  In this work electron backscattered diffraction (EBSD)-assisted slip trace
analysis and transmission electron microscopy have been utilized to investigate
the interaction of basal dislocations with precipitates in the Mg alloys
Mg-1%wt.Mn-0.7%wt.Nd (MN11) and Mg-9%wt.Al-1%wt.Zn (AZ91), with the ultimate
aim of determining the origin of their poor precipitation hardening.
Precipitates in these alloys have a plate-shaped morphology, with plates being,
respectively, perpendicular (MgxNdy) and parallel (Mg17Al12) to the basal plane
of the magnesium matrix. Mechanical tests were carried out in solid solution
and peak-aged samples, in tension and compression, both at RT and at moderate
temperature (250C). EBSD-assisted slip trace analysis revealed a clear
dominance of basal slip under a wide range of testing conditions in the
peak-aged MN11 and AZ91 alloys. At room temperature, the origin of the low
precipitation hardening observed lies at the easiness with which precipitates
are sheared by basal dislocations, due to the lattice matching at the interface
with the Mg matrix. At high temperature, dislocation-precipitate interactions
are highly dependent on the deformation mode. In tension, enhanced basal slip
localization gives rise to high stress concentrations at the intersection
between coarse slip traces and particle interfaces, leading to precipitate
fracture; in compression, a more homogenous distribution of basal slip leads to
the dominance of particle shearing. Our study demonstrates experimentally that
basal dislocations are able to shear, and even fracture, the MgxNdy and
Mg17Al12 plates when, for appropriate testing conditions, the local stress due
to dislocation accumulation at particle interfaces exceeds the precipitate
strength.
