Wang Feng, Tzanakis Iakovos, Eskin Dmitry, Mi Jiawei, Connolley Thomas
Brunel Centre for Advanced Solidification Technology (BCAST), Brunel University London, Uxbridge UB8 3PH, UK.
Faculty of Technology, Design and Environment, Oxford Brookes University, Wheatley Campus, Wheatley OX33 1HX, UK.
Ultrason Sonochem. 2017 Nov;39:66-76. doi: 10.1016/j.ultsonch.2017.03.057. Epub 2017 Apr 9.
The cavitation-induced fragmentation of primary crystals formed in Al alloys were investigated for the first time by high-speed imaging using a novel experimental approach. Three representative primary crystal types, AlTi, Si and AlV with different morphologies and mechanical properties were first extracted by deep etching of the corresponding Al alloys and then subjected to ultrasonic cavitation processing in distilled water. The dynamic interaction between the cavitation bubbles and primary crystals was imaged in situ and in real time. Based on the recorded image sequences, the fragmentation mechanisms of primary crystals were studied. It was found that there are three major mechanisms by which the primary crystals were fragmented by cavitation bubbles. The first one was a slow process via fatigue-type failure. A cyclic pressure exerted by stationary pulsating bubbles caused the propagation of a crack pre-existing in the primary crystal to a critical length which led to fragmentation. The second mechanism was a sudden process due to the collapse of bubbles in a passing cavitation cloud. The pressure produced upon the collapse of the cloud promoted rapid monotonic crack growth and fast fracture in the primary crystals. The third observed mechanism was normal bending fracture as a result of the high pressure arising from the collapse of a bubble cloud and the crack formation at the branch connection points of dendritic primary crystals. The fragmentation of dendrite branches due to the interaction between two freely moving dendritic primary crystals was also observed. A simplified fracture analysis of the observed phenomena was performed. The specific fragmentation mechanism for the primary crystals depended on their morphology and mechanical properties.
首次采用一种新颖的实验方法，通过高速成像研究了铝合金中初生相晶体的空化诱导破碎。首先通过对相应铝合金进行深蚀刻，提取出具有不同形态和力学性能的三种代表性初生相晶体类型，即AlTi、Si和AlV，然后将它们置于蒸馏水中进行超声空化处理。对空化气泡与初生相晶体之间的动态相互作用进行了原位实时成像。基于记录的图像序列，研究了初生相晶体的破碎机制。结果发现，初生相晶体被空化气泡破碎主要有三种机制。第一种是通过疲劳型失效的缓慢过程。静止脉动气泡施加的循环压力使初生相晶体中预先存在的裂纹扩展到临界长度，从而导致破碎。第二种机制是由于空化云团中气泡的坍塌而产生的突然过程。云团坍塌时产生的压力促进了初生相晶体中裂纹的快速单调扩展和快速断裂。观察到的第三种机制是由于气泡云团坍塌产生的高压以及枝晶状初生相晶体分支连接点处的裂纹形成而导致的正常弯曲断裂。还观察到由于两个自由移动的枝晶状初生相晶体之间的相互作用而导致的枝晶分支破碎。对观察到的现象进行了简化的断裂分析。初生相晶体的具体破碎机制取决于它们的形态和力学性能。
