Its Because of its application. AZ31 and AZ91 both can be used in various functional application and for past decade in bio-medical application also. They can be used in the implants. There has been a good volume of research you can find under this heading. AZ61 can be used in other applications but not in implants because of its material properties. I think that is the reason why we have much research about the previous one.
I am very sorry, but there are many woks published on this topic. You may find it through data-base searchs, for instance using the web of knowledge.
I send you a PDF document just printed from a very quick search on this data base. With the 50 references you will find here you will have a lot of work to search the cross references.
I would like to share with you an useful experience from my own research. At present, once you think in a research topic, whatever it could be, you may be practically sure that at this same moment there will have many people doing research on it. If this is not the case, you risk to have the next Nobel prize.
I hope this will be useful for your future research life.
The following article may help you to understand the alloy.
R.C. Zeng*, W. Ke, E.H. Han, Influence of load frequency and ageing heat treatment on fatigue crack propagation rate of as-extruded AZ61 alloy. International Journal of Fatigue, 31(3), 2009: 463–467.
Article Influence of load frequency and ageing heat treatment on fat...