Mechanical Properties of High Purity Iron at Low Temperatures

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Abstract
Mechanical properties of high purity irons were investigated by tensile testing at temperatures from room temperature down to 4.2 K. Specimens with the residual resistivity ratio (RRRH) of 1800 were brittle and fractured at grain boundaries at 4.2 K. Specimens with RRRH of 3600, of which the grain structure was almost of the bamboo type, were deformed by slip and often showed complete ductile fracture even at 4.2 K. This is the first observation of slip at 4.2 K without pre-straining at high temperatures in a coarse-grained iron. Specimens with RRRH of 5000, of which the grain structure was of a well-developed bamboo type, cleaved at 4.2 K, and never showed grain boundary fracture. Some plastic properties were investigated on specimens with RRRH of 3600 and 6000. Careful comparison of the results with those reported by other investigators shows that the present specimens are pure enough to reveal some of the intrinsic plastic properties of iron. The yield stress as a function of temperature and the strain rate sensitivity of the flow stress at various temperatures obtained with the present specimens may be considered to be characteristic of iron.