Abstract

The effect of minute Cr additions on dendritic growth directions in Al-20 wt pct Zn alloy has been investigated. As first evidenced by Gonzales and Rappaz (Metall Mater Trans A 37:2797–2806, 2006) Al-Zn alloys exhibit a Dendrite Orientation Transition (DOT), from 〈100〉 below 25 wt pct Zn to 〈110〉 above 60 wt pct, regardless of the solidification speed. The DOT has been interpreted as a modification of the solid–liquid interfacial energy anisotropy (Gonzales and Rappaz, 2006; Haxhimali et al. in Nat Mater 5:660–664, 2006). However, 0.02 to 0.1 wt pct addition of Cr in Al-20 wt pct Zn drastically modifies the dendrite trunk direction from 〈100〉 to 〈110〉 at low solidification speed, typically 200 μm/s. Even more surprising, 〈100〉 dendrite trunks are retrieved in Al-20 wt pct Zn-0.1 wt pct Cr when the speed is increased to 1000 μm/s, but with the concurrent formation of twinned dendrites. Minute additions of Cr have been reported recently to promote icosahedral short range order (ISRO) in liquid Al-Zn, thus reducing the atomic mobility (Kurtuldu et al. in Acta Mater 115:423–433, 2016) and modifying the nucleation kinetics (Kurtuldu et al. in Acta Mater 61:7098–7108, 2013). The present results indicate that ISRO also modifies the attachment kinetics of icosahedral clusters rather than individual atoms, and is most likely responsible of growth direction change in Al-Zn-Cr alloy and of twinned dendrites formation.

中文翻译：

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微量Cr对Al-Zn合金中柱状枝晶生长的影响：液体中二十面体短程有序的影响

抽象的

已经研究了微量Cr添加对Al-20 wt pct Zn合金中枝晶生长方向的影响。最早由Gonzales和Rappaz证明（Metall Mater Trans A 37：2797–2806，2006）的Al-Zn合金表现出枝晶取向转变（DOT），从Zn的<100>低于25 wt％，到Zn的<110>高于60 wt％。 ，无论固化速度如何。DOT被解释为固液界面能各向异性的一种改变（Gonzales和Rappaz，2006； Haxhimali等人，Nat Mater 5：660-664，2006）。然而，0.02至以Al-20 0.1重量PCT添加Cr的重％的Zn急剧改变来自<100>到<110>在低温凝固速度，通常为200枝晶主干方向 μ小姐。更令人惊讶的，<100>枝晶中继线以Al-20重％的Zn-0.1重％的Cr检索时的速度增加到1000 μ米/秒，但与孪晶枝晶的同时形成。最近报道了微量添加Cr可以促进液态Al-Zn中的二十面体短程有序（ISRO），从而降低原子迁移率（Kurtuldu等人在Acta Mater 115：423-433，2016）并改变成核动力学（ Kurtuldu et al。in Acta Mater 61：7098-7108，2013）。目前的结果表明，ISRO还改变了二十面体簇的附着动力学，而不是单个原子，并且最有可能负责Al-Zn-Cr合金的生长方向变化和孪生枝晶的形成。