Abstract
Strengthening by precipitation of second phase is the guiding principle for the development of a host of high strength structural alloys, in particular, aluminium alloys for transportation sector. Higher efficiency and lower emission demands use of alloys at higher operating temperatures (200 °C–250 °C) and stresses, especially in applications for engine parts. Unfortunately, most of the precipitation hardened aluminium alloys that are currently available can withstand maximum temperatures ranging from 150–200 °C. This limit is set by the onset of the rapid coarsening of the precipitates and consequent loss of mechanical properties. In this communication, we present a new approach in designing an Al-based alloy through solid state precipitation route that provides a synergistic coupling of two different types of precipitates that has enabled us to develop coarsening resistant high-temperature alloys that are stable in the temperature range of 250–300 °C with strength in excess of 260 MPa at 250 °C.
Highlights
The amount of strengthening of the Al matrix through dispersion of hard second phase intermetallics depends on the nature of intermetallic/matrix interfaces, hardness of the intermetallic phases, their stability at high temperatures and size and volume fractions[1,2,3,4,5,6,7]
The slower growth kinetics for the quaternary Al-Cu-Nb-Zr alloy as compared to the binary Al-Cu alloy can be discussed on the basis of two factors; additional elastic strain energy minimization and Nb solute-vacancy binding energy
The broad faces of θ′′ precipitates enclose the Al3Zr precipitates leading to formation of “compact” precipitate morphology
Summary
Received: 16 March 2017 Accepted: 21 August 2017 Published: xx xx xxxx aluminium alloys by forming L12 Al3Zr precipitates and nucleating θ′′ precipitates on them. 0.1Nb-0.15Zr in at.% everywhere) was subjected to a unique three step heat treatment It comprised of direct ageing of the as-cast alloy at 400 °C to form the ordered Al3Zr precipitates. This was followed by an optimized higher temperature solution treatment at 535 °C for 30 minutes to dissolve copper solute in the aluminium matrix without affecting the Al3Zr precipitates. The coherent θ′′ precipitates appear in the form of plates with the broad faces having a small positive lattice mismatch (~ + 0.002) and the edges having a large and negative misfit (~ −0.05) with respect to the matrix phase. It can be established that in the compact morphological microstructure, the θ′′ precipitates grow at a significant slower rate compared to the homogeneously formed θ′′ precipitates in binary Al-2Cu alloy
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