Identifier

etd-042012-133156

Abstract

Aluminum-zirconium alloys exhibit superior strength at elevated temperature in comparison to traditional aluminum casting alloys. These alloys are heat-treatable and their strength depends to a large extent on the quenching and aging steps of the heat treatment process. However, measurements show that the critical cooling rate necessary to retain 0.6 wt. pct. zirconium(the minimum amount necessary for significant strengthening) in a super-saturated solid solution with aluminum is 90ºC/s, which is un-attainable with traditional casting processes. On the other hand, the critical cooling rate necessary to retain 0.4 wt. pct vanadium and 0.1 wt. pct. zirconium in a super- saturated solidsolution with aluminum is only 40ºC/s; which suggests that substituting vanadium for zirconium significantly decreases the critical cooling rate of the alloy. This is an important finding as it means that, unlike the Al-0.6Zr alloy, the Al-0.4V-0.1Zr alloy may be processed into useful components by traditional high pressure die-casting. Moreover, measurements show that the hardness of the Al-0.4V-0.1Zr alloy increases upon aging at 400ºC and does not degrade even after holding the alloy at 300ºC for 100 hours. Also, measurements of the tensile yield strength of the Al-0.4V-0.1Zr alloy at 300ºC show that it is about 3 times higher than that of pure aluminum. This increase in hardness and strength is attributed to precipitation of Al3(Zr,V) particles. Examination of these particles with high resolution transmission electron microscopy (HRTEM) and conventional TEM show that vanadium co-precipitates with zirconium and aluminum and forms spherical particles that have the L12 crystal structure. It also shows that the crystallographic misfit between the precipitate particles and the aluminum matrix is almost eliminated by introducing vanadium into the Al3Zr precipitate and thatthe mean radius of the Al3(Zr,V) particles is in the range from 1nm to 7nm depending on the alloy composition and aging practice. Finally, it is found that adding small amounts of silicon to the Al-0.4V-0.1Zr alloy effectively accelerates formation of the Al3(Zr,V) precipitate.

Publisher

Worcester Polytechnic Institute

Degree Name

MS

Department

Materials Science & Engineering

Project Type

Thesis

Date Accepted

2012-04-20

Accessibility

Unrestricted

Subjects

aluminum alloys, high temperature strength, precipitation hardening

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