A Precipitate-Strengthening Model Based on Crystallographic Anisotropy, Stress-Induced Orientation, and Dislocation of S
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CONTROLLING mechanical property anisotropy is very important in aluminum alloys for aerospace applications.[1–3] During the manufacturing process, anisotropic mechanical properties arise from strengthening precipitates, as well as the crystallographic texturing.[4,5] In 2024 series aluminum alloys with plate or lath-like precipitates, applying elastic stress during aging significantly influences the distribution of h¢ or S¢ precipitates, namely, the stress-orienting effect, which leads to anisotropy of the mechanical properties.[6–10]
XIAOBIN GUO, YONG ZHANG, YUNLAI DENG, and XINMING ZHANG are with the School of Materials Science and Engineering, Central South University, Changsha, China and also with the State Key Laboratory of High Performance and Complex Manufacturing, Central South University, Changsha, China. Contact e-mail: [email protected] JIN ZHANG is with the State Key Laboratory of High Performance and Complex Manufacturing, Central South University, Changsha, China and also with the Light Alloy Research Institute, Central South University, Changsha, China. Manuscript submitted December 15, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS A
This stress-aging treatment has a direct relationship with creep age-forming manufacturing processes, which have been applied to lightweight structures, such as wings and ribbed aircraft door protection.[11,12] In stress-aged aluminum alloys, age forming combines age strengthening with metal forming. As fine precipitates disperse in grains and coarsened precipitates form on grain boundaries, the strength and corrosion-resistance properties improve simultaneously.[13–16] However, stresses applied during aging induce the oriented distribution of plate/lath-like precipitates, known as the stress-orienting effect. This effect directly restricts the development of ‘‘age-forming’’ techniques for 2024 aircraft structures.[6–10,12] It has been reported previously[17–20] that external stresses applied during the heating process lead to the oriented distribution of precipitates in some alloys, such as the Ti-hydride phase in Ti-H binary alloys,[17] the Zr-hydride phase in Ziracloy-2 alloys,[18,19] and the Fe16N2 phase in Fe-Ni alloys.[20] All these precipitates distribute preferentially with some variations. Previous investigations[6–10] on stress-aged Al-Cu alloys indicated that the stress-orienting effect depends on the applied stress, temperature, and alloy composition. Hosford and Agrawal[6] studied the stress-orienting
effect of precipitates in Al-4Cu alloy single crystals and found that h¢ precipitates grown on habit planes perpendicular to the applied compressive stress were generated preferentially. However, Eto et al.[7] found contrary results. When the temperature during stress-aging was lower, h¢ precipitates distributed preferentially on the habit planes, which are parallel to the applied compressive stress. The same authors[7] also explained the stress-orienting effect from the precipitates based on the relationship of misfit inclusions and applied stress direction. Skro
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