Microstructure and dislocation analysis after creep deformation of die-cast Mg–Al–Sr (AJ) alloy

The microstructure and creep behavior of Mg/Al composite crankcases cast with three alloy formulations of the Mg–Al–Sr alloy AJ62 have been investigated. Overall 12 components were used within this study. Multi-level creep tests were conducted to evaluate the creep properties at stresses up to 90 MPa and temperatures up to 473 K. Microstructure observations including phase characterization and in-depth dislocation analyses were performed in the as cast condition and after creep testing. The tensile creep testing revealed a distinct primary creep and a high stress exponent up to a value of 10. The threshold stress concept was applied, which yields to an effective stress exponent of 5 indicating a strengthening effect due to particle–dislocation interaction. Transmission election microscopy (TEM) of the microstructure revealed the continuous precipitation of β-Mg17Al12 in the α-Mg matrix near the interdendritic regions during creep. In addition, a fine-dispersed nano-scaled Al–Mn phase, probably Al8Mn5, was observed in the α-Mg matrix in all samples under investigation. According to an in-depth TEM analysis of the dislocation structure, slip of non-basal 〈a〉 dislocations and 〈c + a〉 dislocations is activated in addition to basal slip even at 423 K and very low stress (15 MPa). Furthermore, the TEM images reveal a strong interaction between dislocations and the Mg–Al and Al–Mn matrix precipitates. Hence, matrix strengthening by well-distributed precipitates could be one factor for the excellent creep resistance of AJ-alloys. Despite of the matrix precipitates, the substitution of the eutectic phase Al4Sr by Mg9Al3Sr in one of the alloys seems to be the major difference in the investigated alloys and should therefore account for the differences in creep rate and creep strain.