Low cycle fatigue behavior of a semi-solid processed AM60B magnesium alloy

Strain-controlled low cycle fatigue tests were conducted on a thixomolded (semi-solid processed) AM60B alloy, and its fatigue life, fatigue parameters and cyclic deformation characteristics were evaluated. The microstructure consisted of primary α-Mg and network-like eutectic structure containing β-Mg17Al12 particles. The alloy showed basically symmetrical hysteresis loops in tension and compression and superior fatigue resistance to its die cast counterpart at lower strain amplitudes. The hysteresis loops exhibited a characteristic clockwise rotation, and the elastic modulus decreased with increasing strain amplitude. This was attributed to the non-linear (pseudoelastic) behavior stemming from twinning–detwinning. In comparison with extruded magnesium alloys, while the thixomolded alloy had less extensive twinning during cyclic deformation due to the presence of a large number of β-Mg17Al12 particles along with relatively small grain sizes, two types of twins (namely wider lenticular extension twins and narrower banded contraction twins) still occurred in some favorably oriented and relatively large primary α-Mg grains in the vicinity of fracture surface. With increasing strain ratio fatigue life decreased and the extent of mean stress relaxation increased. While the monotonic and cyclic yield strengths of this alloy were similar, the cyclic strain hardening exponent was higher than the monotonic strain hardening exponent.