The aim of this study was to develop a new castable and heat treatable alloy, based on the A205 alloy composition, with improved thermal stability at 230 °C. Several unique combinations of alloying elements, chosen from groups IIIB, IVB and VB, up to 0.5 wt. %, were added to the A205 alloy. These elements were chosen to nucleate high temperature precipitates, with a cubic L12 structure. The additions were made in group of two, with one elements being always the zirconium. After 1000 hours holding at 230 °C, several of these alloys resulted to have better mechanical properties when compared to the A205 alloy. The IDEAL alloy was chosen and further studies were carried out. This alloy showed the same tensile proprieties of the A205 alloy at peak-aged condition, while at elevated temperature, the IDEAL alloy had 15 % improved UTS and YS. An investigation was carried out to understand the reasons of this improvement. Two strengthening mechanisms were found. Firstly, the microstructure of the alloy showed a reduced size of the Al-Cu precipitates. Secondly, two different high temperature precipitates were nucleated.