The effects of heat treatment and purity on the mechanical properties of monocrystalline NiAl

The yield stress, σy, strain to fracture, εf, and thickness-compensated fracture load of monocrystalline NiAl were investigated as functions of purity, annealing treatment and cooling rates. A miniaturized disk bend test was employed to test disks 3 mm in diameter and approximately 250 μm thick. Specimens in the soft (110) orientation from commercial purity (CP) and high-purity (HP) alloys were tested. The annealing treatments affected σy more strongly than the other mechanical properties. The yield stresses of both of the as-received alloys increased significantly after annealing at 1300 °C followed by furnace cooling. Subsequent annealing at 400 °C for 2 h resulted in a reduction of σy. This behavior is attributed to the role of excess vacancies retained during cooling, which annealed out at the lower temperature. The mean values of the yield strength also tended to decrease with increasing cooling rate from 400 °C, but the effect was small. A substantial increase in σy of the CP alloy was found on prolonged aging at 400 °C, whereas σy of the HP alloy was unaffected. We attribute this behavior to the precipitation of very small particles or solute atom clusters in the CP alloy. The ductility of the CP alloy, and the thickness-compensated fracture loads of both alloys, were relatively insensitive to the heat treatments.