Formation of embedded nanoparticles in an excimer laser treated Mg alloy surface

The surface melting characteristics of AZ91 alloy is investigated using excimer laser (pulses of length 45 ns) under ambient conditions and also a N 2 environment. Experiments are carried out on overlapping spots with close interval. The melt layer hardness for treatment in air is increased to 162 VHN as compared to 73 VHN for the untreated alloy (AZ91 T4 condition), whereas the hardness of the sample treated in an N 2 atmosphere is 116 VHN. A transmission electron microscopy study reveals that the laser treated surface contains nanoparticles of oxide and nitride embedded in the submicron grains of the Mg solid solution. This is responsible for the observed high hardness which gives rise to the improved abrasive resistance. We propose a model to describe the formation of nanoparticles during the laser treatment.