The plastic zone and growth of fatigue cracks in magnesium MA12 alloy at room and low temperatures

The plastic zone formed at the fatigue crack tip and the fracture topography in MA12 magnesium alloy samples, tested at 293 and 140 K in air and in vacuum, were analysed. It was found that the plastic zone formed in vacuum is characterized by a greater size (h) and degree of plastic strain that in air, and the crack growth rate (dl/dN) is lower. Temperature reduction leads to a decrease in h, while dl/dN and the fracture mechanism are affected by temperature ambiguously, depending on the alloy microstructure and the Kmax value. It was established that the size of the plastic zone can be described by the equation: h=A(Kmaxσ0.2ps)2 where A is a coefficient dependent on the alloy structural state, environment and test temperature. Evaluation of the cyclic plastic zone size at Kmax, corresponding to the transition from a low temperature region to a ‘Paris’ region, showed that this transition occurred when the cyclic plastic zon reached the structural parameter of the material.