In the present work, the effect of heat treatments on the microhardness and tensile properties of the Al-0.25 Zr (wt.%) alloy have been investigated. The Al-0.25 Zr (wt.%) alloy was melted in a vacuum furnace, and the molten alloy was poured into crucibles held in a hot filing furnace. Then, the samples were directionally solidified from bottom to top and aged isothermally and isochronally in a muffle furnace. Aging was performed in two ways: using a wide range of temperatures (350-600 degrees C) with a constant aging time (100 h) and a wide range of aging times (3-240 h) with a constant temperature (400 degrees C). The dependence of the microhardness (HV) and ultimate tensile strength (sigma(UTS)) on the aging temperatures and aging times was determined. According to the experimental results, the HV and sigma(UTS) values of the aged samples increase at a certain aging temperature and aging time values, reaching peak values at specific temperatures and aging times. The microhardness and ultimate tensile strength decreased with further increase of aging temperatures and aging times. The microscopic fracture surfaces of the aged samples under different aging conditions were observed using scanning electron microscopy. Fractographic analysis of the tensile fracture surfaces shows that the type of fracture changed significantly from ductile to brittle depending on the aging times. Transmission electron microscopy was also used to characterize the precipitation processes in an Al-0.25 Zr (wt.%) alloy aged at 400 degrees C for 120 h. (C) 2015 Elsevier B. V. All rights reserved.