A method for revealing the localised work hardened region (plastic deformation zone) in Mg-9Al-1Zn alloy (AZ91) has been developed using a new etching technique. With this technique, etching with the solution (9 g picric acid, 30 mL acetic acid, 150 mL ethyl alcohol and 20 mL H O) was executed on the samples after plastic deformation and then heating to 300°C for 3 h. Because of the high strain energy in the plastic deformation zone, the microstructural characteristic in the deformation zone changed substantially after the heating process, where severe precipitation of Mg Al phase occurred in the Mg rich α phase. With the change of microstructural characteristics, the deformation region can be revealed. The plastic zone was revealed by the different degrees of etching response in the deformation and undeformed regions; the plastic zone was slightly etched, whereas the other region was deeply etched. From the different surface height level, the deformation zone was found to be observable even at low magnification. © 2011 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute. 2 17 12