The Effect of Solute Atoms on Aluminum Grain Boundary Sliding at Elevated Temperature
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TRODUCTION
GRAIN boundary sliding (GBS) involves the rigid translation of one grain over another adjacent grain parallel to the grain boundary (GB) interface. GBS mediates the plastic flow of polycrystalline materials,[1,2] especially when grain sizes decrease to nanometer scale.[3] For micron-sized grains, GBS becomes an important deformation mechanism for elevated temperature forming, such as quick plastic forming or superplastic forming.[4–12] It is important to understand how to modify the chemistry at GBs through solute alloy additions to increase the contribution of GBS to total deformation so that the extra processes for refining grain sizes can be avoided. Extensive experimental evidence has suggested that GB chemistry plays an important role in controlling GBS. For example, Vetrano et al.[13–15] have suggested that Sn segregates to GBs and enhances GBS in polycrystalline Al-Mg alloys. In these materials, the Sn in the GB is considered to be a separate phase with lower melting temperature, which decreases the shear strength of the GB. McFadden and Mukherjee[16] showed that superplasticity cannot be achieved in Ni without sulfur, and concluded that the role of sulfur in Ni superplasticity is to inhibit grain growth and enhance GBS by weakening Ni-Ni bonds. The effect of the solute atoms on GBS likely depends on their concentration in the alloy, their segregation level at the GB, and their ability to enhance or inhibit GBS. Many alloying elements in superplastic alloys indeed segregate to GBs, either during static aging or NINGNING DU, Graduate Student, and ALLAN F. BOWER, Professor, are with the Division of Engineering, Brown University, Providence, RI 02912. YUE QI, Staff Research Scientist, is with Chemical Sciences and Materials Systems Lab, General Motors Company, LLC, Warren, MI 48090. Contact e-mail: [email protected] PAUL E. KRAJEWSKI, Group Manager, is with Global Product Development, General Motors Company, LLC. Manuscript submitted December 14, 2009. Article published online June 11, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
deformation. In commercial-purity Al AA5083 alloys, Si segregates to the GBs, and the concentration of Si at the GBs increases with strain and decreases with deformation temperature.[13] Li et al.[17] showed that ultrasonic melt treatment can produce a uniform distribution of Si segregation to the Al GBs. The segregation of Sn and Zr to GBs also has been observed in a polycrystalline Al-Mg-Mn-Sn alloy during static ageing at 723 K (450 C) to 823 K (550 C).[13,14] Mg segregated to the GBs during static aging[18] but depleted during plastic deformation.[13,14] Computational studies have confirmed these observations. For example, Liu et al.[19,20] have performed molecular dynamic simulations of Al-Mg alloys using the embedded atom method (EAM) potential. Their results show that Mg will segregate to GBs and that the level of segregation depends on the structure of the GB; the Mg segregation level is higher near lower angle [110] tilt boundaries but shows minimal variations at [100]
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