The dynamic thermo-mechanical behavior of pure aluminum has attracted renewed interest lately due to experimental observations of an anomalous increase in Hugoniot Elastic Limit (HEL) at incipient plasticity and elevated temperatures in polycrystalline pure metals. In context of current dislocation-mediated plasticity models for metals, this increase in dynamic strength is indicative of a transition in the rate controlling mechanism for dislocation glide from being thermally assisted to phonon-drag restricted due to increase in phonon viscosity at elevated temperatures. Though these studies have helped to shed light on these important mechanisms operative in FCC metals at incipient plasticity, the extent to which they contribute to flow stress, particularly at larger strains, remains unclear. In the present study, we address these questions through a combined experimental and modeling effort focused on investigating the evolution of dynamic flow stress in polycrystalline aluminum using experimental data gathered from a series of combined pressure-and-shear plate impact (PSPI) experiments designed to reveal the flow stress of pure aluminum at strain rates ~ 10⁵/s, plastic strains of up to 40% and temperatures ranging from room to 866 K. In all cases, the flow stress of aluminum, as inferred from the measured transverse particle velocity histories at the free surface of an elastic tungsten carbide target plate, reveals saturation with increasing plastic strains at stress levels that decrease with increasing test temperatures. Numerical simulations are performed to correlate the experimentally observed temperature and strain rate dependence of flow stress at small and large plastic strains using the Austin-McDowell dislocation-mediated plasticity model parametrized to normal plate impact experiments conducted in an earlier study by Zaretsky and Kanel (2012). Extensions to the model are made to better represent dynamic behavior of pure aluminum at larger plastic strains as observed in elevated temperature split Hopkinson Pressure bar (SPHB) experiments of Samanta (1971) and Lindholm and Yeakly (1965), and the combined pressure-and-shear plate-impact experiments conducted in the present study. The main theoretical extension to the Austin-McDowell model made in this paper is the introduction of a new rate- and temperature- dependent dynamic recovery function, which can potentially allow for an effective reduction in the rate of accumulation of dislocations at large plastic strains. The numerical predictions of the revised and re-calibrated plasticity model are brought into agreement with the experimental observations, correlating sufficiently well with dynamic yield stress at incipient plasticity and the flow stress levels at larger plastic strains, plastic strain rates in the range 10³ – 10⁶ /s, and elevated temperatures up to near melt. The model, in concert with the experimental measurements, suggests that at incipient plasticity the rate governing mechanism for plastic flow is phonon-drag restricted dislocation glide, whereas, at higher magnitudes of plastic strain, it transitions to stress-assisted thermally activated glide. The transition between these two rate governing mechanisms is controlled by the evolution of dislocations throughout the deformation process.

最近，由于实验观察到在初始可塑性和高温下多晶纯金属中Hugoniot弹性极限（HEL）异常增加，纯铝的动态热机械行为引起了人们的新兴趣。在当前金属的位错介导的可塑性模型的背景下，动态强度的这种增加表明位错滑移的速率控制机制的转变是由于在高温下声子粘度的增加而被热辅助到声子阻力的限制。尽管这些研究有助于阐明这些在FCC金属中起初塑性作用的重要机理，但它们对流应力的影响程度（尤其是在较大应变下）的程度仍不清楚。在目前的研究中，⁵/ s，塑性应变高达40％，温度范围从室温到866K。在所有情况下，铝的流动应力，从在弹性碳化钨靶板的自由表面上测得的横向粒子速度历史记录推断，结果表明，随着应力水平的升高，随着塑性应变的增加，饱和度随测试温度的升高而降低。使用Austin-McDowell位错介导的可塑性模型进行参数化与Zaretsky和Kanel（2012年早期研究）进行的正常板撞击实验，进行数值模拟以关联实验观察到的大小应力对温度和应变速率的依赖性。 ）。对模型进行了扩展，以更好地表示纯铝在较大塑性应变下的动态行为，如在Samanta（1971）和Lindholm and Yeakly（1965）的高温分流霍普金森压力棒（SPHB）实验以及组合的压力和本研究中进行的剪切板冲击试验。本文对Austin-McDowell模型所做的主要理论扩展是引入了一种新的取决于速率和温度的动态恢复函数，该函数可以潜在地有效降低大塑性应变下位错积累的速率。修正和重新校准的可塑性模型的数值预测与实验观察结果一致，³ – 10 ⁶ / s，温度升高至接近融化。该模型与实验测量结果相吻合，表明在塑性初期，塑性流动的速率控制机制是声子-阻力限制的位错滑移，而在较高的塑性应变下，它转变为应力辅助的热活化滑移。在整个变形过程中，位错的演化控制着这两种速率控制机制之间的过渡。