Lattice strain evolution during age hardening of Mg AZ91 alloy
Sitarama Raju Kada1, P. A. Lynch2, M. R. Barnett3
1Institute for Frontier Materials, Deakin University, Geelong, VIC-3216, Australia.
e: s.kada@deakin.edu.au
In-situ laboratory based transmission X-ray diffraction was utilized to study the
lattice strain evolution for aged AZ91 magnesium alloys. Commercially obtained
AZ91 cast alloy was hot rolled at 400oC to a total strain of 0.8 - resulting in a grain
size of ~10 µm. Aged samples were prepared at 200oC for 30 hours. After rolling, all
alloys exhibit a typical rolling texture characteristic for magnesium alloys such that
the basal poles are aligned along the normal direction of rolled sheet with a spread of
about 40o. In-situ tensile tests were performed along the normal direction of the rolled
sheet in constant strain mode to study the effect of basal plate precipitates on the
age hardenability of basal slip and (1012) twinning modes. From in-situ
measurement of the lattice strain evolution with tensile load, an estimate of the
Critical Resolved Shear Stress (CRSS) for slip and twinning in the as-rolled and peak
aged AZ91 alloys was determined.
The CRSS for basal slip calculated from the lattice strain of the (1013)
reflection increased by about 6 MPa due to ageing comparing with that of the asrolled alloy. The CRSS for deformation twinning was calculated from the drop in the
intensity of (0002) grains by 10%. The CRSS for 10% twinning was increased by
~26 MPa due to ageing when compared with the as-rolled alloy.
These results highlight that the basal plate precipitates in the AZ91 alloy exhibit
poor hardening against basal slip but significant hardening in the case of deformation
twinning.