The effects of heat treatment on very high cycle fatigue behavior in hot-rolled WE43 magnesium

The role of crack initiation and short crack growth on fatigue life in the very high cycle fatigue regime (VHCF) is investigated for three heat treatments of the wrought magnesium alloy WE43. As-received (T5) WE43 with a relatively fine grain size was solution treated and aged to produce precipitation strengthened coarse-grained microstructures in the underaged and peak-aged (T6) conditions. Ultrasonic axial fatigue tests with a cyclic frequency of 20 kHz were conducted using smooth specimens. Heat treatment was shown to have a strong effect on fatigue strength, with the fine-grained, strain hardened T5 condition exhibiting much higher values than the coarse-grained conditions. No significant difference in fatigue strength was observed between the underaged and peak-aged microstructures of equivalent grain size. Crack initiation and short crack growth behaviors in each condition were investigated to determine if one of these behaviors dominated the VHCF lives. It was found that average short crack growth rates for the three conditions were similar and had no clear dependence on microstructural condition. Crack initiation was shown to occur through cyclic slip deformation in particularly large and favorably oriented grains in each condition. Subsurface crack initiation was observed at low stresses and high lifetimes in the coarse-grained conditions, but not in the fine grained T5 condition. Crack growth rates in vacuum were investigated using a unique combination of ultrasonic fatigue instrumentation and scanning electron microscopy (UFSEM) in order to simulate subsurface crack propagation. Environment was shown to have a significant effect on crack growth rate, with rates in vacuum nearly two orders of magnitude lower than in laboratory air. (C) 2016 Elsevier Ltd. All rights reserved.