Nano composites make stronger alloys

Tom Shelley reports on the latest breakthrough in commercially-available metal matrix composites.

Light alloy metal matrix composites are twice as stiff, 30% to 60% stronger and have much greater fatigue resistance than conventional light alloys if they are made by powder metallurgy with sub micron sized reinforcing particles. This development (by AMC) has come of many years of research in developing alloys for the aircraft industry, but the biggest present markets are motorsport and advanced instruments. Most commercially available metal matrix aluminium alloy composites used in typical automotive applications, such as for brake disks, are made by casting, but these are reinforced with relatively large ceramic particles, some of which clump together. But by going to finer sizes, and using powder metallurgy to ensure that the reinforcing particles stay separated and distributed in an even manner, the resulting mechanical properties are greatly improved. Sales manager Andrew Tarrant sums up the properties of the new 'Xfine' range of aluminium silicon carbide products as: high stiffness to unit weight, good heat resistance, good fatigue resistance and easy to machine and finish. Particle size is 0.1 to 1µm, making the materials true nanocomposites. They are currently being produced in two grades: Xfine217 and Xfine225, which as forged plate, offer UTS values of 630MPa and 703MPa respectively, which is increased from 680 to 780MPa if extruded. Stiffnesses – Young's Modulus figures are 100GPa and 115GPa, similar to titanium alloys, but are 40% lighter. Fatigue limits, always of great concern in aircraft manufacture since they decide safe working life, come out at 331 and 353 MPa over 10 million cycles, more than 2.5 times better than equivalent conventional aluminium alloys. When exposed to heat, retained strength is up to 6 times that of conventional aluminium aerospace alloys because there is significant hardening from non precipitation hardening mechanisms. Examples can be seen in a complicated wheel forging, an F1 suspension upright and an extruded high performance bicycle frame made of the new material. Most of the motorsport applications remain guarded – motorsport teams guard their technical secrets with even greater diligence than the military. "One of the keys", according to Tarrant, "s the ability to machine the materials relatively easily." Thread machining, for example, can be undertaken with conventional tools. Some possible applications can be deduced from the revealed applications of the company's earlier 'AMC' products which are also made by powder metallurgy, but with 2 to 3µm silicon carbide particles, which are used to make pistons, connecting rods, valve trains, bearing caps, liners and blocks. Because of the very long development lead times in aircraft development, Xfine products are still in the R&D phase in this sector, but as well as finding favour with "High performance motorsport", Tarrant says they are finding application in optical and other instrumentation, and in high speed machinery. Design Pointers • New metal matrix composite alloys are stiffer than titanium, steel, conventional aluminium and magnesium alloys per unit weight • Retained strength is up to 6 times that of conventional aluminium alloys when subjected to prolonged, elevated temperatures