Thermal Stability of Aluminum-Nickel Binary Alloys Containing the Al-Al₃Ni Eutectic

The capability of Al–Ni binary alloys containing from 3 to 11 wt pct Ni and ranging from hypo-to hypereutectic compositions to retain strength at high temperatures was assessed through the uniaxial compression up to 500 °C and correlated with their solidification characteristics, phase composition and microstructure. The Al-Ni alloys, strengthened by the Al₃Ni eutectic and proeutectic phases, at room temperature reached the yield stress of 70 to 100 MPa, being higher than 26 MPa measured for pure Al but well below 170 MPa achieved for the A380 (Al-8Si-3Cu, wt pct) alloy, tested under identical conditions. During high-temperature compression, the yield stress of Al-Ni alloys reduced at a steady rate with temperature, retaining at 400 °C and 500 °C about 50 pct and 25 pct, respectively, of its initial room temperature level. This yield stress reduction did not correlate with the low coarsening rate of Al₃Ni phases and high hardness retention ratio of 75 to 80 pct, measured after 168 h heating at 500 °C. Although the higher Ni content resulted generally in the higher yield stress, the contribution of Al₃Ni phases to retaining the alloy strength reduced with testing temperature; at all testing temperatures the finer Al₃Ni eutectic phase was more effective than the coarser Al₃Ni proeutectic phase, with an influence of the latter disappearing completely above 300 °C. An implication of the limited role of Al₃Ni eutectic and proeutectic phases in room temperature strengthening of Al-Ni binary alloys and in retaining their strength at increased temperatures for development of novel aluminum alloys is discussed.