In 1983, the magnesium engine cases on Freddie Spencer’s three-cylinder Honda NS500 were run 600 kilometers (372 miles) and then returned to the factory to be remachined so they could be used through a second block of mileage. After that, too weak for reuse, the cases were scrapped.
Googling an assortment of metallurgical papers revealed what went on at that time. Conventional magnesium alloys display what is called “creep,” a gradual yielding under stress, which is accelerated by temperature. This is what allowed Spencer’s crankcases to lose their shape. They were scrapped because the final stage in creep is the formation of voids, which join to form propagating cracks.
Bud Aksland, who worked for years with 500cc GP bikes in Kenny Roberts’ organization, said he liked magnesium crankcases because, under stress, they relaxed to “the shape they liked,” allowing the engine to be freer revving.
Around 1980, magnesium was reevaluated by the automakers, which were under pressure from governments to reduce fuel consumption. Lighter cars need less fuel to accelerate in stop-and-go driving, so the resulting “lightweighting” made magnesium interesting. Steel’s density is 7.8, aluminum’s is 2.8, but magnesium’s is just 1.74—40 percent lighter than aluminum and 78 percent lighter than steel.
Metallurgists saw unexplored possibilities. While the alloy systems of iron, copper, and aluminum have been intensively studied and developed, those of magnesium had barely reached adolescence. Work on magnesium had stagnated because the metal corrodes easily, is subject to stress creep (especially at temps of 150 degrees Centigrade and higher), and the best alloys of the ’60s signed off at low levels of tensile and compressive strength. Mag looked like a lot of work for little gain.
Today, new mag alloys have allowed the light metal to graduate from minor roles in parts, such as seat frames and dashboard instrument arches, to use in the Corvette engine cradle and even in a six-cylinder BMW engine block.
To understand what the structure of metals looks like when magnified, look at any polished granite surface. You will see a jumble of grains, often of several types, all stuck together every which way. Each grain is a crystal in which atoms have arranged themselves into regular planes like stacked sheets of ball bearings. Metals are polycrystalline. Remember when old-timers, holding a broken part, would say, “See, she’s crystallized”? Such statements are nonsense.
Metallurgists calculated what the theoretical strengths of metals should be based upon interatomic forces, but the calculated strengths were many times greater than those of real metals. Why the difference? Good thinkers theorized, and later X-ray diffraction studies confirmed, that real crystals contain defects—incomplete atomic sheets, missing lines of atoms, etc.— called “dislocations” (meaning that some atoms were “dislocated” from their proper places).
To slide one sheet of atoms as a whole across another would require overpowering all of their interatomic bonds simultaneously. But the presence of a defect, a dislocation, was analogous to the backlash in the car-to-car couplers of a long freight train. The locomotive does not have the pulling power to start all the cars in the train simultaneously, but coupler backlash allows it to start the first car, then the second, then the third, until, finally, the whole train is moving.
In the case of metals, applied stress can move these dislocations, allowing one sheet of atoms to gradually slip across its neighbor. Slippage can also take place at grain boundaries. This is how metals yield under stress.
The strengthening of metals, therefore, becomes the problem of preventing applied stress from pushing dislocations through the material. Work hardening is a process of deforming a metal so much that the dislocations in its crystals are driven so far that they become tangled or pinned and cannot be pushed any further. Rolling, forging, or extruding metals can act just as hammering does: hardening a material by immobilizing its dislocations.
BMW 6-cylinder petrol engine with VALVETRONIC – magnesium-aluminum-compound-crankcase
Another method is solid solution hardening. When a few nickel atoms are dissolved in steel, the presence of the different-sized nickel atoms here and there induces stress into the crystal arrangement that makes it harder for dislocations to move. Just before 1900, when Winchester Arms began to alloy nickel into its gun steel, the resulting increase of strength came from this mechanism.
Precipitation hardening seeks the same result as the above: to provide points of resistance to the movement of dislocations under applied stress. But this is accomplished by creating tiny foreign particles to impede dislocations. Many elements have some solubility in metals when they are hot but lose that solubility as the metal cools and solidifies. The foreign element(s) are then forced out of solution and clump together to form precipitated particles in the solid material. In the case of the widely used high-strength stainless steel 17PH, the “PH” stands for precipitation hardening.
As children, most of us performed the experiment of dissolving sugar in a cup of hot water, stirring and adding sugar until no more would dissolve. As the water cooled, the solubility of the sugar in it decreased, and excess sugar was forced out of solution to form sugar crystals (“rock candy”).
Aluminum, alloyed with copper and nickel, shows a similar behavior. When hot, a small amount of copper and nickel can be held in solution in the aluminum, but as the melt cools, some of the two alloying elements is precipitated out as myriad tiny intermetallic particles. Because it takes time for this precipitation to mature, such precipitation-hardened alloys are also termed “age hardening.”
The aluminum alloy Z5D, from which Yamaha built its first aluminum racing chassis in 1980, was just such an age-hardening alloy and, therefore, needed no heat treatment to regain its strength after welding. All it had to do was “rest” for a week or two, allowing the slow thermal diffusion of atoms in the material to form precipitated particles.
In the case of magnesium, recent alloys employ aluminum, silicon, rare-earth elements, strontium, yttrium, or gadolinium to produce such precipitated particles. Older alloys lost strength over time by a process called “over-aging.” Precipitation hardening works best when it forms greater numbers of smaller particles, but over time, and especially at higher temperatures, the smaller particles can clump together to create fewer, bigger ones (just as ice crystals in ice cream become coarser if that pleasant dessert spends too much time in a freezer that’s not quite cold enough). The over-aged material loses strength. This process has in the past made magnesium a poor choice as a cylinder-block material, for example. Cylinders that gradually become less and less round are not desirable!
The turbine blades of early jets suffered the same problem: creeping at temperature. Blades attached to whirling turbine discs gradually grew longer until they began to scrape on the inside of the turbine housing—not acceptable. Strongly precipitation-hardened when originally manufactured, such parts gradually lost strength through over-aging.
The first way to deal with this was to periodically tear the engines down, remove their hot-section parts, and re-heat-treat them. An improved alloy, using a bit of titanium to stimulate the formation of extremely numerous and tiny precipitated particles, was therefore created. After the war, this alloy, the British Nimonic 80A, was adopted as the exhaust-valve material for BSA 500 Gold Stars and Harley-Davidson Big Twins. Can’t have your valves breaking because of uncontrolled precipitate over-aging!
The element yttrium has practically zero solubility in magnesium at room temperature but a small amount is soluble in molten mag. As a magnesium-yttrium alloy cools, the yttrium is pushed out of solution to form a very large number of tiny particles, and these act as nucleation sites on which other alloying elements can precipitate as intermetallics. All these tiny particles not only increase strength by impeding dislocation movement, but their uniformly tiny size makes it harder for any one particle to grow at the expense of smaller particles nearby. This slows the process of over-aging.
There are now magnesium alloys with tensile strengths more than 80,000 psi, a huge improvement over the 24,000 considered good during the ’60s (tensile strength of mild steel is 65,000 psi). Magnesium has come of age, now rapidly catching up in its sophistication with other alloy systems. We can expect to see a lot more magnesium parts in the years ahead.
» Developments in Magnesium, Part 1