Single crystals of copper were deformed at 4.2°K and 77.3°K. At 4.2°K, after a large strain produced by normal slip, jerky flow (discontinuous slip) occurs. From the study of reactor irradiated crystals, it was deduced that a packet of 30 slip lines, each containing 104 dislocations, was released to form each jerk of the discontinuous flow. For samples of certain orientation the region of discontinuous flow was followed by a region of deformation twinning. The fact that deformation twinning was taking place was verified by x-ray methods. The twinning elements were determined to be the (111) plane and the [112] direction. At 77.3°K, discontinuous slip was not observed. In a limited range of orientations, twinning has been observed at 77.3°K. The effects of reactor irradiations on discontinuous slip and on twinning were also studied and it was determined that the occurrence of twinning was unaffected by the irradiations.

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The residual resistivity increase rate as a function of induced resistivity for fast neutron irradiated Al, Cu, Ag, Au, Pt, Fe, Ni, Co and Mo has been studied. Except for Fe the resistivity increase rate as a function of irradiation induced resistivity is non‐linear. The data is best explained with the expression of Balarin and Hauser. Saturation values of the resistivity and defect concentrations plus static and dynamic recombination volumes have been calculated; they differ from those obtained by irradiation in a reactor spectrum. Recombination volumes are very small for the b.c.c. metals Fe and Mo.

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