Latest posts by Dr Beth Cope (see all)
- Teaching Resources Website—explore the resources and join the conversation - 15th July 2014
- Lightweight aluminum beryllium alloys - 30th October 2012
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The 22nd MMPDS Coordination Meeting was held last week (October 23-26), with members meeting to discuss the ongoing development of the Metallic Materials Properties Development and Standardization (MMPDS) data. But what is MMPDS, and who uses it? A good way to find out is to take one of the new materials in the latest release, and ask some questions about why it’s in there. Lightweighting is certainly a hot topic at the moment, so perhaps the lightweight aluminum beryllium alloy (AMS 7911) highlighted in MMPDS-06 would be a good example.
Why alloy aluminum and beryllium?
‘Aluminium’ was once so rare and precious that the Emperor Napoleon III of France had a set of aluminium cutlery made that cost him more than silver, and aluminium bars were proudly displayed at the Universal Exhibition in Paris in 1855. Much has changed since then, not least the name: in the United States it has been officially known as ‘aluminum’ since 1926 (Humphrey Davy’s original suggestion) although many European countries continue to use the spelling ‘aluminium’ (which Humphrey Davy himself had settled upon in 1812). Today, cost effective extraction together with low density, ease of formation, and corrosion resistance have helped to make it the second most important metal in the economy (steel comes first) and the mainstay of the aerospace industry.
Beryllium, an alkaline earth metal, has a high elastic modulus, high heat capacity, and low density, making this rare material appealing for lightweight aerospace applications. However, its usefulness is offset by its brittleness and cost. It is also highly dangerous if inhaled (15% of the population are highly susceptible to Chronic Beryllium Disease, an incurable respiratory disease), so needs careful handling in manufacturing.
Combining aluminum’s excellent specific stiffness and processing characteristics with the lightweight strength of beryllium yields an alloy with high thermal stability and good isotropy in mechanical properties. Aluminum beryllium alloys display high thermal conductivity, light weight, and high specific stiffness. This means they are well suited to semi-conductor assemblies, inspection equipment, avionics, and satellites. The dramatic improvement over the properties of raw aluminum means such alloys can be used in mission critical applications: for example, the high modulus-to-density ratio (3.8 times that of pure aluminum) reduces the chance of mechanically induced failure by minimizing flexure.
Aluminum beryllium is not an alloy in the conventional sense, since beryllium has low solubility in aluminum. Instead powders of the two metals must be pressed together to form a metal matrix composite. This may be done using cold isostatic pressing (CIP) or hot isostatic pressing (HIP), after which more conventional rolling or extrusion may be performed. These methods allow production of highly complex shapes with uniform material properties. Given the health dangers associated with beryllium dust, there are clear advantages to near net shape production which minimizes the need for further machining.
The new data in MMPDS-06 applies to AMS 7911 which is produced using hot isostatic pressing. While extrusion or rolling might give a higher strength, the HIP method allows more complex shapes, uniform isotropic properties, and good dimensional stability. This makes it well suited to applications in optics, such as gimbals and mirror systems.
Why Metallic Materials Properties Development and Standardization (MMPDS)?
The Metallic Materials Properties Development and Standardization (MMPDS) data and its predecessor MIL-HDBK-5 is the preeminent source for aerospace component design allowables relating to alloys and fasteners. MMPDS contains over 2,000 records of statistically-derived design data for aerospace alloys in various forms and thicknesses, as well as information on the temperature dependence of mechanical properties, fatigue curves, and corrosion rankings.
For a material to be included in MMPDS, it needs to be available from multiple suppliers and to be highly qualified. Data is characterized by its statistical provenance for each material specification:
- A Basis data—the lower value of either the statistically calculated number T99 (99% excedence, 95% confidence), or the specification minimum (S-Basis).
- B Basis data—at least 90 percent of the population of values is expected to equal or exceed the statistically calculated mechanical property value, with a confidence of 95 percent
- S Basis data—the specification minimum value specified by the governing industry specification (as issued by standardization groups such as SAE Aerospace Materials Division, ASTM, etc.) or federal or military standards for the material.
MMPDS is continually maintained and updated via an Industrial Steering Group consisting of leading engineering organizations in which Granta is actively engaged. By sharing characterization and qualification data, members maximize the statically validity of the data included in MMPDS, as well as saving time and money. And once a material has been qualified in this way, it can be reused and applied by other organizations seeking to solve new challenges, without having to invest in an expensive qualification process.
So what does this mean for the AMS 7911 aluminum beryllium alloy? Perhaps you could say that for an aerospace material to make it into MMPDS means that it has really arrived.
Data on the AMS 7911 Aluminum Beryllium alloy is included in the MMPDS data module available from Granta, which provides up-to-date values in a structured database that allows data to be searched and manipulated. Data is available as numeric values, as text and, where applicable, as functional and graphical data (for example, capturing temperature dependent properties). The GRANTA MI materials information management system provides quick and easy web browser-based access to the data. This is an extremely cost-effective method for supporting multiple users as they search, compare, and apply this data. You can also output data to simulation programs at a touch of a button and distribute the handbook data to the desktops of engineers who needs it.
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