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

Abstract

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 20kHz 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.

Near‐threshold fatigue crack growth properties of wrought magnesium alloy AZ61 in ambient air, dry air, and vacuum

“…Previous investigations on three cast magnesium alloys (AZ91, AM60, and AS21) by Papakyriacou et al delivered similar results of threshold stress intensity factors in ambient air, which were between 55% and 65% of the respective threshold value measured in vacuum. Adams et al investigated the propagation of short cracks in wrought WE43 alloy and found that growth rates in vacuum are nearly two orders of magnitude lower than in laboratory air. In vacuum, higher stress intensity factor amplitudes were necessary to further prolong cracks that had been previously initiated and propagated in ambient air.…”

“…Threshold stress intensity factor amplitudes are in the range of 1.25 to 1.55 MPam 1/2 in ambient air but 2.1 to 2.7 MPam 1/2 in the inert environment . In differently heat‐treated wrought magnesium alloy WE43, fatigue crack growth of short cracks strongly decelerates or even stops, when the testing environment is changed from ambient air to vacuum …”

“…9 In differently heat-treated wrought magnesium alloy WE43, fatigue crack growth of short cracks strongly decelerates or even stops, when the testing environment is changed from ambient air to vacuum. 10 Atmospheric corrosion of magnesium alloys is highly dependent on the availability of water. Therefore, the amount of humidity in ambient air determines its deleterious effect on the cyclic properties.…”

PRISMS: An Integrated, Open-Source Framework for Accelerating Predictive Structural Materials Science

“…73,132 The parameters of the cohesive law for crack propagation along various slip systems have been obtained using experimental (micro-beach marking) measurements of local crack growth rates within a grain. 127 This is coupled with Fig. 12.…”

“…A unique ultrasonic fatigue scanning electron microscope (UFSEM) was used to quantify the growth rates of cracks as they advance through grains and across grain boundaries. 127,128 Focused ion beam notches were placed in grain neighborhoods with specific characteristics (e.g., Schmid factors, misorientation). Detailed analysis of crack initiation lifetime, small crack propagation rates, crack interactions at grain boundaries and crack pathways in each grain were characterized, 129 an example of which is shown in Fig.…”