On the origin of a remarkable increase in the strength and stability of an Al rich Al-Ni eutectic alloy by Zr addition

The effect of Zr addition to Al rich binary alpha-Al- Al3Ni eutectic cast alloy (Al-3.1 at.% Ni) in enhancing the microstructural stability and strength at high temperature is demonstrated. On subsequent heat treatment after casting, nanometric coherent L1(2) ordered Al3Zr precipitates form inside the alpha-Al that strengthen the alloy. Additionally, remarkable stability of eutectic microstructure was observed even after 100 h of annealing at 400 degrees C. The synergetic effect of the strengthening of the alpha-Al matrix by coherent Al3Zr precipitates and the low coarsening rate of the Al3Ni rods results in a significant increase in high temperature hardness and yield strength of the alloy. The tensile yield strength of the annealed Al-3.1Ni-0.15Zr alloy (400 degrees C, 10 h) tested at 250 degrees C is found to be 185 +/- 10 MPa, which is 1.5 times higher than the corresponding binary Al-3.1Ni alloy. The experimentally determined average rod size (radius) during annealing at 400 degrees C follows the classical matrix diffusion controlled LSW-based coarsening model for both binary Al-3.1Ni and ternary Al-3.1Ni-0.15Zr alloys. The calculated coarsening rate constant values based on modified LSW coarsening model are 10.3 and 4.1 nm(3)/s for Al-3.1Ni and Al-3.1Ni-0.15Zr alloys, respectively. Atom probe tomographic (APT) investigations of the heat-treated ternary alloy unambiguously reveal segregation of Zr solute at the alpha Al/Al3Ni interface in addition to the presence of the strengthening Al3Zr ordered precipitates in the alpha-Al matrix. The segregation hinders the interdiffusion of Al and Ni in the eutectic and, thereby, increasing the stability of the eutectic phase at high temperature. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.