טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentAvraham Shaul
SubjectStrengthening Mechanisms in Mg-Al-Sn-Zn Based Alloys
DepartmentDepartment of Materials Science and Engineering
Supervisors Professor Emeritus Menachem Bamberger
Professor Wayne D. Kaplan
Full Thesis textFull thesis text - English Version


Abstract

Mg alloys are highly attractive for engineering applications thanks to their high specific mechanical properties. The weight reduction in engineering components can result in reduced energy consumption and beneficial environmental implications. The Mg alloy related benefits motivates scientific and engineering interest in development of new creep resistant Mg alloy for elevated temperature applications (T>200°C).

This research is aimed at the processes that take place during high temperature creep deformation in newly developed Mg-Al based alloys. The calculation of phase diagrams method was applied in the development of new creep resistant Mg-Al alloys, model Mg-Sn-Zn alloys for assessing the influence of Zn on the precipitation of Mg2Sn and design of heat treatments.

Different heat treatments were optimized for the Mg-Sn-Zn and the newly developed Mg-Al based alloys. The simulations allowed us to get a maximal hardening response and avoid the presence of a liquid phase during solution treatment. The heat treatments were optimally designed without melting of the eutectic microstructure that forms during casting.

The addition of Zn (Mg-8Sn-1.25Zn) to the binary alloy results in the early appearance of two Vickers micro-hardness maxima in addition to the previously detected Vickers micro-hardness maxima. The initial Vickers micro-hardness maxima can be related to the formation of Zn-rich zones (Zn-vacancy clusters). The second additional Vickers micro-hardness maxima relates to the formation of Mg4Zn7 precipitates. The addition of Zn refines the Mg2Sn precipitates, and thus increases the Vickers micro hardness.

The newly developed castable Mg-Al alloys - New-Base and New-Base (NB & NB) were designed and proved to be creep resistant. The NB alloy is an AM50&AE42 derivative modified with Sn. The NB alloy lacks the Ce Mischmetal, it allows us to differentiate between the influence of Mg2Sn and the Al-RE intermetallics during service. Low Al content is favored in order to reduce the γ-Mg17Al12 phase amount and reduce the homologues temperature distribution. Sn was added in order to promote precipitation hardening by Mg2Sn and RE-elements were incorporated in Al-RE intermetallics that are present at the grain boundary. The predictions related to commercial reference alloys (AZ91, AE42, AM50) and the NB and NB alloys were confirmed by experimental results and available data in the literature.

The principles of the development of creep resistant castabele Mg-Al alloy were applied and proved to be highly accurate. The calculation of phase diagrams method reduces the effort and time involved in alloy design and improves the reliability of the results.