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Room temperature creep and substructure formation in pure aluminum at ultra-low strain rates

机译：纯铝在超低应变速率下的室温蠕变和子结构形成

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The creep behavior in pure aluminum has been investigated by helicoid spring creep tests at strain rates, ε, lower than 10~(-10) s~(-1) and room temperature, 298 K. It was found that the creep behavior at the very low ε depends strongly on grain sizes and impurity concentrations: in high-purity aluminum (5 N Al) with an average grain size of 24 urn, the stress exponent was n ～ 1; while, when the average grain size was larger than 1600 urn, the stress exponent was n ～ 5. Microstructural observation shows the formation of large dislocation cells, 10 um. On the other hand, in commercial low-purity aluminum (2 N Al) with the average grain size of 25 urn, the stress exponent was n = 2. Microstructural observations revealed dislocations emitted from grain boundaries and the formation of lattice dislocation cells. To evaluate creep deformation mechanisms of the pure aluminum, stress change tests were conducted during creep tests. It was revealed that the deformation in the range of the stress exponent, n ～ 5, was controlled by recovery driven by internal stress, σ_i, because instantaneous strains at stress increment were larger than that at stress reduction. While the deformation behaviors in the ranges of the stress exponents, n ～ 1 and n = 2, were in viscous manner, because instantaneous strains at stress increment and reduction were in the same level. Based on those experimental results, the creep mechanisms have been discussed.
机译：通过螺旋弹簧蠕变试验，在应变率ε低于10〜（-10）s〜（-1）和室温298 K的条件下，研究了纯铝的蠕变行为。极低的ε很大程度上取决于晶粒尺寸和杂质浓度：在平均晶粒尺寸为24 um的高纯度铝（5 N Al）中，应力指数为n〜1。当平均晶粒尺寸大于1600 urn时，应力指数为n〜5。显微组织观察表明，形成了10 um的大位错细胞。另一方面，在平均粒径为25微米的商品低纯度铝（2 N Al）中，应力指数为n =2。微结构观察表明，晶界发出的位错和晶格位错单元的形成。为了评估纯铝的蠕变变形机理，在蠕变测试期间进行了应力变化测试。结果表明，应力指数n〜5范围内的变形受内应力σ_i驱动的恢复控制，这是因为应力增加时的瞬时应变大于应力减小时的瞬时应变。在应力指数范围n〜1和n = 2范围内的变形行为是粘滞的，这是因为在应力增加和减小时的瞬时应变处于同一水平。基于这些实验结果，讨论了蠕变机理。

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