Fatigue crack growth study on rolled and cryogenically treated AZ31B Mg alloy plate

•As-rolled AZ31B alloy plate (R) was subjected to shallow cryogenic treatment (SCT) and through-thickness deep cryogenic treatment (DCT).•Cryogenic treatment promoted the formation of microstrain, high dislocation density, and random distribution of secondary phase precipitates (i.e. β-Mg17Al12) along the grain boundaries in the AZ31B plate.•Fatigue crack growth rate (FCGR) testing of as-rolled and cryogenically treated AZ31B alloy specimens was carried out and the results were compared.•Cryogenic treatment lowered the fatigue crack growth resistance of the AZ31B alloy. The fatigue crack growth stability was in the following order: DCT > SCT > R. This paper investigates the fatigue crack growth rate (FCGR) of AZ31B Mg alloy after cryogenic treatments. Shallow and deep cryogenic treatments were carried out separately at −80οC and −196οC for 12 and 72 hrs, respectively. The microstructural characterization of the cryogenically treated AZ31B specimens was carried out by optical microscope (OM) and X-Ray diffraction (XRD). The small crystalline size structure generated in AZ31B alloy due to the cryogenic treatment further promoted the nucleation of Mg17Al12 phase. Accumulation of dislocation density was also observed in the cryogenically treated specimens. CT specimens were prepared for all three conditions to estimate the FCGR. The as-rolled specimen (R) had higher fatigue resistance than the cryogenically treated specimens. The R specimen’s threshold intensity factor (ΔKth) was higher than the cryogenically treated specimens. The exponent (m) value of the R specimen was lower than the shallow and deep cryogenic treated specimens. Hence, the crack growth resistance was marginally higher in the R specimen compared to the cryogenically treated specimens. However, the critical stress intensity factor (ΔKcr) of cryogenically treated specimens was slightly higher than the R specimen. The post-fracture characterization of the specimens was carried out at three different regions by SEM. The cleavage faces, tear ridges, and voids were noticed in all specimens. The fatigue failure of the R specimen was due to ductile and brittle mode, whereas the cryogenically treated specimen was dominated entirely by brittle fracture.