Effects of Plastic Inhomogeneity on the Texture Hardening and Suuperplastic-like Properties of Some Extruded Aluminum Alloys with [111] Fiber Texture

• Main Authors: Uan, Jun-Yen, 汪俊延
• Other Authors: Lui Truan-Sheng, Chen Li-Hui
• Format: Others
• Language: zh-TW
• Published: 1997
• Online Access: http://ndltd.ncl.edu.tw/handle/97127459761555145888

博士 === 國立成功大學 === 材料科學(工程)學系 === 85 === Extrusion of commercial purity aluminum and many Al-based composites often leads to [111] fiber texture. This preferred orientation may not only enhanc e the tensile strength, but also affect other mechanical behaviors such as uniform deformation and formability. Based on this consideration, a widely used 99.8 wt.% pure Al and the Al- Al3Ni eutectic alloy, a potential new structural material, were chosen in this study to explore texture hardeni ng and superplastic-like deformation, with special emphasis on the roles of [111] fiber texture and second phase particle distribution. The tensile tests were performed at room temperature and 500(C, all wi th the tensile direction taken along the [111] fiber axis (i.e. along ex trusion direction). TEM results of the deformed specimens show that primary slip is the main dislocation activity, but it can only be observed in limit ed subgrainsand most of the subgrains are virtually dislocations-free. This p lastic inhomogeneity, which gives rise to the room temperature texture h ardening and 500(C superplastic-like deformation, can be rationalizedby comp aring the Taylor factors of primary slip and multiple slip for the subgr ains of different misorientations. The Taylor factor decrease with in creasing misorientation of the [111] direction from the tensile axis, where the rate of decrease is faster for primary slip. The smaller the Tay lor factor, the easier the dislocation slip. The pole figure data indicate that fewer subgrains have larger misorientation. Owing to the fact that their Taylor factors are smaller, these subgrains should be the se enco untered primary slip of parallel dislocations. The pure Al and the Al-Al3Ni eutectic alloy with pure i ntergranular distribution of the Al3Ni particles in the as-extruded condition and only the latter has superplastic-like behavior at 500 (C. The above two materials reveal a plateau-like flow behavior at the initial stage of yielding at room temperature since primary slip is the main dislocation activity. In contrast to the usual Hall- Petch str engthening, the yielding strength of the extruded aluminum with [111] fib er texture dose not very with subgrain size. At 500(C, the textures of t he Al-Al3Ni eutectic alloy before and after superplastic-like deformation show characteristics that only small portion of the subgrains are misali gned larger from the fiber axis. Therefore, the TEM microstructures show plast ic inhomogeneity in which parallel dislocations can be observed in a f ew subgrains even in the final fracture stage. Rather than the degree of strain hardening, the superplastic-like behavior is associated m ainly with high strain softening resistance. Strain rate harden ing dose not govern the superplastic-like behavior although it p lays a role. Intragranular particles can eliminate the plastic inhomogeneity by causing large amount of dislocations to tangle around the particles, which will not only su ppress texture hardening at room temperature but also easily introduc e dynamic recovery and thus strain softening at 500(C. To restrict th e number of intragranular particles is therefore required in order to have superplastic-like behavior. Through the subgrain refining inv olving particle bonding and ladder-like subgrain formation by pinning of primary slip dislocations, this restriction can be achieved by trapping the particles in the newly formed subgrain boundaries.