as you asked for a free software to construct a ternary phase diagram, I think it is worth to mention the two free to use CALPHAD-projects which I know (CALPHAD means CALculation of PHaseDiagrams):
1. MatCalc-Project of the TU-Vienna (They even provide some free thermodynamic databases. Unfortunately no free database which includes the Zn-Al-Ni-System.):
http://matcalc.tuwien.ac.at/
2. The openCALPHAD-project of the ICAMS at the University of Bochum:
http://www.opencalphad.com/
I don't no, maybe the full CALPHAD-approach is a bit over-sophisticated for your needs. Also I would like to point out again that you need a respective thermodynamic database in order to calculate your ternary phase diagram. This is typically a quite hard thing to get for free, even-though in principle most of the rough data is published.
By the why, if you know any other free or open-source CALPHAD-project Please post I would be interested to know.