Chemistry-structure-property relations in the AlCrFeMnNi alloy system
Restricted (Penn State Only)
- Author:
- Gesualdi, Jarrod
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 17, 2023
- Committee Members:
- Amanda Johnsen, Outside Unit & Field Member
Hojong Kim, Chair & Dissertation Advisor
Allison Beese, Major Field Member
Susan Sinnott, Major Field Member
John Mauro, Program Head/Chair - Keywords:
- electrochemistry
metallurgy
corrosion
microstructure
thermodynamics - Abstract:
- High entropy and compositionally complex alloy research represents a broad domain of metallurgy, corrosion, and thermodynamics research currently in progress in the materials science community. Systematic studies within these domains are critical in understanding the influence of alloy chemistry on mechanical properties, corrosion properties, and thermodynamic properties. The use of relatively lower-cost elements such as Al, Fe, Mn, and Cr as an alloy is of great interest to industries which use metals at large scale, such as for structural members, and careful additions of elements such as Ni are critical for high-performance applications. This thesis outlines the research work in AlCrFeMn, AlCrFeMnNi alloys. Chemistry-structure-property relationships are outlined by systematic control of Al, Cr, and Ni content and observation of bulk phase behavior changes, microstructure changes; and resultant shifts in mechanical properties and corrosion properties in 0.6 M NaCl. This work shows that in AlCrFeMn, Al and Cr effectively stabilize the BCC phase, increasing hardness, and are both effective in improving corrosion resistance in 0.6 M NaCl. While pure Al is not well regarded for its resistance to chloride induced pitting, alloys with relatively low Cr content (5 at% Cr) which did not exhibit passivation in polarization testing, were able to develop a protective passive film as Al content increased from 0–15 at% in the alloy. At higher Cr contents (10, 15 at%), Al addition did not result in a clear benefit to corrosion resistance in polarization testing, but did demonstrate an improvement in corrosion resistance in 24 h exposure tests in air-saturated 0.6 M NaCl, evident by the reduction in frequency of pitting events on the surface. Systematic addition of Ni to Al10Cr15(Fe3Mn)75 to explore the feasibility of high-performance alloys from this system, demonstrated that Ni addition resulted first in ordered phase formation (5, 10 at% Ni), and second in stabilization of the FCC phase (15 at%, 20 at%). The ordered phase consisted of the B2 structure rich in Al and Ni, and deficient in Cr. These ordered phase precipitates increased hardness (~210, ~440, ~465 HV for 0, 5, 10 at% Ni), and promoted high yield strength (YS= 600 MPa, 15 at% Ni). However, their low Cr content resulted in their preferential corrosion from observations after polarization testing. The FCC phase demonstrated superior corrosion resistance. Thermodynamic properties of a selection of AlNi, AlFe, AlFeNi, AlCrFeMn, and AlCrFeMnNi alloys were determined by the electromotive force method between 673–973 K. These measurements demonstrated the decrease in activity of Al in the CCA as Ni content was added (e.g. Al15Cr15(Fe3Mn)70 and Al15Cr15(Fe3Mn)55Ni15, activity of Al (ln a_Al) = -12.5 vs. -17.0 at 773 K). This suggests that Al and Ni undergo a strong chemical interaction even in a compositionally complex alloy. These measurements provide evidence in terms of thermodynamic data for the driving force responsible for consistent ordered phase formation in Al and Ni containing CCAs observed in this work.