Influence of grain size and grain boundary misorientation on the fatigue crack initiation mechanisms of textured AZ31 Mg alloy

TL;DR

This study investigates how grain size and boundary misorientation influence crack initiation mechanisms in textured AZ31 Mg alloy under cyclic loading, revealing the roles of slip systems, twins, and grain boundary characteristics.

Contribution

It provides new insights into the relationship between grain features and fatigue crack initiation mechanisms in AZ31 Mg alloy.

Findings

Cracks initiate in both small and large grains with different mechanisms.

High angle grain boundaries are associated with crack initiation in small grains.

Transgranular cracks dominate in large grains after cyclic deformation.

Abstract

The deformation and crack initiation mechanisms were analyzed in a textured AZ31B-O Mg alloy subjected to fully-reversed, strain-controlled cyclic deformation along the rolling direction after 50 cycles (approximately 33% of the fatigue life). Distinct deformation bands corresponding to pyramidal slip or tensile twins were found in 538 grains out of 2100 grains. Slip trace analysis showed that 72.3% were pyramidal slip bands and 18.4% were twin boundaries. Both pyramidal slip and twinning was only found in 9.1% of the grains with deformation bands. Cracking was widespread after 50 cycles. Grain boundary cracks were found in approximately 15% of the small grains (< 20 $\mu$m) and they were mainly associated with high angle grain boundaries (>40 degrees). Cracking was also found to occur by transgranular cracks parallel to the pyramidal slip bands or twin boundaries in large grains (>45 $\mu$m). The majority (>60%) of these large grains presented transgranular cracks after 50 cycles.