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Exploration of AlCrFeNiTi composition space for precipitation strengthened CCAs

Conventional alloy development methodologies which specify a single base element and several alloying elements have been unable to introduce new alloys at an acceptable rate for the increasingly specialised application requirements of modern technologies. An alternative alloy development strategy searches the previously unexplored central regions of multi-component phase space for alloys whose properties can be tuned with a greater degree of control than previously achievable. The targeted exploration of composition spaces containing five or more elements presents a significant challenge due to the vast number of possible alloy combinations. Novel approaches are required to efficiently map the boundaries of unique phase and morphology formation domains over large regions of multi-principle-element composition space.

Single-principle-element Fe-base alloys containing Al, Cr, Ni, and Ti additions and having a bcc solid solution matrix with hierarchically arranged B2-NiAl and L21-Ni2TiAl precipitate phases have been found to exhibit excellent high temperature strength and creep resistance up to temperatures of 700 °C and stresses of 100 MPa [1-4]. To further improve upon these so-called “Fe-base superalloys”, we have employed a CCA development strategy. Using compositionally graded thin film synthesis coupled with high-throughput micro-beam X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS) composition analysis, and nano-indention (NI), we have screened a large portion of the Fe-rich (30-35 at.% Fe) composition space. In a single thin-film deposition, the phase formation along a continuous concentration gradient from 15 to 25 at.% Al has been obtained. The information gained from this screening has significantly reduced the size of the composition space of interest, allowing targeted bulk-alloy casting in order to determine unique regions of microstructure formation and resulting mechanical properties. Several alloys containing 20-25 at.% Al have been determined to be excellent candidates for applications with a maximum service temperature up to 800 °C, representing a 100 °C improvement relative to the conventionally designed alloys. Several of these alloys have been subjected to in-depth characterization to better understand the mechanisms behind their phase formation and mechanical properties. The alloy development methodology applied here can be used as a template for application-oriented alloy discovery and optimisation.

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