Impact rupture behavior of AZ80 magnesium alloys fabricated by multi-directional forging under decreasing temperature conditions

Abstract

A hot-extruded AZ80 magnesium (Mg) alloy was multi-directionally forged (Multi-Directionally Forged/MDFed) under decreasing temperature conditions from 673 K down to 433 K to cumulative strain of Σδϵ=5.6 (i.e., 7 passes of MDFing) at maximum. The initial average grain size of 20.1 μm was gradually reduced with increasing cumulative strain and average grain size of 0.4 μm could be attained at cumulative strain of Σδϵ=5.6. The Charpy impact tests of MDFed AZ80Mg alloys having various grain sizes were carried out. The impact value first increased up to Σδϵ=0.8, however, it particularly and gradually decreased with increasing cumulative strain over Σδϵ=1.6. Fracture surfaces of the hot-extruded and the MDFed samples to Σδϵ=0.8 exhibited dimples indicating occurrence of ductile fracture. The MDFed samples to Σδϵ=3.2-5.6, however, showed rather flat appearances indicating occurrence of brittle fracture. Because bumps, which sizes were corresponding to grain sizes, were observed on the flat fracture surfaces, it is suggested that cracks propagated along grain boundaries. That is, crack propagation along grain boundaries resulted in the decrease in the impact value, and therefore, it gradually decreased with grain size.