Effect of grain size on fatigue behavior in AZ61 Mg alloys fabricated by MDFing

Abstract

Magnesium (Mg) alloy AZ61 was multi-directionally forged (MDFed) under decreasing temperature conditions using a die. The average grain size decreased with increasing MDFing pass number. The initial grains size of 21.6 μm in the as-annealed specimen decreased gradually during MDFing and an average grain size of 0.3 μm could be attained after MDFing for 8 passes. The tensile strength and Vickers hardness were improved with increasing pass number from 1 to 8. Hall-Petch relationship was held for those static mechanical properties. Subsequently, tension-tension axial loading fatigue tests were performed using the as-annealed specimen and MDFed ones to 1, 3, 6 and 8 passes in which cumulative strains were 0.8, 2.4, 4.8 and 6.4 respectively. Fatigue strengths were highly improved by MDFing with increasing pass number of forging from 1 to 3. However, the improvement looked almost saturated over the pass number of 3. The observed breaking-up of the Hall-Petch relationship concerning with fatigue limits was attributed to grain-boundary sliding followed by crack initiation and propagation along grain boundaries.