Developments In Magnesium, Part 2 Lightweight metal comes of age.
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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