Nagato Ono, Jin Gouya and Sei Miura
Department of Mechanical Engineering, Faculty of Engineering, Sojo University, Kumamoto 860-0082
It is widely recognized that precipitation hardening brings considerable improvements of mechanical properties in 7000 type Al-Zn-Mg-Cu polycrystalline alloys. In order to make clear the effects of grain boundaries on yielding, we have studied the relationship between flow stress and slipped grain size for 7475 alloys by using an in-situ observation of slip bands. Two kinds of specimens were prepared; one was a solution treated at 753 K for 1.1 ks, and the other one was a solution similarly treated and T6 aged at 393 K for 86.4 ks. The in-situ observations in the surface grains of both specimens were continuously performed in deformation ranges from 0 to about 1% plastic strain. The Hall-Petch relation was confirmed between the flow stress and the average slipped grain size in the vicinity of 0.2% proof stress, although each specimen consisted of heterogeneous grains of different sizes. The slope ky in the relation was almost independent of aging, and the increase of frictional stress σ 0 with aging was remarkable. The shear stress for releasing edge dislocation locking derived from the values of ky and σ 0 in an as-quenched specimen was twice as great as the CRSS of 7475 alloy single crystals. Comparing the values of σ 0 in as-quenched and aged specimens, the increase of frictional stress with T6 aging was in agreement with 270 MPa estimated for 0.5% proof stresses of single crystal alloys.
(Received November 18, 2002)
aluminum-zinc-magnesium-copper alloy polycrystal, slip deformation, in-situ observation, Hall-Petch relation, crystal plasticity, dislocation
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