Effect of Ni and Mn Additions on the Ambient and High-Temperature Performance of Zr-Containing Al–Si–Cu–Mg-Based Alloys: Role of Precipitation Hardening

Alloys such as the 354.1 (Al–9 wt% Si–1.8 wt% Cu–0.5 wt% Mg) alloys show a greater response to heat treatment as a result of the presence of both Cu and Mg. These alloys display excellent strength values at both low and high temperatures. Additions of Zr, Ni and Mn are expected to maintain the alloy performance at still higher temperatures. The present study was carried out to investigate the effects of these additions on the tensile properties of 354.1 alloy castings at ambient and at high temperature (250 °C), using different holding times at testing temperature. Alloys were prepared using 0.2 wt% Ti grain-refined 354.1 alloy, comprising alloy R (354.1 + 0.25 wt% Zr) considered as the base or reference alloy, alloy S (354.1 + 0.25 wt% Zr + 2 wt% Ni) and alloy U (354.1 + 0.25 wt% Zr + 0.75 wt% Mn), containing Ni and Mn. The latter two alloys perform better under high-temperature conditions following 1-h stabilization at 250 °C, displaying strength values of 226.64/230 MPa UTS and 225/226 MPa YS, respectively. For a prolonged stabilization time (200 h at 250 °C), however, the strength values are reduced to 111/96 MPa UTS and 90/78 MPa YS. The decrease in strength and increase in ductility observed after prolonged stabilization are related to alloy softening, which occurs because of the overaging conditions during which the equilibrium precipitates form, leading to loss of coherency strain between the precipitates and the matrix.