Abstract
A study was made of the strength-ductility relation-ship at ambient and low temperatures in substitutional iron alloys with about 2 at% Al, Si, Ti, Cr, Mn, Ni or Mo which were cold rolled and subsequently annealed to attain recovered and/or partially recrystallized structure composed largely of polygonized subgrains of 1μm or less. The deformation behavior in tension of these ultrafine-grained iron alloys was shown to be characterized by very small elongation owing to plastic instability, which caused necking immediately after the yielding or during the propergation of the Lüders bands. It was found, however, that some of solute elements manifested a remarkable effect of improving the ductility and that the effect was fully maintained in the presence of the third element, such as Nb, which was added to retain high strength pertaining to recovered state as a result of a well-known retarding effect on recrystallization.
The occurrence of plastic instability was considered to be intimately related to the change in the Lüders strain with the deformation temperature and grain size of material. Accordingly, various factors which may affect the Lüders strain, i. e. the mode of slip, dislocation structure and distribution of interstitial atoms, were investigated in terms of the influence of substitutional solutes. A high dislocation multiplication rate was found to be the most distinctive feature of the alloys which exhibited large elongation at low temperature.
