys of the age-hardenable type2XXX series are extensively used as structural materials in commercial aircraft as well as in aerospace applications, due to their good combination of strength and fatigue resistance.[1] Age-hardening Al alloys have a microstructure that changes during heat treatment. In solid solution, the alloying elements are completely dissolved and the material behaves like a single-phase material. During aging, the alloying elements cluster and start forming different types of small precipitates, depending on the aging temperature. The precipitation sequence in Al-Cu alloys is ao ! a1 þ GuinierPreston ðGPÞ zones ! a2 þ h00 ! a3 þ h0 ! a4 þ h

½1

The phases are as follows: (a) The an is the fcc aluminum. The subscript n denotes each equilibrium with individual precipitates (GP1, h¢¢, h¢, and h); V.M.J. SHARMA, Scientist, and K. SREE KUMAR, Group Director, Material Characterisation Division, Materials and Metallurgy Group, and B. NAGESWARA RAO, Head, Structural Analysis and Testing Group, are with the Vikram Sarabhai Space Centre (VSSC), Trivandrum - 695 022, India. Contact e-mail: bnrao52@ rediffmail.com. S.D. PATHAK, Professor, is with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai - 600 036, India. Manuscript submitted March 22, 2009. Article published online October 27, 2009 3186—VOLUME 40A, DECEMBER 2009

(b) The GP zones are monoatomic layers of Cu on {100}Al; (c) The h¢¢ are thin disks, fully coherent with the matrix; (d) The h¢ are disk shaped, semicoherent on {100}Al, bct CuAl2; and (e) The h are incoherent interface, disk-shaped, CuAl2, complex bct. The GP zones and h¢¢ are weak and coherent and thus shearable by moving dislocations. Further aging leads to coarsening of the microstructure. Larger incoherent precipitates are formed near the peak hardness that are not shearable by dislocations. A change in the dislocation-strengthening mechanism from the particle shearing to bypassing via Orowan looping takes place as the coarsening process proceeds. In addition, precipitatefree zones will form adjacent to the grain boundaries because of the nucleation and growth of coarse precipitates at the grain boundaries. The microstructure thus becomes inhomogeneous and quite complex when the material is age hardened. It is of considerable interest to study the workhardening behavior in the precipitation-hardening systems, because this may offer additional insight into the mechanisms of precipitation and work hardening.[2–4] In addition, work-hardening behavior is of significant relevance to industrial processes such as metal forming.[5] This article examines the influence of the precipitation state on the yield stress and work-hardening behavior by conducting tensile tests on AA2219 in different aged conditions. The work-hardening rate is METALLURGICAL AND MATERIALS TRANSACTIONS A

determined by numerically differentiating the truestress–true strain data. The material constants in the differential equation for the dislocation density are e