I teach introductory materials science and engineering to about 100 mechanical engineers, so I need stories about mechanical designs that are enabled or that fail because of materials. I have heard that (until recently) Mg alloys were not usable at even moderately elevated temperatures (100 C) for automotive applications due to creep. This assertion went with stories about transmission cases creeping out of tolerance in only a few years. Can you confirm this?
What changed in the new alloys from the last 10 years or so to enable magnesium engine blocks in BMW's and magnesium castings for transmission housings? What are the mechanisms for this new creep resistance?

Thanks and best regards,
Bob

Robert W. Hyers, Associate Professor
Department of Mechanical and Industrial Engineering
University of Massachusetts
160 Governors Drive, Amherst, MA 01003
ph: 413-545-2253 fax: 413-545-1027
http://www.ecs.umass.edu/mie/faculty/hyers.html
hyers@ecs.umass.edu

While not addressing the Mg example you raise, two resources listed on the Education community
Digital Resource Center provide examples of other design failures due to materials issues which
may be of use in your intro to MSE class.

“Materials Prognosis as a Tool to Assure Readiness”
Christodoulou, L. and Larsen, J.M.. "Fear is Justified: Materials Failure Matters!" TMS Annual Meeting. San Diego, CA. 2003.
This PDF presentation from TMS 2003 Annual Meeting concerns approaches to predicting materials performance in service and to managing risk of materials failure. Slides 7-10 show photographs of actual transportation disasters linked to micrographs of material defects.

"Metal Fatigue Section." Component Failure Museum. The Open University, Department of Materials Engineering. © 2005 Materials Engineering.
This site includes photographs of failed parts due to metal fatigue along with accompanying text. Failures include truck, car, and bicycle parts. Other sections of the "museum" include failures due to corrosion, manufacturing faults, bicycle component failure, polymer component failure, and failure of a carbon fiber reinforced polymer bicycle frame.

Both resources can be found under the heading:
Visuals / Defects:Visualizations

Cathy Lane Rohrer, Ph.D.
Moderator, Education and ICME Communities
MaterialsTechnology@TMS
crohrer@tms.org

You may be interested in a resource on creep of magnesium alloys that is already on the site. If you go to the Alloy Tab of the Magnesium Digital Resource Center, then to the Alloy Development Folder, you will find an entry for a presentation contributed by Prof. Mihriban Pekguleryuz of McGill University entitled “Magnesium Alloy Development for High Temperature Automotive Applications”. Her presentation discusses new alloys, creep mechanisms etc.

Lynne Karabin
Moderator, Magnesium and Superalloys Communities
MaterialsTechnology@TMS
lkarabin@tms.org

Thanks Lynne! That's exactly what I was looking for.
Bob

In addition to Prof. Mihriban Pekguleryuz presentation there have been several papers published in the magnesium technology proceedings. The basics of creep resistant magnesium alloys for automotive applications go back to the days of the VW Beetle. Several powetrain components were made from AS21. This alloy is extremely castable however, it provides only nominal creep resistance and has a corrosion issue. Lucky the VW Beetle would leak a lot of engine oil and that would help serve as a protective coating on the surface of the casting. The lightweight was critical to the rear engine design of the vehicle. The more modern approach to creep resistance is to add rare earth elements to the base magnesium alloy (AE42, AE44). The rare earth (symbolized as E) is added as naturally occurring mischmetal, usually with amounts of Cerium, Praseodymium, Niobium and others. The mechanisms by which creep properties are influenced are still a subject of debate. However, one of the main suspects has been grain boundary sliding for magnesium alloys. The finely dispersed precipitates have been detected as Mg12Ce particles along grain boundaries have been observed. Please also see alloys such as; MRI 153M, MRI 230D, AS21X, AS31, AXJ530, AJ62 for die casting and MRI 201, MRI 202, SC1 and ML10 for sand casting.